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

Abstract

The photochemistry of several para-substituted derivatives of phenylglyoxylic acid (R–PA, R = OCH₃, CH₃, F, Cl, Br and CN) was studied in polar solvents. The triplet state was detected by laser flash photolysis and phosphorescence in acetonitrile, acetone or acetic acid at room temperature and by phosphorescence in polar glasses at –196 °C. In fluid media the phosphorescence intensity and the triplet lifetime are both reduced on addition of propan-2-ol or water. The rate constant for H-atom abstraction by the triplet of the R–PA from propan-2-ol, 2 × 10⁴–8 × 10⁷ dm³ mol^–1 s^–1 in acetonitrile, increases with the electron-accepting power of the substituent. The intermediate is the corresponding α-carboxy-α-hydroxybenzyl radical. The main transient in neat aqueous solution is the triplet state of the corresponding benzaldehyde as secondary intermediate. Based on results of triplet quenching by water in mixtures with acetonitrile and of time-resolved conductivity measurements, heterolytic α-splitting of the triplet anion involving the benzoyl anion is suggested. Photodecarboxylation occurs in substantial yield for R = CH₃, F, Cl, Br and CN in acetonitrile and Φ(–CO₂) is enhanced in the presence of water, Φ(–CO₂) = 0.4–0.8 at 5–20% H₂O. The values are larger than in neat aqueous solution, where a decreasing dependence vs. pH was generally found. A reaction scheme is presented accounting for the observed intermediates and the dependence of Φ(–CO₂) on the water content and the pH.