Research

Photomechanical crystals have been developed based on photochemical reactions. Photochemical reactions, however, are associated with several drawbacks, for example, slow bending speed (< 5 Hz, > 0.1 s). Here we developed photomechanical crystals driven by a different mechanism, the photothermal effect. Upon UV irradiation to enol-1 crystals, they bent in ~20 ms by the photothermal effect. Upon pulsed UV irradiation, 500-Hz high speed bending was achieved. We hypothesized that this photothermal bending is caused by the nonsteady temperature gradient ΔT along the thickness direction. To elucidate the bending mechanism of the photo-thermal effect, ΔT was calculated using the one-dimensional unsteady heat conduction equation, and the bending angle was simulated from this temperature gradient. As a result, the actual bending behaviour was reproduced with high accuracy, demonstrating that the photothermal bending is caused by ΔT in the thickness direction. (J. Am. Chem. Soc. 2021)

To improve the potential of photomechanical crystals as materials, diversification of mechanical motions is essential. Here, we focused on two polymorphs of enol-2 and investigated their photomechanical behavior. Upon UV irradiation of a thin α-crystal, it slowly bent away with twisting due to photoisomerization. A thin β-crystal, on the other hand, did not bend due to lack of photoisomerization. However, a thick β-crystal oscillated at 500 Hz due to the photothermal effect. A thick α-crystal showed two-step bending by the combination of photoisomerization and photothermal effect. Four different motions could be generated from the same molecular crystals (Chem. Mater. 2022).

Although various photomechanical organic crystals have been developed, crystals that are actuated by either UV, visible or NIR light are extremely rare. Here we have developed broadband light-responsive crystal oscillators driven by photothermally resonated natural oscillations. A black needle crystal of SG3 oscillated at 70 Hz in the first mode of natural vibration to broadband light from 375 to 488, 520, 638, 808 nm and also to white light. The second and third natural vibration modes were induced at higher frequencies of 530 and 1350 Hz respectively, resulting in snake-like movements. The vibration frequencies could be tuned with a pin like a guitar. The crystal could lift a weight 88 times its own weight up and down at 16 Hz for more than 10000 cycles. (Adv. Funct. Mater. 2024)