Kinetic Study of Hydrogen Bonded Exciplex Formation of N₉-methyl Harmane

Abstract

The kinetics of exciplex formation of N9-methyl-1-methyl-9H-pyrido[3,4-b]indole, MHN, in the presence of the proton donor hexafluoro2-propanol, HFIP, in cyclohexane has been studied by UV-vis, steady-state, and time-resolved fluorescence measurements. The results conclusively show the formation of a 1:2 ground-state proton-transfer hydrogen bonded complex, PTC, between the pyridinic nitrogen of the substrate and the proton donor. The formation of these complexes is a necessary prerequisite for the exciplex to be observed. Thus, upon excitation of PTC, an excited-state equilibrium is established between PTC* and a cation like exciplex, CL*, λem. 410 nm. This excited-state reaction is assisted by a proton donor molecule. From the analysis of the multiexponential decays, measured at different emission wavelengths and as a function of HFIP concentration, the excited-state kinetics of this phototautomeric process has been analyzed in detail.