Femtosecond Laser Photolysis Studies on Temperature Dependence of Cyclization and Cycloreversion Reactions of a Photochromic Diarylethene Derivative

Temperature dependencies of cyclization and cycloreversion processes of a photochromic diarylethene derivative, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT), were investigated by steady-state spectroscopy and femtosecond laser photolysis methods. Steady-state measurements revealed that the cyclization reaction quantum yield and the fraction of the conformer with C-2v symmetry favorable for the cyclization (antiparallel, AP conformer) were independent of temperature in the range of 253-343 K. These results indicated that the cyclization reaction of the AP conformer in the open-ring isomer in the excited state had no apparent temperature dependence and suggested that the fate of the excited AP conformer in the open-ring isomer, such as cyclization or deactivation to the ground state, was determined at the conical intersection. On the other hand, the cycloreversion reaction was dependent on the temperature; the reaction quantum yield increased together with a decrease in the lifetime of the excited state of the closed-ring isomer with increasing temperature. On the basis of the adiabatic energy surface for the reaction profiles, it was deduced that the rapid deactivation into the ground state took place in the S-1 state in competition with the activated pathways leading to the conical intersection where the cycloreversion occurred.