4.7 Article

Quenching of a photosensitized dye through single-electron transfer from trivalent phosphorus compounds

Journal

JOURNAL OF ORGANIC CHEMISTRY
Volume 65, Issue 15, Pages 4715-4720

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/jo000448i

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Various types of trivalent phosphorus compounds 1 undergo single-electron transfer (SET) to the photoexcited state of rhodamine 6G (Rho(+*)) in aqueous acetonitrile to quench the fluorescence from Rho(+*). The rate constants k(p) for the overall SET process were determined by the Stern-Volmer method. The rate is nearly constant at a diffusion-controlled limit in the region of E-1/2(1) < 1.3 V (vs Ag/Ag+), whereas log k(p) depends linearly on E-1/2(1) in the region of E-1/2(1) > 1.3 V, the slope of the correlation line being -alpha F/RT with alpha = 0.2. The potential at which the change in dependence of log k(p) on E-1/2(1) occurs (1.3 V) is in accordance with the value of E-1/2(Rho(+*)) (1.22 V) that; has been obtained experimentally. Thus, the SET step is exothermic when E-1/2(1) < 1.3 V and endothermic when E-1/2(1) > 1.3 V. The alpha-value (0.2) obtained in the endothermic region shows that the SET step from 1 to Rho(+*) is irreversible in this region. Trivalent phosphorus radical cation 1(.+) generated in the SET step undergoes an ionic reaction with water in the solvent rapidly enough to make the SET step irreversible. In contrast, the SET from amines 2 and alkoxybenzenes 3 to Rho(+*) is reversible when the SET step is endothermic, meaning that the radical cations 2(.+) and 3(.+) generated in the SET step undergo rapid back SET in the ground state to regenerate 2 and 3.

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