The effect of Si/Al ratio and moisture on an organic/inorganic hybrid material: Thioindigo/montmorillonite

Colored Organic/Inorganic Hybrid Materials (OIHM) with reversible properties were prepared by a solid-state reaction between several montmorillonites (MMT) with different Si/Al ratio (Bentolite L®, Texas, Wyoming and Kunipia deposits) and a neutral organic dye (thioindigo) at 413 K for nine hours. Spectroscopic and thermogravimetric analysis of the above mentioned interactions were evaluated. Results obtained by these techniques revealed an influence of Si/Al ratio in dehydrated MMT, where the intramolecular charge-transfer (IMCT) in thioindigo progressively red-shifted and increased in intensity in the following order: BentoT (Si/Al = 8.2) > TexasT (Si/Al = 7) > WyomingT (Si/Al = 5.6) > KunipiaT (Si/Al = 4.8). Moreover, a disturbance of thioindigo C=O at 1654 cm−1 to lower frequencies occurred due to C=O---Lewis acid sites (LAS) and Brønsted (B) interaction in MMT with high Si/Al ratio. In the presence of water, a smaller C=O shift due to C=O---(H2O)LAS or B interaction was identified. In addition, displacement of the basal spacing (001) in all MMT confirmed the effect of water on the reversible color changes displayed by UV–Vis diffuse reflectance spectroscopy. The existence of different binding strengths in OIHM was also evaluated by TGA and UV–Vis spectroscopy of their ethanol Soxhlet extractions.