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ABSTRACT ZHANG, JINYUAN. Study of Bridge Effects on Electronic Coupling of Donor- Bridge-Acceptor Biradical Complexes

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ABSTRACT ZHANG, JINYUAN. Study of Bridge Effects on Electronic Coupling of Donor- Bridge-Acceptor Biradical Complexes ABSTRACT ZHANG, JINYUAN. Study of Bridge Effects on Electronic Coupling of Donor- Bridge-Acceptor Biradical Complexes. (Under the direction of Dr. David Shultz). A series of Donor-Bridge-Acceptor (D-B-A) (D: S=1/2 ortho-semiquinonate, SQ; A: S=1/2 nitronlynitroxide, NN) biradical complexes featuring different bridges were synthesized to serve as ground state analogues of charge separated excited states as well as molecular analogs of single-molecule break junction devices. The aim is to elucidate bridge-mediated electronic structure contributions to electronic coupling. The study of biradicals with sterically hindered para-phenylene bridges and an “Aviram-Ratner” (bicycle[2.2.2]octane) bridge allowed for an experimentally- determined evaluation of torsionally dependent (π) and torsionally independent (σ) contributions to the electronic and exchange couplings at parity of donor, acceptor and donor-acceptor distance. The torsional dependence was illustrated using a 3-dimensional, “Ramachandran-type” plot that related D-B torsion and B-A torsions to both electronic and exchange couplings. Biradicals with asymmetric thiophene-pyridine bridges were used to study a bridge’s ability to affect current rectification in a molecular electronic device. Our approach uses McConnell’s electronic coupling theory and Nitzan’s correlation of electronic coupling with conductance (= resistance-1). Within the framework of our model, exchange coupling parameters of the biradicals were used to estimate rectification ratios of the unsymmetric bridge. This study showed that the intra-bridge torsion angles and the unexpected symmetry of the bridge LUMO conspired to create a small rectification ratio. However, the “biradical approach” is quite effective to derive key structure-property relationships that allow insight into the choice of bridge fragments for molecular rectification. Study of Bridge Effects on Electronic Coupling of Donor-Bridge-Acceptor Biradical Complexes by Jinyuan Zhang A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy Chemistry Raleigh, North Carolina 2016 APPROVED BY: ________________________ ________________________ David Shultz Elon Ison Committee Chair ________________________ ________________________ Walter Weare Joshua Pierce BIOGRAPHY Jinyuan was born on June 30th, 1988 in Kaiyuan, Yunnan Province, China. He was curious about various natural and scientific phenomena since childhood. After entrance into junior high school, he developed my interest in chemistry and started to set up his own lab at home to do basic chemistry experiments. Thus, he, without hesitation, chose chemistry as his major when he entered Xiamen University in 2006. Xiamen University has the most beautiful campus in China and it was on that campus he first met his wife Jiazhen Song. In 2010, he had a chance to get in the Key Laboratory for Modern Biochemistry of Fujian Province where he conducted my undergraduate project on asymmetric synthesis of a complicated natural product stemofoline under Prof. Peiqiang Huang’s guidance. After graduated he joined Prof. Hongping Zhu’s Group to do research on main-group organometallic chemistry of as a research assistant. And then he decided to pursue a higher level in chemistry in the US, then he received the offer of admission from NC State University and joined the Shultz Group in 2011. His Ph.D research focuses on donor-bridge-acceptor biradical metal complexes and he has accomplished many of the complexes featuring synthetically challenging bridges. ii ACKNOWLEDGEMENT First of all, I would like to express my sincere gratitude to my advisor Prof. David Shultz for the continuous support of my entire Ph.D study. His guidance helped me in both the research leading to and the writing of this dissertation. Secondly, I would thank Prof. Martin Kirk and his group for collaborating with us. Also I would like to thank my committee members: Prof. Elon Ison, Prof. Walter Weare and Prof. Joshua Pierce, for their insightful comments and encouragement that helped me overcome difficulties encountered during my Ph.D studies. My sincere thank also goes to the Shultz Group members: Dan, “Tich,” Dr. Wang and “Shaker” for their help during my research and their effort to make the lab an interesting place. I also thank our staff crystallographer, Roger Sommer as well as Lukasz Wojtas (U. South Florida) for crystal structures, and Prof. Nathaniel Finney for synthetic advice. Finally, I would like to thank my wife Jiazhen Song, for taking very good care of me as well as our two children, and for her devotion and love for our family. iii TABLE OF CONTENTS LIST OF TABLES…………………………………………………………………...vi LIST OF FIGURES……………………………………………….………………vii LIST OF SCHEMES………………………………………………………..…….xii LIST OF ABBREVIATIONS……………………………………...…………....xiv I. General Introduction, Background and Theory………………………………....1 I.1. Introduction of Electron Transfer in Donor-Bridge-Acceptor Systems…………...1 I.2. How to Measure Electronic Coupling…………………………………………….2 I.2.1. Photoinduced Electron Transfer Reactions in D-B-A Systems…………….2 I.2.2. Single Molecule Conductance from STM-BJ Experiment……………...….4 I.2.3 Magnetic Exchange Coupling in D-B-A Biradical Complexes ………….…5 I.2. Introduction to Exchange Coupling in Biradicals……………………………...….7 I.2.1. Definition of Exchange Coupling Parameter J……………………………..7 I.2.2. Measurement of Exchange Coupling JDA in D-B-A Biradical Complexes…8 I.3. Valence Bond Configuration Interaction Method………………………..………11 I.4. Introduction to D-B-A Biradical Complexes in the Shultz Group……………….14 References…………………………………………………………………………....20 II. Determining the Conformational Landscape of σ and π Coupling Using para- Phenylene and “Aviram–Ratner” Bridges………………..………………...…24 II.1. Introduction………………………………………………………………...…24 II.1.1. Introduction to Torsional Dependence on Electronic Coupling …………..24 II.1.2. Introduction to “Aviram-Ratner” Bridge: Bicyclo[2.2.2]octane Bridge.... 26 II.1.3. Introduction to Spin Polarization of σ-Framework…………………...….27 II.1.4. Target Molecules and Expectation…………………………………...…29 II.2. Results and Discussion of Completed Work…………………………………….31 II.2.1. Synthesis of Biradicals with Methyl Substituted Phenylene Bridges…….31 II.2.2. Synthesis of Biradical with Bicyclo[2.2.2]octane Bridge……………..…33 II.2.3. Structural and Magnetometric Study of Torsional Dependence Effect ….41 II.2.4. Spectroscopic and Theoretical Study of Torsional Dependence Effect…..45 II.2.5. Torsional Independence of Bicyclo[2.2.2]octane Bridge……………….51 II.3. Experimental Section………………………………………………….……....55 References……………………………………………………………………………77 III. Donor-Bridge-Acceptor Biradicals as Models of Single Molecule Devices: Determination of Bridge Rectification Ratios…………….…………………80 III.1. Introduction……………………………………………………………..……80 III.1.1. Introduction to Molecular Electronics…………………………….…..…80 III.1.2. Target Molecules and Expectations…………………………………...…84 III.2. Results and Discussion of Completed Work…………………………………88 III.2.1. D-B-A Biradical Complexes with Donor-Acceptor Bridges…………..…88 III.2.2. Synthesis of Biradicals Complexes with Asymmetric Thiophene-Pyridine iv Bridges………………………………………………………………….98 III.2.3. Structural and Magnetometric Study on Rectification Effect of SQ-T-P-NN and SQ-P-T-NN………………………………………………………....103 III.2.4. Spectroscopic and Theoretical Study of Rectification Effect of SQ-T-P-NN and SQ-P-T-NN…………………………………………………………106 III.2.5. Synthesis and Characterization of the Biradicals with Pyridine bridges- The “Parent Compounds” of SQ-T-P-NN and SQ-P-T-NN…….………110 III.3. Experimental Section………………………………….……..……...………..118 References…………………………………………………………………………..138 Appendix.…………………………………………………………………………..140 A. Donor-Bridge-Acceptor Biradicals with Methyl Substituted Thiophene Bridges….……………………………………………………………..…140 B. Experimental Section……………………………………………………..…144 References……………………………………………………………………151 v LIST OF TABLES Table II-1. Different Friedel-Crafts reaction conditions and results………………36 Table II-2. Measured exchange coupling (JDA) and calculated electronic coupling (HDA) for SQ-NN and SQ-Bridge-NN biradicals………………...……50 Table II-E1. Crystallographic details of 1-BCO, 1-PhMe2 and 1-Me2Ph….………59 Table II-E2. Select Torsion Angles for Complexes 1-Ph, 1-MePh, 1-PhMe, 1-pXylyl, 1-PhMe2, 1-Me2Ph and 1-PhMe4……………………………………60 Table III-1. Comparison of different bromination conditions and the results…..…89 Table III-2. Comparison of different Suzuki reaction conditions and the results…91 Table III-E1. Crystallographic details of SQ-P-T-NN and SQ-T-P-NN…………..119 Table III-E2. Select Torsion Angles for Complexes SQ-T-P-NN and SQ-P-T-NN.122 vi LIST OF FIGURES Figure I-1. (A) Photoinduced electron transfer (PET) reaction. (B) Orbital diagram perspective of PET reaction…………………………………………...…2 Figure I-2. Jablonski diagram of PET reaction in D-B-A system……………..……3 Figure I-3. D-B-A system used for PET experiments by Wasielewski.………………4 Figure I-4. Cartoon of typical STM molecular junction measurement.………………5 Figure I-5. Three steps of STM-BJ experiment to measure molecular conductance…5 Figure I-6. Architecture of D-B-A biradical complexes in the Shultz Group………...6 Figure I-7. Cartoon suggesting the utility of D-B-A biradical electronic structure to elucidate molecular structure-property relationships…………………..…7 -1 -1 Figure I-8. Theoretical χparaT vs T plots with 500 cm ≤ JDA ≥ 500 cm in system of 2 unpired electrons……………………………………………………….10
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