Cooperative ππ* ↔ nπ* Trapping Mechanism in the Excited-State Dynamics of 5-Fluorouracil.

Uracil and its derivatives typically achieve photostability through ultrafast internal conversion (IC), and dark nπ* states can play a prominent role in relaxation dynamics. Compared with uracil, 5-fluorouracil (5-FU) exhibits a much higher nπ* quantum yield and a longer nπ*-state lifetime, yet the molecular origin of this enhanced nπ*-state participation remains unclear. Here, we perform electronic-structure calculations and nonadiabatic dynamics simulations to elucidate the excited-state relaxation mechanism of 5-FU. The results reveal rapid ππ* → nπ* transfer within the first ∼38 fs, facilitated by the near-degeneracy and strong nonadiabatic coupling between the two states in the Franck-Condon region. Subsequently, recurrent ππ* ↔ nπ* population exchange establishes a cooperative trapping mechanism that involves both electronic states. This trapping is governed by sustained ππ*/nπ* coupling and dynamic population redistribution, rather than by static dark-state residence or simple barrier-controlled decay. During the ππ* ↔ nπ* exchange, intermittent recovery of ππ* character preserves a dynamical connection to the ππ*/S0 conical intersection pathway. Overall, this cooperative ππ* ↔ nπ* trapping mechanism provides a mechanistic basis for the distinct nπ*-mediated relaxation dynamics of gas-phase 5-FU and suggests that recurrent bright-dark states exchange may contribute to the persistence of long-lived nπ*-associated character.