Insight into the new excited-state intramolecular proton transfer (ESIPT) mechanism of N,N′-bis(salicylidene)-p-phenylenediamine (p-BSP)

Abstract

A new excited-state intramolecular proton transfer (ESIPT) mechanism of N,N′-bis(salicylidene)-p-phenylenediamine (p-BSP) including the single and double ESIPT processes has been proposed using the time-dependent density functional theory (TDDFT) method, which was clearly different from the one proposed previously (Ziółek et al., J. Phys. Chem. A. 2009, 113, 2854). By analyzing the bond lengths, bond angles and IR vibrational spectra, we determined that two intramolecular hydrogen bonds (IHBs) (O1–H2⋯N3 and O4–H5⋯N6) of p-BSP were strengthened in the S1 state, which could facilitate the ESIPT process. Additionally, intramolecular charge transfer based on the frontier molecular orbitals (MOs) and the map of the electron density difference between the S0 and S1 states demonstrated the possibility of ESIPT reaction. To reveal the detailed ESIPT mechanism, we constructed potential energy surfaces (PESs) in the S0 and S1 states, and thus determined that the single and double ESIPT processes coexisted in the S1 state.