Hierarchical Coordination Polymer–Polymer Composites for Robust, Programmable Photomechanical Actuation

ABSTRACT

Photoresponsive coordination polymers offer precise molecular‐to‐macroscopic control of mechanical motion, yet their brittleness and limited processability constrain practical implementation. Here we report a hierarchical composite strategy that embeds a one‐dimensional Zn(II) coordination polymer capable of topochemical [2+2] cycloaddition within a poly(vinyl alcohol) matrix and coats it with poly(ε‐caprolactone), yielding mechanically robust, scalable, and water‐stable photomechanical films. The composites exhibit wavelength‐selective activation and expansion‐programmable deformation, enabling dual‐stage bending and radial expansion with actuation fidelity exceeding 98% over extended storage and radial strengths approaching those of load‐bearing polymeric systems. Comparative analysis across representative photoactuating materials identifies a distinct regime combining crystalline photochemical precision with macroscopic mechanical resilience. These results establish general design principles—topochemical preorganization, hierarchical stress transfer, and protective coating—for translating lattice‐confined photochemistry into durable, expansion‐programmable mechanical actuation, providing a foundation for next‐generation adaptive and deployable material systems.

A composite film (CP1‐PVA)@PCL, exhibiting water resistance, toughness, and load‐bearing capacity, is prepared using the photoresponsive coordination polymer CP1. The films are assembled into slit‐patterned tubular actuators, and by controlling the duration of light exposure, the actuators achieve a dual‐stage expansion behavior with a controllable degree of actuation. These results provide a foundation for next‐generation adaptive and deployable material systems.

1. Introduction

Adaptive materials capable of undergoing controlled, post‐fabrication shape transformations are central to emerging technologies in soft robotics, reconfigurable systems, and deployable architectures [1, 2, 3, 4, 5, 6, 7]. A persistent challenge in this field is the simultaneous achievement of large, programmable actuation, mechanical robustness, long‐term stability, and scalable processing. Meeting these criteria requires materials that efficiently translate molecular‐ or lattice‐level stimuli into macroscopic mechanical work.

Light‐responsive systems are particularly attractive because optical stimuli enable remote, spatially resolved, and temporally precise actuation without physical contact [8, 9, 10, 11, 12, 13]. Coordination polymers (CPs) and molecular crystals that undergo photochemical transformations, such as [2+2] photocycloaddition, offer well‐defined reaction pathways and high actuation precision [14, 15, 16, 17, 18, 19]. However, these crystalline systems are typically brittle, difficult to process, and mechanically incompatible with load‐bearing architectures [20, 21, 22, 23, 24], limiting their translation beyond proof‐of‐concept demonstrations.

Recent efforts have addressed these limitations by integrating photoactive crystalline components within polymer matrices, thereby combining the structural precision of ordered photochemistry with the toughness and flexibility of polymers [25, 26, 27, 28, 29, 30, 31, 32]. Liquid crystalline elastomers (LCEs) [33, 34] exemplify this strategy, efficiently converting external stimuli into macroscopic mechanical motion through mesoscale orientational order (Figure 1a). In parallel, molecular crystals [35, 36] and coordination polymer (CP) crystals [37, 38, 39] enable precise molecular‐level structural transformations through topochemical lattice reactions (Figure 1b), whether used as single crystals or incorporated into composite architectures [40, 41]. Photothermal polymers [42, 43, 44] further expand the actuation landscape via light‐driven thermal pathways (Figure 1c). Despite these advances, achieving the simultaneous integration of crystalline‐level photochemical precision, large programmable deformation, mechanical robustness, and long‐term environmental stability remains challenging [45, 46, 47, 48, 49, 50, 51]. Many hybrid systems exhibit trade‐offs between actuation efficiency and structural durability, or between molecular order and scalable processability. These constraints motivate a central question: can lattice‐confined photochemical reactions be translated into mechanically resilient, scalable materials without sacrificing actuation fidelity?

FIGURE 1.

Concept and materials design of representative photoactuating systems. (a) Schematic of LCEs photoactuators, illustrating soft polymeric deformation under light. (b) Molecular/crystalline CP photoactuators embedded in polymer matrices, showing bending or fracture upon photochemical stimulus. (c) Photothermal polymer systems undergoing light‐induced expansion and contraction. (d) Fabrication workflow of (CP1‐PVA)@PCL tubular actuators and working principles of dual‐stage, expansion‐programmable radial expansion driven by hierarchical photomechanical deformation.

Here we report a hierarchical composite strategy that translates lattice‐confined photochemistry into mechanically robust, scalable actuation. A 1D Zn(II) coordination polymer, {[Zn(OAc)₂(tpeb)]·2H₂O}_n (CP1; tpeb = 1,3,5‐tri‐4‐pyridyl‐1,2‐ethenylbenzene), undergoes topochemical [2+2] photocycloaddition and is integrated within a polymeric architecture engineered for efficient stress transfer and environmental stability. Crystalline CP1 is embedded in a poly(vinyl alcohol) (PVA) matrix to form a mechanically continuous CP1‐PVA composite film, which is subsequently coated with poly(ε‐caprolactone) (PCL) to enhance mechanical stability and aqueous durability, yielding a hierarchical (CP1‐PVA)@PCL structure. To demonstrate programmable macroscopic deformation under mechanical constraint, the composite films are assembled into slit‐patterned tubular actuators capable of time‐governable radial expansion (Figure 1d). Beyond this proof‐of‐concept geometry, we delineate how crystalline photochemical strain can be amplified, stabilized, and programmably expressed across macroscopic length scales, establishing a structural blueprint for integrating molecular‐level precision with mechanically resilient architectures.

2. Results

2.1. Structural Basis and Photochemical Reactivity of CP1

Pale yellow, needle‐like crystals of CP1 were obtained in 80.6% yield via solvothermal reaction of ZnSO₄·7H₂O with tpeb (Figure S1) in an acetonitrile–water solvent system. Single‐crystal x‐ray diffraction (Table S1) revealed that each Zn(II) center is chelated by an acetate ligand and interconnected via pairs of tpeb ligands to form a one‐dimensional helical chain structure (Figure 2a; Figure S2). The acetate ligand likely originates from the hydrolysis of acetonitrile under solvothermal conditions [52]. Critically, adjacent olefinic C═C bonds from neighboring chains in CP1 adopt a head‐to‐tail, parallel arrangement with an intermolecular separation of 3.743 Å, satisfying Schmidt's criteria for topochemical [2+2] photocycloaddition (Figure 2b; Figure S3). Powder x‐ray diffraction (PXRD) confirmed its phase purity, while thermal and optical analyses indicate its stability above operational conditions and strong absorption in the near‐UV region, enabling selective photoactivation at 365 nm (Figures S4–S6).

FIGURE 2.

Crystal structure and topochemical photoreactivity of CP1. (a) One‐dimensional helical chain of CP1 (H atoms omitted for clarity). Atom colors: C, light blue; N, teal; Zn, orange; O, light pink; reactive C═C moieties involved in [2+2] cycloaddition highlighted in gold. (b) Topochemical arrangement of adjacent tpeb ligands satisfying Schmidt's criteria for [2+2] cycloaddition (left: gold dashed lines denote unreacted nonbonded contacts; right: pale purple ring denotes the cyclobutane group). (c) Solution‐state ¹H NMR spectra showing progressive cyclobutane formation under 365 nm UV irradiation, confirming efficient lattice‐confined photochemistry.

Upon UV irradiation at 365 nm, CP1 undergoes a gradual photochemical transformation accompanied by a macroscopic color change from pale to dark yellow and a loss of long‐range crystallinity, yielding an amorphous photoproduct (CP1a) (Figure S7). Solution‐state ¹H NMR spectroscopy (DMSO‐d ₆) confirms the formation of cyclobutane moieties, evidenced by the emergence of new resonances at 4.0–4.5 ppm and concomitant shifts of pyridyl proton signals from 8.9–9.1 to 8.2–8.3 ppm (Figure 2c). Quantitative analysis reveals a time‐dependent conversion, increasing from 24% after 10 min to complete transformation within 90 min of irradiation (Figure 2c; Figures S8–S12). The photomechanical response of CP1 is intrinsically irreversible because it originates from a topochemical [2+2] cycloaddition reaction. Thermal treatment of the fully irradiated material (up to 200°C) and microwave irradiation failed to regenerate the original olefin resonances in the ¹H NMR spectra, indicating the absence of detectable cycloreversion under the investigated conditions. Consequently, the light‐induced deformation represents a permanent photochemical transformation rather than a reversible photoactuation process. This well‐defined kinetic progression indicates that the extent of topochemical [2+2] cycloaddition can be precisely modulated through irradiation duration, providing direct temporal control over the underlying photochemical process. These results demonstrate that lattice‐confined photocycloaddition in CP1 proceeds efficiently under mild conditions and establishes a clear molecular‐scale origin for the photomechanical deformation described below.

2.2. Wavelength‐ and Time‐Programmable Photomechanical Response

The photomechanical response of CP1 single crystals is programmable along both wavelength and temporal dimensions, as revealed by monitoring free‐end deformation under controlled UV irradiation. Figure 3 illustrates how wavelength‐selective activation and irradiation‐time control govern the rate and extent of lattice‐confined [2+2] photocycloaddition, thereby providing orthogonal design parameters for programming the magnitude and kinetics of photomechanical actuation. Under 365 nm illumination (irradiance 260 mW·cm⁻²), CP1 crystals exhibit rapid, large‐amplitude bending, reaching ∼30° within 1 s, ∼70° within 2 s, and forming a complete loop by 3 s. Continued irradiation induces mechanical fracture at approximately 4 s, followed by a brief vibrational recoil (∼0.5 s) (Figure 3a–f; Video S1). In contrast, irradiation at 420 nm produces qualitatively similar deformation modes but with markedly slower kinetics: bending initiates after ∼2 s, progresses to a maximum curvature of ∼70° by 8 s, and stabilizes in a hook‐like geometry upon prolonged exposure (Figure 3g–l; Video S2).

FIGURE 3.

Wavelength‐ and time‐programmable photomechanical response of CP1 single crystals. (a–f) Time‐resolved bending under 365 nm irradiation. The final deformation state can be programmed by controlling irradiation duration, yielding bending angles of approximately 30° (1 s), 70° (2 s), and 180° (3 s), followed by fracture at longer exposure times. (g–l) Time‐resolved bending under 420 nm irradiation, showing similar photomechanical behavior with slower kinetics. Wavelength and irradiation time therefore function as independent programming parameters, governing response kinetics and final deformation state, respectively.

Control experiments confirm that the observed deformation originates predominantly from lattice‐confined photochemical processes rather than photothermal effects. The pronounced wavelength dependence of the actuation kinetics directly correlates with the efficiency of [2+2] photocycloaddition, indicating that macroscopic motion is governed by the rate of bond formation within the crystal lattice rather than by thermal relaxation. Beyond wavelength‐dependent modulation of actuation rate, the sequential deformation behavior—from rapid initial bending to saturation or fracture—demonstrates that time constitutes an independent and programmable control parameter. By regulating irradiation duration, the extent of photochemical conversion and the resulting final deformation state can be precisely preset. These observations establish a direct coupling between crystal packing, photochemical kinetics, and macroscopic mechanical output, providing a molecular‐to‐macroscopic design principle that underpins the hierarchical composite systems described below.

2.3. Translation of Crystalline Photomechanics Into Composite Films

To translate the pronounced photomechanical response of CP1 crystals into processable, mechanically robust formats, CP1 was embedded within a polymer matrix. Pristine CP1 crystals are intrinsically brittle and incapable of sustaining large macroscopic strains, necessitating matrix reinforcement to enable coherent actuation. A 7.7 wt% aqueous PVA solution was used to disperse CP1, followed by casting onto polyethylene terephthalate (PET) substrates and drying at 40°C for 24 h, producing flexible, pale yellow CP1‐PVA films. Scanning electron microscopy (SEM) revealed that the CP1 crystals were fragmented into microcrystalline grains (∼0.5–1 µm) during film preparation and were uniformly dispersed throughout the PVA matrix, forming a ∼50 µm‐thick composite film (Figure 4a; Figures S13–S15), while PXRD confirms preservation of crystalline structure (2θ = 12°, 21°, 23°, 25°; Figure S16).

FIGURE 4.

Hierarchical composite film architecture and mechanical performance. (a,d) Cross‐sectional SEM images of CP1‐PVA and (CP1‐PVA)@PCL films, showing uniform dispersion of CP1 within polymer matrices and conformal PCL coating. (b,e) Representative stress–strain curves of CP1‐PVA and (CP1‐PVA)@PCL films, showing enhanced ductility and load‐bearing capacity. (c,f) Surface roughness (Ra) measurements by AFM for CP1‐PVA and (CP1‐PVA)@PCL films. Hierarchical coating preserves crystalline photochemistry while providing mechanical reinforcement and aqueous stability, enabling programmable, durable photomechanical actuation.

Mechanical testing reveals that the CP1‐PVA composite film undergoes a sequential deformation profile characteristic of tough polymeric materials, including elastic deformation, yielding, strain softening, cold drawing, and eventual fracture (Figure 4b). The film exhibits an elongation at break of 100% and an elastic modulus of 6.1 GPa (Figure 4b), demonstrating the reinforcing contribution of crystalline CP1 domains within the polymer matrix and indicating substantial toughness combined with load‐bearing capability. Atomic force microscopy (AFM) shows a low root‐mean‐square surface (RMS) roughness (Sq = 2.53 nm), consistent with homogeneous morphology and uniform crystal dispersion (Figure 4c). These structural and mechanical analyses indicate strong interfacial coupling between CP1 and the surrounding polymer, which mechanically stabilizes the photoactive phase while enabling efficient transmission and amplification of photochemically generated lattice strain across macroscopic length scales.

Upon 365–450 nm irradiation, CP1‐PVA films exhibit pronounced negative phototropic bending, with curvature increasing at shorter wavelengths (Videos S3–S6 and Figures S17–S20). Single crystals of CP1 exhibit anisotropic photomechanical bending (Figure 3), confirming the directional nature of the lattice‐confined photochemical strain. In the composite films, however, the CP1 microcrystals (∼0.5–1 µm) are randomly oriented, precluding a direct correlation between individual crystallite orientation and the macroscopic bending direction. Instead, the observed negative phototropic bending arises from a photochemical conversion gradient generated by asymmetric light penetration through the film thickness. Because CP1 strongly absorbs 365 nm light, crystallites near the irradiated surface experience a higher degree of [2+2] photocycloaddition than those in the interior, producing a strain gradient across the film thickness. Under the mechanical constraint of the polymer matrix, this asymmetric strain is converted into a net bending moment, resulting in macroscopic bending away from the light source. Specifically, at 365 nm, 4 × 40 mm² films bend to 90° within 2 s and reach 180° by 3 s, corresponding to actuation rates up to 60° s⁻¹. The progressive deformation enables precise control of final curvature through adjustment of irradiation duration, establishing time‐governed mechanical programmability in the composite films. The persistence of wavelength‐dependent actuation confirms that the embedded crystalline photochemistry remains the dominant driving force, while the compliant polymer matrix coherently amplifies lattice‐level strain into large, macroscopic bending.

Collectively, these results demonstrate that embedding topochemically reactive crystals within a polymer matrix achieves simultaneous mechanical reinforcement and preservation of photochemical fidelity, enabling scalable films capable of translating molecular and lattice transformations into robust macroscopic actuation. However, the CP1‐PVA films exhibit limited stability in aqueous environments, motivating the development of a hierarchical coating strategy to achieve long‐term operational robustness.

2.4. Enhancing Water Stability and Mechanical Integrity Through PCL Coating

To address the aqueous sensitivity of CP1‐PVA films while preserving photomechanical functionality, we introduced a protective poly(ε‐caprolactone) (PCL) coating. CP1‐PVA films were repeatedly brushed with 1 wt% PCL in CHCl₃ and dried between cycles, yielding hierarchical (CP1‐PVA)@PCL composites. Systematic optimization of coating cycles established a balance between environmental protection and efficient strain transmission, with five cycles identified as optimal. Long‐term stability was evaluated by immersing the composites in physiological saline (0.9 wt% NaCl) for 90 days, followed by air‐drying prior to characterization. Additional immersion experiments under acidic (pH = 3) and alkaline (pH = 11) conditions revealed progressive degradation of the composite architecture (Figure S21), indicating that the present (CP1‐PVA)@PCL system is most suitable for neutral or mildly aqueous environments. Extension to more aggressive chemical conditions will require matrix materials with enhanced pH resistance. Physiological saline was selected rather than deionized water to assess durability under ionic conditions representative of realistic aqueous environments, where osmotic and ion‐mediated effects can accelerate degradation of hydrophilic polymer matrices. Following immersion, the integrity of the PCL barrier was preserved, confirming its role as an effective hydrophobic encapsulation layer. The control experiments further clarify the distinct functions of the hierarchical composite components. CP1 crystals serve as the source of photochemically generated lattice strain but fracture readily under mechanical constraint because of their intrinsic brittleness. Embedding CP1 within the PVA matrix enables efficient redistribution and transfer of this anisotropic strain, thereby converting localized crystal deformation into coherent macroscopic actuation. In contrast, the PCL coating contributes primarily to environmental protection rather than actuation itself. By suppressing water ingress and preserving the mechanical integrity of the PVA matrix, the coating maintains long‐term strain‐transfer capability and photomechanical performance in aqueous environments. The resulting architecture therefore decouples photochemical function, mechanical amplification, and environmental stability into complementary structural elements, illustrating a general design principle for robust photomechanical composites. Structural analyses demonstrate that the coating does not disrupt CP1 crystallinity or its homogeneous dispersion within the PVA matrix (Figures S22). SEM analysis revealed a continuous and conformal PCL coating with a thickness of ∼3.16 µm, which forms a hydrophobic protective barrier while preserving the integrity of the photoactive CP1‐PVA layer. The overall thickness of the (CP1‐PVA)@PCL film was ∼45 µm (Figures S23–S27). Cross‐sectional SEM images (Figure 4d; Figure S26) further confirmed the well‐defined multilayer architecture of the composite, clearly distinguishing the continuous PCL coating, the CP1‐PVA composite layer, and their interface. Notably, CP1 microcrystals remained uniformly dispersed within the PVA matrix beneath the protective PCL shell (Figure 4d).

After saline treatment, mechanical performance exhibits a moderate reduction but remains sufficient for load‐bearing actuation. The (CP1‐PVA)@PCL films display an elongation at break of 50% and an elastic modulus of 3.1 GPa, following a fracture profile comparable to that of CP1‐PVA (Figure 4e). AFM analysis indicates increased surface roughness (Sq = 18.1 nm) after treatment (Figure 4f), reflecting surface restructuring while maintaining overall structural coherence. Photomechanical function is preserved across 365–450 nm irradiation, with rapid actuation and both wavelength‐selective and time‐governed curvature maintained (Videos S7–S10 and Figures S28–S31). Under 365 nm illumination, a 4 × 40 mm² film reaches 103° bending within 3 s and 180° within 5 s, corresponding to actuation rates up to 30° s⁻¹. The slight reduction in actuation kinetics relative to the uncoated CP1‐PVA film arises from the mechanical constraint imposed by the PCL shell, which enhances environmental durability while efficiently transmitting photochemically generated strain. Photomechanical durability was evaluated by measuring the bending response of (CP1‐PVA)@PCL films after prolonged immersion in physiological saline (Figure S32). The maximum bending angle decreased from 150° prior to immersion to 88° after 90 days, demonstrating sustained actuation capability under extended aqueous exposure. Having established mechanical robustness and environmental durability in planar films, we next examined whether the hierarchical composite could sustain coherent photomechanical deformation under geometrically constrained, load‐bearing configurations.

2.5. Geometry‐Encoded, Expansion‐Programmable of Tubular Actuators

To evaluate how lattice‐confined photochemical strain can be expressed under mechanical constraint, the hierarchical (CP1‐PVA)@PCL films were assembled into slit‐patterned tubular actuators (Figure 5a). Cylindrical geometries provide a mechanically stringent platform in which bending of the film is converted into radial expansion, enabling quantitative assessment of load‐bearing deformation in a closed architecture. This configuration allows geometric encoding of the deformation pathway while amplifying small in‐plane strains into macroscopic circumferential displacement. Tubes with opening diameters ranging from 1 to 8 mm were fabricated by folding slit‐patterned composite films around glass rods of corresponding diameters and fixing the overlapping edges with adhesive tape to preserve cylindrical geometry. The resulting devices, designated by their inner diameters, serve as a model constrained system to probe whether crystalline photochemical transformation can be coherently translated into controlled radial expansion across multiple length scales.

FIGURE 5.

Expansion‐programmable of tubular actuators via accumulated photochemical strain. (a) Conceptual schematic of dual‐stage radial expansion governed by time‐accumulated, lattice‐confined [2+2] photocycloaddition under constant irradiation: initial tubular state, onset of actuation, completion of the first expansion stage (360° rotation), and completion of the second expansion stage following additional irradiation (second 360° rotation). (b, c) Time‐sequence images of horizontally oriented (CP1‐PVA)@PCL tubular actuators with inner diameters of 1 mm (b) and 4 mm (c), illustrating reproducible, staged radial expansion. Each row corresponds to the actuation states schematized in a. As in Figure 3, the final deformation state is encoded by irradiation duration, establishing time as an independent design parameter for programming macroscopic actuation.

Based on the wavelength‐dependent actuation behavior of the composite films (Figure 3; Figure S28), 365 nm irradiation—corresponding to the highest photochemical conversion rate—was selected as the actuation source for the tubular devices. Under constant 365 nm UV illumination, the actuators exhibit reproducible outward radial expansion driven by collective photomechanical deformation of the composite walls. The staged and programmable character of this response arises from the finite kinetics of the lattice‐confined [2+2] photocycloaddition within the embedded CP1 crystals. As confirmed by ¹H NMR analysis, photoconversion proceeds progressively with irradiation time. During the initial exposure period, a fraction of crystalline domains reacts rapidly, generating localized stress that induces the first fast bending and radial expansion of the tube wall. Continued irradiation activates additional domains, and the accumulated photochemical strain cooperatively drives a second expansion stage, resulting in pronounced re‐expansion of the tubular structure. The cumulative expansion ratio depends on tube diameter and irradiation duration, reaching 119–188% during the first stage and 128%–250% after completion of the second stage (Figure 5b,c; Figures S33–S36; Table S2; Videos S11–S18). SEM analysis after two complete expansions (Figure S37) revealed an intact PCL/PVA interface without detectable delamination or significant microcrack formation, confirming the mechanical durability of the protective coating during photomechanical actuation. These findings demonstrate that under fixed spectral conditions, radial deformation is encoded by irradiation time, establishing time accumulation as the operative programming axis in geometrically constrained tubular architectures.

The expansion proceeds via a dual‐stage mechanism. Direct stage‐resolved photoconversion measurements within the composite film are currently unavailable because the embedded CP1 phase cannot be quantitatively analyzed by solution‐state NMR. Nevertheless, thermal controls, wavelength‐dependent kinetics, the presence of a distinct intermediate plateau, the requirement for continued illumination to initiate the second expansion stage, and the known photoconversion kinetics of CP1 collectively support the interpretation that the dual‐stage actuation arises from the progressive accumulation of photochemically generated strain. Accordingly, this mechanism is presented as a well‐supported interpretation rather than definitive proof, and future studies employing spatially resolved conversion measurements will be required for rigorous validation. The first stage involves lattice‐level [2+2] cycloaddition and localized bending, while the second stage amplifies this strain across the macroscopic tube, producing substantial radial dilation. This stepwise deformation allows precise control of actuator geometry and achieves >98% actuation fidelity after months of storage (Figure S38). By translating molecular‐scale photochemistry into mechanically robust, large‐scale motion, these tubular actuators establish a generalizable strategy for expansion‐programmable structures, bridging the gap between precisely ordered crystalline photoactuators and scalable, load‐bearing materials suitable for soft robotics, deployable devices, and adaptive architectures.

2.6. Mechanical Performance of Tubular Actuators

To assess the functional robustness of the (CP1‐PVA)@PCL tubular actuators, their radial strength was quantified as a measure of load‐bearing capacity under light‐induced deformation. For thin‐walled cylindrical structures, the radial strength can be approximated through elastic deformation analysis based on Hooke's law [53, 54]. In this simplified mechanical framework, the radial force per unit length (F _R) is given by F _R = E × t × ε _R, where E represents the effective elastic modulus of the composite wall, t is the wall thickness (Figure S24), and ε _R = ΔD/D ₀ denotes the radial strain, with ∆D and D ₀ corresponding to the change in diameter and the initial diameter, respectively. This approximation is commonly applied to thin‐walled cylindrical systems and provides a useful approach for comparing the radial mechanical performance, despite the neglect of local anisotropy and nonlinear deformation effects. During actuation, the tubular structures were oriented horizontally and irradiated from below, resulting in outward radial expansion of the slit‐patterned composite architecture. The evolution of actuator diameter was monitored in situ as a quantitative measure of macroscopic deformation. By dynamically monitoring the evolution of the diameter during irradiation (Figure 6), the radial strain and the corresponding radial strength were extracted in situ. This experimental setup directly correlates lattice‐confined photochemical strain to macroscopic mechanical output under geometrical constraint.

FIGURE 6.

Mechanical characterization of light‐expanded tubular actuators. Time‐dependent diameter evolution of (CP1‐PVA)@PCL tubular actuators (1–8 mm) under UV irradiation.

For a representative actuator with an initial diameter of 4 mm, UV‐induced dual‐stage expansion yielded a calculated radial strength of 206.9 N·mm⁻¹, while smaller‐diameter actuators (1 mm) achieved radial strengths of 37.9 N·mm⁻¹ (Table S3). The reduced force output observed for smaller actuators reflects geometric scaling effects and the smaller absolute diameter changes achievable at reduced dimensions. Notably, the radial strengths obtained for the smaller actuators approach values reported for biodegradable metallic stents and other load‐bearing tubular biomedical devices [55, 56]. Importantly, all tubular actuators retained structural integrity over repeated light‐triggered expansion cycles, demonstrating the mechanical resilience of the hierarchical composite architecture.

Comparison with other classes of photoactuating materials (Table 1 and Tables S4 and S5) further highlights the distinctive position of the present system. Because actuator performance depends strongly on geometry and deformation mode, geometry‐specific metrics such as radial strength are reported separately from cross‐material benchmarking. Molecular crystals [35, 36] and unreinforced coordination polymers [37, 38, 39] can exhibit rapid photomechanical responses but typically suffer from brittleness and limited mechanical durability. In contrast, LCEs actuators [33, 34], crystalline polymers [40, 41], and photothermal polymers [42, 43, 44] are capable of large deformations but often lack precise wavelength selectivity, directional strain control, or long‐term mechanical stability under aqueous conditions. The (CP1‐PVA)@PCL tubular actuators uniquely integrate crystalline photochemical precision, large programmable deformation, load‐bearing capacity, and aqueous durability within a single materials platform. Accordingly, quantitative strain comparisons are presented only for the tubular actuators developed in this work, thereby ensuring methodological consistency across all reported measurements.

TABLE 1.

Positioning of representative photoactuating material classes, including CP‐polymer composites. Comparison of key performance metrics across major classes of photoactuators: LCEs, molecular crystals, CPs‐based systems, crystalline polymers, photothermal polymers, and the present (CP1‐PVA)@PCL composites. Metrics include actuation mechanism, structural order, elastic modulus, and environmental stability. The hierarchical CP‐polymer system uniquely integrates directional photochemistry, macroscopic strain amplification, load‐bearing capability, and long‐term aqueous stability, occupying an underexplored regime of the photoactuator design landscape.

Material class	Actuation mechanism	Structural order	Elastic modulus	Stability
LCEs [33, 34]	Photoisomerization	Amorphous	0.1–18 MPa	Low
Molecular crystals [35, 36]	Photochemical	Crystalline	0.1–20 GPa	Low
CP photoactuators [37, 38, 39]	Photochemical Lattice change	Crystalline	0.5–20 GPa	Low Moderate
Crystalline polymers [40, 41]	Photochemistry and Matrix	Semicrystalline	1–10 GPa	Moderate
Photothermal polymers [42, 43, 44]	Photo/thermal expansion	Amorphous	0.1–1.5 GPa	Moderate
This work	Photocycloaddition	Crystals and polymers	3.1 GPa	High

These results demonstrate that the CP‐polymer hybrid system bridges the long‐standing gap between soft photoresponsive materials and mechanically resilient deployable structures. By combining lattice‐confined photochemistry with hierarchical stress transfer and protective polymer coating, the present system achieves durable, time‐governable mechanical output at force scales relevant to load‐bearing, constrained structures. This integration establishes a generalizable materials framework for translating crystalline photochemical precision into robust, macroscale adaptive systems.

3. Discussion

This study demonstrates a generalizable materials strategy for converting lattice‐confined photochemical reactions in coordination polymers into macroscale, mechanically robust photomechanical actuators. Central to this approach is the decoupling of photochemical function from mechanical integrity: CP1 crystals provide precise, directional photochemistry, while the PVA matrix and PCL coating ensure stress transfer, durability, and environmental stability.

In the broader landscape of photoactuating materials (Table 1), the (CP1‐PVA)@PCL composites occupy a previously underexplored regime. Unlike soft polymeric or photothermal systems, which achieve large strain at low modulus, or brittle crystalline actuators that exhibit high structural order but limited deformability, these hybrids combine: (i) directional, lattice‐confined photochemistry from crystalline CPs, (ii) load‐bearing capacity and macroscopic strain amplification through polymer matrices, (iii) long‐term stability under prolonged ionic aqueous exposure enabled by hierarchical PCL coating, and (iv) the time‐governed programmable arising from the finite kinetics of crystalline photo‐transformation. The retention of structural and mechanical integrity under prolonged ionic aqueous conditions indicates potential robustness in chemically complex environments beyond ambient settings. These attributes position the system at the intersection of structural precision, programmable actuation, and mechanical resilience. From a conceptual standpoint, four generalizable design principles emerge:

1. Topochemical preorganization. Reactive ligands within CP1 are aligned to enable rapid, quantitative [2+2] cycloaddition, translating molecular‐scale chemistry into directional mechanical strain.

2. Hierarchical stress transfer. Embedding photoactive crystals within a load‐bearing polymer matrix amplifies lattice‐level strain across macroscopic dimensions without catastrophic fracture, demonstrating how microstructure governs bulk mechanical function.

3. Environmental decoupling through coating. Control experiments comparing CP1, CP1‐PVA, and (CP1‐PVA)@PCL demonstrate that the PVA matrix is primarily responsible for strain transfer and mechanical amplification, whereas the PCL layer functions as a protective barrier that preserves these properties during prolonged aqueous exposure. A mechanically compatible, hydrophobic PCL layer preserves photomechanical fidelity while protecting against degradation, illustrating architectural separation of functional and protective roles.

4. Expansion‐programmable actuation. The finite timescale of photoinduced structural transformation enables stepwise deformation through control of irradiation duration, establishing temporal kinetics as an orthogonal design parameter. Although the available evidence supports a photochemically driven strain‐accumulation mechanism, direct spatially resolved measurements of local photoconversion remain necessary to establish a quantitative relationship between reaction progression and staged mechanical output. Future studies combining operando spectroscopic and structural characterization will be valuable for resolving this relationship.

These principles define a materials‐centered framework for integrating crystalline photochemistry into mechanically resilient and environmentally stable architectures, independent of specific device geometries. Importantly, the hierarchical strategy is not restricted to a particular metal center or ligand topology, but instead relies on general features of anisotropic lattice reactivity and polymer‐mediated stress transfer, suggesting adaptability to other crystalline photochemical systems. Coordination polymers offer chemically tunable lattice structures, enabling rational control over photoreactivity, anisotropic strain generation, and mechanical output. When combined with appropriate polymer architectures, this approach bridges the long‐standing gap between the precision of molecular crystals and the robustness of bulk soft materials.

An important characteristic of the present system is that its actuation is irreversible. Unlike photoactuators that rely on reversible molecular isomerization or thermally recoverable phase transitions, the mechanical output here originates from a topochemical [2+2] cycloaddition that permanently alters the molecular structure. The resulting loss of long‐range crystallinity further limits recovery of the initial state. Consequently, the present architecture is best suited for applications in which actuation precision, structural stability, and long‐term retention of the deployed state are prioritized over cyclic reversibility.

Future efforts will focus on expanding photoresponsive coordination polymer libraries, optimizing interfacial architectures to maximize strain transmission efficiency, and developing light‐delivery strategies suitable for complex operational environments. Equally important will be quantitative evaluation of actuator performance, including force generation, stall force, cyclic load‐bearing capacity, and the influence of mechanically induced chain orientation and pre‐strain on photochemical strain transfer. Such studies will help establish quantitative relationships among photochemical conversion, material structure, deformation mode, and mechanical output, providing a foundation for predictive actuator design. Collectively, these insights establish a conceptually rigorous pathway for translating lattice‐level photochemistry into durable macroscopic actuation and provide a framework for the development of next‐generation adaptive materials beyond the specific geometries demonstrated here.

4. Experimental Section

4.1. General Materials and Characterization

All reagents and solvents were purchased from commercial suppliers and used as received unless otherwise noted. Elemental analyses (C, H, and N) were performed using a Carlo Erba CHNO‐S microanalyzer. Powder x‐ray diffraction (PXRD) patterns were collected on a PANalytical X'Pert PRO MPD diffractometer (PW3040/60) using Cu Kα radiation (λ = 1.5406 Å) over 2θ = 3–50° with a step size of 0.02°. Thermogravimetric analysis (TGA) was carried out on a Mettler Toledo STAR system under nitrogen at a heating rate of 10°C min⁻¹. UV–vis absorption spectra were recorded using a Varian Cary 50 spectrophotometer. Atomic force microscopy (AFM) measurements were performed on a Bruker Dimension Icon system operated in ScanAsyst mode, with data processed using NanoScope Analysis v1.5 (Bruker Software, Inc.). Scanning electron microscopy (SEM) images were obtained on a Hitachi S‐4700 cold field‐emission microscope operated at 15 kV. Optical irradiance was measured using a Thorlabs PM100D power meter equipped with an S425C photodiode sensor [57].

4.2. Synthesis of tpeb

The ligand 1,3,5‐tri‐4‐pyridyl‐1,2‐ethenylbenzene (tpeb) was prepared according to the literature method [58]. Anal. calcd for C₂₇H₂₁N₃: C 83.69, H 5.46, N 10.84. Found: C 83.21, H 5.13, N 10.65. ¹H NMR (400 MHz, DMSO‐d ₆): δ 8.95 (dd, 6H), 8.29 (dd, 6H), 8.23 (s, 3H), 8.12 (d, 3H), 7.74 (d, 3H) (Figure S1).

4.3. Synthesis of CP1

ZnSO₄·7H₂O (86.3 mg, 0.24 mmol) and tpeb (23.2 mg, 0.06 mmol) were dissolved in a mixed solvent of acetonitrile (5.4 mL) and deionized water (9.0 mL) in a 15 mL pressure‐resistant thick‐walled vial. The pH was adjusted to approximately 5.0 using 0.1 M HNO₃. The mixture was sonicated for 15 min, sealed, and heated at 150°C for 24 h under static solvothermal conditions. After slow cooling to room temperature (5°C·h⁻¹), pale yellow needle‐like crystals of CP1 were collected by filtration, washed with anhydrous ethanol, and vacuum‐dried. Yield: 32.9 mg (80.6%, based on tpeb). Elemental analysis for C₃₁H₃₁N₃O₆Zn: Calcd (%): C, 61.39, H, 5.12, N, 6.93. Found (%): C, 61.56, H, 5.22, N, 6.98.

4.4. Photochemical Conversion of CP1 to CP1a

Crystalline CP1 powder was evenly spread on a glass substrate and irradiated with 365 nm UV light at room temperature for 90 min, affording a uniform dark yellow powder (CP1a). Elemental analysis confirms identical composition to CP1, consistent with topochemical photodimerization rather than decomposition.

4.5. Isolation of Organic Photoproduct

CP1 (0.5 g) was irradiated at 365 nm for 90 min and subsequently treated with 2 M HNO₃ (20 mL) under stirring at room temperature for 24 h to decompose the coordination framework. The resulting solid was filtered, washed with chloroform (3 × 10 mL), and vacuum‐dried to yield the photodimerized tpeb derivative in 51.2% yield (based on tpeb).

4.6. Fabrication of CP1‐PVA Composite Films

CP1 crystals were ball‐milled for 30 min, dispersed in ethanol (5 mL), ultrasonicated for 5 h, and dried at 40°C. The resulting powder was added to 10 g of a 7.7 wt% aqueous poly(vinyl alcohol) (PVA) solution and stirred for 12 h to obtain a homogeneous suspension. The mixture was cast onto a pre‐cleaned polyethylene terephthalate (PET) substrate within a polytetrafluoroethylene (PTFE) mold and dried at 40°C for 24 h to yield flexible CP1‐PVA composite films.

4.7. Preparation of (CP1‐PVA)@PCL Composite Films

A 1 wt% solution of PCL in chloroform was brush‐coated onto CP1‐PVA films. After each coating cycle, films were dried at 40°C to remove residual solvent. The coating process was repeated until a continuous hydrophobic PCL layer was formed. The resulting (CP1‐PVA)@PCL films were stored under ambient conditions prior to testing.

4.8. Preparation of Physiological Saline Aqueous Solution

Physiological saline was prepared by dissolving NaCl (0.9 g) in deionized water (100 mL). The pH was adjusted to 7.4 using tris–HCl buffer. All immersion experiments were conducted at 37°C.

4.9. Fabrication of Tubular Actuators

Composite films were cut into rectangular strips, folded longitudinally, and patterned with predefined parallel slit arrays along the folded edge. A margin of 0.3 mm was retained on both sides of the slit region to maintain mechanical integrity. Films were unfolded, wrapped around glass rods of defined diameters, and the overlapping edges were fixed using adhesive tape to form cylindrical tubular actuators. Detailed dimensions are provided below.

8 mm actuator: 26 × 40 mm² film; ten slits (2.6 mm width); final length 40 mm.

4 mm actuator: 13 × 40 mm² film; ten slits (1.3 mm width).

2 mm actuator: 7 × 35 mm² film; ten slits (0.7 mm width).

1 mm actuator: 3.5 × 35 mm² film; five slits (0.7 mm width).

4.10. Backlit Irradiation Method for Tubular Actuators

Tubular actuators mounted on glass rods were irradiated from below using a UV light source. Continuous rotation ensured uniform illumination. Radial expansion was controlled by irradiation wavelength and exposure time.

[CCDC 2444991 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.]

Funding

The authors acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. U24A20507, 22271203, and 22403052), the State Key Laboratory of Organometallic Chemistry of Shanghai Institute of Organic Chemistry (2024KF005), Collaborative Innovation Center of Suzhou Nano Science and Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Project of Scientific and Technologic Infrastructure of Suzhou (Grant No. SZS201905).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: adma74238‐sup‐0001‐SuppMat.doc.

Supporting File 2: adma74238‐sup‐0002‐VideoS1‐S18.zip.

Data Availability Statement

The data that support the findings of this work are available from the corresponding author upon request.