New Insights on the Excited-State Proton-Transfer Reactions of Betacarbolines:  Cationic Exciplex Formation

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2004-03, Vol.108 (11), p.1910-1918
• Main Authors: Carmona, Carmen, Balón, Manuel, Coronilla, Antonio Sánchez, Muñoz, María A
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp030829a

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-hexafluoro-2-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.