Photophysics of phenol and pentafluorophenol: The role of nonadiabaticity in the optical transition to the lowest bright (1)pi pi* state

摘要

We report multimode vibronic coupling of the energetically low-lying electronic states of phenol and pentafluorophenol in this article. First principles nuclear dynamics calculations are carried out to elucidate the optical absorption spectrum of both of the molecules. This is motivated by the recent experimental measurements [S. Karmakar et al., J. Chem. Phys. 142, 184303 (2015)] on these systems. Diabatic vibronic coupling models are developed with the aid of adiabatic electronic energies calculated ab initio by the equation of motion coupled cluster quantum chemistry method. A nuclear dynamics study on the constructed electronic states is carried out by both the time-independent and time-dependent quantum mechanical methods. It is found that the nature of low-energy pi sigma* transition changes, and in pentafluorophenol the energy of the first two (1)pi sigma* states, is lowered by about half an eV (vertically, relative to those in phenol), and they become energetically close to the optically bright first excited (1)pi pi* (S-1) state. This results in strong vibronic coupling and multiple multi-state conical intersections among the pi pi* and pi sigma* electronic states of pentafluorophenol. The impact of associated nonadiabatic effects on the vibronic structure and dynamics of the (1)pi pi* state is examined at length. The structured vibronic band of phenol becomes structureless in pentafluorophenol. The theoretical results are found to be in good accord with the experimental finding at both high energy resolution and low energy resolution. Published by AIP Publishing.