Photodecarboxylation of phenylglyoxylic acid: influence of para-substituents on the triplet state properties

The photochemistry of several para-substituted derivatives of phenylglyoxyl ic acid (R-PA, R = OCH3, CH3, F, Cl, Br and CN) was studied in polar solven ts. The triplet state was detected by laser flash photolysis and phosphores cence in acetonitrile, acetone or acetic acid at room temperature and by ph osphorescence in polar glasses at -196 degrees C. In fluid media the phosph orescence intensity and the triplet lifetime are both reduced on addition o f propan-2-ol or water. The rate constant for H-atom abstraction by the tri plet of the R-PA from propan-2-ol, 2 x 10(4)-8 x 10(7) dm(3) mol(-1) s(-1) in acetonitrile, increases with the electron-accepting power of the substit uent. The intermediate is the corresponding alpha-carboxy-alpha-hydroxybenz yl radical. The main transient in neat aqueous solution is the triplet stat e of the corresponding benzaldehyde as secondary intermediate. Based on res ults of triplet quenching by water in mixtures with acetonitrile and of tim e-resolved conductivity measurements, heterolytic alpha-splitting of the tr iplet anion involving the benzoyl anion is suggested. Photodecarboxylation occurs in substantial yield for R = CH3, F, Cl, Br and CN in acetonitrile a nd Phi(-CO2) is enhanced in the presence of water, Phi(-CO2) = 0.4-0.8 at 5 -20% H2O. The values are larger than in neat aqueous solution, where a decr easing dependence vs. pH was generally found. A reaction scheme is presente d accounting for the observed intermediates and the dependence of Phi(-CO2) on the water content and the pH.