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Mechanistic Studies of Cation Photogeneration Loyola University Chicago Loyola eCommons Dissertations Theses and Dissertations 1998 Mechanistic Studies of Cation Photogeneration Mary Frances Clifton Loyola University Chicago Follow this and additional works at: https://ecommons.luc.edu/luc_diss Part of the Chemistry Commons Recommended Citation Clifton, Mary Frances, "Mechanistic Studies of Cation Photogeneration" (1998). Dissertations. 3732. https://ecommons.luc.edu/luc_diss/3732 This Dissertation is brought to you for free and open access by the Theses and Dissertations at Loyola eCommons. It has been accepted for inclusion in Dissertations by an authorized administrator of Loyola eCommons. For more information, please contact [email protected]. This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License. Copyright © 1998 Mary Frances Clifton LOYOLA UNIVERSITY CHICAGO MECHANISTIC STUDIES OF CATION PHOTOGENERATION A DISSERTATION SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY BY MARY FRANCES CLIFTON CHICAGO, ILLINOIS JANUARY, 1998 Copyright by Mary Frances Clifton, 1998 All rights reserved. ACKNOWLEDGMENTS Special thanks are extended to Dr. Mary K. Boyd, Dr. David S. Crumrine, Dr. Kurt W. Field, and Dr. Gerry K. Noren. I am also indebted to the Department of Education for a Graduate Assistantship in Areas of National Need (GAANN) and the Arthur J. Schmitt Foundation for a dissertation fellowship. 111 TABLE OF CONTENTS ACKNOWLEDGMENTS . iii LIST OF TABLES ................................................... vii LIST OF FIGURES ................................................. Vlll LIST OF ABBREVIATIONS . xi ABSTRACT . xiii Chapter I. INTRODUCTION . I A. Carbocations: Definitio°' Applications and Photogeneration . I 1. Definition and Applications . I 2. Photoinitiated Polymerization . 2 a. Photoinitiated Radical Polymerization . 3 b. Photoinitiated Charge-Transfer Polymerization . 6 c. Photoinitiated Cation-Radical Polymerization . I 0 d. Photoinitiated Simultaneous Radical and Cation-Radical Polymerization . 14 3 . Photoinitiators . 15 a. Free Radical Initiators .................. 15 b. Cationic Photoinitiators ................. 21 4. Methods for Photogeneration of Carbocations ...... 22 a. Photoheterolysis of Carbon-Heteroatom Bonds .............................. 25 b. Carbon-Carbon Bond Cleavage . 27 c. Heterolysis ofPhotogenerated Radical Cations ............................. 28 d. Oxidation of Photochemically Generated Radicals ............................. 30 e. Protonation of Photochemically Generated Carbenes . 31 f Photoprotonation of Styrenes, Aromatics and Phenylacetylenes . 3 2 B. Photochemistry of Carbonyl Compounds . 33 1. Norrish Type I Reactions ...................... 38 2. Norrish Type Il Reactions . 40 3. Intermolecular Hydrogen Abstraction . 43 IV 4. Paterno-Biichi Reaction ....................... 45 C. Photochemistry of the Carbon-Carbon Double Bond ...... 45 D. Earlier Work On The Thermal and Photogeneration of Xanthyl Cations . 49 II. RES ULTS . 56 A. 9-Xanthylacetone and 9-Xanthylacetophenone . 56 1. Syntheses . 56 2. Preparative Photolyses ........................ 57 3. Acid Catalysis . 62 4. Triplet Sensitization and Quenching Studies ........ 63 5. Laser Flash Photolysis Studies .................. 67 6. Photoproduct Studies . 77 a. 9-Xanthylidene Acetophenone . 77 i. Syntheses ...................... 77 ii. Quantum Yields . 78 iii. Synthesis of Proposed Tertiary Benzylic Alcohol . 79 a. Method 1 . 79 b. Method 2 ................ 81 7. Fluorescence . 83 B. 2-Methyl-3-(9-xanthyl)propene ....................... 83 1. Syntheses . 83 2. Preparative Photolyses ........................ 84 3. Acid Catalysis .............................. 85 4. Triplet Sensitization .......................... 93 5. Triplet Quenching ........................... 94 6. Fluorescence ............................... 95 III. DISCUSSION . 96 A. 9-Xanthylacetone and 9-Xanthylacetophenone . 96 1. Acid Catalysis . 96 2. M~chanism of the Photogeneration of the Xanthyl Cation from 9-Xanthylacetone and 9-Xanthylacetophenone . 96 3. Triplet Sensitization and Quenching Results ........ 98 4. Product Quantum Yields . 100 5. Photoproduct Studies . 103 a. Mechanism ofFormation of9-Xanthylidene Acetophenone . 103 B. 2-Methyl-3-(9-xanthyl)propene . 111 IV. CONCLUSIONS . 118 v V. EXPERIMENTAL ....................................... 121 A. Materials . 121 B. General Methods . 121 C. General Photolysis Procedure ....................... 122 D. Triplet Sensitized Photolysis: General Method .......... 123 E. Triplet Quenching ................................ 124 F. Laser Flash Photolysis . 124 G. Syntheses . 125 1. Ethyl 2-(9-Xanthyl)-3-oxobutanoate 3 ........... 125 2. 9-Xanthylacetone 1 ......................... 125 3. Ethyl 2-(9-xanthyl)-3-phenyloxopropanoate 4 ..... 126 4. 9-Xanthylacetophenone 2 .................... 126 5. 2-Methyl-3-(9-xanthyl)propene 14 .............. 126 6. 9-Phenylethynylxanthenol 6 ................... 127 7. 9-Xanthylidene acetophenone 5 . 128 8. Attempted Synthesis of Tertiary Benzylic Alcohol to: 9-Hydroxy-(9-xanthyl)acetophenone ............ 128 a. Method 1 . 128 b. Method 2 . 129 9. Product Quantum Yield . 130 VI. SPECTRA ............................................. 133 APPENDIX ........................................................ 152 REFERENCES ..................................................... 180 VITA ............................................................ 188 VI LIST OF TABLES 1. GC Area Percent Analyses of Products Obtained from the Attempted Hydration of 9-Xanthylidene Acetophenone 5 . 80 2. GC Area Percent Analyses Obtained from Products of the Irradiation of 9-Xanthylacetophenone 2 . 82 3. Product Quantum Yields . 102 4. Disproportionation/Combination Ratios kA . 106 vii LIST OF FIGURES Figure Page 1. Examples of cation-radical photoinitiators: aryldiazonium salts, quaternary ammonium salts and onium salts . 11 2. Examples ofType I free radical photoinitiators: a,a'-dialkoxyacetophenones, a-alkoxydeoxybenzoins, dibenzoyl disulfides, azocompounds such as phenylazo-4-diphenylsulfone and dibenzoyl methanes . 16 3. Examples of Type II free radical photoinitiators: benzils, benzophenones, fluorenones, xanthones and camphorquinone . 19 4. Examples of cationic photoinitiators: triarylsulfonium salts, diaryliodonium salts, Bronsted acids and Lewis acids . 23 5. Effect of solvent polarity on wavelength shifts ......................... 36 6. Xanthyl alcohols and xanthyl ketone used to thermally generate the xanthyl cation: 2-(9-xanthyl)ethanol a, 1-(9-xanthyl)-2-methyl-2-propanol b, 1-(9-xanthyl)-2-propanol c, 1-(9-xanthyl)-2-phenyl-2-propanol d and 9-xanthylacetone 1. The trityl alcohol, 4,4,4-triphenyl-2-methyl- 2-butanol e, demonstrated no cation formation. 50 7. Absorption spectra measured following irradiation of9-xanthylacetone 1 in 30% H2S04 (with 33% acetonitrile cosolvent) with an excitation wavelength of300 nm. Spectra were measured at 1 min irradiation time intervals and show growth of the xanthyl cation peaks .......... 58 8. Absorption spectra measured following irradiation of xanthyl cation solutions produced from photolysis of9-xanthylacetone 1in30% H2S04 (with 33% acetonitrile cosolvent) with an excitation wavelength of300 nm. Spectra were measured at 4 min irradiation time intervals and show absorption peaks corresponding to the xanthyl cation decreasing with the concomitant formation of xanthone. 60 Vlll LIST OF FIGURES (Continued) Figure Page 9. Plot of absorption intensity ofxanthyl cation (at 374 nm) versus percent aqueous H2S04 (acid concentration not corrected for cosolvent). The xanthyl cation was produced from irradiation of 9-xanthylacetone 1 with irradiation times of 1 min ( 1. ), 2 min ( • ), and 4 min ( •). These times correspond to cation conversions of 10%, 32%, and 60% respectively. 64 10. Transient spectra obtained following photolysis of9-xanthylacetophenone 2 in 90:8:2 CH3CN:H20:H2S04. Solutions were purged with nitrogen and show decay of the initially formed xanthyl radical. 68 11. Transient spectra obtained following photolysis of 9-xanthylacetophenone 2 in 90:8:2 CH3CN:H20:H2S04. Substrate solutions were purged with air and show clean conversion of the xanthyl radical (A.max=345 nm) to the xanthyl cation (A.max=374 nm) ................................. 71 12. Transient absorption waveforms of9-xanthylacetophenone 2 (solid) and 9- xanthylacetone 1 (dotted) at 370 nm in 90:8:2 CH3CN:H20:H2S04 purged with air. ......................................... 73 13. Transient absorption waveforms for 9-xanthylacetophenone 2 (370 nm) in 90:8:2 CH3CN:H20:H2S04 purged with nitrogen and purged with air. 75 14. Absorption spectra measured following irradiation of2-methyl-3-(9-xanthyl) propene 14 in 30% H2S04 (with 33% acetonitrile cosolvent) with an excitation wavelength of 254 nm. Spectra were measured at 2 min irradiation time intervals and show growth of the xanthyl cation peak. 86 15. Plot of absorption intensity ofxanthyl cation (at 374 nm) versus 1-50% aqueous H2S04 (acid concentration not corrected for cosolvent). The xanthyl cation was produced from irradiation of2-methyl-3-(9-xanthyl)propene 14 after 0 min(•), 4 min ( • ), and 4 min minus 0 min ( 1. ). .......... 89 16. Plot of absorption intensity ofxanthyl cation (at 374 nm) versus 0-20% aqueous H2S04 (acid concentration not corrected for cosolvent). The xanthyl cation was produced from irradiation of2-methyl-3-(9-xanthyl)propene 14 after 0 min(•), 4
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