FLAVIN TRIPLET QUENCHING AND SEMIQUINONE FORMATION BY ALIPHATIC α‐SUBSTITUTED ACETIC ACIDS: INTERMEDIATES IN FLAVIN‐SENSITIZED PHOTODECARBOXYLATION*

Abstract—The quenching of flavin excited states and the production of radical intermediates by aliphatic α‐substituted acetic acids have been investigated using fluorescence and laser photolysis measurements. Stern‐Volmer quenching constants for the flavin triplet are 2–4 orders of magnitude smaller than those found previously for aromatic acids. At the high substrate concentrations required to obtain triplet quenching, singlet quenching, probably via exciplex formation, is also observed. When triplet and radical yields are quantitated, it is found that, for pyruvate, glyoxylate and lactate, quenching occurs virtually exclusively by a one‐electron transfer mechanism. This supports the contention that the flavin‐sensitized photodecarboxylation of these acids proceeds via a radical mechanism.