BETA-CARBOLINES .2. RATE CONSTANTS OF PROTON-TRANSFER FROM MULTIEXPONENTIAL DECAYS IN THE LOWEST SINGLET EXCITED-STATE OF HARMINE IN WATER AS A FUNCTION OF PH

The beta-carbolines present a complex problem involving multiple equil ibria in the excited state in hydrogen-bonding solvents including wate r. 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 emi ssion wavelengths of the three species as a function of the pH, seven unknowns (four rate constants and three reciprocal lifetimes) were det ermined. Data analysis was made both by a previously reported numerica l method and by analytical solution of the differential equation set, The results obtained accurately describe the independently obtained st eady-state fluorescence results. The dramatic modifications of the equ ilibria and rate constants between the ground and excited states can b e understood on the basis of the significative changes in charge densi ties 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.