Liquid-Crystalline\nPhotonic Sandwich: Electroresponsive\nColloids of Clay Nanosheets Loading Photofunctional Dyes

Colloidal clay nanosheets obtained by the delamination\nof layered\ncrystals of smectite-type clay minerals in water form liquid crystals\nbecause of their shape anisotropy. Loading of organic dyes onto the\nliquid crystalline clay nanosheets will enable novel photonic materials,\nwhere photofunctions of the loaded dye are controlled by the liquid\ncrystallinity of the clay nanosheets. However, adsorption of organic\ndyes onto the nanosheets renders the nanosheet surfaces hydrophobic,\nand consequently, colloidal stability of the nanosheets is lost. In\nthis study, this drawback is overcome by sandwiching cationic stilbazolium\ndyes between a pair of synthetic fluorohectorite nanosheets. This\nis realized by the preparation of stilbazolium–clay second-stage\nintercalation compounds characterized by intercalation of dye cations\ninto every other interlayer space of the hectorite clay, where nonintercalated\ninterlayer spaces are occupied by Na<sup>+</sup> ions. The second-stage\nintercalation compounds are obtained by partial ion exchange of mother\nclay mineral incorporating Na<sup>+</sup> ions in all of the interlayer\nspaces and delaminated from the Na<sup>+</sup>-containing interlayer\nspaces to form clay nanosheets sandwiching the dye molecules. Aqueous\ncolloids of the dye-sandwiching clay nanosheets form colloidal liquid\ncrystals, and the dye-sandwiching liquid crystalline clay nanosheets\nrespond to an applied AC electric field to be aligned parallel to\nthe electric field. The assembled structure of the dye-sandwiching\nclay nanosheets under the electric field is characterized by aligned\ndiscrete clay platelets, which is somewhat different from that of\na colloidal liquid crystal of clay nanosheets without dye loading\ncharacterized by macroscopic liquid crystalline domains up to submillimeters.\nThe electric alignment of the clay nanosheets induces alteration of\nlight absorption of the sandwiched stilbazolium molecules, which verifies\na strategy of constructing stimuli-responsive photonic materials of\nclay–organic hybrids.