Dynamic study of excited state hydrogen-bonded complexes of harmane in cyclohexane-toluene mixtures.
Photoinduced proton transfer reactions of harmane or 1-methyl-9H-pyrido[3,4-b]indole (HN) in the presence of the proton donor hexafluoroisopropanol (HFIP) in cyclohexane-toluene mixtures (CY-TL; 10% vol/vol of TL) have been studied. Three 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 hydrogen-bonded cation-like exciplex (CL*) with a stoichiometry of at least 1:3 and a zwitterionic exciplex (Z*). Time-resolved fluorescence measurements evidence that upon excitation of ground state PTC, an excited state equilibrium is established between PTC* and the cationlike exciplex, CL*, lambdaem approximately/= 390 nm. This excited state reaction is assisted by another proton donor molecule. Further reaction of CL* with an additional HFIP molecule produces the zwitterionic species, Z*, lambda(em) approximately/= 500 nm. From the analysis of the multiexponential decays, measured at different emission wavelengths and as a function of HFIP concentration, the mechanism of these excited state reactions has been established. Thus, three rate constants and three reciprocal lifetimes have been determined. The simultaneous study of 1,9-dimethyl-9H-pyrido[3,4-b]indole (MHN) under the same experimental conditions has helped to understand the excited state kinetics of these processes.
Carmona
Carmen
C
Departamento de Química Física, Facultad de Farmacia, Universidad de Sevilla, Spain. carmona@fafar.us.es
Balón
Manuel
M
Galán
Manuel
M
Guardado
Pilar
P
Muñoz
María A
MA
D003510:Cyclohexanes / Q000737:chemistry*; D006247:Harmine / Q000737:chemistry*; D006860:Hydrogen Bonding; D007700:Kinetics; D008956:Models, Chemical; D013050:Spectrometry, Fluorescence; D014050:Toluene / Q000737:chemistry*
D003510:Cyclohexanes; D014050:Toluene; D006247:Harmine