This paper is part of a symposium-in-print dedicated to Professor Eduardo. A. Lissi on the occasion of his 70th birthday.
Proton and Electron Transfer in the Excited State Quenching of Phenosafranine by Aliphatic Amines†
Article first published online: 22 MAY 2007
Photochemistry and Photobiology
Volume 83, Issue 3, pages 535–541, May/June 2007
How to Cite
Broglia, M. F., Bertolotti, S. G. and Previtali, C. M. (2007), Proton and Electron Transfer in the Excited State Quenching of Phenosafranine by Aliphatic Amines. Photochemistry and Photobiology, 83: 535–541. doi: 10.1562/2006-07-31-RA-989
- Issue published online: 22 MAY 2007
- Article first published online: 22 MAY 2007
- Received 31 July 2006; accepted 1 November 2006; published online 9 November 2006
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 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.