Vibronic Effects in the Ultrafast Interfacial Electron Transfer of Perylene-Sensitized TiO₂ Surfaces

We combine ultrafast transient absorption (TA) spectroscopy and nonadiabatic quantum dynamics simulations to describe the real-time unfold of vibronic effects on the photoabsorption of TiO₂ anatase sensitized with the (perylen-9-yl)carboxylate dye (Pe-COOH/TiO₂). The excited state is mapped in time and frequency by ultrafast broadband spectroscopy while atomistic quantum dynamics is used to simulate the self-consistent vibronic effects. The TA map shows the lifetime of the electronic population generated in the S₁ state of the dye and the rise of the absorption D₀-D₁ of the cation. The theoretical analysis reveals that the electron transfer from perylene into TiO₂ is complete within 20 fs, in agreement with the 12 fs experimental measurement. Because of the structural relaxation produced by the photoinduced electron transfer, the optical gap decreases by 390 meV, in agreement with the D₀-D₁ transition band. Furthermore, the reorganization energy estimated to be around 220 meV is mostly due to the energy shift of the HOMO level, since the electron transfer occurs in the wide-band limit with little dependence on reorganization energy modes. By assuming the Condon approximation and by making use of the mixed quantum/classical trajectories of the Pe-COOH/TiO₂ system, the absorption spectrum is calculated, and the broad features of the transient absorption spectrum are correlated to excited-state nuclear reorganization effects of the adsorbate dye. The reorganization energy modes are identified by the power spectrum of the velocity autocorrelation function, which shows the occurrence of nonequilibrium modes within the range 1000-1800 cm^-1 as in-plane asymmetric C-C vibrations in the perylene dye. The vibrational modes with the strongest influence on the optical gap contribute to shifting the absorption spectrum up in energy by ∼2000 cm^-1. The overall agreement between theory and experiment reveals the capabilities of both methods to study vibronic effects in molecular and extended systems.