As intelligent materials responsive to light, photomechanical hydrogels not only possess high‐water content, excellent softness and biocompatibility, but also can accomplish various mechanical motions upon spatiotemporal stimulation of external light, which exhibit great potential in biomedical and underwater bionic fields. Molecular photoswitches have been used broadly in preparation of photomechanical hydrogels owing to their high photosensitivity and reversible molecular structure transformations induced by light. Herein, the current progress of photomechanical hydrogels based on typical molecular photoswitches such as spiropyran, azobenzene, and hexaarylbiimidazole (HABI) are introduced. Especially, as a promising building unit for photomechanical hydrogels, HABI has been highlighted due to the unique molecular structures and reversible photoswitching capability. HABI‐derived polymer hydrogels demonstrate flexible mechanical behaviors upon localized light irradiation. The characteristics and challenges of photomechanical hydrogels based on molecular photoswitches are also prospected.

Photochromic hexaarylbiimidazoles (HABIs) enable reversible transformation between HABI and triphenylimidazole radicals (TPIRs) upon UV light irradiation, which have been applied in photo-patterning, super-resolution imaging, photoinduced self-healing materials and photomechanical hydrogels. Here, we designed and prepared a novel kind of polyurethane (PU) photoresist based on photo-cleavable HABI cross-linkers (HABI-PU), linear PU backbone and additive tert-butylhydroquinone (TBHQ) radical quencher. Upon UV light irradiation, the C-N bond inside HABI unit is broken, causing the transformation of HABI into two TPIRs which are subsequently quenched by TBHQ. Therefore, the cross-linking points of HABI-PU in UV irradiation area were broken, forming flexible PU polymer chains with declined molecular weight, which can be dissolved in developer. HABI-PU films show excellent photosensitivity as a positive photoresist in photolithography. The reaction mechanism between HABI and TBHQ was studied by UV-vis absorption spectra, electron paramagnetic resonance (EPR), mass spectra and nuclear magnetic resonance spectra. The lithographic patterns were characterized through metallurgical microscope and scanning electron microscope with a resolution at about 5 µm.