Photodecarboxylation Study of Carboxy-Substituted N-Alkylphthalimides in Aqueous Solution: Time Resolved UV−Vis Spectroscopy and Conductometry

Abstract

The photophysical and photochemical properties of N-phthaloyltranexamic acid (3b) and N-methylsulfanylmethylphthalimide (4e) as well as several sulfur-containing carboxylates, N-phthaloylmethionine (1), N-phthaloyl-S-carboxymethylcysteine (2c), phthalimidomethylsulfanyl acetic acid (4a), and its derivatives 4b-4d were studied in aqueous solution by time-resolved UV−vis spectroscopy and conductivity using laser pulses at 248 or 308 nm. From the changes in absorption spectra upon continuous irradiation at 290 nm, the quantum yields of substrate decomposition (Φd) were measured for various cases. The quantum yields of formation of the lowest triplet state (Φisc) are small, when Φd is large and vice versa. The quantum yields of decarboxylation, Φ(−CO2), were determined by conductometry. Φ(−CO2) is largest (0.3) for 3b in argon-saturated aqueous solution at pH 6−10 and decreases in the order 4a−c, 2c, 1, and 4d. The increase of Φd vs pH indicates that carboxylic acids exhibit low decarboxylation efficiencies, ...