Ring-closing photoisomerization of some 2,6-diarylstyrenes

The structure, spectroscopy and photochemical reactions of three symmetrical 2,6-diarylstyrenes (aryl = phenyl, 2-furyl and 2-thiophenyl) have been investigated. The ground-state structures are highly nonplanar, having large aryl-phenyl and vinyl-phenyl dihedral angles. All three diarylstyrenes have broad UV absorption bands attributed to allowed, delocalized π,π* (highest occupied molecular orbit-lowest unoccupied molecular orbit) transitions. The 2-furylstyrene is weakly fluorescent, with a large Stokes shift attributed to a change in geometry from the nonplanar ground state to a more planar singlet state. Irradiation in fluid solution results in efficient conversion of the diarylstyrenes to cyclized 9,10-dihydrophenanthrene and 4,5-dihydronaphthofuran or thiophene products, thus extending the scope of the 2-vinylbiphenyl photocyclization reaction to heterocyclic analogs. Irradiation at low temperatures in glassy media permits observation of the UV absorption spectra of the unstable primary photoproducts. Upon warming of the glass, these intermediates undergo rapid hydrogen migration to form the stable dihydroarene products.