Twofold adiabatic cis—trans isomerization of Z,Z-1,6-bisstyrylcycloheptatriene in the triplet state

The three E/Z isomers of 1,6-bisstyrylcycloheptatriene, ZZ, EZ and EE, have been prepared and studied in the triplet state. The triplet species produced on sensitization of each of the three isomers have identical lifetimes (τT = 1.0 μs) and T1−Tn absorption spectra. Furthermore, EZ and ZZ undergo efficient one- and two-fold adiabatic isomerizations (φEZ → EE = 0.82, φZZ → EZ + EE = 0.83) respectively, to yield predominantly EE, the dominating isomer in the photostationary state mixture (97% EE, 3% EZ). Thus, sensitized excitation of any of the isomers produce an equilibrated triplet state dominated by a species with a geometry similar to that of EE. Decay from the triplet state gives predominantly EE.