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 413K 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 1654cm^-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.