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

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 S1 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 S1 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 S1 in polar solvent is, on the other hand, stabilized by the torsional motion of the 4-dimethylanilino group.