Four &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 ( r , r * ) 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.

The photophysical behavior of the P-carbolines norharmane, h m a n e , and harmine has been studied in waterin-oil (w/o) microemulsions in AOT/cyclohexane. With pure solvents, solubility and partitioning studies indicate that these molecules dissolve preferentially in the organic phase but that their solubility in the aqueous phase increases upon decreasing pH as a consequence of protonation. However, in w/o microemulsions these molecules appear to be located predominantly at the surfactant/water interface. Again, decreasing pH increases partitioning into the water region. From absorption and fluorescence spectral studies with the microemulsion system, differences are observed in the predominant species at any "pH' compared with that in pure water. From analysis of fluorescence decays, four kinetically distinguishable species have been observed in the excited state, namely, neutral species in cyclohexane, neutral species in the microemulsion droplets, cation, and zwitterion. For harmine, the effect of waterhfactant ratio (WO) on the photophysical behavior and distribution of these species was studied. Partitioning was observed between aqueous and organic subphases. A study has been carried out on the effect of pH on these processes. In agreement with other studies on "pH' in microemulsions, the cation is observed up to apparent pH values much higher than those for pure aqueous solutions. The results are interpreted in terms of the compartmentalization of the hydroxide ions between the water pools. Effects of changing wo on the decay of the various species are interpreted in terms of "quenching" of the neutral species in the cyclohexane subphase by its trapping within the microemulsion droplets. A kinetic model for this is presented. The effect of changing the polarity (by addition of methanol) upon the partitioning behavior has been studied, and it is shown that addition of methanol favors partitioning into the organic phase.

The -carbolines present a complex problem involving multiple equilibria in the excited state in hydrogenbonding solvents including water. Three excited state species exist: neutral, cation, and zwitterion. Here we examine the multiple equilibria and excited state kinetics of harmine, using time-resolved and steady state fluorescence techniques. From an analysis of the multiexponential decays, measured at the emission wavelengths of the three species as a function of the pH, seven unknowns (four rate constants and three reciprocal lifetimes) were determined. Data analysis was made both by a previously reported numerical method and by analytical solution of the differential equation set. The results obtained accurately describe the independently obtained steady-state fluorescence results. The dramatic modifications of the equilibria and rate constants between the ground and excited states can be understood on the basis of the significative changes in charge densities on the two nitrogen atoms of harmine upon excitation. Mechanisms are proposed for the formation of excited state cation and zwitterion beginning with the excited state neutral molecule.

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