Photophysics and oxygen quenching of transition-metal complexes on fumed silica

Abstract

Pressed fumed silica has unique mechanical, chemical, and optical properties. It forms dense (1 g/mL), optically transparent disks, yet has a surface area of 200 m^2/g, which is the same as the unpressed material. The disks are highly porous and respond quickly to changes in gas composition. Before pressing, the silica can be impregnated by adsorption with a variety of materials. As a means of probing this surface and determining its potential utility, we have adsorbed luminescent RuL_3^(2+) ~(L = α-diimine) complexes and examined their luminescence and oxygen quenching. The adsorption sites are highly heterogeneous as shown by nonexponential decays in the presence or absence of an O_2 quencher. By use of a newly developed method, quenching is shown to be predominantly or exclusively by dynamic rather than static processes. Modeling of the quenching indicates that the results are best fit by surface quenching by adsorbed O_2 that obeys a Freundlich adsorption isotherm, although a two-site model with different quenching constants yields reasonable fits. The high density, transparency, porosity, and ease of preparation of homogeneously dispersed high levels of adsorbed species suggest that pressed fumed silica has potential as a support for luminescence sensors and photocatalysts.

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