Trans → cis photoisomerization of 1-(1-naphthyl)-2-(4-nitrophenyl)ethylene

Abstract

The quantum yields of cis ⇄ trans photoisomerization of 1-(1-naphthyl)-2-(4-nitrophenyl)ethylene were measured as a function of solvent polarity, concentration of quenchers (ferrocene, azulene) and temperature. The quantum yield of fluorescence of the trans isomer is small in non-polar solvents and alcohols at room temperature but shows a significant dependence on the nature of the solvent. A transient, observed by laser flash photolysis in fluid and rigid media, is assigned to the trans configuration of the lowest triplet state ( 3t *). At room temperature 3t * is suggested to be in rapid equilibrium with the twisted configuration ( 3p *). The features of the triplet state are very similar to those of 4-nitrostilbenes whereas the relaxation properties of the excited trans singlet state ( 1t *) are different. Based on a comparison of the effects of quenchers and temperature on the triplet lifetime (and yield) with the effects on φ t→c, a triplet mechanism is proposed for the trans → cis photoisomerization at ambient temperatures. The cis → trans pathway partly involves and partly bypasses the triplet equilibrium.