PHOTOISOMERIZATION AND PHOTOCYCLIZATION REACTIONS OF 1-STYRYLANTHRACENE

On triplet sensitization, 1-styrylanthracene (1SA) undergoes adiabatic cis-->trans one-way isomerization ((3)c(*) --> (3)t(*)) similarly to 2-anthrylethylenes. However, upon direct irradiation, cis-1SA in the singlet excited state mostly undergoes cyclization to a dihydrophenanthrene-type product (DHP), 4a,4b-dihydrobenzo[b]-chrysene, competing with an inefficient intersystem crossing to (3)c(*) followed by one-way isomerization. The produced DHP, in deaerated benzene, is reverted to cis-1SA by a thermal (E(a) = 14.9 kcal mol(-1)) or a photochemical pathway; however, under an oxygenated atmosphere DHP gives benzo[b]chrysene. A failure in the production of a cyclized product upon the excitation of trans-1SA shows that the isomerization really takes place in a one-way fashion.