Mechanism of photoinduced intramolecular charge transfer in aminobenzethynes: An ab initio study

Ab initio electronic structure calculations were performed to characterize the charge transfer (CT) process in benzethyne (BE), 4-amino-benzethyne (ABE) and 4-dimethylamino-benzethyne (DMABE). The Hartree-Fock (HP), configuration interaction scheme (CIS), complete active space self-consistent field (CASSCF) and CASPT2 (multiconfigurational second-order perturbation theory taking CASSCF function as a reference) methods were employed. Geometry optimization of the CT state at the CIS level predicts a planar configuration with a trans bent ethyne group to be the lowest excited singlet state in all systems. The significant stabilization of the CT state in the BE series can be understood as arising from the rehybridization of the carbon atoms of the ethyne group on transfer of an electron. The calculation of the relative energy of the locally excited state and the relaxed CT state using the CASPT2 method reveals that the intramolecular CT process is exothermic in ABEs under isolated molecule conditions, in contrast with the much studied aminobenzonitriles, where interaction with a polar solvent is required for sufficient stabilization of the CT state. Therefore BE and its amino derivatives potentially offer unique opportunities for the investigation of photoinduced intramolecular CT in isolated molecules.