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 dete cted 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 t riplet of n, pi character, is confirmed by the study of the transient s toward the final keto photoproduct, i,e, the benzoylthiophene ethyl derivative (DTPA). At neutral pH, upon adiabatic release of the CO2 fr agment, long-lived triplet biradicals and groundstate intermediates wi th a protonated carbonyl oxygen are formed. Laser flash photolysis of DTPA leads almost quantitatively to the lowest pi,pi triplet, with si milar T-T absorption features as those of TPA. However the DTPA triple t appears essentially unreactive in aqueous medium. In isopropanol H-a bstraction from the solvent is demonstrated by the formation of the ke tyl radical.