Elucidation of Ultrafast Decay, Vibrational Beating, and Slow Decay Processes of Excited Azulene

Azulene's nonradiative decay dynamics and kinetics from its singlet excited-states were studied using mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) combined with trajectory surface-hopping nonadiabatic molecular dynamics (NAMD) and Fermi's golden rule (FGR) rate theory. The NAMD dynamics reproduce experimental observations that the S₁ → S₀ decay is accelerated and exhibits a crossover from mono- to biexponential kinetics with increasing excess vibrational energy. Minimum-energy-path analyses reveal a continuous S₁/S₀ conical-intersection seam slightly above the S₁ minimum, providing readily accessible funnels. Time-resolved normal-mode projections reveal selective energy funneling into C-C stretching modes at 1272, 1528, and 1692 cm^-1. Constructive combinations of the latter two modes appear to promote rapid access to the conical-intersection seam, whereas their beating at ∼ 160 fs imposes a natural limit on the decrease of the decay time upon a further increase of excess energies, suggesting that their interference delays the decay. The FGR rate calculation data for the S₁ → S₀ transition reaffirm that the nonradiative decay proceeds mainly near or through the conical intersection, rather than via simple nonadiabatic derivative coupling around the minimum of S₁. On the other hand, the FGR rates indicate that S₂ decays predominantly to S₀, as the S₂-S₁ vibronic coupling is exceptionally weak, which serves as a primary cause for azulene's characteristic anti-Kasha emission.