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 π,π* 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 –10 kcal/mol energy barrier. The occurrence of decarboxylation from an upper state, likely the second triplet of n, π* 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 CO 2 fragment, long‐lived triplet biradicals and ground‐state intermediates with a protonated carbonyl oxygen are formed. Laser flash photolysis of DTPA leads almost quantitatively to the lowest π,π* 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.