ABSTRACT

Modifications of the Small Molecule Maltol and Photoactivity when Coordinated to Transition Metals

Britain C. Bruner, Ph.D.

Advisor: Patrick J. Farmer, Ph.D.

The family of hetero-substituted maltol chelators, thiomaltol (Htma), dithiomaltol

(Httma), and 3-hydroxypyridine-4-thione (Hopto) have been used to generate complexes with Ru(II), Pt(II), Ti(IV), and P(III) that exhibit unique photochemical and photophysical properties. Photo-excitation into ligand-based absorption bands of

+ complexes [Ru(bpy)2(ttma)] and Zn(ttma)2 engendered electron transfer reactions. Both complexes exhibit long-lived triplet emissions in the near IR spectral region.

+ Photochemical experiments with [Ru(bpy)2(ttma)] formed alcohol and aldehyde products upon photolysis in presence of mild oxidants that do not oxidize in the dark,

3+ 2+ such as methyl viologen, [Ru(NH3)6] and [Co(NH3)5Cl] . A family of new Pt(II) bipyridyl complexes are reported using the maltol-derived ligands as electron-donors.

The [Pt(bpy)L]+ complexes display intense and long-lived luminescences due to Ligand- to-Ligand Charge Transfer (LLCT) states; these luminescences are quenched by electron acceptors such as methylviologen and O2. These compounds are also efficient at singlet 1 oxygen ( O2) generation and quenching. Likewise, a family of Ti(IV) complexes with

maltol-derived chelators has been synthesized to model the use in dye-sensitized solar

cell applications. Lastly, several novel six-coordinate phosphorous complexes with the chelators of the formula P(L)2X2 were synthesized, which also exhibit room temperature

emissions. These several families of photo-active complexes represent a useful palette of

dyes for photochemical applications. Modifications of the Small Molecule Maltol and Photoactivity when Coordinated to Transition Metals

by

Britain C. Bruner, B.A.

A Dissertation

Approved by the Department of Chemistry and Biochemistry

Patrick J. Farmer, Ph.D., Chairperson

Submitted to the Graduate Faculty of Baylor University in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

Approved by the Dissertation Committee

Patrick J. Farmer, Ph.D., Chairperson

Kevin Klausmeyer, Ph.D.

Caleb D. Martin, Ph.D.

Kevin L. Shuford, Ph.D.

Zhenrong Zhang, Ph.D.

Accepted by the Graduate School August 2014

J. Larry Lyon, Ph.D., Dean

Page bearing signatures is kept on file in the Graduate School. Copyright © 2014 by Britain C. Bruner

All rights reserved

TABLE OF CONTENTS

List of Figures ...... vi List of Schemes ...... x List of Tables ...... xi Acknowledgments...... xiii Chapter One ...... 1 References ...... 11

Chapter Two...... 16 Introduction ...... 16 Experimental ...... 18 Results ...... 25 Discussion ...... 38 Conclusions ...... 43 References ...... 44

Chapter Three...... 47 Introduction...... 47 Experimental ...... 49 Results ...... 57 Discussion ...... 72 Conclusions ...... 84 References ...... 86

Chapter Four ...... 89 Introduction ...... 89 Experimental ...... 92 Results ...... 95 Discussion ...... 104 Conclusions ...... 109 References ...... 111

Chapter Five ...... 113 Introduction ...... 113 Experimental ...... 114 Results ...... 118 Discussion ...... 131 Conclusion ...... 132 References ...... 134

Bibliography ...... 141

v LIST OF FIGURES

Figure 1.1 1H NMR stack plot of maltol, thiomaltol, dithiomaltol, Hopto, deferiprone .... 3

Figure 1.2 Room temperature UV-visible absorption spectra (normalized) of thiolated, maltol-derived ligands in CH3CN ...... 5

Figure 1.3 Solid state structures of trismaltolato Fe(III) and tristhiomaltolato Fe(III) ...... 7

Figure 2.1 Crystal structure of Httma ...... 26

Figure 2.2 Crystal structure of [Zn(ttma)] 2, 2-4 ...... 26

Figure 2.3 Cyclic voltammograms of 2-1, 2-4, and Httma ...... 29

+ Figure 2.4 Comparison of normalized UV-vis spectra of [Ru(bpy)2ttma] , Zn(ttma)2 and -1 -1 Ru(bpy)3 molar extinction coefficient (M cm ). Inset: of Httma over same range ...... 30

Figure 2.5 Normalized absorption and emission spectra for compound 2-4 in CH3CN. .. 31

Figure 2.6 Normalized absorption and emission spectra for 2-4 in CH3OH...... 31

Figure 2.7 ESI-MS spectrum of a sample generated by photoexcitation of an anaerobic + 2+ solution of [Ru(bpy)2(ttma)] and MV in CH3CN/CH3OH followed the + addition of 0.1 M NaOH. The species formed is [Ru(bpy)2(ttma-aldehyde)] (m/z = 584)...... 32

Figure 2.8 LCMS analysis of product solution after photolysis of 2-1 for in presence of Ru(NH3)6Cl3 ...... 34

+ Figure 2.9 Internal standard calibration curve for determining percent [Ru(bpy)2ttma] + and percent [Ru(bpy)2ttma-aldehyde] . Peak area ratio is equal to the ratio of + the sample to internal standard, [Ru(bpy)3] ...... 35

Figure 2.10 1H NMR spectra of complexes 2-1 – 2-4, free ligands Htma and Httma...... 35

Figure 2.11 MS analysis of soluble products obtained after photo-oxidations of 2-4 ...... 36

1 Figure 3.1 H NMR spectra (CD3CN) of 3-1 (top), 3-2 (middle) and 3-3 (bottom) ...... 58

Figure 3.2 Cyclic voltammograms of complexes 3-1, 3-2a and 3-3 ...... 60

Figure 3.3 Room temperature UV-vis absorption of 3-1 & emission...... 61

vi Figure 3.4 Room temperature UV-vis absorption of 3-2 & emission...... 61

Figure 3.5 Room temperature UV-vis absorption of 3-2b & emission ...... 61

Figure 3.6 Room temperature UV-vis absorption of 3-3 & emission...... 62

Figure 3.7 Room temperature excitation/emission spectra of 3-1 ...... 63

Figure 3.8 Room temperature lifetime of 3-1 & fit ...... 63

Figure 3.9. Low temperature (77K) excitation/emission spectra of 3-1 ...... 64

Figure 3.10 Low temperature (77K) lifetime of 3-1 & fit in DMM ...... 64

Figure 3.11 Room temperature excitation/emission spectra of 3-2a ...... 64

Figure 3.12 Room temperature lifetime of 3-2a & fit in DMM ...... 65

Figure 3.13 Low temperature (77K) excitation/emission spectra of 3-2a ...... 65

Figure 3.14 Low temperature (77K) lifetime of 2 & fit in DMM ...... 65

Figure 3.15 Visible excitation/emission and near IR emission of 3-2b ...... 66

Figure 3.16 Room temperature excitation/emission spectra of 3-3 ...... 66

Figure 3.17 Room temperature lifetime of 3-3 & fit in DMM ...... 66

Figure 3.18 Low temperature (77K) excitation/emission spectra of 3-3 ...... 67

Figure 3.19 Low temperature (77K) lifetime of 3-3 & fit in DMM ...... 67

Figure 3.20 Quenching of 3-1, 3-2a, and 3-3 luminescence by methyl viologen ...... 69

Figure 3.21 Plot of Pt(II) concentration vs emission intensity ...... 70

Figure 3.22 Self Quenching of [Pt(t-butylbpy)ttma]+ and [Pt(bpy)ttma]+ ...... 70

Figure 3.23 Plots of singlet oxygen luminescence quenching constants (Kobs) vs. concentration of complexes 3-1 – 3-3 ...... 71

1 Figure 3.24 H NMR spectra (CD3CN) of 3-4 (bottom), 3-5 (middle) and 3-6 (top) ...... 80

Figure 3.25 Room temperature UV-vis absorption of 3-4 and emission ...... 81

Figure 3.26 Room temperature UV-vis absorption and emission of 3-5 ...... 81

Figure 3.27 Room temperature UV-vis absorption & emission of 3-6...... 82

vii Figure 3.28 Room temperature excitation, emission, and near IR emission spectra of 3-4 ...... 82

Figure 3.29 Room temperature excitation and emission spectra of 3-5 ...... 83

Figure 3.30. Near IR excitation and emission spectra of 3-5 ...... 83

Figure 3.31 Room temperature excitation, emission and near IR emission spectra of 3-6 ...... 83

Figure 4.1 X-ray structure of [Ti4(maltolato)8(µ-O4)]• 18 H2O ...... 91

Figure 4.2 X-ray crystal structure of 4-5 ...... 96

Figure 4.3 Normalized room temperature UV-vis absorption of 4-1 in CH3CN ...... 98

Figure 4.4 Normalized room temperature UV-vis absorption of 4-2 in CH3CN ...... 99

Figure 4.5 Normalized room temperature UV-vis absorption of 4-3 CH3CN ...... 99

Figure 4.6 Normalized room temperature UV-vis absorption of 4-4 and emission ca. 530 at 77 K in CH3CN ...... 99

Figure 4.7 Normalized room temperature UV-vis absorption of 4-5 and emission at 77K ca. 642 nm in CH3CN ...... 100

Figure 4.8 Overlay of voltammograms for 4-1 ...... 101

Figure 4.9 Overlay of voltammograms for 4-2 ...... 102

Figure 4.10 Overlay of voltammograms for 4-4 ...... 103

Figure 4.11 Overlay of voltammograms for 4-5 ...... 103

Figure 4.12 Characterization of anatase (top) and rutile (bottom) nanorods of TiO2 with different aspect ratios by TEM images (A,B) and XRD Patterns (C). Adapted from Lu et. al34 ...... 106

Figure 4.13 Processed TEM images of rutile (101) and (301) twins...... 107

Figure 4.14 Dyes adsorbed on TiO2 ...... 107

Figure 4.15 TEM images of ZnO nanorods with different aspect ratio (A, B) ...... 108

Figure 4.16 Left, comparison of absorbance of thiomaltol (Htma) and dithiomaltol (Httma) in solution and adsorbed onto ZnO rods; right, FTIR spectra of dithiomaltol in solution and adsorbed onto ZnO rods ...... 108

viii Figure 4.17 Schematic showing ensemble of dye-coated nanoparticles for use on DSSC anode ...... 109

Figure 5.1 X-ray structure (ORTEP) of of 5-1 ...... 120

Figure 5.2 Proton NMR Spectra of 5-1 in (CD3)2CO...... 124

Figure 5.3 Proton NMR spectra of 5-2a in D2O/CD3OD...... 124

Figure 5.4 Proton NMR spectra of 5-2b in CD3OD...... 125

Figure 5.5 Phosphorus NMR of 5-2b ...... 125

Figure 5.6 Proton NMR spectra of 5-3 in CD3OD, excess triethylamine suppressed .... 126

Figure 5.7 Phosphorus NMR of 5-3 in D2O ...... 126

Figure 5.8 Proton-proton correlated 2D spectra of 5-2a in CD3OD ...... 127

Figure 5.9 Proton-carbon correlated 2D spectra of 5-2a in D2O/ CD3OD ...... 127

Figure 5.10 Proton-proton correlated 2D spectra of 5-2b in CD3OD ...... 128

1 13 Figure 5.11 2D HSQC H, C NMR of 5-2b in D2O/ CD3OD ...... 128

Figure 5.12 Overlay of voltammograms for 5-2b ...... 129

Figure 5.13 Overlay of voltammograms for 5-3 ...... 129

Figure 5.14 UV-vis absorbance and emission spectra of 5-1 in MeOH ...... 130

Figure 5.15 UV-vis absorption spectra and emission spectra of 5-2a in MeOH ...... 131

Figure 5.16 UV-vis absorbance spectra of 5-2b and 5-3 ...... 131

ix LIST OF SCHEMES

Scheme 1.1 Structures of maltol via conjugation and deprotonation ...... 1

Scheme 1.2 Synthetic strategies for ligand variation ...... 2

Scheme 1.3 Ligands derived from maltol ...... 2

Scheme 1.4 Jablonski Diagram of T1-T2 inversion ...... 5

Scheme 1.5 Schematic of a dye sensitized solar cell78 ...... 9

Scheme 1.6 Diagram of general set-up used for measuring IPCE ...... 10

Scheme 2.1 Ru(II) and Zn(II) complexes investigated in this work ...... 17

Scheme 2.2 Oxidative quenchers used...... 18

Scheme 2.3 Thermodynamic cycle with excited state reduction and oxidation potentials of [Ru(bpy)2ttma][PF6] ...... 37

Scheme 2.4 Thermodynamic cycle with excited state reduction and oxidation potentials of Zn(ttma)2 ...... 38

Scheme 2.5 C-H activation through thiopyrillium intermediate ...... 39

+ Scheme 2.6 Oxidative quneching reaction of [Ru(bpy)2ttma] ...... 40

Scheme 3.1 General synthesis and structure of Pt(bpy)(electron-donor) complexes...... 48

Scheme 3.2 Mixed, O,S and O,O chelating ligands ...... 48

Scheme 3.3 Proposed thio-pyrillium intermediate in C-H activation mechanism...... 79

Scheme 4.1 Proposed anthocyanin-Ti(IV) complex ...... 90

Scheme 4.2 Titanium complexes and their assignments ...... 92

Scheme 5.1 General structure of a spirophosphorane ...... 113

Scheme 5.2 General structure of complexes described ...... 114

Scheme 5.3 Reaction schemes for 5-1 and 5-2a ...... 118

Scheme 5.4 General structure of complexes 5-1 – 5-3 with proposed hydrogen connectivity ...... 132

x LIST OF TABLES

Table 1.1 Structural, electronic, and Fe(III) affinities of Fe(III) complexes of maltol, thiomaltol, deferiprone, and hopto ...... 8

Table 2.1 Crystallographic data of Httma, 2-3, and 2-4 ...... 26

Table 2.2 Structural data of complex 2-1 ...... 27

Table 2.3 Structural data of complex 2-4 ...... 28

Table 2.4 Electrochemical data. All potentials adjusted vs. NHE in CH3CN, 0.1 M TBAPF6 ...... 29

Table 2.5 Photochemical Data ...... 30

Table 2.6 Photolysis yields, % yielda (isolatedb) ...... 33

Table 2.7 Aromaticity comparison ...... 40

Table 2.8 Potential vs. photolysis yields ...... 42

Table 2.9 Calculated absorption energies and excited state reduction and oxidation potentials vs. NHE ...... 43

Table 3.1 Redox potentials of compounds 3-1 - 3-3 ...... 59

Table 3.2 Electronic absorption maxima of complexes 3-1 - 3-3 ...... 62

Table 3.3 Excitation and emission peak wavelengths, lifetimes (τ) at 298 K and 77 K, and quantum yield (φF) for complexes 3-1, 3-2 and 3-3 ...... 67

Table 3.4 Stern-Volmer constants (Ksv) calculated for quenching of complex 3-1 – 3-3 luminescence by [MV]2+ ...... 68

Table 3.5 Optimal concentrations of compounds 3-1 – 3-3 (µmol/L) for maximum emission ...... 70

1 Table 3.6 Quantum yields for O2 generation () and rate constants for quenching (kT) by compounds 3-1 – 3-3 ...... 72

Table 3.7. Summary of photochemical data for complexes 4-6 ...... 84

Table 4.1 Summary of parameters for data collection and structure refinement of 4-5 ... 96

xi Table 4.2 Selected Chemical shifts in Ti(IV) complexes with average aromaticity ...... 97

Table 4.3 Absorption bands’ absorptivities and assignments for 4-1 – 4-5 ...... 98

Table 4.4 Low temperature 77 K excitation & emission peak wavelengths (nm) in anhydrous CH3CN ...... 100

Table 4.5 Redox potentials vs. NHE corrected with ferrocene vs. NHE ...... 104

Table 5.1 Crystallographic data for 5-1 ...... 120

Table 5.2 Bond lengths in 5-1 ...... 122

Table 5.3 Bond angles in 5-1 ...... 122

Table 5.4 Ligand vinylic proton1H NMR chemical shifts (ppm)...... 123

xii ACKNOWLEDGMENTS

I would like to thank Dr. Farmer. His knowledge and support throughout my

graduate career has been invaluable. While administrative duties have kept him busy over

the years, his heart, mind, office door and were always open to his students.

To my committee, I am appreciative for your patience, support, and availability.

Dr. Klausmeyer, for his strong will to examine each and every crystal, no matter the

quality. Dr. Martin, for his insight and neighborly support through the writing process.

Dr. Shuford, for arming me with tools to better understand computation chemistry. Dr.

Zhang, for her support and availability to step in and keep my defense on track.

I would also like to thank the Department of Chemistry at Baylor University.

Nancy Kallus, Adonna Cook, Virginia Hynek, and Barbara Rauls, whom in their busy schedules take the daily obstacles each graduate student faces and makes it effortless and uncomplicated.

I would also like to thank the American Chemical Society Petroleum Research fund for the generous financial support. Without the funds available to purchase equipment and chemicals or to support myself, I would never have made it by this time to the point I am at now in my academic career.

I must also thank my colleagues in the Farmer Research Group, without whom, things would have been much darker in the lab. My roommates and friends, for the timely laughs and distractions to keep me well adjusted. My family, for feeding my soul and body to keep me moving towards my goals. My wife’s family, for the joys and strengths of a growing family. My wife and one true love, for putting up with playing second fiddle

xiii to chemistry, supporting me through long nights and weekends spent training for the

Ironman Triathlon, for being there at the finish line then, and now. Without her my heart would be empty and my work would be fallow.

Lastly, I would like to give acknowledgment and thanks to those that have lead me down my path. Mrs. Marshall, my high school chemistry teacher who inspired me to think deeply and ask not just why, but how. Dr. Smucker, my undergraduate research advisor who showed me how to work in a lab setting and allowed me learn from my own mistakes and successes.

Every day these people gave me their support in the means they were best suited.

xiv CHAPTER ONE

An Introduction and Background on the Chemistry of 3-Hydroxy-2-Methyl-4H-Pyrone and Its Use Towards a Photochemical Device

Introduction

Chemistry and Structure

Maltol, an FDA approved food additive. Maltol (3-hydroxy-2-methy-4H-pyran-

4-one, Scheme 1.1) is the oxidized product of the disaccharide maltose. It is a naturally

occurring organic compound that is readily found in pine needles, the bark of larch trees,

and roasted malt.1,2 Maltol is approved for flavor enhancement in the United States by the

Food and Drug Administration (FDA). A semi-volatile organic compound, maltol imparts odors of cotton candy and caramel to the foods in which it is added. Maltol-

Fe(III) complexes have also been extensively studied due to their stabilities and solubility.3–19

OH O O O- OH OH O- O- +H -H

Protonation Deprotonation O O O O (Hydroxy-pyrillium) (Maltol) (Oxo-pyrillium) Scheme 1.1 Structures of maltol via conjugation and deprotonation

The structure of maltol is based on the organic compound 4H-pyrone (-pyrone)

with the formula C6H6O3. The nature of the conjugation in the pyrone’s ring and the -

1 hydroxy ketone allow for easy delocalization of the π-electrons, forming the tautomers

and resonance structures shown in Scheme 1.1. The stable bond arrangement for each

20 structure prevent the accurate determination of the bond arrangements when in solution.

A solvent’s effects on the bond lengths can be observed by the change in ring current and the amount of deshielding at the vinylic protons in 1H NMR experiments.21

Hydroxy-pyrillium Maltol OH O OH Aqueous HCl OH (pH 1)

O O

Nu- Polar aprotic or Reflux ≥72 hours Non-polar solvent X = N-R P2S5 or Lawesson's Reagent,  O S Polar protic or OH OH non-polar solvent P2S5,  X X X = N-Me, deferiprone O, thiomaltol S, dithiomaltol Scheme 1.2 Synthetic strategies for ligand variation

Ligands derived from maltol. Small organic molecules can be derived from

maltol by functional group transformations both on and in the ring. In-ring heteroatom

substitutions are possible when a strong nucleophile is reacted with maltol in acid or base

solutions.22–25 Substitution at the carbonyl O-atom is observed with thionating or

selenating reagents.22,26–30 Scheme 1.2 outlines the methods available in literature for

variations on the maltol motif as demonstrated in studies on maltol-derived ligands.

2 S S O S O S OH OH OH OH OH OH

O S N N N N Ph Ph (Htma) (Httma) (Deferiprone) (Hopto) (Hppp) (Htppp)

Scheme 1.3 Ligands derived from maltol

The earliest synthesized derivatives of maltol, thiomaltol and 3-hydroxy-1,2-

dimethylpyridin-4(1H)-one (deferiprone, Scheme 1.3) are obtained from simple, low

yield reactions. Deferiprone is generated in dilute hydrochloric acid via the addition of

methylamine to maltol.23 Alternative primary amines and other pnictogens are also viable

options.24,31 Thiomaltol and the thiolated deferiprone analogue, Hopto, can be obtained

using any non-polar or polar-aprotic solvent while refluxing in the presence of a

thiolating reagent, such as red phosphorous and sulfur, phosphorous pentasulfide, or

Lawesson’s Reagent.21,22,27–30,32

Ring substitution of the O-atom occurs sequentially to the carbonyl substitution

when experiments are performed with more than two molar equivalents of Lawesson’s

Reagent, producing dithiomaltol.30 Analogous chalcogens may allow for different carbonyl O-atom substitutions.32 Thiomaltol and dithiomaltol are noticeably different

from maltol in both color and odor; S-substitution shifts the characteristic maltol

absorptions to lower energies, with dithiomaltol having the lowest energy absorption.

Chemical shift and heteroaromaticity. The differences in 1H NMR chemical

shifts ( = ppm) of the vinylic protons on maltol, thiomaltol, and dithiomaltol (Figure

2 1.1) are shown as the chemical shifts of similar pyrones and thiopyrones have also been

indexed and reported.33–35

Heteroaromaticity is the degree of delocalization within a heterocycle.33–36 The

most frequently used method for comparing heteroaromaticity has been the Harmonic

Oscillator Model of Aromaticity (HOMA) index which is determined by subtracting the bond length elongations (BLE) and alterations (BLA) from the value one, Eq. 1.1.33–36

Thus the index assigns non-aromatics as equal to zero (= 0) and aromatics equal to one (=

1).33 Over the last decade, the Nuclear Independent Chemical Shift index (NICS) has

been the alternative to the HOMA index, with over 300 citations.37 This computational

method is based on values of magnetic shielding by the ring center.35,38,39 Alternatively,

the general or overall displacement of the average chemical shift can be used for a

comparison of ligands heteroaromaticity as shown in Eq. 1.2, where n corresponds to

the number of chemical shifts observed in the aromatic region.40

Figure 1.1 1H NMR stack plot of maltol (a), thiomaltol (b), dithiomaltol (c), Hopto (d), deferiprone (e)30

3 HOMA1BLEBLA 1.1

1 1.2 ⋯

The heteroaromaticity of pyrones and thiopyrones have been studied and predicted by DFT calculations.21,34,36,40 It is suggested that the cationic free ligand is the most aromatic while the anionic is the least aromatic. In their natural form, thiobenzenes are more aromatic. This is due to a combination of an increased ring size and change in electronegativity which give higher conjugation.40,41

Htma Hopto Httma Normalized Intensity Intensity Normalized 200 300 400 500 600 Wavelength (nm)

Figure 1.2 Room temperature UV-visible absorption spectra (normalized) of thiolated, maltol-derived ligands in CH3CN

Unique photochemistry of thiopyrones due to thiocarbonyl. The thiopyrones and their polyaromatic analogues, thioflavones, have unique photophysical and photochemical behaviors.42–48 Figure 1.2 shows how the visible absorption spectra of the

4 free thiolated ligands thiomaltol, dithiomaltol, and hopto vary with the simple transformations in structure. Polar or acidic solvents can affect the room temperature emissions implying multiple states, excited state lifetimes, and quantum yields of each electronic transition.49

Specifically, previous phosphorescence studies of aromatic thiones in solution at room temperature show that in some cases an excited state with the orbital configuration

43,44 of a T2 (π,π*) can be populated from a S0→S1, due to a T1–T2 energy inversion. The

Jablonski diagrams in Scheme 1.4 illustrate the orbital inversion of the T1-T2 excited states. The interpretation of this behavior suggests that the former T2 (π,π*) excited state is now directly populated by inner system crossing (ISC) over the former T1 (n,π*) state.

Characteristic emissions can be further enhanced when coordinated to transition metals with paired electrons.46 Understanding the manipulation of the triplet states (e.g., from n,

π* to π,π*) suggests applications in photosensitive materials for green pesticides, fluorimetric analysis of ion concentrations, imaging of bio-membrane structures, and

OLEDs.47–51

S2 * S2 * T2(π,π ) T2(n,π )

S1 IC S1 IC IC * IC * T (n,π ) T1(π,π ) ISC 1 ISC NR/Q NR/Q Solvent F F P P Energy → Energy

Energy →Energy NR/Q NR/Q

S0 S0

Scheme 1.4 Jablonski Diagram of T1-T2 inversion

5 Chelation and Coordination

Transition metal complexes. Transition metal and main group complexes with

maltol, thiomaltol, and other pyridone derivatives have been studied for the applications in medicine, metal transport/chelation drugs, treatment for Alzheimer’s, diabetes, anticancer applications, and tissue imaging.6,8–13,15,17–19,24,28,52–62 These metal complexes

are typically characterized by solid-state crystallography, and spectrochemical

characterizations. The results show a variety of behaviors that can be optimized for

specific roles, as previously cited.

The most extensive studies in literature examine maltol and deferiprone as iron-

specific-chelator pharmaceuticals. These drugs offer a potent means to correct the levels

of iron in the blood.3–6,8,9,12–15,17–19,27,29 Thiomaltol, and Hopto have been shown

comparable affinities for Fe(III) in aqueous solutions as to the O,O ligands (maltol and

deferiprone). The alpha-hydroxy ketone present in maltol and deferiprone chelate Fe(III)

similarly as catechols do but with greater Fe-O bond lengths variance, ca. ±0.04 Å and distortion in the twist angle at the octahedral Fe(III) center.27,29,8,18,63 These structural variances from catechols are attributed to the methyl group and ring oxygen.29 Structural

differences between the O,O complexes (maltol and deferiprone) and the O,S complexes

(thiomaltol and hopto) show larger chelate bite angles, longer Fe-S bond lengths than the

corresponding Fe-O bond length, and less trigonal twist.8,12,27,29 These studies imply a strong trans influence from the O,S ligands due to these structural assignments. Structures of tris-maltolato Fe(III) and tris-thiomaltolato Fe(III) shown in Figure 1.3.

6

a) b)

Figure 1.3 Solid state structures of trismaltolato Fe(III)8 (a) and tristhiomaltolato Fe(III)27 (b)

The Fe(III) complexes of maltol, thiomaltol, deferiprone, and hopto have unique

visible absorptions with maxima ranging from 410-615 nm.18,27,29,63 Lower energy

absorptions as in the Fe(III) complexes are not seen in the free O,O ligands. The

absorptions ca. 350 nm observed in both O,S give the free ligands a characteristic yellow

color. When Fe(III) is coordinated the observed changes in absorption bands depend on

the ligand’s substitution pattern. The higher energy absorptions are typically ligand-

centered and observed to increase in intensity and split while the lower energy

absorptions are mostly weaker in intensity charge-transfers (LMCT, MLCT, d-d

transitions) red-shifted from the free ligands.

From previous studies, deferiprone shows the greatest value for affinity towards

Fe(III). Deferiprone was only recently approved by the FDA for the use of iron-overload

treatment in the United States under the drug name Ferriprox®.64 Maltol has been used to solubilizes Al(III) for studies examining neurodegenerative disorders related to

Alzhiemers.52 Compared to maltol and deferiprone, thiomaltol shows comparably strong

7 affinities but is still bested by the O,O ligands due to the disfavored Fe(III) or Al(III) hard-soft-acid-base interactions imparted by the new thione.18,27,65,66

Table 1.1 Structural, electronic, and Fe(III) affinities of Fe(III) complexes of maltol, thiomaltol, deferiprone, and hopto

Maltol Deferiprone Thiomaltol Hopto Bite Angle (deg.) 80.5 av. 80.8 av. 81.5 av. 80.9 av. Fe-O Bond Length (Å) 1.987 av. 1.998 av. 1.979 av. 2.007 av. Fe-O/S Bond Length (Å) 2.065 av. 2.046 av. 2.499 av. 2.471 av. Octahedral Twist (deg.) 50.4 44.9 48.2 43.6 UV-vis (λmax) 468 460 615 550 pFe 16.9 14.1 20.2 15.6

Dye Sensitized Solar Cells

Sensitization of photosensitive semiconductors. One possible use for these maltol-based compounds (Scheme 1.3) may be in DSSCs as photoreductants adsorbed onto titania (TiO2) particles at the anode surface. The low cost, photo activity, and stability have made titania popular in use of light-to-energy applications such as those studied by Michael Grätzel et al.67–77 on dye-sensitized solar cells, (DSSCs). This photovoltaic device is essentially an electrochemical cell with modified anodes and cathodes bridged by a hole-transport material (HTM), which is often a solution of an oxidizable electrolyte such as an iodide/triiodide mixture, Scheme 1.5.78 The anode is a transparent indium-doped tin oxide layer (ITO) on glass with a porous coating of titania

(TiO2) on which a photoreductant dye is adsorbed. Often the cathode is a Pt surface or an

ITO plate, modified with carbon or soot to facilitate triiodide reduction. Excitation of the photoreductant dye leads to generation of an excited state with the proper energetics from which electrons are injected to the conduction band of TiO2, and are ultimately

8 transferred to the anodic ITO plate, k1 in Scheme 1.5. The oxidized dye is then reduced by iodide at the anode/HTM interface, k2, and the hole filled by reduction of triiodide at the HTM/cathode interface, k3. Thus, a photoreduction is initiated at the dye/TiO2 surface

(or alternative metal oxide nanoparticles crucial to facilitating charge separation), and the hole is carried back to the cathode surface by the HTM. The overall potential realized by such a system depends on the conduction band energy of the injected electron and the reduction potential of the hole-transporter.

Scheme 1.5 Schematic of a dye sensitized solar cell78

Grätzel’s collection of works have focused on understanding the electron transfer mechanisms that govern incident photon conversion efficiency (IPCE) and the properties of the materials used to favor efficient electron transport.67,71,72 The DSSC outlined in

Scheme 1.5 draws the favored route for the electron cascade, not shown are the

9 disfavored back electron transfers that work against the current. Measuring the IPCE with instrumentation outlined in Scheme 1.6 have lead identified devices comparable in efficiency to modern commercial silicon-type photovoltaics.76

Plot Monochromatic Light Source Analog/Digital Converter



+- Lock-In V R Amplifier

DSSC

Scheme 1.6 Diagram of general set-up used for measuring IPCE

Maltol-derived ligands have chelating α-hydroxy ketone functionalities analogous to that of anthocyanin, a red dye found in berries with unusually high efficiency for

DSSCs.67 The basis of using maltol-based ligands as sensitizers for DSSCs applications is presented in the following chapters by examining and comparing the photochemical and photophysical behaviors of their complexes with transition metals. The results in each set of experiments with the late transition metals, Ru(II) and Pt(II), exhibit unique photochemistry and photophysical behaviors. Initial results with Ti(IV) show late transition metals are not needed for photochemical reactivity, thus suggesting applications in DSSCs.

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15 CHAPTER TWO

Ligand-based Photooxidations of Dithiomaltolato Complexes of Ru(II) and Zn(II): Photolytic CH Activation and Evidence of Singlet Oxygen Generation and Quenching

Portions of this chapter published as: Bruner, B.; Walker, M. B.; Ghimire, M. M.; Zhang, D.; Selke, M.; Klausmeyer, K.; Omary, M.; Farmer, P. J. Dalton Trans. 2014, 43, 11548- 11556.

Introduction

Dye Research and Development

Much effort has gone into the development of new photochemical dyes, which induce electron transfer reactions upon exposure to light for uses in dye sensitized solar cell (DSSC) and other photochemical applications.1 Most commonly used are

luminescent compounds based on the well-known families of Ru(diimine)x and porphyrin-based species that form long-lived triplet states (3ES) and undergo facile

2 2+ electron-transfer reactions. In particular, the [Ru(diimine)2L] family of complexes has

been extensively studied due to photochemical redox reactivity involving electron

transfer reactions from long-lived luminescent states.3–9

Anthocyanin, a red dye found in many berries, was shown to have an unusually

good efficiency as a DSSC dye when chelated to the surface of titania.10 Chemically

similar hydroxy-pyrone chelates such as maltol have been studied for use as diagnostic

tools,11 anticancer drugs,12,13 and metal transport.14–16 Thiomaltol, discovered in 1969,17 in which the pyronal ketone is replaced with a thione, has been used for similar applications.18,19 A ring substituted derivative, 3-hydroxy-2-methyl-4H-thiopyran-4-

thione, dithiomaltol or Httma, was first reported by Brayton and displayed unusual

16 aromaticity characterized by a downfield shift in the vinylic proton peaks in the 1H NMR

spectrum.20

Recently, the Farmer group reported that the bis-bipyridyl Ru(II) complex of

dithiomaltol, [Ru(bpy)2(ttma)][PF6], 2-1, undergoes C-H activation at a pendant alkyl

+ position upon electrochemical or bulk oxidation, yielding [Ru(bpy)2(ttma-alcohol)] , 2-2,

+ 21 and [Ru(bpy)2(ttma-aldehyde)] , 2-3, as products (Figure 2.1). This chapter examines

the photo-initiation of similar oxidative reactivity for compound 1 and the homoleptic

Zn(ttma)2 complex, 2-4 (Figure 2.1), using flash quench methodology employing electron acceptors 1,1'-dimethyl-4,4'-bipyridinium ([MV]+2), pentamminechlorocobalt(III)

chloride (Co(NH3)5Cl3), hexaammineruthenium(III) chloride (Ru(NH3)6Cl3), and 2,3-

dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), Scheme 2.2.22

+

N R N O S O S S Ru+2 Zn+2 S N S O S N Complex -R 2-4 2-1 -CH3 2-2 -CH2OH 2-3 -CHO

Scheme 2.1 Ru(II) and Zn(II) complexes investigated in this work

17 Cl Cl NH3 3 NH3 3 NH3 NH3 +3 +3 H3N Ru NH3 H3N Co Cl

H3N H3N H3N H3N

Cl2 O N N N Cl

Br2 Cl N N N O

Scheme 2.2 Oxidative quenchers used

Experimental

Materials and Methods

Reagents. The dithiomaltol ligand, [Ru(bpy)2(ttma)][PF6], Zn(ttma)2 and 4,4’-

dimethyl-1,1’-trimethylene-2,2’dipyridinium dibromide, [DTDP][Br2], were prepared

16,23 +2 according to published procedures. Electron acceptors, [MV] , Co(NH3)5Cl3,

Ru(NH3)6Cl3, DDQ and all other chemicals used were purchased from Sigma-Aldrich.

Air-sensitive manipulations were carried out using Schlenk techniques or in an anaerobic dry glovebox.

Physical measurements. Electronic absorption spectra were recorded by Perkin

Elmer Lambda 900 or a Hewlett Packard 8453 Diode Array spectrophotometers.

Emission and excitation spectra were recorded on a Hitachi F-4500 fluorescence spectrometer. All photochemical reactions were performed in anaerobic quartz fluorescence cells or sealed jacketed beakers.

18 X-Ray diffraction. Crystallographic data was collected on crystals with

dimensions 0.31 x 0.18 x 0.08 mm for 2-1, 0.28 x 0.17 x 0.14 mm for 2, 0.28 x 0.17 x

0.14 mm for 2-4, and 0.31 x 0.18 x 0.08 mm for Httma. Data was collected at 110 K on a

Bruker X8 Apex using Mo-K radiation (λ = 0.71073 Å). All structures were solved by

direct methods after correction of the data using SADABS.24–26 All the data were

processed using the Bruker AXS SHELXTL software, version 6.10.27

Electrochemistry. Redox potentials were measured by cyclic voltammetry under anaerobic conditions using a CHI-760B potentiostat in dry-degassed CH3CN with 0.1 M

tetrabutylammonioum hexafluorophosphate (TBAPF6) as the supporting electrolyte. The

cells consisted of a glassy-carbon working electrode (3.0 mm dia.), an AgCl coated Ag

reference wire, and a coiled Pt wire auxiliary electrode. Measured potentials were

corrected using a ferrocene standard, with the Fc/Fc+ couple set to 642 mV NHE.

Photolysis. Large-scale photolysis experiments were carried out using an Oriel

Apex Quartz Tungsten Halogen Source with a 150 W Xe Arc Lamp and filter

accessories. Samples were stirred under N2 at 5˚C for 3 hr. Smaller scale photolysis were

performed anaerobically in a 3 mL anaerobic quartz fluorescence cells for 45 min

followed by a bench top workup.

Quantification by mass spectrometry. All mass spectra were collected on an

Accela Bundle Liquid Chromatograph (LC) coupled to a Thermo Electron Linear Trap

Quadropole Orbitrap Discovery mass spectrometer (Orbitrap) using positive electrospray

ionization (+ESI) or by Direct Infusion (DI) with +ESI. Data was collected and processed

19 using Xcalibur v.2.0.7 software. Standards for LC separation were used to calibrate

reaction yields for compounds 2-1 and 2-3. Separated products were collected by column-chromatography inside a glove box for isolated yields. Samples were collected

and stored under inert atmosphere, removed from the glovebox, dried in vacuo, and

returned to the glovebox to record mass and prepare samples for 1H NMR spectroscopy.

+ + [Ru(bpy)2ttma] and [Ru(bpy)2(ttma-aldehyde] were quantified from photolysis

reactions by separation on a 15 cm × 2.1 mm (5 μm, 300 Å) ZORBAX extend-C18

column (Agilent Technologies). A binary mobile phase gradient containing 0.1 %

(vol/vol) formic acid in water (A) and CH3CN (B). The elution conditions are expressed

as percentages A/B: 0-2 min, 97/3; 2-15 min, 97/3-2/98; 15-18 min, 2/98; 18-24, 2/98-

97/3; flow rate: 350 µL/min, temperature: 24˚C. Autosampler operations were set to a

10.0 µL injection volume at 24˚C. A switch valve was used between the Accela Bundle

LC and the Orbitrap and programmed as follows: 0-2 min position A (waste), 2-20 min

Position B (collect), 20-24 min position A. Full-scan accurate mass spectra (m/z range

50–1000) of eluting compounds were obtained at high resolution (30,000 FWHM) on the

Orbitrap mass analyzer. Tris(2,2’-bipyridine)ruthenium(II) chloride [Ru(bpy)3][Cl]2 was

used as an internal standard for calibration.

Standard solutions of 0.0, 0.1, 1, 5, 7, 10, 15, 20 and 30 ppm were prepared for

the analysis of reaction percent yield (% yield) by external calibration with an internal

standard Ru(bpy)3 of 5 ppm for added internal calibration. External calibration curves

+ were plotted with the concentrations of known [Ru(bpy)2ttma] and [Ru(bpy)2ttma- aldehyde]+ against the corresponding peak area. Internal calibration curves were plotted with the known concentrations of analyte against the analyte/internal standard peak area

20 + ratio. Matrix spike and matrix spike duplicates of [Ru(bpy)2ttma] and [Ru(bpy)2ttma- aldehyde]+ were added to the reactions to verify instrument response and sample

integrity. Peak areas were integrated using the Xcalibur Qualbrowser Software.

Calibration curves and statistics were performed with Mircosoft Excel for Mac 2011.

Photophysical and photochemical studies. Near IR emissions were measured in

acetonitrile (CH3CN) with a Photon Technology International (PTI) QuantaMaster Model

QM-4 scanning spectrofluorometer equipped with a 75-watt xenon flash lamp, emission

and excitation monochromators, excitation correction unit, and a near-infrared (NIR)

PMT from Hamamatsu. Lifetimes were collected using a pulsed Xenon source with a

pulse repetition rate of 300 pulse/sec and a PTI-supplied Gated Voltage-Controlled

Integrator to interface to the NIR PMT.

Excited-state oxidation and reduction potentials were determined by the observed

E00 for singlet spin states, and approximated for the triplet spin states based on the

observed emission wavelength. Thermodynamic cycles were then drawn with the excited

state reduction and oxidation potentials calculated from the equations 2.1 and 2.2.

∗ 2.1

∗ 2.2

21 Experiments with Dithiomaltol

Synthesis of dithiomaltol, Httma. Dithiomaltol was synthesized using a

modification of previously reported methods.28 A sample of 3-hydroxy-2-methyl-4- pyrone (maltol, 1.00 g, 7.93 mmol) was combined with 1.1 eq of hexamethyldisiloxane

(HMDO, 1.85 mL, 87.2 mmol) in 60 mL of anhydrous toluene and gently heated for 30 min. A sample of Lawesson’s Reagent (3.53 g, 87.3 mmol) was then added before fitting the reaction flask with a condenser, and heated to reflux under nitrogen for 1.5 h. A black precipitate was removed by vacuum filtration, and the filtrate was concentrated to give a dark brown solid. The product was purified by silica flash column chromatography eluting with 4% CH3OH in CH2Cl2, as a bright orange band. After removal of solvent by

reduced atmosphere, an orange solid was isolated, which gave dithiomaltol as orange

iridescent flakes. Yield: 70%. Slow evaporation of the solid in hexanes gave orange

1 needles suitable for x-ray diffraction studies. H NMR (500 MHz, CDCl3): 9.34 (s,

1H), 8.17 (d, 1H), 7.51 (d, 1H) 2.49 (s, 3H); ESI MS: m/z, 159 ([M + H]+); UV-vis:

-1 -1 CH3CN, ƛmax(ε in M •cm ) 221 (ε = 4,300), 285 (ε = 1,700), 397 (ε = 6,600).

Synthesis of [Ru(bpy)2ttma][PF6], 2-1. [Ru(bpy)2(ttma)](PF6) was synthesized

21 from the same procedure previously reported. Ru(bpy)2Cl2 (100. mg, 0.206 mmol) and

Httma (35.9 g, 0.227 mmol) were refluxed for 1 hour in 5.0 mL ethylene glycol under N2.

The mixture was then poured into a solution of 200 mL DI H2O and excess KPF6. A purple solid was obtained from filtration. Purification by column chromatography on alumina with a dichloromethane/acetonitrile/methanol mixture (1:1:0.1) as the eluent yielded 113.7 mg (0.159 mmol, 77.2%). ESI MS: m/z 571 ([M]+); 1H NMR (500 MHz,

22 CD3CN δ 9,39 (d, J = 5.6 Hz, 1H), 8.53 (d, J = 5.5 Hz, 1H), 8,42 (d, J = 8.2 Hz, 1H),

8.39 (t, J = 16.1, 8.1 Hz, 2H), 8.26 (d, J = 8.1 Hz, 1H), 7.98 (t, J = 16.4, 8.2 Hz, 2H), 7.96

(d, J = 9.2 Hz, 1H), 7.86 (t, J = 15.7, 7.9 Hz, 1H), 7.78 (d, J = 5.6 Hz, 1H), 7.69 (t, J =

15.7, 7.9 Hz, 1H), 7.60 (d, J = 5.6 Hz, 1H), 7.55 (d, J = 9.2 Hz, 1H), 7.52 (t, J = 13.5, 7.2

Hz, 1H ), 7.48 (t, J = 13.2, 7.3 Hz, 1H), 7.20 (t, J = 13.2, 7.0 Hz, 1H), 7.02 (t, J = 13.3,

7.2 Hz, 1H), 3.24 (s, 3H).

Synthesis of Zn(ttma)2, 2-4. As adapted from previous report, a pellet of NaOH

(0.238 g, 5.95 mmol) was added to MeOH (200 mL) to make a 29.8 mM stock solution.

Httma (0.097 g, 0.613 mmol) was added to 19 mL of a stirred 29.8 mM NaOH in a

MeOH solution turning it a red color. Five minutes later ZnCl2 (36.8 mg, 0.291 mmol) and 20 mL of DI H2O was added to the stirred solution, a light yellow precipitates formed immediately, and the reaction was left to stir overnight. Filtering the reaction solution yielded 0.092 g (83%) of a yellow solid. The compound was crystallized in CH2Cl2 by

slow evaporation to produce samples suitable for X-ray diffraction. 1H NMR (500 MHz,

CDCl3): δ 2.64 (s, -CH3, 6H), 7.58 (d, J = 9.1 Hz, 2H), 8.43 (d, J = 9.1 Hz, 2H).

Electrospray MS: 401 ([M+Na]+).

Chemical oxidation of 2-1 with DDQ. The complex [Ru(bpy)2(ttma)][PF6] (4.21 mg, 5.87 µmol) and DDQ (1.33 mg, 5.86 µmol) were placed in a 50 mL flask with 10.0 mL anhydrous CH3CN. The flask was sealed, degassed with N2, then stirred for 45 min

before the addition of NaOHaq (1 mol eq), after which the solvent was removed under N2 purge, and separated by column chromatography inside a glove box using alumina as the separation media and anhydrous acetonitrile as the eluent. The isolated yield of aldehyde

23 1 2-3 was 3.9 mg (92 %). ESI MS: m/z 584.99. H NMR (500 MHz, CD3CN): d 9.96 (d, J

= 1.5, Hz, 1H), 9.18 (d, J = 5, Hz, 1H), 8.56 (d, J = 5, Hz, 1H), 8.46 (d, J = 7.21, Hz, 1H),

8.44 (d, J = 8.3, Hz, 1H), 8.43 (d, J = 9.21, Hz, 1H), 8.32 (d, J = 8.3, Hz, 1H), 8.08 (d, J =

8.0, 1.3, Hz, 2H), 8.05 (d, J = 9.3, Hz, 1H),7.96 (d, J = 8.0, 1.5, Hz, 1H), 7.88 (d, J = 5.7,

Hz, 1H), 7.79 (d, J = 8.0, 1.4, Hz, 1H), 7.76 (t, J = 9.3, 1.5, Hz, 1H) 7.62 (d, J = 7.3, 1.3,

Hz, 1H), 7.61 (d, J = 6.0, Hz, 1H), 7.58 (d, J = 7.3, 1.3, Hz, 1H), 7.3 (d, J = 7.3, 1.2, Hz,

1H), 7.13 (d, J = 7.3, 1.3, Hz, 1H).

Large scale photochemical oxidations of 2-1 and 2-4. In a typical experiment, samples of [Ru(bpy)2(ttma)][PF6] (34.9 mg, 48.8 µmol) and Co(NH3)5Cl3 (13.0 mg, 51.9

µmol) were placed in a 50 mL jacketed flask with 10.0 mL anhydrous acetonitrile. The

flask was sealed, degassed with N2, and cooled to 5˚C with a circulating cooling bath.

The reaction mixture was photolyzed for 45 min with a 400 nm low-pass filter before the

addition of NaOHaq (1 mol eq), after which the solvent was removed under N2 purge and separated on an alumina column inside a glove box with anhydrous CH3CN as eluent.

The isolated yield of aldehyde 2-3 was 17.9 mg (50.4 %). ESI MS: m/z 584.99 ([M]+); 1H

NMR (500 MHz, CD3CN):  9.96 (d, J = 1.5, Hz, 1H), 9.18 (d, J = 5.0, Hz, 1H), 8.56 (d,

J = 5.0, Hz, 1H), 8.46 (d, J = 7.2 Hz, 1H), 8.44 (d, J = 8.3 Hz, 1H), 8.43 (d, J = 9.2 Hz,

1H), 8.32 (d, J = 8.3 Hz, 1H), 8.08 (d, J = 8.02 Hz, 2H), 8.05 (d, J = 9.3 Hz, 1H), 7.96 (d,

J = 8.0 Hz, 1H), 7.88 (d, J = 5.7 Hz, 1H), 7.79 (d, J = 8.0 Hz, 1H), 7.76 (d, J = 9.3 Hz,

1H) 7.62 (d, J = 7.3 Hz, 1H), 7.61 (d, J = 6.1 Hz, 1H), 7.58 (d, J = 7.3 Hz, 1H), 7.3 (d, J =

7.3 Hz, 1H), 7.13 (d, J = 7.3 Hz, 1H). Analogous experiments were carried out with

+2 [MV] and Ru(NH3)6Cl3 as described in text.

24 Photochemical oxidations of 2-4. Following the same procedures as in

photochemical oxidations of 2-1. A mixture of 2-4 (12.6 mg, 33.0 µmol) and

Co(NH3)5Cl3 (7.4 mg, 41 µmol) were placed in a 50 mL jacketed flask with 25.0 mL

anhydrous CH3CN. The flask was sealed, degassed with N2, and cooled to 5˚C with a

circulating cooling bath. The reaction was photolyzed for 45 min with a 400 nm low-pass

filter before the addition of NaOHaq (1 mol eq). With formation of an insoluble red precipitate; the remaining solution was analyzed by ESI-MS. Analogous experiments were done with DDQ as described in text.

Small scale photochemical oxidations of 2-1 and 2-4. These followed similar procedures as above. A stock solution of [Ru(bpy)2ttma][PF6] was prepared from 17.0 mg

(23.7 µmol) in 5.0 mL anhydrous acetonitrile (4.75 mM); a sample of this solution (220.

uL, 0.924 µmol) was mixed with 200 equivalents of MV+2 (4.76 mg, 0.185 mmol) in a purged quartz cell and irradiated for 15 min with a 400 nm low-pass filter. The reaction

was then mixed with excess NaOH (0.25 mL of 0.143 M) and examined by LCMS for

characterization and quantification of products.

Results

Studies on Ru(II) and Zn(II) Complexes With Dithiomaltol

Synthesis and characterization of Httma and Zn(ttma)2, 2-4. Coordination complexes of Httma with metal ions are typically synthesized by deprotonation at room temperature or heating at high temperature in polar protic solvents. Both methods were used in synthesizing previously reported compounds [Ru(bpy)2(ttma)][PF6], 2-1, and

25 Zn(ttma)2, 2-4. X-ray diffractometry was used to determine the structure of Httma, shown in Figure 2.1 and of 2-4 in Figure 2.2. Details of the crystal parameters, data collection, and refinement are summarized in Figure 2.2 and Table 2.1.

Figure 2.1 Crystal structure of Httma

Figure 2.2 Crystal structure of [Zn(ttma)] 2, 2-4

26 Table 2.1 Crystallographic data of Httma, 2-3, and 2-4

Httma 2-3 2-4 Empirical formula C6 H6 O S2 C30 H29 F6 N4 O3 P Ru S2 C13 H11 Cl3 O2 S4 Zn Formula mass 158.23 803.73 499.18 a (Å) 5.3629(4) 10.7114(12) 7.5568(9) b (Å) 8.6723(7) 12.7589(15) 25.901(4) c (Å) 13.5524(11) 12.9779(15) 7.6967(10) α (°) 90.00 109.440(2) 73.404(3) β (°) 114.730(4) 96.084(2) 79.141(3) γ (°) 90.00 101.515(2) 75.735(3) 3 V (Å ) 1368.3(3) 1610.07 921.33(16) Z 8 2 2 Crystal System Monoclinic Triclinic Triclinic Space Group P2(1)/c P-1 P-1 T(K) 110(2) 163(2) 110(2) -3 Dcalcd.(g/cm ) 1.536 1.658 1.536 -1 μ(mm ) 0.684 0.71073 2.224

2θmaz (°) 28.32 28.32 28.32 Reflections measured 3347 17841 4491 Reflections used 2983 7767 3850 Data/restraints/parameters 2983/0/173 7767/0/399 2850/0/210

R1 [I > 2σ(I)] 0.0317 0.0434 0.0383

wR2 [I > 2σ(I)] 0.0757 0.1128 0.0993 2 R(F o) (all data) 0.0366 0.0564 0.0486 2 Rw (F 0) (all data) 0.0799 0.1218 0.1199 2 GOF on F 1.046 1.043 1.091

Table 2.2 Structural data of complex 2-1

Bond (Å) Angle (deg.) Ru-O(1) 2.10817 O(1)-Ru-S 84.33 Ru-S(1) 2.32028 N(2)-Ru-O(1) 172.88 N(2)-Ru-S(1) 170.31

27 Table 2.3 Structural data of complex 2-4

Bond (Å) Angle (deg.) Zn(1)-O(1) 1.957(2) O(1)-Zn(1)-O(2) 106.80(9) Zn(1)-O(2) 1.979(2) O(1)-Zn(1)-S(3) 123.38(6) Zn(1)-S(3) 2.2871(8) O(2)-Zn(1)-S(3) 88.79(6) Zn(1)-S(1) 2.2968(8) O(1)-Zn(1)-S(1) 89.48(6) O(2)-Zn(1)-S(1) 119.95(7) S(3)-Zn(1)-S(1) 129.30(3)

Ligand constraint in compound 2-4 distorts metal-centered tetrahedral geometry with O(1)-Zn(1)-S(3) angle at 123.38(6)˚ and O(2)-Zn(1)-S(3) at 88.79(6)˚. The dihedral angle between the two ligands is 93° while the bite angles of the ligands are < 90°.

Average C-C bond lengths are 1.4071625 Å (C-C 0.0919). The structures of 2-1 and 2-

3 were previously reported;21 bond lengths, angles, and interatomic distances are given in

Table 2.2 for 2-1, and Table 2.3 for 2-4.

Electrochemical measurements. The ground state redox properties of Httma, 2-1

and 2-4 were assessed using cyclic voltammetry, shown in Figure 2.3 and Table 2.4. The

Httma undergoes a characteristic irreversible reduction at -0.98 V NHE, with similar

reductions seen at ca. -1.12 V for the metal complexes; a related oxidation wave is seen

on the return scan, at ca. -0.1 V for Httma, and -0.4 and -0.6 V for compounds 2-4 and 2-

4, respectively. Compound 2-4 also displays quasi-reversible oxidation at 0.7 V, assigned to the Ru2+/3+ couple, and reduction at ca. -1.5 V, assigned to the bipyridine moieties.

28 1.5 1.0 0.5 0 ‐0.5 ‐1.0 ‐1.5 ‐2.0 V vs. NHE

Figure 2.3 Cyclic voltammograms of 2-1 (solid, middle), 2-4 (dashed, top), and Httma (dotted, bottom) in anhydrous CH3CN with 0.1 M TBAPF6 as the supporting electrolyte on glassy carbon disc electrode, scan rate 100 mV/s

Table 2.4 Electrochemical data for 2-1 and 2-4. All potentials adjusted vs. NHE in CH3CN, 0.1 M TBAPF6

E’Ox E’Red Httma -0.156(ir) -0.969(ir) 2-1 +0.693 -1.117(ir) -1.488 2-4 +1.200(ir) -1.119(ir) -1.359

Photophysical and photochemical studies. The normalized absorption spectra of

2+ metal complexes 2-4, 2-4 and well-known [Ru(bpy)3] are compared in Figure 2.4. The

absorption of metal-coordinated ttma itself is demonstrated by the band at 420 nm in the

spectrum of 2-4. Several other ttma complexes show similar ligand-based absorption bands with extinction coefficients on the order of 10 to 20 x 103 M-1cm-1 per ttma

ligand.29 The spectrum of 2-4 exhibits mid-range bands from 450 to 750 nm, which are

significantly shifted and broadened in comparison to the analogous bands of

29 2+ [Ru(bpy)3] . These, as well as other determined photochemical properties of complexes

2-4 and 2-4, are given in Table 2.5. ) 1 -

) 40 Httma

cm 6 -1 35 -1 cm

L•mol 4 3 -1 30

. 10 (x 2 25 L•mol coeff

3 0 20 Ext. 400 600 800 nm + 15 Ru(bpy)2(ttm) 1 Zn(ttm) 2 4 10 2+ Ru(bpy)3 5 Ext. Ext. coeff10 (x . 0 300 400 500 600 700 800 nm

+ Figure 2.4 Comparison of normalized UV-vis spectra of [Ru(bpy)2ttma] (−), Zn(ttma)2 -1 -1 (•••) and Ru(bpy)3 (---) molar extinction coefficient (M cm ). Inset: of Httma over same range

Table 2.5 Photochemical data of 2-1 and 4-1

Ext. Coef. Ex. λmax Complex Abs λmax (nm) Em (nm) Em (µs) (103•M-1•cm-1) (nm) 2-1 355 16.5 410 420 0.01 537 12.1 1080 8.5 2-4 422 37.1 400 450 0.005 1088 20 1271 210

Excitation of 2-4 in CH3CN solution at wavelengths >400 nm generates no observable emissions, but excitation at 355 nm band obtains a short-lived fluorescence

(λem = 420 nm, em = 10 ns) which increases in intensity ca. 100 fold at 77 K.

30 Measurements in the near IR identified an emission at λem= 1080 nm with em = 8.5 s,

Figure 2.5. Addition of MV2+ to anaerobic solutions of 2-4 significantly decreases the intensity of the 1080 nm emission but does not shorten its lifetime.

(a) alized Intensity Nor m

310 510 710 910 1110

) Wavelength (nm)

(b) nits (AU nits Arbi t rary U

105 120 135 150 165 180 195 Time (µs)

Figure 2.5 Normalized absorption (solid line) and emission (dotted) spectra for compound 2-4 in CH3CN.

Excitation of 2-4 into the 422 nm band (ε = 37100 M-1cm-1) obtains a short lived

(τ ~ 5 ns) emission with maximum at 450 nm, which is quenched in the presence of

excess of MV2+. Complex 2-4 also has phosphorescent emissions in the NIR at 1271 nm,

em = 210 s, and 1088 nm, em = 20 s, Figure 2.6.

Figure 2.6 Normalized absorption (solid line) and emission (dotted) spectra for 2-4 in CH3OH.

31 Bulk oxidations by flash quench photolysis. Initial flash quench experiments at

room temperature showed that irradiation of 2-4 in CH3CN/CH3OH mixtures with a

large 200-fold excess of the electron-acceptor methyl viologen, [MV]2+, using a Hg lamp

and a 400 nm low-pass filter generates a blue product solution characteristic of reduced

+ [MV] in ca. 15 min; addition of NaOH(aq) to the product solution gives [Ru(bpy)2(ttma- alcohol)]+ 2-2 in ca. 10% yield by ESI-MS analysis, Figure 2.7. Analogous irradiation

experiments using a 400 nm high-pass filter showed no observable changes. Photolysis of

+ [Ru(bpy)2(ttma)] at room temperature in H2O in the absence of an electron transfer agent

at room temperature leads to displacement of the Httma ligand to form the

+ 28-29 [Ru(bpy)2(H2O)(OH)] complex, as previously reported for analogous complexes.

Subsequently, all photolysis experiments using complex 2-4 were conducted at 5 ˚C in

the absence of water, with addition of basic water afterwards.

Figure 2.7 ESI-MS spectrum of a sample generated by photoexcitation of an anaerobic + 2+ solution of [Ru(bpy)2(ttma)] and MV in CH3CN/CH3OH followed the addition of 0.1 + M NaOH. The species formed is [Ru(bpy)2(ttma-aldehyde)] (m/z = 584).

32 Bulk photolysis of CH3CN solutions of 2-4 in the presence of varied potential

electron acceptors Co(NH3)5Cl3 and Ru(NH3)6Cl3 generated color changes indicative of

reduction of the oxidants; the reactions were quenched with the addition of basic water after photolysis. Separation under the chromatographic conditions resulted in

+ + [Ru(bpy)2ttma] and [Ru(bpy)2ttma-aldehyde] eluting at 9.21 and 8.76 min respectively

with the internal standard eluting at 5.64 min. Selected In Monitoring (SIM) was used to

assign Total Ion Current (TIC) peaks to the products separated. Nine-point external and

internal calibration curves showed good linearity with R2>0.99 in all cases, Figure 2.9.

The recovery of materials from the matrix spiked samples averaged 92.9% for

+ + [Ru(bpy)2ttma] and 93.2% for [Ru(bpy)2ttma-aldehyde] . Yields were comparable

between NMR and mass spec as shown in Table 2.6.

Table 2.6 Photolysis yields, % yielda (isolatedb)

Oxidant 1 2 3 MV+2 73 (61.4) 15 12 (7.15) +3 Ru(NH3)6 45 (77.7) 9 34 (19.3) +3 Co(NH3)5 45 (45.7) 16 39 (50.4) a LCMS yields using ca 1.1 fold excess of oxidant b isolated yields using ca. 1.4 excess oxidant, 200 fold for MV2+.

Definitive characterization of the C-H oxidation products was obtained by

chromatographic separations following analogous larger scale reactions over 3 hr. Proton

NMR spectra of the isolated 2-3 was used to confirm the structural assignment by

comparison to the original reported spectra as well as to that of new samples of 2-3

generated by dark oxidation of 2-4 using the organic oxidant DDQ (Figure 2.8). Yields of

33 the alcohol 2-2 were not obtained due to its decomposition during aerobic purification and manipulations.

Figure 2.8 LCMS analysis of product solution after photolysis of 2-1 for in presence of Ru(NH3)6Cl3 at 1:1.1 stoichiometry in CH3CN. (A) Region of the total ion current chromatogram showing product peaks; selective ion mass chromatogram peaks (B) and + + corresponding ion mass spectra (m/z=1) (C) of [Ru(bpy)2ttma-OH] , [Ru(bpy)2ttma=O] , + and [Ru(bpy)2ttma]

34 1.E+00 R² = 0.99869 1.E+00

1.E+00

8.E-01

6.E-01 R² = 0.99610

Peak Area Ratio Area Peak 4.E-01

2.E-01

0.E+00 0 5 10 15 20 25 Concentration (ppm)

+ Figure 2.9 Internal standard calibration curve for determining percent [Ru(bpy)2ttma] + (X) and percent [Ru(bpy)2ttma-aldehyde] (+). Peak area ratio is equal to the ratio of the + sample to internal standard, [Ru(bpy)3]

Figure 2.10 1H NMR spectra of complexes 2-1 – 2-4, free ligands Htma and Httma recorded in CD3CN at room temp. “*” Indicates the doublets from the vinylic hydrogens on the ttma ligand

35 Bulk photochemical oxidation of the homoleptic Zn complex 2-4 was assayed in a

similar manner. Photolysis of 2-4 in the presence of [MV]2+ does not produce any

noticeable color change indicative of irreversible reduction. Photolysis in the presence of

Co(NH3)5Cl, Ru(NH3)6Cl3 and DDQ initiated a change in color of the reaction mixtures

indicating reduction of the oxidants. Analysis of the photolyzed solutions by MS showed

+ a mixture of oligomeric CxHyOz species as the main components (Figure 2.11); the

structures of these species have not been determined.

413.2745 441.3058 831.6039 859.6352 (c ) C19H41O9 C21H45O9 C42H87O15 C44H91O15

Simulation

441.3122 859.6330 z=1 z=1 (b ) 831.6009 4 w/DDQ 413.2798 z=1 z=1

(a ) 441.3117 859.6330 z=1 z=1

4 w/Co+3 413.2798 831.6009 z=1 z=1

100 200 300 400 500 600 700 800 900 m/z

Figure 2.11 MS analysis of soluble products obtained after photo-oxidations of 2-4 in CH3CN

Singlet excited states E00 transition were determined for 2-1 to using the short-

wavelength edges of the emissions estimate the energy between the lowest triplet state

and ground state.30 Using equations 2.1 2.2, the HOMO/LUMO band gap (1ES00) was calculated to be 3.2 V. Excited state oxidation and reduction potentials were calculated to be -2.51 V and 2.08 V respectively. The low energy wavelength of emission band’s was

36 used to approximate the 3ES00 for 2-1 as there were not any observable isosbestic points

observed.

*+2

N 1E00 = 2.08 eV at 388 nm 1E00 = -2.51 eV at 388 nm N O 3 00 S 3 00 E = 0.12 eV ca 1000 nm Ru+2 E = -0.55 eV ca 1000 nm N S N

+1 +3

N N N O 1 00 N O S E = 3.20 eV at 388 nm S Ru+2 Ru+2 3E00 = 1.24 eV ca 1000 nm N S N S N N

+2

N E = -1.17 V N O E = 0.65 V R S o Ru+2 N S N

Scheme 2.3 Thermodynamic cycle with excited state reduction and oxidation potentials of [Ru(bpy)2ttma][PF6]

The E00 for 2-4 were determined in the same manner, where the 1ES00 was

observed to be 435 nm and the 3ES00 approximated to be 1220 nm. A 2.84 V

HOMO/LUMO band gap was calculated for 1ES00 transition and a much lower 1.02 V

band gap was calculated for the 3ES00.

37 *

S O S Zn+2 1 00 S 1 00 E = 1.65 eV at 435 O S E = -1.64eV at 435 3E00 = -0.17 eV ca 1000 nm 3E00 = 0.18 eV ca 1000 nm

1 00 -1 E = 2.84 eV at 435 +1 3E00 = 1.02 eV ca 1000 nm S O S O S S Zn+2 Zn+2 S S O S O S

S O S ER = -1.12 V Zn+2 Eo = 1.2 V S O S

Scheme 2.4 Thermodynamic cycle with excited state reduction and oxidation potentials of Zn(ttma)2

Discussion

Analysis of Ru(II) and Zn(II) Complexes with Dithiomaltol

Photophysical and photochemical behaviors. Previous work demonstrated that outersphere oxidation of 2-1 generated a darkly colored species, which reacted with water or other nucleophiles to give products oxidized at the pendant methyl. The C-H activation of compound 2-1 was proposed to be facilitated by a thiopyrillium tautomer of this oxidized species, effectively a ligand to metal charge transfer as illustrated in Scheme

2.5.31–33 Evidence of aromaticity of the bound ttma ligand was provided by the shift of its two ring protons in 1H NMR spectra, as well as the smaller variance of the C-C bonds within the ring as compared to analogous thiomaltol complexes. We can now use similar comparisons including both the free Httma ligand and the homoleptic Zn complex 2-4.

38 +2+2 +1 S S S H2O Ru+3 S Ru+3 S+ Ru+2 S O O O

HO Scheme 2.5 C-H activation through thiopyrillium intermediate

A downfield shift in the chemical shift for the vinylic ring protons in

thiopyranthiones is known in the literature; for maltol, two doublets appear at 6.45 and

7.73 ppm, for thiomaltol, the signals are at 7.33 and 7.59 ppm, and for dithiomaltol, the

resonances are at 7.52 and 8.16 ppm.20,34,35 The magnitude of ring current in a system can be measured from the average chemical shift, (δa + δb)/2, which is δ 7.75 for 2-1 compared to δ = 8.00 for Httma and 8.01 for 2-4. This suggests that both the free ligand and complex 2-4 have greater ring current and thus more delocalization than 2-1.

Likewise, the difference between C-C bond lengths within the ttma ring should be

minimized by delocalization, and the variance is much larger for 2-1 than for Httma or 2-

4. Thus though aromaticity is clearly a characteristic of the thiopyran-thione moiety in these compounds, it does not correlate with the photochemical reactivity described. Table

2.7 gives the average ppm of the two vinylic protons and the variance in ring C-C bond lengths.

N N - N S h + OH (aq) N S Ru2+ Ru2+ S S N O N O N N Q H+ + Q- R = 2,2’-bipyridine N N R = CH3 (2-1), CHOH (2-2), CHO (2-3)

+ Scheme 2.6 Oxidative quneching reaction of [Ru(bpy)2ttma] .

39 The initiation of this oxidative chemistry outlined in Scheme 2.6 by photo-

excitation is analogous to the well-known [Ru(bpy)3]2+.5,36 Unexpectedly, we find no

reactivity upon excitation into the MLCT absorption of 2-4, but excitation into the ttma- based absorption band does induce similar electron transfer reactivity.

Table 2.7 Aromaticity comparison

a b Compound H1 H2 Havg Avg C-C(Å) C-C(Å) Httma 8.13 7.87 8.00 1.39775 0.03635 2-1 7.97 7.52 7.75 1.40173 0.04652 2-4 8.43 7.58 8.01 1.40675 0.03052 a average of all C-C bonds in thiopyranyl ring b standard deviation of C-C bonds in thiopyranyl ring

The MLCT-like absorption of 2-4 exhibits bands are significantly broader than

2+ that of [Ru(bpy)3] , suggesting strong electronic coupling between the Ru(II) and ttma

ligand but no analogous luminescence is obtained, Figure 2.4. We found excitation into a

band tentatively assigned to the ttma ligand generates a short-lived singlet emission and a

long-lived emission in the near IR, more characteristic of organic triplet

phosphorescence. The homoleptic Zn complex 2-4 was used as a simple test of these

assignments; and excitation of 2-4 into the analogous ttma band at 422 nm did indeed

generate similar fluorescent and phosphorescent emissions. Photo-excitation of Httma

alone yields no products, thus coordination to a metal center is essential for the observed

electron transfer chemistry to take place.

The possible involvement of trace oxygen contamination was also investigated.

Both compounds 2-1 and 2-4 are efficient sensitizers of singlet oxygen; compound 2-1

also functions as an efficient singlet oxygen desensitizer. These may be examples of

40 singlet oxygen sensitization by a simple Forster mechanism, i.e. by an

emission/absorption overlap. Excitation of 2-1 under air generates no observable yields of

the C–H activation products 2-2 or 2-3, indicating that singlet oxygen itself is not

1 37 involved in the C–H activation process, consistent with the known reactivity of O2.

Thus excitation of 2-1 produces a photostate which is capable of irreversibly

reducing [MV]2+ (E0′ = −440 mV NHE) but not DTDP]2+ (E0′ = −691 mV). This highly

reducing photostate must be relatively short-lived, as any species capable of reducing

2+ 3+ 2+ [MV] should quantitatively reduce both [Ru(NH3)6] and [Co(NH3)5Cl] . The yields

for oxidative flash quench experiments of compound 2-1 are shown in Table 2.8. Also notable is that an excess of [MV]2+ effectively quenches the emissions of the homoleptic

Zn adduct 2-4 but does not irreversibly oxidize it, as occurs in analogous photolysis

3+ 2+ experiments with [Ru(NH3)6] and [Co(NH3)5Cl] quenchers. Thus complex 2-1 is a more powerful photo-reductant than 2-4, roughly following the observed ground state reduction potentials which are assigned to the ttma moiety in these species.

Table 2.8 Potential vs. photolysis yields

E0’ Zna Oxidant Yield (%) (V NHE) (+/-) MV2+ -0.440 b 27 - 3+ c Ru(NH3)6 -0.159 43 + 2+ c Co(NH3)5Cl 0.341 55 + a Photoreduction by 4 indicated by +/-. b reference 35, c reference 30

The thermodynamic cycle for 2-1 (Scheme 2.3) shows ground state electrochemical oxidation and reduction potentials in conjunction with excited state

41 oxidation and reduction potentials. From such cycle, a comparison can be made between

the oxidizing/reducing strength of the excited states versus the ground states. Scheme 2.3

presents the data for 2-4 in the same manner, data summarized in Table 2.9. From these

values, the excited state of 2-1 is predicted to be competent to reduce [MV]2+ while that

of 2-4 is not, as is observed. But similarly, the triplet excited state of 2-4 should not

3+ reduce [Ru(NH3)6] , counter to experiments herein. As previously noted, the addition of

[MV]2+ to anaerobic solutions of 2-1 prior to photo-excitation significantly decreases the

intensity of the near IR emission without shortening its lifetime. This implies that once

formed, the emissive state is not involved in the electron transfer, i.e., the photo-state

which reduces [MV]2+ must precede the phosphorescent state.

Table 2.9 Calculated absorption energies and excited state reduction and oxidation potentials vs. NHE

00 00 O R Complex E (nm) E (eV) E* (V NHE) E* (V NHE) 2-1 387.5 3.20 -2.51 2.08 ca. 1000 1.24 -0.55 0.12 2-4 435 2.84 -1.64 1.65 ca. 1220 1.02 0.18 -0.17

Thus, the nature of the reductive excited states of compounds 2-1 and 2-4 remain obscure. Certain aromatic thiones demonstrate unusual phosphorescence,29,36,38–40 for example T2 emissions have been described for uncomplexed aromatic thiones, as well as

39–43 T1–T2 energy inversion caused by interactions of the excited state with solvent. A thione similar to Httma, 3-hydroxyflavothione demonstrates a long-lived emission when coordinated to transition metals with paired electrons,44,45 very similar to the behavior described here.

42 Conclusions

The dithiomaltol complexes 2-1 and 2-4 undergo photo-oxidation in the presence of electron acceptors; the reactivity stems from excitation of the ttma-based transitions.

Coordination to a metal center is essential for the observed electron transfer chemistry to

take place, as photo-excitation of Httma alone yields no products. Attempts to identify

the obscure transitions responsible concluded that a dark/non-emissive state, with a

tunable excited-state oxidation potential, is dependent on the coordination chemistry.

These results suggest the use of hetero-substituted maltol derivatives for applications

which do not rely on luminescent intermediates for photo-induced electron transfers.

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46 CHAPTER THREE

Synthesis and Photophysical Studies of Pt(II)(bipyridyl) Complexes with Heteroatom- Substituted Maltol Derivatives: Long-lived Room Temperature Luminescence

Introduction.

Square Planar Platinum(II) Complexes

Eisenberg complexes. The Eisenberg group first described the photochemistry of

II square planar Pt (bpy)Ln complexes, which arise from an unusual metal-assisted ligand-

1–3 to-ligand charge transfer (LLCT). As shown in Scheme 3.1, the ancillary ligand Ln acts as an electron donor, typically dithiols or acetylides, while the bipyridine acts as electron

2+ acceptor, as it does in the well characterized [Ru(bpy)3] complex and related

II luminophores. The Pt (bpy)Ln complexes demonstrate exceptionally long lived

emissions, quite rare for d8 square planar complexes known at that time. Subsequently, a number of Pt(bpy)(electron-donor) complexes have been reported with emissions in the range of 540-650 nm with lifetimes varying from nanosecond at room temperature to microsecond when cooled to 77 K.4–7 Aromatic dithiolate donors demonstrate some of

the longest lived (µs) excited state lifetimes for this luminophore motif.6,8–12 Alternating

the chelating atoms (O,O, O,S) and the in-ring heteroatom of the electron-donor allows

tuning of the photophysical and photochemical properties, such as HOMO/LUMO band

gaps, excited state lifetimes, near IR emissions, and singlet-oxygen sensitization.13–15

47 e- hv

N Cl X Ethylene Glycol N X Pt + Pt N Cl Y  N Y

Electron Electron Acceptor Donor

Scheme 3.1 General synthesis and structure of Pt(bpy)(electron-donor) complexes.

II In this work, a series of Pt (bpy)Ln complexes have been synthesized using thiomaltol (Htma), dithiomaltol (Httma) and hydroxypyridinthione (Hopto) ligands,

Scheme 3.2. Previous reports have shown these hydroxythiones to be strong metal chelators for biomedical applications.16–19 As previously noted, unusual photoreductive chemistry by Ru(II) and Zn(II) complexes of dithiomaltol suggests the reactivity resulted from pseudo-aromaticity within the hetero-substituted hydroxypyridthione ring.20 The complexes [Pt(bpy)tma][PF6] (3-1), [Pt(bpy)ttma][PF6] (3-2), and [Pt(bpy)(hopto)][PF6]

(3-3) which represent the first Pt(bpy)(electron-donor) complexes with mixed O,S atom chelation and as will be shown, they exhibit some of the longest luminescence lifetimes for this structural motif.

S S S O O O OH OH OH OH OH OH

O S N O N N Ph Htma Httma Hopto Maltol Deferiprone Hppp

Scheme 3.2 Mixed, O,S and O,O chelating ligands

48 Experimental

Materials and Methods

Reagents. Compounds such as 3-hydroxy-2-methyl-4H-pyran-2-one (maltol,

Aldrich), Lawesson’s reagent, (2,2’-bipyridine)dichloroplatinum(II), (Alfa Aesar), and

phosphorous pentasulfide (P4S10) (Acros) were used as received from vendors. 3-

hydroxy-2-methyl-4H-pyran-4-thione (thiomaltol, Htma), 3-hydroxy-2-methyl-4H-

thiopyran-4-thione (dithiomaltol, Httma), and 3-hydroxy-1,2-dimethylpyridine-4(1H)-

thione (Hopto) were synthesized by previously reported methods.16–18,21 Acetonitrile

(CH3CN, Fischer) and dichloromethane (CH2Cl2, Fischer) were purified and dried on

glass columns under argon. Ethylene glycol (BDH) was degassed before each use.

Physical measurements. Electronic absorption, steady-state luminescence,

relative quantum yields,22 self-quenching, and steady-state Stern-Volmer experiments

were carried out in degassed, anhydrous CH3CN on a HP Agilent 8453 UV-Visible

spectrophotometer and a Hitachi Fluorescence Spectrophotometer F-7000. Self-

quenching experiments for emission intensity maxima as a function of concentration (10-

4–10-5 mol/L) were used for room temperature emission lifetime studies and measured in

CH3CN. Low-temp lifetimes (77 K) were measured in a dimethylformamide-methylene

chloride-methanol (DMM) solution on a PTI Spectrofluorometer with steady-state, nano-

and micro-second lifetime accessories at the Omary Lab. Elemental analysis of each

sample was performed by Atlantic Microlab Inc.

49 Electrochemistry. Redox potentials were measured by cyclic voltammetry (CV) under anaerobic conditions using a CHI-760B potentiostat in dry-degassed CH3CN with

0.1 M tetrabutylammonioum hexafluorophosphate (TBAPF6) as the supporting electrolyte. The cells consisted of a Pt-disk working electrode, AgCl coated Ag reference wire, and a coiled Pt wire auxiliary electrode. All electrochemical measurements were corrected using the ferrocene/ferrocenium couple (Fc/Fc+ = 642 mV vs. NHE). 1H NMR

(499.78 MHz) spectra were recorded on a Varian 500 NMR System in deuterated- acetonitrile (CD3CN), unless noted otherwise. Accurate masses were resolved by electrospray ionization mass spectrometry (ESI-MS) on a Thermo Scientific LTQ

Orbitrap Discovery in methanol (CH3OH) solutions of 1-10 ppm.

Photophysical studies. Lifetime studies were performed on solutions in deuterated-acetonitrile (CD3CN) with absorbances between 0.05–0.3 AU at the 355 nm excitation wavelength. The laser pulse energy was 1–2.5 mJ at 355 nm. Room temperature emission lifetimes were measured in CH3CN while low-temp lifetimes (77

K) were measured in a dimethylformamide-methylene chloride-methanol (DMM) solution on a PTI Spectrofluorometer with steady-state, nano- and micro-second lifetime accessories.

1 Singlet oxygen. Measurements of O2 quantum yields and luminescence

1 quenching rates of O2 were determined by the Selke Lab using a time-resolved Nd:YAG laser set-up (excitation pulse duration 4–6 ns at 355 nm and 5–7 ns at 532 nm, Minilase

II, New Wave Research Inc.) and a liquid N2 cooled Ge photodetector (Applied Detector

Corporation Model 403 S). A Schott color glass filter (model RG850, cut-on 850 nm,

50 Newport, USA) was taped to the sapphire entrance of the detector to block ultraviolet and

visible radiation. The port opening to the detector contained a long wave pass filter

(silicon filter model 10LWF ~ 1000, Newport, USA) which transmits in the range of

1100–2220 nm and blocks in the range of 800–954 nm, a band pass filter (model BP-

1270–080-B*, CWL 1270 nm, Spectrogen, USA) which blocks in the UV, visible, and IR

regions and only transmits in the range of 1200 to 1310 nm with a maximum

transmission of 60% at 1270 nm. Signals were digitized on a LeCroy 9350 CM 500 MHz

oscilloscope and analyzed using Origin software. All experiments were carried out at

ambient temperature and air. UV-visible spectra were recorded on a Cary 300 Bio

Spectrophotometer (Varian).

The absorbance of the reference sensitizer (perinaphthenone) and the PtII

1 complexes were matched within 80%. The initial O2 intensity was extrapolated to t = 0.

The data points of the initial 0–5 s were not used due to electronic interference signals

1 from the detector. The quenching rates of O2 were measured by Stern–Volmer analysis

II using methylene blue as sensitizer at 532 nm in (CD3CN). The concentration of the Pt complexes used in the measurements ranged between 0.01–0.6 mM.

Experiments with (2,2’-Bipyridine)Platinum(II) and Thiolated Chelates (O,S)

Synthesis of thiomaltol, 3-hydroxy-2-methyl-4-pyranthione, Htma. In a glove box

3.54 g maltol (0.0280 mol) and 3.80 g P2S5 (0.00855 mol) were measured and removed in

a sealed flask. Hot, dry dioxane was added to the flask and refluxed under N2 for 2.5 hr.

250 mL of DI H2O was added to the dark green solution and stored in refridgeration over

night. Filtration gave yellow powder/crystalline mixture, 1.58g (39.7%). ESI MS: m/z,

51 1 NMR H NMR (500 MHz, CDCl3) δ 7.77 (s, 1H), 7.58 (d, J = 5.0 Hz, 1H), 7.32 (d, J =

-1 -1 5.0 Hz, 1H), 2.45 (s, 3H). UV-vis: CH3CN, ƛmax (ε in M •cm ) 206 (ε =12,000), 275 (ε

=14,000), 359 (ε =19,000).

Synthesis of 3-hydroxy-2-methyl-4H-thiopyridinone (HOPTO). Thiomaltol, 0.502 g (3.98 mmol) was dissolved into 20.0 mL of deionized water and degassed. Then, 3.5 mL MeNH3 (40% in water) was added by syringe resulting in an amber solution. The colored solution was then refluxed under anaerobic conditions for 54 hours. Cooling to room temperature resulted in a red precipitate collected by filtration, 0.432 g (70.0 %

1 yield). H NMR (500 MHz, CDCl3) δ 8.77 (s, 1H), 7.47 (d, J = 6.7 Hz, 1H), 7.11 (d, J =

-1 -1 6.7 Hz, 1H), 3.78 (s, 3H), 2.50 (s, 3H). UV-vis: CH3CN, ƛmax (ε in M •cm ) 213 (ε =

2,790), 257 (ε = 1,630), 349 (ε = 4,210).

Synthesis of (2,2’-bipyridine)thiomaltolatoplatinum(II) hexafluorophosphate,

[Pt(bpy)(tma)][PF6], 3-1. A 0.127 g (0.300 mmol) sample of Pt(bpy)Cl2 and 0.0433 g

(0.304 mmol) thiomaltol were dissolved in 5.0 mL ethylene glycol and refluxed for 30

min. The mixture was then poured into a solution of 200 mL DI H2O and KPF6 1.10 g

(5.98 mmol). The orange-red solid was collected after refrigeration (2 hr) and washed with DI H2O. Purification by chromatography on basic alumnia with CH3CN/CH2Cl2 as the eluent (1:1) yielded 0.126 g (0.197 mmol, 64.8% based on Pt). Product was

1 characterized by H NMR (500 MHz, CD3CN) δ 8.90 (d, J = 4.9 Hz, 1H), 8.70 (d, J = 5.6

Hz, 1H), 8.25 (td, J = 7.8, 1.5 Hz, 1H), 8.16 (m, 2H), 8.03 (d, J = 4.7 Hz, 1H), 7.72 (ddd,

J = 7.3, 5.6, 1.4 Hz, 1H), 7.47 (d, J = 4.7 Hz, 1H), 7.42 (ddd, J = 7.7, 5.8, 2.0 Hz, 1H),

+ -1 -1 2.66 (s, 3H), ESI-MS m/z 492 ([M] ). UV-vis: CH3CN, max ( = M •cm ) 268 (22,000),

52 309 (10,600), 321 (12,000), 340 (6,340), 430 (13,300); Anal. for [C16H14N2O2PtS][PF6]: calculated(found) C, 30.15(31.09): H, 2.06 (2.09): N, 4.39 (4.29).

Synthesis of (2,2’-bipyridine)dithiomaltolatoplatinum(II) hexafluorophosphate,

[Pt(bpy)(ttma)][PF6], 3-2a. A 0.0988 g (0.234 mmol) sample of Pt(bpy)Cl2 and 0.0452 g

(0.286 mmol) dithiomaltol were dissolved in 5.0 mL ethylene glycol and refluxed for 30

min with color change from yellow to red. The mixture was then poured into 200 mL of

DI H2O and 0.557 g (3.03 mmol) of KPF6. The new solution was allowed to stand in an

ice bath, forming a purple precipitate that was collected as a red-orange solid and washed

with DI H2O. Purification by chromatography on basic alumnia with CH3CN/CH2Cl2 as the eluent (1:1) yielded 0.122 g (0.186 mmol, 79.5% based on PtII). 1H NMR (500 MHz,

(CD3)2SO) δ 8.72 (d, J = 5.2 Hz, 1H), 8.57 (d, J = 5.5 Hz, 1H), 8.44 (d, J = 9.0 Hz, 1H),

8.36 (d, J = 8.1 Hz, 1H), 8.28 (t, J = 7.8 Hz, 1H), 8.15 (t, J = 7.8 Hz, 1H), 8.06 (d, J = 9.0

Hz, 1H), 7.75 (t, J = 6.3 Hz, 1H), 7.42 (t, J = 6.6 Hz, 1H), 2.60 (s, 3H), ESI-MS m/z 508

+ -1 -1 ([M] ), UV-vis: CH3CN, max( = M •cm ) 247 (15,800), 268 (18,800), 307 (9,970), 320

(4,380), 458 (15,500); Anal. for [C16H14N2OPtS2][PF6]: calculated(found) C,

29.41(29.20), H, 2.01(1.98), N, 4.29(4.17).

Synthesis of (4,4’-diterbutyl-2,2’-bipyridine)dithiomaltolplatinum(II)-

hexafluorophosphate (3-2b). A mixture of 4,4’-di-tert-butyl-bipyridineplatinum(II)

dichloride (102.1 mg, 0.1909 mmol) and dithiomaltol (32.0 mg, 0.202 mmol) were

combined into a round bottom flask. 10.0 mL of ethylene glycol, degassed, was added to

the mixture and stirred. The suspension was set to reflux under N2 for 6 hrs. The

reaction’s mixture was poured into a 200 mL solution of 0.2 M KPF6 and stored

53 overnight at 1.1˚C. The red-orange precipitate was filtered off and washed with 3 x 10

mL portions of DI H2O. Purification by column chromatography on silca with 5% MeOH

in DCM yielded 50.6 mg (0.0661 mmol, 34.6 %, based on PtII). 1H NMR (500 MHz,

CD3CN) δ 9.00 (d, J = 5.9 Hz, 1H), 8.69 (d, J = 6.2 Hz, 1H), 8.31 (s 1H), 8.25 (s, 1H),

8.12 (m, 2H), 7.83 (d, J = 6.04 1H), 7.51 (d, J = 6.1 Hz, 1H), 2.74 (s, 3H), 1.49 (s, 9H),

1.44 (s, 9H).

Synthesis of (2,2’-bipyridine)(3,4-HOPTO)platinum(II) hexafluorophosphate,

[Pt(bpy)(HOPTO)][PF6] 3-3. A sample of 0.225 g of Pt(bpy)Cl2 (0.549 mmol) was

added to a solution of 3,4-HOPTO, 0.0826 g (0.532 mmol) dissolved in 5.0 mL of

ethylene glycol and refluxed. After refluxing for 30 min, 3.51 g KPF6 (19.1 mmol) were

dissolved in 300 mL DI H2O and added to the reaction solution. A brown precipitate was collected by filtration and washed with DI H2O. Purification by chromatography on basic alumnia with CH3CN/CH2Cl2 as the eluent (1:1) yielded 0.230 g (0.354 mmol, 64.5%

II 1 based on Pt ). H NMR (500 MHz, CD3CN) δ 9.56 (d, J = 5.8 Hz, 1H), 9.04 (d, J = 5.5

Hz, 1H), 8.72 (d, J = 5.6 Hz, 1H), 8.18 (m, 2H), 7.72 (m, 1H), 7.62 (m, 1H), 7.46 (d, J =

6.6 Hz, 1H), 7.40 (td, J = 5.7, 3.3 Hz, 1H), 7.28 (d, J = 6.4 Hz, 1H), 3.93 (s, 3H), 2.61 (s,

+ -1 -1 3H), ESI-MS m/z 505 ([M] ), UV-vis: CH3CN, max ( = M •cm ) 277 (9,560), 309

(6,660), 316 (5,990), 360 (3,730), 440 (2,360), Anal. for [C17H17N3OPtS][PF6]: calculated(found) C, 31.39(32.85), H, 2.48(2.93), N, 6.46(6.29).

54 Experiments with (2,2’-Bipyridine)Platinum(II) and Oxygen Chelates (O,O)

Synthesis of 3-hydroxy-2-methyl-1-phenyl-4-pyridinone, Hppp. Maltol (4.0331 g,

0.03198 mol) and aniline (6.3631 g, 0.06833 mol) were combined into a 250 mL round-

bottom-Schlenk flask and diluted with 100 mL of 1% HCl(aq). The mixture was sealed

and set in a heated silicone oil bath for 3 days at 120˚C. The off-white precipitate was

separated by filtration and decolorized with black carbon in hot-methanol. The slurry was then filtered through a glass frit. Isolation by crystallization yielded 1.7977 g (8.9384 mmol, 25.88%). 1H NMR (500 MHz, acetone) δ 7.60 (m, 3H), 7.51 (d, J = 7.3 Hz, 1H),

7.48 (m, 2H), 6.27 (d, J = 7.3 Hz, 1H), 2.04 (s, 3H).

Preparation of diaqua(2,2’-bipyridine)platinum(II) nitrate, [Pt(bpy)

(H2O)2][(NO3)2]. To a vigorously stirred flask of 2,2’bipyridineplatinum(II)chloride

(0.513 g, 1.22 mmol) in degassed nano-pure water (144 mL), 2.29 molar equivalents of

silver nitrate (0.467 g, 2.78 mmol) was added. The suspension was stirred under N2 for

14 hr at room temperature forming a green/grey slurry. The suspension was filtered three times through a celite plug to remove silver chloride. The mixture was used as collected for synthesis of (2,2’-bipyridine)(maltolato)platinum(II)hexafluorophosphate.

Synthesis of (2,2’-bipyridine)(maltolato)platinum(II)hexafluorophosphate,

[Pt(bpy)(ma)][PF6] 3-4. To the 8.44 mM solution of Pt(bpy)(NO3)2 • 2H2O (0.607

mmol) previously prepared, >1 molar equivalent of maltol (532.1 mg, 4.22 mmol) and

triethylamine (0.625 mL, 4.48 mmol) dissolved in nano-pure H2O (72.0 mL) and added.

After stirring for 12 hr, 1.42 g KPF6 (7.71 mmol) dissolved into 500 mL DI H2O and

55 added to the reaction mixture and refrigerated for 4 hr. Filtration and purification on

1 alumina with MeCN/H2O collected 140 mg of a bright red solid, 37.1 % Yield. H NMR

(500 MHz, (CD3)2CO) δ 9.07 (dd, J = 16.0, 5.3 Hz), 8.57 (d, J = 7.6 Hz), 8.47 (d, J = 7.9

Hz), 8.31 (d, J = 5.1 Hz), 7.03 (d, J = 5.1 Hz), 2.67 (s), ESI-MS m/z 476 ([M]+), UV-vis:

-1 -1 CH3CN, max ( = M •cm ) 256 (4,020), 304 (1,760), 316 (1,700), 348 (870), 397 (941),

Synthesis of 2,2’-bipyridine(3-hydroxy-1,2-dimethyl-4-pyridinone)platinum(II)

hexafluorophosphate, [Pt(bpy)(HOPO)][PF6] 3-5. A solution of Pt(bpy)(NO3)2 • 2H2O

(0.241 mmol) and, triethylamine, 0.40 mL, and 40.1 mg of HOPO were combined and stirred for 12 hr. The reaction was then added to a solution of 317 mg KPF6 (2.02 mmol)

dissolved into 200.0 mL DI H2O and allowed to precipitate in the refrigerator. Filtration

1 afforded 58.0 mg, 54.3 % yield. H NMR (500 MHz, CD3CN) δ 8.95 (d, J = 5.8 Hz, 1H),

8.85 (d, J = 4.7 Hz, 1H), 8.26 (t, J = 7.9 Hz, 2H), 8.17 (d, J = 8.0 Hz, 2H), 7.64 (m, 2H),

6.95 (d, J = 2.1 Hz, 1H), 6.87 (s, 1H), 2.51 (s, 3H), 2.19 (s, 3H), 13C NMR (125 MHz,

CD3CN) δ 213.45, 155.55, 150.95, 148.49, 146.40, 141.39, 141.12, 133.44, 132.66,

128.04, 127.89, 124.04, 123.80, 122.78, 105.32, 23.84; ESI-MS m/z 489 ([M]+), UV-vis:

-1 -1 CH3CN, max ( = M •cm ) 239 (30,600), 258 (32,300), 301 (17,000), 312 (16,200), 423

(5,260).

Synthesis of 2,2’-bipyridine(3-hydroxy-2-methyl-1-phenyl-4-pyridinone)

platinum(II)hexafluorophosphate, [Pt(bpy)(hppp)][PF6] 3-6. To an 8.44 mM solution of

Pt(bpy)(NO3)2 (0.607mmol), >1 molar equivalent of 3-hydroxy-2methyl-1-phenyl-4-

pyridinone (hppp, 901,8 mg, 4.48 mmol) and triethylamine (0.625 mL, 4.48 mmol)

dissolved in nano-pure H2O (72.0 mL) and added. After stirring for 12 hr, 1.42 g KPF6

56 (7.71 mmol) dissolved into 500 mL DI H2O and added to the reaction mixture,

refrigerated for 4 hr. Filtration collected a dark red solid, 226 mg, 58.6 % yield. 1H NMR

(500 MHz, CD3CN) δ 8.75 (dd, 16.1 ,5.7 Hz, 2H), 8.20 (ddd, J = 16.1, 8.2, 1.5 Hz, 2H),

8.07 (dd, J = 7.7, 3.1 Hz, 2H), 7.65 (m, 3H) 7.58 (dtd, 25.31, 5.66, 1.33 Hz, 2H), 7.49 (d,

J = 6.9 Hz, 1H), 7.44 (ddd, J = 5.5, 3.4, 1.5 Hz, 2H), 6.61 (d, J = 6.8 Hz, 1H), 2.14 (s,

13 3H), C NMR (125 MHz, CD3CN) δ 207.34, 176.03, 160.60, 156.90, 156.60, 156.34,

149.50, 149.31, 141.32, 140.28, 140.01, 137.26, 134.79, 130.41, 130.16, 128.02, 127.83,

+ - 126.59, 123.75, 123.63, 110.31, ESI-MS m/z 551 ([M] ), UV-vis: CH3CN, max ( = M

1•cm-1) 257 (18,600), 303 (8,240), 316 (8,030), 422 (4,280).

Results

Studies with (2,2’-Bipyridine)Platinum(II) and Thiolated Chelates (O,S)

Synthesis and characterization of 3-1 – 3-3. The ligands Htma, Httma, and Hopto

(Scheme 3.2) were synthesized from methods in reported literature.16,24–26 The syntheses

of Pt-complexes 3-1 – 3-3 were adapted from previous reports.6,9–11,27–29 The ligands were

mixed with stoichiometric Pt(bpy)Cl2 or Pt(tbu-bpy)Cl2 and refluxed in ethylene glycol

until a color change was observed. The reaction mixture was diluted in water, and the Pt-

containing products precipitated as hexafluorophosphate salts which are air stable for

extended periods. Crude products were separated by column chromatography and the

purified samples characterized by 1H NMR spectra, ESI-MS, and elemental analysis, and

13C NMR spectra where elemental analysis is unavailable.

57 NMR Characterization. The conversion of a pyrone to thiopyrone shifts the

vinylic proton signals in 1H NMR spectra downfield, implying increased aromaticity of

the heterocyclic system.30 A measure of the aromaticity is obtained using Eq. 3.1, the

averaged chemical shifts of vinylic ring protons. As seen in Figure 3.1, the apparent

aromaticity of compounds vary with 3-2 > 3-1 > 3-3, with average vinylic chemical shifts in ppm of 8.32, 7.76 and 7.30 respectively. By this measure, all three complexes with the maltol-derivatives have aromaticity greater than that of toluene-3,4-dithiolate in the

II 6 classic Eisenberg Pt (bpy)Ln complex, with an average of 6.83 ppm.

2⁄ 3.1

8.59.09.5 8.0 7.5 6.57.0 ppm

1 Figure 3.1 H NMR spectra (CD3CN) of 3-1 (top), 3-2 (middle) and 3-3 (bottom) from 9.5 ppm – 6.0 ppm.

Redox properties. Cyclic voltammograms of complexes 1-3 in CH3CN solution

are shown in Figure 3.2, and the determined reduction potentials are given in Table 3.1.

58 In general, voltammograms for all three complexes show reductions attributable to both

the heterosubstituted maltol and the bipyridine ligands.31

Previously reported Pt(diimine)(dithiolate) complexes show irreversible oxidations ca. 0.6 V NHE, which were attributed to Pt-centered oxidation.6,10 The oxidations observed for complexes 3-1 – 3-3 are decidedly more positive, between 1 to 2

V vs. NHE, but assignment of ligand- or metal-based is difficult.

The reductions of the maltol-derived ligands are electrochemically irreversible with following oxidation waves, likely due to structural changes upon reduction. For instance, the cyclic voltammogram of 3-1 shows a small oxidation current at ca. -0.4 V, which is only seen after sweeping through the reductive peak at -0.79 V. Similar irreversible reductive and following oxidative waves were observed in voltammograms of

20 Httma, Zn(ttma)2 and (ttma)Ru(bpy)2(PF6). The more negative current waves are

attributable to reduction of the Pt-coordinated bipyridines, as have been previously reported in analogous compounds. For complex 3-3, the irreversible maltol-based reduction appears to overlap that of the bipyridine, ca. -0.92 V.

Table 3.1 Redox potentials of compounds 3-1 – 3-3

cmp EOx (V NHE) L ERed (V NHE) Bpy (Ep) -0.79ir 3-1 1.94ir -1.09rv (0.143) -0.18f -0.70ir 3-2a 1.76ir -1.15rv (0.061) -0.47 1.37ir 3-3 -0.92ir -0.80rv (0.079)

Annotations: rv = chemically reversible, ir = irreversible, f = following wave. All potentials referenced to Fc/Fc+ at 642 mV vs. NHE.

59 2.0 1.0 0.0 -1.0 -2.0 E (V vs. NHE)

Figure 3.2 Cyclic voltammograms of complexes 3-1 (top), 3-2a (middle) and 3-3 -1 (bottom) under anaerobic conditions in CH3CN and 0.1 M TBAPF6, v = 100 mV s

Photochemical characterization. Electronic absorption spectra of compounds 3-1

– 3-3 in CH3CN, shown in Figure 3.3-6, have identifiable bands attributable to both the bipyridine and maltol-derived ligands, from 250-400 nm, as well as characteristic LLCT bands between 400-500 nm. The peak absorption wavelengths and tentative assignments are given in Table 3.2. The wavelengths of absorption assigned to the maltol-derived ligand in the complexes vary in similar order as their free ligand absorptions, with 3-1 <

3-2a < 3-3, while those assigned to the LLCT follow as 3-1 < 3-3 < 3-2a, with compound

3-2a having a much weaker and broader LLCT absorption.

60 Normalized Intensity Normalized Intensity 250 350 450 550 650 750 Wavelength (nm) Figure 3.3 Room temperature UV-vis absorption of 3-1 (–-) & emission (---) Normalized Intensity Normalized 250 350 450 550 650 750 Wavelength (nm) Figure 3.4 Room temperature UV-vis absorption of 3-2 (–-) & emission (---) Normalized Intensity

250 350 450 550 650 750 Wavelength (nm) Figure 3.5 Room temperature UV-vis absorption of 3-2b (–-) & emission (---)

61 NormalizedIntensity

250 350 450 550 650 750 Wavelength (nm)

Figure 3.6 Room temperature UV-vis absorption of 3-3 (–-) & emission (---)

Table 3.2 Electronic absorption maxima of complexes 3-1 - 3-3

3 -1 -1 # ƛmax (nm) ε (10 M •cm ) assignment 3-1 268 22.0 bpy 340 6.34 tma 430 13.3 LLCT 3-2a 268 18.9 bpy 355 4.38 ttma 458 15.5 LLCT 3-3 277 9.56 bpy 360 3.73 hopto 442 2.36 LLCT

For solutions of all three complexes studied, excitation into the low energy LLCT band gave a strong emission at room temperature, shown in Figure 3.3-6, with data listed in Table 3.3. The quantum yields and Stokes shifts varied in the order of 3-2a > 3-1 > 3-

3, which also follows the pyranthione and pyridthione aromaticity as determined by

NMR. The excited state lifetimes reported in Table 3.2 were attained from purified samples in airtight cuvettes while minimizing emission-quenching contaminants; plots of the emission decay are given in Figure 3.8, Figure 3.10, Figure 3.12, Figure 3.14, Figure

62 3.17, Figure 3.19. The emission of 3-2a though significantly weaker than the others, is still easily observable at room temperature. The emission lifetimes measured in frozen

DMM glass (DMF/CH2Cl2/CH3OH, 1:1:1 by volume) at 77K were significantly longer

and essentially identical for all three complexes. Arbitray Units (AU) (AU) Arbitray Units

300 400 500 600 700 Time (µs) Figure 3.7 Room temperature excitation/emission (–/(– –) spectra of 3-1, 430 nm/630 nm at 2.2 × 10-5 M Arbitray Units (AU)

90 110 130 150 170 190 Time (µs) Figure 3.8 Room temperature lifetime of 3-1 (–-) & fit (---)

63 Arbitray Units (AU) (AU) Units Arbitray

300 400 500 600 700 Time (µs)

Figure 3.9. Low temperature (77K) excitation/emission (–/(– –) spectra of 3-1, 430 nm/630 nm at 2.2×10-5 M in DMM Arbitray Units (AU) (AU) Units Arbitray

90 190 290 390 Time (µs)

Figure 3.10 Low temperature (77K) lifetime of 3-1 (–-) & fit (---) in DMM Arbitray Units (AU) (AU) Units Arbitray

400 500 600 700 800 Wavelenth (nm)

Figure 3.11 Room temperature excitation/emission (–/(– –) spectra of 3-2a, 460 nm/690 nm at 8.4×10-5 M in DMM

64 Arbitray Units (AU) Arbitray Units

90 110 130 150 170 190 Time (µs)

Figure 3.12 Room temperature lifetime of 3-2a (–-) & fit (---) in DMM Arbitray Units (AU)

400 500 600 700 800 Wavelenth (nm)

Figure 3.13 Low temperature (77K) excitation/emission (–)/(– –) spectra of 3-2a, 458 nm/645 nm at 8.4×10-5 M in DMM Arbitray Units (AU) Arbitray Units

90 190 290 390 Time (µs) Figure 3.14 Low temperature (77K) lifetime of 3-2 (–-) & fit (---) in DMM

65 Arbitrary Units (AU)Units Arbitrary

350 650 950 1250 Wavelength (nm)

Figure 3.15 Room temperature visible excitation/emission (–/– –) and near IR emission (– –) of 3-2b in DMM Arbitray Units (AU)ArbitrayUnits

350 450 550 650 750 Wavelenth (nm)

Figure 3.16 Room temperature excitation/emission (–/– –) spectra of 3-3, 440 nm/630 nm at 2.6×10-4 M in DMM ArbitrayUnits (AU)

100 120 140 160 180 200 Time (µs)

Figure 3.17 Room temperature lifetime of 3-3 (–-) & fit (---) in DMM

66 Arbitray Units (AU) (AU) Units Arbitray

350 450 550 650 Wavelenth (nm)

Figure 3.18 Low temperature (77K) excitation/emission (– –/ –) spectra of 3-3, 435 nm/600 nm at 2.6×10-4 M in DMM Arbitray Units (AU)

90 190 290 390 Time (µs)

Figure 3.19 Low temperature (77K) lifetime of 3-3 (–-) & fit (---) in DMM

Table 3.3 Excitation and emission peak wavelengths, lifetimes (τ) at 298 K and 77 K, and quantum yield (φF) for complexes 3-1, 3-2a and 3-3

Ex. Em. τ (µs) a τ (μs) # φF (nm) (nm) 298 K 77 K 3-1 430 630 1.7 0.021 44 3-2 460 690 2.7 0.068 46 3-3 450 640 0.76 0.050 44

The room temperature fluorescence quantum yields for each complex were

2+ measured by comparing the emission spectra to [Ru(bpy)3] with a known fluorescence

67 quantum yield (φF = 8 %) and are reported as derived from Eq. 3.3 as relative quantum yield (F), where A is the absorbance at excitation, F is the area under the emission

curve, and n is the refractive index of the solvents used. Subscripts s and x refer to the

standard and unknown.

Quenching of luminescence. Initial emission studies of complexes 3-1 – 3-3

displayed wide variability in lifetimes and intensities. For instance, emissions were

dramatically reduced by exposure to air. Samples capped with standard plastic, silicon-

lined lids for fluorescence cells also exhibited variable emission intensities. Reproducible

data were obtained by using samples contained in high-throughput quartz tubes prepared

in a glovebox, capped with PTFE lids, and then sealed with wax.

Systematic quenching studies were first undertaken using methylviologen

2+ ([MV] ). Plots of the relative emission intensity (I0/IQ) as a function of methylviologen

2+ concentration [MV] , obtained the Stern-Volmer constant, Ksv, calculated from Eq. 3.2

and reported in Table 3.4. Luminescence intensity ratios increased linearly with

2+ -5 -1 -1 increasing [MV ] with 3-3 having the largest Ksv, at 2.2 x 10 M •s .

Table 3.4 Stern-Volmer constants (Ksv) calculated for quenching of complex 3-1 – 3-3 luminescence by [MV]2+

5 -1 -1 # Ksv (10 M s ) 3-1 1.2 3-2 1.9 3-3 2.2

68 Figure 3.20 Quenching of 3-1, 3-2a, and 3-3 luminescence by methyl viologen

⁄ 1 3.2

Self-quenching was also investigated using plots of concentration (mol/L) vs. emission intensity (AU). All three complexes show self-quenching above ca. 90 uM

(Figure 3.21). For this reason, lower complex concentrations were used for all data reported.

69 3-2 3-3 3-1 Normalized Intensity Normalized Intensity

0 100 200 300 400 500 600 Molarity (µM)

Figure 3.21 Plot of Pt(II) concentration vs emission intensity

Table 3.5 Optimal concentrations of compounds 3-1 – 3-3 (µmol/L) for maximum emission intensities

Compound Concentration (µmol/L) 3-1 55 - 75 3-2 80 -120 3-3 90 - 100 Samples measured in anhydrous acetonitrile at room temperature. Intensity (AU)

0 10 20 30 40 50 60 70 Concentration (µmol/L)

+ + Figure 3.22 Self Quenching of [Pt(t-butylbpy)ttma] (), and [Pt(bpy)ttma] (◆)

Singlet oxygen production and quenching. The observed sensitivity to air- contamination led us to examine the possibility of singlet oxygen sensitization by these

70 complexes. Singlet oxygen quantum yields were measured by monitoring the

1 characteristic O2 emission in the near IR (1270 nm region) and comparison of the

emission signal with perinaphthenone as a reference compound with a known singlet

1 oxygen quantum yield ( of 1.0. The O2 quantum yields vary significantly, from

almost unity for complex 3-2a to just 0.18 for 3-3, Table 3.6.

1 The ability of these compounds to quench O2 was determined by using

Methylene Blue as external sensitizer. All were moderate to strong quenchers of singlet oxygen. Total rates of singlet oxygen removal (kT) were obtained by time-resolved near

IR luminescence quenching experiments; kT represents the sum of both physical and

chemical quenching of singlet oxygen removal by complexes. The results are

summarized in Table 3.6, and typical Stern-Volmer plots are shown in below. The

1 dithiomaltol complex 3-2a was both the best sensitizer and slowest quencher of O2.

Figure 3.23 Plots of singlet oxygen luminescence quenching constants (Kobs) vs. concentration of complexes 3-1 – 3-3

71 Table 3.6 Quantum yields for singlet oxygen generation () and rate constants for singlet oxygen quenching (kT) by compounds 3-1 – 3-3

 -1 -1  kT (M s ) 1 0.53 ± 0.04 1.1 ± 0.1  107 2 0.95 ± 0.05 2.5 ± 0.2  106 3 0.18 ± 0.02 1.8 ± 0.1  107

Accurate lifetimes reported in Table 3.3 were attained from purified samples in airtight cuvettes while minimizing emission-quenching contaminants. Emission spectra were recorded in DMM glass (DMF/CH2Cl2/CH3OH, 1:1:1 by volume) showing a tenfold increase in the lifetime magnitude. Fluorescence quantum yields for each complex were

2+ measured by comparing the emission spectra to Ru(bpy)3 with a known fluorescence

quantum yield (F = 8%) and are reported in Table 3.3 from Eq. 3.3 as relative quantum yield (F where A is the absorbance at excitation, F is the area under the emission

curve and n is the refractive index of the solvents used. Subscripts s and x refer to the

standard and unknown.

⁄ ⁄ ⁄ 3.3 Φ Φ

Studies with (2,2’-bipyridine)platinum(II) and Oxygen Chelates (O,O)

Synthesis and characterization of 3-4 - 3-6. The ligand 1-phenyl-2-methyl-3-

hydroxy-4-pyridinone was synthesized from methods in literature.35,36 Ligand

coordination with platinum was adapted and modified from previous methods and

methods within this work, drastic changes in color were observed.4 Column

72 chromatography gave optimal separation. The resulting complexes are very sensitive to

light and oxygen in solution, as observed by the metallic coating left on the glassware.

These platinum materials (3-4 – 3-6) are not air stable like complexes 3-1 – 3-3 in their solid form. The same precautions as in experiments of complexes 3-1 – 3-3 were taken regarding samples fluorescence intensity.

Aromaticity. As before, the aromaticity was measured using Eq. 3.1. The averaged chemical shifts giving the trend 3-4 > 3-5 > 3-6, where δ (ppm) = 7.67, 6.91 and

6.84 respectively, Figure 3.24. Experiments with 5 show reproducible results of sample degradation in solution by NMR spectra. Time lapsed NMR experiments were not compiled. In each sample, a percentage of product was observed with significantly large amounts of material resembling platinum coordinated to bipyridine.

1 Figure 3.24 H NMR spectra (CD3CN) of 3-4 (bottom), 3-5 (middle) and 3-6 (top) from 9.5 ppm – 6.0 ppm

73 Photochemical characterization. Experiments of samples 3-4 - 3-6 yield similar

high-energy absorbances ca. 240-400 nm. Only one low-energy absorption is observed in

3-4 (Figure 3.25) 3-5 (Figure 3.26) and 3-6 (Figure 3.27). The spectrum of 3-1 most closely resembles the band structure in the spectra of 3-1 – 3-3. The molar absorptivity of the high-energy absorbances for 3-4 – 3-6 reported is comparable in intensity to the absorptivity of 3-3 with mixed-MLCT absorption bands. Normalized Intensity 200 300 400 500 600 700 800 Wavelength (nm)

Figure 3.25 Room temperature UV-vis absorption of 3-4 (–-) & emission (---), excitation ca. 369 nm. Normalized Intensity 200 300 400 500 600 700 800 Wavelength (nm)

Figure 3.26 Room temperature UV-vis absorption (–) and emission (---) of 3-5, excitation ca. 375 nm.

74 Normalized Intensity 200 300 400 500 600 700 800 Wavelength (nm)

Figure 3.27 Room temperature UV-vis absorption of 3-6 (–-) & emission (---), excitation ca. 450 nm.

Steady state emission experiments performed at room temperature in the absence of oxygen, on the platinum complexes 3-4 – 3-6 showed that excitation into the low energy absorption band gives weak emissions at room temperature for 3-4, 3-5, and 3-6 at

541 nm (Figure 3.28), 490 nm (Figure 3.29), and 540 nm (Figure 3.31). Each sample also exhibited a near IR emission after exposure to atmospheric oxygen. Excited state lifetimes of samples in solution at room temperature, collected in Table 3.7, show significantly shorter-lived excited states than complexes 3-1 – 3-3. Arbitrary Units (AU)

300 500 700 900 1100 1300 1500 Wavelength (nm) Figure 3.28 Room temperature excitation spectra at 369 nm (–) and emission spectra (– – –) of 3-4 at 600 nm and near IR emission (– –) in CH3CN

75 300 400 500 600 700 Wavelength (nm)

Figure 3.29 Room temperature excitation spectra of 3-5 at 343 nm and 375 nm(–) with emission spectra (– – –) at 493 nm and 530 nm in CH3CN Arbitrary Units (AU)

300 500 700 900 1100 1300 1500 Wavelength (nm)

Figure 3.30. Near IR excitation, ca. 425 nm, and emission, ca. 980 nm, spectra of 3-5 in CH3CN at room temperature. Arbitrary Units (AU)

300 500 700 900 1100 1300 1500 Wavelength (nm)

Figure 3.31 Room temperature excitation/emission (–/– –) spectra of 3-6, 450 nm/630 nm and near IR emission spectra in CH3CN at room temperature.

76 Table 3.7. Summary of photochemical data for complexes 4-6

# Abs. (nm)  (M1•cm-1) Em. (nm) τ (µs) 298 K 600 0.317 3-4 397 941 980 493 0.676 3-5 423 4770 540

980 493 0.477 3-5 422 232 540 980

Discussion

Analysis of (2,2’-Bipyridine)Platinum(II) and Thiolated Chelates (O,S)

Photophysical and photochemical behaviors. In this work, thione-substituted

maltol derivatives are used as electron donors in the classic Eisenberg motif of

II Pt (bpy)Ln complexes. These Pt(II) complexes exhibit long-lived emissions from unusual

ligand-to-ligand charge transfer absorptions. The previously reported room temperature

II emission lifetimes of such Pt (bpy)Ln complexes range from 1 ns to 1 µs for the

dithiolate adducts and 300 to 700 ns for the diacetylide complexes.6,10,27 Analogous

lifetimes of compounds 3-1 – 3-3 at both room temperature and 77 K are significantly

II longer than both the Pt (bpy)Ln complexes and Pt(II) complexes 3-4 – 3-6 with 1 having

the longest at 2.7 µs at room temperature and an absolute quantum yield of ca. 7%.

The use of maltol-derived donors in the Eisenberg motif was suggested by unusual oxidation chemistry described in previous work.20,29 For instance, the outersphere

+ oxidation of [Ru(bpy)2(ttma)] engenders C-H oxidation at the pendant methyl rather

than S-based oxidation at the thione or ring sulfide, Scheme 3.3.14,20,29 As previously

noted, the C-H oxidation products are also obtained by photo-induced electron transfer in

77 flash quench experiments.20 Such pendant methyl oxidation resembles benzylic C-H

activations in aromatic systems, and suggested an aromaticity in the metal-bound

dithiomaltol ligand. The Zn(ttma)2 complex was also shown to engender photo- reductions from a long-lived emission in the near IR.20 The calculated aromaticity of the

related thiopyriliums, an oxidized tautomeric form of the ligand shown in Scheme 3.3, is

second only to that of benzene.32–34

O

O O +2 S Oxidant +2 S (bpy)2Ru (bpy)2Ru NaOH(aq) S S

O + +3 S (bpy)2Ru S

Scheme 3.3 Proposed thio-pyrillium intermediate in C-H activation mechanism

We suggest that LLCT may engender similar aromaticity in the heterosubstituted-

maltolato ligands of compounds 3-1 – 3-6. For these Pt(bpy)(electron-donor) complexes,

the apparent aromaticity of the donor ligands correlates with several photophysical

properties observed: the molar absorptivity of low energy LLCT absorptions and the

Stokes shift and quantum yields of the long-lived emissions.

The long-lived emissive states of compounds 3-1 – 3-3 also facilitate energy

2+ transfer, e.g., the luminescence quenching demonstrated by [MV] and O2. Indeed,

compound 3-2a shows a remarkable ability to generate singlet-oxygen and of the three

78 compounds it has the lowest rate of singlet-oxygen quenching, an order of magnitude

lower than 3-1 and 3-3.

With each material, the collisional rate constants (Ksv) are of the same order of

magnitude, 105 M-1s-1 due to the negligible changes in overall size and shape. However

the largest Ksv is reported for 3-2b with the terbutyl- functional groups added to the

bipyridine, increasing the rate by altering the occurrence of collisional quenching. Studies on the formation and quenching rate of singlet-oxygen show 3-2a to have the highest return on formation (95%) of singlet-oxygen indicating use, or applications for singlet- oxygen sensing. The lowest quenching rate constant is observed for 3-2a by an order of magnitude compared to 3-1 and 3-3. It is likely that quenching rate studies with methylviologen and singlet-oxygen are less dependent on the aromaticity of the donor-

ligands and more on the ability of the hetero-atom in the ring to donate electrons or the

collisional frequency due to size.

Aromaticity. As before, the aromaticity was measured using Eq. 3.1. The

averaged chemical shifts giving the trend 3-4 > 3-5 > 3-6, where δ (ppm) = 7.67, 6.91 and

6.84 respectively, Figure 3.24. Experiments with 5 show reproducible results of sample degradation in solution by NMR spectra. Time lapsed NMR experiments were not compiled. In each sample, a percentage of product was observed with significantly large amounts of material resembling platinum coordinated to bipyridine.

79 1 Figure 3.24 H NMR spectra (CD3CN) of 3-4 (bottom), 3-5 (middle) and 3-6 (top) from 9.5 ppm – 6.0 ppm

Photochemical characterization. Experiments of samples 3-4 - 3-6 yield similar

high-energy absorbances ca. 240-400 nm. Only one low-energy absorption is observed in

3-4 (Figure 3.25) 3-5 (Figure 3.26) and 3-6 (Figure 3.27). The spectrum of 3-1 most closely resembles the band structure in the spectra of 3-1 – 3-3. The molar absorptivity of the high-energy absorbances for 3-4 – 3-6 reported is comparable in intensity to the absorptivity of 3-3 with mixed-MLCT absorption bands.

80 Normalized Intensity 200 300 400 500 600 700 800 Wavelength (nm)

Figure 3.25 Room temperature UV-vis absorption of 3-4 (–-) & emission (---), excitation ca. 369 nm. Normalized Intensity 200 300 400 500 600 700 800 Wavelength (nm)

Figure 3.26 Room temperature UV-vis absorption (–) and emission (---) of 3-5, excitation ca. 375 nm. Normalized Intensity 200 300 400 500 600 700 800 Wavelength (nm)

Figure 3.27 Room temperature UV-vis absorption of 3-6 (–-) & emission (---), excitation ca. 450 nm.

81 Steady state emission experiments performed at room temperature in the absence of oxygen, on the platinum complexes 3-4 – 3-6 showed that excitation into the low energy absorption band gives weak emissions at room temperature for 3-4, 3-5, and 3-6 at

541 nm (Figure 3.28), 490 nm (Figure 3.29), and 540 nm (Figure 3.31). Each sample also exhibited a near IR emission after exposure to atmospheric oxygen. Excited state lifetimes of samples in solution at room temperature, collected in Table 3.7, show significantly shorter-lived excited states than complexes 3-1 – 3-3. Arbitrary Units (AU)

300 500 700 900 1100 1300 1500 Wavelength (nm)

Figure 3.28 Room temperature excitation spectra at 369 nm (–) and emission spectra (– – –) of 3-4 at 600 nm and near IR emission (– –) in CH3CN

300 400 500 600 700 Wavelength (nm)

Figure 3.29 Room temperature excitation spectra of 3-5 at 343 nm and 375 nm(–) with emission spectra (– – –) at 493 nm and 530 nm in CH3CN

82 Arbitrary Units (AU)

300 500 700 900 1100 1300 1500 Wavelength (nm) Figure 3.30. Near IR excitation, ca. 425 nm, and emission, ca. 980 nm, spectra of 3-5 in CH3CN at room temperature. Arbitrary Units (AU)

300 500 700 900 1100 1300 1500 Wavelength (nm) Figure 3.31 Room temperature excitation/emission (–/– –) spectra of 3-6, 450 nm/630 nm and near IR emission spectra in CH3CN at room temperature.

Table 3.7. Summary of photochemical data for complexes 4-6

# Abs. (nm)  (M1•cm-1) Em. (nm) τ (µs) 298 K 600 0.317 3-4 397 941 980 493 0.676 3-5 423 4770 540

980 493 0.477 3-5 422 232 540 980

83 Analysis of (2,2’Bipyridine)Platinum(II) and Oxygen Chelates (O,O)

Here, the maltol, HOPO, and Hppp ligands are used to generate new O,O chelates

in the Eisenberg motif. These Pt(II) complexes exhibit shorter-lived emissions than the

O,S analogues but are comparable to the emissive square planar platinum complexes

previously reported.6,10,27 The apparent aromaticity of the donor ligands still correlates

with several photophysical properties: weaker or lesser intensities in the molar

absorptivity, Stokes shift and the emission lifetimes, as pyrones and pyridinones are still

quite aromatic.37–39

Conclusions

This work represents the first application of sulfur-oxygen and oxygen-oxygen

II chelate donor ligands to study of Eisenberg’s square planar Pt (bpy)Ln. These bidentate

chelates derived from hetero-atom substitution of maltol demonstrate unusual

photochemistry. In application with the O,S chelates, the Pt(II) complexes provide the

longest lived luminescence yet reported for this ligand-to-ligand-charge-transfer (LLCT)

motif. It is also shown that the aromaticity of the heteroaromatic ligands correlate with

the observed photochemical lifetimes and quantum yields. As a result of the

1 luminescence lifetimes of these compounds, they demonstrate unusual activity as O2 sensitizers.

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87 CHAPTER FOUR

Synthesis and Photophysical Characterization of Ti(IV) Complexes with Maltol and Maltol-derived Ligands

Portions of this chapter published as: Lu, W.; Bruner, B.; Casillas, G.; He, J.; Jose- Yacaman, M.; Farmer, P. J. CrystEngComm 2012, 14, 3120–3124.

Portions of this chapter published as: Lu, W.; Bruner, B.; Casillas, G.; Mejía-Rosales, S.; Farmer, P. J.; José-Yacamán, M. Nanotechnology 2012, 23, 335706.

Introduction

Background and Development of DSSCs

Titanium is the ninth most abundant element in the Earth’s crust. The most common and naturally occurring form of titanium is titanium dioxide (TiO2, titania),

which is used in an array of materials and food products as pigmentation, an inorganic

UV blocker, and n-type semi-conductor.1 Titanium(IV) complexes with oxygen chelating ligands have been extensively studied for uses as polymer catalysts.2–8

Fujishama and Honda first suggested its use in semiconductor photocatalysis,

showing titania as a viable material for splitting water, but it was Vogel who first

demonstrated the science behind sensitization.9–12 The chemistry of photography has come a long way since Vogel’s discovery. James Moser first reported dye sensitized photocurrent, erythrosine on silver bromide,13 to undergo electron transfer of Rhodamine

B to zinc-oxide,12 and finally photocurrent densities and electrolyte pH dependencies for

dye sensitized electrodes.14 A complementary goal to this project is to investigate the

88 surface adhesion and photochemistry of the maltol-based dyes on semiconductor surfaces and nanoparticles.15

The titania film in the standard DSSC plays an essential role in both collecting the

16 charge and scattering the light. The standard TiO2 powder used (P-25, Degussa) contains both anatase and rutile phases in a ratio of about 3:1. The charge carrying capacity is attributed to the anatase phase, and the smaller rutile nanoparticles increase the light scattering within the porous films.17

Scheme 4.1 Proposed anthocyanin-Ti(IV) complex

Much effort has gone into the development of new dyes applicable to DSSCs.

Luminescent compounds based on the well-known families of Ru(bpy)x and porphyrin- based complexes are used most commonly, which form long-lived triplet states (3ES) that undergo facile electron-transfer reactions.18 The titanium(IV)maltol complex model the

Grätzel-type, dye-sensitized solar cells (DSSCs) where the anthocyanins dyes found in

89 berries, red-cabbage, and beets are used as sensitizers adsorption to titania particles,

Scheme 4.1.19

Figure 4.1 X-ray structure of [Ti4(maltolato)8(µ-O4)]• 18 H2O

We have previously reported that related compounds based on the food-additive maltol undergo facile photo-oxidations when coordinated to metal ions.20 These compounds have chelating -hydroxy ketone or thione functionalities analogous to that of anthocyanins. Developing interests in anti-cancer drugs have lead to the discovery of a new titanium(IV)maltol (Figure 4.1) as a stable, water-soluble species with milli-molar

21–25 IC50 values. This chapter reports the syntheses of Ti(IV) complexes of hetero-atom substituted derivatives complexed with Ti(IV) that model their use in DSSC applications,

(Scheme 4.2).

90 Complex X, Y (4-1) Maltol (O, O) (4-2) HOPO (O, N-Me) O Y Cl (4-3) Hppp (O, N-Ph) Cl Ti (4-4) Thiomaltol (S, O) X (4-5) Dithiomaltol (S, S) Cl Scheme 4.2 Titanium complexes and their assignments

Experimental

Materials and Methods

Reagents. Titanium(IV) chloride (TiCl4, Alfa Aesar), 3-hydroxy-2-methyl-4H-

pyran-2-one (maltol, Tokyo Chemical Industry), and 3-hydroxy-1,2-dimethyl-4H-

pyridin-2-one (deferiprone, Sigma-Aldrich) were used as received from vendors. 3-

hydroxy-2-methyl-1-phenyl-4H-pyridin-2-one (Hmpp), thiomaltol (Htma), and dithiomaltol (Httma) were synthesized by previously reported methods.26–29 Anhydrous

toluene (Fischer) and hexanes (Fischer) were degassed with N2 by displacement before

each use.

Physical measurements. Room temperature UV-vis absorbance spectra were

performed in dry-degassed acetonitrile (CH3CN) on a HP Agilent 8453 UV-Visible spectrophotometer while steady-state luminescence spectra were collected on an Olis

RSM 1000F4. Redox potentials were measured by cyclic voltammetry (CV) under anaerobic conditions using a CHI-760B potentiostat in dry-degassed CH3CN with 0.1 M

tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. The

cells consisted of a Pt-disk working electrode, AgCl coated Ag reference wire, and a

coiled Pt wire auxiliary electrode. All electrochemical measurements reported were

91 collected at a scan rate of 100 mV/s and corrected using the ferrocene/ferrocenium couple

(Fc/Fc+ = 642 mV vs. NHE). 1H NMR (499.78 MHz) spectra were recorded on a Varian

500 NMR System in deuterated acetonitrile (CD3CN), unless noted otherwise. Accurate masses were resolved by electrospray ionization mass spectrometry (ESI-MS) on a

Thermo Scientific LTQ Orbitrap Discovery in methanol (CH3OH) solutions of 1-10 ppm.

Experiments with Titainum(IV) Chloride

Synthesis of trichloro(maltolato)titanium(IV), 4-1. Titanium(IV)tetrachloride

(189 µL, 189 mg, 1.00 mmol) and anhydrous toluene (10.0 mL) were transferred by

cannula to a flask containing maltol (126.0 mg, 1.000 mmol ) under N2. The mixture was

stirred for 24 hrs before reducing the volume to approximately 1/3 by vacuum. Hexanes

(30.0 mL) were added to the concentrate forming a red/orange precipitate. The precipitate

was filtered off and further washed with hexanes (3 x 10 mL) and dried under vacuum.

1 Yield 192 mg, 0.688 mmol, 68.8%. H NMR (500 MHz, CD3CN) δ 8.27 (d, J = 5.2 Hz,

13 1H), 6.70 (d, J = 4.8 Hz, 1H), 2.43 (s, 3H), C NMR (125 MHz, CDCL3) 188.86, 156.39,

+ 140.935, 133.04, 121.96, 27.47; ESI MS: m/z, 235 ([M -3Cl + 2CH3OH] ); UV-vis:

-1 -1 CH3CN, ƛmax(ε in M •cm ) 212 (ε = 13,100), 289 (ε = 6,000) 358 (ε = 3,390).

Synthesis of trichloro(deferiprone)titanium(IV), 4-2. Titanium(IV)tetrachloride

(334 µL, 577 mg, 3.03 mmol) and anhydrous toluene (10.0 mL) were transferred by cannula to a flask containing deferiprone (423.7 mg, 3.044 mmol ) under N2. The mixture was stirred for 24 hrs before reducing the volume to approximately 1/3 by vacuum.

Hexanes (30.0 mL) were added to the concentrate forming a red/orange precipitate. The

92 precipitate was filtered off and further washed with hexanes and dried under vacuum.

1 Yield 324.3 mg, 1.11 mmol, 36.6%. H NMR (500 MHz, CD3CN) δ 7.66 (d, J = 6.8 Hz,

1H), 6.40 (d, J = 6.6 Hz, 1H), 3.80 (s, 3H), 2.32 (s, 3H); ESI MS: m/z, 248 ([M -3Cl +

+ 13 2CH3OH] ), C NMR (125 MHz) δ 171.02 155.38, 154.24, 138.57, 122.81, 42.91,

-1 -1 11.86; UV-vis: CH3CN, ƛmax(ε in M •cm ) 218 (ε = 10,160), 260 (ε = 6,510), 394 (ε =

3310).

Synthesis of trichloro(ppp)titanium(IV), 4-3. Titanium(IV)tetrachloride (500.0

µL, 865.0 mg mg, 4.560 mmol) and anhydrous toluene (30.0 mL) were transferred by cannula to a flask containing hmpp (0.9376 g, 4.660 mmol) under N2. The mixture was

stirred overnight at room temperature. Precipitation occurred after storage at 0˚C for 24

hours. The precipitate was filtered off and further washed with diethyl ether before drying

1 under vacuum. Yield 1.386 g, 3.720 mmol, 81.58 %. H NMR (500 MHz, CD3CN) δ 7.73

(d, J = 6.9 Hz, 1H), 7.65 – 7.57 (m, 3H), 7.51 – 7.43 (m, 2H), 6.58 (d, J = 6.9 Hz, 1H),

13 2.07 (s, 3H), C NMR (125 MHz, CD3CN) δ 183.00, 165.61, 156.03, 151.72, 117.50,

+ 111.23, 106.77, 100.99, 10.63; ESI MS: m/z, 310 ([M -3Cl + 2CH3OH] ); UV-Vis:

-1 -1 CH3CN, ƛmax(ε in M •cm ) 227 (ε = 13,770), 275 (ε = 14,250), 408 (ε = 5,290).

Synthesis of trichloro(tma)titanium(IV), 4-4. A portion of a 10.5 mM of

Titanium(IV) tetrachloride in anhydrous toluene (1.00 mL, 19.9 mg, 0.105 mmol) was added to 14.9 mg (0.105 mmol) of thiomaltol by syringe. The mixture was sealed under

N2, briefly stirred and allowed to precipitate overnight. Washing with hexanes

precipitated a dark red crystalline solid. Collection by filtration yielded 30.3 mg, 83.7 %.

1 H NMR (500 MHz, CD3CN) δ 8.13 (d, J = 4.8 Hz, 1H), 7.36 (d, J = 4.8 Hz, 1H), 2.50 (s,

93 13 3H), C NMR (125 MHz, CD3CN) δ 180.85, 153.14, 152.22, 120.13, 119.83, 15.49; ESI

+ -1 -1 MS: m/z, 251 ([M -3Cl + 2CH3OH] ); UV-Vis: CH3CN, ƛmax(ε in M •cm ) 200 (ε =

27,600), 250 (ε = 22,500), 275 (ε = 21,500), 352 (ε = 15,700), 450 (ε = 4,360).

Synthesis of trichloro(ttma)titanium(IV), 4-5. Following the procedure for the synthesis of 4-4, 1.00 mL of 10.5 mM of Titanium(IV) tetrachloride in anhydrous toluene

(19.9 mg. 0.105 mmol) was added to 16.6 mg (0.105 mmol) of dithiomaltol. The mixture

was sealed under N2, briefly stirred and let sit overnight. Washing with hexanes

precipitated a dark red crystalline solid. Collection by filtration yielded 20.2 mg, 62.0 %.

1 H NMR (500 MHz, CD3CN) δ 8.30 (d, J = 9.2 Hz, 1H), 8.04 (d, J = 9.2 Hz, 1H), 2.56 (s,

13 3H), C NMR (125 MHz, CD3CN) δ 167.62, 134.16 131.41, 128.87, 114.69, 13.6; ESI

+ -1 -1 MS: m/z, 268 ([M -3Cl + 2CH3OH] ); UV-Vis: CH3CN, ƛmax(ε in M •cm ) 224 (ε =

17,500), 278 (ε = 12,300), 397 (ε = 7,700), 460 (ε = 3,760).

Results

Studies with Titainium(IV) Chloride Complexes

Synthesis and characterization. Complexes 4-1 - 4-5 were synthesized similarly

to the previously reported synthesis of dichlorobismaltolatotitanium(IV) complex, at

room temperature in non-polar aprotic solvents, e.g. toluene or xylenes.6,25 A sharp color

change indicated progress of the reactions. The composition and coordination were

determined by 1H NMR spectroscopy, mass spectrometry, and X-ray diffractometry.

94 X-ray crystallography. Crystals suitable for X-Ray diffractometry were isolated by allowing reaction mixtures of TiCl4 in toluene with maltol-based ligand to slowly cool in a sealed vial. Crystal parameters of 4-5, data collection and refinement are summarized in table Table 4.1. The structure of 4-5 is shown in Figure 4.2.

Figure 4.2 X-ray crystal structure of 4-5

Table 4.1 Summary of parameters for data collection and structure refinement of 4-5

Empirical formula C6 H5 Cl3 O S2 Ti T(K) 296(2) -3 Formula mass 311.47 Dcalcd.(g/cm ) 1.876 -1 a (Å) 11.1470(8) μ(mm ) 1.836

b (Å) 8.0321(6) 2θmaz (°) 28.26 c (Å) 13.0873(9) Reflections measured 2217 α (°) 90.00 Reflections used 5403 Data / restraints / β (°) 109.745(3) 2217 parameters

γ (°) 90.00 R1 [I > 2σ(I)] 0.0195 3 V (Å ) 1102.86 wR2 [I > 2σ(I)] 0.0505 2 Z 4 R(F o) (all data) 0.0218 2 Crystal System Monoclinic Rw (F 0) (all data) 0.0519 2 Space Group P 2(1)/n GOF on F 1.043

95 NMR Characterization. Characteristic chemical shifts of the free ligands, maltol, deferiprone, hmpp, thiomaltol, and dithiomaltol, were compared with those of the Ti complexes. The ligand’s vinylic protons in 4-1 divergently shift from maltol’s chemical shift 7.72 and 7.40 to 8.27 and 6.70, respectively. The methyl group shifts downfield from 2.38 to 2.43 ppm. The chemical shifts for compounds 4-2 - 4-5 shift downfield and are summarized in Table 4.2.

Table 4.2 Chemical shifts in Ti(IV) complexes of the methyl and vinylic protons with average aromaticity

Cmp. Me H1 H2 Aromaticity  (H1-H2) 4-1 2.43 8.27 6.70 7.79 1.57 4-2 2.32 7.66 6.40 8.32 1.26 4-3 2.07 7.73 6.58 7.30 1.15 4-4 2.50 8.13 7.36 7.08 0.77 4-5 2.56 8.30 8.04 7.46 0.26

Photophysical characterization. Absorption spectra of complexes 4-1 - 4-5 in anhydrous acetonitrile show multiple high-energy absorptions characteristic of ligand-to- metal absorptions (Ti-Cl) and ligand absorptions.21,31,32 A new charge transfer absorption band, lower in energy, is also seen for each complex. Absorption maxima and tentative assignments are given in Table 4.3.

Absorption spectra for the complexes are shown in Figure 4.3-4.7 normalized to absorption peaks at or near 212 nm. For example, the titanium complex, 4-1, exhibits three distinct peaks at 212, 289, and 358 nm with a peak shoulder at ca. 250 nm.

96 Table 4.3 Absorption bands’ absorptivities and assignments for 4-1 – 4-5

# ƛmax (nm) (103 M-1•cm-1) Absorption type 1 212 15.7 Cl—Ti 242 7.24 Ligand 289 4.04 Ligand 358 3.720 LMCT 2 218 21.5 Cl—Ti 250 15.8 Ligand 394 4.46 LMCT 3 202 25.6 Cl—Ti 227 20.7 Ligand 275 13.2 Ligand 408 4.44 LMCT 4 200 27.6 Cl—Ti 250 22.5 Ligand 275 21.5 Ligand 352 15.7 Ligand 450 4.36 LMCT’ 5 224 17.5 Cl—Ti 278 12.3 Ligand 397 7.70 Ligand 460 3.76 LMCT’

Normalized Intensity

200 300 400 500 600 700 800 900 Wavelength (nm)

Figure 4.3 Normalized room temperature UV-vis absorption of 4-1 in CH3CN

97 Normalized Intensity

200 300 400 500 600 700 800 900 Wavelength (nm)

Figure 4.4 Normalized room temperature UV-vis absorption of 4-2 in CH3CN Normalized Intensity

200 300 400 500 600 700 800 900 Wavelength (nm)

Figure 4.5 Normalized room temperature UV-vis absorption of 4-3 CH3CN Normalized Intensity

200 300 400 500 600 700 800 900 Wavelength (nm)

Figure 4.6 Normalized room temperature UV-vis absorption of 4-4 and emission ca. 530 at 77 K in CH3CN

98 Normalized Intensity

200 300 400 500 600 700 800 900 Wavelength (nm)

Figure 4.7 Normalized room temperature UV-vis absorption of 4-5 and emission at 77K ca. 642 nm in CH3CN

Emissions were only detected in 4-4 and 4-5 at low temperatures at 77K, Table

4.4. Emission spectra were limited due to limations the Olis RSM 1000F4 which required excitation into a high energy ligand-based band of complex 4-4 and lower energy absorption for 4-5.

Table 4.4 Low temperature 77 K excitation & emission peak wavelengths (nm) in anhydrous CH3CN

Ex. (nm) Em. 4 352 530 5 365 642

Electrochemistry. Electrochemical reduction and oxidation potentials were

determined from cyclic voltammograms (CV) on each sample, with 0.1 M

tetrabutylammonium hexafluorophosphate as the supporting electrolyte in anhydrous

acetonitrile under nitrogen atmosphere. Potentials were recorded against Ag/AgCl wire

reference electrode and corrected to NHE with a ferrocene internal standard at the end of

each experiment (Fc/Fc+ = 642 mV vs. NHE). As multiple reduction peaks are observed

99 in voltammograms of each sample, multiple experiments for each sample were carried

out to identify metal based reductions. For example, voltammograms of 4-1 exhibit three

semi-reversible reductions indicated as ER(ΔEP) in mV: -45(100), -288(73), and -533(76).

Two quasi-reversible peaks are observed in 4-3 when sweeping from positive potentials >

200 mV, Experiments to isolate peak potentials, scanning through 200 to -1750 mV do

not reproduce the reduction wave initially observed ca. 100 mV. Complexes 4-4 and 4-5

each show two clusters of reductions with overlapping peak potentials. Isolated

experiments in complex 4-4 show two quasi-reversible reductions with ER= -336 mV

(ΔEP = 85 mV) and -1200(87) mV. Similar experiments in 4-5 resemble that of 4-4 with

peaks now at 382(69) mV and -1139(86) mV. Results are summarized in Table 4.5.

(c) (d)

(b)

(a)

1.5 1.0 0.5 0.0 -0.5 -1.0 E (V vs. NHE)

Figure 4.8 Overlay of voltammograms for 4-1 with scans from (a) 1.4 V to -0.8V, (b) 0.20 to -0.82 V, (c) 0.20 to -0.40 V, (d) -0.41 to -0.66 V.

100 (c) (d)

(b)

(a)

2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 E (V vs NHE)

Figure 4.9 Overlay of voltammograms for 4-2 with scans from (a) 1.4 to -1.50 V, (b) 0.20 to -1.55, (c) -0.42 to -0.71 V, (d) -0.72 to -0.92 V.

(c) (d)

(b)

(a)

1.5 1 0.5 0 -0.5 -1 -1.5 -2 E (V vs NHE)

Figure 4. Overlay of voltammograms for 4-3 with scans from (a) 1.40 to -1.70 V, (b) , 0.20 to -1.70, (c) -0.38 to -0.75 V, (d) -0.98 to -1.75V.

101 (d) (e) (f)

(c) (b)

(a)

1.5 1 0.5 0 -0.5 -1 -1.5 E (V vs. NHE)

Figure 4.10 Overlay of voltammograms for 4-4 with scans from (a) 1.4 to -1.50 V, (b) 1.36 to 0.20 V, (c) 0.20 to -1.50 V, (d) , -0.15 to -0.59 V, (e) -0.59 to -1.03 V, (f) -1.20 to -1.50 V.

(e) (f)

(d) (c)

(b)

(a)

0.0 0.5 1.0 1.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 E (V vs. NHE)

Figure 4.11 Overlay of voltammograms for 4-5 with scans from (a) 1.4 to -1.70 V, (b) 1.40 to 0.60 V, (c) 0.20 to 1.50 V, (d) 0.18 to -1.40 V, (e) -0.17 to -0.60 V, (f) -1.10 to - 1.50 V

102 Table 4.5 Electrochemical data for Ti(IV) complexes. All potentials adjusted vs. NHE in CH3CN, 0.1 M TBAPF6

Ox Ti(IV) → Ti(III) (Ep) Ligand (Ep) 4-1 -0.045(0.1) -0.553(0.076), 4-2 -0.819(0.081) -0.617ir 4-3 -0.062(0.015) -0.592(0.077) 4-4 1.186 -0.336(0.085) -1.252ir 4-5 0.702 -0.382(69) -1.034

Discussion

Analysis of Titainium(IV) Complexes

In both the O,O and O,S ligands, no variance in synthesis was needed for isolation

of compound of the general structural formula Ti(L)Cl3. These complexes were of similar

size, arrangement, and solubility. Mass spectral analysis in methanol does not show any

presence of chlorine by isotope analysis, however due to reaction with the solvent MeOH

during ionization. 1H NMR spectra of samples that have been handled with methanol do

show signs of solvent displacement of the halogen ligands.

The 1H NMR experiments show pseudo-aromatic chemical shifts for the vinylic protons similar those characterized in the the Ru(II), Zn(II) and Pt(II) complexes.

Estimations of ligand’s aromaticity in 4-4 and 4-5 by this method differ only slightly from values for Pt(tma)+ and Pt(ttma)+. In this regard, the average chemical shifts for the maltol complex 4-1 is, interestingly, as it is greater than that of the Pt(hopto)+ complex.

Our assumption is that ligand aromaticity correlates to propensity for electron transfer;

thus these tentative results suggest complexes 4-1, 4-4, and 4-5 may be viable for DSSC

applications.

103 Spectral data from UV-vis absorption gives insight into the electronic properties of complexes 4-1 - 4-5. Each compound exhibits three distinct absorptions, varying from

narrow ranges 200-220 nm, 260-280 nm, to the broader 360-400 nm attributed to the

ligand absorptions. Additional lower energy absorptions are observed at wavelengths

greater than 400 nm; for complexes 4-1 - 4-3 they are long, trailing absorptions while for

complexes 4-4 and 4-5, a shoulder and new peak are visible.

Luminescence was observed in compounds 4-4 and 5-5 at 77 K, where few

vibronic states are accessible. Complex 4-4 exhibited emission upon excitation in to the

ligand absorption band at 350 nm; the Stokes shift of 168 nm suggests it is

phosphorescence from a ligand based triplet state. Similar behavior is observed for

complex 4-5, excitation into the ligand absorbance at 360 nm yields an emission at 642

nm, with a larger Stokes shift of 280 nm. This is likely due to a significant change in the

triplet state energy of the dithiomaltol complexes compared to those of thiomaltol.

Voltammograms of the titanium compounds exhibit at least one quasi-reversible

reduction wave, i.e. one with a corresponding return wave with a peak separation less

than 100 mV. Complex 4-1 is similar to that of a previously reported titanium bis-

maltolato complex, which was reported to have two metal centered reversible reductions

at 480 mV (ΔEP = 80 mV vs. NHE) for the Ti(IV) → Ti(III) and at 720 mV (ΔEP = 70

mV )for the Ti(III) → Ti(II) (200 mV and 460 mV vs. NHE). The observed reductions for

the mono-chelate occur at -288 mV and -553 vs. NHE with comparable return wave

potentials.

104 Applications to Metal Oxide Materials.

In an important breakthrough, Dr. Weigang Lu in our lab synthesized anatase and rutile nanorods with controlled diameters and aspect ratios.32 The diameter of anatase nanorods in Figure 4.12 are around 5~8 nm and 6~10 nm while the aspect ratio increase from 6~8 to 8~12, varying the reaction time allowed control of the aspect ratios. Altering the reaction temperature produced rutile nanorods, varying the reaction time allowed control of the aspect ratios.

Figure 4.12 Characterization of anatase (top) and rutile (bottom) nanorods of TiO2 with different aspect ratios by TEM images (A,B) and XRD Patterns (C). Adapted from Lu et. al32

Transmission electron microscopy of these nanorods, Figure 4.13, performed in the Yacaman lab at UTSA, allowed the atomic structures of the rutile and anatase nanocrystals to be directly resolved, with no need for calculation or image simulation using atomic resolution STEM techniques.33 The locations of oxygen rows in rutile twin’s boundaries are directly determined. To the best of our knowledge, this is the first time oxygen columns have been distinguished in rutile twin boundaries.

105 Figure 4.13 Processed TEM images of rutile (101) and (301) twins. A) (101) twins, the brighter features correspond to Ti row; the lighter features are O rows. The arrow indicates the position of the (101) twin boundary. B) The intensity of profile line shows distances between Ti and O rows in image A. C) (301) twins, the arrow indicates the {301} twin boundary. D) The intensity of line profile shows distances between Ti and O rows in image C33

In collaboration with Dr. Lu, the adsorbance of maltol, thiomaltol and dithiomaltol on bulk TiO2 or ZnO nanorods was investigated. As shown below in Figure

4.14, the colors of the adsorbed dye in bulk TiO2 suspensions resembles that of the molecular TiLxCly complexes in solution; the maltol complex appears orange, the thiomaltol brown and the dithiomaltol deep red.

Figure 4.14 Dyes adsorbed on TiO2

106 a) b) c)

Figure 4.15 TEM images of ZnO nanorods with different aspect ratio (A, B)

Dr. Lu also synthesized ZnO metal oxide nanoparticles, shown in Figure 4.15, and

the adsorption of the maltol-based dyes onto the ZnO nanorods was assayed. After

soaking the nanoparticles in a dye/chloroform solution over a day, the adsorbed particles

were isolated and washed to remove excess dye, then re-suspended for absorbance

measurement. As shown in Figure 4.16, the characteristic absorbance of thiomaltol

(Htma) and dithiomaltol (Httma) significantly shifted upon adsorption onto ZnO, similar

to that seen upon metal-coordination; maltol treated ZnO is yellow, thiomaltol treated is

brown, dithiomaltol is dark red.

HTTMA ZnO ZnOHTTMA HTMA ZnOHTTMA HTTMA ZnO ZnOHTMA Adsorption

300 400 500 300 400 500 4000 3500 3000 2500 2000 1500 1000 Wave length(nm) Wave length (nm) Wave Number(cm-1)

Figure 4.16 Left, comparison of absorbance of thiomaltol (Htma) and dithiomaltol (Httma) in solution and adsorbed onto ZnO rods; right, FTIR spectra of dithiomaltol in solution and adsorbed onto ZnO rods

107 The dye-treated ZnO powders were also assayed by FTIR using Attenuated Total

Reflectance (ATR). Figure 4.16 (right) shows the comparison of ATR-IR patterns for

dithiomaltol in solution and adsorbed onto ZnO. The dye’s spectral signal intensity on

ZnO nanorods is identifiable although weak due to a low dye:ZnO ratio. However, C-H

stretches at 3000 cm-1 can be observed, and two strong absorbances at 1000 cm-1 and

1100 cm-1 are also observed, perhaps resulting from bonds (C-O, C=S) involved in

coordination with the ZnO surface, Figure 4.16.

Ultimately, we envision using ensembles of dye-coated nanoparticles as photo-

anode modifications, each type with its own unique absorbance and conductance band

energies that would enable harvesting energy throughout the usable solar spectrum,

Figure 4.17.

Dye 1

Dye 2

Dye 3

Figure 4.17 Schematic showing ensemble of dye-coated nanoparticles for use on DSSC anode

Conclusions

In this study, five Ti(IV) complexes were synthesized in good yields. Experiments

in characterization resolved structures and indicated stabilities, electronic absorptions and

emissions, and electrochemical reduction and oxidation potentials. Trends uncovered by

108 characterization and spectral analysis show the ability to tune the HOMO-LUMO band gaps, allowing for strategies towards new dyes. Separately, the adsorption of ligands onto titania particles in suspension ought to translate to an easy and straightforward assembly process for DSSC devices with the possibility of blending semiconductor-dye to maximize visible light absorption.

Most works on light-to-energy conversion in scientific literature have focused on the optimization in the design of DSSCs by fine-tuning the sensitizer, semiconductor, or electrolyte.16-19 This chapter shows that a library of dyes can be developed and employed in the design of a single device in conjunction with a system to create, design and, evaluate DSSCs that allows for variations to be easily made at multiple levels of design.

Further works are needed to test the last phase of device construction by manufacturing simple DSSCs and varying dyes, dye mixtures, semiconductors, and semiconductor mixtures to maximize light-to-energy conversion.

109 REFERENCES

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(6) Szafert, S.; Utko, J.; Ejfler, J.; Jerzykiewicz, L.; Sobota, P. In Organometallic Catalysts and Olefin Polymerization; Blom, D. R.; Follestad, A.; Rytter, P. E.; Tilset, P. M.; Ystenes, P. M., Eds.; Springer Berlin Heidelberg, 2001; pp. 240– 250.

(7) Hamaki, H.; Takeda, N.; Nabika, M.; Tokitoh, N. Macromolecules 2012, 45, 1758–1769.

(8) Mehta, A.; Tembe, G.; Parikh, P.; Mehta, G. Polym. Int. 2014, 63, 206–213.

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(10) Vogel, H. W. The chemistry of light and photography; New York : D. Appleton, 1875.

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111 CHAPTER FIVE

Synthesis and Characterization of Novel Phosphorus(III) Dihalide Complexes with Maltol and Thiomaltol

Introduction

Compounds of Phosphorous(III) and (V)

Six–coordinate phosphorus(III) and phosphorus(V) compounds were once

thought to be unusual.1 Now, complexes such as spirophosphoranes, Scheme 5.1, are

studied for the potential uses as peptide coupling agents and ATP analogues for energy

release and storage (adenosine triphosphate).2,3 Most reports have focused on the five- coordinate species, analogous to pentavalent phosphorus intermediates in biochemical phosphoryl transfer.4

O

O P R O

O

= Any O,O atom chelate O O R = any alkyl or halogen

Scheme 5.1 General structure of a spirophosphorane

The six-coordinate phosphorus(III) compounds remain less common.5 Several neutral six-coordinate phosphorus compounds with O,O chelates are known,6–8 but none

112 with maltol and thiomaltol, Scheme 5.2. This chapter will discuss the methods developed for a straightforward synthesis of these new phosphorus complexes, their initial characterization and examination of their photochemistry.

Y Y Compounds X, Y O O (5-1) O, Fl P (5-2) O, Cl O O (5-3) S, Cl X X

Scheme 5.2 General structure of complexes described

Experimental

Materials and Methods

Reagents. 3-hydroxy-2-methyl-4H-pyran-2-one (maltol, TCI), Lawesson’s reagent, (2,2’-bipyridine)dichloroplatinum(II) (Alfa Aesar), phosphorus pentasulfide

(P4S10) (Acros) and phosphorus(III) chloride (Alfa Aesar) were used as received from vendors. 3-hydroxy-2-methyl-4H-pyran-4-thione (thiomaltol, Htma), 3-hydroxy-2- methyl-4H-thiopyran-4-thione (dithiomaltol, Httma) and 3-hydroxy-1,2- dimethylpyridine-4(1H)-thione (HOPTO) were synthesized by previously reported methods.9,10,11 Tetrahydrofuran (THF, Fischer) was purified and dried on glass columns under argon. Anhydrous toluene (Alfa Aesar) was used as provided. Water/ice/salt baths were used to cool reactions to -10.0˚C. A nitrogen atmosphere was maintained by

Schlenk manifold or glove box where anaerobic conditions were needed.

113 Physical measurements. Steady-state UV-vis absorbance, and luminescence,

were carried out in degassed, anhydrous CH3OH on a HP Agilent 8453 UV-Visible

spectrophotometer and a Hitachi Fluorescence Spectrophotometer F-7000. Elemental

analysis of each sample was performed by Atlantic Microlab Inc. Redox potentials were

measured by cyclic voltammetry (CV) under anaerobic conditions using a CHI-760B

potentiostat in degassed methanol/water (1:1) with 0.1 M NaCl as the supporting

electrolyte. The cells consisted of a Pt-disk working electrode, AgCl coated Ag reference

wire, and a coiled Pt wire auxiliary electrode. All electrochemical measurements were

corrected using the ferrocene/ferrocenium couple (Fc/Fc+ = 642 mV vs. NHE). 1H NMR

(499.78 MHz) spectra were recorded on a Varian 500 NMR System while 31P{H} NMR

were recorded on a Bruker 360 System. Accurate masses were resolved by electrospray

ionization mass spectrometry (ESI-MS) on a Thermo Scientific LTQ Orbitrap Discovery

in methanol (CH3OH) solutions of 1-10 ppm.

Experiments with Phosphorous(III)

Synthesis of PF2(maltol)2, 5-1. Maltol (100.3 mg, 0.7953 mmol) and 20.0 mL anhydrous THF were combined into a Schlenk flask connected to a Schlenk manifold.

Under a blanket of positive pressure nitrogen flow, 1.1 molar equivalent of BF3•Et2O

0.1075 mL, 0.8749 mmol) was added to the maltol solution. Upon cooling the suspension

to 0˚C with a water/ice/salt bath mixture, H2PPh (0.08721 mL, 0.7930 mmol) was added

under N2 by syringe. The reaction mixture was allowed to stir to room temperature

overnight. Solvent was removed by vacuum, leaving a colorless oil. The organic

extraction from water and chloroform resulted in an off-white solid after drying in a

114 vacuum desiccator, 0.152 mg (59.9 %). Colorless crystals suitable for X-ray diffraction

1 grown in methanol. H NMR (500 MHz, (CD3)2CO) δ 7.99 (d, J = 5.5 Hz, 4H), 7.54 (s,

1H), 6.41 (d, J = 5.5 Hz, 4H), 6.18 (s, 1H), 2.33 (s, 12H).

Synthesis of PCl2(maltol)2, 5-2a. Maltol (126.2 mg, 1.00 mmol) and 20.0 ml of a

1:1 solution of dichloromethane and toluene were combined into a Schlenk flask

connected to a Schlenk manifold. Upon cooling the suspension to 0˚C with a

water/ice/salt bath mixture, 0.5 molar equivalent of PCl3 70.0 µL, (0.500 mmol) was added to the maltol solution. The reaction mixture was allowed to stir to room temperature overnight. A precipitate formed and the solvent was removed by cannula.

After drying in a vacuum desiccator the off-white solid was collected, 106.2 mg (57.5 %).

- 1 ESI MS: m/z, 283 ([M - 2Cl); H NMR (500 MHz, (CD3OD) δ 8.00 (d, J = 5.5 Hz, 4H),

7.53 (s, 1H), 6.48 (d, J = 5.5 Hz, 1H), 6.20 (s, 1H), 2.33 (s,12), 13C NMR (125 MHz,

31 CD3OD) δ 174.57, 155.66, 153.05, 142.43, 113.27, 13.55, P{H} NMR (146 MHz,

-1 -1 (CD3OD) δ 6.26 (s), 1.87 (s). UV-vis: CH3OH, ƛmax(ε in M •cm ) 214 (ε = 10,400), 279

(ε = 6,620).

Synthesis of [Et3N•HCl]4[PCl2(maltol)2], 5-2b. Maltol (504.9 mg, 4.004 mmol)

and THF (25.0 mL) were combined into a Schlenk flask inside of a N2 filled glove box.

The reaction vessel was sealed and removed from the glove box to connect to a Schlenk

manifold. Under a blanket of positive pressure nitrogen flow, 0.5 molar equivalent of

TEA (279 µL 2.00 mmol) was added to the maltol/THF mixture. Upon cooling the suspension to 0˚C with a water/ice/salt bath mixture, PCl3 (174 µL, 2.05 mmol) was added under N2 by syringe. The new reaction mixture was allowed to stir to room

115 temperature before setting to reflux for 3.5 hr. Solvent was removed after reflux by

reducing the atmosphere, leaving a brown solid. The crude solid was washed with Et2O

and collected. Purification by sublimation of the crude product separated maltol from

-1 -1 product, yield 229 mg (23.5 %). UV-vis: CH3CN, max( = M •cm ) 212 nm (6650), 276

1 nm (5270). H NMR (500 MHz, D2O) δ 7.89 (d, J = 5.4 Hz, 2H), 7.39, (s) 6.40 (d, J = 5.5

Hz, 2H), 6.03 (s) 3.21 (q, J = 7.3 Hz, 24H) 2.35 (s, 48H), 1.08 (t, J = 7.4 Hz, 72H), 13C

NMR (125 MHz, D2O) δ 176.65, 158.80, 158.14, 144.08, 115.45, 49.00, 16.63, 10.69,

31 P{H} NMR (146 MHz, D2O) δ 4.05 (d, = 644 Hz).

Synthesis of [Et3N•HCl]4[PCl2(thiomaltol)2], 5-3. In the same method as before, thiomaltol (414.4 mg, 2.91 mmol) and THF (25.0 mL) were combined into a Schlenk flask. Outside of the glove box, 0.5 molar equivalent of TEA (205 µL 1.45 mmol) was added to the thiomaltol/THF mixture. Upon cooling the suspension to 0˚C with a water/ice/salt bath mixture, PCl3 (127 µL, 1.45 mmol) was added under N2 by syringe.

After reaching room temperature the mixture was set to reflux for 3.5 hr. Solvent was

removed after reflux by reducing the atmosphere, leaving a dark brown solid. The crude

solid was washed with Et2O and collected. Purification by sublimation of the crude

product separated maltol from product, yield 194 mg (27.4 %). UV-vis: CH3CN, max( =

-1 -1 1 M •cm ) 275 nm (11,900), 361 nm (21,800). H NMR (360 MHz, CD3OD) δ 7.86 (d, J

= 5.0 Hz, 2H), 7.76 (s, 6H), 7.34 (d, J = 5.0 Hz, 2H), 5.93 (s, 6H), 2.45 (s, 24H).

116 Results

Studies with Phosphorous(IV) Complexes

Synthesis and characterization of complexes. As shown in Scheme 5.3,

Compound 5-1 was generated in moderate yield by reacting stoichiometric amounts of

maltol and phenyl phosphine with 0.1 molar excess of BF3•Et2O under anaerobic

conditions in an attempt to synthesize the phosphorus-containing version of the phenyl-

pyridinone ligand Hppp, as examined in Chapter 3 and Chapter 4. The reaction yielded a

blue-emitting, six-coordinate phosphorus complex, 5-1.

O 5-1 - 59.9%

1) THF, BF3•EtO2, 0˚C O O O 2) H PPh, 0˚C - RT P 2 F F O O OH O

1) DCM/Tol. 0˚C 2) 0.5 eq PCl 0˚C - RT O - O 3 Cl O Cl Maltol P O O O 57.5% 5-2a O Scheme 5.3 Reaction schemes for 5-1 and 5-2a

A more rational route yielded compounds 5-2 and 5-3. The dichloro-derivative, 5-

2a, was synthesized using maltol and 0.5 M equivalence of phosphorus trichloride (PCl3) by mixing at -10.0˚C and stirring to room temperature. Triethylamine was employed in

117 synthesis of 5-2b and 5-3 yielding a triethylammonium salt. Low yields are reported for both 5-2b and 5-3. While solutions of samples 5-1 – 5-3 react with protic solvents, their solid samples are stable on bench-top for several weeks. Samples stored under vacuum or in the absence of humidity or oxygen proved to last much longer.

X-ray crystallography. Crystals of 5-1 suitable for X-ray diffractrometry were

grown in deuterated acetone. Structure is shown in Figure 5.1 and details of the crystal

parameters, data collection, and refinement are summarized in Table 5.1, Table 5.2, and

Table 5.3. The determined structure contains hydroxyl protons on oxygen atoms O2 at fifty percent occupancy; thus the empirical formula contains only one hydroxyl proton per complex.

Figure 5.1 X-ray structure (ORTEP) of of 5-1

Distortion is evident in the octahedral geometry around the six-coordinate phosphorus center with F(1)-P(1)-F(2) angle at 93.56(9)˚ and with F(1)-P(1)-O(2)A angle at 94.28 (6)˚. The dihedral angle between the two ligands is 90.5˚ while the bite angles of the ligands are < 90˚. The average C-C bond lengths are 1.3898 Å (C-C 0.0307). P(1)-

118 F(1) and P(1)-F(1)A bond lengths are both long, 1.65 Å. The phosphorus bonds lengths are longer than the average P-O and P-F bond lengths12 (avg. P-O 1.819675 Å, avg.

1.6517 Å P-F), but are comparable to known neutral six-coordinate phosphorous compounds.5,13

Table 5.1 Crystallographic data for 5-1

Empirical formula C12 H11 F2 O6 P Formula mass 313.17 a (Å) 10.1167(9) b (Å) 9.6548(9) c (Å) 12.3697(11) á (°) 90 â (°) 114.730(4) ã (°) 90 3 V (Å ) 1200.20(19) Z 4 Crystal System Monoclinic Space Group C2/c T(K) 110(2) -3 Dcalcd.(g/cm ) 1.766 -1 ì(mm ) 0.284

2èmaz (°) 26.35 Reflections measured 5243 Reflections used 1217 Data / restraints / parameters 1217 / 0 / 97

R1 [I > 2ó(I)] 0.0395

wR2 [I > 2ó(I)] 0.1156 2 R(F o) (all data) 0.0414 2 Rw (F 0) (all data) 0.1173 GOF on F2 1.086

119 Table 5.2 Bond lengths in 5-1

Bond Å P(1)-F(1) 1.6517(12) P(1)-F(1)A 1.6517(12) P(1)-O(2) 1.7708(13) P(1)-O(2)A 1.7708(13) P(1)-O(1) 1.8685(14) P(1)-O(1)A 1.8686(14) O(1)-C(1) 1.288(2) O(2)-C(2) 1.335(2) O(3)-C(4) 1.339(2) O(3)-C(3) 1.364(2) C(1)-C(5) 1.414(3) C(1)-C(2) 1.418(3) C(2)-C(3) 1.364(3) C(3)-C(6) 1.478(3) C(4)-C(5) 1.362(3)

Table 5.3 Bond angles in 5-1

Angle Deg. Angle Deg. Angle Deg. F(1)-P(1)-F(1)A 93.56(9) F(1)-P(1)-O(1)A 176.52(6) C(5)-C(1)-C(2) 118.86(17) F(1)-P(1)-O(2) 94.28(6) F(1)A-P(1)-O(1)A 88.57(6) O(2)-C(2)-C(3) 125.04(18) F(1)A-P(1)-O(2) 90.42(6) O(2)-P(1)-O(1)A 88.45(6) O(2)-C(2)-C(1) 114.16(16) O(2)A-P(1)- F(1)-P(1)-O(2)A 90.42(6) 86.68(6) C(3)-C(2)-C(1) 120.79(18) O(1)A F(1)A-P(1)-O(2)A 94.28(6) O(1)-P(1)-O(1)A 89.43(9) C(2)-C(3)-O(3) 118.48(18) 173.14(10 O(2)-P(1)-O(2)A C(1)-O(1)-P(1) 110.52(12) C(2)-C(3)-C(6) 126.91(18) ) F(1)-P(1)-O(1) 88.57(6) C(2)-O(2)-P(1) 112.05(12) O(3)-C(3)-C(6) 114.60(16) F(1)A-P(1)-O(1) 176.52(6) C(4)-O(3)-C(3) 121.52(15) O(3)-C(4)-C(5) 123.30(17) O(2)-P(1)-O(1) 86.67(6) O(1)-C(1)-C(5) 126.64(18) C(4)-C(5)-C(1) 116.96(18) O(2)A-P(1)-O(1) 88.45(6) O(1)-C(1)-C(2) 114.49(17)

120 1 NMR characterization. The H NMR spectrum of 5-1 in d6-acetone exhibits peaks, at 7.54, 6.18, 3.63, and 1.79 ppm, Figure 5.2. Other peaks due to free maltol ligand are also seen.14–17 The 1H NMR spectrum of 5-2a in Figure 5.3 closely resembles that of

5-1 while the spectra of 5-2b, Figure 5.4, and 5-3, Figure 5.6, show additional peaks from

triethylammonium at 3.25 and 1.35 ppm. Enhancement of the hydroxyl proton signal in

Figure 5.6 is due to the signal suppression of the excess triethylamine peaks, further work

is needed. The maltol-based ligand vinylic proton chemical shifts for these complexes are

reported in Table 5.4, with averages used to depict an increase in aromaticity between the

free ligand to the complexes as described in Chapter 1.

Table 5.4 Ligand vinylic proton1H NMR chemical shifts (ppm).

# Me Ha Hb Average Maltol 2.38 7.72 6.44 7.08 Thiomaltol 2.45 7.59 7.33 7.46 5-1 2.33 7.99 6.41 7.20 5-2a 2.33 7.99 6.41 7.20 5-2b 2.35 7.89 6.40 7.15 5-3 2.45 7.89 7.34 7.62

New doublet peaks are seen for each sample, with coupling values (J = Hz) ca.

1.3 ppm, 650 Hz. The doublet observed in the 31P{H} NMR is indicative of coupled to hydrogen (J ca. 650 Hz). Literature values for coupling between phosphorous and

hydrogen is observed in NMR spectra with coupling constants (J = Hz) between 600-700

Hz for one bond coupling while four and five bond couplings are typically < 10 Hz.5,19,20

Gradient correlated and hetero correlated spectroscopy experiments of 5-2a (Figure 5.8,

121 Figure 5.9) and 5-2b (Figure 5.10, Figure 5.11) show coupling between the vinylic proton resonances but do not show connectivity to the aromatic signals neighboring the doublets.

Figure 5.2 Proton NMR Spectra of 5-1 in (CD3)2CO.

Figure 5.3 Proton NMR spectra of 5-2a in D2O/CD3OD.

122 d H d Cl Cl O O+ P O O O O

H H a H H a b b

d

a c b c

Figure 5.4 Proton NMR spectra of 5-2b in CD3OD.

Figure 5.5 Phosphorus NMR of 5-2b in D2O with H3PO4 in CD2Cl2 standard set to 0.0 ppm

123 Figure 5.6 Proton NMR spectra of 5-3 in CD3OD, excess triethylamine suppressed

Figure 5.7 Phosphorus NMR of 5-3 in D2O

124

Figure 5.8 Proton-proton correlated 2D spectra of 5-2a in CD3OD

Figure 5.9 Proton-carbon correlated 2D spectra of 5-2a in D2O/ CD3OD

125 Figure 5.10 Proton-proton correlated 2D spectra of 5-2b in CD3OD

1 13 Figure 5.11 2D HSQC H, C NMR of 5-2b in D2O/ CD3OD

126 Electrochemical characterization. Cyclic voltammograms (CVs) of 5-2 and 5-3

collected in MeOH/H2O (1:1) with 0.1 M NaCl do not show reversible behavior for

reduction or oxidation, Figure 5.12. Analogous investigations of 5-3 performed do not show any signs of reversible reduction or oxidation.

(d) (c) (b)

(a)

2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 E (V vs. NHE)

Figure 5.12 Overlay of voltammograms for 5-2b (a) from 1.56 to -0.94 V (b) from 1.65 to 0.25 V (c) from 0.16 to -1.0 V (d) from 0.16 to -0.64 V

(c) (d) (b)

(a)

2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 E (V vs. NHE)

Figure 5.13 Overlay of voltammograms for 5-3 (a) from 1.88 to -1.37 V (b) from 2.00 to 0.03 V (c) from 1.13 to 0.03 V (d) 0.03 to -1.6 V

127 Cyclic voltammograms sweeping from 0.03 V to -1.27 V vs. NHE do show one

reduction wave at -1.17 V without any return wave, anodic current Figure 5.13. Cyclic

voltammetry of 5-3, sweeping through positive potentials 0.03 to 2.00 V vs. NHE, shows

a large peak current at 1.53 V vs. NHE with a weaker return wave, cathodic current that

is potential dependent as indicated by the more selective scan.

Photophysical studies. UV-visible spectra of compound 5-1 shows absorbances at 228 nm and 261 nm, Figure 5.14. A unique, blue emission is also observed at room temperature both in solution and as a solid when irradiated with long wave UV radiation from a hand lamp. UV-vis spectra for 5-2a shows two absorptions of energies at 214 and

279 nm, Figure 5.15. Excitation and emission spectra of 5-2a (Figure 5.15) show two emissions, max = 329 nm and 616 nm, originating from the higher energy absorption,

max = 229 nm. The excitation/emission spectra for triethylamine salts 5-2b and 5-3 show

no room temperature emissions in the visible spectrum. Excitation and emission spectra

of 5-2b shows two high energy absorbances with λmax 212 nm and 276 nm with molar

absorptivities of 6650 M-1•cm-1 and 5270 M-1•cm-1 respectively with a weaker shoulder

ca. 360 nm, Figure 5.16. Normalized Intensity Intensity Normalized 200 300 400 500 600 Wavelength (nm)

Figure 5.14 UV-vis absorbance (–) and emission (---) spectra of 5-1 in MeOH

128 Normalized Intensity 200 400 600 800 Wavelength (nm) Figure 5.15 UV-vis absorption spectra (––) and emission spectra (---) of 5-2a in MeOH

Normalized Intensity Intensity Normalized 200 300 400 500 600 700 Wavelength (nm) Figure 5.16 UV-vis absorbance (––) spectra of 5-2b and 5-3 (---)

Discussion

Analysis of Phosphorous(III) Complexes

In this work, four new six-coordinate P(III) complexes are reported. The data for the crystal structure of 5-1, Figure 5.1, confirms the six-coordinate environment at the phosphorus center. While Figure 5.1 shows both ligand’s hydroxy groups as protonated, the data suggests two protons at half occupancy on both oxygens, or one proton per phosphorus complex. The additional proton balances the charge for P(III) at the center of the complex.

Proton and proton decoupled phosphorus NMR integration experiments with complexes 5-1 – 5-3 indicate the presence of one additional proton for every two ligands.

129 1 31 A doublet with JPH ca. 650 Hz, is seen in H and P{H} NMR experiments while the 2D

homo (1H,1H) and hetero NMR (1H,13C) do not show any connectivity to other protons or

carbons. The assignment of the 31P-1H coupling is on the order of magnitude seen in

cationic P-H bonds (1J = 600-700 Hz). It is more reasonable to attribute the appearance of

the doublet to a symmetric difference between the phosphorous environments generated

by alternating protonated ligands. The general connectivity is proposed in Scheme 5.4 as structural data does not support a seven-coordinate phosphorous center.20–22

Y Compounds X, Y H Y O O (5-1) O, Fl (5-2) O, Cl (5-3) S, Cl P

X X

Scheme 5.4 General structure of complexes 5-1 – 5-3 with proposed hydrogen connectivity

As previously illustrated in this dissertation, maltol-derived complexes are

capable of generating unusual photophysical and chemical properties, possibly due to

23–25 pyrollium-like resonance structures. The high energy emissions, max < 400 nm in 5-1

and 5-2 are likely due to the O>O ligands. The lack of emission for 5-2b or 5-3 can be attributed to quenching by the triethylammonium counter ion, as has been previously described.27

Conclusion

Four new phosphorus(III) compounds have been synthesized and characterized,

though more thorough work is needed. The direct synthesis through PX3 should be useful

130 in synthesizing other complexes with maltol-derived complexes. By varying the maltol- based ligand used, the structural, photo-, and electrochemical properties in these phosphorus complexes may be altered, allowing for specific applications in light emitting diodes or a signal-detection by emission.

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