Hybrid mesoporous silica grafted with photoisomerizable 2-hydroxychalcones

• Photochromic mesoporous materials functionalized with flavylium were synthesized.
• The new hybrids show pH-dependent reflectance spectra.
• Photochromic behavior with thermal recovery was observed.

Hybrid photochromic mesoporous materials based on MCM-41 and SBA-15 were synthesized by covalent attachment of 3′-butoxy-7-hydroxyflavylium (Fl-OH) and 3′-butoxy-7-metoxyflavylium (Fl-OCH3) hydrogensulfates. The pristine materials were initially grafted with 3-chloropropyl groups, reacted with 3′-hydroxyacetophenone and finally condensed with appropriate salicylaldehydes to yield the new hybrids MCM-41-Fl-OH and SBA-15-Fl-OCH3. The materials were characterized by powder X-ray diffraction, N2 adsorption, solid-state 13C CPMAS NMR spectroscopy, and thermogravimetric and elemental analyses, which confirm the successful covalent bonding of the flavylium moieties with loadings of 16.90 ± 0.05% and 11.78 ± 0.04% (w/w) for MCM-41-Fl-OH and SBA-15-OCH3, respectively. Flavylium compounds originate in solution a multiequilibria reaction network than can be actuated by pH and light, defining pH-coupled photochromic systems. The new hybrids show pH-dependent reflectance spectra resembling those observed in solution, but shifted to higher pH ranges, indicating a higher stability of the grafted flavylium cations. Irradiation of these materials equilibrated at adequate pH values where the photoisomerizable trans-chalcones predominate shows formation of the respective flavylium cations that recover back to the initial compositions upon standing in the dark, leading these new organic–inorganic hybrids as pH-dependent photochromic materials.

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