Mechanisms of nonadiabatic transitions in photoisomerization processes of conjugated molecules: Role of hydrogen migrations

Mechanism of nonadiabatic transition in the C=C torsional photoisomerization process of ethylene and polyenes is investigated by using the ab initio configuration interaction calculation method. We have calculated the low-lying singlet state potential energy surfaces and their nonadiabatic couplings. A multidimensional search for the molecular configurations yielding strong nonadiabatic couplings is performed to find the origin of very fast photoisomerization kinetics, which are experimentally observed to be typically in the order of a few or a few tens of picoseconds. It is found that the ‘‘pseudo’’ migration motion of a hydrogen adjacent to the twisted C=C bond causes a potential surface crossing of the low-lying excited and ground states and thus induces a sufficiently large nonadiabatic coupling to explain this experimental evidence. The hydrogen migration motion is facilitated by the so-called zwitterionic character of the low-lying excited states near the 90° C=C twisted conformation, proceeds almost without an energy barrier and involves large molecular conformational changes. The shape of potential surface crossing is found to be multidimensional in nature and so that, once a molecule reaches this crossing region by the hydrogen migration, almost all the internal molecular motions cause the strong nonadiabatic couplings between the excited and ground states. The role of the ‘‘2 1A−g’’ state in the photoisomerization is also discussed.