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Proton and Electron Transfer in the Excited State Quenching of Phenosafranine by Aliphatic Amines †
Author(s) -
Broglia Martín F.,
Bertolotti Sonia G.,
Previtali Carlos M.
Publication year - 2007
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 - 10.1562/2006-07-31-ra-989
Subject(s) - chemistry , flash photolysis , quenching (fluorescence) , photochemistry , electron transfer , excited state , reaction rate constant , acetonitrile , ultrafast laser spectroscopy , redox , deprotonation , triplet state , singlet state , kinetics , inorganic chemistry , fluorescence , atomic physics , molecule , ion , laser , organic chemistry , physics , quantum mechanics , optics
The quenching of the excited singlet and triplet states of phenosafranine by aliphatic amines was investigated in acetonitrile and methanol. The rate constants for the quenching of the excited singlet state depend on the one‐electron redox potential of the amine suggesting a charge transfer process. However, for the triplet state, quenching dependence on the redox potential either is opposite to the expectation or there is not dependence at all. Moreover, in MeOH the first‐order rate constant for the decay of the triplet state, k obs presents a downward curvature as a function of the amine concentration. This behavior was interpreted in terms of the reversible formation of an intermediate excited complex, and from a kinetic analysis the equilibrium constant K exc could be extracted. The log K exc shows a linear relationship with the p K b of the amine. On the other hand, for the triplet state quenching in acetonitrile k obs varies linearly with the amine concentration. Nevertheless, the quenching rate constants correlate satisfactorily with p K b and not with the redox potential. The results were interpreted in terms of a proton transfer quenching, reversible in the case of MeOH and irreversible in MeCN. This was further confirmed by the transient absorption spectra obtained by laser flash photolysis. The transient absorption immediately after the triplet state quenching could be assigned to the unprotonated form of the dye. At later times the spectrum matches the semireduced form of the dye. The overall process corresponds to a one‐electron reduction of the dye mediated by the deprotonated triplet state.