Construction of Photoresponsive 3D Structures Based on Triphenylethylene Photochromic Building Blocks

Photoresponsive materials have been widely used in sensing, bioimaging, molecular switches, information storage, and encryption nowadays. Although a large amount of photoresponsive materials have been reported, the construction of these smart materials into precisely prescribed complex 3D geometries is rarely studied. Here we designed a novel photoresponsive material methyl methacrylate containing triphenylethylene (TrPEF2-MA) that can be directly used for digital light processing (DLP) 3D printing. Based on TrPEF2-MA, a series of photoresponsive 3D structures with reversible color switching under ultraviolet/visible light irradiations were fabricated. These complex photoresponsive 3D structures show high resolutions (50 μm), excellent repeatability (25 cycles without fatigue), and tunable saturate color degrees. Multicomponent DLP 3D printing processes were also carried out to demonstrate their great properties in information hiding and information-carrying properties. This design strategy for constructing photoresponsive 3D structures is attractive in the area of adaptive camouflage, information hiding, information storage, and flexible electronics.

3D printing, a technology that refers to the additive manufacturing process of building 3D solid objects from digital models, shows the capability of customizing complex 3D structures [38][39][40][41][42]. Along with recently rapid developments in 3D printing, a few attempts have been made to fabricate photoresponsive structures with complex geometries by 3D printing, which showed enormous potential in the applications of adaptive camouflage, information hiding, information storage, soft robotics, and flexible electronics [43][44][45][46][47]. For instance, the commercial 3D-printed powders (i.e., nylon 12) mixed with WO 3 uncoated nanoparticles were used to create photoresponsive 3D structures by sintering-based 3D printing [48]. Hybrid organic-inorganic polyoxometalates were stabilized and solubilized by polymeric ionic liquid matrices, which could be further processed by 3D printing into photochromic 3D geometries [49]. Photoswitchable donor-acceptor Stenhouse adducts (DASAs) were postmodified on a two-photon laser processing complex microstructure [50]. However, due to the employed doping or postmodifying methods, the photoresponsive 3D structures suffered from unquantified and nonhomogeneous functional partials and poor solvent or thermal resistances. In addition, multicomponent 3D printing processes are hard to conduct with postmodify photochromic materials. Recently, triphenylethylene derivatives, which own admirable repeatability and simple chemical structures, are considered as promising photochromic materials [51][52][53][54]. Moreover, the easily modified chemical structure makes them promising candidates for preparing ultraviolet (UV) curable photochromic resins that are suitable for UV-based 3D printing technologies to directly establish macromolecular cross-linking network, such as digital light processing (DLP)-based 3D printing, a technology that is recognized as one of the most efficient 3D printing method to realize high-accuracy 3D structures with high speed [55][56][57].
Herein, we demonstrate a simple and novel photoresponsive material, methyl methacrylate containing triphenylethylene (TrPEF 2 -MA), which can be directly used for DLP 3D printing for the first time ( Figure 1). The high-resolution 3D objects show great solvent/thermal resistance and precisely controllable photoresponsive properties. TrPEF 2 -MA with a simple chemical structure exhibits controllable photochromism and fast response. More importantly, the MA moiety in TrPEF 2 -MA endows the molecule with UV curable ability, which could be directly copolymerized during the DLP 3D printing process (Figure 2(a)). By adding molecule TrPEF 2 -MA and photoinitiator into poly(ethylene glycol) diacrylate (PEGDA), liquid resins for high-resolution DLP 3D printing were successfully prepared. A series of highly complex and precisive 3D structures with high resolution up to 50 μm were fabricated. Under the alternating UV and visible light irradiation, these printed stable macromolecularbased 3D structures featured reversible color switching between transparent and yellow (absorption at ca. 470 nm) with good repeatability. Besides, the saturate color degree of the photoresponsive 3D structures could be precisely controlled by adjusting the composition ratio of TrPEF 2 -MA in the resin. To further demonstrate the potential applications of the directly DLP 3D-printable photoresponsive material TrPEF 2 -MA, multicomponent 3D printing processes were carried out for the construction of the information (QR codes) hiding 3D structures, multicolor photoresponsive 3D structures, and information-carrying 3D structures. This study provides a novel strategy to directly design DLP 3Dprintable photoresponsive material and reveals the potential applications of the areas of adaptive camouflage, information hiding, information storage, and flexible electronics.

Synthesis and Photochromic Properties of TrPEF 2 -MA.
The synthetic details of 4-(2,2-bis(4-fluorophenyl)vinyl) benzyl methacrylate (TrPEF 2 -MA) are described in the Supporting Information. All compounds were characterized with 1 H NMR spectroscopy and high-resolution mass spectrometry. The 3D-printable photochromic material TrPEF 2 -MA was achieved with decent synthetic yields of 37.3% through a simple three-step synthetic route as shown in Scheme S1 in Supporting Information. The low-cost starting materials, simple synthetic route, and high synthetic yields provided the possibility of large-scale preparation of TrPEF 2 -MA.
Upon UV excitation (365 nm), noticeable photochromism of TrPEF 2 -MA was observed both in solution and thin film. The color changes of TrPEF 2 -MA was ascribed to the reversible ring-closure reaction of triphenylethylene group [51], as illustrated in Figure 2(a), according to previous literatures with similar structures. In THF solution, a low-energy absorption band with the maximum at 461 nm was observed, which gradually increased with the increasing irradiation time. After irradiation for ca. 260 seconds, enhancement of the absorption band stopped (Figure 2(b)). With consequent visible light irradiation, the color of TrPEF 2 -MA solution gradually faded to its original color in 2520 seconds ( Figure 2(c)). Moreover, thermal-back reaction (ring-opening reaction in dark) of TrPEF 2 -MA in THF (1:0 × 10 −1 mol/L) was also achieved according to the fitting curve of the absorbance band at 461 nm in the time-dependent absorption studies. The thermal-back reaction is relatively slow, and the halflife in THF solution is 597.5 seconds in dark at 303 K ( Figure S6). In order to provide evidence for photochromic mechanism and identify the ring-closure structure of TrPEF 2 -MA. By exposing TrPEF 2 -MA under UV irradiation in oxygen atmosphere for 12 hours, dehydrogenated product TrPEF 2 -MA(O) was obtained. As shown in Scheme S2, it can be concluded from the chemical structure of TrPEF 2 -MA(O) that the photochromic mechanism is caused by the ring-closure reaction. In addition, compound TrPEF 2 -MA displayed an excellent reversibility during the photochromic process, which shows no obvious fatigue even after 20 photochromic and bleaching cycles (Figure 2(d)). The excellent recyclability and slow thermal-back reaction indicate its huge potential applications as rewritable and reversible photoresponsive materials.
The fact that TrPEF 2 -MA is an oily liquid at room temperature, the photochromic properties of TrPEF 2 -MA films added into polymer matrix were also investigated. 2-mmthick photochromic films with the doping concentration of 10 wt% in polymer were prepared by photocuring the 3Dprintable resin. The preparation details of the polymer films are described in the Supporting Information. Photophysical studies of TrPEF 2 -MA films were listed in Figure S1. The transparent flexible films showed good flexibility and photochromic properties. Upon UV light irradiation for ca. 50 seconds, the polymer films changed from colorless to 2 Research yellow; simultaneously, the emission properties of the polymer films were quenched. Similar to the TrPEF 2 -MA in solution, a reverse process occurred when irradiated by visible light. The UV-Vis absorption spectra as a function of the photochromic bleaching time in films are shown in Figure S5. It can be clearly seen that the films faded within 2310 seconds upon visible light irradiation. Compared with the solution state (THF), the absorption bands of the doped film were slightly red-shifted to 482 nm, which is mainly due to the environmental constraints in the rigid polymer matrix [58]. The light transmittance of this film reached over 80% in daylight, while drastic decrement was observed in the range of 400 to 600 nm when exposed to UV light, as shown in Figure 2(e). The distinctive transmitting properties of the polymer films provide numerous potential applications, such as photoswitchable patterning and anticounterfeiting applications in solid surface and interface [59]. Moreover, the stability of the doped and copolymerized photoresponsive polymer samples was investigated as shown in Figure 2(f). The fabrication procedure of polymer films was described in details in the Supporting Information [51]. After soaking polymer films in THF solution overnight, the photochromic behavior of the doped polymer films disappeared, while the copolymerized films retained (Figure 2(g)). It reveals that the photoresponsive functional groups in doped polymer film are unstable when dissolved in organic solvent. In copolymerized films, the grafted triphenylethylene groups are much more stable under organic solvent corrosion. Moreover, the differential scanning calorimetry (DSC) and thermal gravimetric analyses (TGA) studies were carried out to study thermal properties of photoresponsive polymers ( Figure S2 and S3). The copolymerized film showed a slightly higher degrading temperature (358°C) than that of the doped polymer film (271°C), indicating its better thermal stability. It could be ascribed to the strong covalent bonds between the cross-linking network. In addition, the scanning electron microscopy (SEM) studies of the two films were conducted as shown in Figures 2(h) and 2(i). It can be clearly seen that TrPEF 2 -MA can be more uniformly dispersed in the polymer matrix by copolymerization than direct doping. Hence, it could be concluded that the fabrication of photoresponsive 3D structures based on directly printable photoresponsive resins showed greater advantages both in thermal and chemical stabilities than those doped photoresponsive resins.

Constructions of Photoresponsive 3D Structures.
To fabricate various photoresponsive 3D structures, the photochromic liquid resins were directly used in DLP 3D printing process as shown in Figure 3 Table S1.
As illustrated in Figure 3(b), the solidified 3D structures were macromolecular-based networks, which were composed of cross-linked PEGDA and grafted TrPEF 2 -MA functional groups. The copolymerized chemical structures were further verified by the Fourier transform infrared (FTIR) spectra as shown in Figure S4. With 3D-printable liquid resin C, a simple "light-responsive flower" 3D structure was fabricated (Figures 3(a) and 3(c)). This transparent "light-responsive flower" could gradually turn to bright yellow upon UV irradiation (50 seconds) and recover to transparent by visible light irradiation (2310 seconds).

Research
According to previous results, the photochromic property of the 3D printing structure could be ascribed to the ring-closure reaction of the TrPEF 2 -MA functional groups as shown in Figure 3(d) [51]. Likewise, a more complex 3D structure "light-responsive tree" was also printed with the same resin as shown in Figure 3(e). The specific printing parameters and operational details are presented in Table S2. This "light-responsive tree" gradually turned from transparent to orangish yellow within 50 seconds under 365-nm UV light. Thus, the color saturations of the photoresponsive 3D structures could be precisely manipulated by adjusting the UV irradiation time. In addition, SEM studies of the printed 3D structures were performed as shown in Figure 3(  5 Research is consistent with the preset slice thickness, indicating the high fidelity during the DLP 3D printing process. One of the most important advances for DLP 3D printing technology is that it can achieve the manufacture of complex hollow structures. A series of high-resolution hollow structures were fabricated with these photoresponsive 3D-printable resins containing different mass fractions of TrPEF 2 -MA. These printed honeycomb structure (resin B), modified hollow cube (resin C), and porous hollow sphere (resin D) showed excellent printing resolutions and smooth surfaces, indicating the excellent performances for these resins in fabricating complex structures (Figure 4(a)). Under sufficient UV irradiations, these 3D hollow structures with different proportions of TrPEF 2 -MA turned to pale yellow (resin B), bright yellow (resin C), and orangish yellow (resin D), respectively, as shown in Figure 4(a).
Moreover, UV/Vis reflectance spectra of these 3D hollow structures were collected. As shown in Figures 4(b)-4(d), the absorption maxima of resins B, C, and D are 468, 482, and 471 nm, respectively. It can be clearly seen that the absorption intensities significantly boosted with the increasing proportions of TrPEF 2 -MA, 0.032 in resin B, 0.108 in resin C, and 0.194 in resin D. Besides, the color maintaining times were prolonged from 960 seconds (resin B) to 3650 seconds  Figure S7-S9. Moreover, through time-dependent UV-Vis absorption test at different temperatures, it was found the half-life of photochromism shortened with the increase of temperature as shown in Figure S10-S13. Thus, the color saturations and maintaining times of the photoresponsive 3D structures could be precisely controlled and easily adjusted by tuning the proportion of TrPEF 2 -MA in resins and temperature. In addition, all these complex hollow structures showed excellent printing resolutions and smooth surfaces, indicating the excellent stability for these resins during DLP 3D printing. Furthermore, the 3D hollow structures also displayed an excellent photochromic reversibility. After 25 photochromic and bleaching cycles, insignificant fatigue was detected (Figures 4(e)-4(f) and Movie S1). These superiorities of TrPEF 2 -MA indicate the potential applications in information hiding, information carrying, and others ( Figure S14).

Multicomponent Photoresponsive 3D Structures.
To clearly illustrate the potential applications of the photoresponsive 3D structures, multicomponent 3D printing processes were also performed with different resins to realize information hiding and information carrying in 3D structures. Firstly, a double-layered QR code structure was printed by automatically switching liquid resins during the printing process as shown in Figure 5(a)   7 Research directly scanned with a cellphone to get the information "IFE." The scannable QR code could be reversibly turned to transparent within 2310 seconds under visible light irradiation. In addition, a four-tier pyramid was printed with different resins containing TrPEF 2 -MA of 0, 5, 10, and 20 wt% from the bottom to the top, respectively ( Figure 5(b)), as the color saturations of the photochromic 3D structure could be precisely tuned by manipulating the proportion of TrPEF 2 -MA in the resins. The photoreponsive four-tier pyramid altered to gradient-yellow upon UV light and recovered to transparent under visible light. The photoresponsive gradient-color 3D structures are quite striking in adaptive camouflage. As shown in Figure 5(c), a transparent framework was printed with resins A (0 wt% TrPEF 2 -MA) and C (10 wt% TrPEF 2 -MA) to realize information carrying and encryption. By rational arrangement of printing resins for the framework, information could be successfully hidden in the 3D structure. By irradiating the 3D framework with UV light, components printed with resin C turned to yellow, while the remaining component maintained transparent. Thus, four different numbers 6, 2, 9, and 0 appeared on each surface of the framework and disappeared under visible light irradiation. According to this demonstration, key information could be carried or hidden in the designed 3D structures. Therefore, the development of these new photoresponsive 3D-printable resins paves a new way to endow a wide variety of practical functions such as advanced anticounterfeiting, information hiding or carrying, and adaptive camouflage to traditional printed 3D structures.

Conclusion
In summary, we have proposed a rational strategy to construct photoresponsive prescribed complex 3D structures. By attaching the UV curable MA group to photochromic triphenylethylene, a photoresponsive material TrPEF 2 -MA that can be used for directly DLP 3D printing was realized. A series of photoresponsive 3D structures with high resolution (up to 50 μm) were fabricated. These 3D structures showed a striking color switching between transparent and yellow under alternating UV/visible light irradiation with good repeatability. To illustrate the information hiding and information-carrying properties, multicomponent DLP 3D printing processes were employed. Information hiding (QR codes), multicolor photoresponsive, and information-carrying multimaterial-based 3D structures were successfully printed. This work provides a novel and efficient strategy to construct photoresponsive 3D structures for information hiding and paves the way for more potential applications in adaptive camouflage, information storage, and flexible electronics.

Data Availability
All relevant data that support the findings are available within this article and supporting information and are also available from authors upon reasonable request.