The cis—trans isomerization of nitrostilbenes XV: mixed singlet and triplet mechanism for trans → cis photoisomerization of 4-nitro-4′-dialkylaminostilbenes in non-polar solvents

Abstract

The quantum yields of direct cis ⇌ trans photoisomerization (φc → t and φt → c) and of fluorescence of the trans isomers (φf) of three 4-nitro-4′-R-stilbenes (R  amino (1), dimethylamino (2) and diethylamino (3)) were measured in several saturated hydrocarbons. Formation and decay of the lowest triplet state was observed by nanosecond laser flash photolysis. The triplet yield (φT), the triplet lifetime (τT), φt → c and φf were measured as a function of temperature and of the concentration of the quenchers ferrocene, azulene (Q) and oxygen. Twisting in the triplet, involving a 3t* ⇌ 3p* equilibrium, analogous to that in other 4-nitrostilbenes, is suggested on the basis of the effects of temperature and quenchers on φT and τT. The trans → cis photoisomerization of 1 follows the triplet route almost completely. The existence of a singlet pathway (20% – 30% contribution) for 2 and 3 in non-polar solvents at room temperature is concluded from the non-linear dependence of the φ0t → c/φt → c ratio on the concentration of Q. For these two nitrostilbenes a mixed singlet—triplet mechanism for the trans → cis photoisomerization is suggested.