Indirect Intersystem Crossing (S1 → T3/T2 → T1) Promoted by the Jahn-Teller Effect in Cycloparaphenylenes.

Vibronic-coupling effects play a key role for excited-state charge- and energy-transfer processes in organic molecular systems. Here, we demonstrate how the Jahn-Teller effect in triplet excited states of highly symmetric cycloparaphenylenes triggers an indirect intersystem crossing deactivation pathway. Strong Jahn-Teller distortion in the doubly degenerate second excited triplet state (T2) brings the molecular system energetically close to the lowest triplet state (T1), thereby opening the possibility for an extremely rapid internal conversion. Quantum dynamics simulations reveal an initial T2 → T1 population decay within 50 fs. Experimental observation of size-dependent intersystem crossing rates of cycloparaphenylenes is explained based on the proposed S1 → T3/T2 → T1 mechanism.