Photoisomerization of polyenes: Potential energy surfaces and normal mode analysis

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Abstract
Photoisomerization processes of polyenes in the triplet mechanism are investigated theoretically. Global features of the lowest triplet and the ground state potential surfaces are evaluated for butadiene, pentadiene, hexatriene, and the protonated Schiff base of butadiene in an ab initio method and for longer polyenes in a semiempirical method. The steric relaxations along the isomerization are carefully examined. To understand the mechanism of radiationless transitions (intersystem crossings) concomitant with photoisomerizations, the normal mode analyses are performed and the overlap integrals of the Franck–Condon type are calculated. The torsional modes around the certain C–C bonds are found to be the main acceptor of the electronic energy in the deactivation process and some other modes such as H flappings act as minor but non‐negligible acceptors.