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Light- and humidity-responsive chiral nematic photonic crystal (PC) films containing cellulose nanocrystals (CNCs) were fabricated. A photoactive polymer with hydrophilic groups, poly-(3,3′-benzophenone-4,4′-dicarboxylic acid dicarboxylate polyethylene glycol) ester, was coassembled with CNCs to form flexible iridescent films with a tunable chiral nematic order. In the coassembly process, the intermolecular hydrogen bonds of CNCs were weakened, which facilitated the fine regulation of the chiral PC nanostructure. The PC films displayed sensitive responses to both light and humidity. With increasing humidity from 30 to 100%, the chiral nematic helix pitch increased from 328 to 422 nm. The color of the PC films changed from blue to green, yellow, orange, and dark red with increasing relative humidity. Over 15 min of light irradiation, the absorption intensity of the films increased gradually. The light and humidity responses of the films were reversible. The films maintained their variable cholesteric liquid crystal texture and helical lamellar structure after light irradiation at different humidities. These PC films are expected to be useful in intelligent coatings and 3D printing.

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Chiral liquid crystal materials that are responsive to environmental stimuli are in demand. A chiral photonic crystal membrane based on cellulose nanocrystals (CNCs) was prepared by molecule assembly in the present work. A fluorescent molecule containing a cationic group, [N-(3-N-benzyl-N,N-dimethylpropyl ammonium chloride)-1,8-naphthalimide]­hydrazine, was assembled on the surface of the CNCs. The new chiral photonic crystal membrane possesses supersensitive multiresponses to small molecules, such as water and formaldehyde molecules. The appearance, liquid crystal texture, fluorescence, and color of the chiral membrane have sensitive changes induced by small molecules. By increasing RH from 30 to 100%, the reflectance peak of the membrane red-shifted from 498 to 736 nm. In particular, the iridescent texture and fingerprint structure of the membrane could change markedly under trace amounts of formaldehyde, and the chiral membrane can form an extremely sensitive off–on fluorescence switch. The relationship between the fluorescence intensity and the trace concentration of formaldehyde satisfied the linear equation with the association coefficient of 0.9997. The changes in fluorescence and color are visible to the naked eye, and the membrane can quantitatively recognize trace formaldehyde at a molecular level in a humid environment. The mechanism by which the fluorescence switch operates was investigated using density functional theory at the B3LYP/6-31G­(d) level. The membrane has potential for use in the fields of advanced functional materials and biomaterials.