posted on 2022-03-15, 15:36authored byTian-Yi Xu, Fei Tong, Hui Xu, Meng-Qi Wang, He Tian, Da-Hui Qu
Photomechanical molecular crystals
are promising candidates for
photoactuators and can potentially be implemented as smart materials
in various fields. Here, we synthesized a new molecular crystal, (E)-3-(naphthalen-1-yl)acrylaldehyde malononitrile ((E)-NAAM), that can undergo a solid-state [2
+ 2] photocycloaddition reaction under visible light (≥400
nm) illumination. (E)-NAAM microcrystals
containing symmetric twinned sealed cavities were prepared using a
surfactant-mediated crystal seeded growth method. When exposed to
light, the hollow microcrystals exhibited robust photomechanical motions,
including bending and dramatic directional expansion of up to 43.1%
elongation of the original crystal length before fragmentation due
to the photosalient effect. The sealed cavities inside the microcrystals
could store different aqueous dye solutions for approximately one
month and release the solutions instantly upon light irradiation.
A unique slow–fast–slow crystal elongation kinematic
process was observed, suggesting significant molecular rearrangements
during the illumination period, leading to an average anisotropic
crystal elongation of 37.0% (±3.8%). The significant molecular
structure and geometry changes accompanying the photocycloaddition
reaction, which propels photochemistry to nearly 100% completion,
also facilitate photomechanical crystal expansion. Our results provide
a possible way to rationally design molecular structures and engineer
crystal morphologies to promote more interesting photomechanical behaviors.