Impact of the Dynamic Electron Correlation on the Unusually Long Excited-State Lifetime of Thymine

Non-radiative relaxation of the photoexcited thymine in the gas phase shows an unusually long excited-state lifetime, and, over the years, a number of models, i.e., S₁-trapping, S₂-trapping, and S₁&S₂-trapping, have been put forward to explain its mechanism. Here, we investigate this mechanism using non-adiabatic molecular dynamics (NAMD) simulations in connection with the recently developed mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) method. We show that the previously predicted S₂-trapping model was due to an artifact caused by an insufficient account of the dynamic electron correlation. The current work supports the S₁-trapping mechanism with two lifetimes, τ₁ = 30 ± 1 fs and τ₂ = 6.1 ± 0.035 ps, quantitatively consistent with the recent time-resolved experiments. Upon excitation to the S₂ (ππ*) state, thymine undergoes an ultrafast (ca. 30 fs) S₂→S₁ internal conversion and resides around the minimum on the S₁ (nπ*) surface, slowly decaying to the ground state (ca. 6.1 ps). While the S₂→S₁ internal conversion is mediated by fast bond length alternation distortion, the subsequent S₁→S₀ occurs through several conical intersections, involving a slow puckering motion.