TEMPERATURE EFFECTS ON THE PHOTOREACTIVITY AND ROTAMERISM OF (Z)-1-STYRYLANTHRACENE IN NONPOLAR AND POLAR-SOLVENTS

The photochemical and photophysical behaviour of (Z)-1-styrylanthracen e (cis-1-StAn) has been investigated as a function of temperature and solvent polarity. In non-polar solvents above 260 K, cis-1-StAn mostly undergoes photocyclization to benzo[b]chrysene through an activated s inglet pathway (18 kJ mol(-1)). The cis --> trans photoisomerization o ccurs with very low quantum yield through an adiabatic mixed (singlet and triplet) mechanism above room temperature or through an adiabatic triplet mechanism below 260 K. Above room temperature, a second cycliz ation photoproduct was observed, formed from a different conformationa l isomer. The presence of conformational equilibria of cis-1-StAn in t he ground and excited singlet states is also supported by the dependen ce of its spectral properties on temperature and by calculations. In p olar solvents, the cis- --> trans photoisomerization, which occurs mai nly through a diabatic singlet mechanism, is competitive with cyclizat ion in the relaxation of the lowest excited singlet state.