Sensitized triplet photochemistry of E- and Z-1,3,5-hexatriene

The sensitized cis-trans photoisomerization of 1,3,5-hexatriene in argon-saturated n-hexane solution at room temperature is investigated by continuous illumination and gas chromatographic analysis of product distributions. Initial quantum yields of isomerization exhibit a marked dependence on hexatriene concentration, especially for the Z → E isomerization where the yields reach values well above unity for high hexatriene concentrations. Correspondingly, the isomeric composition at the photostationary state is dependent on hexatriene concentration. Also, the isomeric composition at the photostationary state varies with azulene concentration. The concentration dependence is interpreted in terms of a quantum chain mechanism, involving the excited triplet state of hexatriene as the chain-carrying species. The photostationary state is found to be essentially independent of sensitizer triplet energy, in agreement with expectations based on the similar spectroscopic triplet energies of the two isomers. The shape of the torsional potential energy surface of isomerization via the triplet state is discussed on the basis of the results.