Sol-gel phase transitions in a photochromic spiropyran-modified material by incorporation in a hydrogel

Photochromic spiropyran-modified material 1 and hydrogelator 2 bearing sugar and D-alanine moieties were prepared and their thermal- and photo-responsive properties in aqueous solution were investigated by NMR, CD, PXRD, DSC, and UV-vis spectroscopy. The xerogels [1 + 2] (1:1-20 wt%) with various ratios obtained from water showed well-developed fibril structures with diameters of 50-100 nm and length of several hundred micrometers. Interestingly, after irradiation with UV light, these aggregates were destroyed because the closed form (SP) of spiropyran unit of 1 changes to the corresponding merocyanine form (MC). In contrast, further heating led to the original hydrogels [1 + 2], due to the transformation of the opened form (MC) of 1 into the corresponding spiropyran form (SP). Thus, the spiropyran unit of 1 played an important role in the gelation of water by interacting with the gelators in a host-guest interaction that could modify the superstructures and sensitively affect their stability.