Chemical and Photophysical Behaviour of π-Extended Tropyliums

Non-benzenoid annulenes have long fascinated organic chemists, due to their chemical properties, aromaticity, and photophysical behaviour. When incorporated into a polycyclic aromatic framework, such non-hexagonal rings can give rise to nonplanar structures with modified optical properties and improved solubility compared to planar, defect-free graphene. Among non-benzenoid annulenes, the tropylium cation has been of special interest, due to its unique blend of reactivity (owing to its positive charge) and stability (a result of its aromaticity). Indeed, this cation has found wide utility as a versatile ligand , stimulus-responsive dye , and an organocatalyst. Yet, reports of polycyclic aromatics featuring this heptagonal annulene are sparse. This Thesis employs tropylium and its neutral, nonaromatic homologue, cycloheptatriene (Figure 1) as key structural motifs within an extended π-framework to gain fundamental insights into the electronic and optical properties of non-benzenoid and charged polycyclic aromatics. We find that judicious engineering of strain into the framework of sterically overcrowded tropyliums can cause its aromaticity to rupture, forming an “aromatic-to-nonaromatic” equilibrium at room temperature. Moreover, modifying the conjugation length in a series of cycloheptatriene-rotors was found to vastly alter their photoluminescence properties, allowing for new modes of chemical reactivity. We also report our synthetic forays toward a highly warped redox-active warped nanographene, as well as preliminary findings on the excited-state proton transfer dynamics in a series of hydroxybenzotropyliums. 1. T. Murahashi, M. Fujimoto, M. A. Oka, Y. Hashimoto, T. Uemura, Y. Tatsumi, Y. Nakao, A. Ikeda, S. Sakaki and H. Kurosawa, Science, 2006, 313, 1104–1107 2. U. P. N. Tran, G. Oss, D. P. Pace, J. Ho and T. V. Nguyen, Chem. Sci., 2018, 9, 5145–5151. 3. D. J. M. Lyons, R. D. Crocker and T. V. Nguyen, Chem.—Eur. J., 2018, 24, 10959–10965. 4. P. K. Saha, A. Mallick, A. T. Turley, A. N. Bismillah, A. Danos, A. P. Monkman, A.- J. Avestro, D. S. Yufit and P. R. McGonigal, Nature Chem., 2023 15, 516–525. 5. A. T. Turley, P. K. Saha, A. Danos, Aisha N. Bismillah, A. P. Monkman, D. S. Yufit, B. F. E. Curchod, M. K. Etherington, and Paul R. McGonigal, Angew. Chem. Int. Ed., 2022, 61, e202202193.