BETA-CARBOLINE PHOTOSENSITIZERS .1. PHOTOPHYSICS, KINETICS AND EXCITED-STATE EQUILIBRIA IN ORGANIC-SOLVENTS, AND THEORETICAL CALCULATIONS

Four beta-carbolines have been examined regarding absorption, emission, and excited-state equilibria in several classes of organic solvents. In hydrocarbon and nonprotic solvents, excitation results only in an excited-state neutral tautomer and fluorescence. In a neutral protic solvent (methanol), excitation of the neutral tautomer (except harmaline) results in multiple excited-state tautomers-neutral, cation, and zwitterion-and emission. The excited cation and zwitterion are formed from the neutral in and during the lifetime of the lowest (pi,pi*) singlet state. A kinetic model has been developed for the complex decays, and an analysis is presented for the neutral protic solvent case. Absorption, emission, and kinetic decay data for the cations are also presented. Only a single excited-state tautomer exists and emits independent of the solvent. Theoretical calculations are carried out regarding (1) state orders and (2) ground- and excited-state charge densities, the latter of which helps rationalize the existence of multiple excited-state tautomers. The impact of the results on photosensitizer aspects of the compounds is given.