Unveiling the Role of Hot Charge-Transfer States in Molecular Aggregates via Nonadiabatic Dynamics

Exciton dynamics governs energy transfer and charge generation in organic functional materials. We investigate high-energy nonadiabatic excited-state dynamics for a bithiophene dimer to describe time-dependent excitonic effects in molecular aggregates. We show that the lowest excited states are populated on the subpicosecond time scale. These states are localized and unproductive in terms of charge separation. Productive high-energy charge-transfer (CT) states are populated within 50 fs during exciton deactivation, but they are short-lived (similar to 100 fs) and quickly transfer their population to lower states. Our simulations offer molecular-level insights into ultrafast photoinduced charge separation potentially triggered by hot CT states in solid-state organic materials. Design rules are suggested to increase hot exciton lifetimes, favoring the population of CT states as gateways for direct charge generation. These rules may boost the CT quantum yield by depleting unproductive recombination channels.