Abstract
Dynamic molecular crystals showing light-triggered macro-movements have attracted great attention due to their unique ability for light–force conversion. These molecular crystals are driven remotely without any intermediary devices like wires and motors, which can transform light energy into mechanical work directly. However, the limited space restricts molecular rotation and motion in the crystalline state; thus, realizing macro-movements in molecular crystal systems is still a formidable challenge. In this review, we aim to focus on the underlying working mechanism of the photo-controllable macroscopic motion of molecular crystals with special focus on their practical applications. In detail, we discuss the basic principles and macroscopic photomechanical effects of these dynamic molecular crystals, including their deformation (i.e., bending, twisting, curling); complex motion (i.e., crawling, rotating, rolling); and disintegration (i.e., photosalient effect). Then, we introduce the most promising applications of photomechanical molecular crystals in the fields of all-optical devices, crystal actuators, and biomimetic artificial muscles. Therefore, this review will provide inspiration to develop state-of-the-art dynamic molecular crystals by bridging the disciplines of physics, chemistry, and engineering science.
Highlights
Considering the rapid advances in the field of photomechanical molecular crystals, we believe it is timely to summarize the basic principles, methods, and materials systems relevant to this emerging research field
Structural strain is characterized by a strain tensor, which is defined by the values of principal components and direction of orthogonally principal axes with respect to crystallographic axes, where principal components offer the values of linear strain in the direction of principal axes
In 2018, this group found that by changing the UV light direction on the crystals, photomechanical twisting modes can be tuned from helicoid to cylindrical helix, which indicates that the strain tensor produced by photochemical reactions on the crystal surface can be controlled by the excitation direction [Fig. 5(c)]
Summary
Photochemical reactions in crystals will induce the distortion of crystal lattice, which generates structural strain and mechanical stress. Due to their anisotropic property, photochemical reactions take place unevenly in crystals, resulting in different degrees of transformation and stress from the surface layers to the interior of crystals. When crystal structure expands under the influence of photochemical reactions, the transformed surface-product layer will undergo compressive stress because of the pulling force from the interior unchanged part. Shrinkage of crystal structure would cause tensile stresses in the irradiated surface-product layer. The sign of stress depends on the detailed distribution of product in the whole crystal as well as the induced structural strain by phototransformation.[24,68]
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