Visible-Light Driven Dynamic Crystal Actuators: From [2+2] Photodimerization-Induced Fracturing to Elastic Cyclobutane Solvates

Light-responsive organic molecular crystal materials
can efficiently
convert light energy to mechanical energy. Currently, most light-responsive
organic crystals respond only to ultraviolet (UV) irradiation. The
inherent biological hazards and environmental risks with high-energy
UV light necessitate the advancement of visible-light-responsive organic
crystal materials. We synthesized BTPA, a novel pyridine-based propenenitrile
derivative engineered with a donor−π–acceptor
(D−π–A) structure. This compound demonstrates
broadband optical absorption spanning the UV and visible regions of
the spectrum. Upon illumination, the BTPA crystals undergo photocycloaddition
reactions concomitant with photomechanical effects, specifically manifested
as directional cracking along the (043)/(04¯3¯) crystallographic planes. Recrystallization
of the photocycloaddition product produced needle-like crystals, identified
by SCXRD as a cyclohexane solvate. Notably, these solvate crystals
exhibit reversible elastic deformation on (100)/(1¯00) facets. Finally, by leveraging the oriented
photomechanical cracking of BTPA crystals, we developed a visible-light-powered
actuator for remote steel ball manipulation. The system demonstrates
remarkable performance (force-to-weight ratio ∼300, force density
= 3.42 × 105 N/m3, power density = 32.1
J/m3), rivaling commercial voice coil actuators. This work
showcases the potential of organic molecular crystals as advanced,
light-responsive smart materials.