Synthesis, Characterization, Thermodynamic, and Kinetic Studies of Vapochromic Pt(II) Complexes

Synthesis, Characterization, Thermodynamic, and Kinetic Studies of Vapochromic Pt(II) Complexes.

A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the Degree of

Doctor of Philosophy (Ph.D.)

in the Department of Chemistry of the College of Arts and Sciences

2018

Mahmood Karimi Abdolmaleki

M.S., University of Cincinnati, 2016 M.Sc., Isfahan University of Technology, Iran 2008 B.Sc., Arak University, Iran 2004

Committee Chair: Dr. William B. Connick

Abstract

Vapochromic materials, undergoes a reversible color change upon exposure to certain volatile organic compounds (VOCs). Since color changes are easily detected by eye, these materials are potentially useful in the detection of specific hazardous volatile compounds (e.g.,

NH3, pyridine, CO, H2S). Therefore, there is considerable interest in understanding the properties of vapochromic materials and their potential practical uses. The most well-known class of vapochromic materials are solids containing molecular platinum(II) complexes, which have been the focus of numerous studies. The tendency of solids containing square-planar d8-electron platinum metal centers to exhibit vapochromic properties is related in part to the propensity of these complexes to form close non-covalent Pt…Pt contacts, which have a strong influence on the solid-state color (e.g., yellow, orange, red, purple) and spectroscopy of these materials. The color changes upon vapor absorption and desorption typically are not a result of chemical reaction.

Instead, they are related to vapor absorption/desorption induced changes in crystal packing and intermolecular interactions. The resulting changes in Pt…Pt distances have a dramatic effect on the color and luminescence properties of these materials. However, despite considerable progress, the discovery of vapochromic platinum(II) systems remains largely serendipitous.

Moreover, the vapochromic mechanism, energetics, morphological factors, and the relationship between spectroscopic properties, structure, and the sorption/desorption kinetics and thermodynamics are not fully understood. This lack of understanding is preventing researchers from rationally tailoring the response time, selectivity, and sensitivity, as required for sensing applications. Chapter 2 describes the use of a cell phone camera and the CIELAB method (color space specified by the International Commission on Illumination) to characterize the color change in different vapochromic systems. In this study we have developed a semi-automatic

color change analysis software that digitally analyzes images (e.g., video frames) collected while a vapochromic material is absorbing or desorbing vapor. Chapter 3 reports a detailed investigation of [Pt(dcmbpy)Cl2].CH2Cl2, which exhibits the unusual behavior of responding to certain vapors without vapor uptake. Specifically, red, solid [Pt(dcmbpy)Cl2].CH2Cl2 changes to green and finally to a yellow material upon exposure to acetone, THF, and methanol vapors.

Chapter 4 reports the first example of the determination of the enthalpy of vaporization (ΔHvap) and activation energy (Ea) for solvate loss for a homologous series of vapochromic materials. In addition to these parameters, we have evaluated parameters describing sensitivity, speed of color change during vapor desorption, speed of color change during vapor absorption, and the volume attributed to the acetonitrile molecule in the solvate structures of nine salts having very similar compositions.

Acknowledgements

The work detailed in this dissertation would not be possible without the support of some truly wonderful people.

First and foremost, I would like to thank my research advisor, Professor Bill Connick., not only for his tremendous academic support, but for his excellent guidance, enthusiasm, motivation, patience, , professionalism, and immense knowledge. I have been incredibly fortunate to have an advisor who gave me the freedom to explore new area in my research. His guidance has helped me to be more independent, both professionally and as a person. I will carry his philosophy and vision throughout my professional career and life. I also would like to thank my dissertation committee members, Prof. Bruce Ault, Prof. Michael J. Baldwin, and Dr. Robert Streicher for their valuable feedback and comments and recommendation letters. My special thanks to Dr. Streicher who gave me the opportunity to work in his group at NIOSH which was a valuable experience during my PhD.

I am grateful to Dr. Kaval Necati for always being encouraging and helpful. His comments and feedback were great help to improve the results of this work. I would like to thank Dr. Pablo Rosales and Dr. Melodie Fickenscher for SEM and DSC-TGA training. I also would like to thank our departmental crystallographer Dr. Jeanette Krause for the crystallographic work reported in this dissertation. I would like to thank my dear friend Sadegh Riasi for his contribution in color change analysis.

I would also like to thank the past and present members of the Connick Group. Vikas, Amie, Kumudu, and Daoli were very welcoming and great help while I was getting started in the lab. I would also like to thank Angela, Jessie, Nate, Spencer, and Caroline for being great labmates. I would like to thank my undergraduates Mark Bovee, Kelsey Mengle, Xi Lin, Alexander Cassandra, and Rajiv Karani for their valuable contribution in different projects.

Finally, I would like to thank my family for their unconditional support, and for believing in me. I owe everything I have to my parents who I admire and love.

To my parents, Ali and Farah.

Table of Contents

Table of Contents i List of Tables iii List of Schemes iv List of Figures v List of Abbreviations and Symbols xi

Chapter 1 Introduction 1 References 10

Chapter 2 A Digital Imaging Method for Evaluating the Kinetics of 20 Vapochromic Response

Introduction 20 Experimental sections 23 Materials and methods 23

Synthesis and imaging of [Pt(tpy)Cl]ClO4.H2O at different humidity 23 levels. - - - Synthesis and imaging of [Pt(tpy)X ]YF6 (X = Cl , Br and I ; Y = P, 24 As and Sb) salts.

Synthesis and imaging of [Pt(dcmbpy)Cl2].CH2Cl2. 25 Color Change Analysis 26 Results and Discussion 30 Determining the influence of vapor pressure on vapochromic 30 response. Simultaneous measurements of vapochromic response for 33 comparative kinetic studies Detection of an intermediate in a vapochromic process. 37 Conclusion 39 References 41 Appendix 51

Chapter 3 Vapochromic materials that do not incorporate vapors in the 66 crystal lattice and form Nano fibers

Introduction 66 Experimental 68 Materials 68

Synthesis of Pt(dcmbpy)Cl2.CH2Cl2 (R1.CH2Cl2 and R1.CH2Cl2), 68 and Pt(dcmbpy)Cl2 (Y). Characterization and Methods 69

Structure Determination 70 Results and discussion 73

Dichloromethane Solvates of Pt(dcmbpy)Cl2 73 Vapochromism 74 Characterization of Materials 76 Vapoluminescence 79 Vapor-Induced Recrystallization 81 Morphological Changes 82 Conclusion 87 References 88 Appendix 90

Chapter 4 Thermodynamics and Kinetics of a Series of Closely Related 92 Vapochromic Platinum(II) Salts

Introduction 92 Experimental sections 94 Materials 94 - - - Synthesis of [Pt(tpy)X ]YF6 (X = Cl , Br and I ; Y = P, As and Sb) 94 salts. - - - Kinetics of response of [Pt(tpy)X ]YF6 salts (X = Cl , Br and I ; Y = 94 P, As and Sb)

Determination of Enthalpies of vaporization (ΔHvap). 95 Determination of Activation Energies 95 Evaluation of Sensitivities 95

Void volume measurement 96 Results and Discussion 96

Correlation between ΔHvap, Ea, 푡1/2 of absorption, 푡1/2 of 101 desorption, limiting vapor pressure of acetonitrile (Vp) (mm Hg), - - and void volume (VV) of the [Pt(tpy)X ]YF6 salts (X = Cl , Br and I-; Y = P, As and Sb). Conclusion 104 References 105 Appendix 109

List of Tables

Chapter 2

Table 1 The vapochromic response was monitored over time, and the rate of 36 color change plot was used to determine the onset, end, and duration of the absorption.

Table A1 The vapochromic response was monitored over time, and the rate of 63 color change plot was used to determine the onset, end, and duration of the desorption.

Table A2 The vapochromic response was monitored over time, and the rate of 63 color change plot was used to determine the Smax, and Area under the S versus time of the absorption and desorption.

Chapter 3

Table 1 Crystallographic Data for Pt(dcmbpy)Cl2.CH2Cl2 (R1.CH2Cl2) and 72 Pt(dcmbpy)Cl2 (Y) at (150 K)

Chapter 4

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Table 1 Values of the enthalpy of vaporization (ΔHvap), activation energy 100 (Ea), 푡1/2absorption, 푡1/2 desorption, limiting acetonitrile vapor - pressures (Vp), and solvate void volumes for [Pt(tpy)X]YF6 (X = Cl , - Br- and I ; Y = P, As and Sb) salts, (Note: error bars are ±2σ).

Table 2 Table 2. Pearson correlation coefficients (r) for relationships between 102 Ea, ΔHvap, 푡1/2 of absorption, 푡1/2of desorption, limiting acetonitrile vapor pressure (Vp), and void volume (VV).

List of Schemes

Chapter 1 Scheme 1 Line drawing of Pt(tpy)Cl+. 4

Scheme 2 [Pt(dcmbpy)Cl2].CH2Cl2 9

Chapter 2

Scheme 1 Line drawing of [Pt(tpy)Cl]ClO4.H2O 23

+ - - - Scheme 2 Line drawing of [Pt(tpy)X ]YF6 (X = Cl , Br and I ; Y = P, As and Sb) 24

Scheme 3 Line drawing of [Pt(dcmbpy)Cl2].CH2Cl2 25

- - - Scheme 4 Array of [Pt(tpy)X]YF6 ([X]YF6) (where X = Cl , Br and I ; Y = P, As 34 and Sb) salts and their qualitative colors in the absence (upper left of diagonal) and presence (lower right of diagonal) of acetonitrile vapor, which produces the acetonitrile solvate. [Cl]SbF6 and [Br]SbF6 do not absorb acetonitrile vapor, as indicated by the solid diagonal line.

Chapter 3

Scheme 1 [Pt(dcmbpy)Cl2].CH2Cl2 68

Scheme 2 Vapochromic properties of the Pt(dcmbpy)Cl2 system. (i): R1CH2Cl2, 74

(ii): R2CH2Cl2

Chapter 4

- - - Scheme 1 Line drawing of [Pt(tpy)X]YF6 (X = Cl , Br and I ; Y = P, As and Sb) 94

Scheme 2 Array of [Pt(tpy)X]YF6 complexes and their response to acetonitrile 97 Scheme 3 Schematic showing the thermodynamic and kinetics parameters for the 101 release of acetonitrile from [Pt(tpy)X]YF6.CH3CN.

List of Figures

Chapter 1

Figure 1 Qualitative molecular orbital diagram showing the origin of the 3 MMLCT for two closely interacting square planar platinum(II) terpyridyl complexes.

Figure 2 Pt(mbzimpy)Cl+ and the vapochromic response of Pt(mbzimpy)Cl+ 4 salts. Figure 3 Isothermal gravimetric (—) and DT (—) analysis of 7 [Pt(tpy)Cl]ClO4.H2O at 22°C for 90 minutes.

Chapter 2

Figure 1 (A) Color of [Pt(tpy)Cl]PF6 during exposure to acetonitrile: original 30 colors from digital camera (top) vs. colors after curve fitting (bottom). (B) Original and fitted curves of L, a and b.

Figure 2 (A) Rate of the color change (S(t)) with respect to time at different 31 humidity levels of 38% ( ), 57% ( ), 64% ( ), 75%( ), 100% ( ), and (B) the plot of the t1/2 vs relative humidity (%).

Figure 3 UV-visible absorption spectra of thinly dispersed crystallites of 33 [Pt(tpy)Cl]ClO4 recorded during exposure to water vapor. The red and blue colours are intended to draw attention to the fact that the earliest spectra have an approximate isosbestic point shifted slightly from that of the later spectra.

Figure 4 (A) Average color within the region of interest of each [Pt(tpy)X]YF6 36 salt during exposure to acetonitrile vapor for 225 minutes. (B) Rate of

color change (S(t)) as a function of time for the nine salts.

Figure 5 Crystals of [Pt(dcmbpy)Cl2].CH2Cl2 (A) before and (B) after acetone 37 exposure viewed through a microscope at 200X magnification. During the 1-hour vapor exposure period, most of the crystals changed color to green, and some changed color to yellow. All crystals changed color to yellow during exposure for 24 hours.

Figure 6 Plot of the color of [Pt(dcmbpy)Cl2].CH2Cl2 during the acetone vapor 39 exposure. Black solid line shows the color change (CC) vs time, black dashed line shows the rate of the color change (S) vs time.

Figure A1 X-ray powder diffractograms of the ground yellow [Pt(tpy)Cl]PF6 (—), and 51 the simulated pattern (—).

Figure A2 X-ray powder diffractograms of the ground yellow [Pt(tpy)Br]PF6 (—), and 51 the simulated pattern (—).

Figure A3 X-ray powder diffractograms of the ground yellow [Pt(tpy)I]PF6 (—), and 52 the simulated pattern (—).

Figure A4 X-ray powder diffractograms of the ground yellow [Pt(tpy)Cl]AsF6 (—), and 52 the simulated pattern (—).

Figure A5 X-ray powder diffractograms of the ground yellow [Pt(tpy)Br]AsF6 (—), and 53 the simulated pattern (—).

Figure A6 X-ray powder diffractograms of the ground yellow [Pt(tpy)I]AsF6 (—), and 53 the simulated pattern (—).

Figure A7 X-ray powder diffractograms of the ground yellow [Pt(tpy)Cl]SbF6 (—), and 54 the simulated pattern (—).

Figure A8 X-ray powder diffractograms of the ground yellow [Pt(tpy)Br]SbF6 (—), and 54 the simulated pattern (—).

Figure A9 X-ray powder diffractograms of the ground yellow [Pt(tpy)I]SbF6 (—), and 55 the simulated pattern (—).

Figure A10 The overlaid graphs of the Rate of the color change (S), Color Change (CC) 56 and Average color bar of [Pt(tpy)Cl]ClO4 at 38% humidity.

Figure A11 The overlaid graphs of the Rate of the color change (S), Color Change (CC) 57 and Average color bar of [Pt(tpy)Cl]ClO4 at 57% humidity.

Figure A12 The overlaid graphs of the Rate of the color change (S), Color Change (CC) 58 and Average color bar of [Pt(tpy)Cl]ClO4 at 64% humidity.

Figure A13 The overlaid graphs of the Rate of the color change (S), Color Change (CC) 59 and Average color bar of [Pt(tpy)Cl]ClO4 at 100% humidity.

Figure A14 The overlaid graphs of the Rate of the color change (S), Color Change (CC) 60 and Average color bar of [Pt(tpy)X]YF6 (X = Cl-, Br- and I-; Y = P, As and Sb) salts. Figure A15 Rate of color change with respect to time for the nine salts as acetonitrile 61 vapor was desorbed.

Figure A16 The overlaid graphs of the Rate of the color change (S), Color Change (CC) 62 - - - and Average color bar of [Pt(tpy)X]YF6 (X = Cl , Br and I ; Y = P, As and Sb) salts as acetonitrile vapor was desorbed.

Chapter 3

Figure 1 X-ray powder diffractogram of R2.CH2Cl2 (—), and the simulated 73 pattern of (R1.CH2Cl2) (—).

Figure 2 X-ray powder diffractogram of R1.CH2Cl2 produced by exposure of 75 R2.CH2Cl2 to acetone vapor to give G, followed by exposure to dichloromethane vapor (—). The simulated pattern based on the single-crystal structure of R1.CH2Cl2 also is shown (—).

Figure 3 X-ray powder diffractograms of Y formed by exposure of R1.CH2Cl2 76 to (A) acetone vapor, (B) THF vapor, and (C) acetonitrile vapor. The simulated pattern based on the single-crystal structure of Y also is shown (—). Figure 4 ORTEP diagrams of (A) red Pt(dcmbpy)Cl2.CH2Cl2 (R1.CH2Cl2) , 78

and (B) yellow Pt(dcmbpy)Cl2 (Y) at 150K.

Figure 5 X-ray powder diffractograms of G formed by exposure of R1.CH2Cl2 79 to (A) Acetone, (B) THF or (C) methanol vapors.

Figure 6 Solid-state emission spectra recorded during exposure of R1.CH2Cl2 80 to acetone vapor at room temperature (λex = 436 nm) with showing the (A) red to green conversion and the subsequent (B) green to yellow conversion.

Figure 7 Solid-state emission spectra of R2.CH2Cl2 recorded during exposure 81 to acetone vapor at room temperature (λex = 436 nm) (A) red to green, and (B) green to yellow conversion. Figure 8 Plot of the number of equivalents acetone (—) and dichloromethane 82 (—) present during the exposure of the R1.CH2Cl2 to the acetone vapor.

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Figure 9 Representative SEM images of (A,B) red, (C,D) green after acetone 84 exposure, (E,F) yellow after acetone exposure, and (G,H) spaghetti red after (A,B) exposed to dichloromethane vapor.

Figure 10 Representative SEM images of green after (A) acetone, (B) methanol, 86 and (C) THF exposure , (D,E,F) yellow after THF exposure, and (G,H) red spaghetti form.

Figure A1 Solid-state excitation spectra of R1.CH2Cl2 (—), and R2.CH2Cl2 (— 90 ) are shown in (A), and solid-state excitation spectra of green powders (—), and yellow powders (—) during the exposure of R2.CH2Cl2 to acetone vapor at room temperature (λex = 436 nm) are shown in (B).

Chapter 4

Figure 1 (A) DSC thermograms of [Pt(tpy)Br ]PF6.CH3CN at different heating 98 2 rates and (B) Plot of ln(β /Tp ) against 1000/Tp. (Note error bars are ±2σ, and some error bars are smaller than the height of the point)

Figure 2 (A) graph of Ea against Speed of desorption, and (B) Graph of ΔHvap 103 against limiting vapor pressure of the acetonitrile (Vp). (Note error bars are ±2σ, and some error bars are smaller than the height of the point)

Figure A1 Representative DSC thermograms and and ΔHvap values of [Pt(tpy)X 109 - - - ]YF6 salts (X = Cl , Br and I ; Y = P, As and Sb)

- - Figure A2 Representative DSC thermograms of [Pt(tpy)X ]YF6 salts (X = Cl , Br 110 and I-; Y = P, As and Sb) at ramp rates of 2( ), 3( ), 5( ), 7( ), and 10( ) °C/min. 2 Figure A3 Plot of ln(β /Tp ) against 1000/Tp for the [Pt(tpy)Cl]PF6.CH3CN salt. 111 Note error bars are ±2σ, and some error bars are smaller than the height of the point. 2 Figure A4 Plot of ln(β /Tp ) against 1000/Tp for the [Pt(tpy)Br]PF6.CH3CN salt. 111 Note error bars are ±2σ, and some error bars are smaller than the height of the point.

2 Figure A5 Plot of ln(β /Tp ) against 1000/Tp for the [Pt(tpy)I]PF6.CH3CN salt. 112 Note error bars are ±2σ, and some error bars are smaller than the height of the point.

2 Figure A6 Plot of ln(β /Tp ) against 1000/Tp for the [Pt(tpy)Cl]AsF6.CH3CN salt. 112 Note error bars are ±2σ, and some error bars are smaller than the

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height of the point.

2 Figure A7 Plot of ln(β /Tp ) against 1000/Tp for the [Pt(tpy)Br]AsF6.CH3CN salt. 113 Note error bars are ±2σ, and some error bars are smaller than the height of the point.

2 Figure A8 Plot of ln(β /Tp ) against 1000/Tp for the [Pt(tpy)I]AsF6.CH3CN salt. 113 Note error bars are ±2σ, and some error bars are smaller than the height of the point.

2 Figure A9 Plot of ln(β /Tp ) against 1000/Tp for the [Pt(tpy)Br]SbF6.CH3CN salt. 114 Note error bars are ±2σ, and some error bars are smaller than the height of the point.

2 Figure A10 Plot of ln(β /Tp ) against 1000/Tp for the [Pt(tpy)I]SbF6.CH3CN salt. 114 Note error bars are ±2σ, and some error bars are smaller than the height of the point.

Figure A11 Scatterplot of Ea and ΔHvap. Note error bars are ±2σ, and some error 115 bars are smaller than the height of the point. Figure A12 Scatterplot of t1/2 for desorption and ΔHvap. 115 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A13 Scatterplot of t1/2 for absorption and ΔHvap. 116 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A14 Scatterplot of t1/2 for absorption and Ea. 116 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A15 Scatterplot of t1/2 for absorption and Ea. 117 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A16 Scatterplot of Vp and Ea. 117 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A17 Scatterplot of Vp and ΔHvap. 118 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A18 Scatterplot of Void volume and Ea. 118 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A19 Scatterplot of t1/2 for absorption and Void volume. 119 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A20 Scatterplot of t1/2 for desorption and Void volume. 119 Note error bars are ±2σ, and some error bars are smaller than the height of the point.

Figure A21 Scatterplot of Vp and Void volume. 120 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A22 Scatterplot of Void volume and ΔHvap. 120 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A23 Scatterplot of Vp and t1/2 for absorption. 121 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A24 Scatterplot of Vp and t1/2 for desorption. 121 Note error bars are ±2σ, and some error bars are smaller than the height of the point. Figure A25 Scatterplot of t1/2 for absorption and t1/2 for desorption. 122 Note error bars are ±2σ, and some error bars are smaller than the height of the point.

List of Abbreviations and Symbols

ΔHvap Enthalpy of vaporization

ΔSvap Entropy of vaporization

- AsF6 Hexafluoroarsenate

2D Two dimensional

A Absorbance

Å Angstrom a, b and c Three-dimensional unit cell edges b Pathlength (cuvette)

β Heating rate

Br- Bromide

Br2 Bromine

C Carbon

C Degree celsius

CC Color change path

CH3OH Methanol

CH3CN Acetonitrile

CH2Cl2 Dichloromethane

CIELAB color space specified by the International Commission on Illumination

Cl- Chloride

- ClO4 Perchlorate cm Centimeter cm-1 Wavenumber

COD 1,5-cyclooctadiene

bipyridine-׳bpy 2,2 d Intradimer interplanar spacing / interatomic distance dcmbpy 4,4'-dicarboxymethyl 2,2'-bipyridine

DSC Differential scanning calorimetry

DMF Dimethyl formamide

DMSO Dimethyl sulfoxide

DTA Differntial thermal analysis

Ea Activation energy

EtOH Ethanol

Et2O Diethyl ether

 Molar extinction coefficient / molar absorptivity (M-1 cm-1)

EPR Electron paramagnetic resonance et al. And others fwhm full width at half maximum g Gram h Hour

H Hydrogen

HCN hydrogen cyanide Hz Hertz (s-1) h Hour

HOMO Highest occupied molecular orbital

Hz Hertz (s-1)

IL Intraligand

I- Iodide

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K Kelvin

L Ligand

LC Ligand centered

LLCT Ligand-to-ligand-charge transfer

LMCT Ligand-to-metal-charge transfer

LAB Channel for lightness (L), channel for red to green scale (a), and channel for the blue to yellow scale (b). nm Nanometer

NMR Nuclear magnetic resonance

λabs max Wavelength of maximum absorbance

λem Emission wavelength

λex Excitation wavelength

LF Ligand field

LUMO Lowest unoccupied molecular orbital

M Molarity m/z Mass/charge

Me Methyl mbzimpy 2,6-bis(N-methylbenzimidazoly-2-yl) mg Milligram ml Milliliter

L Microliter

M Metal

M Molarity

Me methyl

MLCT Metal-to-ligand charge transfer

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M Micromolar mm Millimeter mM Millimolar

MMLCT Metal-metal-to-ligand charge transfer mol Mole

MS Mass spectrometry

MS-ESI Mass spectrometry – electron spray ionization

MHz Megahertz or (106 Hz) mV Millivolts

N Nitrogen

NH3 Ammonia nm Nanometer nM Nanomolar

NMR Nuclear magnetic resonance

P Phosphorus

- PF6 Hexafluorophosphate

Ph Phenyl ppm Parts per million

- (PF6) Hexafluorophosphate

Pt Platinum

Pt···Pt Intradimer Pt…Pt distance

Pt···Pt···Pt Angle formed by three consecutive Pt atoms along the stacking axis.

 Pi

RGB Reg, Green and Yellow color

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ROI Region Of Interest

Ru Ruthenium

S speed of color change

- SbF6 hexafluoroantimonate

SEM Scanning electron microscope

SI Supplementary information

TGA thermogravimetric analysis

Tp Heat maximum tpy 2,2′:6′,2″-terpyridine

 Theta (for Angle)

UV-vis Ultra violet -visible

 Stretching frequency/ vibrations

VOC Volatile organic compounds vs. Versus

XRPD X-ray powder diffraction

Chapter 1

Introduction

The design and synthesis of stable, selective, and reversible gas sorbing materials have drawn considerable attention due to the potential application of these materials in environmental analysis, catalysis, and chemical sensing devices.1-9 One class of such materials, known as vapochromic materials, undergoes a reversible color change upon exposure to certain volatile organic compounds (VOCs).10-29 Since color changes are easily detected by eye, these materials are potentially useful in the detection of specific hazardous volatile compounds (e.g., NH3,

29-83 pyridine, CO, H2S). Therefore, there is considerable interest in understanding the properties of vapochromic materials and their potential practical uses.

The most well-known class of vapochromic materials are solids containing molecular platinum(II) complexes, which have been the focus of numerous studies.84-89 However, despite considerable progress, the discovery of vapochromic platinum(II) systems remains largely serendipitous. Moreover, the vapochromic mechanism, energetics, morphological factors, and the relationship between spectroscopic properties, structure, and the sorption/desorption kinetics and thermodynamics are not fully understood. This lack of understanding is preventing researchers from rationally tailoring the response time, selectivity, and sensitivity, as required for sensing applications.

During the late l990’s the field of vapochromism ignited by Mann and co-workers with

2- their studies of double salts composed of the stacked square-planar Pt(CN)4 dianions and

2+ Pt(tetracyanide)4 dications which undergo rapid and reversible color changes in the presence of volatile organic compounds. The vapochromic response is believed to be related to changes in non-covalent Pt…Pt separations, as well as specific analyte-cyanide ligand interactions and changes in the dielectric constant of the material.5,11-17,19,78

Virtually all known platinum(II) vapochromic systems involve incorporation of vapor.

But there is one interesting exception which was reported by You et al., who described the

90 vapochromic behavior of two enantiomeric chiral platinum(II) complexes. Pt(La)(C≡C ̶ Ph) and

׳ ׳ -bipyridine, Lb = (+)- 4,5-pinene-6 -phenyl-׳Pt(Lb)(C≡C ̶ Ph) (La = (-)- 4,5-pinene-6 -phenyl-2,2

bipyridine). The complexes selectively responded to dichloromethane vapor changing color-׳2,2 from yellow to orange. However, single crystal X-ray diffraction data showed that dichloromethane was not incorporated in either of the yellow or orange forms. The color change was attributed to increased … and Pt…Pt interactions in the orange form. The authors suggested that the high selectivity for dichloromethane vapor is a result of the abundant attractive and exoergic non-convalent interactions between dichloromethane vapor and the yellow forms which can affect the molecular structure and packing mode during the crystal growth.

Interestingly, this is the only example where vapor induces a color change without becoming incorporated into the crystal lattice.

Platinum(II) complexes with the tridentate 2,2';6',2"-terpyridine ligand (tpy) also have received considerable attention (Scheme 1).91-100 Spectroscopic and structural studies reveal that the vapochromic response for these square-planar platinum(II) complexes involves the absorption of a VOC that changes the crystal packing, including non-covalent Pt···Pt and/or -

2 stacking interactions. The emission from platinum(II) complexes with short Pt···Pt interactions originates from a lowest mixed-metal-to-ligand charge-transfer (MMLCT) state, involving a filled HOMO d*orbital derived from the antibonding combination of the 5dz2 Pt orbitals of adjacent complexes and an unoccupied * LUMO mainly located on the aromatic heterocyclic ligand (Figure 1).51,101

p*

6pz 6pz

p *

d*

2  2 5dz 5dz

d

Figure 1. Qualitative molecular orbital diagram showing the origin of the MMLCT for two closely interacting square planar platinum(II) terpyridyl complexes.102

It was the detailed work of Bailey that demonstrated the influence of the anion on the crystal packing, colors and emission spectroscopy of Pt(tpy)Cl+ salts (Scheme 1).95 Notably, the

MMLCT emission shifts to longer wavelength with increasing Pt…Pt interactions. The work of

Grove et al. showed that changing the counter ion in closely related Pt(mbzimpy)Cl+ salts

(Figure 2) was a convenient means of changing the vapochromic selectivity. For example, the

3 chloride salt was found to undergo a colorimetric response to methanol vapor but not acetonitrile

- vapor. By contrast, the PF6 salt changed color in the presence of acetonitrile vapor, but not methanol vapor (Figure 2).51 Castellano and coworkers leveraged this discovery by preparing a sensing array of 18 salts of platinum(II) terpyridyl chloride complexes (Scheme 1). It was found that each solid showed a unique colorimetric selectivity for vapors, and therefore pattern recognition could in principle be used to identify which vapors were present in a gas sample.57

Scheme 1: Line drawing of Pt(tpy)Cl+.

Grove et. al., JACS, 2004

Figure 2. Pt(mbzimpy)Cl+ and the vapochromic response of Pt(mbzimpy)Cl+ salts.51

One of the most significant structural studies of vapochromic platinum(II) complexes was

27 reported by Wadas and coworkers. It was found that [Pt(Nttpy)Cl](PF6)2 (Nttpy=4′-(p-

4 nicotinamide-N-methylphenyl)- 2,2′:6′,2″-terpyridine) changes from red to orange upon

… exposure to methanol vapor. The red crystals of [Pt(Nttpy)Cl](PF6)2 have short Pt Pt distances of 3.301 and 3.360 Å with a Pt…Pt…Pt angle of 171.9°. By contrast, the orange form contains methanol solvate in the lattice void with longer Pt…Pt distances of 3.622 and 3.964 Å (Pt…Pt…Pt,

126.7°.) This is an important example in which both the structure of the non-solvate and solvate have been determined by X-ray crystallography before and after vapor exposure. In other cases, the structures with and without solvate have been inferred from structural studies of crystals precipitated from solution under different conditions.81

Despite these efforts there have been few studies focused on evaluation of the thermodynamics and kinetics of vapochromic systems.17,78,103-105

In one example, Kobayashi et al. reported that S-coordinated orange

[Pt(SCN)2(H2dcbpy)].H2O (H2dcbpy =4,4′-dicarboxy-2,2′-bipyridine) complex changed to red

N-coordinated [Pt(NCS)2(H2dcbpy)].3DMF complex upon exposure to DMF vapor.

Thermogravimetric and differential thermal analysis revealed that the red sample releases 3 equivalents of DMF during an endothermic process when heated to 250°C. 105

In another example Grate et al. reported the kinetics of vapor sorption/desorption by a

78 double salt, [Pt(CN-cyclododecyl)4][Pt(CN)4]. This material reversibly absorbs three molecules of water per formula unit at room temperature. Bulk gravimetric analysis showed that exposure of the complex to 100% humidity resulted in rapid (<1 min) uptake of 3±0.1 water molecules per formula unit. Reduction of humidity resulted in a rapid loss of 0.5 water molecules, followed by a slower process to restore the original value. These observations were in agreement with the

QCM (quartz crystal microbalance) and spectroscopic analysis of thin films of the complex. A

5 key contribution in this study was demonstration of a method for simultaneous measurement of the gravimetric and optical properties of vapochromic materials.

Mann and coworkers also reported the kinetics of vapor sorption/desorption of Pt(CN-p-

(C2H5)C6H4)2(CN)2 complex. Recrystallization of Pt(CN-p-(C2H5)C6H4)2(CN)2 gives a crystalline orange polymorph and an amorphous purple polymorph.17 The orange form changes color in the presence of various solvent vapors (e.g., ethanol, toluene, benzene, chlorobenzene) whereas the purple form does not. TGA and single-crystal X-ray diffraction studies showed that there are no solvate molecules in either the orange or purple polymorphs. Gravimetric analysis of the bulk samples was used to follow absorption of one equivalent of toluene by the orange form.

The mass gain occurred in two steps. The first step occurred within six minutes, and the mass gain was consistent with uptake of 0.5(1) equivalents of toluene molecules per complex. The second step required more time (10 hours), resulting in a total mass gain of 0.9(1) equivalents of toluene molecule per complex. By contrast, the desorption process under the purged N2 gas occurred in three steps. Gravimetric analysis showed that the first step occurred quickly and the mass loss was 0.5(1) equivalents of toluene. The second step occurred over three hours, corresponding to loss of an additional 0.25 equivalents. Additional long-term N2 purging or heating was required to reach the dry mass.

In a more recent study, Taylor and coworkers reported that [Pt(tpy)Cl]ClO4.H2O changes from red to yellow upon dehydration.103 Spectroscopic, diffraction, gravimetric and calorimetric data demonstrated that the hydration and dehydration processes are more complicated than a simple A→B process. Neither solid-state room-temperature emission spectra nor UV-visible absorption spectra showed conclusive evidence of intermediate formation during hydration or dehydration. However, XRPD studies of the dehydration process revealed at least two crystalline

intermediates, and room-temperature isothermal TGA/DTA showed four thermal events suggesting the formation of as many as four intermediates (Figure 3). 106 The DSC measurements show three endothermic events with onset temperatures of ~0, 5 and 15°C. These observations(I) illustrate the importance of experimental conditions when attempting to elucidate vapochromic mechanistic details.

(II)

Time (min)

Figure 3. Isothermal gravimetric (—) and DT (—) analysis of [Pt(tpy)Cl]ClO4.H2O at

22°C for 90 minutes. 103

As suggested by this review of key studies in the field, knowledge of the thermodynamics and kinetics of vapochromic responses remains scarce. One challenge is identification of a suitable system for study. In this respect, the previous work by Taylor is encouraging.106 Those studies focused on salts of Pt(tpy)X+ (X- = Cl-, Br- or I-). The vapochromic properties of these materials are modulated by the counter-anion, the ancillary ligand (X-), as well as substitutions on the tridentate ligand.106 Equally important, the structures of more than a few Pt(tpy)X+ salts have been determined, which suggests the feasibility of using Pt(tpy)X+ salts for delineation of the structural factors that govern the vapochromic processes. The work presented in this 7 dissertation, capitalizes on the promise of the Pt(tpy)X+ salts and related systems in order to elucidate the relationships between sensitivity, morphology, structure, sorption/desorption kinetics and thermodynamics for a series of platinum(II) complexes.

Dissertation Chapters: Chapter 2 describes the use of a cell phone camera and the

L*a*b method (color space specified by the International Commission on Illumination)107 to characterize the color change in different vapochromic systems. In this study we have developed a semi-automatic color change analysis software that digitally analyzes images (e.g., video frames) collected while a vapochromic material is absorbing or desorbing vapor. The advantages of using this method, as compared to reflectance spectroscopy or transmission spectroscopy through a thin film, include low cost, convenience, portability, ease of sample preparation, the absence of need for specialized equipment, and the ease of simultaneously collecting data on different samples under identical conditions. In addition, this method arguably provides direct insight into what a human would observe when monitoring these color changes by eye.

Limitations of the method also are discussed in Chapter 2.

Chapter 3 reports a detailed investigation of [Pt(dcmbpy)Cl2].CH2Cl2, which exhibits the unusual behavior of responding to certain vapors without vapor uptake (Scheme 2). Specifically, red, solid [Pt(dcmbpy)Cl2].CH2Cl2 changes to green and finally to a yellow material upon exposure to acetone, THF, and methanol vapors. The green and yellow materials were characterized by several techniques, including XRPD and SEM. Results show that exposure to acetone, THF or methanol produced the same stable green intermediate and the same final yellow material. The crystal structure of the yellow material confirmed that there is no solvate molecule incorporated in the crystal lattice. Interestingly this is the second example of the vapochromic response in platinium(II) complexes without vapor uptake. SEM images of the

8 green and yellow materials show evidence for the formation of remarkable nanoscale architectures induced by vapor exposure. The vapor-induced formation of a nanoscale structure during the vapochromic process has not previously been documented.

Scheme 2. [Pt(dcmbpy)Cl2].CH2Cl2

Chapter 4 describes the use of the differential scanning calorimetry (DSC) to determine the enthalpy of vaporization (ΔHvap) and activation energy for vapor desorption (Ea) of the

- - - [Pt(tpy)X]YF6.CH3CN (X = Cl , Br and I ; Y = P, As and Sb) series. In addition, we determined the minimum vapor pressure of acetonitrile required to induce a colorimetric response for each

[Pt(tpy)X]YF6 material. Subsequently, we examined the relationships between the solvate/non- solvate structures, kinetics of colorimetric response (absorption and desorption), sensitivity, activation energies, and enthalpy of vaporization. The study is innovative because it represents the first example of the determination of the thermodynamic basis for the sensitivity, selectivity and kinetics of vapochromic response in a series of materials in which the chemical properties have been systematically varied.

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Chapter 2

A Digital Imaging Method for Evaluating the Kinetics of Vapochromic Response

Introduction

Color change plays a central role in many phenomena including potential practical effects, such as the response of acid-base indicators and other chemical sensing systems,1-10 electrochromic materials and devices,11-16 biophotonic tools in tissue and cell diagnosis,17,18 and thermochromic liquid crystals. 19,20 UV-Visible absorption and reflectance spectroscopy are often used to monitor changes in absorption of light at different wavelengths, as well as the appearance and disappearance of bands associated with specific electronic transitions.21-26 However, these methods require relatively expensive instrumentation and user expertise to prepare the sample, operate the instrument and interpret the spectra. In addition, variations in lighting, color contrast, and other effects make it exceedingly challenging to use spectra to predict perception of color experienced by a human. For these reasons, there is a growing interest in evaluating color of functional materials and other substrates rather than merely recording their spectra. For example, a UV-visible spectrophotometer has been used to quantify color measurements for quality control of wine and other beverages.27-30 In one study, the quality of the wine was evaluated by color intensity and hue.27 However, even with these applications, there was the requirement of relatively expensive technologies and significant expertise. As a result there is continued interest in developing low-cost direct methods of color evaluation.

The distribution of the counterfeit medicines and drug products (medicines without active ingredients, with insufficient ingredients, or with fake packaging and inadequate dosage) is a major health concern in developing countries.31 To deter counterfeiting, tablets have been

20 manufactured with identifiable and unique shapes and codes, however these tablet features are relatively easy to mimic. As a result there is an ongoing effort to develop an inexpensive and rapid technique which can be operated by a lay person to distinguish the genuine product from the counterfeit medicines. Nazzal et al. demonstrated that the quantitative measurement of the color on the surface of tablets with a colorimeter could be used as a unique product identifier.31

In other studies researchers have developed intelligent rapid, low cost and portable systems that take advantage of a digital imaging approach to detect the discoloration status of steel bridge coating by using smartphones.32-35

Three models, RGB (red, green, and black), CMYK (cyan, magneta, yellow, and key), and CIELAB, are commonly used to express color. RGB is an additive color space in which each color is generated by adding Red, Green and Blue hues in suitable proportion. A shortcoming is that this model is device dependent in that detection and reproduction of color may vary.

Luminance also limits the gathering of saturation and hue of colors from RGB parameters, as well as information about lightness and shadows, and RGB is digitized for only 256 x 256 x 256 colors. CMYK which is the subtractive color model used in color printing, and defines four colors: Cyan, Magenta, Yellow (and Black). White color is the default in the absence of light.

Other colors are made by subtracting (absorbing) suitable wavelengths. Both RGB and CMYK cannot display the full range of color seen by the human eye. A third model is CIELAB color space which has been specified by the International Commission on Illumination. It describes all the colors visible to the human eye and was created to serve as a device independent model, which means that the numerical number change is close to the visually sensed change. This color space consists of one channel for lightness (L) and two color channels (a and b) where a represents a value in the red to green scale and b represents a value in the blue to yellow scale. It

21 also should be noted that neither RGB nor CMYK fully encompass the color gamut of CIELAB.

However, most imaging devices report color in RGB space, and therefore even if the color coordinates are converted to CIELAB space, the many potential benefits of CIELAB space are lost. However, even under these circumstances, it is possible that variations in the L channel may facilitate direct detection of artifacts due to transient shadows.

Color change is central to the sensing properties of vapochromic materials, which are a class of compounds that exhibit a colorimetric response to certain chemical vapors.36-42 In other words, these materials change color upon exposure to certain volatile organic compounds. The most common class of these materials consist of square-planar platinum(II) complexes.36,38,42-46

Since color changes are easily detected by eye, these materials are potentially useful in the

47-101 detection of specific hazardous volatile compounds (e.g., NH3, pyridine, CO, H2S).

Therefore, there is considerable interest in understanding and learning to modulate the properties of vapochromic materials for practical use. Visible absorption/reflectance spectroscopy have provided valuable insights into the properties of these materials.102-106 However, despite considerable effort, there is surprisingly little detailed understanding of the kinetics and mechanisms of the vapochromic responses of these systems. A contributing factor is the expense in terms of resources and time required to undertake detailed kinetics studies in which different samples are exposed to identical conditions in order to allow for comparisons to be made.

Inspired by this constellation of circumstances, we describe here the development and evaluation of a comparatively inexpensive, rapid and portable strategy for simultaneously monitoring the color space coordinates of multiple vapochromic materials under the same vapor exposure conditions. This approach utilizes a digital camera and a semi-automatic color change analysis software that digitally analyzes the vapochromic behavior of chemical compounds

merely based on a series of images (e.g., video frames) recorded throughout the color change

experiment. The results demonstrate that this strategy is effective in producing quantitative

information about the kinetics of processes that qualitatively resembles human visual perception.

Experimental Sections

Materials and Methods. K2PtCl4 was purchased from Pressure Chemical. COD (1,5-

cyclooctadiene), 4,4'-dimethyl- 2,2'-bipyridine, and 2,2′:6′,2″-terpyridine (tpy) were obtained

from Aldrich and used as received. NH4PF6, NaAsF6 and NaSbF6 were purchased from Aldrich.

+ NH4ClO4 was purchased from Fisher Scientifics. [Pt(tpy)Cl]ClO4.H2O, Pt(COD)Cl2, [Pt(tpy)X]

salts (X=Cl,Br, I), and 4,4'-dicarboxymethyl 2,2'-bipyridine (dcmbpy) were prepared according

to published procedures.107-110

Images were collected using a Keyence Digital Microscope VHX-1000 with the

microscope light, or the digital camera of an iPhone 6 cell phone with room light. In the case of

the cell phone, care was taken to avoid approaching the experimental setup during data

collection, because the color derived from images was found to be very sensitive to shadows.

Synthesis and imaging of [Pt(tpy)Cl]ClO4.H2O at different humidity levels.

110 [Pt(tpy)Cl]ClO4.H2O was prepared according to the published procedures. Red needles of

107 [Pt(tpy)Cl]ClO4.H2O were grown from an acetone/water mixture. (Scheme 1)

Scheme 1. Line drawing of [Pt(tpy)Cl]ClO4.H2O

The crystals were isolated by filtrations. To reduce the potential hazard of an explosion, very small batches of crystals (~10mg) were individually ground to a fine uniform powder and then dried in a desiccator. For each humidity experiment, 10 mg of the dehydrated yellow powder was placed in aluminum weigh boats. Each sample was placed in chamber set to a relative humidity level (38%, 57%, 64%, 75% and 100%) using saturated aqueous solutions of different salts.111

The color of the sample was monitored for 10 to 60 min by taking a photo every 0.1 to 3 seconds using a stationary-mounted cell phone camera.

- - - Synthesis and imaging of [Pt(tpy)X ]YF6 (X = Cl , Br and I ; Y = P, As and Sb) salts.

Starting materials, Pt(COD)Cl2 and [Pt(tpy)Cl]Cl.2H2O, were prepared according to published

109,110 procedures. Pt(COD)Br2 and Pt(COD)I2 were prepared by adding excess equivalents of

112 LiBr and NaI, respectively, to a acetone solution of Pt(COD)Cl2. [Pt(tpy)Br]Br and [Pt(tpy)I]I

110 were prepared following the procedure used for [Pt(tpy)Cl]Cl.2H2O. Samples of

- - - 112 [Pt(tpy)X](YF6) (X = Cl , Br and I ; Y = P, As and Sb) were prepared by anion metathesis.

(Scheme 2). The XRPD pattern for the salts matched the simulated pattern for the previously determined crystal structures112 (Appendix Figures A1-A9).

+ - - - Scheme 2. Line drawing of [Pt(tpy)X ]YF6 (X = Cl , Br and I ; Y = P, As and Sb)

For vapor absorption studies, bright yellow crystalline samples of each of the nine [Pt(tpy)X]YF6 salts (Scheme 2) were grown by the partial evaporation of a (1:1) acetone-water mixture at room

24 temperature. The crystals were filtered, dried, and ground to give fine uniform powders. Samples of each of the powders (10 mg) were placed in separate aluminum weigh boats. The nine weigh boats were placed inside a glass chamber containing a reservoir of acetonitrile. After sealing the chamber, the colors of the samples in the weigh boats were monitored by taking a photo of all nine samples simultaneously every 5 seconds using a stationary-mounted cell phone camera

(iPhone 6). 2702 photos were collected over a period of 225 minutes. For the vapor desorption process, nine yellow powder samples in aluminum weigh boats were exposed to acetonitrile vapor for 24 h. Each of the samples changed color to red, except for [Pt(tpy)Cl]SbF6 and

[Pt(tpy)Br]SbF6, which are not vapochromic. The desorption of acetonitrile from the samples in air was monitored by taking a photo every 10 seconds using the stationary camera. 1200 photos were collected over a period of 200 minutes.

Synthesis and imaging of [Pt(dcmbpy)Cl2].CH2Cl2. To 30 ml of 1:1 dichloromethane -

113 methanol solution was added 20 mg of Pt(DMSO)2Cl2 and 270 mg of 4,4'-dicarboxymethyl

2,2'-bipyridine (dcmbpy).108 The resultant solution was stirred under argon for a day. This afforded a dark red precipitate which was filtered, and washed with diethyl ether. Red crystals of

[Pt(dcmbpy)Cl2].CH2Cl2 were grown from a dichloromethane solution at room temperature

(Scheme 3).

Scheme 3. Line drawing of [Pt(dcmbpy)Cl2].CH2Cl2

In order, to monitor the kinetics of the color change and stability of the green intermediate, red crystals of [Pt(dcmbpy)Cl2].CH2Cl2 were placed on a glass microscope slide.

The sample was exposed to acetone vapor and a video was recorded over a period of 1 hour using a Keyence Digital Microscope VHX-1000 with 200 times (200X) magnification.

Color Change Analysis

To analyze the change in color of the above mentioned vapochromic materials, a semi- automatic software was developed in MATLAB® R2016b (MathWorks, Inc.) The input data for the software is the series of images or the video recorded throughout the experiment. The software was set up to read .jpg images and mp4 videos, but it can be easily modified to read other commonly used image/video file formats. The analysis performed by the color change analysis software results in several color change indicators such as color change path, speed of color change, color change, as well as the onset time, end time and half-time.

The following describes the algorithm and deliverables of the color change analysis software:

Step 1. Given the image file provided by user, the software reads and displays the

first image (or the first video frame) and prompts user to select one or more

n×n pixel ranges, which are the Regions of Interest (ROIs) for each

vapochromic sample in the image. These ROIs are subsequently used to

collect color samples for each compound. In this study, we used n=10 for the

macroscopic images obtained using a iPhone 6 and n=3 for the microscopic

video recording obtained using a Keyence Digital Microscope. Throughout

the remainder of this discussion, the term “image” is used for both the .jpg

images and video frames.

Step 2. Next, the software loads the remaining images using MATLAB’s

imread/read function. The output of imread/read function is in RGB color

model. At each time step (i.e., for each image), the software reads and

averages the R, G, and B values within each ROI. The outputs of this step are

four one-dimensional arrays for each ROI: time (푡), average red color

channel (푅(푡)), average green color channel (퐺(푡)), and average blue color

channel (퐵(푡)).

Step 3. Next, the RGB data at each time are converted to the standard CIELAB color

space using MATLAB’s rgb2lab function. CIELAB is a color space

specified by International Commission on Illumination to mimic the human

vision.31 This color space consists of one channel for lightness (L) and two

color channels (a and b). The outputs of this step are the three arrays of 퐿(푡),

푎(푡) and 푏(푡), where 푡 is time and 퐿, 푎 and 푏 are the lightness and color

channels in CIELAB color space.

Step 4. After this step, the software finds the best analytical fits for 퐿(푡), 푎(푡) and

푏(푡) data. For the vapochromic sensors synthesized in this study, a multi-step

logistic function in the form of

푁 푁 퐾푖 푓(푡) = ∑푖=1 푓푖(푡) = ∑푖=1 (1) 1+푒(−훼푖(푡−훽푖))

provided good fits (R2 > 0.90). In equation 1, 푁 is the number of logistic

functions 푓푖(푡). 퐾푖 and 훼푖 are the carrying capacity and growth rate of each

logistic function, respectively, and 훽푖 is the time in which 푓푖(푡) reaches 퐾푖/

2. The number of logistic functions (푁) is manually set to 2 and 3 for two-

and three-step color change cases, respectively. For each ROI, the above

nonlinear regression is solved using MATLAB’s fitnlm function for 퐿(푡),

푎(푡) and 푏(푡) individually. The outputs of this step are three one-

dimensional arrays of 퐿′(푡), 푎′(푡) and 푏′(푡). It should be noted that the

software can be readily modified to incorporate other functions (c.f., a multi-step

logistic function) in order to describe different behaviors than those

encountered here. Figure 1 shows an example of a one-step logistic function

fitted on recorded color change of a sample salt.

Step 5. Given the fitted curves of 퐿′(푡), 푎′(푡) and 푏′(푡), the software produces the

following information for each compound:

a. Color change path is the 3D path represented by values of 퐿′(푡), 푎′(푡)

and 푏′(푡) in 퐿′-푎′-푏′ space. This curve shows the color path that the

vapochromic material follows throughout the experiment.

b. Speed of color change, 푆(푡), is the rate of change of color along the

color change path at time t, which is defined as:

푑퐿′ 2 푑푎′ 2 푑푏′ 2 푆(푡) = √( ) + ( ) + ( ) (2) 푑푡 푑푡 푑푡

푑퐿′ 푑푎′ 푑푏′ where , and are the time derivatives of 퐿′(푡), 푎′(푡) and 푑푡 푑푡 푑푡

푏′(푡). Large values of S(t) are associated with periods when the color

of the substrate is changing rapidly, whereas low values are associated

with periods when color is changing very slowly or not at all.

28 c. Color change, 퐶퐶(푡), is the distance between the initial color and the

color at time t in L-a-b space, defined as:

푡 퐶퐶(푡) = 푆(푡) 푑푡 (3) ∫0

From the geometrical viewpoint, 퐶퐶(푡) is the distance between

[퐿′(0), 푎′(0), 푏′(0)] and [퐿′(푡), 푎′(푡), 푏′(푡)] along the color change

path.

d. Onset time,푡0, end time, 푡푓, and half-time, 푡1/2, quantify the

responsiveness and duration of color change events. Onset time is

defined as the time at which 푆(푡) reaches 10% of its peak upon the

initiation of color change. End time is defined as the time at which

푆(푡) descends to 10% of its peak before reaching its plateau. Finally,

푡 −푡 half-time is calculated as 푓 0. In cases where 푆(푡) starts from a value 2

larger than 10% of its peak, t0 is defined as zero.

Figure 1. (A) Color of [Pt(tpy)Cl]PF6 during exposure to acetonitrile: original colors from digital camera (top) vs. colors after curve fitting (bottom). (B) Original and fitted curves of L, a and b.

Results and Discussion

Determining the influence of vapor pressure on vapochromic response. Using a cell phone camera, images of yellow powder samples of [Pt(tpy)Cl]ClO4 were recorded immediately after placing each sample in a chamber at a specific relative humidity level (38%, 57%, 64%,

75% and 100%). For each humidity, the images were analyzed by identifying a 10 x 10 pixel region of interest (ROI), averaging the RGB colors of the pixels within the region for each frame, and then converting the colors to CIELab color space. The evolution of each of the L, a and b parameters in time was found to be well-described by the sum of two logistic functions (see

Experimental Section). From these fits, the rate of color change (S(t)) as function of time was determined; plots of S(t) are shown in Figure 2a. The onset time (t0) for color change was taken 30 as the time point at which S(t) reached 10% of its maximum value or was assigned a value of zero if S(t=0) already exceeded 10% of the maximum. The end time (tf) was defined as the time at which S(t) dropped to 10% of its maximum value. The average of these two times (t1/2) provided a measure of the duration of response (t1/2). The dependence of t1/2 on relative humidity

(%) also is shown in Figure 2b.

(A) (B)

Figure 2. (A) Rate of the color change (S(t)) with respect to time at different humidity levels of 38% ( ), 57% ( ), 64% ( ), 75%( ), 100% ( ), and (B) the plot of the t1/2 vs relative humidity (%).

The overlaid graphs of the rate of the color change (S), color change (CC) and the average color bar of [Pt(tpy)Cl]ClO4 at different humidity levels are shown in Figures A10-A13.

The t1/2 values at different humidities are 1.2 (100%), 2.1 (75%), 4.3 (64%), and 15.5 min (57%).

There was no response at 38%. A similar trend occurs for the maximum of S(t), which is the inflection point of CC(t). Thus, the results show that the speed of response increases as the vapor pressure increases. This suggests the notion that relative rates of change in such vapochromic systems could be used as a measure of analyte concentration, which may be a more

31 reliable parameter given the challenge of assessing color parameters in real world settings with wide variation in lighting and contrast.

Interestingly, the analysis of the video images did not reveal any evidence of the formation of intermediates, which would be expected to manifest as shoulders or additional peaks in the profile of S. This conclusion is in reasonable agreement with previous measurements of UV-visible absorption spectra of thinly dispersed crystallites of

114 [Pt(tpy)Cl]ClO4 recorded during exposure to water vapor. At first glance, it was not obvious from those spectra that water absorption was more complicated than an A→B process. However, close examination of the spectra revealed two pseudo-isosbestic points at ~500 and ~510 nm, as emphasized by the red and blue coloration of spectra in Figure 3. On their own, these data were not entirely convincing, since solid samples are prone to scattering artifacts. However, diffraction, gravimetric, and calorimetric data ultimately corroborated the presence of

114 intermediates during hydration and dehydration of [Pt(tpy)Cl]ClO4 complex. The absence of detailed spectral information is an obvious limitation of the method described here. However, it is noteworthy that preparation of quality thin films for measurements of the type shown in Figure

3 is considerably more tedious and far less reproducible than sample preparation using the approach described here.

0.30

0.26

0.22

0.18 400 500 600 Wavelength (nm)

Figure 3. UV-visible absorption spectra of thinly dispersed crystallites of [Pt(tpy)Cl]ClO4 recorded during exposure to water vapor. The red and blue colours are intended to draw attention to the fact that the earliest spectra have an approximate isosbestic point shifted slightly from that of the later spectra.

Simultaneous measurements of vapochromic response for comparative kinetic

- - - studies. Scheme 4 shows an array of nine [Pt(tpy)X]YF6 (X = Cl , Br and I ; Y = P, As and Sb) complexes and their responses to acetonitrile vapor. Scheme 4 shows the approximate colors of the nine [Pt(tpy)X]YF6 salts and the corresponding acetonitrile solvates. Seven of the nine bright yellow [Pt(tpy)X]YF6 solids are vapochromic, turning from the yellow to orange upon exposure to air saturated with acetonitrile vapor for 48 hours at room temperature. Dashed diagonal lines indicate that the salts absorb vapor and as a result change color. Two of the yellow materials,

[Pt(tpy)Cl]SbF6 and [Pt(tpy)Br]SbF6, did not change color when exposed to acetonitrile vapor and stayed yellow over the 48 hour vapor exposure period. However, crystals of

[Pt(tpy)Cl]SbF6.CH3CN and [Pt(tpy)Br]SbF6.CH3CN are readily prepared by evaporation of acetonitrile solutions.

- - - Scheme 4. Array of [Pt(tpy)X]YF6 ([X]YF6) (where X = Cl , Br and I ; Y = P, As and Sb) salts and their qualitative colors in the absence (upper left of diagonal) and presence (lower right of diagonal) of acetonitrile vapor, which produces the acetonitrile solvate. [Cl]SbF6 and [Br]SbF6 do not absorb acetonitrile vapor, as indicated by the solid diagonal line.

Samples of each of the nine salts were placed in a chamber of acetonitrile-saturated air.

The colors of the salts were monitored by recording an image using a cell phone every 5 seconds over a 220 min period. The resulting 2702 images were analyzed by identifying a 10 x 10 pixel region of interest for each of the nine samples, averaging the RGB colors of the pixels within each region for each frame, and then converting the colors to CIELab color space. Two of the samples, [Pt(tpy)Cl]SbF6 and [Pt(tpy)Br]SbF6, remained the same color throughout the duration of the experiment. For the remaining seven samples, the evolution of each of the L, a and b parameters in time was found to be well-described by the sum of two logistic functions (see

Experimental Section and Figure 1). From these fits, the rate of color change (S(t)) as function of time was determined. The colors of each sample at each time point and plots of S(t) are shown in Figure 4. The overlaid plots of the colors, rate of the color change (S), and color change (CC) of each of the nine salts are shown in Figure A14. Despite rather small chemical

34 differences between these materials, the response behaviors varied markedly. Notably, the range of the durations of response (i.e., t1/2 values) spanned more than 90 minutes, and in the case of one complex, there was a significant delay in the onset of response (t0). Thus, the results of a single videographic experiment can provide unambiguous, quantitative information about the relative rates of response of a series of vapochromic compounds.

For the desorption process, the color change was monitored by recording an image every

10 seconds after exposing the nine red-orange [Pt(tpy)X]YF6.CH3CN salts to air at room temperature. The resulting 1200 images were analyzed (Appendix Figure A15). Again, given their chemical similarities, the materials exhibited a wide variation in kinetic parameters.

Notably, the range of the durations of response spanned more than 140 minutes. Interestingly, in the case of one complex, the desorption process was still not complete after one day. This conclusion was not obvious when observing by eye, but it was readily evident in plots of S(t) and

CC(t), thus demonstrating that electronic monitoring of color can provide additional insights.

The overlaid plots of the rate of the color change (S), color change (CC) and the average color bar of all nine salts are shown in Appendix Figure A16. Values of t0, tf and t1/2 are collected in

Table A1. The values of Smax and area under the graph of S versus time for both absorption and desorption process is summarized in Table A2. For both the absorption and desorption experiments, the fact that the evolution of color in time was well described using two logistic functions is consistent with an A→B type process. Of course it is possible that the mechanisms of vapor absorption/desorption are more complex than this, but it is evident that, in this case, those details are not amplified in color space.

Figure 4. (A) Average color within the region of interest of each [Pt(tpy)X]YF6 salt during exposure to acetonitrile vapor for 225 minutes. (B) Rate of color change (S(t)) as a function of time for the nine salts.

Table 1. The vapochromic response was monitored over time, and the rate of color change plot was used to determine the onset, end, and duration of the absorption

Sample Onset/min End/min Duration/min

[Pt(tpy)Cl]PF6 28.0 117.3 89.3

[Pt(tpy)Br]PF6 14.4 48.6 34.2

[Pt(tpy)I]PF6 5.9 23.9 18.0 [Pt(tpy)Cl]AsF6 49.0 183.4 134.4

[Pt(tpy)Br]AsF6 19.3 46.8 27.5

[Pt(tpy)I]AsF6 22.8 48.1 25.3

[Pt(tpy)Cl]SbF6 m

[Pt(tpy)Br]SbF6

[Pt(tpy)I]SbF6 38.8 92.3 53.5

Detection of an intermediate in a vapochromic process. Red crystals of

Pt(dcmbpy)Cl2.CH2Cl2 change color to green and lose one equivalent of CH2Cl2 when exposed to acetone vapor. Upon further exposure, the resulting green Pt(dcmbpy)Cl2 converts to a yellow form of Pt(dcmbpy)Cl2 (Figure 5).

Figure 5. Crystals of [Pt(dcmbpy)Cl2].CH2Cl2 (A) before and (B) after acetone exposure viewed through a microscope at 200X magnification. During the 1-hour vapor exposure period, most of the crystals changed color to green, and some changed color to yellow. All crystals changed color to yellow during exposure for 24 hours.

This process was captured in a one-hour video. The resulting 86262 frames were analyzed by converting the average color of the sample in the ROI of each image to CIELAB color space. The evolution of the L, a and b parameters in time required a sum of three logistic functions (c.f., two functions used in the previous examples). This observation is consistent with the formation of an intermediate in the vapochromic response, which matches observations made by eye. From the fit, the rate of color change, S(t), as function of time and the color change

37 function, CC(t), were determined. These resulting functions are plotted in Figure 6. The red crystals immediately started to turn green and were fully converted to the green color after ~8 min. The red-to-green conversion is characterized by a t1/2 value of 4 min. The green intermediate was stable for ~30 min under these conditions, as indicated by the flat S(t) and

CC(t) functions over this time range (Figure 6.) After ~40 min the color within the ROI had turned yellow, however it was evident from inspection of other regions of the images that the rate of conversion from green to yellow varied widely. However, within 48 hours exposure to acetone vapor, all crystals had turned yellow. For the region monitored in this experiment, the green-to-yellow conversion exhibited a t1/2 value of 20 min. The behavior of the S(t) and CC(t) functions clearly captures the A→B→C behavior observed by eye. But in addition, this analysis provides insights into the stability (i.e., duration) of each phase as well as the speed at which the transitions occur. Interestingly, identification of other ROIs for characterization can lead to different t1/2 values for the red to green and green to yellow conversion which shows another potentially highly significant application of this method which allows us to evaluate spacial heterogeneity in the response properties of a material.

Figure 6. Plot of the color of [Pt(dcmbpy)Cl2].CH2Cl2 during the acetone vapor exposure. Black solid line shows the color change (CC) vs time, black dashed line shows the rate of the color change (S) vs time.

Conclusion

In this study we set out to evaluate the use of a rapid, low cost, portable strategy for monitoring changes in color during exposure of vapochromic materials to chemical vapors. We developed a semi-automatic color analysis software that digitally analyzes color in regions of interest, allowing for comparisons of thousands of images collected over time. The results demonstrate that this strategy is effective in producing quantitative information about the kinetics of processes that qualitatively resembles human visual perception. Though this approach does

39 not provide wavelength information typical of absorption/reflectance spectroscopy, it offers significant advantages, including convenience and the capability to simultaneously evaluate color changes in multiple samples under identical conditions. In addition, this method allows for evaluation of spatial heterogeneity in the response properties of a material.

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Appendix

Figure A1. X-ray powder diffractograms of the ground yellow [Pt(tpy)Cl]PF6 (—), and the simulated pattern (—).

Figure A2. X-ray powder diffractograms of the ground yellow [Pt(tpy)Br]PF6 (—), and the simulated pattern (—).

Figure A3. X-ray powder diffractograms of the ground yellow [Pt(tpy)I]PF6 (—), and the simulated pattern (—).

Figure A4. X-ray powder diffractograms of the ground yellow [Pt(tpy)Cl]AsF6 (—), and the simulated pattern (—).

Figure A5. X-ray powder diffractograms of the ground yellow [Pt(tpy)Br]AsF6 (—), and the simulated pattern (—).

Figure A6. X-ray powder diffractograms of the ground yellow [Pt(tpy)I]AsF6 (—), and the simulated pattern (—)

Figure A7. X-ray powder diffractograms of the ground yellow [Pt(tpy)Cl]SbF6 (—), and the simulated pattern (—).

Figure A8. X-ray powder diffractograms of the ground yellow [Pt(tpy)Br]SbF6 (— ), and the simulated pattern (—).

Figure A9. X-ray powder diffractograms of the ground yellow [Pt(tpy)I]SbF6 (—), and the and the simulated pattern (—).

Figure A10. The overlaid graphs of the Rate of the color change (S), Color

Change (CC) and Average color bar of [Pt(tpy)Cl]ClO4 at 38% humidity.

Figure A11. The overlaid graphs of the Rate of the color change (S), Color

Change (CC) and Average color bar of [Pt(tpy)Cl]ClO4 at 57% humidity.

Figure A12. The overlaid graphs of the Rate of the color change (S), Color

Change (CC) and Average color bar of [Pt(tpy)Cl]ClO4 at 64% humidity.

Figure A13. The overlaid graphs of the Rate of the color change (S), Color

Change (CC) and Average color bar of [Pt(tpy)Cl]ClO4 at 100% humidity.

Figure A14. The overlaid graphs of the Rate of the color change (S), Color Change (CC) and

- - - Average color bar of [Pt(tpy)X]YF6 (X = Cl , Br and I ; Y = P, As and Sb) salts.

Figure A15. Rate of color change with respect to time for the nine salts as acetonitrile vapor was desorbed.

Figure A16. The overlaid graphs of the Rate of the color change (S), Color Change (CC) and Average color bar of

- - - [Pt(tpy)X]YF6 (X = Cl , Br and I ; Y = P, As and Sb) salts as acetonitrile vapor was desorbed.

Table A1. The vapochromic response was monitored over time, and the rate of color change plot was used to determine the onset, end, and duration of the desorption.

Sample Onset/min End/min Duration/min

[Pt(tpy)Cl]PF6 0.00 40.0 40

[Pt(tpy)Br]PF6 11.5 23.5 12

[Pt(tpy)I]PF6 0.00 - -

[Pt(tpy)Cl]AsF6 8.70 23.2 14.6

[Pt(tpy)Br]AsF6 16.0 62.0 46

[Pt(tpy)I]AsF6 0.00 48.2 48.2

[Pt(tpy)Cl]SbF6

[Pt(tpy)Br]SbF6

[Pt(tpy)I]SbF6 12.8 117.0 104.2

Table A2. The vapochromic response was monitored over time, and the rate of color change plot was used to determine the Smax, and Area under the S versus time of the absorption and desorption.

Sample Smax of absorption Area Smax of desorption Area

[Pt(tpy)Cl]PF6 0.12 6.00 0.14 2.98

[Pt(tpy)Br]PF6 0.07 1.46 1.27 8.21

[Pt(tpy)I]PF6 0.66 6.53 0.03 3.35

[Pt(tpy)Cl]AsF6 0.06 4.83 1.21 9.69

[Pt(tpy)Br]AsF6 0.25 3.73 0.34 8.61

[Pt(tpy)I]AsF6 0.24 3.29 0.23 6.19

[Pt(tpy)Cl]SbF6 -

[Pt(tpy)Br]SbF6

[Pt(tpy)I]SbF6 0.20 6.21 0.14 7.88

X-ray Powder Diffraction

Data for yellow materials were collected at the Advanced Photon Source Station 11-BM,

Argonne National Laboratory using 0.413620 Å photons in transmission geometry as the sample was rotated. The incident photons were selected with platinum striped mirrors and a double

Si(111) monochromator with adjustable sagittal focus. The scattered beams were selected with

(12) perfect Si(111) analyzer crystals. Discrete detectors covering an angular range from -6 to

16º 2θ were scanned over a 34º 2θ range, with data points collected every 0.001º 2θ at a scan speed of 0.01º/s.

Chapter 3

Vapochromic materials that do not incorporate vapors in the crystal lattice and form Nano fibers

Introduction

In the late 1990s, Mann and co-workers ignited the field of vapochromism with their studies of platinum(II) double salts that undergo distinct color changes and changes in luminescence properties upon exposure to certain volatile organic compounds.1-7 Since then, it has been found that the vapochromic effect in nearly all of the more than one hundred vapochromic platinum(II) complexes investigated is associated with absorption and desorption of the vapor by the solid.8-13 Spectroscopic and structural studies reveal that absorption of vapor can modify crystal packing, resulting in changes in non-covalent Pt···Pt and - stacking interactions. The wide variation in colors of materials composed of stacked platinum(II) complexes with polypyridyl ligands (e.g., 2,2'-bipyridine) is a consequence of an intense, spin- allowed mixed-metal-to-ligand charge-transfer (MMLCT) transition, involving a filled d*

HOMO derived from the antibonding combination of the Pt(5dz2) orbitals of adjacent complexes and an unoccupied * LUMO of the aromatic heterocyclic ligand.14,15 The energy of this band is strongly dependent on non-covalent Pt···Pt interactions and therefore strongly affected by rearrangements in solid-state packing induced by vapor uptake. The luminescence from these systems frequently originates from the corresponding spin-forbidden MMLCT state.

Virtually all known platinium(II) vapochromic systems involve incorporation of vapor.

But there is one interesting exception which was reported by You et al., who described the

16 vapochromic behavior of two enantiomeric chiral platinum(II) complexes. Pt(La)(C≡C ̶ Ph) and

׳ ׳ -bipyridine, Lb = (+)- 4,5-pinene-6 -phenyl-׳Pt(Lb)(C≡C ̶ Ph) (La = (-)- 4,5-pinene-6 -phenyl-2,2

bipyridine). The complexes selectively responded to dichloromethane vapor changing color-׳2,2 from yellow to orange. However, single crystal X-ray diffraction data showed that dichloromethane was not incorporated in either of the yellow or orange forms. The color change was attributed to increased … and Pt…Pt interactions in the orange form. The authors suggested that the high selectivity for dichloromethane vapor is a result of the abundant attractive and exoergic non-covalent interactions between dichloromethane vapor and the yellow forms which can affect the molecular structure and packing mode during the crystal growth.

Interestingly, this is the only example where vapor induces a color change without becoming incorporated into the crystal lattice.

This chapter reports a detailed investigation of Pt(dcmbpy)Cl2.CH2Cl2, which exhibits the unusual behavior of responding to certain vapors without vapor uptake (Scheme 1). Specifically, red, solid Pt(dcmbpy)Cl2.CH2Cl2 is indefinitely stable at RT, and thermal gravimetric analysis shows that the CH2Cl2 molecules is not lost until ~105 °C. However, upon exposure to acetone,

THF, or methanol vapors, the material changes color to green and finally to a yellow color. The green and yellow materials were characterized by several techniques, including XRPD and SEM.

Results show that exposure to acetone, THF or methanol produced the same stable green intermediate and the same final yellow material. The crystal structure of the yellow material confirmed that there is no solvate molecule incorporated in the crystal lattice. SEM images of the green and yellow materials show evidence for the formation of remarkable nanoscale architectures induced by vapor exposure. The vapor-induced formation of a nanoscale structure during the vapochromic process has not previously been documented.

MeO

N Cl Pt CH2Cl2 N Cl MeO

Scheme 1. Pt(dcmbpy)Cl2.CH2Cl2

Experimental

Materials. K2PtCl4 was purchased from Pressure Chemical (Pittsburgh, PA).

[Pt(DMSO)2Cl2] and 4,4'-dicarboxymethyl 2,2'-bipyridine (dcmbpy) were prepared according to

17,18 published procedures. Deuterated DMSO-d6 solvent were purchased from Cambridge Isotope

Laboratories. All other reagents were purchased from Sigma-Aldrich.

Synthesis of Pt(dcmbpy)Cl2.CH2Cl2 (R1.CH2Cl2 and R1.CH2Cl2), and

Pt(dcmbpy)Cl2 (Y). To 30 ml of 1:1 dichloromethane-methanol solution was added 20 mg of

18 Pt(DMSO)2Cl2 and 270 mg of 4,4'-dicarboxymethyl-2,2'-bipyridine (dcmbpy). The resultant solution was stirred under argon for 24 h. The resulting dark red precipitate was isolated by filtration, and washed with diethyl ether. Red solvated crystals, Pt(dcmbpy)Cl2.CH2Cl2

(R1.CH2Cl2), were grown by partial evaporation of a dichloromethane solution at room temperature. Grinding this material produced a red powder, which is referred to as R2.CH2Cl2.

Yellow non solvated crystals, Pt(dcmbpy)Cl2 (Y), were grown by partial evaporation of a dichloromethane-acetone mixture at room temperature. Both R1.CH2Cl2 and Y were

1 characterized by X-ray crystallography. H NMR of R1CH2Cl2 and R2CH2Cl2: (DMSO-d6, δ):

9.75 (2H, d, J=4.9 Hz), 9.13 (2H, s), 8.27 (2H, d, J=6.8 Hz), 5.76 (2H, s, CH2Cl2), 4.01 (6H, s, -

-1 -1 CH3). UV-vis (dichloromethane): λmax/nm (ԑ/mol cm ): 302(25084), 424(4314).

Characterization and Methods

1H NMR spectra were recorded at room temperature using a Bruker AC 400 MHz

1 instrument. H NMR spectra of samples of R1.CH2Cl2 were recorded during the acetone vapor exposure. This was accomplished by removing each solid sample as rapidly as possible from a sealed chamber containing a reservoir of acetone and then quickly dissolving the sample in

DMSO-d6.

TGA measurements were carried out using a NETZSCH STA 409 PC instrument. An open aluminum oxide ceramic crucible was used for sample loading. The instrument was continuously purged with argon gas at a 30 ml/min flow rate. The temperature was increased from ~24°C to 155°C at 10°C/min. XRPD data for yellow materials were collected at the

Advanced Photon Source Station 11-BM, Argonne National Laboratory using 0.413620 Å photons in transmission geometry as the sample was rotated. The incident photons were selected with platinum striped mirrors and a double Si(111) monochromator with adjustable sagittal focus. The scattered beams were selected with (12) perfect Si(111) analyzer crystals. Discrete detectors covering an angular range from -6 to 16º 2θ were scanned over a 34º 2θ range, with data points collected every 0.001º 2θ at a scan speed of 0.01º/s. The XRPD data for the red and green materials were collected in Bragg-Brentano geometry (θ-2θ) using a Philips X'Pert

Powder Diffractometer operated at 40 kV, 40 mA with CuKαradiation (λ=1.54 Å). Room- temperature steady-state emission spectra were collected using a SPEX Fluorolog-3 fluorimeter equipped with a double emission monochromator and a single excitation monochromator.

Crystal or powder samples of Pt(dcmbpy)Cl2.CH2Cl2 were sprinkled on a thin strip of grease on

69 a microscope slide. The slide was gently shaken to remove unadhered material. Emission spectra were collected by exciting the sample at 436 nm. All spectra were corrected for instrument response. For SEM studies, samples were sputter-coated with gold-palladium mixture using a

Denton Desk II Sputter Coater and imaged using a FEI/Phillip XL 30 ESEM-FEG at a 15 kV accelerating voltage. Red, green, and yellow crystals were covered by a gold-palladium mixture prior to SEM imaging.

Structure Determination:

Red needle-like crystals of R1.CH2Cl2 were grown from dichloromethane. Yellow crystals of Y were grown from CH2Cl2-acetone. For X-ray examination and data collection, a suitable crystal (R1.CH2Cl2: 0.14 x 0.02 x 0.01 mm; Y: 0.040 x 0.025 x 0.015 mm) was mounted in a loop with paratone-N and transferred immediately to a goniostat bathed in a cold stream. Intensity data were collected at 150K at Beamline 11.3.1 at the Advanced Light Source

(Lawrence Berkeley National Laboratory) with λ=0.7749Å using a Bruker APEX2 CCD detector for R1.CH2Cl2 and a Bruker PHOTON100 CMOS detector for Y. Data frames (R1.CH2Cl2: 2s frames at 0.3°  intervals; Y: 1s frames at 0.5  intervals) were collected to ~60° in 2 using

APEX2 and processed using the program SAINT routine within APEX2. The data were corrected for absorption and beam corrections based on the multi-scan technique as implemented in SADABS for R1.CH2Cl2 and TWINABS for Y. The structures were solved by a combination of direct methods involving SHELXTL v6.14 and the difference Fourier technique, and refined by full-matrix least squares on F2. Non-hydrogen atoms were refined with anisotropic displacement parameters. The H-atom positions were calculated and treated with a riding model in subsequent refinements. The isotropic displacement parameters for the H-atoms were defined as a*Ueq of the adjacent atom (a=1.5 for methyl and 1.2 for all others). In the case of

R1.CH2Cl2, the largest residual electron density peak resides in a chemically-unreasonable position between C1-H1 of the substituted bipyridyl ring and Cl1 bound to Pt. Attempts to model it as a potential water molecule, methyl-C or hydroxyl-O gave unreasonable results with respect to displacement parameters and location. Crystal data are summarized in Table 1.

Table 1: Crystallographic Data for Pt(dcmbpy)Cl2.CH2Cl2 (R1.CH2Cl2) and Pt(dcmbpy)Cl2 (Y) at (150 K)

Formula C14H12N2O4Cl2Pt.CH2Cl2 (Red C14H12N2O4Cl2Pt (Yellow form) form)

Formula weight 623.17 538.25 Temperature 150(2) K 150(2) K Wavelength 0.77490 Å 0.7749 Å Crystal system Monoclinic Monoclinic

Space group P21/c P21/c

a = 17.276(2) Å; α = 90° a = 13.2604(11) Å; α = 90° Unit cell dimensions b = 6.7419(8) Å; β = 115.075(1)° b = 24.953(2) Å; β = 105.940(2)° c = 17.665(2) Å; γ = 90° c = 9.6391(8) Å; γ = 90°

Volume 1863.6(4) Å3 3066.8(4) Å3 Z 4 8 Density (calculated) 2.221 Mg/m3 2.332 Mg/m3 Absorption coefficient 10.086 mm-1 11.805 mm-1 F(000) 1184 2032 Crystal size 0.14 x 0.02 x 0.01 mm3 0.040 x 0.025 x 0.015 mm3 θ range for data collection 3.59 to 28.97° 2.490 to 31.172° Index ranges -21 ≤ h ≤ 21, -8 ≤ k ≤ 8, -21 ≤ l ≤ 22 -17 ≤ h ≤ 17, -33 ≤ k ≤ 33, -12 ≤ l ≤ 12

Reflections collected 15914 61883

Independent reflections 3804 [Rint = 0.0558] 61883 Completeness to θ = 67.60° 99.7 % Completeness to θ = 27.706° 99.9 % Absorption correction Multi-scan Semi-empirical from equivalents Max. and min. transmission 0.9059 and 0.3325 0.970 and 0.803 Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Data / restraints / 3804 / 0 / 235 61883 / 0 / 420 parameters Goodness-of-fit on F2 0.984 1.023 Final R indices [I>2σ(I)] R1 = 0.0409, wR2 = 0.1064 R1 = 0.0335, wR2 = 0.0781

R indices (all data) R1 = 0.0508, wR2 = 0.1109 R1 = 0.0448, wR2 = 0.0833 Largest diff. peak and hole 3.721 and -2.657 eÅ-3 1.896 and -0.926 eÅ-3

Results and discussion

Dichloromethane Solvates of Pt(dcmbpy)Cl2. Red crystals of Pt(dcmbpy)Cl2.CH2Cl2 were grown from CH2Cl2 and are referred to as R1.CH2Cl2. Grinding this material produced a red powder, which is referred to as R2.CH2Cl2. Interestingly, solutions of R1.CH2Cl2 and

1 R2.CH2Cl2 exhibited identical properties (e.g., H NMR spectra), however the XRPD diffractogram of R2.CH2Cl2 does not match that predicted based on the single-crystal structure of R1.CH2Cl2 (Figure 1). Thus, the results establish the conversion of R1.CH2Cl2 to

R2.CH2Cl2 is an example of a mechanically induced phase change. It noteworthy that, despite the low boiling point of dichloromethane (39.6°C), both of these solvate materials are indefinitely stable at room temperature. In point of fact, thermal gravimetric analysis shows that the loss of one equivalent of dichloromethane occurs at 109 and 101°C for R1.CH2Cl2 and

R2.CH2Cl2, respectively.

Q-Space (1/A°)

Figure 1. X-ray powder diffractogram of R2.CH2Cl2 (—), and the simulated pattern of (R1.CH2Cl2) (—).

Vapochromism. Scheme 2 shows the surprising complexity of this vapochromic system.

Exposure of either of these solids to vapors of acetone (~0.3 h), tetrahydrofuran (~1 h), or methanol (~24 h) at room temperature produced green forms of solid Pt(dcmbpy)Cl2. These vapochromic products are referred to as G. Interestingly, regardless of whether starting with

R1.CH2Cl2 or R2.CH2Cl2, the green material converts back to R1.CH2Cl2 when exposed to dichloromethane vapor (~12 h). Even if it is already a powder, re-grinding R1.CH2Cl2 produces

R2.CH2Cl2. (Figure 2). In other words, the system exhibits a qualitatively reversible cycle, involving (1) conversion of R1.CH2Cl2 to R2.CH2Cl2 by mechanical grinding, (2) conversion of

R2.CH2Cl2 to G upon exposure to acetone, tetrahydrofuran, or methanol, and (3) conversion of

G to R1.CH2Cl2 by exposure to dichloromethane vapor.

Dichloromethane vapor (i)

Acetone, THF, or Methanol vapor Pt(dcmbpy)Cl2 Grinding

(ii)

Pt(dcmbpy)Cl2

Scheme 2. Vapochromic properties of the Pt(dcmbpy)Cl2 system.

(i): R1CH2Cl2, (ii): R2CH2Cl2

Q-Space (1/A°)

Figure 2. X-ray powder diffractogram of R1.CH2Cl2 produced by exposure of R2.CH2Cl2

to acetone vapor to give G, followed by exposure to dichloromethane vapor (—). The simulated pattern based on the single-crystal structure of R1.CH2Cl2 also is shown (—).

On the other hand, prolonged exposure of G vapors acetone (~48 h), tetrahydrofuran (~6 weeks), or methanol (~15 weeks) caused G to convert to a yellow form of Pt(dcmbpy)Cl2, referred to as Y. Likewise, exposure of R1.CH2Cl2 or R2.CH2Cl2 to acetonitrile vapor directly produces Y (~3 days) without observable formation of G as an intermediate. The XRPD patterns of these yellow products matched that predicted from the crystal structure determined for yellow crystals of Pt(dcmbpy)Cl2 grown from dichloromethane-acetone mixtures (Figure 3). Not surprisingly, Y is readily redissolved in dichloromethane and crystals of R1.CH2Cl2 form during evaporation. However, we were unable to convert Y to R1.CH2Cl2 by exposure to dichloromethane vapor.

Figure 3. X-ray powder diffractograms of Y formed by exposure of R1.CH2Cl2 to (A) acetone vapor, (B) THF vapor, and (C) acetonitrile vapor. The simulated pattern based on the single-crystal structure of Y also is shown (—).

Characterization of Materials. Crystals of R1.CH2Cl2 contain discrete square planar complexes that form dimers with Pt…Pt distances of (3.4625(4)Å) resulting in a Pt…Pt…Pt angle of

153.6(2)°. There are short interactions (2.897Å) between H atom of CH2Cl2 and the Cl atom of

1 Pt(dcmbpy)Cl2. The presence of one equilvalent of dichloromethane solvate was verified by H

NMR spectroscopy of solutions of R1.CH2Cl2. In crystals of Y, there are two independent molecules without solvent in the lattice (Figure 4). The shortest Pt…Pt distance is 5.844 Å. It should be noted that R1.CH2Cl2 and Y, as well as R2.CH2Cl2, each dissolve to give yellow

1 solutions, with identical H NMR spectra with the exception of the CH2Cl2 resonance. Likewise, the UV-visible absorption spectra of these solutions are indistinguishable, exhibiting the expected lowest spin-allowed, solvent-sensitive MLCT band near ~410 nm19 (methanol, 408 nm; dichloromethane, 424 nm; acetone, 419 nm; THF, 437 nm; acetonitrile, 408 nm).

The accumulated evidence supports that the difference in colors of R1.CH2Cl2 and Y is attributable to solid-state intermolecular interactions, as previously described for platinum(II)

20 diimine complexes. Notably, the presence of short Pt…Pt interactions in crystals of R1.CH2Cl2 are consistent with the red color of this material. Specifically, these interactions are expected to stabilize lowest mixed-metal-to-ligand charge-transfer (MMLCT) states, involving a filled

HOMO d*orbital derived from the antibonding combination of the 5dz2 Pt orbitals of adjacent complexes and an unoccupied * LUMO mainly located on the aromatic heterocyclic ligand.

Conversely, the absence of short Pt…Pt interactions in crystals of Y are consistent with the yellow color and the expectation that MMLCT transitions are absent.

(A)

(B)

Figure 4. ORTEP diagrams of (A) red Pt(dcmbpy)Cl2.CH2Cl2 (R1.CH2Cl2) , and (B) yellow Pt(dcmbpy)Cl2 (Y) at 150K.

Efforts to obtain single-crystal X-ray diffraction quality crystals of the green form of

Pt(dcmbpy)Cl2 (G) were unsuccessful, and we were only able to prepare this material by exposure to vapors of acetone, methanol, or tetrahydrofuran. However, as shown in Figure 5, the

XRPD patterns of G prepared by each of these methods were indistinguishable, indicating that these materials have the same crystal form. Additional analytical data were consistent with formulation as Pt(dcmbpy)Cl2.

(C)

(B)

(A)

Q-Space (1/A°)

Figure 5. X-ray powder diffractograms of G formed by exposure of R1.CH2Cl2 to (A)

Acetone, (B) THF or (C) methanol vapors.

Vapoluminescence. The sequential conversion of the red materials, R1.CH2Cl2 and

R2.CH2Cl2, to G and then Y in the presence of acetone vapor was readily monitored by emission spectroscopy (Figures 6 and 7). The room-temperature solid-state emission spectra of

R1.CH2Cl2 (685 nm) and R2.CH2Cl2 (690 nm) exhibit nearly identical maxima. The longest

79 wavelength excitation maxima (R1.CH2Cl2, 530 nm; R2.CH2Cl2, 565 nm) lie at much longer wavelengths than the unimolecular MLCT transition observed in fluid solution (410-420 nm), indicative of intermolecular, solid-state interactions (Figure A1 Appendix). By analogy to related systems,19-22 the emission is assigned as originating from a lowest, predominantly spin-forbidden

MMLCT excited state. The comparatively similar emission bands for R1.CH2Cl2 and

R2.CH2Cl2 suggests somewhat similar short Pt…Pt interactions in these materials. Upon exposure of these red materials to acetone vapor, the emission band loses intensity and shifts to

~710 nm upon formation of G and subsequently ~680 nm for Y. The broad long-wavelength

-1 emission from Y (FWHM, 3000 cm ) is strikingly similar to that observed for Pt(bpy)Cl2

(emission maximum, 625, FWHM, 3600 cm-1). By analogy to earlier studies, we assign the emission from Y to a lowest predominantly spin-forbidden ligand field state. Thus, vapor exposure induces a distinct change in the lowest excited state electronic structures of these materials.

(B)

Wavelength (nm) Wavelength (nm)

Figure 6. Solid-state emission spectra recorded during exposure of R1.CH2Cl2 to acetone vapor at room temperature (λex = 436 nm) with showing the (A) red to green conversion and the subsequent (B) green to yellow conversion.

(A) (B)

Wavelength (nm) Wavelength (nm)

Figure 7. Solid-state emission spectra of R2.CH2Cl2 recorded during exposure to

acetone vapor at room temperature (λex = 436 nm) (A) red to green, and (B) green to yellow conversion.

Vapor-Induced Recrystallization. To evaluate vapor uptake and release of dichloromethane, red crystals of R1.CH2Cl2 were placed in an acetone saturated air atmosphere.

At intervals, a solid aliquot was removed and quickly dissolved in DMSO-d6. Subsequently, the

1H NMR spectrum was recorded. The results show that exposure of the red crystals to acetone vapor causes the color to change to green and is accompanied by substantial uptake of acetone

(≥7 equivalents) and concomitant and nearly complete loss of dichloromethane within ~40 minutes (Figure 8). Most of the crystals stayed green for 24 hours with some beginning to change color to yellow. All crystals had turned yellow within 48 hours, and greater uptake of acetone (~20 equivalents) was associated with the green-to-yellow conversion process. The high vapor accumulation is reminiscent of deliquescence in which a solid absorbs moisture from air.

However, it is important to note that the solubility of the complexes in acetone is ~4.2 x 10-4 M, corresponding to >3.2 x 105 equivalents of acetone. In other words, even at the highest number of observed equivalents of acetone (~20 equivalents), <0.1% of the solid could be dissolved at one time. Thus, because neither acetone, methanol or tetrahydrofuran are incorporated into the

81 resulting green and yellow solids, we infer that dichloromethane rapidly partitions into the absorbed vapor and then the surrounding atmosphere, resulting in a collapse (or recrystallization) of the stacked Pt(dmcbpy)Cl2 complexes of R1.CH2Cl2 (or R2.CH2Cl2) to give the green polymorph (G). Over time, G steadily dissolves and re-precipitates as the Y polymorph, ultimately completing the conversion to the yellow form.

Figure 8. Plot of the number of equivalents acetone (—) and dichloromethane (—) present during the exposure of the R1.CH2Cl2 to the acetone vapor.

Morphological Changes

SEM images recorded of R1.CH2Cl2 and R2.CH2Cl2 at various stages of vapor exposure revealed vapor-induced formation of remarkable nanoarchitectures (Figures 9 and 10). For example, SEM images of R1.CH2Cl2 shows that these crystals are large and needle shaped with smooth surfaces (Figures 9A, 9B). Upon exposure to acetone, THF or methanol vapors the crystals changed color to green and eventually yellow. SEM images of the G materials consistently revealed the formation of gradually curving nanofibers protruding from the original crystal block, as if vapor exposure shredded and splintered the original crystal. The fibers were

82 thousands of nanometers in length, but with narrow widths in the ~80-200 nm range (Figures 9C,

9D). Interestingly, these fibers appeared to wave as the electron source of the SEM instrument came into close proximity during imaging. This is attributed to charge build-up on the fibers, resulting in repulsion of the fibers by the electron beam. In keeping with this interpretation, thicker Au/Pd coatings of the sample resulted in less motion which is attributable to more efficient conduction and less charge buildup.

Upon exposure to dichloromethane, G converts to R1.CH2Cl2, resulting in a distinctly different morphology than found for the original red crystals. SEM images reveal long, approximately parallel fibers with thickness in the 1-2 µm range. The resulting clumps of fibers in the magnified images look like of bundles of dry spaghetti (Figures 9G, 9H). Thus, the nanofibers of the green form appear to anneal to form bundles of thicker fibers, but without restoration of the larger faceted needles precipitated from dichloromethane solution.

50µm 105µmµm 5µm 2µm

(A) (B) (C) (D)

5µm 50µm 5µm 20µm

(E) (F) (G) (H)

Figure 9. Representative SEM images of (A,B) red, (C,D) green after acetone exposure, (E,F) yellow after acetone exposure, and (G,H) spaghetti red after (A,B) exposed to dichloromethane vapor.

SEM images of the Y materials also reveal a distinctly different morphology than found for G or

Pt(dcmbpy)Cl2.CH2Cl2 (Figures 9E, 9F). These yellow blocks with dimensions in the 1.5-3 µm range are smaller than the starting red needles. These observations are consistent with the notion that prolonged exposure to vapor induces dissolution of Pt(dcmbpy)Cl2 and reprecipitation as microscale crystalline blocks of Y. It should be noted that G and Y formed by exposure of red

Pt(dcmbpy)Cl2.CH2Cl2 (R1.CH2Cl2 or R2.CH2Cl2) to methanol or tetrahydrofuran vapor exhibited morphologies that are indistinguishable from those found upon exposure to acetone.

These observations indicate that the morphologies of the green and yellow materials are essentially independent of whether produced using acetone, methanol, or tetrahydrofuran (Figure

10).

2µm 5µm 2µm 5µm (D) (A) (B) (C)

20µm 10µm 20µm 20µm

(H) (E) (F) (G)

Figure 10. Representative SEM images of green after (A) acetone, (B) methanol, and (C) THF exposure , (D,E,F) yellow after THF exposure, and (G,H) red spaghetti form.

Conclusion

This study reports a detailed investigation of Pt(dcmbpy)Cl2.CH2Cl2, which exhibits the unusual behavior of changing color upon exposure to certain vapors without incorporation of vapor into the resulting crystal lattice. Specifically, upon exposure to acetone, THF, or methanol vapors, red Pt(dcmbpy)Cl2.CH2Cl2 loses solvate and converts first to a green polymorph and then

1 a yellow polymorph of Pt(dcmbpy)Cl2. Interestingly, H NMR spectra of Pt(dcmbpy)Cl2.CH2Cl2 recorded during exposure to acetone vapor show significant vapor accumulation. Nevertheless, the solubility of the complex in acetone, methanol and tetrahydrofuran is very low which is in agreement with the observation that the solid does not appear to dissolve when exposed to these vapors. However, SEM images of the resulting green and yellow solids show evidence of the formation of remarkable nanoscale architectures induced by vapor exposure. The green material consists of nanoscale fibers, whereas the yellow material consists of microscale blocks.

Therefore, we propose that vapor uptake and loss of dichloromethane induces a collapse (or recrystallization) of the stacked Pt(dmcbpy)Cl2 complexes to give the green polymorph. Upon further exposure, the green polymorph steadily dissolves and re-precipitates as crystals of the yellow polymorph. These observations suggest a previously undocumented mechanism of vapochromic response in which deliquescent-like vapor uptake induces recrystallization to particles with distinctly different morphologies, colors and emission properties.

References:

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Appendix:

(A) (B)

Figure A1: Solid-state excitation spectra of R1.CH2Cl2 (—), and R2.CH2Cl2 (—) are shown in (A), and solid-state excitation spectra of green powders (—), and yellow powders (—) during the exposure of R2.CH2Cl2 to acetone vapor at room temperature

(λex = 436 nm) are shown in (B).

Chapter 4

Thermodynamics and Kinetics of a Series of Closely Related

Vapochromic Platinum(II) Salts

Introduction

During the late 1990s, Mann and co-workers effectively founded the field of vapochromism with their discovery of platinum(II) double salts that change color upon exposure to volatile organic compounds.1-5 Since that early work, more than one hundred platinum-based vapochromic materials have been prepared,6-19 including stacked platinum(II)8,20 and mixed platinum(II)-palladium(II) double salts, 1,21 as well as dinuclear platinum(II) complexes.22,23 The tendency of solids containing square-planar d8-electron platinum metal centers to exhibit vapochromic properties is related in part to the propensity of these complexes to form close non- covalent Pt…Pt contacts, which have a strong influence on the solid-state color (e.g., yellow, orange, red, purple) and spectroscopy of these materials.24-28 The color changes upon vapor absorption and desorption typically are not a result of chemical reaction. Instead, they are related to vapor absorption/desorption induced changes in crystal packing and intermolecular interactions.29-31 The resulting changes in Pt…Pt distances have a dramatic effect on the color and luminescence properties of these materials.7,8

In 2008, Castellano and coworkers demonstrated that Pt(tpy)Cl+ salts can be incorporated into microarrays that can be used for vapor sensing.32 Studies such as this and others,7,33-35 have demonstrated that substitution of different counter-anions and modifications of the terpyridine ligand results in materials with vastly different vapor selectivities. Despite these advancements, there have been few mechanistic studies of vapochromic systems. Moreover, thermodynamic

92 and kinetic data on such systems is scarce.6,8 For example, some researchers have determined the structures of vapochromic materials before and after absorption (and in particular the Pt…Pt distances),29,36 but have stopped short of obtaining kinetic and thermodynamics data which is essential to development of a detailed mechanistic picture. Without this knowledge, vapochromic materials are still discovered exclusively through serendipity, and we are unable to formulate a rational strategy for designing materials with specific selectivities and sensitivities.

To begin to address these challenges, in the current study, we used differential scanning calorimetry (DSC) to determine the enthalpy of vaporization (ΔHvap) and activation energy (Ea)

- - - for the desorption of acetonitrile from [Pt(tpy)X]YF6 (X = Cl , Br and I ; Y = P, As and Sb)

(Scheme 1) which is a series of nine closely related vapochromic salts having similar chemical compositions. In addition, as described in Chapter 2, we have measured the relative kinetics of color change due to vapor absorption and desorption for each of the salts in this series.

Furthermore, we have evaluated the relative sensitivities of these materials by determining the limiting vapor pressure of acetonitrile below which each salt does not undergo a color change during a fixed period of time. We also have estimated the efficiency of the incorporation of acetonitrile by calculating the void volume when acetonitrile is removed from the lattice of

[Pt(tpy)X]YF6.CH3CN crystals. Using this combination of data, we have identified correlations between the enthalpy of vaporization, the activation energy for desorption, the sensitivity, the kinetics of colorimetric response, and the void volume.

- - - Scheme 1. Line drawing of [Pt(tpy)X]YF6 (X = Cl , Br and I ; Y = P, As and Sb)

Experimental sections

Materials. K2PtCl4 was purchased from Pressure Chemical. 1,5-cyclooctadiene (COD) and 2,2′:6′,2″-terpyridine (tpy) were obtained from Aldrich and used as received. NH4PF6,

NaAsF6 and NaSbF6 were purchased from Aldrich. Pt(COD)Cl2, and [Pt(tpy)Cl]Cl.2H2O were prepared according to published procedures.25,37

- - - Synthesis of [Pt(tpy)X ]YF6 (X = Cl , Br and I ; Y = P, As and Sb) salts. Pt(COD)Br2 and Pt(COD)I2 were prepared by adding excess LiBr and NaI, respectively, to an acetone

38 solution of Pt(COD)Cl2 as previously described. [Pt(tpy)Br]Br and [Pt(tpy)I]I were prepared

37 - following the procedure used for [Pt(tpy)Cl]Cl.2H2O. Samples of [Pt(tpy)Cl](YF6) (X = Cl ,

- Br- and I ; Y = P, As and Sb) (Scheme 1) were prepared by anion metathesis in water.

- - - Kinetics of response of [Pt(tpy)X ]YF6 salts (X = Cl , Br and I ; Y = P, As and Sb).

The experimental details of how the onset 푡0, end time 푡푓 and half-time 푡1/2 for the absorption and desorption of acetonitrile vapor were obtained are given in Chapter 2. The half-time (푡1/2),

푡푓−푡0 defined as , provides a measure of the duration of the color-change event, and values of 푡 2 1/2 are summarized in Table 1.

Determination of Enthalpies of vaporization (ΔHvap). To determine enthalpy of vaporization (ΔHvap), crystalline samples of each of the red [Pt(tpy)X]YF6.CH3CN materials were prepared by slow evaporation of acetonitrile solutions. Each sample was subjected to differential scanning calorimetry (DSC) analysis using a NETZSCH DSC 200F3 instrument.

Data were collected at a furnace ramp rate of 2°C /min from -20°C to 120°C. The enthalpy was obtained from the area under the endotherm (Appendix Figure A1).39,40 Measurements were made in triplicate and duplicate to obtain an estimate of uncertainty (2σ=±2.56) using pooled standard deviations.41 The resulting values are reported in Table 1.

Determination of Activation Energies. To determine activation energy, crystalline samples of each of the red [Pt(tpy)X]YF6.CH3CN materials were grown by slow evaporation of acetonitrile solutions. Samples were subjected to differential scanning calorimetry (DSC) analysis using a NETZSCH DSC 200F3 instrument for a range of furnace ramp rates (2, 3, 5, 7, and 10°C/min) (Appendix Figure A2). Data typically were collected from -20°C to 140°C. In each case, the peak temperature in the endothermic process (Tp) was obtained from the DSC trace. Measurements were done in triplicate or duplicate at 10°C/min for salts to obtain an estimate of uncertainty. The activation energy (Ea) for vapor desorption for each salt was

42,43 estimated using the Kissinger method. Values of Ea are summarized in Table 1.

Evaluation of Sensitivities. To measure the sensitivity of the salts to acetonitrile vapor,

- - - bright yellow crystals of each of the nine [Pt(tpy)X]YF6 (X = Cl , Br and I ; Y = P, As and Sb) salts were grown by partial evaporation of a (1:1) acetone-water mixture at room temperature.

For each salt, the crystals were filtered, dried, and ground to fine uniform powders. About 10 mg of each of the powders were placed into 9 different weighing boats. The samples were exposed to different vapor pressures of acetonitrile for 48 hours. Acetonitrile vapor pressure was changed

95 by using different ratios of water-acetonitrile mixtures. Photographs were recorded at each vapor pressure to help evaluate changes in color. The minimum vapor pressure required to induce a color change in each salt was estimated by interpolating data from a previous study of

44 acetonitrile-water mixtures. Limiting acetonitrile vapor pressures (Vp) required to induce a color change in each salt are summarized in Table 1.

Void volume measurement

CrystalExplorer.3.1 software was used to determine the void volumes of acetonitrile in the lattice.45 The solvated acetonitrile molecule was removed from the structure prior to calculation. The isovalue of 0.0003 au (1 au equals to 1.496 x 1021 Å) was used to define the boundary surface of the void.46 The results are summarized in Table 1.

Results and Discussion

Scheme 2 shows an array of [Pt(tpy)X]YF6 complexes and their responses to acetonitrile vapor. Seven of the nine bright yellow [Pt(tpy)X]YF6 solids are vapochromic, turning from the yellow to orange upon exposure to air saturated with acetonitrile for 48 hours at room temperature. Dashed diagonal lines indicate that the salts absorb vapor and as a result change color. Two of the yellow materials, [Pt(tpy)Cl]SbF6 and [Pt(tpy)Br]SbF6, did not change color when exposed to acetonitrile vapor and stayed yellow after 48 hours.37

Scheme. 2 Array of [Pt(tpy)X]YF6 complexes and their response to acetonitrile

The enthalpy of vaporization (ΔHvap) and activation energy (Ea) for each salt was determined using differential scanning calorimetry, which is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature.47-49 Loss of solvent from the sample is expected to lead to an increase in required heat (i.e., endothermic event), by analogy to the cooling effect of sweat evaporating from skin. The enthalpy of vaporization was obtained from the area under the endotherm.39,40 The activation energy of the salts were estimated using the

Kissinger method,42,43 which assumes that for a given DSC curve with the heating rate, β, one observes the maximum reaction rate at the peak temperature, Tp. The Kissinger relationship is:

2 ln(β/Tp ) = -Ea/RTp + ln(K0 Ea/R)

−1 where R is the gas constant (8.314 J/(mol K) and K0 (s ) is the rate constant for the desorption

2 of the acetonitrile vapor from salts. Values of Ea obtained from plots of ln(β /Tp ) against

1000/T p (Figures 1A and 1B and Appendix Figures A1-A8) are summarized in Table 1.

(A) (B)

Figure 1. (A) DSC thermograms of [Pt(tpy)Br ]PF6.CH3CN at different heating rates and

2 (B) Plot of ln(β /Tp ) against 1000/Tp. (Note error bars are ±2σ, and some error bars are smaller than the height of the point)

The Kissinger method has been used to determine activation energies more extensively than any other multiple-heating rate method. This method offers a significant improvement in the

50 accuracy of Ea values as compared to the Ozawa–Flynn–Wall method. However this method produces a single value for the activation energy for any process regardless of its actual kinetic complexity. As a result, the activation energy determined can adequately represent only single- step kinetics. We selected this method to evaluate the activation energy because the accumulated data (e.g., DSC thermograms and color imaging data) for these systems suggests that the absorption/desorption processes are single step reactions without any chemical interactions between the vapor molecules and the platinium(II) centers.

The Ea and ΔHvap values for solvent release from the non-vapochromic

[Pt(tpy)Br]SbF6.CH3CN were significantly lower than found for the vapochromic salts, which indicates that the acetonitrile solvate is less stable in the crystal lattice. Vapor release from the

98 other non-vapochromic salt, [Pt(tpy)Cl]SbF6.CH3CN, was too rapid for reliable measurements of

Ea and ΔHvap.

limiting MeCN

vapor pressures ΔHvap (J/gr) 풕ퟏ/ퟐ absorption/min 풕ퟏ/ퟐ desorption/min Void volume Sample Ea(Kj/Mol) (mm Hg) (Vp) 3 (Average) (Å )

[Pt(tpy)Cl]PF6 44.7 20.0 50.33 ± 2.38 64.58 ± 4.83 52.3 ± 1.7 28.25

[Pt(tpy)Br]PF6 17.1 6.0 75.30 ± 1.50 64.39 ± 4.59 33.4 ± 1.8 29.25

[Pt(tpy)I]PF6 9.0 - 90.36 ± 1.06 90.06 ± 8.14 25.0 ± 2.0 33

[Pt(tpy)Cl]AsF6 67.2 7.3 39.62±3.04 60.84 ± 4.67 68.6 ± 0.8 27.31

[Pt(tpy)Br]AsF6 13.8 23.0 45.51 ± 0.028 64.19 ± 4.91 61.5 ± 0.9 28.69

[Pt(tpy)I]AsF6 12.7 24.1 61.22 ± 2.42 63.11 ± 4.31 37.8 ± 2.6 33.92

[Pt(tpy)Br]SbF6 - - 20.18 ± 4.38 50.07 ± 3.60 - 27.87

[Pt(tpy)I]SbF6 26.8 52.1 56.90 ± 0.084 75.37 ± 6.56 45.1 ± 1.3 30.86

Table 1. Values of the enthalpy of vaporization (ΔHvap), activation energy (Ea), 푡1/2absorption, 푡1/2 desorption, limiting acetonitrile - - - vapor pressures (Vp), and solvate void volumes for [Pt(tpy)X]YF6 (X = Cl , Br and I ; Y = P, As and Sb) salts, (Note: error bars are ±2σ).

100

Correlation between ΔHvap, Ea, 풕ퟏ/ퟐ of absorption, 풕ퟏ/ퟐ of desorption, limiting vapor pressure of acetonitrile (Vp) (mm Hg), and void volume (VV) of the [Pt(tpy)X ]YF6 salts (X

= Cl-, Br- and I-; Y = P, As and Sb). Scheme 3 shows a simplified reaction coordinate for release of acetonitrile (CH3CN) from Pt(tpy)X]YF6.CH3CN. Ea describes the height of the energy barrier that must be surmounted in order for vapor to be released, and ΔG is the driving force for solvate release. We anticipate that the rate of release will decrease as this barrier increases. On the other hand, we anticipate that the relative sensitivity, as measured by limiting vapor pressure of the acetonitrile, will increase with ΔG. The enthalpic contribution to the driving force, ΔHvap, describes the heat released during vapor desorption (enthalpy of vaporization). We hypothesize that the entropic contribution to ΔGvap (i.e., -TΔSvap), is dominated by the conversion of trapped acetonitrile into vapor and is likely similar for each material. In other words, based on this simplified model, sensitivity (as measured by the minimum vapor pressure required for a response) is expected to increase with ΔHvap for vapor release.

Scheme 3. Schematic showing the thermodynamic and kinetics parameters for the release of acetonitrile from [Pt(tpy)X]YF6.CH3CN.

101

To test these hypotheses and identify other possible relationships, Figure 2 and Figures

A11-A25 in the Appendix show scatterplots for pairs of the following parameters: Ea, ΔHvap,

푡1/2 of absorption, 푡1/2of desorption, limiting acetonitrile vapor pressure (Vp), and void volume

(VV). Pearson correlation coefficients (r) summarizing these relationships are collected in the correlation matrix shown in Table 2. We have examined the correlations between the Smax and area of the S versus time (Chapter 2) and ΔHvap, Ea, 풕ퟏ/ퟐ of absorption, 풕ퟏ/ퟐ of desorption, limiting vapor pressure of acetonitrile (Vp) (mm Hg), and void volume (VV) values and didn’t observed any strong correlations.

ΔHvap Ea 풕ퟏ/ퟐ of absorption 풕ퟏ/ퟐof desorption Vp VV

ΔHvap 1 0.84 -0.67 0.64 -0.96 0.68

Ea 0.84 1 -0.45 0.97 -0.65 0.61

풕ퟏ/ퟐof absorption -0.67 -0.45 1 -0.46 0.72 -0.70

풕ퟏ/ퟐof desorption 0.64 0.97 -0.46 1 -0.58 0.63

Vp -0.96 -0.65 0.72 -0.58 1 -0.76 VV 0.68 0.61 -0.70 0.63 -0.76 1

Table 2. Pearson correlation coefficients (r) for relationships between Ea, ΔHvap,

푡1/2 of absorption, 푡1/2of desorption, limiting acetonitrile vapor pressure (Vp), and void volume (VV).

These analyses show that, consistent with our working hypothesis, there is a good correlation between t1/2 for desorption and Ea (r = 0.97; Figure 2a). In other words, the duration of the color change decreases with activation energy for desorption. There also is a good correlation between Vp and ΔHvap (r = -0.96; Figure 2b). Thus, the vapochromic sensitivity increases as the enthalpy of vaporization decreases.

102

(A) (B)

Figure 2. (A) graph of Ea against Speed of desorption, and (B) Graph of ΔHvap against limiting vapor pressure of the acetonitrile (Vp). (Note error bars are ±2σ, and some error bars are smaller than the height of the point)

We were particularly interested in identifying relationships between structure, as measured by void volume, and thermodynamic and kinetic parameters. An intriguing complication is that for eight of the nine salts, the non-solvates are isostructural, crystalizing in the same triclinic space group. By contrast, [Pt(tpy)I]AsF6 crystallizes in a monoclinic space group. Therefore, it was of interest to identify instances where data for this compound deviate from those of the other salts. Of the 15 possible correlations, [Pt(tpy)I]AsF6 was an outlier according to the Thompson tau test for only the VV vs. Ea and VV vs. Hvap correlations.

Exclusion of [Pt(tpy)I]AsF6 from these analyses resulted in significantly strong correlations

(VV/Ea, r=0.94 vs. 0.61; VV/Hvap, r=0.85 vs. 0.64). The VV vs. Ea correlation is consistent with the expectation that, as cavity volume available to the acetonitrile solvate increases, the barrier to escape of the solvate is lowered. The VV vs. Hvap correlation suggests that, for this series of salts, increasing the cavity for the acetonitrile tends to stabilize the solvate structure.

On the other hand, it is evident that most correlations involving the 6 parameters investigated

103 here are not strongly sensitive to the different non-solvate packing arrangement for

[Pt(tpy)I]AsF6. Notably, it appears that the VV vs. t1/2(absorption) and VV vs. Vp correlations are not sufficiently strong for [Pt(tpy)I]AsF6 to be identified as a statistical outlier.

Conclusion

In this study, we report the first example of the determination of the enthalpy of vaporization (ΔHvap) and activation energy (Ea) for solvate loss for a homologous series of vapochromic materials. In addition to these parameters, we have evaluated parameters describing sensitivity, speed of color change during vapor desorption, speed of color change during vapor absorption, and the volume attributed to the acetonitrile molecule in the solvate structures of nine salts having very similar compositions. The results can be understood in terms of a simplified reaction coordinate that relates kinetics of desorption to activation energy and volume occupied by the acetonitrile solvate. Likewise, the sensitivity of these materials is strongly related to the enthalpy of vaporization. The results also suggest that there are limitations to the use of acetonitrile solvate volume as a structural parameter for identification of correlations because this parameter does not take into account differences in non-solvate packing arrangements. Going forward, it would be valuable to develop one or more structural parameters that better capture the structural changes accompanying absorption and desorption of vapor.

104

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Appendix Cl Br I

PF6

AsF6

SbF6

- - - Figure A1. Representative DSC thermograms and and ΔHvap values of [Pt(tpy)X ]YF6 salts (X = Cl , Br and I ; Y = P, As and Sb)

109

Cl Br I

PF6

AsF6

SbF6

- - - Figure A2. Representative DSC thermograms of [Pt(tpy)X ]YF6 salts (X = Cl , Br and I ; Y = P, As and Sb) at ramp rates of 2( ), 3( ), 5( ), 7( ), and 10( ) °C/min.

110

2 Figure A3. Plot of ln(β /Tp ) against 1000/Tp for the [Pt(tpy)Cl]PF6.CH3CN salt.