Theory of photochemical isomerization in polyenes

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Abstract
In this paper we investigate the nonradiative transition rate from excited states of molecules which have several internal torsional degrees of freedom. A model is developed for the description of cis–trans photoisomerization in solutions of polyenes. Using as an example potential energy surfaces calculated for butadiene, we assess the effects introduced by dynamical coupling between torsional degrees of freedom. We find that because of the excited state geometry, an electronic excitation will result in no more than one cis–trans isomerization. In addition, the distribution of electronic energy in conjugated polyenes is expected to be quite different from that predicted for molecules with one or several uncoupled torsions. In contrast to the latter cases, the torsional modes of conjugated polyenes are found to be poor acceptors of electronic energy. This energy is thus made available to other vibrational modes, providing a possible mechanism for hydrogen transfer reactions.