The function of SiO2 colloids in photoinduced redox reactions. Interfacial effects on the quenching, charge separation, and quantum yields

I. Willner, Jer Ming Yang, Colja Laane, John W. Otvos, Melvin Calvin*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

87 Scopus citations

Abstract

The function of negatively charged colloidal SiO2 particles in controlling photosensitized electron-transfer reactions by means of electrostatic interactions between the different components in the system has been studied under steady-state conditions of illumination and by flash photolysis. In particular, the photosensitized reduction of the zwitterionic electron acceptor, propyl viologen sulfonate, PVS0 (1), using tris(2,2′-bipyridinium)ruthenium(II), Ru(bpy)32+, as sensitizer has been investigated in the SiO2 colloid and compared to the homogeneous system. Fluorescence quenching studies indicate that the quenching reaction of Ru(bpy)32+ with the neutral PVS0 is not affected by electrostatic interactions exerted by the negatively charged SiO2 interface (kq = 1.5 × 109 M-1 s-1). In contrast, the quenching of Ru(bpy)32+ by the positively charged methylviologen, MV2+, is strongly enhanced by the SiO2 colloid (kq = 5 × 1010 M-1 s-1). This efficient quenching is attributed to electrostatic adsorption of the quencher on the negatively charged interface. Flash photolysis experiments indicate that the back-electron-transfer reaction between the intermediate photoproducts, Ru(bpy)33+ and PVS-•, is substantially retarded in the SiO2 colloid (kb = 5.7 × 107 M-1 s-1) as compared to the homogeneous system (kb = 7.9 × 109 M-1 s-1). This retardation is attributed to electrostatic repulsion of the reduced product, PVS-•, from the oxidized sensitizer adsorbed on the SiO2 particles. The back-electron-transfer process was examined at different temperatures and an activation energy of 4.2 kcal mol-1 for the recombination reaction was estimated. The importance of the surface potential of the SiO2 colloids in retarding back reactions was demonstrated by varying the ionic strength and the pH of the medium. The results confirm that the surface potential is the dominant factor in improving the quantum yield for PVS0 reduction.

Original languageEnglish
Pages (from-to)3277-3282
Number of pages6
JournalJournal of Physical Chemistry
Volume85
Issue number22
DOIs
StatePublished - 1981
Externally publishedYes

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