Proton and Electron Transfer in the Excited State Quenching of Phenosafranine by Aliphatic Amines†

Abstract

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 pKb 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 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.

Comparison of the Kinetics of Electron Transfer in the Diffusion Limit for the Singlet and Triplet Quenching of Eosin Y by Quinones

“…However, in the present case, at low quencher concentrations and in a low viscosity solvent, we did not find evidence of exciplex intermediacy in the triplet quenching. The participation of an excited state complex in the triplet quenching leads to a downward curvature in the plots of 1/τ vs [Q] . In our case, all kinetics plots were linear as shown in Fig.…”

Probing electron transfer dynamics of phenosafranine with iodide

“…4a represents the transient absorption (TA) spectrum of PSF in acetonitrile medium at 1 ms after the excitation under degassed condition. The observed TA spectrum is in accordance with previously reported TA spectrum of PSF [30][31][32][33]46]. Peaks at 730 nm and 820 nm correspond to triplet-triplet absorption of PSF.…”

“…The choice of phenosafranine (3,7-diamino-5-phenylphenazinium chloride, PSF), Scheme 2, is due to its strong absorption in the region of 500-550 nm with a high molar absorption coefficient (4.4 Â 10 4 M À 1 cm À 1 ). It has been extensively employed as a photosensitizer in electron transfer reactions in homogeneous media [29][30][31][32][33] and in semiconductor devices [34,35]. The use of PSFas a sensitizer in solar energy conversion was also discussed elsewhere [36][37][38][39][40].…”

New Developments in the Mechanism of Drug Action and Toxicity of Conjugated Imines and Iminiums, including Related Alkaloids

“…Fipronil (27), parasiticide; Flavopereirine (28), anticancer [25]. Oxythioquinox (38), fungicide; Perisoxal (39), analgesic; Phenosafranine (40), electron transfer agent [26]. …”