Mechanically Responsive Organic Crystals by Light

Abstract

Mechanically responsive molecular crystals that exhibit macroscopic motions such as bending, twisting, and locomotion by light and heat have been studied intensively over the past decade. Photoisomerization has been applied to induce various motions, especially the bending of typical photochromic crystals, e. g., diarylethene and azobenzene. Phase transition is another mechanism underlying crystal actuation. Moreover, photothermal effect is a promising mechanism that has the potential to actuate any crystals that absorb light, including those for which actuation cannot be achieved by photoisomerization or phase transitions. Molecular crystals have an advantage over polymers and gels in terms of having a higher elastic modulus and stronger output force. However, previous studies of mechanical crystals have been limited mostly to basic research. There is a need to address the practical application of such mechanically responsive crystals in sensors, switches, actuators, and soft robots.

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

“…Considering that the photothermal effect occurs in almost any crystal that absorbs light, there is the potential for rapid actuation of any crystal using light in the ultraviolet (UV), visible, and infrared (IR) regimes. Mechanically responsive crystals have been reviewed in several publications; − several mathematical models have been proposed to quantify mechanical motions. − Notably, the term “crystal adaptronics” has been proposed in reference to the emerging research field of adaptive (i.e., stimuli-responsive) crystals. ,, …”

Crystal actuation switching by crystal thickness and light wavelength

“…[1][2][3][4][5] Despite their hard, brittle appearance, molecular crystals exhibit macroscopic deformation upon irradiation with light. 1,[6][7][8][9][10][11] Compared with polymers, molecular crystals are more advantageous in light-driven actuators and soft robots because of their higher elastic modulus and stronger output force. 9 Thus far, various light-fuelled (i.e., photomechanical) crystals that exhibit bending, [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] twisting, 23,29,30 bending with twisting, 31,32 curling, 33 rotation, 34,35 peeling, 36,37 and jumping 38,39 have been developed; these characteristics are mainly based on the photoisomerisation (sometimes photodimerisation) of molecules.…”

“…1,[6][7][8][9][10][11] Compared with polymers, molecular crystals are more advantageous in light-driven actuators and soft robots because of their higher elastic modulus and stronger output force. 9 Thus far, various light-fuelled (i.e., photomechanical) crystals that exhibit bending, [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] twisting, 23,29,30 bending with twisting, 31,32 curling, 33 rotation, 34,35 peeling, 36,37 and jumping 38,39 have been developed; these characteristics are mainly based on the photoisomerisation (sometimes photodimerisation) of molecules. 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.…”

“…Despite their hard, brittle appearance, molecular crystals exhibit macroscopic deformation upon irradiation with light. 1,6–11 Compared with polymers, molecular crystals are more advantageous in light-driven actuators and soft robots because of their higher elastic modulus and stronger output force. 9 Thus far, various light-fuelled ( i.e.…”

Photomechanical crystalline materials: new developments, property tuning and applications

“…Then, the property tuning and enhancement strategies of photomechanical crystals are emphasized, including the adjustment of mechanical properties (flexibility) and photomechanical behaviors, which was less discussed in previous reviews. [33][34][35][36][37] Finally, we summarize the current application progress of photomechanical crystals, focusing on from only the photomechanical behavior itself to the composite functions of photomechanical properties and other properties, such as fluorescence and optical waveguides. This functional combination is one of the current development trends of photomechanical crystalline materials.…”