New Insights into Intersystem Crossing in Substituted Aromatics: Singlet-Triplet Conversion in Carbonyl-Substituted...

A new study is presented to elucidate the photodynamics of model carbonyl-substituted polyaromatics targeting the relevance of carbonyl-group orientation and torsional re-equilibration on intersystem crossing (ISC). Our experiments focused on 9-acetylanthracene (9AA) using femtosecond resolved spectroscopy. In the ground state of this molecule, steric interactions force the carbonyl substituent into a near-perpendicular orientation relative to the aromatic system. The time-resolved signals from 9AA show that ISC takes place after spectral shifts that reflect the evolution of the carbonyl group to a slanted geometry as it adjusts to a dihedral angle of around 40° with respect to the aromatic plane. Depending on the solvent, in 9AA manifold crossing takes place on the 3 to 25 ps time-scale. On the other hand, for 2-acetylanthracene (2AA) which is coplanar in both S₀ and S₁, the emission lifetimes can reach several nanoseconds. Analysis of these systems at the highest available theoretical levels reveals further insights into the excited-state dynamics. For 9AA and in contrast with previous publications, it is established that for all relevant geometries, the first excited singlet retains a ππ* character and decays through ISC with no involvement of other singlet states. The manifold crossing involves the interaction with the triplet manifold through states which's transition orbitals are partially localized at the acetyl substituent. Specifically, the slanted geometry of the carbonyl group in 9AA and the potential energy surface around the equilibrium S₁ geometry implies significant spin-orbit interactions and accelerated manifold-crossings. The present results highlight the relevance of substituent reorientation and their slanted geometries which appear to be a dominant feature in carbonyl and nitrated aromatic systems which show rapid ISC dynamics. In the article, we include details on the differences in the mechanisms operating in these two kinds of systems which show the fastest ISC rates among organic chromophores.