Photoprocesses of molecules with 2-nitrobenzyl protecting groups and caged organic acids

Article abstract of DOI:10.1111/j.1751-1097.2007.00215.x

The 308 nm photoinduced formation of the nitroso product and the intermediacy of the aci-nitro form(s) were studied for a series of 2-nitrobenzyl alkyl and aryl esters (1a-4e) and bis-(nitrophenyl)methyl acetates (5a-6b) by time-resolved UV-vis spectroscopy. A triplet state appears as major transient, when 2-nitrobenzyl derivatives 1 are substituted by 4,5-dimethoxy (2) and 4,5-methylenedioxy (3/4) groups. This triplet of charge transfer character is, however, not part of the route via the aci-nitro into the 2-nitroso form. The activation energy and preexponential factor of the longest lifetime component (τ_aci), i.e. the major part of the aci-nitro decay, were determined. The carboxylic acids as leaving groups have rather small effects on τ_aci. An additional nitrated phenyl ring in α-position (5) leads generally to shorter τ_aci value. Otherwise, the photogeneration of nitroso products is similar. The quantum yield (Φ_d) varies only moderately with structure, the yield of the aci-nitro form and Φ_d are correlated and little affected by solvent properties.