A laser flash photolysis study of fenofibric acid in aqueous buffered media: unexpected triplet state inversion in a derivative of 4-alkoxybenzophenone

Laser excitation of aqueous solutions of fenofibric acid (FA) at pH 7.4 show the formation of two reaction intermediates, the triplet state and the hydrated electron. The former is longer lived in water than in acetonitrile; its anionic form decays irreversibly by decarboxylation to give a carbanion that protonates before or after rearrangement. Several spectroscopic and quenching studies suggest that in aqueous media the triplet state of FA has a pi,pi* character, in comparison with an n,pi* character in organic media. Further, the known chemistry of the triplet, including decarboxylation and hydrogen abstraction, occurs predominantly from the n,pi* state, and as a consequence, activation energies are higher when the lowest triplet has a pi,pi* character. Photoionization is more important in aqueous than in organic media and involves a biphotonic process. Hydrated electrons are trapped by FA, leading to the corresponding ketyl radical after protonation.