Features such as color, brightness and fluorescence are extremely important in applications of pigments. Hybrid materials inspired by the ancient Maya Blue pigment are a promising alternative to improve the properties and applicability of natural and synthetic dyes. In this work, we report the preparation, photophysical properties, and stability of several fluorescent hybrid pigments based on flavylium cations (FL) adsorbed on palygorskite (PAL). Five flavylium cations were investigated, viz., the 3',4',7trimethoxyflavylium (FL1), 7-hydroxy-4'-methoxy-flavylium (FL2), 7-hydroxy-4methylflavylium (FL3), 5,7-dihydroxy-4-methylflavylium (FL4) and 7-methoxy-4methylflavylium (FL5) cations. Only FL1 and FL2, without a methyl substituent at the 4position that could hinder inclusion in palygorskite channels, adsorbed strongly on PAL, producing fluorescent hybrid pigments with attractive colors. The spectroscopic and fluorescence properties of the FL1/PAL and FL2/PAL hybrid pigments were characterized. The color of the adsorbed dyes was somewhat more resistant to changes in external pH, photochemical stability was maintained and the thermal lability was markedly improved in the FL/PAL hybrid pigments, pointing to flavylium cations as promising chromophores for the development of fluorescent hybrid pigments with attractive colors.
Adsorption of flavylium and pyranoflavylium cations on sepiolite clay produces highly fluorescent hybrid pigments with improved color and thermal stability.
The pyranoanthocyanins present in red wine display great potential as photosensitizers in bio-inspired Dye-Sensitized Solar Cells (DSSCs). Following a biomimetic approach, a series of amino-π-bridgepyranoanthocyanin derivatives were employed as dye sensitizers in DSSCs. The dimethylamine group was selected to take advantage of its electron-donor character and the possibility of 'dual-mode anchoring' (-OH vs. dimethylamino) to titanium dioxide. The increase in π-conjugation via insertion of C=C bonds affected molecule flexibility, electron-donor ability and the pH-dependent equilibria of the pyranoanthocyanin derivatives. The current vs. potential properties of photoanodes using these dyes pointed to essential features of the relationship between power conversion efficiency and dye structure. These included the influences of the dimethylamine group, of π-conjugation and of substitution in ring B on the adsorption of the dyes to TiO2 and on the overall performance of the DSSCs prepared from them with and without added acid. An overall efficiency of 2.55% was obtained for the best performing compound, 4-(dimethylamino)-cinnamyl-pyranocyanidin-3-O-glucoside (JO3), which consolidates the importance of this family of compounds as potential dye-sensitizers for DSSC applications.
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