Hybrid Nanoscale Organic Molecular Crystals Assembly as a Photon‐Controlled Actuator

Abstract

Molecule deformation to macroactuation: In a novel hybrid assembly photoisomerization causes microscale deformation of molecules that is amplified to macroscale bending of a composite membrane. The nanoscale molecular crystals, which are unevenly distributed in a functional polymer matrix, provide a new strategy for designing higher performance actuators that combine the advantages of both molecular crystals and liquid crystal elastomers. Micro- and nanoscale machines and robots with simple structures that can directly transform environmental energy into mechanical work are in high demand. Traditional electromotors usually have the inductor, controller, generator, motor, and transmission integrated into one system. Until now, a variety of energy forms, including electricity, heat, light, and ion transfer, have been widely applied to drive actuators.1–4 Among these energy forms, light is a multiparameter-adjustable stimulus, that is, the monochromaticity, wavelength, intensity, polarization, and the position of illumination can be precisely tuned to manipulate the actuator.5–7 In addition, unlike for other stimuli, light parameters can be controlled remotely without the need for direct contact with the actuator, thus making it possible to simplify the device by getting rid of complex components, reducing the size and weight, and minimizing the structure in a freestanding configuration. Photoisomers have been used in several types of photoresponsive actuators, for example, azobenzene in liquid crystal elastomers (LCEs), and diarylethene and anthracene derivatives in microscale molecular crystals. In LCEs, the driving force is the phase inversion caused by photoisomerization of azobenzene mesogens.8 LCE actuators have high frequency oscillation, wavelength selectivity, and polarization selectivity and significant properties derived from large strain and stress.7, 9, 10 The grafting of azobenzene moieties onto a polymer skeleton should change the intrinsic properties of a material, however, the phase inversion process usually takes a relatively long time to cause deformation of the whole LCE. In comparison with LCEs, molecular crystals can be actuated directly as a result of photoisomerization and subsequent lattice deformation of dye molecules, thus presenting much better repeatability, fatigue resistance, and faster response rates. Nevertheless, molecular crystal actuators are fragile and are highly restricted in performance by the crystal size and shape.11–18 Macroscale molecular crystals rarely present ideal deformation properties and it is still a challenge to use molecular crystals in macroscale actuators. Herein, we report the development of a hybrid nanoscale organic molecular crystal assembly as a photoresponsive actuator. In this novel system, rod-like nanocrystals of 2-hydroxynaphthylidene-1′-naphthylamine (HNAN) are selectively oriented and unevenly distributed in a compact polyvinylidene fluoride–hexafluoropropylene (PVDF–HFP) copolymer membrane. Compared with pure PVDF–HFP, which does not show any response to UV light, the composite membrane presents UV-induced bending, even with only 1 wt % of HNAN. Such an assembly approach would be a practicable way to combines the advantages of both LCEs and molecular crystals for actuator design. HNAN is synthesized by a previously reported method.19 This kind of Schiff base molecule will undergo isomerization after being exposed to UV light of the appropriate wavelength, and recover its initial state when the UV light is turned off. In our experiment, HNAN (2 mg) and PVDF–HFP (200 mg) are dissolved in DMF (2 mL) to form a homogeneous viscous solution. After being cast onto a 75 mm×25 mm glass slide, the mixture is heated at 80 °C until the DMF is completely evaporated, to leave a yellow, transparent composite membrane approximately 52 μm in thickness. An SEM image of a section of the membrane (Figure 1 a) shows that the hybrid structure mainly consists of rod-like nanoparticles