Vertically aligned mesoporous silica films bearing covalently bound tris(2,2′-bipyridyl)ruthenium(II) redox photocatalysts

Abstract

This work reports a strategy to prepare mesoporous silica films with preferential vertical alignment of pore channels with covalently attached tris(2,2′-bipyridyl)ruthenium(II) redox photocatalysts, i.e. [Ru(bpy)₂(bpy’)]²⁺ (prepared from a precursor grafting agent containing 2,2′-bipyridine (bpy) and derivatized 4,4′-dimethyl-2,2′-bipyridine (bpy’) units). This was achieved by the electrochemically assisted self-assembly of vertically-aligned mesoporous silica films containing varying amounts of azide groups and their subsequent derivatization by click coupling with an alkyne-bearing [Ru(bpy)₂(bpy’)]²⁺ reagent. The effectiveness of the functionalization processes was monitored by infrared spectroscopy, indicating a best compromise reached for an azide content large enough to immobilize a high content of [Ru(bpy)₂(bpy’)]²⁺ moieties, but not too large to avoid steric constraints and leading to lower yields of the click reaction. The structural integrity (hexagonal order and mesopore orientation) after functionalization was checked by transmission electron microscopy and grazing-incidence small-angle X-ray scattering. The films were generated on transparent indium-tin oxide electrodes and cyclic voltammetry experiments demonstrated a good electrochemical response via electron hopping between adjacent [Ru(bpy)₂(bpy’)]²⁺ moieties immobilized in the nanochannels, which was dependent on the functionalization level and proved stable upon repetitive potential cycling as a result of their covalent attachment to the silica walls. These films also exhibit luminescence, which can be exploited for photoelectrochemical applications, e.g., detection by electrochemiluminescence or electrochemical regeneration of photocatalyst, as illustrated here from preliminary experiments.