Photothermally Driven High-Speed Crystal Actuation and Its Simulation

Abstract

Mechanically responsive crystals have been increasingly explored, mainly based on photoisomerization. However, photoisomerization has some disadvantages for crystal actuation, such as a slow actuation speed, no actuation of thick crystals, and a narrow wavelength range. Here we report photothermally driven fast-bending actuation and simulation of a salicylideneaniline derivative crystal with an o-amino substituent in enol form. Under ultraviolet (UV) light irradiation, these thin (<20 μm) crystals bent but the thick (>40 μm) crystals did not due to photoisomerization; in contrast, thick crystals bent very quickly (in several milliseconds) due to the photothermal effect, even by visible light. Finally, 500 Hz high-frequency bending was achieved by pulsed UV laser irradiation. The generated photothermal energy was estimated based on the photodynamics using femtosecond transient absorption. Photothermal bending is caused by a nonsteady temperature gradient in the thickness direction due to the heat conduction of photothermal energy generated near the crystal surface. The temperature gradient was calculated based on the one-dimensional nonsteady heat conduction equation to simulate photothermally driven crystal bending successfully. Most crystals that absorb light have their own photothermal effects. It is expected that the creation and design of actuation of almost all crystals will be possible via the photothermal effect, which cannot be realized by photoisomerization, and the potential and versatility of crystals as actuation materials will expand in the near future.

Crystal actuation switching by crystal thickness and light wavelength

“…The threshold thickness (33 mm) for crystal actuation through the photothermal effect in this study was slightly higher than the threshold thickness (20 mm) reported in a previous study. 41 The thickness dependence of the bending and straightening time constants of enol-1a crystals is shown in Fig. 7b; the time constants for photothermally induced bending (B0.01 s) were two orders of magnitude smaller (i.e., faster) than the time constants for photoisomerisation-induced bending (B1 s), which is consistent with previous reports.…”

“…50 Actuation switching is another requirement for the practical application of photomechanical crystals. Among salicylideneaniline crystals, photoisomerisation actuates thin crystals, whereas the photothermal effect actuates thick crystals; 41,50 the photoisomerisation mechanism can be induced by ultraviolet (UV) light alone, whereas the photothermal effect mechanism can be induced by both UV and visible light. 41 Therefore, we hypothesised that crystal actuation could be switched by changing the crystal thickness or light wavelength.…”

“…Recently, we reported rapid bending of molecular crystals through the photothermal effect, in which thermal energy is produced by photoexcitation of a material; 40 we unveiled that the bending mechanism was driven by a non-steady temperature gradient in the thickness direction. 41 Compared with photoisomerisation, the photothermal effect has several advantages in crystal actuation, including higher speed, greater light wavelength range, and actuation of thicker crystals. 41 To increase the applicabilities of these photomechanical crystals in actuators and soft robotics, there is a need to diversify their mechanical motions.…”

“…41 Compared with photoisomerisation, the photothermal effect has several advantages in crystal actuation, including higher speed, greater light wavelength range, and actuation of thicker crystals. 41 To increase the applicabilities of these photomechanical crystals in actuators and soft robotics, there is a need to diversify their mechanical motions. One method for achieving this goal involves utilising polymorphism, in which one type of molecule forms multiple crystalline structures.…”

Analysis of molecular photomechanical performance using a one-dimensional harmonic model

“…Photomechanical materials have the property that their constituent nuclei change position after photon absorption, generating a force and displacement that can be harnessed to perform mechanical work [1][2][3]. This change in atomic coordinates could result from heating (photothermal) [4][5][6], a change in electronic state (photostrictive) [7], or a chemical reaction (photochemical) [8].…”