Transient species in the photochemistry of tiaprofenic acid and its decarboxylated photoproduct

Photoexcitation of tiaprofenic acid (TPA) in aqueous medium leads with almost unitary efficiency to the lowest pi,pi* triplet, which is detected by transient absorption. The deactivation of this state occurs in the microsecond time domain and is dominated by a thermally activated spin-allowed process with similar to 10 kcal/mol energy barrier. The occurrence of decarboxylation from an upper state, likely the second triplet of n, pi* character, is confirmed by the study of the transients toward the final keto photoproduct, i,e, the benzoylthiophene ethyl derivative (DTPA). At neutral pH, upon adiabatic release of the CO2 fragment, long-lived triplet biradicals and groundstate intermediates with a protonated carbonyl oxygen are formed. Laser flash photolysis of DTPA leads almost quantitatively to the lowest pi,pi* triplet, with similar T-T absorption features as those of TPA. However the DTPA triplet appears essentially unreactive in aqueous medium. In isopropanol H-abstraction from the solvent is demonstrated by the formation of the ketyl radical.