Mapping the potential energy surfaces of the 1,6-diphenyl-1,3,5-hexatrieneground and triplet states

The relative energies of the ground state isomers of 1,6-diphenyl-1,3,5-hex atriene (DPH) in benzene are determined from the temperature dependence of the equilibrium isomer composition obtained with the use of diphenyl disele nide as isomerization catalyst. In the triplet state, DPH exists as an equi librium mixture of all-trans (ttt), trans,cis,trans (tct), cis,trans,trans (ctt), and cis,cis,trans (cct) isomers. Under degassed conditions, photoiso merization of the triplets is primarily bimolecular, involving a quantum ch ain process. Oxygen eliminates the quantum chain process by efficient deact ivation of DPH triplets thereby revealing the triplet state isomeric compos ition. The temperature dependencies of the fluorenone-sensitized photoisome rization quantum yields and photostationary states for DPH in air-saturated benzene provide two independent measures of the temperature dependence of the equilibrium contribution of the isomeric triplets. They reveal the rela tive energies of the DPH triplet isomers. Together with the known 34 kcal/m ol triplet energy of ttt-DPH, these results define points on the potential energy surfaces of the ground and triplet states corresponding to the equil ibrium geometries of the four observed DPH isomers. At these geometries the two surfaces roughly parallel each other. Complete equilibration of isomer ic triplets within 100 ns requires that the energies of triplet biradical t ransition states be no higher than 40.3 kcal/mol. Estimated radical stabili zation energies give 40.2 and 41.6 kcal/mol for the energies of biradical t ransition states for central and terminal bond isomerization, respectively, in the ground state of ttt-DPH.