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 (νSO) 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.