The photostability of silylated Methyl Red (MR-GPTMS) dyestuff at 360 nm and under Visible light was investigated in water solution, in comparison to that of pure Methyl Red (MR). Both the dyestuffs were found relatively stable under visible light, while substantial decomposition, with different reaction times, occurs under UV irradiation, also on dependence of the pH value. In order to elucidate the mechanism of the color fading, a detailed spectroscopic study of the chemical structure of both molecules (i.e. MR and MR-GPTMS) was performed by means of UV–Vis, IR and NMR techniques, together with the employment of semi-empirical calculations. In the case of the functionalized MR-GPTMS dyestuff, an interesting behavior was found at acidic pH in comparison to the free MR: the presence of the bound GPTMS produces a decrease of the dyestuff decomposition rate of one order of magnitude. As a matter of fact, the functionalized azo-dye retains its pH-sensing behavior, while it improved significantly the sol-gel functionalized MR-GPTMS dye photostability. The rates of decolorization were found to fit a pseudo-first-order kinetic model. The enhanced stability is due to the covalent bond between the carboxyl group of azo dye and the opened epoxy group of GPTMS. This functionalization increase the polarity of silylated Methyl Red in the region of the azo group and consequently the surrounding electron density making it less susceptible to cleavage by hydroxyl radicals attack.

The analysis of the photolyzed products using Fourier Transform Infrared Spectroscopy (FTIR) further confirms that the ultraviolet degradation of both dyestuffs proceeded by following the same reaction pathway.