SOLVENT EFFECT ON THE REACTIVITY OF MONOSUBSTITUTED PHENOLS TOWARDS SINGLET MOLECULAR-OXYGEN (O2(1-DELTA-G)) IN ALKALINE MEDIA

The influence of solvent polarity on the dye-sensitized photooxidation (singlet molecular oxygen (O2(1DELTA(g))) mediated) of a series of pa ra-substituted phenolates was studied. Kinetic evidence obtained shows that the overall and the pure chemical interactions, phenolate-O2(1DE LTA(g)), depend on the solvent polarity in a different way. This is cl early reflected in the efficiency Of O2(1DELTA(g)) photooxidation of t he substrates: surprisingly, the photooxidation quantum yield increase s as the overall quenching rate constant decreases. The substrate phot ooxidation quantum yields obtained ranged from 0.05 to 0.15, the upper limit corresponding to a moderately polar medium (a benzene-methanol mixture) and the lower to an aqueous medium. We conclude that a high s olvent polarity favours only the obtainment of the encounter complex ( O2(1DELTA(g))-phenolate), whereas the reactive step is affected in muc h the same way as those processes where charges are neutralized along the reaction pathway. The results obtained are discussed in terms of a partly polar excited state complex between O2(1DELTA(g)) and the phen olates. The rate constant for the reactive pathway in both organic and aqueous media is correlated with the Hammet-type substituent constant R-.