Interplay between Aromaticity and Photophysics in Novel Pro-aromatic Thioamide Chromophores

The molecular aromaticity of π-conjugated annulenes can be perturbed using chemical functionalization to afford novel systems that exhibit attractive properties. Classical aromatic systems are expected to obey the Hückel rule in the ground state. However, upon photo-excitation of those Hückel-type molecules, a change/reversal in aromaticity characters in the excited state could compromise the stability of the transient species. To improve the photo kinetics and stability of transient species derived from Hückel-type molecules or chromophores, one can (i) perturb the intrinsic global aromaticity of the ground-state molecules or (ii) introduce pro-aromatic moieties in the backbone of these molecules. The latter strategy would afford molecular systems with moieties having varying degrees of aromaticity. Expectedly, upon photo excitation, aromaticity reversal would create excited transients having enhanced local aromaticity, ensuring greater photostability and extended excited state kinetics. Alternatively, one can devise molecular systems that feature both global Hückel and local Baird aromaticity such that the photo excited properties of these novel systems can be influenced by Baird aromaticity, which could facilitate efficient singlet-to-triplet intersystem crossing (ISC).This dissertation focuses on pro-aromatic quinoidal scene thioamides (QDM 1) and its derivatives that were prepared to elucidate the role of molecular aromaticity and π-conjugation on ground-state stability and excited-state behaviors. QDM 1 was used as a light-harvesting triplet chromophore in/for photon-energy management and non-linear photophysical processes. While the unique photophysics of QDM 1 was found to be influenced by aromaticity reversal, it was also found that the presence of the thioamide functionality induced non-adiabatic photophysics, ensuring efficient ISC. The synergy between molecular aromaticity and “sulfur-effect” was explored with derivatives 2–8: Derivatives 2–4 were prepared to interrogate the role of ground-state aromaticity imbalance (aromaticity extension) on the stability and triplet photophysics of QDM 1, derivatives 4–6 were investigated to unravel the effect of the chalcogen atoms on the triplet photo-behavior of QDM 1, and derivatives 7 and 8 were studied to understand the interplay between pro-aromaticity or π-regression and “sulfur-effect” on the excited-state photo behavior of QDM 1. The structural alterations of QDM 1 have shed light on the influence of ground- and excited-state aromaticity on photo-behavior(s) of novel pro-aromatic thioamide chromophores that were investigated throughout my graduate research journey. My dissertation will present a chronological insight into the design, synthesis, photophysics, and computational investigations of pro-aromatic quinoidal acenethioamide QDM 1 and its derivatives.