Theoretical investigation of potential energy surfaces relevant for excited-state hydrogen transfer in o-hydroxybenzaldehyde

The potential energy functions of the electronic ground state as well as the lowest n π^* and ππ^* excited single states of o-hydroxybenzaldedhyde (OHBA) have been theoretically characterized along the proton transfer (PT) reaction coridnate as well as the reaction coordinate leading to the prefulvenic form. The calculations have been performed with the ab initio complete-active-space self-consistent-field (CASSCF) method and with second-order perturbation theory, employing the CASSCF wave function as the reference state. It is found that the ¹ππ^* state is almost isoenergetic with the ¹nπ^* state after vertical excitation of OHBA. The ¹ππ^* potential energy function is found to be barrierless along the PT reaction coordinate, while the ¹nπ^* potential energy exhibits a significant barrier. An efficient non-radiative pathway has been identified resulting from strong non-adiabatic interactions between the ground and the excited singlet states along the reaction coordinate to the prefulvenic form of OHBA. It is argued that the consideration of multi-dimensional vibronic interactions is essential for the understanding of the excited state dynamics of OHBA.