Ab initio study on the photochemical behavior of 4-dimethylamino,4 ′-cyanostilbene

Abstract

Ab initio complete active space self-consistent field (CASSCF) and the second order perturbation (MRMP2) calculations have been performed to examine the photochemical behavior of 4-dimethylamino,4 ′-cyanostilbene (DCS) in the low-lying excited states. The potential energy curves with respect to four torsional angles at the CASSCF level is possible to give a reasonable interpretation on the experimental findings and the MRMP2 calculation well reproduces the experimental observable such as the excitation energy. The S 1 state in the Franck–Condon region is the internal charge transfer (ICT) state where the electron is partially transferred from the 4-dimethylanilino into the 4-cyanostyryl groups. Only the potential energy curve in S 1 with respect to the torsional motion of the ethylenic double bond has a small barrier to the perpendicularly twisted conformation leading to the cis–trans photoisomerization, though the others becomes energetically much unstable. DCS in S 1 in polar solvent is, on the other hand, stabilized by the torsional motion of the 4-dimethylanilino group.