EXCITED-STATE PROTOTROPIC REACTIONS OF TETRAHYDROHARMANE

Steady-state and time-resolved fluorescence measurements of THHN H-1-m ethyl-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 det ermine the excited state pK(a) values for the three prototropic proce sses: N-A, C-N and DC-C reactions. The dicationic and anionic forms, o bserved outside the 0-14 pH range, are extremely weakly fluorescent sp ecies and only the second-order rate constants for the protonation and deprotonation processes, respectively, could be determined. In the ca se of the C-N process, studied in near-neutral media, excited-state pr oton exchange is much slower than fluorescence emission and the inflec tion points in the fluorimetric titration curves correspond quite clos ely to the ground state pK(a)(G).