Mechanical motion of molecular crystals induced by [4 + 4] photodimerisation

Abstract

The photomechanical bending of crystals of a stilbene-type compound substituted with anthracene and indanone groups, (E)-2-(9-anthrylmethylene)-1-indanone (trans-1), was investigated. When a narrow platelike microcrystal was irradiated with ultraviolet (UV) light at 365 nm, the crystal gradually bent away from the light source and finally reached a semicircular shape after more than 10 min. A larger rod-like crystal, approximately 10 mm in length, also exhibited a slight bending motion. The cessation of UV irradiation caused the bent crystals to return very slowly to their straight form. In the crystals, the anthracene planes of two neighbouring trans-1 molecules are arranged in a head-to-tail parallel manner, with a short planeto-plane distance of only 3.72 Å. The 1 H nuclear magnetic resonance spectra of trans-1 crystals before and after UV irradiation revealed the intermolecular [4 + 4] photodimerisation of the two anthracene planes, while the trans-to-cis photoisomerisation was not significant. The UV-vis absorption spectra of the trans-1 powder crystals, obtained using a diffuse reflectance spectrophotometer, showed a gradual increase in absorbance between 200 and 500 nm with increasing UV irradiation time, reaching a maximum after 1 h. Thermal back-monomerisation was very slow in the dark, not recovering the initial spectrum even after 25 days. The fluorescence spectra at 570 nm, derived from the anthracene excimer, decreased in intensity with increasing UV irradiation time due to a decrease in the amount of anthracene chromophore via photodimerisation. In situ X-ray measurements revealed that the bending of the crystals was caused by slight elongation of the b axis of the unit cell, corresponding to the long axis of the rod-like crystals. Calculations revealed that the observed crystal elongation could be explained by an optimised head-to-tail [4 + 4] orientation of the anthracene dimer.

Fabrication of Light‐Triggered Soft Artificial Muscles via a Mixed‐Matrix Membrane Strategy

“…The projection distance of adjacent BA2DA dimers along the stacking direction is 7.201 Å, while the corresponding anthracene groups in BA2DA is 6.922 Å after geometrical optimization (Figure S21). This result suggested that the crystal length could elongate approximately 4.0 % along the b axis after intermolecular [4+4] photodimerization, which is consistent with our observation in SCXRD measurement . In addition, the bending behavior of BA2DA crystal was reversible by thermal treatment .…”

Fabrication of Light‐Triggered Soft Artificial Muscles via a Mixed‐Matrix Membrane Strategy

“…The[4+ +4] photoreaction of BA2DA can form dimmer, oligomer or polymer.I no rder to simplify the calculation task, we proposed an aggregation model of BA2DA-dimer to explain the experiment results.T he projection distance of adjacent BA2DA dimers along the stacking direction is 7.201 ,w hile the corresponding anthracene groups in BA2DA is 6.922 after geometrical optimization ( Figure S21). [14] In addition, the bending behavior of BA2DA crystal was reversible by thermal treatment. [14] In addition, the bending behavior of BA2DA crystal was reversible by thermal treatment.…”

“…This result suggested that the crystal length could elongate approximately 4.0 %a long the b axis after intermolecular [4+ +4] photodimerization, which is consistent with our observation in SCXRD measurement. [14] In addition, the bending behavior of BA2DA crystal was reversible by thermal treatment. [15] The disappearance of characteristic 1 HNMR signals ascribed to anthracene dimerization ( Figure S14) revealed BA2DA can completely recover after thermal treatment.…”

Photomechanical Molecular Crystals of an Azopyridine Derivative and Its Zinc(II) Complex: Synthesis, Crystallization and Photoinduced Motion

“…When 6cazpy crystals were irradiated, the intensities of some peaks decreased. For example, the intensity of the peak at 2θ = 4.782 • decreased to 50.6% of the original value (Figure 9a,b), likely due to the trans-to-cis photoisomerization and a corresponding deterioration of crystallinity [21]. The intensity eventually recovered to 64.6% of the original value at 10 min after the UV irradiation was stopped.…”

“…Among all the external stimuli, light which can be remotely, instantly and precisely controlled is an attractive source to adjust material properties. Therefore, many photomechanical materials such as azobenzene [11][12][13][14][15], salicylideneaniline [16], diarylethenes [17][18][19], anthracene [20,21], naphthalene diimides [22], furylfulgide [23] and even metal organic complex crystals [24,25] have been investigated for the realization of converting light energy to mechanical motion of bending [11,12], twisting [20,26,27], coiling [28,29], expanding [30], jumping [10] and popping [24].…”