Excited-state prototropic reactions of tetrahydroharmane

Abstract

Steady-state and time-resolved fluorescence measurements of THHN (9H-1-methyl-1,2,3,4-tetrahydropyrido[3,4-b]indole), have been carried out as a function of the acidity or basicity of the medium. In these media, four different THHN species have been detected, namely: dication (DC), cation (C), neutral (N) and anion (A). Attempts have been made to determine the excited state pK^*_a values for the three prototropic processes: N–A, C–N and DC–C reactions. The dicationic and anionic forms, observed outside the 0–14 pH range, are extremely weakly fluorescent species and only the second-order rate constants for the protonation and deprotonation processes, respectively, could be determined. In the case of the C–N process, studied in near-neutral media, excited-state proton exchange is much slower than fluorescence emission and the inflection points in the fluorimetric titration curves correspond quite closely to the ground state pK^G_a.