An ab initio MO study on a model system for photochromic compounds containing a dithienylethene unit is presented. On the basis of the obtained potential energy profile, a rationalization is provided for the proposed mechanism of the experimentally observed stepwise multiphoton process in the ring-opening cycloreversion reaction. An explanation, which correlates the experimental quantum yields with the calculated properties as a function of substituent effects, is provided. A method has been developed which can be utilized as a guiding principle for future molecular design.

The mechanism of the photochromic cycloreversion reactions is theoretically examined in a model system of dithienylethenes by means of the CASSCF and CASPT2 methods. The structures of its conical intersections (CIs), which are the branching points of the internal conversions, were obtained. The analyses of the minimum energy paths from the Franck-Condon states and the CI points suggest that the cycloreversion reaction occurs during the intramolecular vibrational energy redistribution (IVR) toward the quasi-equilibrium on the 2A state. The current study of the model system will provide a basic insight for the photochromic molecular design.