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 (tau(T)=1.0 mu s) and T-1-T-x absorption spectra. Furthermore, EZ and ZZ undergo efficient one- and two-fold adiabatic isomerizations (phi(EZ-->EE)=0.82, phi(ZZ-->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.