Dynamical calculations on the photoisomerization of small polyenes in a nonadiabatic formalism

Abstract

The dynamics of the cis–trans isomerization in 10° monopyramidalized ethylene, butadiene, and hexatriene has been studied in a nonadiabatic treatment including explicitly the twisting motion. The electronic energy curves, (transition) dipole moments, and nonadiabatic couplings have been obtained from ab initio CI calculations. An initially (Franck–Condon) excited wave function is constructed from the final vibrational eigenfunctions and energies. The appearance of the calculated prepared state is discussed in relation to the increasing line broadening vs decreasing chain length of the primary vibrational bands in the 1 1B+u absorption spectra of polyenes. It is found that this feature can be explained from the increasing tendency of shorter polyenes to be nonplanar in the excited state. The time evolution of the molecules is monitored by calculating the radiationless decay, the dipole moment, and the contributions of the electronic states to the total wave function. The role of the nonabsorbing 2 1A−g state is discussed. It is demonstrated that the dynamics of the cis–trans isomerization are directly correlated to the shape of the potential energy curve of the absorbing excited state. Ethylene is found to rotate once about the double bond in 0.075 ps and butadiene in 0.2 ps, whereas hexatriene is excited to an almost stationary wave function. Because of the limitations of the present model, the molecules do not show any substantial (‘‘sudden’’) polarization upon inclusion of the nonadiabatic coupling. For the same reason, the contribution of the electronic ground state to the total wave function is small.