Temperature effects on the photoreactivity and rotamerism of (Z)-1-styrylanthracene in non-polar and polar solvents

Abstract

The photochemical and photophysical behaviour of (Z)-1-styrylanthracene (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 singlet pathway (18 kJ mol ^-1 ). The cis→trans photoisomerization occurs 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 cyclization photoproduct was observed, formed from a different conformational isomer. The presence of conformational equilibria of cis-1-StAn in the ground and excited singlet states is also supported by the dependence of its spectral properties on temperature and by calculations. In polar solvents, the cis→trans photoisomerization, which occurs mainly through a diabatic singlet mechanism, is competitive with cyclization in the relaxation of the lowest excited singlet state.