Nanochannel Confinement Governs the Stabilization of Thioindigo–Sepiolite Maya Blue-Type Pigments

Thioindigo, a sulfur-substituted analogue of indigo, provides a useful molecular probe for evaluating the role of guest–host hydrogen bonding in Maya Blue-type hybrid pigments. In this work, thioindigo–sepiolite hybrid pigments were prepared by ball milling followed by thermal treatment from room temperature to 500 °C. Diffuse reflectance spectroscopy, colorimetry, powder X-ray diffraction, FT-IR spectroscopy, and N2 physisorption were combined to track the temperature-dependent evolution of the host–guest structure. The results show that heating induces progressive migration of thioindigo from external sites into the sepiolite nanochannels, accompanied by stepwise dehydration of the clay. Channel incorporation is supported by the decrease in accessible micropore volume, the retention of the sepiolite (110) reflection at high temperature through a pillaring effect, and the systematic evolution of thioindigo-related vibrational bands. Pigments prepared at 100–300 °C exhibit the best overall performance, combining high color saturation with strong resistance to concentrated HNO3, DMSO extraction, and 600 h of visible-light irradiation. First-principles molecular dynamics simulations based on a hydrated sepiolite channel model further show that thioindigo adopts a flat-lying configuration within the channel and remains mobile mainly along the channel axis. The distances between thioindigo carbonyl oxygen atoms and coordinated-water hydrogen atoms fluctuate substantially and do not support the formation of persistent hydrogen bonds on the simulated time scale. These results indicate that physical nanochannel confinement, assisted by nonspecific van der Waals interactions, is the dominant stabilization mechanism in the thioindigo–sepiolite system, thereby broadening the molecular design space for new Maya Blue-type hybrid pigments.