Polymorph-Derived Diversification of Crystal Actuation by Photoisomerization and the Photothermal Effect

Abstract

Mechanically responsive materials have been investigated extensively over the past two decades. Diversification of actuation modes is essential for the practical application of mechanical materials. Polymorphic crystals with different crystal structures composed of the same compound can exhibit distinct mechanical motions. Here, we focused on two polymorphs of a salicylideneaniline derivative with a 4-fluoro substituent in enol form, 1α and 1β, and investigated their different photomechanical behaviors. Under ultraviolet (UV) light irradiation, the thin plate-like 1α crystal bent away gradually and strongly from the light source, with some twist caused by enol–keto photoisomerization. In contrast, the thin, needle-like 1β crystal did not bend by photoisomerization; however, the thick 1β crystal bent away quickly from the light source because of the photothermal effect, ultimately achieving 500 Hz high-speed bending under pulsed UV laser irradiation. Moreover, the thick plate-like 1α crystal exhibited two-step motion: fast bending forward by the photothermal effect and then slow bending away by photoisomerization. We succeeded in creating four motions using two polymorphic crystals and two distinct mechanisms, thereby providing a novel approach to diversify the mechanical motions of molecular crystals and expanding the potential and versatility of molecular crystals as actuation materials.

Crystal actuation switching by crystal thickness and light wavelength

“…, faster) than the time constants for photoisomerisation-induced bending (∼1 s), which is consistent with previous reports. 41,50 These results suggest that the bending and straightening time constants may be used as indicators to distinguish photoisomerisation-driven bending ( τ = ∼1 s) from photothermal-driven bending ( τ = ∼0.01 s). Overall, these findings demonstrate that crystal bending direction and speed can be readily switched by altering crystal thickness.…”

“…[45][46][47][48][49] Our research group created four motions from different crystal forms of the same molecule by combining two polymorphs (a and b) and two actuation mechanisms (photoisomerisation and the photothermal effect). 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.…”

“…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.…”

“…45–49 Our research group created four motions from different crystal forms of the same molecule by combining two polymorphs (α and β) and two actuation mechanisms (photoisomerisation and the photothermal effect). 50…”

Heterogeneous Photochemical Reaction on Optical Length Scale in Photomechanical Molecular Crystals

“…Under such a background, photomechanical molecular crystals have been attracting much attention for application to remote controllable microactuators that can directly convert light energy to mechanical energy without any physical contacts 7,8 . As important factors to determine the mode of photomechanical responses, molecular structures 9 , molecular packings 10 , illumination conditions 11 , crystal morphologies 12 , and crystal sizes 13 have been investigated so far because the actuation mode is determined by the strain tensor induced by photochemical reactions 14 .…”