Photomechanical responses of coordination polymers regulated by precise organization of the photoactive centers†

Jia-Wei Wu‡ ^a, Zhen Qin‡ ^a, Qing-Shu Dong‡ ^a, David James Young ^b, Fei-Long Hu *^a and Yan Mi *^a
aKey Laboratory of Chemistry and Engineering of Forest Products, State Ethnic Affairs Commission, Guangxi Key Laboratory of Chemistry and Engineering of Forest Products, Guangxi Collaborative Innovation Center for Chemistry and Engineering of Forest Products, Guangxi Minzu University, Nanning 530006, China. E-mail: hflphd@163.com
^bGlasgow College UESTC, University of Electronic Science and Technology of China, Chengdu 611731, China

First published on 14th August 2024

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Photomechanical smart materials have potential for applications as diverse as information storage, sensors, actuators,^1,2 artificial muscles,^3–6 and self-healing materials. Photomechanical smart materials can be triggered by reversible cyclization,⁷ [4+4] dimerization, isomerization of azobenzene derivatives, [2+2] photocycloaddition^8–11 and other light-induced reactions.^12–14

[2+2] photodimerization reactions based on coordination polymers (CPs) eliciting photomechanical responses are less observed.¹⁵ In particular, the single crystal to single crystal (SCSC) [2+2] photodimerization reaction of CPs with a photomechanical response is less often reported.¹⁵ Photomechanical effects, including cracking, splitting,¹⁶ jumping, twisting, shape deformation, explosion,¹⁷ curling, bending¹⁸ and even rocking can be driven by [2+2] photocycloaddition.¹⁷

Naumov et al. have reported that the crystal size, shape and surface strain tensor influences the intensity and mode of photosalient movement.^19,20 They report that the inner structure of the crystal is ultimately responsible and dependent on precise arrangement of the photoactive centers.^7,21 Inner structure includes the packing mode of photoactive centers, the asymmetric arrangement of photoactive centers and the alignment of photoactive centers relative to the specific crystallographic axes with the associated generation of strain during the photochemical reaction.²² Controlling this photo-mechanical behavior by tuning the crystal structure has posed a significant challenge.^23,24

Polymers²⁵ can be combined with photosalient materials to generate “artificial muscles” with elasticity and motility, and have demonstrated reversible, controllable mobility in response to external stimuli. Compositing crystalline CP_S crystal with organic polymers can overcome the disadvantage of the former's brittleness and large Young's modulus to give practical macroscopic devices.

In this paper, we report the synthesis of pale rod-like crystals of CP₁, [Cd(Fsbpe)(DBBA)₂]·2DMF (CP₁) (Fsbpe = 4-(3-(4-fluorostyryl)-5-(pyridin-4-yl)phenyl)pyridine) from the solvothermal reaction of 3CdSO₄·8H₂O, DBBA and Fsbpe in a mixed solvent at 140 °C for 12 h. The phase purity of the bulk crystalline material was confirmed by comparison of the experimental powder X-ray diffraction pattern with that of the simulated powder X-ray diffraction pattern derived from the single crystal data (Fig. S7, ESI†). TGA indicated that CP₁ lost the lattice solvent of DMF molecules at about 230 °C and was then thermally stable up to 370 °C (Fig. S8, ESI†). Single crystal X-ray analysis revealed that CP₁ crystallized in the orthorhombic space group Pbcm. The asymmetric unit of CP₁ contained one Cd(II) atom, one DBBA ligand and Fsbpe ligand. Each Cd(II) center was ligated by five O atoms from three auxiliary carboxyl acids and two N atoms from the Fsbpe ligand forming a pentagonal-bipyramidal geometry (Fig. S9, ESI†). The secondary building unit (SBU) consisted of two cadmium ions bridged by four DBBA ligands. These SBUs were also linked by Fsbpe ligands to form a 1D structure along the c axis (Fig. S10, ESI†). A pair of Fsbpe ligands were aligned in a head-to-head (HH) manner and the distance between the centroids of the olefin bonds was 4.251 Å (Fig. S11, ESI†), which does not meet the requirements of Schmidt's rule.²⁶ The seriously disordered fluorostyrene groups of Fsbpe suggested that the dynamic molecular motion would temporarily allow the reaction centers to approach each other for the [2+2] photocycloaddition.²⁷ When single crystals of CP₁ were subjected to UV light (365 nm), the solution ¹H NMR spectrum of the resulting organic material showed the disappearance of the olefin proton signals at 7.63 and 7.41 ppm and the presence of new peaks at 4.91 and 4.79 ppm, indicating cyclobutanes were produced by [2+2] photocycloaddition. Likewise, the shift of the signals for the pyridyl protons from 8.72 and 7.94 to 8.56 and 7.65 ppm respectively, supported this structural transformation (Fig. S12, ESI†). Complete conversion of the Fsbpe ligands to the corresponding cyclobutanes was achieved after 13 h of UV light irradiation.²⁸

A microscope equipped with a high-resolution camera was used to monitor photo-induced mechanical properties. Crystals of CP₁ under UV irradiation displayed multiple photo-mechanical motions, including cracking, splitting and jumping (Fig. 1 and 3 and Fig. S13, Video SV1, ESI†). Parallel cracks appeared in the (002) direction perpendicular to the main elongation (b axis) of the crystal (Fig. S14, ESI†). We suggest that the build-up of strain due to [2+2] photocycloaddition is released by cleavage along the crystal plane.²³

To further probe the role of the photoreactive CC groups in the mechanical motion, an alkyne bond (–CC–) containing ligand was incorporated into analogue CP₂ [Cd(Fepbpe)(DBBA)₂]·2DMF using the same experimental conditions employed for CP₁. TGA indicated that CP₂ released the lattice solvent of DMF molecules at about 220 °C and then decomposed at about 350 °C (Fig. S8, ESI†). Fsbpe was replaced by alkynyl containing ligand 4,4′-(5-((4-fluorophenyl)ethynyl)-1,3-phenylene)dipyridine (Fepbpe) and the resulting CP₂ also crystallized in the orthorhombic Pbcm space group with nearly the same cell parameters identical to CP₁. A pair of Fepbpe ligands from two adjacent moieties were arranged face-to-face and further ligand coordination similar to CP₁ resulted in a 1D chain backbone²⁹ (Fig. S15, ESI†). The ¹H NMR spectrum showed no changes before and after UV light (365 nm) irradiation (Fig. S16, ESI†) indicating no reaction occur.

As expect, crystals of CP₂ showed no obvious photo-mechanical behavior upon UV light irradiation (Fig. S17 and Video SV2, ESI†) confirming that the photomechanical behavior of CP₁ can be attributed to [2+2] photocycloaddition.

We next investigated the effect of incorporating multiple reactive centers on photomechanical behavior. Ligand Fsbpeb was used to construct of CP₃, [Cd(Fsbpeb)(DBBA)₂] (Fsbpeb = 4-(3-(4-fluorostyryl)-5-((E)-2-(pyridin-4-yl)vinyl)styryl)pyridine). TGA indicated that CP₃ was thermally stable up to 350 °C (Fig. S8, ESI†). A pair of Fsbpeb molecules were arranged face-to-face and two CC pairs were aligned in parallel fashion, and at an appropriate distance of 3.860 Å to satisfy Schmidt's criteria.²⁶ The third fluorobenzene CC pair were arranged crisscrossed (Fig. S18, ESI†). A close inspection of the crystal structure indicated that the fluorobenzene group was seriously disordered due to absence of the binding interactions. The ¹H NMR spectrum of CP₃ after exposure to UV light for 24 h displayed a new set of signals at 4.72 and 4.65 ppm attributed to cyclobutane resonances (Fig. S19, ESI†).³⁰ The SCSC transformation of CP₃ to CP₃′ (Fig. 2), resulted in the two pairs of CC bonds attached to the pyridines converting to a cyclobutane. After inspecting the crystal structure more closely reveal that a significant expansion on crystallographic c axis was observed upon UV light irradiation for 24 h, indicating large molecules movements occurred during the photoinduced cycloaddition reaction.

CP₃ exhibited various photomechanical behaviors upon UV light (365 nm) irradiation, including violent splitting, jumping, bending, breaking and popping (Fig. 3 and Video SV3, ESI†). Crystals of CP₃ underwent light-induced bending behavior when the leaf shaped crystals were subjected to UV light (365 nm). Upon irradiation, the lower end of the crystal bend rapidly away from the light source, reaching a maximum angle of 80° (Fig. S20, ESI†). Interestingly, when subsequently irradiated from the opposite side, the bent crystal recovered its original shape and then bent further from the light source. This process could be repeated over several cycles (Fig. S21 and Video SV4, ESI†). This bending behavior occurred in the (200) plane consistent with Bravais–Friedel–Donnay–Harker (BFDH) calculations (Fig. S22, ESI†). The gradient shielding effect led to more photodimerization of the upper face than in the lower face,³¹ resulted in the bending behavior for CP₃.

It is remarkable that the photoactive centers in CP₃ could be recovered by heating CP₃′ at 240 °C for 20 h (Fig. S23, ESI†). This was confirmed from the ¹H NMR spectrum (Fig. S23, ESI†), in which the chemical shifts at 4.72 and 4.65 ppm belonging to cyclobutanes disappeared. Importantly, the recovered reactive centers repeated these photomechanical behaviors when again exposed to UV irradiation.

Comparing photosalient effects (Fig. S24, ESI†), crystals of CP₁ displayed cracking, splitting and jumping under UV irradiation for three minutes (Video SV5, ESI†). Crystals of CP₂ did not display any photoactuation behavior under the same irradiation conditions (Fig. 3), although they bear similar crystal morphology. It suggested that the photo inert of –CC– group couldn’t lead to the photo response behavior compared with CP₁. Crystals of CP₃ exhibited a variety of movements upon UV light irradiation, including flipping, standing, exploding and bending (Fig. 3). This discrepancy in the photo-mechanical behaviours of CP₁–CP₃ depend on the nature and orientation of the photoactive centres. BFDH calculation placed the photoactive centers in the widest crystal plane (200) in CP₁ and CP₃ (Fig. S22, ESI†). A gradient shielding effect leads to the bending behavior of CP₃ which is reversed by heating.³¹

In order to improve the photosalient performance of the crystals for practical use, the photoreactive CP_S was combined with PVA to form a composite membrane.^12,32 The CP₃–PVA composite membrane was successfully constructed by mixed-matrix membrane strategy.³³ The FTIR and PXRD patterns of a CP₃–PVA composite membrane matched well with the combined patterns of PVA and CP₃ (Fig. S25 and S26, ESI†). This composite membrane responded rapidly to irradiation, bending and wrapping towards the light source (Videos SV6 and SV7, ESI†). Furthermore, we fabricated an octopus-inspired CP₃–PVA robot with multiple arms which could grasp objects (47.7 mg) upon visible light irradiation (Fig. 4 and Video SV8, ESI†). The robot was firstly placed above the object, and then each arm of the robot octopus started to bend upon UV light irradiation for a few seconds. Finally, the object was completely embraced by the robot octopus. The embraced robot octopus was then lifted away from the ground with artificial force in a fishing fashion. There was sufficient contact force between the object and the robot to prevent the object from dropping under gravity.¹⁴ The lifted object didn’t drop during the further transfer process.

In summary, CP₁–CP₃ with similar one-dimensional chain motifs exhibited completely different photomechanical behaviors in response to UV light. It illustrated that the photomechanical behaviors of CP can be regulated by arranging the photoreactive centers, including the types and amount of functional groups. The results showed that more photoresponsive functional groups produce more intense photomechanical behaviors. The orientation of the photoactive centers relative to the specific crystallographic axis determined the degree of strain resulting in particular movements, which could be reversed by heating. Therefore, the modulation of CP_S photoactuation can be achieved through the accurate arrangement of photoactive center.

The authors are grateful for financial support from the National Natural Science Foundation of China (No. 22361004), Natural Science Foundation of Guangxi (No. 2024GXNSFDA010057), Xiangsi Lake Young Scholars of Guangxi Minzu University (No. 2021RSCXSHQN04), the training program for thousands of backbone young teachers in Guangxi universities, natural science foundation cultivation project of Guangxi Minzu University (2023MDKJ003) and the Open Fund of Guangxi Key Laboratory of Agricultural Resources Chemistry and Biotechnology (2022KF02).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2362977–2362980. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc03471f

‡ These authors contributed equally to this work and should be considered co-first authors.

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