A theoretical study on vibronic spectra and photo conversation process of protonated naphthalenes

The equilibrium structures and vibrational frequencies of the ground state and several singlet low-lying excited states of alpha- and beta-protonated naphthalenes (α- and β-HN⁺) have been studied by time-dependent density-functional theory (TD-DFT). Within the Franck-Condon approximation, vibronic absorption spectra of α-HN⁺ and β-HN⁺, together with the vibronic emission spectrum of α-HN⁺, have been calculated. The obtained good agreement between the theoretical and experimental spectra enables to correctly assign vibronic features in both absorption and emission spectra. Moreover, the non-radiative deactivation pathway from the low-lying excite states to the ground state in α-HN⁺ and β-HN⁺, as well as the photo-induce proton transfer pathway, are investigated at the CASPT2/CASSCF/6-31G* level. Our study is helpful for understanding the photochemical behavior of these important polycyclic aromatic hydrocarbon molecules.