Proton and electron transfer in the excited states quenching of phenosafranine by aliphatic amines.

MARTÍN F. BROGLIA, SONIA G. BERTOLOTTI AND CARLOS M. PREVITALI

PHOTOCHEMISTRY AND PHOTOBIOLOGY

Elsevier Science

Lugar: Amsterdam; Año: 2007 vol. 83 p. 535 - 541

0031-8655

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, kobs 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 Kexc could be extracted. The log Kexc shows a linear relationship with the pKb of the amine. On the other hand, for the triplet state quenching in acetonitrile kobs varies linearly with the amine concentration. Nevertheless, the quenching rate constants correlate satisfactorily with pKb 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 absorpti on 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.kobs 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 Kexc could be extracted. The log Kexc shows a linear relationship with the pKb of the amine. On the other hand, for the triplet state quenching in acetonitrile kobs varies linearly with the amine concentration. Nevertheless, the quenching rate constants correlate satisfactorily with pKb 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 absorpti on 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.