Aromaticity can influence light‐harvesting ability and optoelectronic properties of π ‐conjugated annulenes and heteroaromatics. Here, we designed and compared the triplet photophysics of aromatic thioamide naphthoquinodimethyl‐ bis ‐thioamide ( QDM 1 ) to those of the π ‐regressed derivatives 3,8‐dimethyl‐3,8‐dihydro‐3,8‐phenanthroline‐4,7‐dithione ( p ‐QDM 1 ) and 4,7‐dimethyl‐4,7‐dihydro‐4,7‐phenanthroline‐3,8‐dithione ( p ‐QDM 2 ). Given the unique topology of its aromatic core, we first established that the triplet excited state of QDM 1 exhibits features similar to those of a naphthyl or pyrene fragment with an extended triplet lifetime (τ T ≈ 9 µs) due to a reversal or enhancement of aromaticity in the triplet manifold. On the other hand, the triplet lifetimes of the p ‐QDM derivatives ( τ T ≈ 0.088–0.15 µs) are far below that of QDM 1 , indicating that the presence of the pseudoquinoidal moiety in the structure of QDM 1 aids in extending the triplet lifetime. Furthermore, the π ‐regression and the position of the thiocarbonyl group (C─S) in the p ‐QDM derivatives were also shown to influence their optoelectronic properties. Computational investigations employing anisotropy of the induced current density analysis and multicenter index calculations provide complementary magnetic and electronic assessments of aromaticity that are in agreement with the experimental observations that π ‐conjugation/topology influence the excited triplet behavior(s) of these thioamides. The present investigation revealed fundamental aspects of π ‐conjugation and aromaticity modulation, and their impact on molecular photophysics. Moreover, our findings may inform the rational design of new photoactive organic materials based on aromaticity modulation.
Guang et al. (Thu,) studied this question.