Solvent‐Assisted Preparation of Recyclable Multidimensional Fluorescent Actuators

ABSTRACT

Light‐responsive elastic crystals and polymers are widely used in the fabrication of smart actuators. However, crystal morphology, which is strongly dependent on the molecular self‐assembly, remains difficult to control, while the performance of polymers is significantly affected by their components, limiting their practical applications. To balance the advantages of crystal‐ and polymer‐based materials, in this study, solvent molecules were doped into small‐molecule photosensitive materials (E‐BI‐TPA‐CS) to weaken the intermolecular interactions and enable the precise, facile, and fast preparation of flexible multidimensional photoactuators through simple artificial manipulations such as stretching. The as‐prepared actuator combines the high energy density of crystalline materials with the tailorable properties of polymers. Upon UV or visible light irradiation, E‐BI‐TPA‐CS undergoes E‐Z isomerization, driving actuator deformation. The reversible isomerization and simple fabrication process make the actuators recyclable. Additionally, these photoresponsive materials can adhere to metal surfaces to facilitate the construction of photoswitches without complex operations, such as metal deposition. Furthermore, the introduction of the aggregation‐induced emission (AIE) unit enables visual tracking of the actuation process. This resin‐like material offers a new perspective on multifunctional materials by effectively bridging the gap between crystalline and polymeric systems, optimizing the preparation of actuators, and improving their performance.

In this work, an AIE‐active D‐π‐A molecule was synthesized to construct recyclable fluorescence actuators. With the help of solvent molecules, 1D fiber, 2D film and 3D architecture were facilely, rapidly and precisely prepared and exhibited flexible deformation driven by UV/vis light induced E‐Z isomerization.

1. Introduction

Photoresponsive smart materials, capable of altering their luminescence [1, 2, 3, 4, 5] and morphology [6, 7] through photochemical reactions, hold revolutionary potential for flexible optoelectronic devices, actuators [8, 9], artificial muscles [10, 11], and soft robotics [12]. Compared to other photochemical actuators, such as polymers [13, 14], liquid crystal elastomers [15, 16, 17], and gels [18], small‐molecule photoactuating materials, particularly crystalline ones, have been extensively studied because of their tightly packed molecular structures, high energy transfer efficiency, and rapid response speed (Figure 1A). However, the inherent brittleness and poor fatigue resistance of crystalline materials significantly limit their practical applications. Although recent advances [19, 20, 21, 22] have been made to modulate molecular flexibility through strategies such as substituent engineering [23, 24], the introduction of long alkyl chains [25], co‐crystal design [26, 27], and the incorporation of solvent molecules [2, 28] as plasticizers, challenges remain in precisely controlling the morphology and size of the crystals [29, 30], thereby limiting their broader applicability.

FIGURE 1.

Advantages and drawbacks of (A) crystalline materials and (B) polymers. (C) Molecular design strategy for E‐BI‐TPA‐CS. (D) As‐prepared 1D, 2D, and 3D actuators and their performance. (E) Schematic of the preparation and bending mechanism of the fibers.

Previous reports indicate that varying parameters such as solvent, temperature, and the rate of solvent evaporation can yield crystals with different morphologies [31]. However, crystals growth relies heavily on molecular self‐assembly [32], which introduces unpredictability regarding their size, morphology, and photoinduced motion. Moreover, the growth of target crystals is often a time‐consuming process with a low success rate. Research on polymers has demonstrated that it is possible to develop photoactuators with controllable deformation and tailorable morphologies by employing pre‐stretching treatments [33] and cutting processes [34]. However, significant challenges remain regarding small‐molecule materials. To fabricate programmable actuators, a recent report [35] demonstrated the integration of crystals with polymers to create photomechanical actuators that, not only overcome the inherent brittleness of crystalline materials, but also retain the high energy density and rapid response speed of crystals while incorporating the excellent operability of polymers. However, these systems often rely on intricate and demanding fabrication processes. Therefore, there is an urgent need for a simple and convenient method to process small‐molecule materials for the precise, facile, and fast fabrication of flexible actuators with desired morphologies.

Compared to crystalline materials, polymers maintain their integrity during processing because of their abundant interchain interactions, which serve as robust yet reversible crosslinks and sacrificial bonds that facilitate significant energy dissipation and recyclability [36]. Although numerous intermolecular interactions in crystalline materials enable crystal elasticity, these crystals cannot be processed via stretching in the same manner as polymers. This study aims to incorporate solvent molecules into organic small‐molecule materials to weaken rigid intermolecular interactions, thereby increasing the number of reversible crosslinks and sacrificial bonds and enabling these materials to be processed like polymers.

Photoactuators often suffer from aggregation‐caused quenching or photobleaching, which hinders visual monitoring of their actuation. Fluorescent actuators with balanced fluorescence properties and actuation performance are promising for bionics and soft robotics [37]. Recent advances in aggregation‐induced emission luminogens (AIEgens) have provided new opportunities [38, 39, 40]. In solution, AIEgens easily reach conical intersections (CIs) owing to intramolecular motion in the excited state [41], causing excited‐state energy to return to the ground state through non‐radiative transitions, which quenches or weakens fluorescence. In contrast, in the solid state, restricted intramolecular motion (RIM), especially the restriction of double‐bond rotation, inhibits CI crossing, ensuring a high fluorescence quantum yield. Therefore, AIEgens are suitable for the preparation and study of fluorescent actuators. By introducing multiple rotors into the molecular framework, the steric hindrance of the molecule can be increased, ensuring fewer π–π interactions and sufficient space for the molecule to adjust its structure in the excited state. The introduction of bulky steric units, such as triphenylamine units, disrupts effective molecular conjugation. This results in minimal differences in photophysical properties before and after isomerization, thereby preventing photobleaching. Moreover, weak interactions, such as C–H···π interactions and H–H interactions, can be introduced to ensure the processibility of the materials.

Cyanostilbenes exhibit excellent photoactivity and high fluorescence quantum yields [42] and are widely used in the development of AIEgens and photoactuators [43]. The cyano unit can also act as a hydrogen‐bond acceptor, facilitating the formation of an intermolecular network. Here, through ingenious molecular design, we introduced triphenylamine and phenyl‐benzimidazole as substituents of cyanostilbene to selectively synthesize a donor (D)‐π‐acceptor (A) structured molecule, E‐BI‐TPA‐CS (Figure 1C). This molecule exhibits both twisted intramolecular charge transfer (TICT) and AIE characteristics [44] and undergoes reversible E‐Z isomerization in solution. With the help of solvent molecules, we achieved the facile fabrication of photoresponsive actuators spanning 1D fibers, 2D films, and 3D architectures (Figure 1D). Driven by E‐Z isomerization, the as‐prepared 1D, 2D, and 3D actuators exhibited bending, curling, and contracting behaviors. By capitalizing on the intrinsic adhesion of E‐BI‐TPA‐CS, we demonstrated that it can be directly coated onto metallic substrates to construct hybrid heterostructure actuators, as exemplified by a light‐gated circuit switch. These resin‐like materials bridge the gap between crystalline and polymeric actuators, offering innovative approaches to the efficient and straightforward production of high‐performance actuators.

2. Results and Discussion

2.1. Synthesis and Characterization

Multiple rotors were incorporated into the phenylbenzimidazole‐substituted cyanostilbene skeleton to form a D‐π‐A molecule with AIE characteristics [45]. The synthetic pathway for E‐BI‐TPA‐CS is illustrated in Scheme S1. Initially, precursor 3 was synthesized via the reaction between N‐phenyl‐1,2‐phenylenediamine and 2‐cyanoacetamide. This precursor was then subjected to a Knoevenagel condensation with 4‐(diphenylamino) benzaldehyde to give E‐BI‐TPA‐CS in a 57% yield. The molecular structures of E‐BI‐TPA‐CS and all intermediates were characterized by nuclear magnetic resonance (NMR), high‐resolution mass spectrometry (HRMS), and single‐crystal x‐ray diffraction analysis (Figures S1–S6 and Table S2).

2.2. Photophysical Properties

The absorption and emission spectra were measured to investigate the photophysical properties of E‐BI‐TPA‐CS in solution and the solid state. As shown in Figures 2A, E‐BI‐TPA‐CS exhibited two absorption peaks in dimethyl sulfoxide (DMSO). The absorption peak at 291 nm originated from the π–π* transition, while the peak at 411 nm was attributed to intramolecular charge transfer (ICT) absorption [44]. Compared to E‐CN‐BIFPh, which was reported in our earlier work [45] with a narrow Stokes shift of 20 nm and a full width at half maximum (FWHM) of 98 nm (Figure S7), E‐BI‐TPA‐CS displayed a larger Stokes shift of 163 nm, a large FWHM of 112 nm, and a broad emission band at 574 nm in DMSO. As the polarity of the solvent increased, the absorption band of E‐BI‐TPA‐CS shifted from 395 nm in n‐hexane to 423 nm in acetonitrile, and the emission peak shifted from 474 nm in n‐hexane to 575 nm in DMSO (Figure 2B,C), indicating the existence of the TICT effect. When water was added to the DMSO solution, the emission intensity initially decreased, accompanied by a redshift in the emission peak owing to the increased polarity of the solution, which facilitated the TICT effect (Figure 2D). As the water content further increased, the emission intensity gradually increased owing to aggregate formation and RIM facilitated by abundant intermolecular interactions; this inhibited the TICT process, leading to a blueshift of the emission band and confirming typical AIE properties (Figure 2E,F). To investigate aggregate formation, dynamic light scattering (DLS) measurements were performed at various water content levels. As shown in Figure S8, when the water fraction reached 60%, aggregates began to form with a size of 266 nm. A further increase in the water fraction led to a decrease in the nanoparticle size. Owing to the strong intermolecular interactions in the crystal, the absorption peaks of E‐BI‐TPA‐CS in the solid state (Figure S12) were slightly redshifted to 304 and 453 nm, whereas the emission peak barely changed compared to its behavior in DMSO.

FIGURE 2.

(A) Absorption and emission spectra of E‐BI‐TPA‐CS in DMSO (concentration: 10⁻⁵ M; λ _ex = 365 nm). (B) Normalized absorption and (C) emission spectra of E‐BI‐TPA‐CS in different solvents. Abbreviations: Hex = n‐hexane, Tol = toluene, THF = tetrahydrofuran, ACN = acetonitrile, DMF = dimethylformamide, and DMSO = dimethylsulfoxide. (D) Emission spectra (λ _ex = 410 nm) and (E) fluorescence images of E‐BI‐TPA‐CS in DMSO/H₂O mixtures with different water fractions (f _w). (F) Plots of relative emission intensity (α _AIE, α _AIE = I/I ₀) and wavelength versus the composition of the DMSO/H₂O mixtures.

2.3. Investigation of Driving Forces

Most light‐responsive actuators derive their driving force from the stress generated by structural changes that occur during the chemical reactions of molecular motors [46]. Therefore, it is essential to investigate the ability of a molecule to undergo a chemical reaction, the specific type of reaction it facilitates, and the efficiency of the process. Given that, the H‐H COSY (600 MHz) and time‐dependent ¹H NMR spectra were recorded in DMSO‐d ₆ (Figures S13 and S14). As illustrated in Figure 3A, the signals for H_c, H_d, H_e, H_a, and H_b in E‐BI‐TPA‐CS appeared at 7.75, 7.64, 7.19, 7.16, and 6.86 ppm, respectively. Upon UV irradiation, these signals gradually decreased, and new signals emerged within the pink box at 7.73, 7.52, 7.49, 7.07, and 6.62 ppm. After separation and purification, the photoreaction products exhibited identical mass‐to‐charge ratios (Figure 3C). By combining the ¹H and ¹³C NMR spectra (Figures S9 and S10), HRMS (Figures S6 and S11), and single‐crystal XRD analysis (Figure 3C), we concluded that E‐BI‐TPA‐CS underwent photoisomerization in DMSO‐d ₆ to yield its Z‐isomer (Z‐BI‐TPA‐CS). Based on the integration of H_c and H_c’ in the time‐dependent ¹H NMR spectra (Figure S14), the isomerization conversion ratio (%) versus irradiation time is plotted in Figure 3B. After 60 min of UV irradiation, the isomerization conversion ratio reached a plateau of 39%, referred to as the first plateau. Upon prolonged UV irradiation, the absorption band of E‐BI‐TPA‐CS in acetonitrile blueshifted from 424 to 415 nm (Figure 3D). The blueshift was derived from the interruption of conjugation caused by the dihedral angle change from 18.1 to 60.41° (Figure 3C). However, the emission intensity of E‐BI‐TPA‐CS in dilute acetonitrile was slightly enhanced after 365 nm light irradiation without an obvious blueshift (Figure S15), which ensured that the fluorescence of the actuator was not quenched during the actuation process. Interestingly, this plateau was also achieved upon irradiation of an acetonitrile solution of Z‐BI‐TPA‐CS with UV light (Figure S16). Notably, the low isomerization conversion at the plateau was related to the relatively high absorbance of Z‐BI‐TPA‐CS at 365 nm compared with that of E‐BI‐TPA‐CS.

FIGURE 3.

(A) ¹H NMR spectra of E‐BI‐TPA‐CS in DMSO‐d ₆ before (black) and after (blue) 365 nm UV irradiation (60 mW cm⁻²) for 60 min. The red spectrum represents E‐BI‐TPA‐CS in the solid state after 365 nm UV irradiation for 60 min, followed by dissolution in DMSO‐d ₆. (B) Photoconversion from E‐BI‐TPA‐CS to Z‐BI‐TPA‐CS at different time points evaluated from the ¹H NMR spectra in Figure S14. (C) HPLC spectra of E‐BI‐TPA‐CS in acetonitrile before and after 365 nm UV irradiation for 1, 5, and 30 min. The HPLC spectrum of E‐BI‐TPA‐CS in the solid state after 365 nm UV irradiation for 20 min. The molecular configuration of E‐BI‐TPA‐CS (left) and Z‐BI‐TPA‐CS (right) in the single crystal state. (D) Absorption spectra of E‐BI‐TPA‐CS in acetonitrile before and after 365 nm UV irradiation (60 mW cm⁻²) at different times.

The ¹H NMR spectra of E‐BI‐TPA‐CS in the solid state after irradiation with 365 nm light (60 mW cm⁻²) for 60 min, followed by dissolution in DMSO‐d ₆, were also recorded. After irradiation, new peaks appeared at the same positions as those observed after the direct irradiation of the solution (Figures 3A and S17), indicating that the same reaction occurred in both the solid state and solution phase. After the separation and purification of the Z‐isomer, its photophysical properties were studied (Figure S18). With an increase in solvent polarity, Z‐BI‐TPA‐CS exhibited a typical redshifted emission, similar to that of E‐BI‐TPA‐CS, which is consistent with its TICT properties. Notably, the Z‐BI‐TPA‐CS powder exhibited a relatively blueshifted emission (540 nm) compared to the E‐BI‐TPA‐CS powder (575 nm).

Theoretical calculations were also performed to gain further insight into this isomerization. As depicted in Figure 4A, the highest occupied molecular orbital (HOMO) of E‐BI‐TPA‐CS is delocalized across the entire molecule, whereas the lowest unoccupied molecular orbital (LUMO) is primarily localized on the cyano double bond, indicating a hybrid of local excitation (LE) and charge transfer (CT) states. To further confirm the formation of this hybrid state, the hole and electron distributions of BI‐TPA‐CS in the three forms during isomerization were calculated; the results are summarized in Figure S19. The holes in the E‐form are concentrated on the TPA unit, whereas the electrons are primarily distributed in the cyanovinylene moiety. The pronounced spatial separation between the hole and electron distributions provides clear evidence of the CT character. Moreover, a distinct overlap between the holes and electrons is observed at the middle phenyl ring, indicating the presence of an LE state. These observations collectively confirm that the excited state of the E‐form exhibits a hybridized character comprising both LE and CT features. Because of the donor–π–acceptor electronic effects, the energy gap between the HOMO and LUMO of E‐BI‐TPA‐CS was found to be 3.00 eV. When the planar conjugation is broken, the HOMO of Z‐BI‐TPA‐CS is mainly localized on the triphenylamine group with an energy gap of 3.16 eV, which is consistent with the change observed in the absorption spectra. Additionally, time‐dependent density functional theory (TD‐DFT) calculations were performed to explore the energy and configuration changes of the molecules upon excitation. As illustrated in Figure 4B,C, E‐BI‐TPA‐CS underwent structural relaxation upon excitation. When the dihedral angle (C1‐C2‐C3‐C4) rotated from 179.43 to 86.99°, the double bond weakened, forming a transient state (Ts*). Subsequently, the excited transition state underwent vibrational relaxation and returned to the ground state, followed by rotation around a single bond, leading back to E‐BI‐TPA‐CS or yielding Z‐BI‐TPA‐CS. The change in molecular configuration produced during isomerization provides the driving force for the subsequent photomechanical deformation of the actuator. Calculations revealed that Z‐BI‐TPA‐CS exhibited a relatively higher free energy (0.09 eV) than E‐BI‐TPA‐CS, which allowed for the selective synthesis of E‐BI‐TPA‐CS. Nevertheless, the higher free energy indicates that Z‐BI‐TPA‐CS is more sensitive to light. As shown in Figure S20, with continuous 365 nm irradiation, signals belonging to the E‐isomer appeared at 7.78, 7.76, 7.72–7.58, 7.23–7.19, 6.88, and 6.86 ppm, indicating that Z‐E isomerization occurred. After 15 min of irradiation, the isomerization conversion fraction exceeded 40%, which was much faster than E‐Z isomerization, indicating lower photostability of the Z‐isomer compared to the E‐isomer.

FIGURE 4.

(A) HOMO and LUMO of E‐BI‐TPA‐CS and Z‐BI‐TPA‐CS calculated using the Gaussian package at the B3LYP/6‐311 g (d, p) level. (B) Calculated isomerization process from E‐BI‐TPA‐CS to Z‐BI‐TPA‐CS in acetonitrile. (C) Dihedral angle (C1‐C2‐C3‐C4) in the E‐form, transition state (Ts*), and Z‐BI‐TPA‐CS. (D) Visual intermolecular interactions calculated based on the π‐dimers in the crystalline state. (E) Hirshfeld surface and (F) 2D fingerprint plots of E‐BI‐TPA‐CS in the crystalline state. Energy frameworks corresponding to the (G) Coulomb, (H) dispersion, and (I) total interaction energy components in E‐BI‐TPA‐CS crystals.

2.4. Intermolecular Interactions in Crystals

To gain an intuitive understanding of the conformation and intermolecular interactions of E‐BI‐TPA‐CS, its crystal structure was analyzed. As shown in Table S2, the E‐BI‐TPA‐CS crystal (CCDC: 2403633) [47] belongs to the monoclinic crystal system with a P2₁/n space group containing four molecules per unit cell. The double bond and the two directly connected aromatic rings are essentially coplanar, with a dihedral angle (θ ₂) of only 18.1° (Figure 5A). Notably, an intramolecular C─H···π interaction (d ₁ = 2.395 Å) results in a large dihedral angle (θ ₁ = 72.8°) between the benzimidazole unit and the adjacent phenyl ring. Triphenylamine, a typical AIE moiety, exhibits significant dihedral angles between its phenyl rings (θ ₃ = 72.51° and θ ₄ = 63.04°). This steric hindrance leads to weak π–π interactions (d ₃ = 3.458 Å) between adjacent molecules. Additionally, C─H···N (d ₂ = 2.501 Å), C─H···π interactions (d ₇ = 2.678 Å), and numerous H─H interactions (d ₄, d ₅, and d ₆) occur between neighboring molecules. Such a twisted structure and the high density of intermolecular interactions endow E‐BI‐TPA‐CS with its AIE properties. Viewed along the b‐axis, molecules color‐coded similarly pair to form building blocks that stack in a layered arrangement within the ac plane.

FIGURE 5.

Single crystal structures of (A, B, and C) E‐BI‐TPA‐CS and (D, E, and F) Z‐BI‐TPA‐CS, viewed from different directions.

Theoretical calculations further revealed the presence of π–π interactions and hydrogen bonds between the molecules, as illustrated by the green regions in Figure 4D. The Hirshfeld surface (Figure 4E) exhibited multiple red spots, suggesting the presence of various interaction sites among the molecules. The 2D fingerprint plot demonstrates the presence of C─H···N, C─H···π, and H···H interactions, accounting for 12.2%, 30.6%, and 51.8% of the total interactions, respectively (Figure 4F). To quantify the magnitude of the intermolecular interactions, the CrystalExplorer17 [48] software was used to calculate the intermolecular interaction energies in the E‐BI‐TPA‐CS crystal using the B3LYP/6‐31G(d, p) basis set. The results, displayed in Figures 4G–I, and S21, categorize the total interaction energy into electrostatic, polarization, dispersion, and repulsion components, calculated according to the following formula:

$$E_{tot}=k_{ele}{E}_{ele}+k_{pol}{E}_{pol}+k_{dis}{E}_{dis}+k_{rep}{E}_{rep}$$

Where, k represents the optimum scale factors for the benchmark energy models. The total interaction energy between the two nearest molecules was calculated to be −87.6 kJ mol⁻ [1]. These intermolecular interactions are sufficiently strong that the E‐BI‐TPA‐CS crystal resists plastic deformation under external forces.

2.5. Multi‐Dimensional Macroscopic Photoactuator Performance

Upon concentrating the ethyl acetate solution of E‐BI‐TPA‐CS, the compound exhibited a resin‐like consistency. By employing a drop‐casting procedure [49], the material was fabricated into one‐dimensional fibers, as shown in Figure 6A. Scanning electron microscopy (SEM) images revealed that the surfaces of the as‐prepared fibers were smooth, with a diameter of approximately 8 µm. Under the application of an external force, the fibers bent beyond 180° without fracturing. The SEM images indicated that no cracks appeared in the bent regions, demonstrating the excellent flexibility of the fibers. As illustrated in Figure 6B, the bent fiber showed a large bending curvature of 323 cm⁻¹, further confirming its remarkable flexibility.

FIGURE 6.

(A) Image of an E‐BI‐TPA‐CS fiber prepared using the drop‐casting procedure. Inset: Image of E‐BI‐TPA‐CS fiber after bending under an external force. The SEM images (right and left) correspond to the original and bent fibers, respectively. (B) SEM image of a curved fiber of E‐BI‐TPA‐CS. (C) Polarized optical micrograph (POM) of an E‐BI‐TPA‐CS fiber. Abbreviation: A = analyzer and P = polarizer. (D) Photomechanical motion of an E‐BI‐TPA‐CS fiber under 365 nm UV irradiation (60 mW cm⁻²) and an SEM image of the deformed fiber. The blue arrows indicate the irradiation direction; the white solid line represents the original position before irradiation. (E) Calculated molecular dimensions of E‐BI‐TPA‐CS and Z‐BI‐TPA‐CS in the crystalline state. (F) Photomechanical motion of fibers prepared from recycled materials. (G) Photomechanical motion of 2D films and (H) 3D architectures of E‐BI‐TPA‐CS under 365 nm UV irradiation (60 mW cm⁻²). The blue arrow indicates the irradiation direction, and the red arrow indicates the stretching direction. The schematic illustration of the (I) separated smart switch and (K) smart switch in circuit under 365 nm UV irradiation. (J) Photomechanical motion of aluminum foil (6 µm) coated with E‐BI‐TPA‐CS under 365 nm UV irradiation (60 mW cm⁻²). (L) Application of the photo‐driven switch in an electrical circuit.

Polarized optical microscopy (POM) was conducted to further investigate the effect of stretching on molecular packing within the fiber. As shown in Figure 6C, the fiber exhibited anisotropy when the angle between the fiber stretching direction and the polarized light was altered, indicating that the molecular packing within the fiber became more ordered. To better understand how external forces influence the molecular alignment in the aggregates, the fiber was exposed to UV irradiation (60 mW cm⁻², Figure 6D and Video S1). When the UV lamp was turned on, the fiber exhibited bright‐yellow fluorescence. During the initial 0.8 s, the fiber bent rapidly away from the light source. With continued irradiation, the fiber bent toward the light source. Within 26 s, the bending angle reached 78°, corresponding to an average bending speed of 3.1°/s. This fiber‐like fluorescent actuator exhibited strong fluorescence and excellent actuation performance. The SEM images of the bent region revealed a smooth surface without any cracks, confirming that the fiber underwent ductile bending under UV irradiation. Upon altering the direction of the light irradiation, the fiber first returned to its initial state and subsequently bent toward the light source. The photomechanical motion of fibers with different thicknesses was also studied (Figure S22). When exposed to 365 nm light, the thicker fiber (∼ 20 µm) exhibited a relatively slow bending speed (0.9°/s). However, the thinner fiber (∼ 1 µm) showed a faster bending speed of 9.6°/s, which is comparable to that of crystalline actuators. Therefore, similar to a crystalline actuator, the fiber exhibits size‐dependent photomechanical motion [12, 50, 51].

The photothermal properties of E‐BI‐TPA‐CS were examined to investigate the bending mechanism of the fibers. Because of the time‐consuming nature of fabricating large quantities of one‐dimensional fibers, two‐dimensional thin films (described later) were used as substitutes. Before UV irradiation, the surface temperature of the thin film was 20.3° C (Figure S23A). After 2 s of irradiation, the temperature increased by 6.6 °C, reaching a maximum of 32.6 °C at 11 s. With prolonged irradiation, the temperature stabilized without further increase. Combined with the differential scanning calorimetry (DSC) curve (Figure S23B), no phase transition peaks were observed in the 20–35 °C range, indicating that the temperature change induced by UV irradiation did not cause a phase transition. Therefore, at the initial stage of UV irradiation, the temperature increase on the irradiated surface caused thermal expansion, whereas the non‐irradiated side remained unchanged. This differential expansion led to the bending of the fiber away from the light source [52]. To prove this, the thermal effect on fiber bending was investigated. As shown in Figure S24, when the heating platform was brought close to the fiber, it bent away from the heat source owing to the differential thermal expansion. Subsequently, the fiber returned to its original shape as uniform heating eliminated the thermal gradient. Therefore, the initial photoinduced bending away from the light source was caused by the photothermal effect.

The endothermic peak observed at approximately 66° C in the DSC curve (Figure S23B), along with the mass loss in the thermogravimetric analysis (TGA) curve (Figure S23C), indicates the presence of ethyl acetate molecules in the material. To determine the role of these molecules, an E‐BI‐TPA‐CS molecule with four ethyl acetate molecules was optimized using the B3LYP functional and the 6–311G(d, p) basis set. As indicated by the green regions in Figure S25, E‐BI‐TPA‐CS formed multiple hydrogen bonds with ethyl acetate. The presence of these solvent molecules weakened the intermolecular interactions among E‐BI‐TPA‐CS, enabling the resin‐like material to be easily stretched into fibers. To verify whether the subsequent bending toward the light source originated from photothermally induced solvent loss, fibers were prepared directly from the molten crystalline state of E‐BI‐TPA‐CS. By heating the crystals to 170° C and maintaining the temperature between 150 and 160 °C, fibers were smoothly drawn using the drop‐casting procedure (Figure S26C) because the intermolecular interaction can be easily broken at such high temperature. When these solvent‐free fibers were exposed to UV light, photoresponsive bending was observed, similar to the solution‐drawn fibers. Because of the absence of solvent, it can be concluded that the intrinsic driving force for the photoresponsive bending was the E‐Z isomerization of E‐BI‐TPA‐CS, independent of residual solvent molecules.

Unlike the fibers fabricated via the solvent‐assisted drop‐casting procedure, the bending rate of the solvent‐free fiber was only 3.6°/min. To explore the difference in bending rates between the two types of fibers, ¹H NMR spectra of E‐BI‐TPA‐CS crystals were analyzed after UV irradiation. As shown in Figure S26A, the ¹H NMR spectral changes observed after 30 min of UV irradiation of the crystals were consistent with those observed in solution. The integration of the ¹H NMR signals indicated that the conversion after 30 min of irradiation was only 7%. The low isomerization conversion in the crystalline state is attributed to the densely packed structure, which restricts the rotation of the double bond. In contrast, the solvent molecules in the fibers produced via drop‐casting acted as plasticizers, increasing intermolecular spacing and providing sufficient space for isomerization, thereby enhancing the conversion.

To explore the relationship between E‐Z isomerization and photoinduced bending of the fiber toward the light source, the crystal structure of Z‐BI‐TPA‐CS (CCDC: 2403634) [47] was analyzed. As shown in Table S2 and Figures 5, Z ‐BI‐TPA‐CS crystallized in a triclinic system with a P‐1 space group, containing two molecules per unit cell. Within the molecule, weak π–π interactions (d ₈ = 2.562 Å) and H─H interactions (d ₉ = 2.863 Å) help maintain the conformation. Intriguingly, despite the absence of chiral centers, adjacent molecules adopted enantiomeric configurations owing to steric constraints (Figure S27). Adjacent enantiomers stack together through C─H···N interactions (2.475 Å). Along the a‐axis (Figure 5F), the two enantiomers connect head‐to‐tail to form a pore‐like motif with n‐hexane molecules residing within the pores. These motifs assemble via C─H···N (2.475 Å) and H···H (2.562, 2.794 Å) interactions. Molecular dimensions (Figure 6E) derived from the crystal structures of E‐BI‐TPA‐CS and Z‐BI‐TPA‐CS reveal that the E‐isomer measures 18.47 Å (length), 8.81 Å (width), and 10.90 Å (height). Isomerization contracted the molecular long axis to 14.58 Å while expanding the width and height to 10.94 and 13.05 Å, respectively.

Based on the results, the proposed mechanism is illustrated in Figure 1E. Initially, E‐BI‐TPA‐CS powder was dissolved in ethyl acetate and then heated to concentrate it into a resin‐like state, where the molecules were randomly oriented. Subsequently, fiber fabrication via drop‐casting aligned the molecular long axes parallel to the fiber axis under the influence of an external force. During the initial stage of UV irradiation, photothermal expansion of the fiber surface caused the fiber to bend rapidly away from the light source. With continued irradiation, the E‐BI‐TPA‐CS molecules on the fiber surface underwent E‐Z isomerization, resulting in molecular contraction along the longitudinal axis of the fiber. This generated a contraction force along the length of the fiber, leading to bending toward the light source. This mechanism further confirms that external forces effectively regulate the alignment of molecular orientation within the aggregates.

To systematically investigate the effects of solvent type, evaporation rate, and initial solution concentration on the performance of the fiber‐based photoactuators, a series of experiments was conducted. As illustrated in Figures S28–S30, the photoactuation performance of fibers prepared from 10 mg/mL solutions of E‐BI‐TPA‐CS in DCM, EA, acetonitrile, and DMF was evaluated. Although the actuator dimensions varied across batches owing to manual fabrication, all samples exhibited significant bending toward the light source under UV irradiation. Notably, the use of high‐boiling‐point solvents (e.g., DMF) required higher processing temperatures, which may limit practical applications. Furthermore, the influence of the evaporation rate—controlled by varying the temperature during solvent evaporation—was examined using EA solutions of E‐BI‐TPA‐CS (10 mg/mL) concentrated at 80, 100, and 140° C. Although the resulting actuators showed visible variations in size, no clear trend in photoactuation performance was observed across different evaporation temperatures. Finally, actuators were fabricated using EA solutions with varying initial concentrations (5, 10, and 50 mg/mL). Under UV irradiation, batch‐to‐batch variations in actuator size were evident, however, these did not significantly affect the photoactuation performance.

Fatigue resistance and reversibility are critical parameters for evaluating the practical visibility of photomechanical materials. A fiber was fabricated and anchored at one end of the capillary tube (Figure S31). Under alternating UV light irradiation, the fiber bent toward the light source, resulting in repetitive left‐to‐right motion of the fiber tip. Initially, the tip displaced 8 mm within 3 s. However, as the number of cycles increased, the time required to achieve this 8 mm displacement increased significantly; by the 10^th cycle, 88 s were required. These results indicate that while the fibers exhibited some reversibility, their fatigue resistance was insufficient for sustained practical applications. To improve the stability, fibers were fabricated using DMF. As shown in Figure S32, these fibers bent back and forth under alternating UV irradiation. By the 20th cycle, the time required to achieve the same displacement increased from 3 to 21 s. The significantly enhanced stability of the fiber is attributed to the low volatility of DMF, which minimizes solvent loss during actuation.

Owing to the advantages of visible light over ultraviolet light in practical applications, such as improved environmental compatibility and reduced photodamage, the molecular response to white light was investigated. After the first plateau was exposed to ambient light for several days, ¹H NMR spectroscopy was performed. The results in Figure S14 indicate that the Z‐BI‐TPA‐CS content in DMSO‐d ₆ increased with exposure duration. As illustrated in Figure S33A, after 10 days of natural light exposure, the E‐Z isomerization conversion stabilized at approximately 83%, referred to as the second plateau. Further experiments replacing ambient light with white light revealed that continuous exposure resulted in a further blueshift in the absorption band to 406 nm (Figure S33B). The second plateau is attributed to the significantly higher absorbance of E‐BI‐TPA‐CS under white light compared to that of Z‐BI‐TPA‐CS. The higher conversion fraction under white light compared to UV light demonstrates the strong photoresponsiveness of E‐BI‐TPA‐CS to white light, enabling the use of white light to drive actuators. As shown in Figure S34, upon irradiation with white light (90 mW cm⁻²), the solvent‐assisted fiber bent toward the light source at a rate of 2.4°/min.

To study the reversibility of the isomerization, the thermostability of the photoproduct was investigated. Figure S36 shows that after heating the second plateau in the DMSO‐d ₆ solution at 70° C for 8 h, the Z‐BI‐TPA‐CS content decreased from 83% to 64%. Continued heating for approximately six days resulted in a further reduction in Z‐isomer content to 18%, indicating that the E‐Z isomerization was thermally reversible. This reversible isomerization and simple fabrication process allow for the recycling of fatigued photomechanical materials through thermal processing, thereby mitigating the limitations of insufficient fatigue resistance. To shorten the time required for Z‐E isomerization and increase the practicality of the actuators, a series of ¹H NMR spectra of the E/Z‐BI‐TPA‐CS mixture was recorded after heating at various temperatures. As shown in Figure S37, to simulate the recovery of fatigued materials, the initial content of Z‐BI‐TPA‐CS in the samples was set to 44%. This sample was dissolved in DMSO‐d ₆ and heated at 140 °C for various durations. As shown in Figure S37C, the Z‐isomer content decreased to 30% after 6 h of heating. When the solid sample was heated at 180° C for 20 min, the Z‐isomer content decreased by approximately 20%. After 6 h at this temperature, the content was reduced to approximately 10%, indicating that increasing the temperature significantly reduces the time required for material recovery. To evaluate recyclability, the actuator was subjected to repeated cycles. In the first cycle, the irradiated material was collected, heated at 180° C for 3 h, and redissolved in EA. After concentration, a new fiber was fabricated (Figure 6F) and exposed to 365 nm light. Within 12 s, the fiber exhibited significant bending toward the light source. Even after five cycles (fifth recycling), the fiber maintained significant photoinduced bending within 35 s, demonstrating excellent recyclability.

Given that the resin‐like state of E‐BI‐TPA‐CS can be drawn into fibers, we investigated whether similar methods could be employed to fabricate two‐dimensional films. As illustrated in Figure S38, the resin‐like material was placed between two glass slides. By slowly moving the slides apart and allowing the material to dry naturally, a film was produced (Figure S38A), demonstrating the excellent film‐forming capability of E‐BI‐TPA‐CS. Nanoindentation tests were performed to characterize the mechanical properties of these films. As shown in Figure S39, a maximum displacement of 250 nm was observed under a load of 0.5 mN. Upon unloading, the displacement recovered to 200 nm, indicating the elastic behavior of the film. From the fittings of the initial unloading curves, the Young's modulus (E) and hardness (H) of film were determined to be 8.36 and 0.3 GPa, respectively.

To investigate the molecular packing within the film, the emission spectra, fluorescence quantum yield, and fluorescence lifetime of E‐BI‐TPA‐CS (Figure S40) were analyzed in its crystalline, thin‐film, and rapidly cooled solid state (i.e., amorphous form). The photophysical data are summarized in Table S3. The absorption and emission peaks of the film were observed at 449 and 571 nm, respectively, which are comparable to those of the crystalline form (453 and 574 nm). However, the fluorescence lifetime (1.22 ns) and quantum yield (7.2%) of the film were more consistent with the amorphous state (1.27 ns and 6.6%, respectively). Additionally, powder X‐ray diffraction (PXRD) patterns revealed that the film exhibited two weak bands at 2θ values of 12° and 21° (Figure S40C). These correspond to the diffraction peaks associated with the dominant crystal facets, indicating that the molecular packing within the film lies between that of the crystalline and amorphous states.

The prepared film was cut to an appropriate size, with one end anchored along the stretching direction and placed under UV light (Figure S38B). Within the first 0.2 s, the film initially underwent rapid bending away from the light source. With prolonged exposure, the film gradually bent toward the light, achieving a bending angle exceeding 90° within 88 s. The curling behavior of the film was analogous to the bending observed in the fibers, suggesting that the underlying mechanisms were identical. Figure S38C illustrates the film preparation and its corresponding bending mechanism. The application of an external force aligned the molecular long axes within the film nearly parallel to the stretching direction. When illuminated from below, the lower surface of the film experienced thermal expansion owing to the photothermal effect, resulting in rapid bending away from the light within 0.2 s. Continued irradiation induced E‐Z isomerization of the molecules on the illuminated surface, generating a contraction force along the stretching direction, which caused the film to bend downward.

When the stretching direction was altered, as indicated by the red arrow in Figure 6G, the film was cut into an eagle silhouette. During the initial 3 min of UV irradiation, the film curled toward the light source, akin to an eagle flapping its wings (Video S2). When the irradiation direction was changed, the film returned to its planar state, similar to an eagle spreading its wings. The entire process can be visualized as the motion of an eagle during its flight. Regardless of the stretching direction, the curling always occurred along the stretching direction, further confirming that the molecular orientation within the aggregates can be artificially controlled.

Interestingly, the resin‐like state of E‐BI‐TPA‐CS formed 3D architectures during decompression (Figure 6H). Atomic force microscopy (AFM) images (Figure S41A) show that the surfaces of the resulting 3D architectures were smooth, with a thickness of less than 900 nm (Figure S41B). When the bowl‐shaped architecture was subjected to UV irradiation, it contracted inward (Video S3), reminiscent of the swimming motion of a jellyfish. These 3D architectures were subsequently cut into films (Figure S41D), which similarly exhibited initial bending away from the light source, followed by bending toward the light upon continued UV irradiation.

2.6. Application

Beyond simulating biological movements, the potential applications for the actuators developed in this study were further explored. Because stretching induces molecular orientation within the aggregates, we speculate that directional friction could similarly facilitate molecular alignment. Leveraging the excellent film‐forming ability and adhesive properties of E‐BI‐TPA‐CS, an intelligent optical switch was designed and fabricated. As illustrated in Figure 6I, the resin‐like E‐BI‐TPA‐CS was coated onto aluminum foil (6 µm) and subsequently oriented through friction to form the switch. Initially, the responsive performance of the optical switch was evaluated. Under UV irradiation, the switch initially bent away from the light source owing to the photothermal effect. Continued irradiation induced E‐Z isomerization of the E‐BI‐TPA‐CS molecules, causing the switch to bend toward the light. Additionally, temperature measurements of pure aluminum foil under UV irradiation (Figure S42) showed that its temperature remained nearly unchanged. This confirms that the observed bending is directly attributed to the photothermal and photochemical responses of the E‐BI‐TPA‐CS coating.

Based on the above mechanism, a circuit was constructed as depicted in Figure 6L, comprising a battery, conductive glue, an indicator light, and the optical switch. Prior to irradiation, the circuit was disconnected, and the indicator light remained off. Upon activation of the UV lamp, the optical switch exhibited yellow fluorescence, benefiting from AIE properties, which enabled remote monitoring of the switch's status under the excitation light. After 2 s of continued irradiation, the optical switch bent away from the light source because of the photothermal effect, connecting the circuit and illuminating the indicator light. Under continuous irradiation, the switch bent toward the light source, resulting in circuit disconnection and deactivation of the indicator light.

Furthermore, E‐BI‐TPA‐CS was coated onto a thicker aluminum foil surface (0.2 mm) using the same method (Figure S43). Under UV irradiation, the aluminum foil bent toward the light due to the isomerization effect of E‐BI‐TPA‐CS. This mechanism was similarly applied in the circuit; the bending toward the light caused by isomerization connected the circuit, illuminating the indicator light within 80 s. Crucially, the isomerized molecules remained stable after the UV source was deactivated, allowing the switch to maintain its state and keep the indicator continuously illuminated. E‐BI‐TPA‐CS underwent Z‐E isomerization upon heating, endowing the material with good recyclability. To demonstrate this, the irradiated material was collected, heated at 180 °C for 3 h, and subsequently re‐coated onto aluminum foil to fabricate a new photoswitch. As shown in Figure S44, when this recycled switch was exposed to UV light for 25 s, the circuit closed and the red indicator light was illuminated. Notably, the indicator light remained active even after the UV source was removed. This recyclable photoactuator, not only aligns with the principles of sustainable development, but also offers a novel strategy for the design of next‐generation responsive actuators.

3. Conclusion

In this study, we developed a recyclable fluorescent photoactuator based on an AIE‐active D‐π‐A molecule (E‐BI‐TPA‐CS), which exhibits reversible E‐Z isomerization. The solvent helped enable the fabrication of multi‐dimensional fluorescent soft actuators, ranging from 1D fibers to 3D architectures. Their anisotropic photoinduced bending, curling, and contraction behaviors demonstrate that mechanical stretching effectively regulates molecular alignment within the aggregates. Leveraging reversible isomerization and rapid fabrication methods, fatigued actuators can be fully recycled and reused through a simple dissolution‐heating‐concentration cycle. Furthermore, the adhesive nature of the resin‐like material allows it to be directly coated onto metallic surfaces to construct functional photoswitches without the need for complex operations such as metal deposition. By bridging the gap between crystalline and polymeric systems, this material integrates the advantages of both while drastically reducing the actuator fabrication time. This advancement offers a versatile platform for smart switches, microrobots, and bioinspired actuators, surpassing conventional photoresponsive materials and paving the way for scalable, intelligent material design.

Author Contributions

Cheng Liu: conceptualization, methodology, writing – original draft, investigation, validation, data curation, and formal analysis. Weijian Li: data curation, investigation. Xinwen Ou: investigation, data curation. Herman H.‐Y. Sung: software, investigation. Xuedong Xiao: data curation, investigation. Haoran Wang: supervision, writing ‐ review and editing, conceptualization, and validation. Jin Wang: data curation, investigation. Shengyi Yang: writing – review and editing, investigation, formal analysis, data curation, and validation. Ian D. Williams: software, resources. Fei Wang: data curation, investigation. Ryan T. K. Kwok: writing – review and editing, project administration, and supervision. Jacky W. Y. Lam: writing – review and editing, project administration, and supervision. Ben Zhong Tang: writing – review and editing, resources, project administration, supervision, and funding acquisition.

Conflicts of Interest

The authors declare no competing financial interest.

Supporting information

Detailed instrumentation, materials, methods, synthetic procedures, NMR spectra, mass spectra, optical spectra, microscopic images, SEM images, PXRD, additional experimental details, and x‐ray crystallographic data (CCDC numbers: 2403633 and 2403634).

Supporting File 1: anie72709‐sup‐0001‐SuppMat.docx.

Supporting File 2: anie72709‐sup‐0002‐cif.zip.

Supporting File 3: anie72709‐sup‐0003‐VideoS1.mp4.

Supporting File 4: anie72709‐sup‐0003‐VideoS2.mp4.

Supporting File 5: anie72709‐sup‐0004‐VideoS3.mp4.

Data Availability Statement

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