Excited-state inter- and intramolecular proton transfer in methyl 3-hydroxy-2-quinoxalinate: effects of solvent and acid or base concentrations

Absorption, fluorescence excitation and fluorescence spectroscopy, combined with time-dependent spectroscopy and semi-empirical (AM1) and density functional theory using Gaussian 98 program calculations have been used to study the effects of solvent and acid or base concentration on the spectral cha...

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Bibliographic Details
Published in:Journal of luminescence 2005-08, Vol.114 (2), p.101-117
Main Author: Dogra, S.K.
Format: Article
Language:English
Subjects:
Absorption spectrum
Atomic and molecular physics
ESIPT
Exact sciences and technology
Fluorescence and phosphorescence
radiationless transitions, quenching (intersystem crossing, internal conversion)
Fluorescence spectrum
Methyl 3-hydroxy-2-quinoxalinate (M3HQ)
Molecular properties and interactions with photons
Physics
Prototropic equilibriums
Theoretical calculations
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Summary:Absorption, fluorescence excitation and fluorescence spectroscopy, combined with time-dependent spectroscopy and semi-empirical (AM1) and density functional theory using Gaussian 98 program calculations have been used to study the effects of solvent and acid or base concentration on the spectral characteristics of methyl 3-hydroxy-2-quinoxalinate (M3HQ). M3HQ is present as enol in less polar solvents and as keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to the phototautomer, formed by the excited-state intramolecular proton transfer, whereas fluorescence is only observed from keto in the polar solvents. In aqueous and polar solvents the monocation (MC5/MC6) is formed by protonating the carbonyl oxygen atom in the ground (S 0) and the first excited singlet states (S 1). Dication is formed by protonating one of N– atom of MC5/MC6. Monoanion is formed by deprotonating the phenolic proton of enol in the basic solution. p K a values for different prototropic equilibriums were determined in S 0 and S 1 states and discussed.
ISSN:0022-2313
1872-7883
DOI:10.1016/j.jlumin.2004.12.011