The temperature- and viscosity-dependent photostationary E/Z ratio in triplet-sensitized photoisomerization of cyclo-octene

In the triplet-sensitized photoisomerization with benzene, toluene, or p-xylene, the photostationary E/Z ratio of cyclo-octene, (E/Z)pss, dramatically increases with lowered irradiation temperature (+ 75 to –123 °C) or with increased solvent viscosity, whereas the (E/Z)pss ratio of oct-2-ene stayed unchanged over the temperature range +25 to –78 °C. Theoretical examinations indicate that the temperature-dependent (E/Z) value is not a simple function of solvent viscosity but also depends upon the solvent structure. The Stern–Volmer studies further revealed that the variable quenching rate constants for Z and E isomers are responsible for the temperature- and viscosity-dependent photostationary state.