Stabilization of flavylium dye by incorporation into Fe-containing mesoporous silicate

Flavylium dye, an analogue of natural anthocyanin, is an expectedly safe coloring material. However, their color instability limits its practical use. We have prepared Fe-containing HMS type mesoporous silicate, and have made a composite of the FeHMS and the flavylium dye. The color stability of the flavylium under visible light irradiation has been enhanced by making the composite with the FeHMS. During 8 h irradiation, 45% of the absorption of the flavylium adsorbed on HMS has been lost, while only 5% of the absorption has been lost on the FeHMS with 2 mol% Fe. From the UV–vis spectra, FeHMS has been confirmed to contain the Fe3+ species highly dispersed in the silica framework. Since the Fe3+ species works as an acid site in the mesopore of HMS, the enhancement of the stability has partially been attributed to the solid acidity of the FeHMS. Under strong light irradiation (>800 klx), the degradation rate of the flavylium has been independent of the light irradiance, because the diffusion rate of the oxygen into the mesopore cannot satisfy the demand to oxidize the flavylium dye under irradiation. This means that the inhibition of the contact with the external oxygen also contributes to the stabilization of the flavylium dye incorporated in the mesopore.

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► Anthocyanin-like flavylium dye is adsorbed on Fe-containing mesoporous silicas.
► Stabilization effect of the flavylium by the acid site of tetrahedral Fe3+ in the silica framework.
► Inhibition of the oxygen diffusion into the mesopore contributes to the stabilization of the dye.