University of Nevada, Reno Synthesis of contorted nanographenes via multi-fold alkyne benzannulation reactions A dissertation submitted in partial fulfillment of the Requirements for the degree of Doctor of Philosophy in Chemistry by Paban Sitaula Prof. Dr. Wesley. A. Chalifoux/ Dissertation Advisor: May 2020 Copyright by Paban Sitaula 2020 All Rights Reserved THE GRADUATE• SCHOOL We recommend that the dissertation prepared under our supervision by entitled be accepted in partial fulfillment of the requirements for the degree of Advisor Committee Member Committee Member Committee Member Graduate School Representative David W. Zeh, Ph.D., Dean Graduate School i Abstract Nanographenes (NGs) of unique shape, size and properties are always at the center of attraction because of their potential application as semiconducting materials in organo-electronic devices. Contorted NGs have gained increased attention because of their fascinating molecular packing, reduced π-π interaction, enhanced solubility and lower band gap compared to the planar analogues. We have synthesized a library of contorted NGs by utilizing the non-oxidative, alkyne benzannu- lation reaction catalyzed by indium chloride and silver bis triflimide by exploiting high energy content of carbon-carbon triple bonds of the diyne precursors under a mild-reaction conditions. We employed two-fold InCl3/AgNTf2-catalyzed alkyne benzannulation reaction to afford a broad collection of highly functionalized, laterally π-expanded, [5]helicene-like naphtho[1,2-a]pyrene derivatives in moderate to very good yields. We were able to utilize this method to expand conju- gation of the HBC core to get a variety of π-extended HBC NGs. The Suzuki cross-coupling of the halogen(s) substituted smaller polycyclic aromatic hydrocarbons with diyne boronic ester gave polyalkyne precursors, which were subjected to multi-fold alkyne benzannulation reaction to af- ford larger, highly soluble contorted NGs. This work also proved the applicability InCl3 and AgNTf2 in the synthesis of up to six benzene ring in one-pot reaction condition. In most of the cases, the chiral, helically twisted skeletons were unambiguously determined through X-ray crys- tallography. We are also very close (two-steps away) towards the synthesis of longer pyrenacenes such as quateropyrene and quinteropyrene following our previous protocol for the synthesis of pyrene, peropyrene and teropyrene. ii a) Chiral naphtho[1,2-a]pyrenes b) Extended chiral HBC NGs c) Longer functionalized chiral pyrenacenes iii Acknowledgements I would like to express the most sincere appreciation to my advisor, Prof. Dr. Wesley A. Chalifoux, for his willingness and enthusiasm in helping me to become the researcher that I am today. I would also like to thank Dr. Chalifoux for always holding the bar high and helping me to grow my chem- ical knowledge. He has been absolutely instrumental regarding my success. He has taught me how to become a great researcher, an inspiring teacher and a good human being. Without his guidance, continuous encouragement, persistent motivation and throughout help, this dissertation would not have been possible. I would like to express my sincere thank and deep appreciation to my graduate committee members, Dr. Robert S. Sheridan, Dr. Sean Casey, Dr. Shamik Sengupta, and Dr. Mark A. Pinsky for their guidance, time, and support during my candidacy exam and dissertation de- fense. I’m very grateful to Prof. Lawrence T. Scott for his thoughtful suggestions every week in our group meeting which really helped me to drive my research projects ahead. I would also like to thank Dr. Vincent Catalano and Dr. Stephen Spain for instrumentation training and for always being there to troubleshoot problems. I would like to acknowledgement to our collaborators Gio- vanna Longhi, Sergio Abbate, Eva Gualtieri, Andrea Lucotti, Matteo Tomassini, Roberta Frazini and Claudio Villani for their contributions in studying optical properties of our compounds. I would like to thank my lab mates in the Chalifoux lab, particularly Dr. Wenlong Yang and Dr. Khagendra Hamal for their help when I started working. I would like to thank Punyanuch Sophanpanichkul, Amber Senese, Kelsie Magiera, Ryan Malone, Stephen George and for their help and support and being nice labmates. I would like to acknowledge faculties, staff, and friends in the UNR Chemistry department for helpful discussions, support and assistance when I needed it most. I would like to thank my parents, whose love and guidance and prayers are with me in whatever I pursue. I’m much grateful to my siblings Bimala and Pawana for always keeping my iv desire at highest priority than their needs. Most importantly, I wish to thank my loving and sup- portive wife, Bimala, and my son Parjanya, who provide unending inspiration. v Table of Contents 1. INTRODUCTION…………………………………………………………………………….1 1.1 Graphene……………………………………………………………………………………...1 1.2 Nanographenes (NGs)………………………………………………………………………. .2 1.3 Clar’s Rule……………………………………………………………………………………4 1.4 Classification of the NGs……………………………………………………………………..5 1.4.1 NGs with zigzag edges……………………………………………………………………...6 1.4.2 NGs with arm-chair edges…………………………………………………………………..6 1.4.3 Zigzag-armchair hybrid NGs……………………………………………………………….7 1.4.4 planar and twisted NG………………………………………………………………………8 1.5 Molecular architectures of chiral NGs……………………………….………………………..9 1.5.1 Helicenes……………………………………………………………………….……..……10 1.5.2 Twistacene……………………………………………………………………….…………10 1.5.3 Propeller-shaped NGs………………………………………………………………………11 1.5.3 Nanohoops and nanotubes (cylindrical NGs)…………………………………….………...12 1.6 Synthetic methods of NGs……………………………………………………………………13 1.7 Alkyne benzannulation reactions………………………………………………….………….16 1.8 Mechanism of electrophilic alkyne benzannulation……………………………….………….24 1.9 Conclusion…………………………………………………………………………………....25 1.10 References………………………………………………………………………………......25 2. SYNTHESIS, CHARACTERIZATIONS AND PROPERTIES OF [5]HELICENE-LIKE π-EXTENDED NAPHTHO[1,2-a]PYRENES………………………………………………...34 2.1 Helicenes……………………………………………………………………………………..34 vi 2.1.1 Laterally π-extended helical NGs………….……………………………………………….35 2.2 Naphtho[1,2-a]pyrenes……………………………………………………………………….37 2.2.1 Our route towards naphtho[1,2-a]pyrenes……………….…………………………………37 2.2.2 Synthesis of diyne boronic ester……………………………………………………………38 2.2.3 Synthesis of 3-bromophenantharenes………………………………………………………40 2.2.4 Two-fold alkyne benzannulation reaction………………………………….………………40 2.3 Photophysical properties……………………………………………………………………..44 2.3.1 UV-vis/fluorescence spectra………………………………………………………………..44 2.3.2 Separation of enantiomers……………………………..…………………………………...46 2.3.3 CD spectra of 2.7d and 2.7i…………………………………………………………………49 2.3.4 FTIR Spectroscopy…………………………………………………………………………51 2.3.5 Raman Spectroscopy……………………………………………………………………….52 2.4 Conclusion and future directions………………………………………..……………………54 2.5 Experimental section…………………………………………………………………………56 2.5.1. General experimental……………………………………………………………………...56 2.5.2. Synthesis and characterizations……………………………………………………………57 2.6 Reference…………………………………………………………………………………….90 3. SYNTHESIS OF HBC-BASED π-EXTENDED NANOGRAPHENES VIA ALKYNE BENZANNULATION REACTIONS………………………………………………….….…...94 3.1 Introduction…………………………………………………………………………………..94 3.2 Our design of the π-extended HBC NGs…………………………………………………….98 3.3 Synthesis of π-extended HBC NG analogues………………………………………………100 3.3.1 Mono π-extended HBC NG 3.18………………………………………………………….100 vii 3.3.1.1 Synthesis of pseudo-1-bromoHBC……………………………………………………...100 3.3.1.2 Two-fold alkyne benzannulation reaction towards HBC NG 3.13……………………..102 3.3.1.3 X-Ray crystal structure and racemization barrier of HBC NG 3.18…………………...104 3.3.2 Pseudo-1,2-π-extended HBC NG 3.19……………………………………………………105 3.3.3 Pseudo-1,4-π-extended HBC NG 3.20……………………………………………………107 3.3.3.1 Synthesis of soluble pseudo-1,4-dibromoHBC………………………………………….108 3.3.3.2 Four-fold alkyne benzannulation towards HBC NG 3.15……………………………….109 3.3.3.3 Possible isomers………………………………………………………………………...110 3.3.3.4 Crystal structures and racemization barrier studies of compound 3.20………………...113 3.3.4 UV-vis spectroscopy……………………………………………………………………...115 3.3.5 Attempted synthesis of pseudo-1,3-extended HBC NG…………………………………..116 3.3.6 Pseudo-1,3,5-extended Propeller-shaped HBC NGs……………………………………...118 3.3.6.1 Propeller-shaped NGs (PNGs)………………………………………………………….118 3.3.6.2 Our design of HBC PNG………………………………………………………………..122 3.3.6.3 Synthesis of pseudo-1,3,5-triiodoHBC 3.75…………………………………………….123 3.3.6.4 Six-fold alkyne benzannulation reaction towards HBC NGP 3.21……………………..124 3.3.6.5 Isomers of HBC PNG 3.21……………………………………………………………...125 3.3.6.6 Synthesis of soluble pseudo-1,3,5-triiodoHBC………………………………………...126 3.3.6.7 Attempted synthesis of HBC PNG starting from soluble triiodoHBC 3.88…………….128 3.4 Conclusion…………………………………………………………………………………..131 3.5 Experimental section………………………………………………………………………..132 3.5.1 General Methods………………………………………………………………………….132 3.5.2 Synthesis and characterizations…………………………………………………………..133 viii 3.6 References…………………………………………………………………………………..146 4. SYNTHESIS OF LONGER PYRENACENES…………………………………………...152 4.1 Introduction…………………………………………………………………………………152 4.2 Our design of longer pyrenacenes…………………………………………………………..156 4.3 Attempted synthesis of quateropyrene……………...………………………………………157 4.3.1 Convergent approach toward the synthesis of chiral diiodoperopyrene…………………..157 4.3.1.1 Synthesis of chiral diiodoperopyrene 4-19……………………………………………...157 4.3.1.2 Eight-fold alkyne benzannulation towards the synthesis