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 almos...

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