Photochromism of 1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene in a Single-Crystalline Phase. Conrotatory Thermal Cycloreversion of the Closed-Ring Isomer

1,2-Bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) was found to undergo a thermally
reversible photochromic reaction in solution as well as in the single-crystalline phase. Upon irradiation with
ultraviolet light the hexane or toluene solution containing 1a and single-crystal 1a turned blue along with the
formation of closed-ring isomer (1b). The blue solution and single crystal returned colorless by irradiation
with visible light (λ > 500 nm) or heating above 100 °C. The bleaching is due to the cycloreversion of 1b to
1a. The thermal cycloreversion activation energies were measured in the toluene solution and in the single-crystalline phase above 100 °C. The activation energy from photogenerated 1b to 1a in the solution was 128
kJ mol-1, while it decreased to 120 kJ mol-1 in the crystal of the open-ring isomer 1a. In the crystal of the
closed-ring isomer 1b the energy increased to 137 kJ mol-1. The cycloreversion reaction was found to be
controlled by the environmental conditions. The thermal cycloreversion reaction in the crystal was directly
followed by X-ray crystallography and revealed to proceed in a conrotatory mode, which is opposed to the
general Woodward−Hoffmann rules.