Photoproducts and triplet reactivity of 4′-nitro- and 2′,4′-dinitro-substituted 4-hydroxystilbenes

The properties of triplet-excited trans-4-hydroxy-4′-nitrostilbene were studied by photochemical means. This species produces singlet molecular oxygen with a quantum yield of ΦΔ = 0.4 in benzene and 0.05 in acetonitrile. Population of the triplet state was also detected by flash photolysis. The triplet yield is substantial in benzene or toluene and small in polar solvents. The major photoprocess is trans → cis isomerization, whose quantum yield is Φt-c = 0.45 in benzene and smaller in more polar media. The effects of solvent polarity on the triplet reactivity were revealed. For trans-4-hydroxy-2′,4′-dinitrostilbene, ΦΔ and Φt-c are much smaller. The photoreaction to the corresponding hydroxyphenylisatogen was examined and mechanistic aspects were discussed.