PHOTOCHEMISTRY OF 1-NAPHTHYLGLYOXILIC AND 2-NAPHTHYLGLYOXYLIC ACID

The photophysical deactivation properties of excited 1- and 2-naphthyl glyoxylic acid (1- and 2-NGA) in polar solvents and their photodecarbo xylation were studied by steady-state and pulsed methods. The naphthyl glyoxylic acids exhibit fluorescence in low quantum yield (less than o r equal to 0.02) and, in glassy media, also phosphorescence in substan tial quantum yield (0.2-0.6). The tripler lifetime at -196 degrees C, measured by phosphorescence and laser flash photolysis, lies in the 0. 2-0.7 s range. In fluid solution the T-T absorption was observed (lamb da(max) approximate to 410-540 nm); the triplet lifetime at room tempe rature lies in the 2-10 mu s range and is reduced in the presence of o xygen. Emission from singlet molecular oxygen was observed for 1- and 2-NGA in acetonitrile with quantum yields of 0.7 and 0.6, respectively . Both triplet lifetime and yield are reduced on addition of water to acetonitrile solution. Photochemical conversion of 1- and 2-NGA into t he corresponding naphthaldehydes occurs in acetonitrile, water and mix tures thereof, but only in aqueous acetonitrile (1-25% water) is the p hotoreaction selective and the quantum yield substantial (Phi = 0.2-0. 4). A reaction scheme is presented which accounts for the observed dea ctivation processes of the excited molecules and the photodecarboxylat ion via an ionic mechanism in aqueous acetonitrile. (C) 1998 Elsevier Science S.A.