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Dynamic Study of Excited State Hydrogen‐bonded Complexes of Harmane in Cyclohexane–Toluene Mixtures ¶
Author(s) -
Carmona Carmen,
Balón Manuel,
Galán Manuel,
Guardado Pilar,
Muñoz María A.
Publication year - 2002
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2002)0760239dsoesh2.0.co2
Subject(s) - cyclohexane , toluene , excited state , chemistry , photochemistry , hydrogen , organic chemistry , physics , atomic physics
Photoinduced proton transfer reactions of harmane or 1‐methyl‐9 H ‐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 cationlike 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*, λ em ∼ 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*, λ em ∼ 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‐9 H ‐pyrido[3,4‐ b ]indole (MHN) under the same experimental conditions has helped to understand the excited state kinetics of these processes.