Triplet State and Photodecarboxylation of Phenylglyoxylic Acid in the Presence of Water

Article abstract of DOI:10.1021/j100333a010

The photodecarboxylation of phenylglyoxylic acid (PA) was studied by quantum yield (Φ_d) and time-resolved conductivity measurements in polar solvents at room temperature. Φ_d is substantial (>/=0.3) in the presence of 3-30 M water in acetonitrile and at pH a = 1.1 in neat water.The triplet states of PA, of its ethyl ester, and of 4-carboxybenzaldehyde were observed by nanosecond laser flash photolysis and emission spectroscopy.The initial transient obtained from PA (e.g., in acetonitrile λ_max = 322 nm; lifetime >/= 3 μs) is assigned to the n,?^* triplet state, and a second transient (in 2-propanol; λ_max = 313 nm; t_1/2 > 100 μs) is assigned to the Ph-COH-COOH radical.Phosphorescence was observed in glassy matrices at -196 deg C and in acetonitrile, acetone, and acetic acid at 25 deg C.In the latter ("inert") solvents the phosphorescence intensity and the triplet lifetime (up to 20 μs) are reduced by addition of alcohols or water.The rate constant of H-atom abstraction by triplet PA from 2-propanol is 1.5 * 10⁶ M^-1 s^-1 in acetonitrile.Addition of water to PA in the "inert" solvents results in a non-Stern-Volmer behavior for triplet quenching.Excitation of PA in neat aqueous solution yields the triplet of benzaldehyde, the excited photoproduct, as main transient whereas triplet PA could not be detected.A mechanism accounting for the dependences of Φ_d on the pH and on the H2O concentration in mixtures with acetonitrile is proposed.Water governs the photochemistry of PA in three respects, via the acid-base equilibrium, as triplet quencher, and by reducing the quantum yield of intersystem crossing to the anion triplet.