Cycloreversion of Aryl-Substituted Cyclobutene by Direct Photolysis and Pulse Radiolysis

Article abstract of DOI:10.1246/bcsj.66.698

The cycloreversion of 1,5-diphenylbicyclo<3.2.0>hept-6-ene (1) to 1,4-diphenyl-1,3-cycloheptadiene (2) has been studied by direct photolysis and radiolysis.The γ-radiolysis of 1 in benzene gave 2 quantitatively up to a stationary state where the ratio of 1 to 2 is 0:100.However, no isomerization of 2 to 1 occurred.The triplet state of 2 (λ_max=380-385 nm; τ=2.7-2.8 μs) was formed by pulse radiolysis of 1 in benzene.It has been found that cycloreversion proceeds by an adiabatic mechanism via the triplet state.On the other hand, the quantum yield for the formation of 2 by the irradiation of 254 nm light to 1 was determined to be 0.64 in hexane.The fluorescence spectra of 1 and 2 also indicated that diabatic cycloreversion occurred.The singlet and triplet energy values of 1, as well as those of 2, were estimated to be 102, 78, 75, and 40 kcal mol^-1, respectively.The energy difference between the ground states of 1 and 2 was determined to be 20 kcal mol^-1 from DSC measurements.