Decoding the Curious Tale of Atypical Intersystem Crossing Dynamics in Regioisomeric Acetylanthracenes

Mapping the primary photochemical dynamics and transient intermediates in functional chromophores is vital for crafting archetypal light-harvesting materials. Although the excited state dynamics in 9-acetylanthracene is well explored, the origin of near-quantitative triplet population and the atypical intersystem crossing (ISC) rate as compared with the regioisomeric analogs (1-/2-acetylanthracene) have rarely been scrutinized. We present a comprehensive account of the photoinduced dynamics in three regioisomeric monoacetylanthracenes using ultrafast transient absorption and quantum chemical calculations. The conjoint experimental and computational investigations suggest that (i) greater stabilization of the ¹nπ* relative to ¹ππ* state, (ii) dissimilar ¹ππ* → ¹nπ* crossover barriers, and (iii) the strong spin-orbit coupling (ν_SO) of the ¹nπ* state with the receiver ³ππ* state command the divergent triplet population in 1-/2-/9-acetylanthracenes. A tacit understanding of the subtle structural-alteration-facilitated contrasting ISC dynamics in carbonylated arenes can act as a stepping stone for the evolution of potent photofunctional materials.