CYCLOREVERSION OF 1-(1-NAPHTHYL)-5-PHENYLBICYCLO[3.2.0]HEPT-6-ENE TO 1-(1-NAPHTHYL)-4-PHENYL-1,3-CYCLOHEPTADIENE BY TRIPLET SENSITIZATION AND DIRECT IRRADIATION

The cycloreversion of 1-(1-naphthyl)-5-phenylbicyclo[3.2.0]hept-6-ene (Ib) to 1-(1-naphthyl)-4-phenyl-1,3-cycloheptadiene (2b) upon benzophe none (BP) sensitization and direct irradiation has been investigated. The triplet sensitization of Ib in benzene gave 2b quantitatively up t o a stationary state where the quantum yield was 0.96. The triplet sta te of 2b (lambda(max) = 455 nm, tau = 4.6 mu s) was formed by the puls e radiolysis of Ib in benzene. It has been found that cycloreversion p roceeds by an adiabatic mechanism via a triplet state. The quantum yie ld for the formation of 2b by the irradiation of Ib at 313 nm was dete rmined to be 0.17 in hexane. The fluorescence spectra of Ib and 2b ind icated that diabatic cycloreversion occurred via a singlet state. The singlet and triplet energies of Ib, as well as those of 2b, were estim ated to be 88, 60, 72, and 39 kcal mol(-1), respectively. The effect o f a substitution of the naphthalene chromophore on bicyclo[3.2.0]hept- 6-ene was estimated.