New insights on the excited-state proton-transfer reactions of betacarbolines:: Cationic exciplex formation

The photoinduced proton-transfer reactions of two betacarbolines, 9-methyl-9H-pyrido[3,4-b]indole, MBC, and 1,9-dimethyl-9H-pyrido[3,4-b]indole, MHN, in the presence of the proton donor 1,1,1,3,3,3-hexafluoro2-propanol, HFIP, are compared. Although MBC is more reactive than MHN, the behavior of both substrates is similar which indicates that a general mechanism in the exciplex formation of these derivatives operates. Three reactive excited-state species have been identified: a 1:2 hydrogen-bonded proton-transfer complex, PTC, between the pyridinic nitrogen of the substrate and the proton donor, a cation-like exciplex, CL, formed by the interaction of excited PTC with a new HFIP molecule, and finally, excited-state cations, C, which can be formed by an excited-state reaction from the CL exciplexes and/or simply by direct excitation of ground-state cations. With the help of the steady-state and time-resolved fluorescence results, the mechanism of these excited-state reactions has been established and the kinetic parameters determined.