Effects of 4,5-dimethoxy groups on the time-resolved photoconversion of 2-nitrobenzyl alcohols and 2-nitrobenzaldehyde into nitroso derivatives.

The photoinduced conversion of the aci-nitro in the nitroso form was studied with four compounds containing the o-nitrobenzyl moiety in solution at ambient temperature using time-resolved UV-vis spectroscopy. For 4,5-dimethoxy-2-nitrobenzyl alcohol (2) and 4,5-methylenedioxy-2-nitrobenzyl alcohol (3) the absorption spectra are red-shifted and, in contrast to the parent 2-nitrobenzyl alcohol (1), a triplet state with CT character was detected after the 308 nm laser pulse. The other photochemical properties of 1-3 are similar. The aci-nitro form of 1-3 in acetonitrile or ethanol is quenched by water, the rate constant is (0.3-1.7) x 10(5) M(-1) s(-1). A CT triplet state and the nitroso product but no aci-nitro form were observed for 4,5-methylenedioxy-2-nitrobenzaldehyde (4). The conversion of the aci-nitro into the nitroso monomer and eventual dimer formation were studied by FTIR spectroscopy. The common features and specific differences in the photoreaction mechanisms of 1-4 are discussed.