Photostability of laser dyes in sol-gel-derived hosts
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Authors Suratwala, Tayyab Ishaq, 1970-
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PHOTOSTABILITY OF LASER DYES IN SOL-GEL DERIVED HOSTS
by
Tayyab Ishaq Suratwala
Copyright © Tayyab Ishaq Suratwala 1996
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING
In Partial Fulfillment of the Requirements
For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1996 DMI Nxjmber: 9720676
Copyright 1996 by- Sura twala, Tayyab Ishaq
All rights reserved.
UMI Microform 9720676 Copyright 1997, by UMI Company. All rights reserved.
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THE UNIVERSITY OF ARIZONA ® GRADUATE COLLEGE
As members of the Final Examination Committee, we certify that we have read the dissertation prepared by Tayyab Suratwala entitled Photostability of Laser Dyes in Sol-Gel Derived Hosts
and recommend that it be accepted as fulfilling the dissertation requirement for the Degree of Doctor of Philosophy
W. Bickel Date C Date ////J /9^ Date
Date
Date
Final approval and acceptance of this dissertation is contingent upon the candidate's submission of the final copy of the dissertation to the Graduate College.
I hereby certify that I have read this dissertation prepared under my d^ect;i9n and recommejid—trKat it be accepted as fulfilling the dissertation
Dissertation Director D.R. Uhlmann Date 3
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fiilfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgement of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the copyright holder.
SIGNED: 4
ACKNOWLEDGEMENTS
First of all, I would like to thank my parents. Their love, feith, prayers, and encouragement have been a foundation for my successes. Without their contributions, I would not be where I am today.
I would like to thank Professor D-R. Uhlmann "boss" for his advisement, his enthusiasm, and his support. His knowledge and insight on science, technology, industry, and an infinite number of other topics has made discussions with him invaluable. He is a wonderfiil mentor and role model, and I have benefited greatly as a scientist by being his student.
Research help and collaboration have been crucial to the successes of this work. Thank you to Professor Zack Gardlimd for the synthesis of the silylated dyes and for the endless number of discussions and advice about the research project and life in the industrial world. Thank you to Kevin Davidson for his help with the lab work and his devotion to the project. Thanks to the collaborators at Optical Sciences, especially Jason Watson and Sandra Bonilla, for the optical measurements and their contributions to this project. I would also like to thank Professor Nasser Peyghambarian for the use of his facilities.
Thank you to the other members of my committee. Professor Brian Zelinski, Professor William Bickel, and Professor Elizabeth Krupinski, for reading my dissertation and your comments.
Thank you to the financial supporters of this work including the Air Force Office of Scientific Research, the Coming Foundation Fellowship, and the Chapman Fellowship.
I would like to thank all my fiiends here in Arizona including my roommates (Sajiv, Joe, Ryan, and JeflO who have made evenings eventful and stress relieving and AML fiiends who were always willing to help with those little things. They made my time spent in Tucson fim and exciting. Finally, thank you to Mustafe, a life-long fiiend, who has always provided me motivation and has driven me to strive for the best of everything. 5
TABLE OF CONTENTS UST OF FIGURES 7
LIST OF TABLES 12
ABSTRACT 14
1) INTRODUCTION/BACKGROUND 16 1.1) Dye Lasers 16 1.2) Ruorescence and Dye/Host Interactions 18 1.3) Photochemistry 23 1.4) Polymer Hosts 29 1.5) Inorganic Hosts 30 1.6) Polyceram Hosts 33 1.7) Coumarin Dyes 34 1.8) Pyrromethene-BFzDyes 36 1.9) SUylated Dyes 38 1.10) Research Objectives 39 2) EXPERIMENTAL 40 2.1) Synthesis of Laser Dyes 40 2.2) Host Chemistry 45 2.2.1) Synthesis ofXerogel Films 45 2.2.2) Synthesis of Polyceram Monoliths 46 2.3) Optical Characterization 55 2.4) FTIR Monitoring of Hydrolysis Reaction Rates 57 2.5) Dye Extraction Measurements 58 2.6) Surface Area and Porosity Measurements 58 2.7) NMR Spectroscopy 58 3) COVALENT BONDING OF LASER DYES TO HOST 60 3.1) Hydrolysis Reaction Rate 60 3.2) Dye Extraction/Thermal Stability 65 3.3) SoUd-State ^Si NMR 71 3.4) Fluorescence EfiBciency 74 3.5) Photostability 76 3.5.1) Fluorescence Photostability 76 3.5.2) Absorption Photostability 79 3.5.3) Effect of Prehydrolysis of the Dye 82 3.5.4) Xerogel Films 84 3.5.5) Polyceram Films 90 3.5.6) Neat Films 94 6
3.6) Spectroscopy 97 3.6.1) Structure Effects of Dye 97 3.6.2) pH Effects 99 3.6.3) Host Effects 109 3.6.4) Drying/gelling effects 116 3.7) Summary: Advantages of Covalent Bonding 121 4) MECHANISM OF PHOTODEGRADATION 122 4.1) Role of Oxygen 122 4.2) Photodegradation Chemistry 125 4.2.1) Degradation Products ofPM-567 125 4.2.2) Acid vs Base Environment 127 4.2.3) Multi-Step Degradation 129 4.3) Types of oxidation processes 131 4.4) Role of Oxygen in Different Hosts 133 5) POROSITY AND PHOTODEGRADATION KINETICS 137 5.1) Control ofPorosity as a Function of Sol-Gel Processing 137 5.2) Control ofPorosity as a Function of Composition 146 5.3) Photostability 155 5.4) Structural Model of the Polycerams 157 5.5) Models to Explain Fluorescence Photostability Behavior 162 5.5.1) Thermal Degradation 163 5.5.2) Singlet-Triplet Equilibration 168 5.5.3) Dyes Located on the pore surface of host 172 6) ADDITIVES TO IMPROVE PHOSTABILITY 184
7) HOW TO MAKE A PHOTOSTABLE DYE/HOST SYSTEM 194
CONCLUSIONS 198
FUTURE WORK 201
APPENDIX: GAUSSIAN MODEL 203
REFERENCES 207 7
UST OF FIGURES
Figure 1.1: Energy diagram for a typical organic molecule' 19
Figure 1.2: Decays pathways for an excited organic molecule 20
Figure 1.3: Photooxidation of 7-(dialkylamino)-4-methyIcoumarin 36
Figure 1.4: Structure of a pyrromethene-BFi complex ^ 37
Figure 2.1 Structure of coumarin dyes: (a) Coum (b) derCoum (c) 2derCoum (d) 2Coum (e) derPCoum (f) 2derPCoum 43
Figure 2.2: Structure ofPyrromethene 567 44
Figure 2.3: Structure of silanol terminated PDMS (n=6-8) 46
Figure 2.4a: Synthetic Route 1: Xerogel Films 49
Figure 2.4b: Synthetic Route 2: Neat Films 50
Figure 2.4c; Synthetic Route 3: Polyceram Acid Route 51
Figure 2.4d: Synthetic Route 4: Polyceram Acid/Base Route: Non Synersis 52
Figure 2.4e: Synthetic Route 5: Polyceram Acid/Base: Synersis 53
Figure 2.4f: Synthetic Route 6: 3AS Polycerams 54
Figure 2.5: Photostability measurement setup 56
Figure 2.6: Photostability setup using a Nd:YAG laser pump 57
Figure 3.1: FTIR spectra of (a) TEOS and (b) derCoum at different hydrolysis times.... 62
Figure 3.2: Hydrolysis rate of TEOS and the silylated coumarin dyes 63
Figure 3.3: Ball and stick model of derCoum in its lowest energy form 64
Figure 3.4: Dye extraction of derCoum and Coum xerogel films at r=0.10 synthesized by Route 1 66
Figure 3.5: Dye extraction of derCoum xerogel films at r=0.10 synthesized by Route 1 with various TM0S:H20 ratios 67 8
Figure 3.6: Dye extraction of Coum xerogel films at r=0.05 synthesized by Route 1 with various TM0S;H20 ratios 68
Figure 3.7: Dye extraction fi-om a bifunctionaiized dye 2derCoum in xerogel films at r=0.10 synthesized by Route 1 under partial and fiill hydrolysis conditions 69
Figure 3.8; Loss of dye upon heating Coum and derCoum doped Polycerams at r=0.001 synthesized by Route 3 71
Figure 3.9: Possible silicate structures formed in the sol-gel process ^ 72
Figure 3.10: CP-MAS ^Si NMR of a derCoum xerogel at r=0.10 synthesized by Route 1.73
Figure 3.11: Relative fluorescence eflSciency as a fimction of concentration for Coum and derCoum xerogel films synthesized by Route 1 76
Figure 3.12: Typical fluorescence photostability curve of a Coumarin xerogel at r=0.05 synthesized by Route 1 77
Figure 3.13: Photostability of a derCoum xerogel film synthesized by Route I illustratmg the partial reversibility of the initial decay in photostability 78
Figure 3.14: Absorption spectra of (a) coumarin xerogel at r=0.05 synthesized by Route and Pyrromethene doped Si02:PDMS Polyceram at r=0.5*10"® synthesized by Route 4 upon exposure to UV light 80
Figure 3.15: Normalized absorption spectra area as a fimction of the normalized maximum absorption of a derCoum xerogel at r=0.05 synthesized by Route 1 and a PM- 567 Si02:PDMS Polyceram at 1=0.5*10"^ synthesized by Route 4 82
Figure 3.16: Photostability FOM of derCoum xerogel fikns at r=0.05 synthesized by Route 1 as a Sanction of prehydrolysis time of the dye 84
Figure 3.17: UV photostability of Coum and derCoum xerogel at 0.05 M 86
Figure 3.18: Proposed structures of Coum and derCoum in xerogel hosts 88
Figure 3.19: UV Photostability of coumarin dyes in MeOH at 10"^ M measured in quartz cuvettes 90
Figure 3.20: UV photostability of Coum and derCoum Polycerams at r=10"^ synthesized by Route 3 92
Figure 3.21: Predicted dye-dye distances as fimction of dye:Si mole ratio 93 9
Figure 3.22: Fluorescence photostability of a derCoum xerogel synthesized by Route 1 and a derCoum Polyceram synthesized by Route 3 at t=0.10 94
Figure 3.23: Effect of silylated dye hydrolysis time on the FOM of photostability for neat films synthesized by Route 2 96
Figure 3.24: Absorption and Fluorescence spectra of Coumarin dyes in MeOH at 10'^ M.98
Figure 3.25: Fluorescence spectra of coumarin dyes: (a) Coum (b) derCoum (c) derPCoum (d) 2Coum (e) 2derCoum (Q 2derPCoum in different chemical environments. 104
Figure 3.26: Electron transfer in a urethane linked silylated dye 108
Figure 3.27: Energy diagrams for dye molecules in solution (A) and in solid hosts (B). Ill
Figure 3.28: Fluorescence spectra of coumarin dyes in MeOH and xerogel host (synthesized by Route 1) when pumped at (1) 320 nm, (2) 337 nm, (3) 365 nm 115
Figure 3.29: Fluorescence spectra of a derCoum xerogel solution (synthesized by Route 1) pumped at 337 nm 116
Figure 3.30: Fluorescence spectra of derCoum in MeOH/water mixture (9:1): (1) 0.10 M HCl (2) Neutral (3) 0.10 M NaOH 117
Figure 3.31: Fluorescence spectra of a dip coated derCoum xerogel solution (synthesized by Route 1) at different drying times and heating times when pumped at 337 nm 118
Figure 3.32: FTIR transmittance spectra of a derCoum xerogels (synthesized by Route 1) air dried and after heating at 125 C 120
Figure 4.1: Fluorescence photostability of a derCoum Xerogel synthesized by Route 1 in different atmospheres 123
Figure 4.3: Structure of 3-ethyl-2-methyhnaleimide 126
Figure 4.4: FTIR spectra of PM-567 and degraded PM-567 in chloroform 127
Figure 4.5: Oxidation routes of alkylpyrrole to produce a maleimide 130
Figure 4.6: Proposed degradation route for PM-567 131
Figure 5.1; Gel time as a function of pH for aqueous silicates. After Iler 140
Figure 5.2: Differences m drying behavior of Polyceram solutions processed using Route 4 and 5 142 10
Figure 5.3: CP-MAS ^Si NMR spectra for SiOarPDMS Polycerams synthesized by (a) Route 4 and (b) Route 5 at 40 vol % PDMS 143
Figure 5.4: Surface area of SiOaiPDMS Polycerams synthesized by Routes 4 and 5 as fiinction of the PDMS content 148
Figure 5.5: CP-MAS ^Si NMR for the Route 5 Polycerams with varying polymer contents 151
Figure 5.6: (a) Density of PM-567 Polycerams as a function of PDMS content processed by Route 4 and 5 (b) same as (a) with fits to data 154
Figure 5.7: Absorption photostability as flmction of PDMS content for PM-567 Polycerams processed using Route 5 156
Figure 5.8: Absorption photostability as fiinction of PDMS content for PM-567 Polycerams processed using Route 4 157
Figure 5.9: Proposed structures of Si02:PDMS Polycerams synthesized by route 4 for different polymer contents 159
Figure 5.10: Proposed structures of Si02:PDMS Polycerams (a) at 40 vol % PDMS and (b) at 60 vol % PDMS synthesized by route 5 162
Figure 5.11: Sample geometry and coordinate system the heat dissipation calculations with a Coumarin doped xerogel film 164
Figure 5.12: Temperature rise in fluorescing volume of the a xerogel film as a fiinction of pump pulses 166
Figure 5.13: Effect of temperature on the excitation and emission spectra of PM-567 in EtOHat 10*^ M 168
Figure 5.14: A 3-level energy diagram fi^om a laser dye molecule 170
Figure 5.15: Energy state populations as at (a) short times (b) at longer times 172
Figure 5.16: Effect of sample porosity on the fluorescence photostability of PM-567 in Si02:PDMS Polycerams 174
Figure 5.17: Photostability rate constant distribution of PM-567 within porous and nonporous Si02:PDMS Polycerams determined by the Gaussian Model 176
Figure 5.18: Photostability lifetime distribution in units of pulses for PM-567 in porous and nonporous Si02:PDMS Polycerams 176 11
Figure 5.19: Type of cages possible in hybrid organic/inorganic materials 179
Figure 6.2: Absorption photostability of PM-567 in EtOH at 10"' M with various additives at 1 wt % 189
Figure 6.3: FTIR spectra of photo-degraded PM-567 in a basic environment 191
Figure 6.4: Absorption photostability of PM-567 in EtOH at 10'^ M with 1 wt % 3AS and with various additives at 1 wt % 192
Figure 6.5: Absorption photostability of PM-567 in a Si02:PDMS Polyceram at 40% PDMS synthesized by Route 4, in a 3AS:TM0S Polyceram synthesized by Route 6, and in a 3AS:TM0S Polyceram synthesized by Route 6 + 2,2,6,6-tetramethyipiperidine 193
Figure 7.1: A schematic view of silica precursor to result in a photostable dye/host system 197 12
LIST OF TABLES
Table 1.1: Various Photochemical reactions 28
Table 1.2: Basic properties of different optical materials. After Manekov et.al.® 30
Table 2.1. Properties of coumarin dyes studied 42
Table 2.2: Absorption and Fluorescence Properties of Pyrromethene 567 44
Table 3.1: Chemical shifts and the fraction of various silicate structures measured by CP- MAS ^Si NMR for a derCoum xerogel at r=0.10 synthesized by Route 1 73
Table 3.2: Photostability of coumarin-doped xerogel films at r=0.05 synthesized by Route 1 85
Table 3.3: Photostability FOM for Coumarin dyes in Polycerams at 1=10"^ synthesized by Route 3 91
Table 3.4: Photostability results for Neat Coumarin fihns 97
Table 3.5: Chemical forms and spectral properties of Coum 101
Table 3.6: Fluorescence Maxima of 2Coum, 2dCoum, and 2dPCoum in MeOH at different acidic and basic environments 107
Table 3.7: Fluorescence maxima of dyes in solvent host and xerogel host 109
Table 4.1: and '^C NMR of degradation products of PM-567 126
Table 4.2: Effect of catalyst on optical properties of the Polyceram solutions 129
Table 4.3: Solubility and diSiisivity of oxygen in various hosts 134
Table 5.1: Methods explored to control porosity in sol-gel matrices 138
Table 5.2: Chemical shifts from MAS-CP ^Si NMR for PM-567 doped Polycerams... 144
Table 5.3: Properties of PM-567 doped Si02:PDMS Polycerams at various PDMS contents synthesized by Route 4 and Route 5 147
Table 5.4: Comparison of the number of pump pulses to the dye concentration for PM- 567 Polyceram monoliths and Coumarin xerogels films 170
Table 5.5: Fitting parameters to Gaussian model curve fit 175 13
Table 5.6: Photostability of PM-567 in various hosts and pump conditions 182
Table 6.1: Properties of antioxidant and base additives 188 14
ABSTRACT
Improving the photostability of laser dyes within sol-gel derived hosts has been the
focus of this study. To accomplish this, synthetic routes were established to incorporate
laser dyes within various sol-gel matrices, the mechanisms of dye photodegradation were
determined, and molecular engineering techniques were employed to improve the
photostability of the dye doped sol-gel hosts.
Various Coumarin (silylated and unsilylated) and Pyrromethene laser dyes were
incorporated within sol-gel derived hosts ranging from Si02 xerogel fihns to SiOarPDMS
Polyceram monoliths which were optically transparent, crack-free, and polishable.
Processing parameters, such as the water content and the pre-hydrolysis of the silylated
Coumarin dyes, greatly affected the degree of dye bonding. The chemical stability of the
Pyrromethene laser dye was also greatly affected by processing parameters, such as the acid/base content.
Both the Coumarin and Pyrromethene dyes were found to degrade by photo- oxidation processes. Therefore, it was expected that the photostability would improve by incorporating the dye molecules within molecular cages within the solid hosts, thereby preventing interactions of the dye with photo-reactive impurities such as oxygen.
The photostability was found to improve using the following molecular engineering methods: (1) by covalently bonding the dye to the host matrix, where the photostability improvement was attributed to the greater probability of obtaining dye caging with the silylated dye; (2) by removing porosity within the host through control of 15
sol-gel processing and composition, where the photostability improvement was attributed
to the elimination of highly photo-reactive dyes located in the pores of the host; (3) by
incorporating additives such as bases and hindered amine antioxidants which slowed the steps of the photodegradation process.
The fluorescence photostability (fluorescence output intensity as a function of pump pulses) of the dye doped films and monoliths showed a characteristic behavior in the fluorescence output, signified by a rapid initial decay (attributed to dyes located within pores of the matrix) and then a slower long-term decay (attributed to photostable dye molecules located within Si02 cages). A model, which applied a Gaussian distribution of the photostabilities of the dye molecules, quantitatively described the observed photostability behavior of the dye doped samples. 16
1) INTRODUCTION/BACKGROUND
1.1) Dye Lasers
Lasing can take place in a variety of media, including solids, liquids, gases, and even
plasmas. Liquid dye lasers represent an important and unique class of lasers. Many families of
lasing dyes exist such as coumarins, oxazoles, xanthenes, cyanines, and pyrromethenes. Most
commercial lasing dyes are dissolved at low concentrations of 10'^ to 10"^ M in solvents such as
alcohols, HjO, or ethylene glycol. Flow of the dye/solvent mixture through the resonator
chamber is required to maintain photostability. This allows the dye molecules to move in and
out of the pump beam within the resonator and give the excited dye molecules time to return to
ground state, minimizing dye degradation. Excited dye molecules are prone to degradation
from excited singlet and triplet states through a number of photochemical mechanisms.
Organic lasing dyes for use as gain media in liquid laser systems have a number of
advantages over inorganic crystals and glasses used as gain media Most solid-state ionic lasers are limited to discrete emission of fundamental wavelengths and their harmonics (e.g.,
NdiYAG, Ruby). Organic lasing dyes offer a larger bandwidth G e., larger cross section for absorption and emission) and therefore offer greater tunability of the lasing wavelength In addition, organic la^g dyes require lower threshold powers for lasing than their inorganic counterparts.
In the last several decades there has been a great deal of interest in developing a solid- state organic gain medium. Since dye/solvent laser systems require fluid pumping systems, and have limited lifetimes due to photo-decomposition and chemical instability, a solid-state 17
medium would be a good alternative. Also, a solid-state gain medium would avoid problems
associated with dye/solvent systems such as convection, evaporation, flow fluctuations, solvent
poisoning, and dye poisoning A solid-state medium would provide ease of use and
replacement, along with expanded applications as slab waveguide lasers, tunable fiber-optic
lasers, and solid-state dye laser rods. Also, dye/host interactions could enhance control of the
fluorescence spectra, improve photostability, and even improve the quantum efficiency of
fluorescence.
A tunable solid-state laser would have numerous applications in medicine. Dye/solvent lasers have played a role in dermatology for the removal of vascular lesions, in cardiology for the break up of blood clots through coronary laser thrombolysis, and in urology for the break up of urinary stones. In the past ten years, the use of lasers in surgical and medical practices is booming; this laser market is striving for wavelength agility, low cost, and compactness of the laser system. A photostable solid-state laser could provide all of these. The output of multiple wavelengths not only allows for selective heating by preferential light absorption by the target material (e.g., ojq^hemoglobin, de-o^ofhemoglobin, melanin, various tattoo ink pigments), but also allows for different penetration depths of the laser light. The low cost and compactness stems from the simplicity of the laser system; solid-state dye lasers would no longer require a fluid pumping system^^. Also, the control of gain media geometry can allow for flashlamp pumping as opposed to laser pumping which decreases cost and the size of the laser system.
The use of a solid-state dye laser for commercial applications has been limited due to the poor photostability of the gain medium. Understanding the mechanism of photodegradation 18
and finding methods to improve photostability is greatly desired and is therefore the focus of
this study.
In the following introduction and background, the origin of fluorescence from a dye
molecule and the interactions of the dye with its environment are discussed. Next, a short
review of photochemical interactions which can destroy dye molecules is provided, followed by
a review of different types of hosts (e.g., polymers, inorganics, Polycerams) which have been
explored for incorporating laser dyes. Finally, the laser dyes classes explored in the present
study and the concept of covalently attaching laser dyes to the host are discussed.
1.2) Fluorescence and Dye/Host Interactions
The origin of lasing stems from the luminescent properties of the gain medium itself
Lasing is achieved by forcing stimulated emission by placing the luminescent material within a
resonator. Improving the lasing photostability characteristics of a particular gain material then requires improvement in the fluorescence photostability.
An energy diagram which illustrates the origin of fluorescence (a radiative process which does not involve an overall change in spin) and phosphorescence (a radiative process which involves an overall change in spin) is shown in Figure 1.1. Organic molecules not only have electronic energy levels, but also have a number of vibrational and rotational energy states corresponding to each level. This gives a "band like" appearance to each energy level. Each ener®^ state can occupy two electrons in which the electron can be paired (singlet state) or unpaired (triplet state). In the diagram the ground state is taken as a singlet level (So), and the 19
first and second excited singlet levels are depicted as Si and S2, respectively. The first triplet
state is given by Ti, which nomially has a slightly lower energy than its corresponding singlet
level (Si).
vibntiooal lelaxation -fc TJE—. intemalinta conveoion
mteisystem CTOssmg s,•im
Energy tnfwnal and external t conversion • K Absoiption Fluorescence Phosphorescence
Figure 1.1: Energy diagram for a typical organic molecule'.
At room temperature most organic molecules within a solvent are in the ground state.
When light reaches the dye molecule, absorption takes place over a range of wavelengths represented by A,i and X2. This process is very fest (10"^'* to 10"'^ sec) relative to all the decay processes. The excited dye molecule can return to ground state by a number of pathways (see
Figure 1.2). Excited Molecule
Intramolecular Intermolecular X I Photophysical Photochemical Photochemical Deactivation Quenching Change to a diflerent Change to a diflerent Change to a diflerent Reversible chemical A» + B-> electronic stale compound compound ctuinge A + B or (B») / \ A* + B-> A* + B* Non-radiative Radiative Isomerization Dissociation
Exchange Dipole-Dipole Exciplex Short Range- Coulombic-Long Range metastable Triplet Energy Singlet Energy Transfer Transfer
Phosphorescence Fluorescence spin forbidden spin allowed Addition Reactions
Internal Intersystem Oxidation Conversion Crossing Substitution Hydrogen Dimerization Reactions Abstraction Figure 1.2: Decays pathways for an excited organic molecule. 21
Immediately after light absorption, collisions between the dye and the solvent result in
vibrational relaxation. Because vibrational relaxation has a very short lifetime (10'^ sec), the
other relaxation processes take place from the lowest vibrational state of the excited electronic
level. The upper vibrational states of Si generally overlap with the ground state of S2;
therefore, the molecule can decay from 82 to Si by internal conversion. Further vibrational
relaxation causes the molecule to return to lowest vibrational state of Si.
Fluorescence can now occur by emitting a photon, returning the dye molecule to So.
Because So has a number of vibrational states, fluorescence takes place over a range of
wavelengths given by X3. The wavelengths of fluorescence are longer than the wavelengths of
absorption due to energy loss through vibrational relaxation. This wavelength shift is referred
to as the Stoke's shift.
Fluorescence competes with several other processes (see left side of Figure 1.2).
Energy can be transferred from the excited molecule to the other dye molecules or solvent species through external conversion, returning the excited molecule to ground state. The efl5ciency of this process is greatly dependent on the number of collisions between molecules.
Therefore, external conversion can be minimized by lowering the temperature or increasing the viscosity. Fluorescence also competes with intersystem crossing where the molecule is a transferred from the singlet to the triplet state. Decay to the ground state from the triplet state with the emission of a photon is called phosphorescenceV Fluorescence lifetimes generally are
3-4 nanoseconds, while phosphorescence lifetimes are much longer (10-100 microseconds) 22
The quantum yield of fluorescence describes the eflSciency of molecular fluorescence. It is defined as the ratio of the number of molecules that fluoresce to the number of excited molecules. In terms of rate constants, the quantum yield (({)f) is:
^ kfJ I ec ic where kt is the fluorescence rate constant, k; is the intersystem crossing rate constant, kec is the external conversion rate constant, and kjc is the internal conversion rate constant. Fluorescence quantum yield is maximized when internal conversion, external conversion, and intersystem crossing from the Si level are minimized ^
The fluorescent properties of a dye molecule are dependent both on its molecular structure and its chemical environment. Molecular fluorescence is generally seen in fused benzene rings/ heterocyclic organic molecules. Molecular structure eflfects on the fluorescent properties have been extensively investigated A number of environmental fectors aflfect the fluorescence properties; pump intensity, temperature, pH of the host, viscosity of the host, and the polarity of the host, as well as physical and chemical interactions of the dye with other species (e.g., other dye molecules, heavy atoms, quenchers, o^q^gen).
The influence of other species on the fluorescence efficiency of the dye molecules can be described by the following:
1) Inner Filter Effects are caused by absorption of the pump light or the fluorescent
emission by the host medium. Re-absorption of fluorescent light can occur by the 23
host material itsel:^ or by other dye molecules present in the gain medium. The loss
of fluorescence eflficienQr can be avoided by choosing a medium which is optically
transparent to the absorbing and fluorescing wavelengths and by keeping the dye
concentration low.
2) Static Quenching is mainly caused by the aggregation of the fluorescing species in
the ground state. These aggregates often become non-fluorescing which reduces
the quantum yield. This can be avoided by using low dye concentrations which
limits dye aggregation.
3) Dynamic or Diffusional Quenching is caused by intermolecular interactions of the
dye molecules in the excited state (Figure 1.2). Dynamic interactions include
processes where energy transfer occurs with other spedes returning the molecule
to ground state and processes where photochemical interactions result in the
permanent destruction of the dye molecules. Dynamic quenching is largely
difiiision-dependent. Therefore, an increase in the viscosity and rigidity of the host
medium should result in a decrease of these dynamic processes^\
1.3) Photochemistry
Organic photochemistry has been a broad, well-studied field of science for the past 30 years. Photochemistry is defined as the chemical change brought about by light absorption. An exhaustive list of photochemical interactions has been well reported^^'^; those relevant to the present study are discussed here. 24
A molecule in its excited state can return to ground state by number of pathways as
illustrated in Fig. 1.2 The pathways can occur through intramolecular or intermolecular
interactions. Intramolecular interactions include photophysical processes (radiative (1^) and
non-radiative (k,-, kec kic) transitions), and photochemical interactions (isomerization and photo-
dissociation reactions).
Isomerization reactions, which involve the structural rearrangement of the molecule
without change in composition, consist of two major types: cis-trans isomerization and ring
isomerization. Cis-trans isomerizations are common in molecules having isolated double bonds such as olefins, stilbenes, and azo compounds^"*. These reactions are often reversible and are the mechanism for reversible photochromic properties of a number of photochromic dyes^.
Ring isomerization reactions involve the structural rearrangement of substituents (e.g., allq'I, fluoro, perfluoroallq'l) on aromatic compounds. The proposed mechanism of these types of reactions involves the formation of an intermediate valence isomer which is cyclic and nonaromatic^^ Examples of both types of isomerization reactions are shown in Table 1.1.
Photo-dissociation reactions are caused by the cleavage of single or double bonds within the dye molecule. Double bond dissociation typically occurs only in organic molecules possessing weak double bonds such as diazo (C=N-N) and azide (N=N-N) compounds; single bond dissociation occurs in organic molecules which possess single bonds between atoms that have multiple bonds, lone pairs, or empty orbitals. Single bonds of this type have a high topicity (the total number of active orbitals at the two terminal atoms of the bond) For laser dye molecules, high pump intensities (>10' W/cm^) can result in excited-state singlet-singlet absorption (absorptions to higher energy singlet levels fi-om lower energy singlet levels) or 25
excited-state triplet-triplet absorptioa (absorptions to higher energy triplet levels from lower
energy triplet levels). This can cause bond cleavages resulting in the permanent destruction of
the dye molecule".
Intermolecular interactions between the excited dye and an impurity consist of
photochemical reactions (permanent chemical alteration of the dye), deactivation processes
(formation of new metastable chemical species which reverse reverting to the original
molecular species), and quenching processes (energy transfer from the dye to the impurity
resulting in an excited impurity and a dye molecule in the ground state). A number of
intermolecular photochemical reactions can occur such as photosubstitution, hydrogen abstraction, photoaddition, and photo-oxidation reactions (see Figure 1.2). These (Cerent types of photochemical reactions result in the permanent loss of photostability.
Substitution reactions of aromatic compounds from the ground-state take place by electrophilic substitution where an "electron-seeking" substituent attacks the aromatic ring and substitutes for one of its hydrogens. On the other hand, the majority of photosubstitution reactions in the excited-state take place by nucleophilic substitution where a "nucleus-seeking" substituent attacks the aromatic ring and substitutes for one of its hydrogens. A number of mechanisms have been proposed for a variety of photo-substitution reactions^. One of the possible mechanism is when the excited aromatic molecule is attacked by the nucleophile resulting in the formation of a anion intermediate, followed by loss of the leaving group to form the product. Common leaving groups are halides, methoxide anions (MeO"), and nitrite anions
(NO2'), and common nucleophiles are hydroxides (OHT), cyanides (NC"), ammonia (NH3), and amines (RNH2)^\ 26
Hydrogen abstraction reactions involve the removal of a hydrogen atom from a donor molecule to reduce the dye molecule. A common example is the photoreduction of a carbonyl
(>C=0) containing compound (see Table 1.1). An photo-excited caibonyl group can induce an a bond (bond between the carbon in >C=0 and another carbon) cleavage or remove a hydrogen from a suitable donor such as primary and secondary alcohols, alkyl benzenes, and some alkenes Intramolecular hydrogen abstraction is also possible where hydrogen is abstracted from the y position 0.e., hydrogen bonded with third carbon away from the carbonyl carbon) on an aliphatic ketone. A variety of products can form including olefins, enols, and cycloalkanoP.
Photoaddition reactions encompass a large class of photoreactions. Commonly, water, alcohols, or carbojqriic attack the 7t component of a carbon-carbon double bond in alkenes, aromatics, or cyclic compounds^\ When two identical dye molecules undergo an addition reaction with each other, it called a dimerization reaction. For example, photodimerization can occur with coumarin (Table 1.1). The formation of chemical or physical dimers, trimer and other higher aggregates generally cause shifting in the absorption band. Typically, dimers are only weakly fluorescent, thus reducing the quantum yield and increasing cavity losses through re-absorption of the fluorescence^.
Photo-oxidation reactions are the most important class of dynamic interactions for the present study. Molecular o^g'gen plays a special role in organic photochemistry. 02 has a triplet ground-state and a low energy occited singlet-state. Therefore, ground-state O2 is easily excited to its singlet state by quenching an excited molecule in its triplet state. Excited singlet- 27
state O2 is then very reactive towards many organic compounds. For example, singlet oxygen
can react with allylic hydrogens to form allylic hydroperoxides (Table 1.1)^^ ^ Other
mechanisms for photooxidation also exist; these will be discussed in greater detail in Section 4.
In addition to photo-oxidation and the other photochemical interactions, dye
degradation can occur thermally. Upon input of high intensities of light, absorption by the host
and the dye can produce suflSdent heat for thermal oxidation. Although the reaction products
via thermal and photo oxidation are similar, the mechanisms by which th^^ occur can be much
diflferent. Thermal oxidation occurs from the formation of an allyl radical by the abstraction of
the allylic hydrogen by the oxygen
With all the mechanisms discussed above which lead to photodegradation, it seems
reasonable that improvement in photostability can be achieved by caging or isolating the dye
by preventing it from interacting with other species. Also, when heat plays a role, choosing a
host with good heat dissipation properties (i.e., high thermal conductivities) would also lead to an improvement in the photostability. 28
Table 1.1: Various Photochemical reactions.
Reaction Reaction Chemical Illustration Type Photo- cis-trans isomerization isomerization of stilbene^
ho
Photo- ring isomerization isomerization of a benzene cir—ly-tS -6 derivative^' Photo- a-cleavage of o o dissociation carbonyl II pi L II R* nn compounds^"^ i r\. 1 R' R-
Photo- photo NOj NO2 substitution substitution of l-chIoro-4- [A liqNHs nitrobenzene^' CI NH2 Hydrogen photo- Abstraction reduction of a »• + RH •- R- + V" carbonyl compound^' Photo- Dimerization addition ofcoumarin dye^° 00. ^ d 0 Photo- photo- oxidation oxidation to produce hydroperoxide 21 HOO^ 29
1.4) Polymer Hosts
Soon after lasing was achieved with dye/solvent ^sterns by Sorokin and Lankard^' in
1966, the first plastic-host dye lasers were built by Soffer and McFariand ^ in 1967 and
Peterson and Snavely in 1968. Since then, a number of groups have investigated the use of
polymer hosts for dye molecules, e.g.®' Lasing has been achieved in laser dye/PMMA
rods, but these materials stiU suffer significant photodegradation or photobleaching when
compared with solvent host dye lasers.
A number of routes have been employed to improve photostability. For example, by
using deojtygenated monomers to make Rhodamine 6G/PMMA systems, a 9-foId reduction in
the UV sensitivity of the material was observed''^ Such results suggest that the presence of
oxygen plays a role in the dye degradation. A reduction of photobleaching O-e., improvement in
photostability) was also observed in Rhodamine 6G/PMMA upon pumping at lower
temperatures. For the same times of UV ®q)osure, the sample measured at 300K had 100%
dye bleaching, while at 193K only 22% of the dye molecules were bleached Lower
temperature causes the polymer cage surrounding the dye molecule to become more rigid,
thereby protecting the dye.
Dyamaev et. al. ^ and Gromov et. al. doped xanthene-series lasing dyes in modified
polymethyl methacrylate (MPMMA). The modification involved the addition of low-molecular weight additives which prevent the production of primary radicals which initiate polymer photodegradation. Cylindrical rods, which were pumped at 530 nm with a fluence of 1 J/cm^, suffered catastrophic drops in the lasing efficiency after 200-800 pulses. Due to the low resistance of polymers to high intensity laser damage (although better than the lasing dye 30 alone), polymer hosts still suffer from significant photodegradation. The bulk laser damage thresholds of polymers are generally 1-2 orders of magnitude smaller than that of fused silica.
Even the most photostable polymers (e.g., MPMMA.) still have less than half the laser dam^e threshold of fused silica ^ In addition to the lack of photostability, polymers suffer from high permeability to gases and moisture, low thermal conductivities, lack of UV transparency, and poor optical stabilities (e.g., high dn/dT). Some of these properties are illustrated in Table 1.2.
Table 1.2: Basic properties of different optical materials. After Manekov et.al.®. f I Material P E a Cv kr no g/cm^ lOVcm^ 10-^/K cal/gK W/mK Polymethyl 1.18-lJO 2.9-32 6.30-7.70 0.35 0.18-0.21 1.49 8.5-9.0 methacrylate Polystyrene 1.05-1.10 2.7-3.1 6.30-9.00 0.30-0.32 0.09-0.14 1.59 12.0 Polycarbonate 1.17-1.24 22-2.5 6.00-7.00 .28 .20 1.58 12.0-14.0 Glasses 220-5.90 50-80 0.04-1.10 0.12-021 0.7-1.4 1.46-1.69 0.1-0.3 (inorganic) Sapphire 3.98-4.00 500 0.50-0.66 0.18 23-25 1.76 1.0 p is the density, E is the Young's Modui US, a is the coefficient of linear expansion, Cv ist]be heat capacity, kr is the thermal conductivity, no is the refractive index, dn/dT is the temperature refractive index.
1.5) Inorganic Hosts
Inorganic hosts for laser dyes have also been explored. Due to the limited temperature stability of lasing dyes, conventional methods of doping them into conventional molten glasses cannot be used. Wet-chemical methods (sol-gel chemistry) are thus very attractive. Doping laser dye molecules into a silica host using the sol-gel process was first reported a decade ago by Avnir et. al. Decay in the fluorescence intensity was monitored as a fijnction of time as the samples were pumped with a quartz-halogen lamp to a dosage of 65.2 mW/cml Improved photostability of Rhodamine 6G was observed in the silica host (tiy2=622 hrs) compared to an 31
aqueous host (ti/2= 86 hrs). This was attributed to the reduction of dye aggr^tion at
concentrations as high as 10'^ M and increased dye isolation within the silica xerogel host.
Since then, numerous dyes including various Rhodamines (Rhodamine 6G,
Sulforhodamine B, Sulforiiodamine 640, Rhodamine and Coumarins (Coumarin 4,
Coumarin 1, Coumarin 102, Coumarin 481, Coumarin 540A, Coumarin 30) have been
doped into sol-gel matrices. Two basic approaches for dye incorporation have been explored.
In one approach, known as pre-doping, dye molecules are added to the matrix in the precursor
stage; while in the other q)proach, post-doping, dye molecules are diflRised into pre-fired
porous silica glass The second approach may allow for dye incorporation into a more
mechanically stable host, but the dye molecules vwll only be located within the open pores of
the network where the dye molecules and impurities are mobile. Using the predoping method
allows for the potential of completely isolating the dye molecules, which is believed to improve
photostability by preventing movement of reactive species within the matrix. Using the post-
doping method with a 4-[2-(5-phenyl-oxazolyl)]-l-methyl pyridinium r-toluenesulfonate (4
PyPO-MePTS) lasing dye in silica glass, the pore size and pore size distribution were found to
affect greatly the lasing characteristics of the gain medium^\
In addition to the work by Avnir et. al. a number of other groups have attributed
improved photostability to the cagjng or to the increased rigidity of the dye within the matrix.
For example, McBCieman et al reported superior photostability of xerogel hosts. Rhodamine
6G in an alumino-silicate gel showed a 90% decline in laser output energy after 4000 pump
pulses, while the same dye in a PMMA host showed a 90% decline in about 275-300 pulses. Sol-gel derived laser glasses have been febricated into films and monoliths. Hanivy et.
al.^'* &bricated 25 pun glass films using a &st sol-gel synthetic route incorporating Rhodamine
6G, Coumarin 153, Piyridine 1, and Polyphenyl 1 at high concentrations (>10'^ M). Lo et. al.®'
has febricated rods of Coiunarin 481 doped silica via sol-gel techniques which showed 36.6%
conversion efficiency when excited by 535 nm XeF eximer laser.
Other advantages of sol-gel derived laser glasses have also been observed. Fujii et. al.
showed improved thermal stability of Rhodamine B encapsulated in a SiCb sol-gel matrix. The
fluorescence spectra of the sol-gel sample degraded at a higher temperature of673 K than with
a preheated Rhodamine B powder dissolved in ethanol which showed degradation at 573 K.
Also, sol-gel glasses have a higher thermal conductivity (1.3 W/mK) than typical dye solvents
(0.2 W/mK). This allows for better heat dissipation ifrom high intensity pumping, reducing
thermal d^radation of the dye and/or the host®'.
The pH of the sol-gel solutions influences the fluorescent properties of the dye.
Different chemical forms of the same dye molecule can exist in different pH environments.
Coumarin 4 can exist in a number of forms such as anionic, cationic, zwitterionic, and neutral,
which all have different fluorescent emission spectra Oxazine 170 lasing dye has four
chemical forms in hosts under conditions of different acidity. Only one form has good
fluorescent characteristics. Thus it is important to control or to know the chemical environment that the laser dye will experience. Gvishi et. al. examined different catalysts and catalyst concentrations in Oxazine 170 doped-SiOz xerogels fi'om tetramethoxysUane (TMOS)
precursors. The best quantum yield of 46% was obtained when no catalyst was used. It also has been shown that the type of species present in the sol-gel matrix can be controlled by the 33
concentration of dye doping. For example, only the cation form of fluorescdn was observed at
low concentrations, while all forms of fluorescein were observed at higher concentrations ^.
Fluorescent dye molecules have also been used as luminescent probes to monitor sol-
gel reactions, giving insight into the structure and chemical environment of the dye molecules
^ Pyranine and Coumarin dyes have been doped in Si02 and SiOr-AIiOs matrices as
luminescent probes®" Shifts in the emission maximum of the dyes were observed during
gelling and drying, indicating changes in the chemical environment of the dyes.
A major drawback of using sol-gel glass hosts is their mechanical properties. Due to
the low temperature processing limitations, dye doped sol-gel glasses are not My densified.
The drying of monoliths or thick films without cracking, and the machining (e.g., cutting,
grinding, and polishing) of such materials has proved to be difBcult
1.6) Polyceram Hosts
Recently, organically modified silicates have been used as laser dye hosts
Polycerams (also known as ORMOSILs, CERAMERs) are polymer-modified ceramic materials, in which the organic and inorganic components are combined on a molecular or near-molecular scale. Polycerams are synthesized using sol-gel chemistry techniques and permit the febrication of a wide variety of compositions as thick coatings or crack-fi-ee monoliths.
Si02:PDMS Polycerams have been shown to have exceptional optical properties with losses as low as 0.15 dB/cm in febricated planar waveguides The superb optical quality and the ability to make polishable monoliths make these materials attractive as hosts for laser dyes.
Knobbe et. al. ^ successfully achieved laser oscillation with Coumarin 153 and
Rhodamine 6G in ORMOSILs with peak gain values of 40 cm'\ The photostability of the coumarin doped Polycerams was measured by pumping with a dye laser at a fluence of 1 J/cm^ with a repetition rate of 1 Hz. Laser emission from the Coumarin-doped Polyceram was reduced by fector of 6 after 6000 pulses. This result shows 1-2 orders of magmtude improvement in photostability compared with respect to coumarin doped polymers
Altmann et. al. ' has also demonstrated lasing with a large number of lasing dyes incorporated in ORMOSILs. Photostability of a Rhodamine B doped Polyceram disk pumped by a frequency doubled q-switched Nd:YAG laser (532 nm) at 10 Hz and 3 raT/pulse showed a
50% decline in laser output after 4500 pulses. Rotating the disk dramatically increased the lifetime, with a 50% decay after 110,000 pulses
1.7) Coumarin Dyes
Coumarin dyes are well known for their fluorescence in the blue-green region of the spectrum; and numerous coumarin derivatives have been used as the active media in tunable dye lasers Coumarin 4 or Coum (7-hydroxy-4-methyl coumarin) is particularly interesting because it possesses a hydroxyl group that can react to produce a silylated laser dye
(see section 1.9). Also, Coum possesses acidic and basic substituents which result in the shift of 35
the absorption and fluorescence spectra through proton transfer depending on the molecular
environment surrounding the dye. For these reasons, Coum was selected for the present study.
The photochemistry of CounMrin dyes has been well investigated these dyes
are particularly prone to photodimerization and photooxidation reactions. Coumarin (Table
1.1) and substituted Coumarins at the 4 and 7 positions are particularly prone to
photodimerization'"^. These substituted Coumarins were found to form dimers from both the
singlet and triplet excited states. The dimerization yields vary greatly depending on the solvent
composition and the dye concentration. Dimerization yields as high as 80% were observed.
Although Coum was not specifically investigated in this study, it is expected that similar
dimerization processes can occur'°^. Photo-oxidation reactions have also been well studied for
Coumarin dyes. 4-methyl substituted Coumarins are particularly susceptible to photooxidation
30, 104, 105 jjjg reaction appears to take place between coumarin in the excited-state and O2 by the free radical mechanism. An example of this oxidation process for a Coumarin dye, 7-
(dialkylamino)-4-methylcoumarin, is shown in Figure 1.3. Removal of oxygen from these dye/solvent mixtures upon excitation has resulted in the reduction in the photooxidation
The same has been found for other dye/host systems^' 36
o r(C2H5)2 o r(C2H5)2
CItOH
o r(C2H5)2 o r(C2Hs)2
CHO COOH
Figure 1.3: Photooxidation of 7-(diaIkylarnino)-4-methylcournarin,
1.8) Pyrromethene-BF2 Dyes
Recently, a new class of dye complexes, Pyrromethene-BF2, has received much attention as highly eflBcient, highly photostable laser dyes. These dyes fluoresce in the same r^on of the spectra as Rhodamine dyes (550-650 nm). The basic structure of these dye complexes is shown in Figure 1.4. Lxaw triplet state populations of these dyes (one-fifth of that of other laser dyes) and low triplet-triplet absorptions of these dyes gready enhance the photostability. These molecules are planar, which prevents twisting motions in the excited state and leads to high quantum yields ((|)f=.995). Under synchronously pumped conditions, these dyes show twice the efiBciency of Rhodamine dyes Numerous Pyrromethene derivatives have been examined; substitution on the 2,6 positions with auxochromic groups (organic substituents which results in the red-shifting of the optical spectra of parent molecule) has resulted in the lowest triplet state populations 37
Figure 1.4: Structure of a pyrromethene-BF2 complex
The inherent photostability of Pyrromethenes makes them attractive for doping into solid hosts. In the last several years Pyrromethene-BF2 complexes have been doped in polymer and sol-gel hosts The focus of these studies has been on the lasing properties of the materials. Lasing action with high slope eflSciency (82.5%) had been achieved and dye laser output ener^es have exceeded 280 mj"^. A million pulses of stable output at 560 nm at a pump fluence of 0.5 J/cm^ for a Pyrromethene-doped polymer rod have been achieved A
SiC)2:PMMA Polyceram host has also been investigated, but low photostability was observed.
This was attributed to incomplete polymerization of the PMMA^^.
Polyceram hosts for Pyrromethene dyes have not been studied in detail, and a more scientific understanding of the decomposition mechanisms of laser dyes within solid hosts is still required in order to improve photostability. 38
1.9) Silylated Dyes
Covalently bonding (or grafting) an organic molecule to its host is an attractive route
for producing a solid-state gain media. To attach chemically a molecular species to a sol-gel
synthesized glass, the organic species need to be chemically altered such that it contains
reactive groups (e.g., silicon alkoxides). This silylated dye can then participate in the hydrolysis
and condensation reactions to be covalently bonded to the host matrbc.
The use of silylated dyes in covalently bonding active and passive dyes to their hosts
has been widely explored in the area of nonlinear optics (NLO) a number of silylated dyes, such as silylated Disperse Red 1 silylated para-nitroaniline and silylated
dinitroaniline (TDP) have been incorporated in sol-gel matrices for NLO applications.
Covalent bonding of the NLO dyes to the host matrix has resulted in improved orientational stability (temporal stability) after corona poling. Covalent bonding of dyes to sol-gel matrices has also been studied in areas of colored glasses'^®'^^^ photochromism spectral hole burning
electrodes'^', and chemical sensors^*
Covalently bonding silylated laser dyes into sol-gel matrices also seems attractive for febricating a gain medium for a solid state dye laser. We hypothesize that covalent bonding will; (1) improve the chemical stability of the material by eliminating leaching of the dye from the host, (2) improve the quantum eflBciency by increasing the rigidity and isolation of the silylated dye within its host, and (3) improve the photostability by increasing the probability of the silylated dye being caged. 39
1.10) Research Objectives
The incorporation of laser dyes within solid hosts offers promise for producing gain media for visible, tunable solid-state lasers. A solid-state dye laser would provide for numerous low and high power applications in medical and defense industries'' The lack of photostability has been the major &ctor which has limited their commercial use.
The area of dye-doped glasses is still in its infancy and still much scientific understanding (e.g., parameters to control final structure, dye/matrix interactions affecting final properties) is essential to controllably process such materials.
With these ideas in mind, the following are the research objectives of the present work:
1) To synthesize numerous sol-gel matrices (xerogels, SiO^PDMS Polycerams) with
the incorporation of various laser dyes (Coumarins and Pyrromethenes) to form
films and polishable monoliths.
2) To examine factors which affect the covalent bonding, chemical processing, host
structure, and dye/host interactions which in turn affect the photostability of the
above synthesized samples.
3) To examine the mechanism of photodegradation and propose methods to improve
the photostability.
4) To suggest the requirements for and feasibility of producing a commercial tunable
solid- state dye laser from the point of view of the gain media. 40
2) EXPERIMENTAL
2.1) Synthesis of Laser Dyes
7-(3-Triethoxysilylpropyl)-0-(4- methylcoumarin) urethane (derCoum). Dry, recrystallized 7-hy(Iroxy-4-methyIcoumarin (Coum) (Aldrich) (0.114 mole, 20.0 g) was dissolved in anhydrous tetrahydrofiiran (THF) (175 ml) to yield a clear, colorless solution.
Isocyanatopropyltriethoxysilane (United Chemical Technologies) (0.133 mole, 32.5 g) was added along with several drops of dibutyltin dilaurate (Aldrich). The solution was refluxed for 4 days before cooling to room temperature and flash evaporating the solvent.
Unreacted isocyanatopropyltriethoxysilane was removed by heating under high vacuum.
The synthesis was monitored using Fourier Transform Infrared Spectroscopy (FTIR)
(Perkin-Elmer 1725x) by the disappearance of the isocyanate peak (2290 cm"^) and the increase in the urethane peak (1765 cm"').
7'(3-(Triethoxysilyl)propoxy)-4-methylcouniarin (derPCoum). Recrystallized
Coum (0.12 mole, 20.8 g) was dissolved in methanol (MeOH) (200 ml) containing potassium hydroxide (7.3 g) and potassium bromide (0.7 g). Dissolution was accomplished by gentle heating, and allyl bromide (0.132 mole, 16 g) was added. The clear, yellow solution was refluxed for 5 hours, cooled to room temperature, mixed with water and extracted with diethyl ether to yield 7-allyloxy-4-methylcoumarin. After crystallization from methanol and drjring, the allyloxycoumarin was dissolved in anhydrous toluene (200 ml). The clear solution was treated with triethoxysDane (United Chemical
Technologies) (0.167 mole, 27.4 g) and chloroplatinic acid (0.000052 mole). The reaction was followed by the loss of the -SH peak at 1209 cm"' using F l lK. After 4 days at 70 C,
the yellow solution was flash evaporated to yield derPCoum.
5,7-Di-(N-triethoxysilyipropyl)-0-(4-methyIcoumariii) urethane (IderCoum).
Anhydrous 5,7-dihydroxy-4-niethylcoumarin (Pfaltz & Bauer) (0.052 mole, 10.0 g) was
dissolved in anhydrous THF (100 ml) to yield a dear, orange solution.
Isocyanatopropyltriethojq'silane (0.120 mole, 29.7 g) was added along with ten drops of
dibutyltin dilaurate. The cloudy, yellow solution was refluxed for 24 hours before cooling
to room temperature and flash evaporating the solvent. FTIR was used to confirm
completion of the reaction in the same manner as with derCoum. Unreacted
isocyanatopropyltriethoxysilane was removed vath heating under high vacuum.
S,7-Di-(3-(triethoxysilyl)propoxy)-4-inethylcoumarin (IderPCoum). 5,7-
dihydroxy-4-methylcoumarin (0.108 mole, 19.2 g) was dissolved in anhydrous acetone
(400 ml). Anhydrous potassium carbonate (40 g) and allyl bromide (0.220 mole, 26.6 g)
were added. The mixture was refluxed for 18 hours before cooling to room temperature.
Acetone was removed by flash evaporation and the resulting solid was stirred for 3 hours in dilute aqueous hydrochloric acid. The neutral dispersion was extracted with diethyl ether. After drying over anhydrous magnesium sulfate, the ether was removed and the product, 5,7-diallyloxy-4-methylcoumarin, was recrystallized fi^om absolute ethanol. After drying, the tan solid was dissolved in anhydrous toluene (150 ml). The clear solution was treated with triethoxysilane (0.160 mole, 26.3 g) and chloroplatinic acid (0.000052 mole).
The reaction was followed by the loss of the -SiH peak at 1209 cm"' using FTIR. After 5 42
hours at 110 C the solution was flash evaporated to yield 5,7-di-(trietho^silyIpropyIoxy)-
4-methylcoumarin.
The structures of Coum and the four synthesized dyes are shown in Figure 2.1.
Proton NMR was performed on the synthesized dyes to confirm successful synthesis
(Table 2.1). Differential scanning calorimetry (DSC, Perkm Elmer DSC-7) was used to
evaluate crystallinity and the melting points of the dyes. Coum and derCoum had definite
melting points, while no such peak was observed for derPCoum, 2derCoum, and
2derPCoum in the temperature range of 25-350 C.
Table 2.1. Properties of coumarin dyes studied.
Dye Name Molecular Melting NMR peaks weight point (CDCb) Coum 7-hydroxy-4- 176.17 192 C not measured methylcoumarin derCoum 7-(N-triethoxysilyl)-0- 423.54 95 C 5 0.6 (2H), 1.2 (9H), 2.4 (4- methylcoumarin) (3H),3.1(2H),3.9(6H), urethane 4.1 (2H), 6.1 (IH), 6.9 (3H) 2derCoum 5,7-di(N-triethojq'silyl 686.90 * 6 0.6 (4H), 1.2 (18H), propyl)-0-(4- 2.4 (3H), 3.1 (4H), 3.9 methylcoumarin) (12H), 4.1 (4H), 6.0 urethane (IH), 6.8 (3H) derPCoum 7-(3 -(triethoxysilyl) 380.52 * 5 0.8 (2H), 1.2 (9H), 2.0 propoxy)-4- (2H), 2.6 (3H), 3.9-4.0 methylcoumarin (8H), 6.0 (IH), 6.5 (2H) 2derPCoum 5,7-Di-(3-(triethoxysilyl) 600.85 6 0.8 (4H), 1.2 (18H), propoxy)-4- 2.0 (4H), 2.6 (3H), 3.8- methylcoumarin 4.0 (16H), 6.0 (11^, 6.5 (2H) * No melting found via DSC in the temperature range of25-350 C 43
OH (a)
CHjCHj O O r| I 0^^0,.^^'V^O—C—NH—(CH2)3—Si—O—CH2CH3 XJQi ? CHjCHj CHj CH2CH3 0 1 O^^.O^^^V.^0—(CHJj —Si—O—CH2CH3 1 (0 ?3 CH2CH3 CH,
OH XjO
CH3 OH CHjCH3 O o ll I O—C—NH—(CH2)3—Si—O—CH2CH3 Oj
CHj j C—o I NH (CH2)3I CH3CH2—O—Si-O—CH:CH3 0 1 CH2CH3
CHjCHJ o
_^^^0-(CHd3—Si-O—CHJCHJ (£) ^^-3^ ? CH3CH3 CH3 o (CHJj CH3CH2—O—Si-O—CHjCHj 0 1 CHjCHj
Figure 2.1 Structure of coumarin dyes: (a) Coum (b) derCoum (c) 2derCoum (d) 2Coum (e) derPCoum (f) 2derPCoum. 44
2,6- diethyl- 1,3,5,7,8- pentamethyipyrromethene- difluoroborate complex.
(Pyrromethene 567 or PM-567) was ^thesized via the route described by Shah et. al.'^*.
Kryptopyrrole was reacted with acetyl chloride in dichloromethane to yield a 2,6-diethyl-
1,3,5,7,8-pentametItylpyrromethene hydrochloride salt. The salt was then reacted with boron trifluoride etherate in the presence of triethylamine to yield PM-567. The structure (Figure 2.2) was confirmed by CNMR(5 12.3, 14.3, 14.9, 16.9, 17.1, 131.6, 132.2, 139.7, 151.6). PM-
567 is also commercially available and its absorption and fluorescent properties are well characterized (Pigure 2.2 and Table 2.2).
C2H5
Figure 2.2: Structure of Pyrromethene 567.
Table 2.2: Absorption and Fluorescence Properties of Pyrromethene 567 PROPERTY VALUE Absorption Maximum 518 nm Fluorescence Maximum 547 nm Molar Absorptivity (518 nm) 7.2*10" L/(mole»cm) Fluorescence Quantum Yield 0.83 in ethanol 0.995 in methanol 45
2.2) Host Chemistry
2.2.1) Synthesis of Xerogei Films
Route 1: Xerogei solutions. All the Coumarin dyes at 11 wt % were separately
dissolved in anhydrous TEIF. Deionized H2O (acidified to 0.15 M HCl) was added at HiOidye
mole ratios of 1.5:1 for derCoum, 3:1 for 2derCoum, 1.5:1 for derPCoum, and 3:1 for
2derPCoum. No H2O was added to the Coum;THF mixture. The solutions were refluxed for
various lengths of time. Separately, tetramethoxysilane (TMOS), THF, and deionized H2O
(acidified to 0.15 M HCl) were mixed in a glass vial at a TM0S:H20:THF mole ratio of 1:4:5.
The solution was mixed for a few minutes allowing the TMOS to hydrolyze. The
dye/tlzO/THF solutions were mixed with the TMOS/H2O/THF solutions in proper amounts,
resulting in a dye:Si mole ratio of 0.05:1 (P=0.05) (see Figure 2.4a).
Route 2: Neat dye solutions. It was possible to form films by hydrolysis-condensation
reactions of the silylated dyes in solvent. Other silicon-containing precursors were not needed
to form either a gel network or a film. All the silylated Coumarin dyes in THF at 110 wt %
were separately reacted with H2O at the same mole ratios stated above with the synthesis of the
xerogei solutions. The solutions were refluxed for various times (see Figure 2.4b).
The dye concentrations are reported as the dye:Si mole ratio and not the dye:TMOS
molar ratio, where the "dye" refers to the "Coum" basic ring structure. In other words, the total moles of Si results fi'om both the TMOS and fi^om the Si contributed fi-om the silylated dye. For example, a r=1.0 derCoum xerogei film would represent 100% derCoum composition and a r=0.5 2derCoum xerogei film would represent a 100% 2derCoum compositioa For the 46
Coum xerogels films the dyeiSi is the same as the dye;TMOS ratio; while for the silylated dyes
that is not the case. The dye:Si ratio (r) is a more accurate representation of the dye
concentration within the sol-gel host.
Film formation. After aging for 24 hours, the solutions (whether xerogel or neat) were passed through 0.2 ^m filters and spin coated on pre-cleaned microscope slides (Gold
Seal) at 2000 rpm for 20 sec. The films were dried at 125 C for 48 hours in vacuum. All films were about 0.5 |am in thickness, as measured with a Dektak D profilometer, and were optically transparent in the visible spectrum. Fluorescence in the blue was visually observed upon UV excitation.
2.2.2) Synthesis of Polyceram Monoliths A low molecular weight (MW=400-700) silanol-terminated polydimethylsiloxane
(PDMS) (United Chemical Technologies) was the reactive polymer used to make the
Polycerams (Figure 2.3). The silanol end groups of the PDMS can participate in condensation reactions with the silylated dye, a hydrolyzed metal alkoxide, or another silanol-terminated PDMS. This can result in DYE-PDMS, Metal-PDMS, or PDMS-
PDMS linkages, respectively.
CHs CH3 HO—Si—(—O—Si-)-;rOH• , ' \ I I " CH3 CH3
Figure 2.3: Structure of silanol terminated PDMS (n=6-8). 47
Route 3: Potyceram Acid Route. Tetraethoxysilane (TEOS), PDMS, tetrahydrofuran
(THF), various Coumarin dyes, and H2O (acidified to 0.15 M with HCI) were mixed in a flask
at a dyerTHFJIaOrTEOS mole ratio of 10'*:20:2;1. The final Polycerams contained 80 volume
% polymer. The solutions were refluxed for 1 hour and then concentrated. Monoliths were
prepared by pouring the solutions in polypropylene beakers and drying for ~3 weeks. Thick films were also made by dip coating the solutions on microscope slides. Polishing of the monoliths was accomplished on an Buehler® Ecomet 3 polisher. A typical polishing sequence involved processing the samples with 600 grit SiC paper, 800 grit SiC paper, 6.0 |im diamond suspension, followed by 1.0 and 0.3 iim AI2O3 micropolish paste. The resulting Polyceram disks were clear and colorless with dimensions of 2 cm in diameter and 0.4 cm thick. This synthetic route is shown schematically in Figure 2.4c.
Route 4: Polyceram Acid/Base Route: Non Synersis. TEOS, PDMS, ethanol
(EtOHT), and H2O (acidified to 0.15 M with H2SO4) were refluxed in a flask for 1 hour at a
Et0H:H20:TE0S molar ratio of 35:2:1. PDMS was added at various concentrations ranging from 0% to 99% by volume. A base (e.g., triethylamine, pyridine, 3-aniinopropyltriethoxysilane
(3AS), l,4-diazobicyclo[2.2.2]octane (Dabco)) was added at base:acid mole ratio of 2:1 and the solution was mixed for 15 minutes. PM-567 was then added at a concentration of r = 0.5*10'^ and the solution was concentrated. Monoliths were made by pouring the solution in polypropylene beakers and drying at
75 C for 3 days. After polishing (as described above), the resulting Polyceram disks were 2 cm
in diameter and 0.4 cm thick. Samples were made at various PDMS contents in the solid
Polyceram ran^g from 0 % to 99 % by volume. This synthetic route is shown schematically
in Figure 2.4d.
Route 5: Polyceram Acid/Base Route: Synersis. TEOS, EtOH, and H2O
(acidified to 0.15 M with H2SO4) were refluxed in a flask for 1 hour at a Et0Hil20:TE0S
molar ratio of 35:3:1. A base (e.g., triethylamine, pyridine, Dabco) was added at base:acid mole ratio of 2:1 and the solution was mixed for 15 minutes. PM-567 was added at r=0.5*10'^
, the solution was refluxed for an additional hour, and then the PDMS was added. The solutions were then concentrated and dried in polyethylene beakers with pinhole tops at 75 C for one week. This synthetic route is shown schematically in Figure 2.4e.
Route 6:3AS Potyceram. TMOS, S-aminopropyltriethojq^silane (3AS), PM-567, and distilled water were mixed in a vial at a 1:0.2:1.2* 10'^:4 mole ratio. The solution gelled within a few seconds. The gel was heat to 75 C for 12 hours in a closed vial, where the gel reversed back to a solutiorL The resulting solution was dried in a open vial for 12 hours. The resulting samples were thick films with thickness' of several millimeters. This synthetic route is shown schematically in Figure 2.4f 49
Dye + THF
H2O (.15M HCl) added for silylated dyes (HiOrDYE 3:1)
TMOS, THF, H2O (.15M HCl) Reflux (0-48 hours)
Mix / Mix
Solution aged 24 hrs
spin coat on microscoipe slides
\ / Xerogel Film 0.5 ^m thick
Figure 2.4a; Synthetic Route 1; Xerogel Films. Dye + THF
H2O (.15M HCl) added for silylated dyes (H20:DYE3:1)
nU Reflux (0-48 hours)
Mix
Solution aged 24 hrs
\ / Spin c oat on microscoipe slides
/ Neat Film 0.5 ^m thick
Figure 2.4b: Synthetic Route 2: Neat Films. 51
TEOS, THF, PDMS, Coumarin Dye, H2O (.15 M HCI)
Reflux (Ihr)
Mix
Concentrate
Cast Monolith Dried 3 Weeks at Room Temp.
Polish
Disk: 2 cm Diameter x 0.4 cm Thick
Figure 2.4c: Synthetic Route 3; Polyceram Acid Route. 52
TEOS, EtOH, PDMS, H2O (.15 M H2SO4) (TEOSrHzO 2:1)
Reflux (1 hr)
\ Base Catalyst Added Mix Pyrromethene \ Dye
Concentrate
Dried in Polyethylene Beakers at 75 C for 3 days
Polish
Disk: 2 cm Diam x 0.4 cm Thick
Figure 2.4d: Synthetic Route 4: Polyceram Acid/Base Route: Non Synersis. 53 TEOS, EtOH, H2O (.15 M H2SO4) (TEOSrHzO 3:1)
Reflux (1 hr)
Base Catalyst Added Mix Pyrromethene Dye >J^ Reflux (1 hr)
/ \ PDMS
Concentrate
Pinhole Drying in Polyethylene Beakers at 75 C for 7 days
\/ Cut/ Polish
Disk: 1.0 cm Diam x 0.4 cm Thick
Figure 2.4e: Synthetic Route 5: Polyceram Acid/Base: Synersis. 3AS + TMOS + PM-567 + H2O (Distilled)
\U Heat 75 C for 12 hours (Closed ViaJi
Heat 75 C for 12 hours (Open Vial)
Thick Films 1 cm Diam x 2 mm Thick
Figure 2.4f: Synthetic Route 6; 3 AS Polycerams. 55
2.3) Optical Characterization
Optical Spectra. Absorption spectra were measured using a Perldn-EImer Lambda 3B
UVATS spectrophotometer. A blank microscope slide was used as a reference for the film spectra. Fluorescence spectra were measured by a ISI SPEX FluoroMax2 Fluorescence
Spectrometer. Solutions were measured in transmission, while films and bulk samples were measured in reflection.
UV Lamp Photostability. Samples were exposed to a long wave UV lamp (>300 nm,
8 Watts) located 1.5 inches away. The absorption spectra of the samples were monitored periodically with a UV-VIS spectrophotometer (Perkin-Elmer, Lambda 3B). The maximum absorption of the dye was normalized to the absorption before UV exposure and plotted as a fimction of UV exposure time. The permanent drop in the absorption represents the degradation of the dye.
N2 Laser pumped Photostability. Photostability was also measured by pumping the samples with a N2 laser at 337 nm with a pulse rate of 7.7 Hz, a pulsewidth of 20 nsec, and a fluence of 0.001 J/cm^ (1*10"^ J/pulse, 0.10 cm^ spot area). A solid angle of the fluorescence was focused onto a silicon detector (Laser Precision) after passing through a 385 nm bandpass filter to block the pump light. The fluorescence output energy per pulse was monitored as a fimction of number of pump pulses. A schematic of the setup is shown in Fig. 2.5. 56
Nitrogen Laser pyroelectric Silicon detector detector Mirror
Beam splitter
lenses &- Mirror ^ Sample
Figure 2.5: Photostability measurement setup.
Frequency Doubled Nd:YAG Pumped Photostability. Fluorescence photostability was also measured by monitoring the fluorescence intensity as a fimction of the pump pulses from a Q-switched, frequency doubled NdrYAG laser (Quanta-Ray DCR-11) at 532 nm with a pulse rate of 10 Hz, a pulse width of 6 nsec, a pulse energy of 28 mJ, and a spot size of 0.6 cm in diameter. The samples were pumped while placed within a 6 inch integrating sphere. The exiting light was passed through color filters and a spectrometer in order to collect only the fluorescent light from sample. The spectrometer was adjusted to the wavelength which had the highest fluorescent intensity (near 577 nm). The intensity of the light exiting the spectrometer was monitored by a silicon deteaor. A schematic of the setup is shown in Fig. 2.6. 57
NdrYAG Laser Silicon Detector
Spectrometer
KDP Crystal
Rlter Mirror
532 nm
Integratng Sphere Mirror Collimating lens Sample
Figure 2.6: Photostability setup using a Nd:YAG laser pump.
2.4) FTIR Monitoring of Hydrolysis Reaction Rates
The hydrolysis reaction rate was monitored by Fourier transform infrared
spectroscopy (FTIR) (Perkin Elmer 1725x). The silylated dyes were separately dissolved
in anhydrous THF at 11 wt %. H2O acidified to 0.15 M HCl was added at a 1.5:1
H2O: silylated dye mole ratio for the monofimctionalized dyes (derCoum, derPCoum) and
3:1 for the bifimctionalized dyes (2derCoum, 2derPCoum). The same was done with
TEOS in THF at 2.11 mole % at H20:TE0S mole ratio of 2:1. The solutions were
refluxed and infrared spectra were taken periodically. An IR transparent NaCI sealed cell was used as the solution holder to prevent evaporation allowing for quantitative jinalysis. 58
Standards were run with HzO, EtOH, TEOS and dye separately to identify peaks.
Anhydrous THF was used as the background.
2.5) Dye Extraction Measurements
To characterize the dye incorporation within the sol-gel matrix, dye extraction
measurements were poformed. The sol-gel films were soaked in THF at 50 C for several days.
The THF was kept in an enclosed glass container, and a reflux condenser was used to prevent
loss of solvent by evaporation. The samples were periodically removed and the maximum
absorption of the dye was measured.
2.6) Surface Area and Porosity Measurements
The surfece area, pore volume, and pore size were determined by the N2 (77 K) adsorption BET (TBrunauer, Emmet, Teller) isotherm method"® on a NCcrometrics ASAP 2000 for the solid Si02 J'DMS Polycerams. Typically, 0.5 gm of the solid Polyceram was outgassed for 48 hours at 90 C in vacuum (10"^ Torr) before the adsorption measurement.
2.7) NMR Spectroscopy
Nuclear Magnetic Resonance (NMR) Spectroscopy was performed at the University of
Arizona, Department of Chemistry NMR Facility. 'H and solution NMR (Varian Unity-
300) was performed on the dyes in deuterated Chloroform with tetramethylsilane as the 59 standard. Magic Angle Spinning (MAS) Cross Polarization (CP) ®Si NMR was performed on tlie solid Si02fDMS Polycerams using a Bruker Instruments model MSL-200. For the CP spectra, 5 ms contact time was applied, and for MAS, spinning rate of 3.5 kHz was used. 60
3) COVALENT BONDING OF LASER DYES TO HOST
3.1) Hydrolysis Reaction Rate
To obtain films with high optical quality fi-om multi-alkoxide precursors, it is important to incorporate the silylated dyes homogeneously within the matrix. Caging can be enhanced by increasing the degree of dye bonding to the matrix and minimizing the extent of dye-dye bonding. The best way to control such a structure is to match the reaction rates of the different alkoxide species. Hydrolysis (1) and condensation (2) reaction schemes for a silicon alkoxide are described below:
=Si-0R+H20 —• =Si-OH+ROH (1)
^Si-OH +=Si-OH —• =Si-0—Si= + H2O (2) where R represents an alkyl group (e.g., -CH3, -CH2CH3). Using FTIR spectroscopy the hydrolysis reaction was monitored by following the drop in H2O content in the solution and by the increase in the alcohol (ROH) content in the solution as the reaction proceeds.
Reactions were carried out under acidic conditions to maximize the hydrolysis reactions and to minimize the condensation reactions, which result in H2O formation.
H2O has three relevant absorptions at 1647 cm"' (bending), 3504 cm"' (stretch), and at 3571 cm"' (stretch), while the hydroxyl group in EtOH has one relevant absorption at 3474 cm"' (stretch). The peaks were identified fi"om reference solutions of H2O and
EtOH. The hydrolysis rate of TEGS was monitored by disappearance of the H2O
(bending) peak at 1647 cm"'. The hydrolysis rate of the silylated dye, on the other hand. 61 was monitored by the decrease in the 3571 cm'^ H2O absorption, because other infrared absorptions of the silylated dye overlapped with the 1647 cm"' H2O absorption, and because the 3475 cm"' EtOH absorption overlapped with the 3504 cm"' H2O absorption.
The IR spectra in Figure 3.1 illustrates the decrease in H2O content during the hydrolysis of TEOS and of a silylated coumarin dye (derCoum).
1 O min 2 15 min 3 IJhrs 0.20 4 5.5 hrs 5 10 hrs
0.15 n
3 0.10
0.05
0.00 1800 1750 1700 1650 1600 1550 1500 1450 1400 Wavenumbers (cm-i)
(a) 62
0.60
0.55 O his 0.50 4hrs 0.45 25 his 48 his 0.40
0.35 0.30
0.15 0.10 0.05 0.00 4000 3900 3800 3700 3600 3500 3400 3300 3200 3100 3000 Wavenumbers (cm-'')
(b)
Figure 3.1: FTIR spectra of (a) TEOS and (b) derCoum at different hydrolysis times.
The hydrolysis rate of the silylated dyes was found to be much slower than that of
TMOS. Under acidic conditions and stoichiometric amounts of water, complete hydrolysis of TMOS takes place within a few minutes therefore, the hydrolysis rate of TMOS could not be easily monitored by the technique used in this study.
An ethoxide-fimctional alkoxide (TEOS) hydrolyzes and condenses much more slowly than a methoxide-fiinctional alkoxide (TMOS) due to steric hindrance of the bulkier ethoxy group The hydrolysis rate of derCoum was even slower than that of
TEOS (see Figure 3.2). Alkyl substituted silanes such as methyltriethoxysilane (MTEOS) are known to hydrolyze much faster than TEOS under acidic conditions'"*^ The alkyl 63
group substitution increases the stability of the charged transition state, enhancing the
kinetics of hydrolysis. The reason for the slower kinetics of hydrolysis of derCoum (which
can be considered an alkyi substituted triethoxysilane) is likely due to the large steric
hindrance caused by the alkyl group which is composed of the urethane dye linkage and
the dye (where "dye" refers to the "Coum" basic structure). Large alkyl groups in alkyl
substituted alkoxysilanes have been found to hydrolyze slower than smaller alkyl groups'"*^'
For example, Pohl et. al. found that a cyclo-CeHu trialkoxysilane to hydrolyze 7 times slower than a methyl trialkoxysilane. Also, they reported a propyl trialkoxysilane to hydrolyze 2.6 times slower than a methyl trialkoxysilane^"".
derCoum • 2derCounfi • derPCoum o 2derPCoum X TEOS
•
o -0.1
0 200 400 600 800 1000 1200 1400 1600 Reflux Time (min)
Figure 3.2; Hydrolysis rate of TEOS and the silylated coumarin dyes. 64
To examine the steric hindrance effects in more detail, the lowest energy form of
derCoum was determined by a molecular modeling program. DerCoum curls up in its
lowest energy state, such that the urethane linkage and the dye surround the
triethoxysilane (Figure 3.3). The large blockage of the triethoxysilane group by the dye
and the urethane linkage gives evidence that a steric effect is likely causing the slower
kinetics of hydrolysis of derCoum with respect to TEOS.
Carbon
Hydrogen
Oxygen
Nitrogen O SOicon
Figure 3.3; Ball and stick model of derCoum in its lowest energy form.
The silylated dyes with the propyl linkage were found to hydrolyze faster than the urethane linked dyes (Figure 3.2). The reason for this is not well understood. We speculate that the faster hydrolysis of the propyl linked dyes is because propyl linked dyes have shorter linkages than the urethane linked dyes, resulting in less steric hindrance. For
the same reasons, TEOS hydrolyzed faster than the urethane linked dyes but slower or
equivalent to the propyl linked dyes.
TMOS precursors were used for making the xerogel films (Route l)in the present
study instead of TEOS due to ease and speed of processing. Therefore, to match the
reaction rates of the silylated dyes with TMOS, prehydrolysis of the silylated dyes was
performed.
3.2) Dye Extraction/Thermal Stability
The degree of dye bonding to the inorganic matrix can be inferred fi-om the dye
extraction results. In Figure 3.4, the absorption of the xerogel films (normalized to the film
absorption before soaking) is plotted as a function of the soak time in THF. The derCoum
sample did not show any decline in the absorbance after nearly 4500 min. This indicates that
the dye is bonded to the SiOa matrix. In contrast, the Coum film showed a decrease in absorption of more than 80% within 1500 min of soaking, indicating substantial dye loss. 66
00 —a— derCoum /Si02 1- 0.8- eo —o— Coum /Si02
0.6-
0.4-
0.0 —r— —I— —I— I 0 1000 2000 3000 4000 5000 Soak Time in THF at 50 C (min)
Figure 3.4: Dye extraction of derCoum and Coum xerogel films at r=0.10 synthesized by Route 1.
The extraction of the dye from its hosts depends significantly on the processing conditions (e.g., order of precursor mixing, water content). Prehydrolysis of the derCoum befiare mixing with TMOS resulted in films in which the silylated dye could not be removed
(Figure 3.4). DerCoum hydrolysis is slow compared with the hydrolysis of TMOS; thus pre- hydrolyzing the slowest hydrolyzing species resulted in greater dye-silica condensation.
The water content also played a large role in the incorporation of derCoum within the sol-gel films (Figure 3.5). Large dye loss occurred with a solution processed under partial hydrolysis conditions (TMOSil2C)=I:2), while no dye loss was observed under M hydrolysis conditions (TMOSiHzO = 1:4). For other conditions, where the solution was first processed 67
under partial hydrolysis and then additional water was periodically added to reach
stoichiometric water content, slight dye loss was observed. The more permanent dye
incorporation at high water content is attributed to the greater hydrolysis of the dye and
TMOS, resulting in greater condensation of the dye with the silica matrix.
-a- 1.0
oo 0.8- —a— Full hydrolysis —o— Partial hydrolysis = 0.6- —A— Partial to full hydrolysis
.o < 0.4-
0.0 1000 2000 3000 4000 5000 Soak Time in THF at 50 C (min)
Figure 3.5; Dye extraction of derCoum xerogel films at r=0.10 synthesized by Route 1 with various TM0S:H20 ratios.
In all variations of processing eq)lored, the Coum dye was almost completely extracted fi'om the films within 1500 min of soaking (Figure 3.6). With solutions processed under fiill hydrolysis conditions, Coum was extracted more slowly than under partial hydrolysis conditions. The slower removal of Coum fi^om the host is attributed to increased partial cagjng of Coum. Partial caging is when a dye molecule is not completely surrounded by the silica 68 matrix, but is caged by the matrix enough to slow down dye migration, but not enough to prevent it.
0.8- 00 T— —a— Count 4/Si02 Full hydrolysis CO —o— Coum 4/SI02 Partial hydrolysis 0.6- —A— Coum 4/Si02 Partial to Full hydrolysis
•9 0.4-
1000 2000 3000 4000 5000 Soak Time ia THF at 50 C (min)
Figure 3.6: Dye extraction of Coum xerogel films at r=0.05 synthesized by Route 1 with various TMOS.HaO ratios.
The above extraction results indicate that under proper processing conditions, the silylated dye can be incorporated within a sol-gel matrix to provide a more chemically stable material than with an unsilylated dye. This allows for the incorporation of large concentrations of organic species while maintaining high chemical and environmental stability.
To prevent dye extraction fi-om the host, the dye must be completely caged within the silica network (which is unlikely for most of the dye molecules) or a silylated dye must be covalently bound via at least one reactive group to the matrix. The monofimctionalized dyes 69 have three reactable sites, while the bifimctionalized dyes have six reactable sites. Therefore, ceteris paribus, the probability of getting bonding of the bifunctionalized dyes to the host matrix should be higher than with the monoflinctionalized dyes. Even though full hydrolysis conditions were required to prevent dye ejctraction from the host for a monofimctionalized coumarin dye, derCoum; the bifunctionalized dyes were not extractable from the host regardless of whether partial or full hydrolysis conditions were used (Figure 3.7). Such results reflect the ease of achieving dye bonding with bifimctionalized dyes.
1.0-
E "5? no CO 0.8- s —T— Partial hydrolysis —A— Full hydrolysis ca CO 0.6- € oU) a < 0.4- •a(D N m 0.2-
1000 2000 3000 4000 5000 Soak time in THF (min)
Figure 3.7; Dye extraction from a bifunctionalized dye 2derCoum in xerogel films at r=0.10 synthesized by Route 1 under partial and full hydrolysis conditions.
Dye bonding to the matrix does not necessarily mean the dye is completely caged. The dye may be covalently bonded within the pores of the network where other species can easily 70
(Miise to the dye and initiate photochemical interactions. It is likely that with full hydrolysis conditions a larger number of the six reactable sites have bonded to the silica matrix than with partial hydrolysis conditions. As the number of covalently bonded reactable sites increases, the probability of caging increases.
The mobility of Coum was not only illustrated by solvent leaching, but also by thermal leaching (Figure 3.8). Coum and derCoum Polycerams (Route 3) were heated at 75 C and the maximum absoibance of the dye (320 nm) was monitored. The heating temperature was well below the degradation temperature of the coumarin dyes, and any drop in absorption is attributed to dye sublimation from the host matrix. Dramatic dye loss was observed from the
Coum Polyceram, while little to no dye loss was observed from the derCoum Polyceram. The high Coum mobility was also confirmed from the observation of crystallization on the surface of the xerogel films and Polyceram monoliths at high dopant levels (r>0.10). 71
1 1 ^ ^ 2 1.0 — •
O 0.8 1
o A 0.6 < •o o 0.4 H (0 E 1 Coum Polyceram 2 derCoum Polyceram
0.2- -
0.0 50 100 150 200 Heating time (hours) at 75C
Figure 3.8: Loss of dye upon heating Coum and derCoum doped Polycerams at r=0.001 synthesized by Route 3.
3.3) SoUd-State ^'Si NMR
Solid-state ^Si Nuclear Magnetic Resonance (NMR) is a powerfiil technique for examining the structure of silicates Figure 3.9 illustrates possible silicate structures that can be formed in the sol-gel process. Q represents a quartenary ojQ'gen tetrahedron; T represents a three oxygen, one allq'l group tetrahedron; and D represents a two ojq'gen, two allq'l group tetrahedron. The superscripts denote the number of alkoxide groups that have 72
reacted to form Si-O-Si linkages. Therefore, Q", T", and D° represent unreacted precursors,
while Q"*, T^, and represent completely reacted species.
BrO-^O-R HrO-^-O-X RrO-^-O-X ItO-^-O-X X-O-^-0-^
QO Q' Q2 Ql Q"
CHJ CH3 CHj OTJ Hr<>-SH>-X -V<>-Sh-(>-X ?«>-SS-0-K
«p) "j-i ^ /pj CH3 CHj CHj R-o-a-o-R it-o-sW)-x xo-a^x \3iy CH3 (Hi] D" D'
R = HorCH2CH3 X = Q,T,orD
Figure 3.9: Possible silicate structures formed in the sol-gel process
A NMR spectra for a derCoum xerogel will have T peaks from derCoum and Q peaks
from TEOS. Quantitative evaluation of the T°, T\ T^, and peaks will determine the degree
of dye bonding within the dye matrix. Cross-polarization (CP) and magic angle spinning
(MAS) techniques were used to obtain enhanced signals and resolution with solid-state ^Si
NMR. Cross Polarization enhances the intensity of Si atoms located near hydrogen atoms (<10
A away)^^. The NMR spectra for a derCoum xerogel using 13 hours of dye prehydrolysis is shovm in Figure 3.10 and summarized in Table 3.1. 73
T»
T I I I 10 0 -10 •30 -70 PPK
Figure 3.10: CP-MAS ^Si NMR of a derCoum xerogel at r=0.10 synthesized by Route 1.
Table 3.1: Chemical shifts and the fiaction of various silicate structures measured by CP-MAS ^Si NMR for a derCoum xerogel at r=0.10 synthesized by Route 1.
rjiO Sample Ti T2 T3 Q" Q' Q' 0' derCoum -22 -54 -65.9 — — -93.7 -102.5 -111.0 xerogel ppm ppm ppm ppm ppm ppm (.00) (.08) (.17) (.75) (.00) (.00) (.05) (.69) (.26) (.07)* (.40)* (.53)* * corrected ratio
The chemical shifts of the T and Q peaks were identified by their relative positions and by assigned peak values fiom previous solid-state ^Si NMR work "V There was no identifiable evidence of any unreacted derCoum (T"), indicating all the dye has been dye bonded to the matrix. 75% of the dye was fully condensed to the host determined by the area 74 of the peak, while 25% of the dye was partially hydrolyzed determined by area of the T' and
peaks. Greater dye caging is expected as the number of species increases with respect to
and species. The dye molecules are more likely to be located within the pores of the matrix because th^r are not likely to be caged. It is expected that the amount of dye prehydrolysis time should affect the ratio between T^;T^:T\ AS the species increases, the probability of caging increases and so does the photostability (see Section 3.5.3).
Quantitative analysis of the tetra-fimctional species is a bit more complicated. The raw data from the CP technique reveals that the Cf .Cf .Q* ratio is 0.05:0.69:0.26. There is, however, a low cross polarization eflSciency with the species because there are fewer H atoms nearby to fecilitate transfer of polarizatiorL The Fourier transform (FT) NMR technique does not have this issue because all species are equally observed. Glaser et. al. found that the discrepancy was large between the CP and FT NMR techniques for a 100% TEOS-derived gel, but was very small with PDMS:TEOS Polycerams. Therefore, corrections are only needed for the xerogel samples, not the Polycerams (^Si NMR data for Polycerams are discussed in
Section 4). The corrected Q peaks are shown in Table 3.1. Such a correction was not needed with the T species, because hydrogen atoms are always near the T type Si, because derCoum has hydrogen containing allq^l groups directly bonded to Si (see Figure 2.1).
3.4) Fluorescence Efficiency
The fluorescence quantum yield could not be measured quantitatively with the experimental setup used. The fluorescence efficiency can be described as the ratio of the number of photons emitted to the number of photons absorbed or as: 75
(f} = ^ ' kf + k„ + k, where kf is the fluorescence rate constant, k^ is the internal conversion rate constant, and the kt is the intersystem crossing rate constant. The relative fluorescence eflBciency was detennined for the sol-gel films by dividing the area of its fluorescence spectrum by the absorption at the excitation beam wavelength (337nm). Figure 3.11 is a plot of this fluorescence eflSciency as a fimction of dye concentration for the dye/sol-gel films. The relative fluorescence efiBciency was higher for the derCoum films at all concentrations. Li contrast, both Coum and derCoum in solvent had essentially the same fluorescence intensity, indicating that both have similar relative fluorescence eflBciendes. The maximum in the relative fluorescence eflBciency was observed at higher concentrations for the derCoum films (0.01 M) than for the Coum films (0.001 M). The silylated dye film showed a 2 fold increase in the relative fluorescence eflBciency at 0.01 M concentration with respect to the normal dye. This improvement is believed to be attributed to the greater ri^dity of derCoum (reflecting a reduction of the internal conversion rate constant
(kic)), and greater dye isolation (reflecting a reduction of the intersystem crossing rate constant ^^^ 45,85,152,153 76
5 700000
600000-
500000-
400000-
300000-
200000- —Q—derCoum 4 / Si02 —o—Coum 4/ Si02 100000-9'
1E-4 1E-3 0.01 0.1 Concentration (r)
Figure 3.11: Relative fluorescence efiBciency as a function of concentration for Coum and derCoum xerogel films synthesized by Route 1.
3.5) Photostability
3.5.1) Fluorescence Photostability
A typical fluorescence photostability curve measured by pumping with a N2 laser is shown in Figure 3.12. Initially, the fluorescence intensity dropped rapidly, but then decayed linearly over long times. This type of photostability behavior has also been observed in a number of other dye/solid host systems'*^''*®' Since most samples showed no recoverability in fluorescence, both regimes represent permanent photodecay. Although a derCoum xerogel showed some recoverability, this only represented an 8% rise in fluorescence upon blocidng the pump beam for one hour after 23,000 pulses (see Figure 3.13). The initial decay has been 77 attributed to dye molecules located in the open pores of the network, which are more prone to photo-decay processes such as oxidation (see Section S).
>, 1.0
S 0.8
U- 0.4
O
0.0 0 10000 20000 30000 40000 50000 Pulses from N laser 2
Figure 3.12: Typical fluorescence photostability curve of a Coumarin xerogel at r=0.05 synthesized by Route 1. 78
? 0.9
fq 0.3
10000 20000 30000 40000 50000 Pump Pulses from a N laser 2
Figure 3.13; Photostability of a derCoum xerogel film synthesized by Route 1 illustrating the partial reversibility of the initial decay m photostability.
In this section, the long term decay will be the focus of attention. To compare the
samples quantitatively, a figure of merit (FOM) is defined to describe the rate of permanent
dye degradation in the slow linear process. The FOM is defined as:
m FOM = -r b
where m is slope and 6 is the y-intercept of the linear regression fit of the slow linear
fluorescent decay. Dividing the slope by the y-intercept provided normalized rates of
fluorescence decay, and this allowed the comparison of samples with different fluorescence output intensities. The FOM values are negative, and a value of 0 would mean there is no decay in the fluorescent output in this region of time. 79
3.5.2) Absorption Photostability
Photostability was also measured by pumping with a longwave UV lamp (>300 nm).
UV photostability or absorption photostability monitors the absorbance of the dye as a function of UV lamp exposure time. This method was used with both the Coumarin (320 nm) and
Pyrromethene (365 nm) dyes, since they both had absorption bands within the spectral output of the UV lamp. Figure 3.14 illustrates the decay in the absorption spectra of a Coumarin xerogel (Route 1) and a Pyrromethene polyceram monolith (Route 4) as a fimction of UV exposure time.
1.0
0.8
(D 0.6 0 hr 1 hr \2hr hr 55 0.4 30 hr
72 hr 0.2
0.0 300 350 400 450 500 Wavelength (nm)
(a) 80
400 450 500 550 Wavelength (nm) (b)
Figure 3.14: Absorption spectra of (a) coumarin xerogel at r=0.05 synthesized by Route and Pyrromethene doped SiOaiPDMS Polyceram at r=0.5*10'^ synthesized by Route 4 upon exposure to UV light.
Most dye degradation processes result in the alteration or the destruction of the k electron aromatic system, which results in the loss or the shifting of the absorption of the dye.
The absorbance (A) of the dye through a given sample at a particular wavelength is given by:
A = log 10 -sbc where I is the incoming light intensity, I<,is the exiting light intensity, s is the molar absorptivity
(L/(mole*cm)), b is the thickness of the sample (cm), and c is the concentration (moles/L).
The molar absorptivity is a material property which is dependent on the particular dye used in 81 the study. The absoibance of the sample is linearly related to the concentration and is therefore a direct measure of the concentration. In order to compare accurately the dye degradation rate
between samples, the samples for each experiment had the same dye concentration and sample thickness.
The absorption photostability measurements were typically carried out by measuring only the majdmum absorbance of the dye. Figure 3.15 is a plot of the normalized area of the absorption spectra as function of the normalized maximum in the absorption of the dye. The solid line represents a 1:1 correspondence between the two. Data from two different samples also shows good 1:1 correspondence. The dye concentration is proportional to the area of the absorption spectra. Since the data shows a good 1:1 correspondence, the drop in the maximum absorbance of the dye is also directiy proportional to the dye concentration. Therefore, photostability data can be taken by simply monitoring the drop in the maximum absorbance of the dye, greatly simplifying the experimental procedure. 82
1.0- 1.0
0.8
0.6
1 i 0.4 (S §. E P 02- Oi
0.0 0.0 0.0 02 0.4 0.6 0.8 1.0 Normalized Maximum Dye Absorption
Figure 3.15: Normalized absorption spectra area as a function of the normalized maximum absorption of a derCoum xerogel at r=0.05 synthesized by Route I and a PM- 567 SiOaiPDMS Polyceram at r=0.5*10~* synthesized by Route 4.
3.5.3) Effect of Prehydrolysis of the Dye
The FTIR spectroscopy studies of the reaction rates discussed earlier showed that
the silylated dyes needed to be prehydrolyzed m order to be incorporated homogeneously within the host matrix. To improve photostability, the silylated dye must be caged within the matrix; and caging can be enhanced by increasing the degree of condensation of derCoum to the silica matrix. To explore this, various dye-xerogel fihns were coated from sol-gel solutions using different prehydrolysis times for the derCoum. Fluorescence photostability was measured and the FOM was evaluated to determine the optimum prehydrolysis time. 83
Two different processing routes were explored. The first route is the xerogel
Route 1 (described in Section 2), where the prehydrolyzed dye solution was mixed with a
prehydrolyzed TMOS solution to obtain the final xerogel solution. The second route is a variation of xerogel Route 1, where the prehydrolyzed dye solution was mixed with
TMOS, and then acidified water was added to hydrolyze the whole mixture.
The photostability results for the derCoum xerogel films as a function of prehydrolysis time of the dye are shown in Figure 3.16. Both processing routes showed a maximum in the photostability FOM for 13 hours of prehydrolysis. For the conditions explored here, the silylated dye hydrolyzed enough after 13 hours such that the condensation rate of the dye and TMOS precursor closely match to maximize the dye- silica bonding, dye caging, and photostability. ^Si NMR results confirm the large degree of dye bonding (i.e., large fi"action of T^ silicate species) for a derCoum xerogel sample processed with 13 hours of prehydrolysis (see Section 3.3).
At short prehydrolysis times (<13 hours), the lower photostability is attributed to the lack of dye caging by the network. Dye bonding to the host does not necessarily mean that the dye is caged. The dyes, which are bonded but not caged, are likely located within the pores of the silica network. For long prehydrolysis times (>13 hours), the dye has been hydrolyzed and significant amount of self-condensation has occurred. This will lead to greater dye-dye bonding vs. dye-silica bonding within the xerogel, and hence decreased photostability. 84
-•—Mot All —a—TMOS Prehydrolyzed
IL.
-5 I 1 1 0 10 20 30 40 50 Pre-hydrolysis Time for derCoum (hours)
Figure 3.16; Photostability FOM of derCoum xerogel films at r=0.05 synthesized by Route 1 as a function of prehydrolysis time of the dye.
3.5.4) Xerogel Films
The photostability FOMs of all the dyes in xerogels hosts are summarized in Table
3.2. The higher values of the FOMs (or the smaller absolute values of the FOMs) correspond to higher photostabilities. For comparison, the absorption photostabilities upon exposure to UV light are also summarized in the table; the absorption photostability is reported as the normalized absorption after 72 hours of UV exposure (i.e., absorbance of the dye film after 72 hours of UV lamp exposure divided by the absorbance of the dye film before exposure). 85
Table 3.2: Photostability of coumarin-doped xerogel films at r=0.05 synthesized by Route 1.
Dye FOM Normalized (10"® pulses"') Absoqition After 72 hours of UV Exposure Coum -3.75 0.67 derCoum -1.12 0.81 2derCoum -5.64 0.35 derPCoum -3.81 0.21 2derPCoum -5.71 0.16
With optimized sol-gel processing conditions, a silyiated dye xerogel film, derCoum (FOM= -1.12*10"® pulses'^), showed a 3-fold improvement in the photostability
FOM compared to its unsilylated dye counterpart, Coum (FOM= -3.75*10'® pulses"^). The same photostability trend was observed with the absorption photostability (see Table 3.2 and Figure 3.17); after 72 hours of UV exposure, the Coum xerogel film had only 67% of the dye not degraded, while the derCoum still had 81% of the dye not degraded. 86
1.0-a
E 0.9- c CM 0.8 Si CJ 0.7-1 c o 0.6 e- o 0.5- to 0.4- —CoumXerogel —a— derCoum Xerogel 0.3-
75 0.2-
0.1-
0.0 I T T" -> T I T 10 20 30 40 50 60 70 80 UV Exposure Time (Hours)
Figure 3.17: UV photostability of Coum and derCoum xerogel at 0.05 M.
However, there was not always a 1:1 correspondence between the two
photostability methods. In other words, when the FOM was high for a particular dye xerogel, signifying a high photostability, the absorption photostability would not always signify the same trend in photostability. For example, the derPCoum xerogel had a similar
FOM to that of Coum, but the absorption photostability of the derPCoum was much lower
(see Table 3.2). The mechanism of dye degradation is probably different depending on the method of pumping. The UV lamp excitation is a continuous, broadband pump, while the
N2 laser pumping is pulsed and monochromatic. Thermal degradation is more likely with
UV lamp pumping because it is continuous, not allowing the dye to return to ground state or allowing for heat to dissipate. Also, the UV lamp degradation method monitors the 87
drop in absorption of the films, while the N2 laser method monitors the drop in the
fluorescence intensity. Therefore, the UV method only monitors the degradation
(bleaching) of the dye molecules, and the N2 laser method can monitor the combination of
fluorescence quenching processes and dye degradation.
By eliminating interaction of the dye molecules with other species, a number of dynamic processes are prevented (Figure 1.2). Because a great majority of the unsilylated dye is easily removed during solvent extraction, these dye molecules must be located within the open pore network of the xerogel and are more likely to undergo photodecomposition through dynamic processes such as oxidation and dimerization. This type of reasoning correlates well to the observed improvement in photostability. Using a silylated laser dye provides improved photostability because the silylated dye has a greater tendency to be caged and immobilized within the host. Proposed structures for Coum and derCoum xerogels in Figure 3.18 illustrate the greater caging of derCoum. 88
\^i^—SiC \/
—Si' oLc/ "oh o—Si. '•~0-Si~-o—S
S-OH /\ CH3 0-Si
/^\ /\ / "-^ ^1— /(^-Si-/ *\
(a)
/ I v\HO—Si \ ./ -Ji^ W str 1 OH\SV_Q'X SiQ^Si^
\ 0-Si^ /^O o M ° \P I "--O DH ^ T/^>T \l. -. _ V^ ^'Si^-^Si-O^Si-
O. Xt P "°si^^Si^
(b)
Figure 3.18: Proposed structures of Coum and derCoum in xerogel hosts.
The Coum xerogel and the monofimctionalized dye xerogels (derCoum, derPCoum) had a much higher fluorescence photostability than the biflinctionalized dye xerogels
(2derCoum, 2derPCoum) (see Table 3.2). These results seemingly contradict the above discussion of photostability. With dye bonding possible at six sites with the biflinctionalized 89 dyes, the probability of dye caging is higher, which should lead to superior photostability compared with the monofiinctionalized dyes. However, 2derCoum and 2derPCoum were synthesized from 5,7-dihydroxy-4-methyIcoumarin (2Coum), while the monofiinctionalized dyes were synthesized from Coum. The UV photodegradation of the silylated dyes and their unsilylated dye counterparts dissolved in MeOH at 10"* M indicate that 2Coum is inherently less photostable than Coum (Figure 3.19). Hence the lower photostability of 2derCoum can be associated with the inherent lack of such stability of its underivatized precursor. The UV half life of Coum was about 5.7 hours, while for 2Coum it was only 2 hours (Figure 3.19). The hydro3tyl group substitution on the seven position makes the 2Coum more prone to photodegradation. Therefore, the FOMs of the monofimctionalized dyes and Coum cannot be compared to the FOMs of the bifunctionalized dyes because th^r are chemically derived from precursors with inherently different photostabilities. 90
1
c —•— Coum —Q— derCoum —•— 2coum 0} —o— 2derCoum cu CO £1 o nM < •a
CO E( o Z
0 5 10 15 20 25 UV exposure time (Hours)
Figure 3.19; UV Photostability of coumarin dyes in MeOH at 10"^ M measured in quartz cuvettes.
Figure 3.19 also shows that the photostability of Coum and derCoum in MeOH were essentially identical. Therefore, silylation did not inherently change the photostability properties of the dye. These results indicate that the improved photostability observed with derCoum is associated with the dye/matrix interactions related to the dye caging within the xerogel host and not with the change in the inherent dye photostability upon silylation.
3.5.5) Polyceram Films
The same Coumarin dyes were also incorporated within Si02:polydimethylsiloxane
(PDMS) Polyceram monoliths (Route 3). Quantitative comparison of the photostability FOM 91
of the Polyceram samples with the xerogels films was not possible because of the diflferences in
dye concentration, sample thickness, and pump conditions. Nevertheless, similar photostability
trends were obtained (e.g., derCoum Polyceram had improved photostability compared to the
Coum Polyceram, and the bifiinctionalized dye Polyceram had a worse photostability than the
other Coumarin dye Polycerams) as shown in Table 3.3 and Figure 3.20. One interesting
difference is that the silylated dyes showed greater improvement in photostability b the
Polyceram host than in a xerogel host. DerPCoum illustrates this point very well. In a xerogel
host the FOM of derPCoum is similar to that of Coum, showing little improvement in
photostability (see Table 3.2). In a Polyceram host, by contrast, a significant improvement in the photostability was observed (Table 3.3).
Table 3.3: Photostability FOM for Coumarin dyes in Polycerams at p=10^ synthesized by Route 3.
Dye-doped Photostability FOM Polyceram (10"^ pulses'^) Coum -4.69 derCoum -2.64 derPCoum -3.25 2derCoum -6.90 92
1 Coum Polyceram 2 derCoum Polyceram
2 02-
0.1 - 0.0-1—I I I I I I I I I I 0 2 4 6 8 10 UV Exposure Time (Hours)
Figure 3.20: UV photostability of Coum and derCoum Polycerams at r=10"^ synthesized by Route 3.
This effect is likely caused by the difference in dye concentration. In the xerogel host the dyerSi mole ratio was r=0.05, while in the Polyceram host it was 1=10"^. The dye-dye distances within a sol-gel host were calculated for various dye concentrations assuming 100% reaction of the Si precursors and using 2.05 gm/cm^ as skeletal density of Si02 (Figure 3.21).
At r=10'^ the average dye-dye distance is «85 A, while at r=0.05 it is «12 A. Coum itself is 8 A long, and silyiated coumarins are much larger. At high dye concentrations, dye-dye interactions
(e.g., dimerization) are likely for all the dyes (whether silyiated or unsilylated), and this should result m the decrease in the photostability improvement caused by the use of a silyiated dye. At low dye concentrations, the silyiated dye has a greater probability of being caged because the dye can bond at three sites for the monofiinctionlized dyes (six sites for the bifiinctionalized 93 dyes) to initiate the caging and because there is more silicate species present to surround the dye, resulting in a greater photostability improvement with respect to the unsilylated dye.
200
? 150-
« 100-
50- t=0.05
1E-5 1E-3 0.01 0.1 DyerSi mole ratio
Figure 3.21: Predicted dye-dye distances as function of dye:Si mole ratio.
The composition of the host is not believed to be the cause for the improved performance of the silylated dye in the Polyceram host. A derCoum SiCb J*DMS Polyceram film at the same dye concentration as the xerogel film showed the essentially the same FOM
(Figure 3.22). Hence the concentration effect is believed to be governing the improved photostability of the silylated dye. Regardless, in both studies, the silylated dye doped into a sol-gel host showed improved photostability compared to the unsilylated counterpart. The effect of composition will be discussed in detail in Section 4. 94
2.5x10«
O 2.0bc10^
Ul 1Ac1(H Pofyceiam film c
Figure 3.22: Fluorescence photostability of a derCoum xerogel synthesized by Route 1 and a derCoum Poiyceram synthesized by Route 3 at r=0.10.
3.5.6) Neat Films
Neat films were synthesized by carrying out the hydrolysis/ condensation reactions on a neat silylated Coumarin precursor as described by Route 2. Optically transparent films were obtained with all neat silylated Coumarin dye films. The neat films have a high dye concentration. The dye: Si mole ratio for the monofimctionalized dyes is r=1.0, and the mole ratio for the bifimctionalized dyes is r=0.5. Xerogel films with the unsilylated dye at this concentration could not be obtained due to crystallization of Coum in the film. At such high concentrations with the neat films, dye-dye interactions are expected to be high
(Figure 3.21). The relative fluorescence efficiencies (fluorescence output intensity / absorbance of film) of the neat films were typically 3-5 times lower than those of the same dyes within xerogel films at r=0.05. This indicates that a large amount of fluorescence quenching is occurring in the neat fihns.
The N2 photostability FOMs of the neat fihns could not be compared directly to those of xerogels samples due to the differences in concentration. It was observed, however, that the FOMs of the neat films increases with hydrolysis time for each of the silylated dyes (Figure 3.23). The improvement in photostability is attributed to the increase in the degree of crosslinkmg within the system with hydrolysis time. For the neat samples, the degree of crosslinldng is the same as the degree of dye bonding. A greater increase in the FOM as fimction of hydrolysis time was observed with the bifimctionalized dyes (2derPCoum and 2derCoum) than with the monofiinctionalized dye (derPCoum).
Because the bifimctionalized dyes have six bonding sites vs. the three bonding sites of the monofianctionalized dye, the amount of caging possible for the bifimctionalized is greater. 96
0.00 I I I I I • 2derCoum • -1.00- • 2derPCoum
n -6.00- (0 S o -7.00- Q. -8.00 0 2 4 6 8 10 12 14 Hydrolysis Tfme (hours)
Figure 3.23; Effect of silylated dye hydrolysis time on the FOM of photostability for neat films synthesized by Route 2.
Even though a high FOM of -0.879*10"^ pulses'' was obtained for the 2derPCoum xerogel processed with 13 hours of hydrolysis, poor photostability was observed by UV degradation of this sample (Table 3.4). Only 15% of the dye remained after 72 hours of
UV excitation. The cause of this is attributed to the difference in pumping. The UV lamp pumping was continuous and broadband, which would result in more sample heating than the nanosecond pulses from the N2 laser. The neat samples are probably more prone to thermal degradation and also have lower thermal conductivity due to the large volume fraction of organic within the film. 97
Table 3.4: Photostability results for Neat Coumarin films.
Neat Film FOM Normalized (10"^ pulses') Absorption after 72 Hours ofUV Exposure derCoum derPCoum -.879 0.150 2derCoum -3.615 0.268 2derPCoum -6.25 0.220
3.6) Spectroscopy
3.6.1) Structure Effects of Dye
The absorption spectra of all the Coumarin dyes in methanol used in the present study were found to be similar, with an absorption maximum around 320 nm (Figure
3.24). The fluorescence spectra showed a distinct difference in spectra properties; 2Coum and the bifimctionalized dyes were red shifted with respect to Coum and the monofimctionalized dyes (Figure 3.24). This red shift of 2derCoum and 2derPCoum is attributed to the inherent optical properties of their parent dye molecule, 2Coum.
Comparing Coum to derCoum and derPCoum, the spectra are ahnost identical with a fluorescence maximum around 385 nm. Consequently, silylation of the dyes did not inherently alter the fluorescent properties of the aromatic portion of the silylated dye.
Therefore, it is possible to synthesized new dyes from a parent dye without destroying or even altering the absorption and fluorescent properties of the parent dye. 98
Coum deiCoum derPCoum aa 2Coum bb 2derCoum cc 2derPCoum
Wavelength (nm)
Figure 3.24: Absorption and Fluorescence spectra of Coumarin dyes in MeOH at 10'^ M.
Since the bifunctionalized dyes were not synthesized directly from Coum, but from
2Coum, direct comparison should only be made of the silylated molecules with their
parent molecule. Therefore, Coum and the monofiinctionalized dyes will often be discussed separately from 2Coum and the bifimctionalized dyes. 2Coum and 2derCoum also had very similar fluorescence spectra, with a fluorescence maximum around 435 nm,
However, 2derPCoum showed anomalous behavior in its fluorescence spectra by exhibiting a 9 nm blue shift with respect to its parent dye, 2Coum. 99
3.6.2) pH Effects
To understand the effects of dye/matrix interactions on the fluorescent properties,
one needs to explore how a dye behaves in different molecular environments. For a solute
in a solvent (liquid or solid), the environmental effects on fluorescence can be divided into
two major categories; (1) interaction of a permanent or induced dipole moment of the dye
molecule with the permanent or induced dipole moment of the solvent and (2) chemical effects (e.g., hydrogen bonding, pH, electron transfer, complex formation, intermolecular electronic energy transfer)
The combination of all the above solute-solvent interactions can be summed up in the true term of polarity. As a result, the polarity of a solvent or solute is very difficult to describe by just one parameter. Many have described the "polarity" (in quotes to signify that it is not the true term of polarity) of a solvent or solute as the four dipole interactions
(permanent-permanent, permanent-induced, induced-induced, and induced-permanent) discussed above, which depend on the dipole moment (|j.) and the polarizability (a) of
both the solvent and solute^^^ "Polarity" parameters, such as Et^ and the Kosower
Z value, have been established to compare quantitatively the "polarity" of different solvents^^"*' Specifically, the Kosower Z values is an empirical scale based on the transition ener^es of l-ethyl-4-carbomethoxypyridium iodide in various solvents which is utilized as a standard of solvent "polarity"'^^ Linear shifts in the optical spectra for many dyes have been observed as a fimction of these "polarity" parameters. 100
Organic molecules which have acidic or basic substituents are often prone to
chemical effects through hydrogen donating or accepting altering the optical properties.
The prediction of spectral shifts upon hydrogen bonding is not well understood. Typically, distinct changes in the fluorescence spectra occur"*
A multiparameter approach for describing spectral shifts due to a number of environmental eftects has been reported by Kamlet and Taft The wavenumber (v) of the absorption or fluorescence maximum can be written as:
y=v +S^ -vaa+bB... o ^ where vo is the wavenumber of the absorption or fluorescence maximum in a reference solvent, %* is a measure of polarity/polarizability efiects of the solvent, a is the hydrogen donor capability (i.e., acidity) of the solvent, |3 is the hydrogen acceptor capability (i.e., basicity) of the solvent, and S, a, and b are coeflBcients describing the strength of each of these contributions Only three major terms in the equation above have pertinence to the present study.
Coum is a laser dye which has been well studied for its optical properties. The dye is known to adopt a number of chemical forms (associated with hydrogen donating and hydrogen accepting) depending on its molecular environment (e.g., pH), each of which have different optical spectra. Three of the ground state forms - neutral, anionic, and cationic - absorb at 320,
365, and 340 nm, respectively, and fluoresce at 400, 455, and 420 nm, respectively. The
2witterionic ecciplex, which only exists in the excited state, fluoresces at 475-480 nm
The structure of chemical forms of Coum and their optical properties have been well 101
established (Table 3.5). Consequently, Coum can be tuned to the desired fluorescence by
adjusting its molecular environment.
Table 3.5: Chemical forms and spectral properties of Coum.
Name Structure Absorption Fluorescenc Conditions Maximum e Maximum Neutral 322 nm 385 nm Foimd in neutral and slightly acidic enviroimients. CHs Anionic 365 nm 455 nm Found in basic environments
CHs Cationic 350 nm 415 nm- Found in
•^HO. .0. .OH 430 nm strongly acidic environments
CHs Zwitterionic 480 nm Found in acidic (exciplex) environments in the presence of water. CHj
Silylating Coumarin dyes should affect the hydrogen donating and accepting capabilities of the dye and hence their fluorescent properties in diflferent pH environments.
To explore this, the fluorescence spectra of coumarin dyes in MeOH in various acidic
(10'^, 10'^, and 10*^ M HCl) and basic (10'*, 10'^, and 10"^ M NaOH) environments were examined (Figures 3.25 a-Q. Coum Ex 320 nm pH
a 10-' M HCI b 10^ M HCI c 10^ M HCI d Neutral e lOfi M NaOH f 10^ M NaOH g 10-1 M NaOH
350 400 450 500 550 600 Wavelength (nm)
(a)
1 ' 1 ' 1 • • ' - T— - 1- - -
• b/~ 3 derCoum Ex 320 nm pH 600000 - - a 10-1 M HCI d.o-i/yi^A\ / \ b 10-3 M HCI 500000 c 10^ M HCI d Neutral 400000 - e 10-5 M NaOH • w % / ^ \ f 10-3 M NaOH 300000 - 9 10-1 M NaOH
V\a 200000 - -
100000 -
0 - I.I.I. » 1 . 1 350 400 450 500 550 600 Wavelength (nm)
(b) 103
-1 r
derPCoum Bx 320 nm pH
10-1 M HCI 10-3M HCI
10-5 M HCI Neutral 10-5 M NaOH 10-3 M NaOH 10-1 M NaOH
350 400 450 500 550 600 Wavelength (nm)
(C)
2Coum Ex 320 nm pH
a 10-1 M HCI (S 100000 b 10-3 M HCI c 10-5 M HCI d Neutral e 10-5 M NaOH r 10-3 M NaOH
g 10-1 M NaOH
Wavelength (nm)
(d) 104
400000
2dCou[n Ex 320 nm pH
300000 a 10-1 M HCI b 10-3 M HCI c 10-5 M HCI d Neutral O 200000 e 10-5 M NaOH f 10-3 M NaOH g 10-1 M NaOH
100000
Wavelength (nm)
—1 ^ T -I • • r - • p -1- -1 — 1 • I •• • I —
300000 - 2derPCoutn Ex 320 nm pH -
' a 10-1 M HCI (D b 10-3 M HCI O 200000 // U ^ \ c c 10-® M HCI // ® - • - . . l-_ . J. . 1 . 1 1 . 1 350 400 450 500 550 600 Wavelength (nm) (Q Figure 3.25: Fluorescence spectra of coumarin dyes: (a) Coum (b) derCoum (c) derPCoum (d) 2Coum (e) 2derCoum (f) 2derPCoum in different chemical environments. 105 Coum has a complex fluorescence behavior upon change in pH (Figure 3.2Sa). The neutral form of the dye is present under neutral and acidic conditions, as indicated by the fluorescence at 385 mn. Under highly acidic conditions at 10"' M, the Zwitterionic form of Coum is clearly present with fluorescence at 480 nm. Under basic conditions of 10'^ and 10"' NaOH, fluorescence from the neutral form (385 nm) disappears, and fluorescence from the anionic form (450 nm) appears. The fluorescence behavior of Coum in the present study matches the previously reported pH behavior of Coum shown in Table 3.5 75 DerCoum has a similar acid/base behavior to Coum. The major difference is that derCoum is more resilient to the formation of the anionic species from the neutral species upon making the environment more basic (Figure 3.25b). At 10"^ M NaOJI, both the neutral and anionic species are present for derCoum, while only the anionic species is present for Coum. The fluorescence behavior of derPCoum showed even more resilience to the formation of the anionic form and the zwitterionic forms (Figure 3.25c). Under highly basic conditions (10*^ M NaOH), more of the nuetral form is present than the anionic form. Also, a very small amount of the zwitterionic form was present in derPCoum compared to Coum and derCoum at 10'^ M HCl. To summarize, the silylated coumarin dyes are more resilient to hydrogen donating or accepting, and therefore are less effected by their molecular environment. A quite different acid^ase fluorescence behavior was observed for the bifimctionalized dyes and its parent dye, 2Coum (Figures 3.25d,e,f). The fluorescence 106 behavior of 2Coum has not been as well characterized as Coum in the literature; therefore different chemical forms of these could be identified. The acid^ase behavior is summarized in Table 3.6 by showing the fluorescence maximum of the 2Coum and its derivatives as a function of the acid or base environment. The table clearly indicates that the dye consistently red shifts on going from an acid environment to base environment. The shift in the spectra is more gradual than with Coum and the monofimctionalized dyes. It is still believed that difierent chemical forms of 2Coum and the bifimctionalized dyes exist because they possess acid and base substituents on the structure just as Coum does. The different chemical forms may have fluorescence spectra that overlap; so the observed spectra shifts of 2Coum and the bifimctionalized dye maybe due to the convolution of different spectra of different chemical forms. The silylated dyes had a slightly less red shift than their parent molecule upon change in the pH (Table 3.6). 2Coum showed a 30 nm shift in spectra from a strong acidic environment (10"' M HCl) to a strong basic environment (10'^ M NaOH), while 2derCoum had a 27 nm shift and 2derPCoum had 25 nm shift. These data also support the above conclusion that the silylated dyes are more resilient to change in their optical properties under different molecular environments compared to their parent molecule. 107 Table 3.6: Fluorescence Maxima of 2Couni, 2dCoum, and 2dPCoum in MeOH at Cerent acidic and basic environments. Dye 10"^ M 10^ M 10 Neutral 10 10^ M 10'M Red HCl HCl Ha NaOH NaOH NaOH shift * 2Coum 425 nm 427 nm 438 nm 436 nm 441 nm 448 nm 455nm 30 nm 2derCoum 427 nm 427 nm 437 nm 436 nm 439 nm 451 nm 454nm 27 nm 2derPCoum 416 nm 419 nm 421 nm 418 nm 419 nm 422 nm 441nm 25 nm *Redshift represents the change in fluorescence maxima from 10"' M HCl to10"' M NaOH The silylated Coumarin dyes with propyl linkages (derPCoum, 2derPCoum) were less affected by their environment than the corresponding urethane linked dyes (derCoum, 2derCoum) (see Figure 3.27 and Table 3.6). This can be explained by the fact that the urethane linkage can experience greater electron transfer with the environment than the propyl linkage. For example, the anionic form of the dye can only exist when an electron is transferred to the oxygen located on the 7 position of ihe basic Coumarin ring structure. The existence of the anionic form in the urethane linked dye (derCoum) is seen in Figure 3.26. Therefore, it is relatively easy to form the different chemical forms of the dye with a urethane linkage compared to the propyl linked dyes; and the nature and degree of functionalization of the dye molecules can be used to control the amount of dye/matrix interactions and thereby to control optical spectra. 108 CH2CH3 /-Ov O rii>^ I O NH—(CH2)3—p—O—CH2CH3 CH3 CH2CH3 0 0 II o .0, .0—c—N—(CH2)3—Si—O-CH2CH3 0 1 CH2CH3 CH3 Figure 3.26: Electron transfer in a urethane linked silylated dye. The propyl linked dyes should not be able to form the anionic or zwitterionic species based on the fact that the propyl linkage should not allow electron transfer to the oxygen located at the 7 position on the aromatic structure; but the amonic and zwitterionic forms were observed with derPCoum, a propyl linked dye. Since the propyl linked dyes could not be re-crystallized upon synthesis to completely isolate the product, it is believed that the presence of some unreacted Coum still exists in the final product of derPCoum. The presence of some Coum in derPCoum may be causing the slight presence of the anionic peak (455 nm) in basic environments and the zwitterionic peaks (480 nm) in acidic environments. 109 3.6.3) Host Effects Solvent vs. Xerogel. The fluorescence maximum of all the dyes in solvent and xerogel hosts is summarized in Table 3.7. There is a dramatic change in the fluorescence spectra upon changing from solvent host to solid host. In solvent host, the neutral and anionic species are present; while in the xerogel host, it appears that only the cationic form is present. The cationic form is only found in highly acidic environments, and it has only been seen in concentrated sulfiiric acid environments'^'. The fluorescence maxima for the cationic form of the Coumarin dyes were determined by measuring the spectra in concentrated sulfuric acid (Table 3.7). Table 3.7: Fluorescence maxima of dyes in solvent host and xerogel host. Dye MeOH Si02 Xerogel Host Concentrated (Neutral) H2SO4 Coum 384, 448 nm 432 nm 410-425 run derCoum 384, 450 nm 438 rmi 410-425 nm derPCoum 384, 449 nm 430 nm 414 nm 2Coum 446 nm 465imi 2derCoum 447 imi 462 nm 468 nm 2derPCoum 424 nm 437 nm 480 nm It is reasonable that the cationic form can exist with all the silylated dyes. The cationic form exists when the carbonyl group on the 2 position of the aromatic system accepts a proton. Based on the structure of the silylated dyes, the carbonyl group is not chemically blocked, so a proton could eflfectively attach to the carbonyl group just as in Coum. 110 Since little is understood about the dye/solvent interactions of 2Coum and the biflmctionalized dyes; it will be even more difficult to discuss the effects of a solid host on the fluorescence spectra. Therefore, the results for 2Coum and the bifiuictionalized dyes are only presented, but not discussed. The cationic form of the dye appears to be present in the xerogel host, although the fluorescence maximum appears to be red shifted compared to the maximum of the cationic form in H2SO4. The discussion below will suggests possible reasons for this. The rigidity of the matrix (or viscosity in the case of solutions) can affect the degree of interaction between the host and the solute. Consider a typical energy band diagram for an organic molecule consisting of a singlet ground state (So) and a first excited singlet state (often called the Frank-Condon (F-C) ground state (Si)) ' (Figure 3.27). The absorption process is very fast (10"'^ sec), which is too fast for the molecule or its environment to move or relax during the transition. After absorption, the molecule is excited to the excited F-C State (Si*). Next, vibrational relaxation of the solute and the solvent occur with an average lifetime of 10"'^ sec to the F-C ground state (Si). Fluorescence (Tf=10'' sec) then occurs from the F-C ground state. The viscosity of the environment can affect the degree of solvent relaxation (molecular rearrangement of the solvent cage surrounding the dye molecule that takes place in the time frame of Xf) from the F-C excited-state to the F-C ground-state. In viscous solvents and in even more viscous media (e.g., solids), solvent relaxation is slowed, resulting In emission from the F-C excited state and a blue shift in fluorescence However, such results are contradictory to the presently observed red shifts going from solution to xerogel hosts. Ill F-C Excited State RnJvpnf relaxation \ Equiiibrium Excited State Energy Absorption B t \Fluorescence F-C Ground State Equilibrium ground state Figure 3.27; Energy diagrams for dye molecules in solution (A) and in solid hosts (B). The expected blue shifts may be overshadowed by other effects. The presently observed red shift can be explained by examining the effects of solvent "polarity" on the dye molecule (see discussion above). Organic molecules with the (n,7C*) transition as their lowest energy transition display a blue shift with increase m "polarity" of the host. In polar solvents, there is greater dipole-dipole interaction with the ground state than with the excited (n,;c*) state, lowering the energy of the ground state. The transition energy increases, and a blue shift occurs. On the other hand, a red shift is generally observed with increase in solvent "polarity" for molecules having the transition as their lowest energy transition. The (ir,7C*) excited state is more polar and more polarizable than the ground state, resulting in greater lowering of the excited state and reducing the transition energy Coum's lowest energy transition is the (%,k*) transition; and therefore it experiences a red shift upon increase in solvent polarity''. In fact, fluorescence is only 112 observed in polar solvents (e.g., water, alcohol) and not in nonpolar solvents (e.g., dioxane, toluene, heptane). The observed red shift with the Coumarin dyes in xerogel hosts is consistent with the highly "polar" nature of the silica surface. In the context of this discussion, the silica surface represents the silica cage surrounding the dye molecule. The "polar" silica surface, which stems from the highly polar silanol groups present at the surface, has been extensively investigated"*'' ^ The Kosower Z value (defined in Section 3.6.2) has been assigned as Z=88 for the silica surface, which is between that of water (Z=94.6) and MeOH (Z=83.6) Numerous examples of a red shift associated with laser dyes in polymer and sol-gel hosts have been reported"*'' ^ 165 To summarize, we hypothesize that a red shifted cationic form of the Coumarin dyes exist in the xerogel host. The presence of the cationic species is justified by the highly acidic nature of the silica surface. The red shift is justified by the "polar" nature of the silica surface. Previous studies of Coumarin in xerogel hosts suggest that the cationic form is present in xerogel hosts when prepared under acidic conditions It is likely that the Coumarin dyes are adsorbed on the silica surface, such that it accepts a proton from the silanol surface to form the cationic species. The hypothesis that both hydrogen bonding effects and "polarity" effects are contributing to changes in the optical spectra is well supported by the drying/gelling study discussed in Section 3.6.4. Homogeneity of chemical forms. It is possible for more than one chemical form of Coum to exist at a particular pH. By optically pumping the samples at different wavelengths of 320, 337 nm, 365 nm, we were able to probe distinct species in the host 113 because the different forms of the dye have different absorption spectra. The heterogeneity of the types of dyes present in the host can then be determined by observing changes in the fluorescence spectra due to variations in the pump wavelength. The results for this type of probing for all the dyes in solvent host clearly suggest the presence of numerous forms of the dye(Figure 3.28). The results for the dyes in the xerogel host were quite different. Regardless of the pump wavelength, the fluorescence maximum did not change. Only a change in the intensity of the fluorescence was observed, which was caused by the difference in absorptivity of the dye at different wavelengths. This suggests that only one form of the dye exists in the solid xerogel host, namely the cationic form. m Coum lO-^M MeOH Coum xerogel film- c3 (Q O co o 350 400 450 500 550 600 350 400 450 500 550 600 Wavelength (nm) Wavelength (nm) (a) (b) I ' I ' I ' T" derCoutn 10-^ M MeO' derCoum xerogel filn \2 1 I I I I I I I I—I—.—I- -I I I I I L. 350 400 450 500 550 600 350 400 450 500 550 600 Wavelength (nm) Wavelength (nm) (C) (d) T—I—I—I—I—•—I—I—I——r- -I—I—I—1—I—I—I—«—r derPCoum 10^ M MeOH derPCoum xerogel film t I I I I- 350 400 450 SCO 550 600 350 400 450 500 550 600 Wavelength (nm) Wavelength (nm) (e) (f) 115 p—r"»—I—I—I—I—I—«—I—«—r 2derCoum lO-^M MeOH 2derCoum xerogel film 09 C XI CO 0 a cQ) O 0)(0 w 3o 350 400 450 500 550 600 350 400 450 500 550 600 Wavelength (nm) Wavelength (nm) (g) (h) 2derPCoum 10^ M MeOH 2derPCoum xerogeifilm 3 Xk (Dw O O c o a> 350 400 450 500 550 600 350 400 450 500 550 600 Wavelength (nm) Wavelength (nm) (i) 0) Figure 3.28; Fluorescence spectra of coumarin dyes in MeOH and xerogel host (synthesized by Route 1) when pumped at (1) 320 nm, (2) 337 nm, (3) 365 nm. 116 3.6.4) Drying/gelling effects Before the drying behavior is discussed, a reference fluorescence spectrum of the xerogel solution (i.e., synthesized solution by route 1 before spin coating) itself is deserving of discussion (Figure 3.29). The spectrum of the derCoum xerogel solution reveals the presence of neutral (385 nm) and the exciplex form (480 nm) of the dye. The neutral form is known to be present m mildly acidic conditions. The pH of the xerogel solution was determined to be approximately 2.2. The exciplex form is known to exist in acidic conditions with the presence of water (Table 3.5 and Figure 3.30). The xerogel solution must then contain some unreacted water. Based on the ratio of the intensities, there is more neutral form than the exciplex form in the xerogel solution. I 350 400 450 500 550 Wavelength (nm) Figure 3.29; Fluorescence spectra of a derCoum xerogel solution (synthesized by Route 1) pumped at 337 nm. 117 CO C3 w (D O C o C/) 0)w o3 JL 350 400 450 500 550 600 Wavelength (nm) Figure 3.30: Fluorescence spectra of derCoum in MeOH/water mixture (9:1): (1) 0.10 M HCl (2) Neutral (3) 0.10 M NaOH. The nature of the silica cage was examined in more detail by timed spectroscopy during drying and gelling of one of the soi-gei solutions. A microscope slide was dip coated with a derCoum sol-gel solution made in MeOH solvent, and the fluorescence spectra were measured as a function of time (Figure 3.31). 118 2 2 min 3 4 min 4 6 min 5 20 min 6 30 min 7 heated 125C for 30 mif 8 heated 125C for 60 mir —I 1 1 I I I 400 450 500 550 Wavelength (nm) Figure 3.31: Fluorescence spectra of a dip coated derCoum xerogel solution (synthesized by Route 1) at different drying times and heating times when pumped at 337 nm. Immediately after dip coating, the fluorescence was light blue in appearance, with a fluorescence maxima at 385 nm and 480 nm range, representing the presence of the nuetral form and the exciplex form of the dye. This spectrum is very similar to the xerogel solution, except that there is more exciplex present. More water must be present due to coadensation reactions upon initial drying. Further drying causes the exciplex form (480 nm) of the dye to decrease due to the consumption of water though fiirther hydrolysis or evaporation. Also, the neutral peak at 385 nm is red shifted to 390 nm, caused by the increase in the "polarity" of the environment. The "polarity" of silica is higher than that of MeOH as discussed above'^^' Further drying leads to the increase in the neutral peak 119 and the decrease in the exciplex peak. The fluorescence spectra then stabilizes after 30 min, where no marked change in the spectra is observed. Hydrolysis and condensation reactions occur upon drying, causing gelation and resulting in the formation of a porous network where the nonbonded dye, MeOH, and H2O are located. The presence of the neutral species suggest that the most of the dyes are surrounded by a MeOH environment. Such an effect is justified by the much greater solubility of the Coumarin dyes studied in MeOH versus H2O; solubility of Coum within MeOH is 10 mg/ml, while in H2O it is only 0.1 mg/ml^®®. Also, some of the dye must be partiaUy surrounded by the silica surface due to the red shift in the spectra. Heating the dip coated film caused dramatic changes in the fluorescence spectra (Figure 3.31 curves 7,8). The neutral peak (385 nm) decreases and the cationic (425-430 nm) peak increases. If this change in the spectra were caused by a change in the "polarity" and not chemical effects, we would expect to see a gradual red shift in the spectra. This is clearly not the case; as heating takes place, the neutral peak decreases and the cationic peak increases, confirming that a chemical effect is occurring. Heating of the film forces the removal of the Me0H/H20 fi'om the pores as well as shrinkage of the pores, which causes the environment of the dye to change from a mostly MeOH environment to silica environment. Also, the formation of the cationic species is likely caused by the adsorption of the dye on silanol surface which allows the carbonyl group on the dye to accept a proton from the silica surface. FTIR spectra of the same solution (derCoum xerogel solution synthesized by Route 1) dip coated on a CaFl2 disc reveal dramatic decrease of 120 the broad -OH band (3400 cm'^) due to MeOH and/or H2O in the film after annealing at 125 C (Figure 3.32) which coincides with the above hypothesis. Air dried xerogel 0 I 1 I I I I I I I I 1 I 4000 3500 3000 2500 2000 1500 1000 Wavenumbers (cm-1) Figure 3.32: FUR transmittance spectra of a derCoum xerogels (synthesized by Route 1) air dried and after heating at 125 C. To summarize the spectroscopy work, complex fluorescence behavior was observed with all the Coumarin dyes due to the formation of different chemical forms of the dye upon change in the molecular environment. Silylation of the dyes did not destroy the fluorescence, but in fact provided little change in the fluorescence characteristics under neutral conditions. The pH behavior indicated that both the monofiinctionalized and bifunctionalized dyes were more resilient to change in chemical forms of the dyes when compared to their unfiinctionalized parent dye. Placing the dyes within a solid host 121 reduced the heterogeneity of chemical forms that exist within the matrix. Drying/gelling studies revealed dramatic changes in the molecular environment of the dye which corresponded to changes in chemical forms of the dye. The neutral form of the dye was present in the wet gel and in the unheated xerogel. Upon heating, the dye changed to the cationic form revealing greater interaction between the dye and the acidic silica environment. 3.7) Summary: Advantages of Covalent Bonding The use of silylated dyes allows the dye to be covalently bonded to various host matrices. Numerous silylated Coumarin dyes have been synthesized and incorporated within xerogels at high concentrations. The use of a silylated laser dye resulted in (1) improved solubility of the silylated dye with respect to its unsilylated counterpart, allowing for higher concentration of active molecules within a sol-gel matrix; (2) higher fluorescence eflSciency, attributed to the greater rigidity and isolation of the silylated dye within its host; (3) improved chemical stability, associated with the lack of leachability of the dye from the host; (4) control of dye/matrix interactions affecting optical spectra; and (5) improved photostability. 122 4) MECHANISM OF PHOTODEGRADATION 4.1) Role of O^gen Organic dyes can degrade by a number of photochemical reactions as discussed in Section 1.3. In order to find methods to improve the photostability of a particular dye/host system, one must understand the mechanism by which the dye degrades. In this section, the mechanisms of photodegradation for Coumarin and Pyrromethene dyes are discussed. Review of the literature reveals that various organic dyes often degrade by oxidation. This hypothesis was tested for both the Coumarin dyes m xerogel films and PM-567 in Polyceram monoliths. A derCoum xerogel film synthesized by Route 1 was placed within a small sealed chamber with quartz windows, and fluorescence photostability was measured using the Na laser setup (Figure 2.5). The photostability was first measured in an air atmosphere within the chamber; then the photostability of a different portion of the same film was measured by evacuating the chamber and flooding it with nitrogen gas. The results are shown in Figure 4.1. The derCoum xerogel film in a N2 atmosphere had a FOM of -3.47*10'^ pulses'\ 71% better than the FOM of the sample measured in an air atmosphere. Similar results were obtained by Lin et. al.^®, where Rhodamine dyes in sol-gel hosts illustrated a 76% improvement in photostability when measured in a N2 atmosphere compared to when measured in air. These results confirm the hypothesis that oxygen plays a large role in the degradation process for the Coumarin dyes. 123 Normal Atmosphere FOM=-534 o> Nitrogen Atmosphere FOM=-3.47 0.0 0 10000 20000 30000 40000 50000 Pump Pulses from Laser Figure 4.1: Fluorescence photostability of a derCoum Xerogel synthesized by Route 1 in different atmospheres. The initial fluorescence decay in Figure 2.5 was much pronounced with the sample in a nitrogen atmosphere, even though the long term fluorescence decay in normal atmospheres was more pronounced. The rapid initial decay of the fluorescence in a nitrogen atmosphere may be due to dye molecules trapped in the triplet state of the dye. The structure of derCoum and the other Coumarin dyes used in the present study all have a methyl group at the 4 position. This methyl group is particularly prone to oxidation in many Coumarin dyes, often yielding carboxylic acids (see Section 1.7). Before the photostability measurements, the Coumarin xerogel films were colorless; after degradation, the films had a faint yellow color caused by absorption at blue wavelengths. The formation of a carboxylic acid during the oxidation of the 4-methyI group in 124 Coumarin dyes is known to cause a 50 nm red shift in the absorbance of the dye, likely causing the yellow discoloration . Oxygen also played a large role in the photodegradation of PM-567 within the Si02:PDMS Polycerams synthesized by Route 4 . Samples were placed in sealed fiised silica cuvettes, and an over pressure of compressed air, oxygen, or argon was passed through the cuvettes while the samples were exposed to a broadband UV light (>300 nm). The measured absorption photostability is shown in Figure 4.2. The presence of oxygen caused a dramatic decrease in the photostability. In an argon atmosphere only 16% of the dye degraded after 74 hours; while in an oxygen atmosphere, >90% of the dye degraded in the same time period. In air, the photodegradation rate was similar to that of the sample in an oxygen atmosphere. T-lO 0.8 CO 07 'S3 B- 0.6 —•—Argon Atmosphere O 0.4- —•—Oxygen Atmosphere —A—Air 0.3- 0 10 20 30 40 SO 60 70 80 UV Exposure Time (hrs) Figure 4.2: UV photostability of PM-457 Polycerams synthesized by Route 4 in different atmospheres. 125 Unlike the Coumarin dyes, little is understood about the degradation chemistry for the pyrromethene laser dyes. A more detailed investigation of the oxidative and other degradation processes of PM-567 is warranted. 4.2) Photodegradation Chemistry 4.2.1) Degradation Products of PM-S67 Now that it is confirmed that oxygen plays a large role in the degradation process, a number of questions arise: Is it simply the dye that is oxidizing? Is the host playing a role in the degradation? What is the mechanism of the oxidation, free radical or singlet oxygen? Is it a single or multi-step reaction? Some of these questions can be answered by determining the degradation products of PM-567 through chemical analysis techniques such as NMR and FTIR spectroscopy. Performing chemical analysis of photo-degradation products is often diflBcult because the dye is dissolved in hosts at fairly low concentrations of 10'^ to 10"^ M, resulting in very small yields of degradation products. To maximize the yield, a custom quartz sealed cell was designed and constructed, offering a short path length (0.3 cm), a large surface area (10 cm x 10 cm) for UV lamp excitation, and a relatively large volume (30 ml compared to a conventional quartz cell with 3 ml). PM-567 was dissolved in EtOH at a concentration of 4*10'^ M, and a high intensity UV Lamp (365 nm, 7000 |iW/cm^) was used to degrade the PM-567/EtOH solution for 48 hours. Before degradation, the 126 solution was opaque red in white light; after degradation, the solution was transparent yellow. The product was isolated by evaporating the solvent. 'H and NMR spectroscopy were used to identify the product (see Table 4.1). 3-Ethyl-2- methylmaleimide was identified as the single major product (Figure 4.3). The FTIR spectra also supported the presence of this degradation product, by observing the presence of C=0 absorption at 1725 cm"' and the decrease in the C=C absorption at 1555 cm"' after degradation (See Figure 4.4). Table 4.1: 'H and NMR of degradation products of PM-567. 'HNMR 'HNMR '^CNMR "CNMR Measured Chemical shifts for Measured Chemical shifts for Chemical Shift 3-ethyl-2- Chemical Shift 3-ethyl-2- (ppm) methylmaleimide (ppm) methylmaleimide 168 1.144 (t) 1.15 (t, 3H) 8.414 8.3 1.363 (t) — 12.614 12.5 1.621 (s) 17.009 16.9 1.968 (s) 1.98 (s, 3H) 137.615 137.6 2.40 (q) 2.41 (q, 2H) 143.309 143.3 7.23 (s) — 171.961 172 7.90 (s, br) 8.08 (s, IH, br) 172.374 172.4 Figure 4.3: Structure of 3-ethyl-2-methylmaleimide. 127 100 3300 cm-< 80 00) 1 60 E in cCO 1555 cnr' ^ 40 degraded PM-567 1725 cm-i -PM-S67 20 4000 3000 2000 1000900 800 700 Wavenumbers (cm-i) Figure 4.4: FTIR spectra of PM-567 and degraded PM-567 in chloroform. 4.2.2) Acid vs Base Environment In addition to obtaining photostability of a dye within a solid host, tiie chemical stability must also be maintained. The chemical stability of PM-567 in the Polyceram solutions was very dependent on the catalyst(s) used to promote the sol-gel hydrolysis and condensation reactions. Reactions carried out with a highly acidic catalyst (e.g., HCl, H2SO4) resulted in complete bleaching (or d^radation) of the dye molecules during synthesis. Weaker acids used as catalysts (e.g., p-toluenesulfonic acid, fINOs), also resulted in significant dye degradation, indicated by a large drop in dye absorptivity and a blue shift in the spectrum of the dye. Such 128 processing conditions resulted in little or no visible fluorescence from the solutions upon UV lamp excitatioiL By examining the structure of PM-567, the pyrromethene-BF2 complex bond is probably the weakest link in the structure. During synthesis of the dye, BF3 was added to 2,6-diethyl-l,3,5,7,8-pentametItylpyrromethene hydrochloride salt under basic conditions to form the pyrromethene-BFa complex. When the dye is exposed to acidic environments, the complex bond will break, resulting in the removal of toxic BF3 gas from the mixture. This has been observed experimentally in our laboratory and reported by Canva et. al."®. In order to insure the stability of the PM dye and to maintain the high optical quality of the sol-gel host, hydrolysis was first carried out under acidic conditions (pH=2.2), and a base (e.g., pyridine, triethylamin^ 3-aminopropyltriethoxysilane(3AS)) was added to neutralize partially the solution (pH=5-6). This procedure (synthetic Routes 4 and 5) allowed for safe incorporation of PM-567 within the Polyceram host. The effect of different catalyst(s) used during the chemical synthesis of the Polyceram solutions on the absorption properties are summarized in Table 4.2. Dye degradation is reduced or prevented upon the addition of a base. 129 Table X2\ Effect of catalyst on optical properties of the Polyceram solutions. Synthetic Catalyst pK. Solution Absorption Absorption Fluorescence Route used Color Maximum at 517 nm (nm) 3 HCl -7.0 colorless none 0.00 no 3 p-toluene -6.0 red/ 498 0.50 no sulfonic acid brown 3 HNO3 -1.4 red/ 501 0.40 minimal orange 4 HCl /pyridine 5.25 orange 369, 516 1.90 yes 4 HCl/ 11.0 orange 372, 517 0.97 yes triethylamine 4 HCl/3 AS 10.7» orange 370,517 not yes measured *pKa of propylamine 4.2.3) Multi-Step Degradation PM-567 has been found to degrade in oxygen environments and acidic environments. Knowing the final oxidation product of this dye (3-ethyl-2- methylmaleimide), a multi-step degradation route is proposed by backtracking through the possible intermediate products fi"om the maleimide compound to the original dye. Pyrrole, especially allqrlpyrroles, are susceptible to oxidation. Upon oxidation, these compounds form products that often contain carbonyl groups located at the 2 and 5 positions of the cyclic ring (i,e., a maleunide)'®'. For example, 2,4-dimethyl-3-ethylpyrrole (kryptopyrrole) can oxidize in MeOH solvent by singlet O2 oxidation to yield 3-ethyl-2- methylmaleimide (see Figure 4.5) Interestingly, this product is the same as the final degradation product of PM-567 determined earlier. Therefore, it is proposed that an 130 alkylpyrrole is an intermediate product during the photodegradation of PM-567, and that this alkylpyrrole oxidized by singlet O2 oxidation to yield 3-ethyl-2-methylmaleiinide. The allq^lpyrrole can only be formed by the chemical splitting of the pyrromethene at the carbon bridge connecting the two pyrrole rings. It is hypothesized that this is caused by the oxidation of the C=C located between the two cyclic rings. This oxidation step can probably occur whether the BF2 is attached or unattached to the pyrromethene. The Pyrromethene-BFi complex bond greatly stabilizes the dye by forcing it to be planar. A planar molecule is much more resilient to transformations to higher energy twisted forms of the dye, which tend to be more reactive. Therefore, it is proposed that the dye is oxidized more easily when the BFi-complex bond is broken. O 0 3 R \ / Figure 4.5: Oxidation routes of alkylpyrrole to produce a maleimide. 131 To summarize, the proposed multi-step degradation of PM-567 is then; (1) acidic removal of the BF2, (2) oxidation of the C=C located at the bridge between the cyclic rings, and (3) oxidation of the resulting alkylpyrroles to form a maleimide (See Figure 4.6). Figure 4.6: Proposed degradation route for PM-567. 4.3) Types of oxidation processes Examining the different types of oxidation processes is a useful exercise, since it may provide clues to prevent oxidation reactions. Photo-oxidation processes can be divided into three major categories: (1) Free radical (autoxidation) oxidation, (2) Singlet O2 oxidation, and (3) Electron transfer free radical oxidation^. The first step of a free radical oxidation reaction involves a photo-excited sensitizer (S), which could be the dye molecule or the solvent, abstracting hydrogen from 132 another dye molecule. This forms a free radical which then reacts with molecular oxygen to form hydroperoxides. Further oxidation of the hydroperoxide often results in the formation of ketones. A possible reaction scheme involvuig free radical oxidation of a dye molecule (D) is shown below: S + hv 'S* ^ + DH D* + SH* D* + O2 D-0-0* D-O-O* + DH D-O-OH + D* D-0-0* + HOH D=0 + H2O2 where the superscripts 1 and 3 represent the singlet and triplet states of that molecule, signifies that the molecule is in the excited state, and signifies that the molecule is a free radical. Unlike most organic molecules, O2 has a triplet ground-state and a very low energy excited singlet-state. Excited-state singlet O2 is produced by quenching a sensitizer in the excited triplet-state. The sensitizer can be an excited dye molecule or an excited impurity, typically carbonyl containing compounds. Oxidation then occurs when singlet O2 reacts with the dye, often by an addition reaction of singlet O2 across a C=C. The oxidation products greatly depend on the structure of the dye molecule itself A generic reaction scheme involving singlet O2 is shown below: 133 S + hv -» 'S' 's' ^'s* + O2 •» S + 'O2* D + ^02*-^ Products The energy of formation of singlet O2 is 22.6 kcal/mole corresponding to an absorption at 1262 nm^. The intrinsic lifetime of singlet O2 is 45 min, but it is rapidly quenched by water vapor or moisture"'*. Lifetimes of singlet O2 in solvent are typically 20-200 |isec while the lifetime of singlet O2 in PDMS is 46 |isec^'^. The third major oxidation mechanism, electron transfer free radical oxidation, involves the electron transfer from a dye molecule to a photo-excited sensitizer. This results in the formation of a radical cation which could directly react with molecular oxygen or with a superoxide anion. The reaction scheme is illustrated below: S* +D->S'* + D^ s- + O2 -> S + O2- + O2 or 02" Products. 4.4) Role of Oxygen in DifTerent Hosts Since it has been established that O2 plays a strong role in the degradation of the dye, it is useful to discuss the implications of the presence of O2 in solvent and solid hosts. 134 The concentration and the difiiisivity of o^q^gen in different hosts is summarized in Table 4.3. Comparing the ojqrgen concentration in air to the ojQ'gen concentration in solvent and PDMS, it is clear that a large amount of oxygen is present in the host. In fact, the dye concentration of PM-567 at 5*10'^ M in a PDMS host is 6*10^^ cm'^, while the concentration of oxygen in PDMS is 4.7*10^' cm'^. The concentration of oxygen exceeds the dye concentration by a factor of 8. Clearly, at low dye concentrations there is more than enough oxygen ateady present in the host to oxidize all the dye. Table 4.3: Solubility and difiiisivity of oxygen in various hosts. Host Solubility or DifTusivlty of O2 concentration of O2 Air (or pores in 6*10^''cm"' 10"' cmVsec solid host) Solvent 1.2*10''cm-^ «10"*-10'^ cmVsec (2*10-^ M) ™ PDMS 4.7*10'®cm-^ 1.7»10-^cm^/sec SiOj glass «10''^ cmVsec'^" 10'^ cm^/sec'*' The difiiisivity of O2 decreases by orders of magnitude from air to solvent to PDMS (Table 4.3). However, the difiusivity of O2 in PDMS compared to most solid polymers is very high at 1.7*10'^ cm^/sec. Most glassy polymers have O2 diflfusion coeflBcients around lO'^-lO"' cm^/sec, while crystalline polymers have values of lO'^-lO'"" cm^/sec"«•"^ In a dense flised silica glass, the difiiision of oxygen is very close to zero at room temperature. Therefore, a dye molecule surrounded by a silica cage should be a good 135 barrier for ojq'gen. No data could be found for the diffusion of o:qrgen through a SiOa xerogel, although it is presumed that the rate of oxygen diffusion is governed by the amount of porosity present in the glass. Within a Si02:PDMS Polyceram, the diffusion of ojQfgen will depend on many factors including the polymer content and the porosity within the nanocomposite. The oxygen difiiisivity within the Polycerams can be estimated as a fimction of polymer content. At low polymer contents, the SiOa matrix is the continuous matrix, and the oxygen diffusion will depend on the porosity within the sample. In section 5, the control of porosity as a fimction of processing is discussed. When pores are present, the diffusion of oxygen through the bulk material is probably near the difiiisivity of O2 in air (10"' cmVsec), while difiusion of oxygen through silica rich regions is probably low. At high polymer contents, the PDMS must be the continuous matrix, and the difiiisivity should be close to the difiiisivity of O2 in PDMS. The Polyceram monoliths typically had thicknesses of 0.4 cm. Using the difiiisivity of O2 in PDMS, the time required for oxygen to difilise half way into the sample (0.2 cm) is about 40 min. Photostability measurements for 50,000 pulses upon pumping at 10 Hz took 1.39 hours. This suggests that during the photostability measurements, there is sufBcient time for O2 to difilise into the sample. However, the concentration of dissolved oxygen already present in the sample exceeds the dye concentration by factor of 8, so that oxygen diffusion is irrelevant because plenty of oxygen is present. At a typical dye concentration 5*10'^ cm"^, the average distance between the dye molecules is 119 A, determined by taking the cubed root of the concentration and converting to Angstroms. At an oxygen concentration of 4.7*10'^ cm"^, 136 the average distance between oxygen molecules is approximately 42 A. Ojq^gen only needs to diffuse a small distance (tens of Angstroms) to reach the dye molecule, and this reflects the need to inorganically cage the dye molecule to prevent dye oxygen reactivity. The structural model in Section 5.4 illustrates these concepts pictorially. 137 5) POROSITY AND PHOTODEGRADATION KINETICS In this section, the photostabilities of laser dyes within sol-gel derived hosts are found to be a large function of the porosity within the host. First, we will discuss how porosity is controlled in Polyceram matrices. Then, the porosity characteristics of these samples will be correlated to the observed photostability properties. Finally, the fluorescence photostability data is correlated to the porosity by a model describing the distribution of dye molecules within the solid host. 5.1) Control of Porosity as a Function of Sol-Gel Processing A number of methods have been used to control the porosity, surface area, and pore size distributions within sol-gel matrices. In a review by Brinker et. al.'^, these different techniques are discussed. A summary of these methods are illustrated in Table 5.1. 138 Table 5.1: Methods explored to control porosity in sol-gel matrices. Method Principle of Method Comments Particle •packing of colloidal particles •uniform pore size obtained Packing creates pores which have sizes with uniform particle size related to particle size •pore volume for spherical particles is always 33% regardless of particle size, assuming dense random packing Aggregation of •pores form through the •works only if clusters don't Fractals aggregation of polymeric sols interpenetrate or if there are no oligomeric species to "fill-in" pores Management of •vary with capillary pressure (Pc) •Methods Capillary to achieve desired pore size •vary ambient pressure Pressure during solvent removal •vary solvent composition •Pc= -2yIt , where y is the •vary surface chemistry surface tension and r is the pore (changes surface tension) radius •go to supercritical conditions •use drying control additives (DCCAs)^'^'^^ Molecular •pore size control by the size of •Example: pore size increases Templating solvent or organic ligand to be fi-om methanol to ethanol to removed isopropanol solvent Relative rates •porous when condensation rate •can control relative rates of of condensation » evaporation rate condensation and evaporation and •less porous or nonporous when through composition, evaporation evaporation rate » processing, aging, and drying condensation rate conditions Sintering and •sintering to higher temperatures Surface reduces pore volume and pore Derivitization size distribution •sintering occurs by the reduction of the solid vapor interfacial energy, which can be controlled by altering the surface of the gel 139 One way to control the porosity is to manipulate the relative rates of condensation and solvent evaporation. When the condensation rate greatly exceeds the evaporation rate during drying, the material will tend to have a higher porosity. In contrast, when the evaporation rate exceeds the condensation rate, the material is more likely to settle, resulting in a less porous material. The condensation rate can be controlled by a number of factors such as the reactivity of the alkoxide, the pH of the solution, the catalyst used, the nature and the amount of solvent present, and the HiOiSi mole ratio. During drying, the evaporation rate can be controlled by solvent, drying temperature, air flow, and surface area of gel exposed to drying. Synthetic Routes 1, 2 and 3 (see Section 2) are processing routes in which sol-gel solutions were processed using only acidic catalysts. These synthetic routes were sufiBcient for incorporating the Coumarin dyes within sol-gel derived hosts. However such a route resulted in the chemical degradation for the PM dye before processing was complete (See Section 4.2.2). The use of a two step acid-base process (Routes 4 and 5) was found to protect the dye by providing a basic or less acidic environment, although the use of these synthetic routes complicates the behavior of the hydrolysis and condensation reactions. The general effect of pH on the gel time for aqueous silicates is simimarized in Figure 5.1'*. The type of behavior has been observed with the hydrolysis/condensation reactions of silicon alkoxides with near stoichiometric water contents and using various catalystsUnder acidic conditions (pH=l-3), both the hydrolysis and condensation reactions are catalyzed, but the hydrolysis rate exceeds the condensation rate. This results 140 in weakly branched structures and relatively long gel times. Under less acidic conditions(pH=3-8 ), the condensation rate increases and the hydrolysis rate decreases, resulting in lower gel times. The hydrolysis becomes rate limiting, so the amount of hydrolysis determines the amount of condensation. The reaction rates of hydrolysis and condensation can then be controlled by hydrolyzing under acidic conditions and then accelerating condensation reactions by adding a base. In this study, synthetic Routes 4 and 5 take advantage of this behavior to obtain optically transparent, polishable monoliths where the PM dye is chemically stable. Ul S 0Ui UJ > 1 cUJ 0 12 pH Figure 5.1; Gel time as a fimction of pH for aqueous silicates. After Der The amount of condensation that takes place in the two step acidA)ase process (Routes 4 & 5) is governed by the degree of hydrolysis in the first step. For example, under partial hydrolysis conditions all the alkoxide groups have not hydrolyzed after the first step. Upon addition of the base, condensation rate is enhanced. However, the water- producing condensation rate is greater than the alcohol producing condensation rate. 141 Therefore, the degree of condensation is largely limited to the number of hydrolyzed species. The conclusion is that one can additionally control the degree of condensation in the acid-base process (Routes 4 and 5) by the amount of water added to the system and by the pH and time of the initial acid treatment- Routes 4 and 5 had specific differences in the processing, which lead to changes in the relative rates of hydrolysis and condensation (See Figures 2.4d-e). First, Route 5 utilized a higher water content with a H20:TM0S mole ratio of 3:1 compared 2:1 ratio used in Route 4. This resulted in a greater degree of condensation to take place in the Route 5 Polycerams. Second, the Route 5 solutions were refluxed for an additional hour after the base catalyst was added, while the Route 4 solutions were not refluxed after the addition of the base. This resulted in the greater condensation rate in Route 5 Polycerams. Third, the Route 5 solutions were pin-hole dried (i.e., dried in a covered polypropylene container with pin holes to allow evaporation), while the Route 4 solutions were open-top dried (i.e., dried in an uncovered polypropylene container) (see Figure 5.2). This resulted in lower evaporation rate for Route 5 solutions allowing more time for condensation to occur than with the Route 4 solutions. The differences in the processing between Routes 4 and 5 lead to dramatic changes in the drying behavior of the Polyceram solutions (see Figure 5.2). A syneresis drying behavior was observed, which is when the gel network contracts and expels the liquid in the pores'"*". The syneresis behavior is attributed to condensation reactions forming new bonds causing gel shrinkage, thus forcing the removal of solvent. The high degree of crosslinking will lead to a high gel strength. This will ultimately lead to less gel shrinkage. 142 and the dried gel will have high porosity and surface area. The drying behavior for the Route 4 Polycerams was quite different. Large solvent removal preceded much of the condensation reactions, because the sample did not gel until most of the solvent was removed. Sample shrinkage was governed by the evaporation; therefore these samples will tend to be less porous or nonporous. Route 4 Route 5 Non-Syneresis Syneresis Figure 5.2: Differences in drying behavior of Polyceram solutions processed using Route 4 and 5. Using N2 adsorption BET to measure the surface area and pore volume, the SiOjiPDMS Polyceram monoliths with 40 % PDMS synthesized using Route 4 had no measurable porosity or surface area, while the monoliths synthesized by Route 5 had a 143 high pore fraction (0.37) and a high surface area (496 m^/gm). These results confirm the behavior discussed above. NMR. CP-MAS ^Si NMR spectroscopy is a powerfiil technique for determining the structure of silicate species. In Section 3, this technique was used to determined the degree of dye bonding within the sol-gel matrix. Here, this technique is applied to determine the degree of silica and PDMS crosslinking within the Route 4 and Route 5 Polycerams. The NMR spectra for Si02;PDMS Polycerams (40 vol % PDMS) synthesized by Route 4 and 5 are shown in Figure 5.3 and summarized in Table 5.2. The silica species are more crosslinked in the Route 5 Polyceram, with 90% of the Q species as and Q"*; while the Route 4 Polyceram had 76% of the Q species as and Q"^. This quantitatively confirms the high degree of condensation of the Route 5 Polycerams. (a) (b) Figure 5.3: CP-MAS ^Si NMR spectra for Si02:PDMS Polycerams synthesized by (a) Route 4 and (b) Route 5 at 40 vol % PDMS. 144 Table 5^: Chemical shifts from MAS-CP ^Si NMR for PM-567 doped Polycerams. Sample D° D^ D^ Q" Q' 0' Q' T5-97b «-4.8 «-13.3 -23.4 — -90.5 -97.6 -104.2 -111.6 40% PDMS ppm ppm ppm ppm ppm ppm ppm Route 4 (.00) («.05) («.95) (.00) (.04) (.20) (.46) (.30) nonsyneresis T5-37b «-4.8 »-13.3 -23.5 -92.9 -97.8 -103.9 -111.5 40% PDMS ppm ppm ppm ppm ppm ppm ppm Route 5 (.00) («.05) («.95) (.00) (.03) (.07) (.54) (.36) Syneresis *Value in parenthesis represents the fraction of D* or Q* species with respect to total amount of D or Q species, respectively, present in the sample. The interpretation of the crosslinking of the D species is more complex. and D' peak assignments have been previously reported with values of -19 to -23 and -12 ppm, respectively^' The PDMS oligomer contains approximately seven Si atoms. That means that five of the Si atoms have a D-D^-D character, while the two Si atoms at the ends of the PDMS can have different types of bonding. In our system it is not possible to obtain the D° peaks which has a chemical shift of -4.8 ppm'^^* only D' and peaks are possible. The amount of D' (-12 ppm) present was very small (<5%) in both samples (Figure 5.3 and Table 5.2). A sharp peak at -23.3 ppm was observed with both samples, which represents both the Si located within the PDMS oligomer and the Si in the silanol groups at the ends of the oligomer which have condensed to another PDMS (D-D^-D). Both samples also had an additional broad D^ peak with chemical shifts between -18 to -21 ppm (see Figure 5.3). The broad peaks stem from the use of the cross 145 polarization technique, which enhances the intensity of Si atoms located near H atoms (<10 The broad peaks correspond to units in more constrained environments; in other words, it represents D species bonding to various Q species. This has been argued from the much longer cross polarization relaxation time for the sharp peak (6.0 msec) to that of the broad peak (0.8 msec)'^^. In other silicate materials, cross polarization times have been observed to decrease when the species are locally constrained. Broad peaks have also been noticed with CP-MAS ^Si NMR spectra of other PDMS/TEOS systems as well as in (dimethyldiethoxysilane) DEDMS / TEOS and DEDMS / (titanium tetraisopropoxide) TIP nanocomposites^*^' Further indication that the broad D^ peak represents cocondensation was suggested by an increase in broad peak with increase in TEOS content in a PDMSiTEOS nanocomposite'®®. The Route 5 Polyceram had a narrower cocondensation peak than the Route 4 Polyceram. The Route 4 Polyceram is more likely to have various D^ species condensing v^dth various Q species (D-D^-Q\ D-D^-Q^ D-D^-Q^, D-D^-Q"*); while the D^ species in the Route 5 Polycerams is more limited to cocondensation with and Q"* species (D-D^- Q^ D-D^-Q'). Overall, for the SiOjrPDMS Polycerams at 40% PDMS, the broad component of the D^ peak signifies that a large degree of cocondensation is occurring between the D and Q species suggesting that organic and inorganic species are mixing at the molecular level, and the Polycerams are molecularly homogeneous. The differences between the two Polycerams (Route 4 and 5) is in the degree of crosslinking and the porosity. Section 5.4 146 illustrates the similarities and dififerences between the Route 4 and 5 Polycerams with a structural model. 5.2) Control of Porosity as a Function of Composition Not only can the porosity be controlled by variations in the synthetic route, but also by the PDMS content. Table 5.3 illustrates the physical properties of SiOa-PDMS Polycerams synthesized by Routes 4 and 5 using various polymer contents. Figure 5.4 is a plot of the surface area of SiOaiPDMS Polycerams synthesized by Route 4 and 5 as a function of polymer content. 147 Table 5^: Properties of PM-567 doped SiOiJ'DMS Polycerams at various PDMS contents synthesized by Route 4 and Route 5. Sample Physical Properties BET Surface Density Pore Volume area (mVg) (gm/cm^) (Volume / Pore Size Fraction pores) Route 5 large syneresis, 828 ±3.5 0.88 ±.04 0.70 cmVgm 0% brittle, very H2O PDMS sensitive 28.8 A (0.65) Route 5 large syneresis, 742 ±3.5 0.62 cmVgm 20% brittle, 1.01+01 PDMS H2O sensitive, clear 28.7 A (0.63) Route 5 large syneresis, hard, 496 ±1.9 0.327cmVgm 40% least tendency to 1.12+01 PDMS crack 26.6 A (0.37) Route 5 large syneresis, 0.27 ±0.14 O.OOlcmVgm 60% hard, clear 1.18+01 PDMS (0.00) Route 5 small syneresis, Not measured Not measured 80% clear 1.15+01 PDMS Route 4 no syneresis 0.91 1.411 0.00 20% water resistant PDMS (0.00) Route 4 no syneresis 0.00 1.315 0.00 40% water resistant PDMS (0.00) Route 4 no syneresis Not 1.271 Not measured 60% slow gelling measured PDMS Hard Route 4 no syneresis Not measured 1.145 Not measured 80% slow gelling PDMS soft 148 800- Route S E 600- Route 4 0- 0 10 20 30 40 SO 60 Volume % PDMS Figure 5.4: Surface area of Si02:PDMS Polycerams synthesized by Routes 4 and 5 as function of the PDMS content. The surface area of the 0 vol % PDMS sample synthesized by Route 5 was very high at 828 mVgm. As the polymer content increased, the surface area and the porosity decreased, and in fact reached zero at 60 vol % PDMS (Figure 5.4). The pore size stayed essentially constant in the range of 26.6 A to 28.8 A. These results indicate that the amount of porosity within a Polyceram composition can be controlled by varying the polymer content in the composition. In contrast, the Route 4 Polycerams were all non- porous regardless of the polymer content. There are two possible explanations of how the polymer content affects the porosity with the Route 5 Polycerams. One, the polymer "fills-in" the pores as the polymer 149 content increases, thereby reducing the porosity. Two, as the polymer content increases, the silica condensation rate decreases due to co-condensation and steric hindrance, resulting in the decrease in the porosity. A decrease in the condensation rate would result from the decrease in the average fimctionality of the total composite as the PDMS content increases, where the TEOS is four functional and the PDMS is two functional. With the Route 5 Polyceram at 0% PDMS, the pore fraction is 0.65. Interestingly, the porosity reached zero upon increase in polymer content near 60 vol %, which is almost the same as the pore fraction in the sample without any polymer. This supports the "fill-in" model. This observation would suggest that the inorganic framework is forming independent of the PDMS. But the CP-MAS ^Si NMR data (see section 5.2) indicate that a significant amount of co-condensation is taking place within material. Therefore, the inorganic network must not form independently from the PDMS. For syneresis drying behavior to occur, it is likely that the continuous matrix must be mostly inorganic. Some co-condensation must occur within the inorganic framework, and the remaining PDMS is forced to occupy the pores. Other arguments also support the "fill-in" model. In the range of 0-60% PDMS, syneresis drying behavior was observed with the Route 5 Polycerams. But at 80% PDMS, the syneresis behavior was reduced, reflected by less shrinkage across the diameter and greater shrinkage along its length. The Route 5 Polyceram at 60% PDMS was nonporous. Therefore, one would expect nonsyneresis drying behavior with this sample, if the porosity reduction was caused by a decrease in the condensation rate. However this was not the case. In fact, identical syneresis behavior was observed in the range of 0% PDMS to 60% 150 PDMS. The polypropylene beaker in which the Polyceram solutions were dried had a diameter of 2.2 cm, while the final syneresis monolith in the polymer content range of 0-60 % PDMS had a diameter of 1.0 cm. Because all the samples shrunk to the essentially the same size, this suggests that in the range of 0-60% PDMS the degree of silica condensation has remained essentially constant. Identical shrinkage and drying behavior is not likely to occur if the condensation rate of the silica species was decreasing with increasing polymer content. CP-MAS ^Si NMR data for the Route 5 Polycerams as a function of polymer content (shown in Figure 5.5) suggest that the degree of silica condensation has remained fairly constant in the range of 20-60% PDMS. 151 80% PDMS Route 5 60% PDMS Route 5 40% PDMS Route 5 20% PDMS Route 5 I—•—I—'—I—'—I—•—I—•—I—I—I—•—I—'—I—•—I—•—I 40 20 0 -20 -40 -60 -80 -100 -120 -140 -160 PPtti Figure 5.5: CP-MAS ^Si NMR for the Route 5 Polycerams with varying polymer contents. The above evidence and arguments support the possibility that the pores are being "fiUed-up" by the excess polymer which are not co-condensed within the continuous matrix. If this is true, than the size of the phase separation between the polymer and Si02 matrix should be equivalent to the pore size. The pore size was very small, 26-28 A; therefore, the size of the phase separation should be 26-28 A. For size comparison, the 152 pore size is about the length of 3-4 PDMS oligomers. A structural model described in Section 5.4 illustrates proposed structures of porous and nonporous Polycerams. Density. The density of the Polycerams described in Table 4.3 correlate well to porosity measurements. The densities of the Route 5 Polycerams increased as the polymer content increased, while the density of the Route 4 Polycerams decreased with increased polymer content (Figure 5.6). The density of the Polycerams (pt) can be described as; Pt ~ ^SiOl PsiOl ^PDMS PpDMS + ^pore Ppore where psi02 is the skeletal density of the silica matrix, Ppdms is the density of the PDMS polymer, Ppore is the density of the pore, VSIO2 is the volume fraction silica, VPDMS is the volume fraction PDMS, and Vpore is the pore volume. The density of the polymer is 0.991 gm/cm^ and the density of the air is 0.001 gm/cm^. The theoretical skeletal density of silica has been reported as 2.05 gm/cm^, although observed values for the skeletal density tend to vary depending on processing conditions (ranging from 1.88 to 2.19 gm/cm^) The skeletal density is not reported for the type of synthetic routes used in the present study. If pores are present within the Polyceram at low polymer contents, one would expect to see an increase in the density upon polymer addition, assuming the polymer occupies the pores of the matrix. When the pores become filled, the density should decrease with fiirther increase in the polymer content. This is because the polymer density is less than the silica density. This type of behavior was observed with the Route 5 Polycerams by showing a rise in density at low polymer contents and a decrease in density at higher polymer contents. By the same reasoning discussed above, the Route 4 153 (nonporous) Polycerams decreased in density with increase in polymer content, as expected. 1.6-r—r Route 4 Non-syneresis Route 5 Syneresis T T T 20 40 60 80 100 volume % PDMS (a) 154 1.6 1.4- c^T" E ii- o a>E 1.0- CO c —•— Route 5 Polycerams 0.6- —Route 4 Polycerams linear fit to Route 4 Polycerams —a— Predicted Route 5 Polyceram densitic 0.4 0 20 40 60 80 100 volume % PDMS (b) Figure 5.6: (a) Density of PM-567 Polycerams as a function of PDMS content processed by Route 4 and 5 (b) same as (a) with fits to data. The density data for the Route 4 Polycerams can be modeled to obtain the skeletal density of silica. Because the Route 4 Polycerams are nonporous, a linear fit is expected of density as a function of the volume % PDMS, as signified by the density equation above with the porosity term excluded. The skeletal density of silica was determined as the y- intercept of a linear fit to the data (shown in Figure 5.6b). A skeletal density of 1.53 gm/cm^ was determined, which is significantly lower than reported values for the skeletal density. Since the data fits fairly well, it is believed that the silica skeletal density does not change with polymer content for the Route 4 for Polycerams. The model can be extended to the Route 5 Polycerams by including the porosity term in the density equation above. If 155 the calculated silica skeletal density is assumed to be the same for the Route 5 Polycerams, then the densities of the Route 5 Polycerams can be predicted using the pore volume measured by BET shown in Table 5.3. However, these values did not fit well to the model, especially at low polymer contents (Figure 5.6b). One explanation of this discrepancy is that the skeletal density changes with polymer content. This is contradictory to the NMR data which suggests that the degree of silica condensation does not change dramatically between 0-60% PDMS (Figure 5.5). It is possible that the skeletal density is then changing with polymer content due to the change in the degree of co-condensation between Si02 and PDMS. 5.3) Photostability The physical and structural properties of the Route 4 and 5 Polycerams have been shown to be quite different. The photostabilities of the dyes within these hosts are now examined to see how variations in the structural characteristics affect the dye/matrix interactions. First, the absorption photostability of the Polycerams synthesized by Route 5 was examined(Figure 5.7). The absorption photostability reveals that as the polymer content increases, the photostability improves. The improvement in the photostability is attributed to the reduction m pore volume (See Table 5.3). The dye molecules which are located within the pores are more susceptible to dye degradation, because O2 has easier access to the dyes in the pores than dye molecules located in polymer or silica cages. 156 20% PDMS 4G%PDMS -•—60% PDMS CO O 0 10 20 30 40 UV Exposure Time (hrs) Figure 5.7; Absorption photostability as function of PDMS content for PM-567 Polycerams processed using Route 5. Absorption photostability of the Polycerams synthesized by Route 4 exhibited intriguing results (Figure 5.8). The photostability improved with increasing silica content, contrary to the results of Route 5 Polycerams. Since all these samples were nonporous, it suggests that the silica caging environment was required to improve further the photostability. A dye caged in a SiOa-rich region is more of an oxygen barrier than a dye caged in a PDMS-rich region. The amount of SiOz-rich regions increases with increase in Si02 content in the total composition. The photostability of the 60% PDMS Polyceram synthesized by Route 5 was the same as the photostability of the 60% PDMS Polyceram synthesized by Route 4. This is expected since both of these samples are nonporous and 157 have the same composition. Also, this indicates that the structural di£ferences between the Route 4 and Route 5 Polycerams aside from the porosity, have little eflfect on the photostability. The 20% PDMS Polyceram showed the best photostability. Unfortunately, this composition has a tendency to crack during drying. The 40% PDMS Polyceram were less prone to cracking during drying. ••—20% PDMS •—40% PDMS ••—60% PDMS •A—80% PDMS 0.0 0 20 40 60 80 100 120 140 UV Exposure Time (hrs) Figure 5.8; Absorption photostability as function of PDMS content for PM-567 Polycerams processed using Route 4. S.4) Structural Model of the Polycerams The photostability results and the measured properties of the Polycerams synthesized by Routes 4 and 5 can be further correlated with a structural model of the Polycerams. In this model, TEOS is represented by a "+" where the four ends correspond 158 to the four functionality of TEOS, and the PDMS is represented by a thick curly line. The sizes of the species present in the Polycerams have been taken into account (1.2 A for an O2 moIecule,1.6 A for a Si-O-Si bond, 8-10 A x 4 A for a PDMS oligomer, and 13 A x 6.4 A for the PM dye molecule). The degree of dye caging was first examined as a function of polymer content. At 40 % PDMS the TEOSiPDMS mole ratio is 25.8:1, and at 90 % PDMS it is 2.1:1. Polycerams synthesized by Route 4 were found to be essentially nonporous and the degree of PDMS condensation is high as illustrated by the CP-MAS ^Si NMR results. Taking into account the degree of silica condensation as reported in Table 5.3, the ratio of is 1:5:11.4:7.5 for the 40% PDMS sample. This gives a guideline of how to connect the "+" species to each other. The structural model is shown in Figure 5.9 for the 40 % and 90 % PDMS Polycerams synthesized by Route 4. The reduction in dye caging by inorganic groups is well illustrated at higher PDMS contents. At 90% PDMS, the dye is more likely to be in a completely PDMS environment; therefore the dye molecules are more mobile and other impurities can difiiise to the dye. Whereas, at 40% PDMS, the dye has a higher probability of being in an inorganic cage, resulting in improved photostability. 159 Dye Silica PDMS 40 vol % PDMS 90 vol % PDMS Figure 5.9: Proposed structures of Si02:PDMS Polycerams synthesized by route 4 for different polymer contents. A more detailed model was developed for some of the SiOa.PDMS Polycerams synthesized by Route 5. The dye concentration within a Polyceram is 5*10'^ M corresponding to 6*10"cm'^. This corresponds to a dye-dye distance of about 120 A. The oxygen concentration within PDMS will be used for the lack of a better value for the Polycerams, corresponding to 4.7*10'^ cm'^ as reported in Table 4.3. This results in an 02:dye ratio of approximately 8:1. From the BET data, the 40% PDMS Polyceram synthesized by Route 5 has an average pore size of 26.6 A and a open pore fraction of 0.37. Also, from the CP-MAS ^Si NMR data, the ratio of is 1:2.3:18:12 and the degree of PDMS condensation is high (Table 5.3 and Figure 5.3). For the 60% PDMS Polyceram, the material is essentially nonporous from the BET data, and the ratio of Q^:Q^:Q^:Q'* is 1:3:16:14 from the NMR data in Figure 5.5. Taking all these factors into account, proposed structures for the Si02:PDMS Polycerams at 40% and 60% PDMS synthesized by Route 5 are shown in Figure 5.10. 160 The oxygen molecules were randomly distributed in the structure, although there is a tendency for the O2 molecules to be located at the pore surface'^' The model illustrates the large presence of ojq^gen in the Polyceram with respect to the dye and its easy access to the dye molecules located in the pores of the porous Polyceram (Figure 5.10a). The size of oxygen with respect to the pores is very small, and the difiusion of o^qrgen through the pores is likely very high. Also, the size or thickness of the silicate regions was largely governed by the ratio of Q species, partly forcing the structure to take the form shown in the Figures 5.10a-b. The structure for the 60% PDMS Polyceram synthesized by Route 5 illustrates a more difficult access of oxygen to the dye and a greater rigidity of the caged dye molecules because of the material's nonporous nature. Also, as discussed in Section 5.2, this structural model illustrates how it is possible to have polymer co-condensed in the structure as well as have the remaining polymer 'filling-up" the small pores in the matrix. Finally, both the structures depict that the oxygen has varying degrees of accessibility to the dye molecules. There is likely a distribution of dye molecules located within the pores and cages of the matrix. This hypothesis is addressed in more detail in the next section, and discussion will continually be referred back to the following structures throughout this document. 120 A Dye O2 • PDMS Si-O-Si (a) 162 120 A to o Dye O2 • PDMS Si-O-Si (b) Figure 5.10; Proposed structures of SiOitPDMS Polycerams (a) at 40 vol % PDMS and (b) at 60 vol % PDMS synthesized by route 5. 5.5) Models to Explain Fluorescence Photostability Behavior In section 3, the long term component of the fluorescence photostability of the Coumarin xerogels was discussed in great detail. The long term decay represented the dyes that were not located in the pores of the sol-gel host, but in areas where O2 did not have easy access to the dye. The focus is now geared towards the rapid initial decay process of the fluorescence photostability (see Figure 3.12). Several groups have observed 163 this initial fluorescence decay with a number of laser dyes in solid host""' Explanations of this phenomena were either very brief or avoided all together. The rapid initial decay in fluorescence has been observed in both the Coumarin xerogel fihns and the PM Polycerams monoliths. Our hypothesis is that the initial decay process in our samples is due to rapid oxidation of the dye located within the pores of the matrix. The following discussion will first disprove other hypotheses, and then will support our hypothesis with experimental data and quantitative fits to the model. 5.5.1) Thennal Degradation The rapid initial decay of the fluorescence may be caused by thermal quenching or degradation of the laser dye as a result of increase in the local temperature. If this were to occur, the temperature of the pump volume (volume of sample exposed to pump light) must rise and equilibrate at the same rate as the decline in the initial decline in the fluorescence. In addition, the temperature rise must be enough to cause the degradation or quenching. To test this hypothesis, one approach is to examine the total energy absorbed in the host upon optical pumping. As the number of pump pulses increases, the energy (or temperature) of the pump volume increases and stabilizes. The increase in temperature could lead to a direct drop in fluorescence by thermally induced degradation or quenching of the dye. 164 The rapid initial fluorescence decay was observed in both the xerogel films and the Polyceram monoliths. For illustration, the following calculations are only shown for the xerogel film. In Figure 5.11, the sample geometry and coordinate system are defined. The assumptions are; (1) the sample absorbs all the pump light, (2) the thermal conductivity for the film and the glass substrate are the same, and (3) heat only dissipates in the z-direction (this assumption is valid since R»L) . The assumptions here exaggerate the conditions resulting in slower heat dissipation, which will only lend greater validity to the final conclusions. Film Substrate r ->2 Pump Volume <-> L Figure 5.11: Sample geometry and coordinate system the heat dissipation calculations with a Coumarin doped xerogel film. For a one dimensional system, the solution to Fourier's second law of heat conduction (analogous to Fick's second law of difiusion) for an instantaneous source of heat on an infinite solid is ( a-z^ Efe.)=f 165 where E(z,t) is the energy concentration (joules/cm^) at position z and time t, Eo is the initial energy concentration, a is the thermal difiusivity of the material. This solution utilizes initial conditions such that E(z,0) = 0 for all z except E(z,0) = Eo for a where L is the thickness of the xerogel film and R is the radius of the spot size of the input pulse. The total energy in joules stored in the pump volume as fiinction of pump pulses is; pulses / ^ E^i^pulses) = where pulses is the number of pump pulses that have hit the sample and v is the pulse rate of the laser in units of Hz. The thermal conductivity (k) for inorganic glasses is 1.3 watts/m*K®', the density of the xerogel film (p) is approximately 1.2 gm/cm^, and the heat capacity (Cp) is 0.75 joule/gm*K Then the thermal diffiasivity (a) can then be calculated fi-om: k This results in a value of 0.014 cmVsec for a. The temperature rise can then be calculated firom Et(pulses)as: 166 E, (pulses) The rise in temperature in the pump volume as flmction of pump pulses was calculated knowing that Eo= 20 joules/cm^, R=0.18 cm, A=0.10 cm^ L=0.00005 cm, and v= 10 Hz (see Figure 5.12). a: ® ® S- 0.2 2000 3000 4000 5000 Pulses Figure 5.12: Temperature rise in fluorescing volume of the a xerogel film as a function of pump pulses. The temperature in the pump volume was found to stabilize slowly, continuing to increase even after 5000 pulses. Experimentally measured rates of the initial fluorescence decay in the fluorescence photostability curves had half lives ranging fi-om 150-2000 167 pulses. Therefore, the calculations indicate the temperature stabilizes too slow to cause the initial drop in fluorescence intensity. There are several other arguments which suggest that the fluorescence decline is not due to temperature effects. First, the ratio of energy in the pump volume just before the second pulse hits the sample to the energy in the pump volume just after the first pulse hits the sample is 5.7*10"^. Therefore, most of the energy introduced by the pump pulse has dissipated before the next pulse of light hits the films. Second, the temperature rise after 5000 pulses is 0.85 C. In a typical xerogel film, the fluorescence intensity had dropped over 50% within 5000 pulses (Figure 3.12). This change in temperature is very small, unlikely causing the large drop in fluorescence observed. This was experimentally confirmed. PM-567 was dissolved in EtOH at 10'^ M and the excitation and fluorescence spectra were measured at 25 C. Then the solution was heated at 35 C and the optical spectra was re-measured (Figure 5.13). A very small drop in the fluorescence output was observed. Therefore, a 0.85 C rise in temperature could not provide the substantial drop in fluorescence observed. 168 1 1 1 1 1 1 1 1 1 r 25 C 35C i2 m *c 3 3 CO Oifi. !» Z3 SMO'CQ c 'S' jO 3- s X LU (0 350 400 450 500 550 600 650 Wavelength (nm) Figure 5.13: Effect of temperature on the excitation and emission spectra of PM-567 in EtOH at 10-^ M. 5.5.2) Singlet-Triplet Equilibration Another hypothesis suggests that the initial decay in fluorescence is due to the excited-state singlet-triplet equilibrium. Upon input of a pulse of light, the dye molecule is excited to the first-excited singiet-state. The dye molecule can then fluoresce or go to the triplet-state by intersystem crossing (see Figure 1.1). Because of the long triplet-state lifetimes, it is possible that not all the dye molecules will return to ground-state before the next input pulse. Therefore, the triplet-state population will accumulate, leaving less of the dye molecules available to fluoresce. The triplet-state population will accumulate as the 169 pump pulses continue to hit the sample, and the population will stabilize, hence stabilizing, the fluorescence output. A simple three level energy diagram for a dye molecule is shown in Figure 5.14. The energy state populations as a function of time are defined as ni(t) where the subscript i is 0, 1, or 2 which represent the ground state, first-excited singlet-state, and the first- excited triplet-state, respectively. The total transition probability per second to all other states fi-om state i is defined as a;. The transition probability per second fi^om state i to state f is given by Ai£, and the mean lifetime of state i is Xi. The rate equations for a 3-level system are as follows: dria — Wj (/) A^q + WT (') A.2Q dn, ~ ~^\ (0 •^10 ~ ^ (0 -^12 dn^ ~ (0 ~ ^2 (0 ^0 Typical values for the transition probabilities (Ay) were assigned for a typical laser dye molecule using the decay times TIJFI"om state i to j (TIO=1 nsec, TI2=10 nsec, and T2O=1000 nsec)^^ and using Aif=l/Tif. The system is assumed to be initially all at ground state, and all the dye molecules are excited to the first excited smglet state just after input of a pulse of light. The initial conditions are then no(t)=0 cm"^, ni(t)=3*10'' cm'^, and n2(t)=0 cm"^. This assumption is valid since the number of photons hitting the sample is approximately equal to the number of dye molecules in the pump volume for a Pyrromethene doped Polyceram at a dye concentration of 5*10"' cm"^ (see Table 5.4). 170 n,(t), Ti tti njCt), Tj Oj Energy t '10 Q u '^0 ,®o Figure 5.14: A 3-level energy diagram from a laser dye molecule. Table 5.4: Comparison of the number of pump pulses to the dye concentration for PM-567 Polyceram monoliths and Coumarin xerogels films. Sample Sample Pump Pump Dye Dyes thickness fluence/ photons/ concentration molecules/ wavelength area area PM 0.5 cm 0.1 J/cm^ 2.7*10'^ 5*10*'M 3*10'^ cm^ Polycerams (532 nm) cm'^ (6*10'^cm-^) Coumarin 0.5 |im 0.001 J/cm^ 1.7*10" r=0.05 2.2*10'° cm^ xerogel (337 nm) cm' (2.2*10^° cm-^) films The populations of each of the energy levels were determined as a function of time after solving the system of differential equations listed above (Figure 5.15a,b). Within several nanoseconds, the ni population drops to ahnost zero. Most of the dye molecules will return to ground state because Xio is much smaller than Xa. The important thing to examine here is how much of the population of n2 remains before the next pulse of light 171 hits the sample. By examining Figure 5.15b, the tiz population becomes non-existent after 38,000 nsec, which is orders of magnitude of time before the next pulse hits the sample, 0.10 seconds later. This means all the dye must return completely to ground state before the next pulse comes in. Therefore, it is not possible for the singlet-triplet equilibration to cause the initial decay in the fluorescence. 3.00E+017 2.50E+017 c 2.00E+017 :g 1.50E+017 P 1.00E+017 5.00E+016 0 2 4 6 8 10 Time (nsec) (a) 172 1000000 800000 - 600000 - c o a a 400000 - oQ. Q. 200000 - 20000 30000 40000 Time (nsec) (b) Figure 5.15: Energy state populations as at (a) short times (b) at longer times. 5.5.3) Dyes Located on the pore surface of host The final hypothesis to explain the initial decay in the fluorescence is that the dye molecules located within the pores of the host are degrading before dye molecules caged in the matrix. Those dye molecules located within the open pores of the matrix are more prone to attack from impurities such as oxygen, because they have easy access to the dyes. Also, dyes located within the pores have a greater number of rotational, vibrational, and translational modes of excitation, and are therefore susceptible to degradation and quenching processes. In contrast, dyes caged in the matrix are less prone to such 173 excitations and interactions with impurities, and are therefore more photostable (see Figure 5.10). To support the above hypothesis experimentally, the fluorescence photostability was compared for both porous and nonporous PM-567 doped SiOaiPDMS Polycerams. Figure 5.16 is a plot of the fluorescence photostability for a 40% PDMS (nonporous) Polyceram synthesized by Route 4, a 40% PDMS (porous) Polyceram synthesized by Route 5, and a 60% PDMS (nonporous) Polyceram synthesized by Route 5. Both of the nonporous samples showed little initial decay in the fluorescence. One of the nonporous samples had the same polymer content and composition as the porous sample, but was synthesized by different processing routes; the other nonporous sample was synthesized by the same processing route, but had a higher polymer content than the porous sample. Regardless, both the nonporous samples showed little initial decline in the fluorescence while the porous sample did. These data support the hypothesis that the dyes located within the pores of the host are the cause for the rapid initial decay in fluorescence. 174 1.0 a Route 5 Polyceram at 60% PDMS - Nonporou: b Route 4 Polyceram at 40% PDMS - Nonporou c Route 5 Polyceram at 40% PDMS - Porous 0)u c O(D (0W 0.8 o£ 3 u. o z 0.4 0 10000 20000 30000 40000 50000 Pump Pulses Figure 5.16: Effect of sample porosity on the fluorescence photostability of PM-567 in SiOaiPDMS Polycerams. The fluorescence photostability data did not fit well to single or double exponential curve fits for many of the dye doped samples studied. Therefore, the system cannot be described simply by one or two different types of dyes molecules within the host having one or two different photostability lifetimes. The dye-host system can be modeled as having a distribution of dye molecules all having different photostabilities within the host. The distribution can be described as a Gaussian distribution of the firee energy of reaction (AG) between O2 and the dye molecule. Therefore, there is Gaussian distribution of the natural log of the reaction rate constant (k), because AG = -RT hi(k). The results of this 175 model are the same as one proposed by Albery et. al.'®^, although a more physical approach in the derivation is shown in Appendix A. The results from the Gaussian model reveal the dye degradation rate as: = -i J exp(-z^) exp(-^ t exp(/ z))dz C(0) ylK% where C(t) is the concentration of fluorescing species at time t, C(0) is the initial dye concentration, k is the average rate constant, y is a parameter describing the distribution of dye molecules. The experimental data fit very well to the model (as shown in Figure 5.16) using just two fitting parameters, k and y. The determined fitting parameters are summarized in Table 5.5 and the dye distributions are illustrated in Figures 5.17 and 5.18. t represents the average lifetime of the dye molecules, determined from t = 1/ k. Table 5.5: Fitting parameters to Gaussian model curve fit. SiOzzPDMS Polyceram Sample y k X 40% PDMS Nonporous (Route 4) 2.2 8*10"® 125,000 pulses pulses'' Porous (Route 5) 3.6 2.3*10-^ 43,500 pulses pulses"' 176 1.0- Nonporous Porous o 0.8- 0.6- p 0.4- 0.0 •20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 ln(k) Figure 5.17: Photostability rate constant distribution of PM-567 within porous and nonporous Si02:PDMS Polycerams determined by the Gaussian Model. 1.0 Non-porous Porous 0.8 0.6 0.4 0.0 101 10« Lifetime (pulses) Figure 5.18: Photostability lifetime distribution in units of pulses for PM-567 in porous and nonporous Si02:PDMS Polycerams. 177 The Route 4 (nonporous) Polyceram has a narrower distribution of dye molecules with Y=2.2 compared with Y=3.6 for the Route 5 (porous) Polyceram. One would expect a narrower distribution with the nonporous sample since there are less number of types of sites for the dye molecule to be located. Also, the nonporous sample has a much smaller rate constant ( K) and therefore a higher T than the porous sample. This means that the average dye molecule within the nonporous Polyceram is more photostable than those within the porous Polyceram. All the Coumarin xerogel films and porous Polycerams exhibited a sharp initial decline in the fluorescence as shown in Figures 3.12 and 5.16. The xerogel films were not measured for their porosity, but it is known that xerogels tend to be highly porous. Only Polycerams with high polymer contents or Polycerams that were measured to be nonporous by BET did not exhibit a sharp declme in the initial fluorescence. These data support the hypothesis that the initial decline in the fluorescence is due to dye molecules located within the pores of the host. The magnitude of the initial decline in the fluorescence should then signifying the fraction of dye molecules located in the pores of the host. The fraction of dyes that are trapped in the pores of the matrix can be estimated from the fraction of fluorescence that has dropped after the rapid initial decline has ended. It is relatively easy to distinguish between the short-term and the long-term decays in the fluorescence with the Coumarin xerogels (see Figure 3.12). When rapid initial decay diminished after approximately 10,000 pulses, the fluorescence intensity had dropped 60-65%. Using the above hypothesis, the 178 fraction of Coumarin dyes located in the open pores of the xerogel films is then 60-65 %. With the fluorescence photostability curves of the PM-567 Polycerams (Figure 5.16), it is more diflBcult to distinguish exactly where the short-term decline ends and the long-term decline begins. In this case, the fraction of dyes in the pores can be estimated from pore fraction in the host, assuming that the dye molecules are randomly distributed within the host. Using the data in Table 5.3, a SiOjrPDMS Polyceram with 40% PDMS synthesized from Route 5 has approximately 37% of the dye molecules located in the pores. The porosity of this composition is well illustrated in a proposed structural model shown in Figure 5.10. By examining Figure 5.16, the number of pulses it takes for the fluorescence to decline by 37% for this same sample is approximately 10,000 pulses. It is then concluded that dye molecules located in the pores of the host have photostabilities less 10"* pulses. This is well illustrated in Figure 5.18. The photostability curves for the nonporous Polycerams in Figure 5.16 have a small amount of initial decline in the fluorescence. Since these samples are nonporous, the decline must be due to dye molecules trapped in incomplete cages, organic cages, or cages which formed with oxygen akeady present in them. Oxygen has the ability to diffuse to and within these dye cages relatively easily. The photostabilities of these dye molecules are estimated to be in the range of 10'*-10^ pulses. The long-term fluorescence decline all decayed linearly with all the Coumarin films and the Polyceram monoliths. The long-term linear decay was described quantitatively by the Figure of Merit in Section 3.5.1. The fluorescence decline in this range is attributed to 179 dye molecules in hybrid cages and highly inorganic cages with photostabilities greater than lO' pulses. From the above discussion, there appears to be a many different types of cages; (1) open pore cages (dyes located in the pore of the matrix); (2) polymer cages; (3) hybrid cages (dyes surrounded by organic and inorganic substituents); and (4) inorganic cages. The dye can be completely or just partially caged, and/or the dye can be trapped in a cage where dissolved oxygen is already present. Figure 5.19 illustrates schematically some possible types of cages. Po^mrCage HyMdCage Iiiois»lcGa0e Dye h Pore Figure 5.19: Type of cages possible in hybrid organicAmorganic materials. The symmetric nature of a Gaussian curve will predict an equal number of less unstable dyes and more stable dyes compared to the average dye. It is likely that the actual 180 dye distribution is not symmetrical. The model discussed here is beneficial because it describes the heterogeneous nature of the dye molecules within the host with respect to their photostabiUty. The model illustrates the need for increasing T by placing all the dye molecules within highly photostable cages to achieve a truly photostable dye/host system. This Gaussian model has been used before to describe fluorescence quenching of Pyrene and 9,10-diphenylanthracene in silica gel by singlet oxygen'^ and the quenching of Ru(bpy)3^^ (Ruthenium tris(bispyridyl) chloride) in porous silica'^®*Experimental fits to the model were physically explained as O2 having varying degrees of accessibility to the dye molecule. Analogous behavior has also been seen in the oxygen quenching of different species (molecular and transition metal) in porous silica. For example, Carraway et. al.'^ measured the quenching behavior of Ru(L)3^^ where L is an a-diimine adsorbed on to porous fumed silica. The quenching rates were multi-exponential, and they modeled the system using the Freunlich Isotherm with a Gaussian distribution of lifetime sites. Avnir et. al.^®^' have described photoprocesses on porous surfaces slightly differently, using a fi-actal approach. The reaction rate has been described as; where k* is a prefi-actor and D is surface fi-actal dimension. The surface fi-actal dimension describes the degree of accessibility of the species adsorbed on the pores These studies illustrate the heterogeneous nature of the pore surface and the varying degrees of accessibility of a reactant to the target molecule. Also, these studies have described quenching phenomena which take place on the nano-second time scale. 181 The heterogeneous nature of the pore surface can be applied to other chemical reactions at longer time scales (i.e., seconds, minutes, hours) such as photodegradation reactions explored in the present work. A number of interactions are possible on porous surfaces ranging from proton transfer to oxidation as described in a review by Dave et. al.'. The photostability of a sample depends not only on the laser dye and the host composition and structure, but also on fartors such as the dye concentration, pump wavelength, pulse rate"^' pump fluence*^ sample thickness'^, and geometry. With all these parameters, it is difficult to compare photostability measurements performed by diflferent research groups. Photostability results on PM-567 in diflferent solid hosts and varying pumping conditions are shown in Table 5.6. The photostability of the dye/host system described in the present study appear to have equivalent or improved photostability compared to other hosts. The present study has been concemed with fluorescence photostability, while many of the other results in the Table represent laser lifetime. Both methods should provide valid measures of the photodegradation process. 182 Table 5.6: Photostability of PM-567 in various hosts and pump conditions Host/ PumD Sample Lifetime Dye Concentration (M) Characteristics Thickness (50%) Pulse rate Fluence (pulses) (Hz) (J/cm^) Ormosil'^ 10 0.12 <12,000 (TMOS:TMSPM:MMA)/ 10 0.46 — <1000 2.0-10"^ Ormosil®^ 1 0.7 10 59,000 (VTEOSy mJ/pulse 10 14,000 10-^ 5 0.7 mJ/pulse Ormosil^®-^ 1 0.20 4 5000 (VTEOSy 5.10-^ SiOziPDMS Polyceram 10 0.10 4 >50,000 (80%PDMS)'^/6.8.10-^ Si02:PDMS Polyceram 10 0.10 4 125,000* (60% PDMS Route 4) (present study)/ 5*10'^ average photostability lifetime predicted by Gaussian model TMSPM=3-trimethoxysilyl propylmethacrylate MMA=methyl methaciylate monomer VTEOS=vinyltriethoxysilane To conclude this section, the structural features which improve the photostability of laser dyes Avithin a sol-gel host are related to possible processing routes. A distribution of diflferent structural cages exists within the sol-gel host. To improve photostability, the amount of porosity needs to be minimized and the degree of inorganic caging needs to be maximized. Porosity can be controlled by a variety of methods used in accordance with sol-gel processing, as discussed in Section 5.1. In particular, the control of the relative 183 rates of condensation and evaporation and the amount of polymer present in the total composition can be used to control the porosity within the SiOaiPDMS Polycerams. To increase the degree of dye caging, several processing options are available. In section 3, the covalent bonding of coumarin dyes to the host provided improvement in photostability which was attributed to higher probability of obtaining dye caging with the silylated dye. Also, increasing the degree of condensation will enhance the degree of dye caging. The amount of condensation can be enhance by using highly reactive alkoxides, pre-reacting the dye with the inorganic alkoxide, and using stoichiometric amounts of water rather than sub-stoichiometric amounts of water. 184 6) ADDITIVES TO IMPROVE PHOSTABIUTY In sections 4 and 5, the mechanisms for photodegradation were examined specifically for pyrromethene dyes. From this evaluation, PM-567 was found to be more susceptible to degradation in an oxygen environment, in an acidic environment, and in a porous environment. In order to improve photostability, several options are available. First, the dye can be caged within the host to prevent photochemical reactions with unpurities. Caging can be increased by covalently bonding the dye to the host and by removing porosity through control of sol-gel processing. Second, the dye can be protected by eliminating impurities fi"om the host. This can be accomplished by performing synthesis in deoxygenated atmospheres and hermetically sealing the host to prevent oxygen firom diffusing in during use. Third, the dye can be placed in an environment which is chemically stable, placing the dye in a basic environment. Finally, the dye can be protected by preventing interaction between the dye and oxygen by adding antioxidants to the host. The latter two methods are explored in this section. Antioxidants are molecular species which interfere with the oxidation process by a variety of mechanisms. There are many types of antioxidants, and choosing the proper antioxidant depends on the host composition, the application, the solubility of the antioxidant in the host, the volatility of the antioxidant, and the mechanism of oxidation. Most antioxidants today are used in the food industry and for polymer stabilization. In this 185 study, antioxidants are used for dye stabilization. Antioxidants can be divided into five major classes listed below and shown schematically in Figure 6.1 1) Absorber: A molecule which preferentially absorbs photons that would be normally absorbed by the dye. 2) Deactivator: A molecule or metal complex that quenches the dye in its excited state. 3) Interceptor: A molecule which slows oxidation by preferentially reacting with oxygen or by quenching excited state oxygen (singlet oxygen). 4) Free radical scavenger (Primary antioxidant): A molecule which preferentially reacts with fi-ee radicals such as alkyl radicals (R") and peroxy radicals (R-O-0*). 5) Decomposer (Secondary antioxidant): A molecule which reacts with hydroperoxides to prevent the formation of fi-ee radicals which accelerate dye degradation. For laser dyes, absorbers and deactivators are not a good choice since they will affect the ability of the dye to fluoresce. Interceptors, decomposers, and fi-ee radical scavengers are the possible choices. D-H + hv or heat D* +O2 ^ D-0-0* + DH DOOH +D* n n rr n (l)-f(2) (4) (4) (5) Figure 6.1; Strategy for attacking preventing oxidation by the fi-ee radical mechanism Interceptors work by physically quenching singlet oxygen or by chemically reacting with oxygen. l,4-diazobicyclo[2.2.2]octane (DABCO) is a well known singlet oxygen 186 quencher and has been used in the past for improving the photostability of laser dyes^®' 1,3-diphenylisobenzofuran is an example of a compound which chemically reacts with molecular oxygen to form 0-dibenzoyl-benzene. Because the fliran compound permanently reacts with O2, this additive is only useflil for a limited time frame if a continual supply of O2 is difiiising into the host. Free radical scavengers encompass a large class of antioxidants which are most commonly used for polymer light stabilization. Hindered amines and hindered phenols are the most common free radical scavengers. Huidered amines tend to be the most effective free radical scavengers, ffindered aliphatic amines, often known as hindered amine light stabilizers (HALS), are useful for protection against high energy radiation for many polymers. A number of HALS, typically substituted piperidines, are conmiercially available^""' The mechanism by which the free radical scavenging takes place for piperidines has been proposed for polymers^"^'^"'^. The same can be applied to a dye molecule in a reaction scheme shown below: 1) >N-H + D-OOH + O2 + hu^ >N-0' 2a) >N-0* + D' ->>N-0-D 2b) D* + O2 D02* 3) >N-0-D + D-O2* -> D-O-O-D + >N-0* First, the amine (>N-H) is oxidized to form a stable nitroxyl radical (>N-0*). The nitroxyl radical, which is one of the most stable free radicals, can then scavenge alkyl radicals (D*), where D represents the dye molecule. This reaction competes with the oxidation of the 187 dye (Reaction 2b). The dye substituted hydoxylamine (>N-0-D) then reacts with an oxidized dye radical (D-C)2*) to regenerate the nitroxyl radical. So piperidines slow the oxidation process by quenching dye free radicals, and the piperidines can be reused unlike 1,3-diphenylisobenzofiiran. Piperidines do not have any singlet oxygen quenching capabilities^"^. The other type of free radical scavengers, hindered phenols, produce hydrogen radicals (H*) which react with the dye radicals (D*). For our purposes, hindered phenols will not be used because of their acidic nature. The degradation of PM-567 has been proposed to occur by a multi-step process: (1) acidic removal of the BF2, (2) oxidation of the C=C located between the cyclic rings, and (3) oxidation of the resulting alkylpyrroles to form maleimides (see Figure 4.6). The last step of the reaction believed to occur by singlet O2 oxidation (see Section 4.2.1). However, littie is known about the mechanism of the second step. To improve photostability, the antioxidant should be chosen to prevent the first oxidation reaction of the dye. It is useless to slow down the last step of the reaction, because the dye will already be degraded. Since the mechanism of the second step is not known, several types of antioxidants were chosen to determine which is the most photostabilizing. This will also provide clues regarding the mechanism(s) of the first oxidation reaction (step 2). The structure and properties of the antioxidants used in this study are shown in Table 6.1. 188 Table 6.1; Properties of antioxidant and base additives. Additive Type Structure Properties Amine mw 221.37 (3AS) base mp 30C 3-aminopropyl bp 122C triethoxysilane pKa 10.7 propylamine [919-30-21 (DABCO) Singlet mw 112.18 1>4- oxygen mp 159C diazobicyclo[2.2.2] quencher A ^940 octane or triethylene pKa 8.8 diamine [280-57-9] HALS mw 141.26 2,2,6,6 bp 152C tetramethylpiperidine Fp24C ^NH^ [768-66-1] (Tinuvui 770) HALS ~7^ 0 o mw481 NH y-0-Ji-(CHd,- Because PM-567 was found to be more chemically stable in a basic envirormient (see section 4.2.2), a primary amine base (3-aminopropyltriethoxysilane or 3AS) was chosen as an additive (see Table 6.1). To avoid possible compatibility problems of the antioxidants with the sol-gel processing and interactions of the additive with the solid hosts, photostability experiments were first carried out in solvent to determine the best additives. The absorption photostability of PM-567 in EtOH at 10'^ M plus 1 wt % of each 189 additive are shown in Figure 6.2. No data were taken for the PM-567 plus the chemical quencher, 1,3-isophenyiisobenzyIfuran. The chemical quencher reacted too rapidly with oxygen, decomposing ail of the fiiran compound before the measurements were even started. All the other additives showed improved photostability with respect to the control (PM-567 in EtOH with no additive). The greatest improvement in the photostability was observed with the 3AS additive. But 3AS is a primary amine and cannot act as an antioxidant. The results suggest the need for a basic environment in order to improve photostability. UV photostability of PM-567 in EtOH (10-* M) with 1 wt% additive x3AS ^ DABCO •A^ piperdine Tinuvin770 ••—Control o 0.0 0 5 10 15 20 25 30 35 40 45 50 UV Exposure Time (hrs) Figure 6.2: Absorption photostability of PM-567 in EtOH at 10'^ M with various additives at 1 wt %. 190 The results above pose a new question: does the dye still degrade by oxidation even in a basic environment? If not, the need for removing oxygen or preventing interaction with oxidation becomes pointless in a basic environment. To examine this question, PM-567 was photo-degraded in EtOH plus 1 wt % ethylenediamine (a strong amine base) in the same manner as discussed in Section 4.2.1. The degradation products were analyzed by FTIR and NMR. The FUR spectra of the degradation product showed a large 1700 cm'^ absorption caused by carbonyl (>C=0) vibrations (Figure 6.3). The presence of carbonyl species in the product suggests that the dye must undergo oxidation. NMR spectroscopy of the same product supports the FTIR results by showing absorptions at 168.78 ppm and 169.2 ppm, which can only be caused by absorption of carbonyl carbons in the product. The degradation product of the dye degraded in a basic environment was not the same maleimide product as when the dye was degraded under neutral conditions (see section 4.2.1). The structure of degradation product when PM-567 was degraded in a basic environment could not be identified due to the large number of peaks observed using NMR (-1.03 ppm, -8.16 ppm, -12.39 ppm, -15.29 ppm, -19.63 ppm, -22.65 ppm, -23.10 ppm, -24.32 ppm, -29.70 ppm, -37.66 ppm, -44.87 ppm, -57.69 ppm, -58.19 ppm, -87.12 ppm, -159.57 ppm, -168.78 ppm, -169.20 ppm). It is suspected that several products of multi-oxidized species were formed. Regardless of whether the dye is in a basic or nuetral or acidic environment, the PM dye degrades by oxidation. The prevention of oxidation therefore is an unportant concern. 191 100 80 60 40 20 0 1— 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers (cm-i) Figure 6.3: FUR spectra of photo-degraded PM-567 in a basic environment. With the above observations, the addition of a proper antioxidant to PM-567 in a basic environment should be effective in improving the photostability. This idea was tested by adding 3AS at 1 wt % to PM-567 at 10"^ M in EtOH plus some antioxidant at I wt %. The results are shown in Figure 6.4. The photostability improved upon the addition of hindered amines 2,2,6,6-tetramethylpiperidine and Tinuvin 770. The photostability slightly decreased when DABCO was added. The results imply that in a basic environment PM- 567 degrades by a free radical mechanism, and the HALS can be effective in further improving the photostability. It could be argued that addition of the antioxidants, which all have a basic character, lead to improvement in the photostability by providing a more basic environment and not by their antioxidant capabilities. This is not likely, because DABCO is a strong base with a pKa value of 8.8, even stronger than a primary amine such as 192 propylamine 10.69. Addition of DABCO did not provide any improvement in photostability. HALS, which are secondary amines, are weaker bases with pKa values near 10.8-11.00. These compounds provided greater improvement in photostability than DABCO. — 3AS + Dabco 3AS + 2,2,6,6-tetramethytpipQrdine •A— 3AS +Tinuvin 770 ^3AS 0 5 10 15 20 25 30 35 40 UV Exposure Time (hrs) Figure 6.4: Absorption photostability of PM-567 in EtOH at 10'^ M with 1 wt % 3 AS and with various additives at 1 wt %. In solvent, the combination of 3AS and 2,2,6,6-tetramethylpiperidine has proven to be an effective photostabilizer system for PM-567. To take advantage of this in a sol- gel host, a new type of Polyceram composition was synthesized from 3 AS and TMOS precursors (Route 6) where the 3AS:TM0S mole ratio was 6.2:1. The piperidine compound was also added to this composition just before drying the solution. At the same dye concentration and sample geometry, the photostability of a nonporous Si02:PDMS 193 Polyceram at 40 % PDMS synthesized by Route 4 was compared to a 3AS:TM0S Polyceram with and without the antioxidant. The results were quite dramatic. Photostability of the 3AS:TM0S Polyceram was significantly better than the Si02:PDMS Polyceram. The addition of the piperidine fiirther improved the photostability, although there was a noticeable reduction in the quantum yield with this Polyceram. Despite the improved photostability, the 3AS;TM0S composition can only be synthesized to date in thick film form and is not polishable, unlike the SiOziPDMS Polycerams. Work is currently in progress to optimize a 3 AS based Polyceram that not only provide improved photostability, but also provide required mechanical properties. E " c 0.8- c o •g. 0.6- O < CD SiO^rPDMS Polyceram— No e 3AS:TMOS Polyceram O 0.2- Z •A—3AS:TMOS Polyceram + 2,2.6»6-tetramethylpiperidin< 0 10 20 30 40 50 60 UV Exposure Time (hrs) Figure 6.5; Absorption photostability of PM-567 in a SiOiiPDMS Polyceram at 40% PDMS synthesized by Route 4, in a 3AS:TM0S Polyceram synthesized by Route 6, and in a 3AS:TM0S Polyceram synthesized by Route 6 + 2,2,6,6-tetramethylpiperidine. 194 7) HOW TO MAKE A PHOTOSTABLE DYE/HOST SYSTEM To make a solid-state dye laser, a number of properties of the gain medium must be considered including the quantum yield, fluorescence lifetime, gain, absorption and fluorescence spectra, tuning range, slope eflBciency, lasing threshold, and photostability. The lack of photostability of laser dyes within solid hosts has been a limiting factor for commercialization of a solid-state dye laser. Improvement in the photostability can be achieved by optimizing the laser system design, by chemically synthesizing a more photostable laser dye, or by controlling the dye/host interactions in the gain medium. The present work has concentrated on the last approach. To make a photostable dye/host gain medium, a number of criteria must be met. First, a laser dye needs to be selected. Dye chemists have synthesized thousands of dyes with varying photostabilities. The dye chosen will depend on factors such as the inherent photostability, the quantimi yield, and the tuning range of the dye as well as the effect of the molecular environment on these properties. Many dyes can be drastically affected by their molecular environment as seen in the present study with the effect of pH on the optical spectra of Coumarin dyes and the effect of pH on the photo- and chemical- stability of PM-567. Second, a proper host needs to be chosen. From the discussion in the introduction and the results of the present study, it was established that an inorganic environment is a more stable environment for the dye than a polymer environment due to lower diffiisivities to gases, greater restriction to dye movement and orientation, higher laser damage 195 thresholds, and greater chemical resistance of inorganic materials. Because of the lack of high temperature stability of laser dyes, they cannot be doped within conventionally processed glasses. Therefore, sol-gel synthesis is attractive because it permits materials to be synthesized at relatively low temperatures (<200 C). A host material must fiilfill a number of requirements. The host must: (1) be optically transparent to the absorption and fluorescing wavelengths; (2) be processable into different shapes; (3) be optically polishable; (4) be chemically compatible with the dye, including issues of solubility and pH stability; (5) be resistant to oxidation; (6) have low solubility of oxygen; (7) have low permeability to oxygen; (8) be able to withstand high intensities of light (high laser damage threshold); (9) have a high thermal conductivity to prevent thermal degradation for high power applications; and (10) have low dn/dT values to prevent thermal lensing effects. Third, after a dye/host system has been selected, dye/host interactions need to modified by controlling molecular structure (i.e., molecular engineering) to maximize the photostabiUty. Dye/host interactions can encompass interactions such as H-bonding, covalent bonding, dye mobility, dye diffusivity, dye aggregation, and acid/base reactions. The dye/host interactions can drastically aflfect the optical properties of the material, including the fluorescence spectra, the quantum yield, the optical scattering, the chemical stability, and the photostability. The key to attacking the photodegradation problem from the perspective of dye/host interactions is to understand the rate limiting mechanism by which the dye degrades. The mechanism of dye degradation will depend the dye/host system considered. In the present study, the Coumarin and Pyrromethene dyes were found to degrade by 196 photo-oxidation processes. A number of techniques were explored to prevent these processes from occurring: (1) covalently bonding the dye, which increased the probability of caging preventing interaction between the dye and oxygen; (2) removing porosity through control of sol-gel processing and composition, which slowed the diffusion of oxygen to interact with the dye; and (3) incorporating additives to the host, which slowed the degradation reactions. Other possible methods include: (1) process in deoxygenated atmosphere and hermetically seal the gain media to prevent oxygen from diffusing into the host; and (2) use energy transfer dye pairs, in which one dye is the absorbing species and the other is the fluorescing species. One possible solution which takes advantages of many of the techniques proposed above to improve photostability would be to design a single precursor which is composed of the dye and additives in a 3-dimensionaI inorganic cage. The illustration of such a precursor is shown in Figure 7.1. Here the dye is already covalently bound to the needed additives to prevent oxidation. In addition, this precursor is surrounded by covalently attached reactable groups represented by "+". For example, in the case of sol-gel processing, the "+" could represent Si-alkoxides. The use of such a precursor would ensure that every dye molecule is caged and protected against possible photodegradation processes. Also, this would prevent any chance of dye aggregation which generally reduces the optical performance of the dye. 197 OUlKHSV Wj Quencher Enasy Transfer Primary Amine Molecule Dye Free radical Scavenger Figure 7.1; A schematic view of silica precursor to result in a photostable dye/host system. This puts the challenge on chemists to synthesize such a silica precursor, and on material scientists to determine what combinations of dye, antioxidant, and energy transfer dye are most compatible. In addition, material scientists need to use such a precursor to obtain a processible, optically transparent, and polishable materials. 198 CONCLUSIONS Laser dyes within sol-gel derived hosts are attractive materials to produce a tunable solid-state dye laser. The commercial applications of such a laser have been limited due to the poor photostability of the dye within the solid host. Improving the photostability of laser dyes within sol-gel derived hosts has been the focus of this study. Various Coumarin and Pyrromethene laser dyes have been incorporated within sol- gel derived hosts ranging from Si02 xerogel films to SiOa-PDMS Polyceram monoliths. Synthetic routes have been developed which have provided for optically transparent hosts which are crack-free and polishable. The mechanisms of photodegradation and methods to improve the photostability of these dyes within a sol-gel derived host were investigated. These methods included; (1) covalently bonding the dye to the host matrix; (2) removing porosity within the host through control of sol-gel processing and composition; and (3) incorporating additives to slow down photodegradation processes A series of Coumarin dyes were synthesized to have an alkoxysilane functionality attached to it. These silylated dye can participate in the hydrolysis and condensation reactions of sol-gel process resulting in the dye being covalently bond to the host matrix. During processing, matching reaction rates by pre-hydrolyzing the silylated dye and by the use of stoichiometric amounts of water were crucial to obtaining a large degree of dye bonding within the host matrix. Covalently attaching the laser dye to the host resulted in: (1) improved solubility of the silylated dye within the sol-gel matrix, allowing for higher concentrations of active molecules within the sol-gel matrix; (2) higher fluorescence 199 eflSciency, attributed to the greater rigidity and isolation of the silylated dye within its host; (3) improved chemical stability, associated with the inability to leach (solvent extract) the dye from the host; (4) control of optical spectra, due to resistance to change in the chemical forms of the dye; and (5) improved photostability, attributed to greater probability of caging of the silylated dye. From photostability measurements in different atmospheres and from chemical analysis, both the Coumarin and Pyrromethene dyes within sol-gel derived hosts were found to decay by photo-oxidation processes. A multi-step degradation process for PM- 567 was proposed as: (1) acidic removal of BF2from the pyrromethene-BFa complex; (2) oxidation of the link between the two pyrrole rings; and (3) oxidation of the resulting alkylpyrroles to form a maleimide. The porosity and surface area within PM-567 doped Si02:PDMS Polycerams were controlled as flmction of sol-gel processing and composition. The porosity control stems from the variations in the relative rates of condensation and evaporation during the drying of the Polyceram solutions. The photostability was found the improve as the porosity decreased, and as the Si02 content increased in the nonporous samples. The fluorescence photostability (fluorescence output as a fimction of pump pulses) of the dye doped films and monoliths showed a characteristic behavior signified by a rapid initial decay and then a slower long term decay in the fluorescence output. The rapid initial fluorescence decay was attributed to dyes located within pores of the Polyceram matrix which are more prone to react with oxygen and other impurities. The slower long term decay was attributed to very photostable dye molecules located within Si02 cages. 200 impermeable to impurities to undergo photochemical reactions. A model, which applied a Gaussian distribution of dye molecules each with different photostabilities, was used to quantitatively describe the behavior of photostability within the solid samples. The model predicts a much smaller distribution of dye molecules and a higher average photostability within the nonporous Polycerams with respect to the porous Polycerams. Finally, the use of additives such as antioxidants and bases were found to improve photostability of the PM-567 dye in solvent and in solid hosts. Basic additives (e.g., primary amines) and hindered amine light stabilizers (HALS) were found to be the most effective. A Polyceram composition synthesized from 3-aminopropyltriethoxysilane (a silylated primary amine base) and TMOS precursors provided the best photostability. 201 FUTURE WORK The results of this investigation reveal several avenues for future work towards improving the photostability of dye/solid host materials. 1) Evaluate the lasing characteristics of the dye doped sol-gel derived hosts with respect to laser performance. This type of characterization involves placing a polished dye doped monolith within a resonator which is then pumped by a laser or a flashlamp. The lasing characteristics such as slope eflSciency, laser threshold energy, output power, and photostability are important parameters to be evaluated for these materials. The results will then be compared to other dye/host systems. 2) Combine the methods to improve the photostability discussed in this study to enhance the photostability. The three molecular engineering methods proposed in this work to improve photostability are: (a) covalently bond the dye to the host; (b) synthesize a nonporous host; and (c) use additives to slow the degradation process. One example which utilizes these techniques simultaneously is to synthesize a silylated Pyrromethene laser dye to be incorporated within a basic, nonporous Polyceram composition with the addition of antioxidant additives. Efforts are currently underway to synthesize a silylated Pyrromethene dye. 3) Investigate other methods to improve photostability, such as: (a) processing in an oxygen free enviroimient and hermetically sealing the dye doped host; (b) using energy transfer dye to minimize photodegradation; and (c) improving resonator design and laser system optics to minimize photostability in a solid-state dye laser system. 202 4) One of the benefits of doping laser dyes in solid hosts, is that the gain medium can be formed into various geometries. The materials can be formed in fiber or films to obtain lasiog waveguides leading to novel optical devices. Future work in this area will include fabricating planar waveguide fi-om laser dye-doped Polyceram films and characterizing the properties of guided fluorescence. 5) The issue of improving photostability of dyes within solid hosts, is not only important for laser applications, but for a number of other applications such as colored glasses, chemical sensors, photochromic materials, and nonlinear optic materials (photorefi-active, harmonic generation, electro-optic). Therefore, in a broader assessment of the future work for dye/host systems, the techniques utilized in this study will also applied to other dye/host systems, benefiting these other areas of research. 203 APPENDIX: GAUSSIAN MODEL For this model, it is assimied that there is a Gaussian distribution of the free energy of reaction (AG) between O2 and the dye molecule. Therefore, there is Gaussian distribution of the natural log of the reaction rate constant (k), because AG = -RTln(k). The result of this model is the same as one proposed by Albery et. al.'®'', although a more physical approach in the derivation is shown. The purpose of this section is to derive a fimction to be fit to experimental data with two parameters, one describing the distribution of the rate constants and the other describing the average rate constant. We will assume that the oxidation reaction is first order. Therefore, for dye molecules with all the same reactivity, its oxidation rate as a fimction of time (r(t)) is: r(t) = •^ = exp(-^) (1) where c(t) is the dye concentration as a function of time, c(0) is the initial concentration, and k is the reaction rate constant. For a system with a distribution of rate constants, the oxidation rate as a fimction of time (R(t)) is: fl(0 =^ = Jexp(-fo)FWa»r (2) where C(t) is the dye concentration as a fimction of time, C(0) is the initial concentration, F(k)dk is the fi-action of species that have rate constants between k and k+dk. Next, equation 2 is rewritten as: 204 /!(/) =^ = |e!q)(-fe)fl3n*) The normalized Gaussian distribution of hi(k) is defined as: expf- (hi ^ - in ^ mt)'. / s— (4) f exp —7 (In kf ,d\a. k 0 ^ r J where y is a term describing the width of the Gaussian distribution and k is the average rate constant. The denominator of equation 4 has an exact solution: Substituting Equation 4 and 5 into Equation 3 we get: =--Vjexp(--YOn^-hi^)']exp(-^yhiA: (6) C(0) \ y J In order to simplify the relationship, a change of variables with: z= — (hiA:-In^) (7) r leads to: C{t) If ^ exp(-z-) exp(-A: t exp(/ z)) dz (8). Equation 8 is the same result as obtained Albery et. al.^®^ Curve fitting experimental data with equation (8) is diflBcult because it is in integral form. Therefore, the equation (8) was modified to make it more usefiil. Using another change of variables: 205 forz>Q z = ~\nX dz = ~^dX /t forz leads to; C(0 C(0)^I^ ^^ InA))(^- ctt + Jexp(-(In2)^)exp(-^/exp(/InA))^-jJi/A (10) which simplifies to: C(/) ^ 1 C(0) J7®*p(~0nA)^)[exp(-A:a '') + exp(-A:/i (11). 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