MECHANISM AND SOLVENT DEPENDENCE FOR PHOTOIONIZATION OF PROMAZINE ANDCHLORPROMAZINE

The mechanism for photoionization of the phenothiazine derivatives, pr omazine and chlorpromazine, has been studied as a function of solvent and excitation conditions. Sequential biphotonic absorption is respons ible for photoionization induced by pulsed laser irradiation at 308 or 355 nm. In order to determine the excited states involved excitation was carried out under conditions where the second photon was exclusive ly absorbed by the lowest excited singlet state (using picosecond exci tation pulses where the pulse duration is much less than singlet lifet ime) or by the lowest triplet state using a two-color, two-pulse excit ation protocol. Photoionization occurs upon two-photon excitation with in the singlet manifold in aqueous solutions and methanol. In addition , photoionization is solvent dependent upon excitation of the triplet state at 355 nm or near its absorption maximum of 460 nm. In water the triplet absorption was bleached concomitant with the generation of ab sorption from the radical cation and hydrated electron. Quantum yields of photoionization of promazine were 0.02 and 0.03 in acetonitrile an d water, respectively. In methanol or other alcohols, photoionization was not observed under the same excitation conditions. Triplet lifetim es, molar absorption coefficients, and quantum yields of intersystem c rossing were determined in various solvents. The experimental results indicate that during single wavelength pulsed irradiation of these com pounds, the second photon is absorbed predominantly by the lowest exci ted singlet state rather than the lowest excited triplet state. The en ergetics of the photoionization of promazine and chlorpromazine were e stimated as a function of solvent.