Fulgide Derivative-Based Solid-State Reversible Fluorescent Switches for Advanced Optical Memory

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Jan 2022Fulgide Derivative-Based Solid-State Reversible Fluorescent Switches for Advanced Optical Memory Yang Jiao, Runqing Yang, Yuchao Luo, Leijing Liu, Bin Xu and Wenjing Tian Yang Jiao State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Runqing Yang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Yuchao Luo State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Leijing Liu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Bin Xu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Wenjing Tian *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000673 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Solid-state fluorescent switches with reversible luminescence characteristics have attracted considerable attention because of their broad applications in advanced photonics, such as anticounterfeiting inks, optical writing and erasing, and biological imaging. Herein, we have fabricated a solid-state reversible fluorescent switch under alternating UV (365 nm) and visible light treatments based on a fulgide (FUL)-functionalized tetraphenylethylene (TPE) derivative (TPE-FUL) containing a photochromic group FUL and aggregation-induced emission (AIE) luminogen TPE. TPE-FUL exhibited excellent reversible absorption and luminescence owing to the interconversion between open TPE-FUL (O-TPE-FUL) and closed TPE-FUL (C-TPE-FUL). Photophysical and theoretical investigations revealed that the luminescence of O-TPE-FUL is based on the local excited state of the TPE moiety, whereas the fluorescence quenching of C-TPE-FUL originates from the intramolecular charge transfer from the TPE to the FUL moiety. The excellent reversible photoswitching properties of TPE-FUL in the solid state allows for its potential use in advanced optical memory applications, such as anticounterfeiting, optical writing and erasing, and information encryption. Download figure Download PowerPoint Introduction Fluorescent switches that change their fluorescence emission and UV–vis absorption on exposure to light have attracted considerable attention because of their potential applications in optical memory.1–7 In recent years, fluorescent switches based on diarylethene, spiropyran (SP), and fulgide (FUL), as well as their derivatives, have been reported.8–20 However, most of the reported fluorescent switches were realized in solution.21–24 Solid-state fluorescent switches are preferable in practical applications, such as anticounterfeiting,25,26 data storage,27,28 optical devices,29 and biological imaging.30,31 For example, diarylethene derivatives exhibiting fluorescent switching properties in the solid state with a high and constant fluorescence on–off switching ratio have been developed and applied in accurate nonvolatile optical memories32–34 and super-resolution imaging.35–37 A tetraphenylethylene-SP (TPE-SP) molecule has been designed and synthesized, and a solid-state fluorescent switch as a near-infrared (NIR) microlaser has been constructed by self-assembling TPE-SP microspherical caps.38 Our group39 reported a solid-state fluorescent switch based on distyrylanthracene-SP (DSA-SP) and its application in anticounterfeiting and super-resolution imaging. FUL and its derivatives exhibit visual changes in color and fluorescence in solution under alternating irradiation by UV and visible light.40–42 Also, they often exhibit good thermal stability, easy synthesis, high fatigue resistance, and reliable photoswitch reactivity in both solution and solid state, which are regarded as efficient fluorescent switches and promising molecules for applications in optical memory devices.43–45 However, to the best of our knowledge, there have been no reports of solid-state fluorescent switches based on FUL and its derivatives due to the aggregation-induced quenching (ACQ) effect caused by the planar structure of the FUL moiety, resulting in weak or no fluorescence in the solid state.46,47 Aggregation-induced emission (AIE) fluorogens with twisted conformations, exhibiting weak emission in solution but strong emission in the solid state, provide a new strategy to improve the fluorescence efficiency of ACQ molecules in the solid state.48–50 Herein, considering the advantages of the FUL moiety and AIE fluorogens, we covalently linked a FUL moiety to a popular AIE molecule, TPE, to construct an effective reversible solid-state fluorescent switch FUL-functionalized TPE derivative (TPE-FUL). TPE-FUL exhibited a reversible change in both color and fluorescence in the solid state under alternating UV and visible light treatments. Experimental and theoretical investigations showed that the fluorescence of open TPE-FUL (O-TPE-FUL) is derived from the local excited (LE) state of the TPE moiety, and the fluorescence quenching of closed TPE-FUL (C-TPE-FUL) is due to the intramolecular charge transfer (ICT) from the TPE to the FUL moiety. The application of TPE-FUL in anticounterfeiting inks, optical erasing, and information encryption indicates that the combination of a photoswitch moiety and AIE fluorogens is an efficient strategy to construct a FUL-based solid-state fluorescent switch. Results and Discussion We synthesized TPE-FUL via a Suzuki coupling reaction, and the detailed synthetic route, ( Supporting Information Scheme S1) as well as the characterization of TPE-FUL, are described in Supporting Information Figures S1–S7. As shown in Supporting Information Figure S8a, before UV irradiation, the absorption spectra of TPE-FUL showed strong absorption bands centered at 301 and 341 nm, assigned to the open form FUL and TPE moieties, respectively, in tetrahydrofuran (THF). After UV irradiation at 365 nm, a new absorption peak appeared at 552 nm, whose intensity increased with irradiation time because of the conversion of the FUL moiety from the open form to the closed form under UV irradiation.37 Accordingly, the color exhibited a visual change from colorless to purple ( Supporting Information Figure S8b). To determine the yield of O-TPE-FUL converted to C-TPE-FUL under the photostationary state (PSS) of UV light, TPE-FUL solution before and after UV light irradiation was analyzed by high-performance liquid chromatography (HPLC) ( Supporting Information Figure S9). By calculating their area ratio, the conversion ratio from O-TPE-FUL to C-TPE-FUL under UV irradiation was 73.6%. The absorption and emission spectra of TPE-FUL in the solid state before and after UV light irradiation are shown in Figures 1a and 1b, respectively. The TPE-FUL powder showed absorption bands at 319 and 432 nm, assigned to the open form FUL and TPE moieties, respectively, before UV irradiation. After UV irradiation, a new absorption band was observed at 570 nm (Figure 1a). The emission spectra exhibited a fluorescence peak at 497 nm with a quantum yield (Φf) of 0.11 and a lifetime of 2.1 ns before UV irradiation ( Supporting Information Figure S10), but the fluorescence was quenched after UV irradiation (Figure 1b). As shown in Figure 1c, the color of TPE-FUL powder gradually changed from yellow to dark purple under ambient lighting, and the cyan fluorescence was quenched after UV irradiation on account of the photochemical conversion of O-TPE-FUL to C-TPE-FUL (Figure 1d). Figure 1 | (a) UV–vis absorption spectra and (b) PL spectra of TPE-FUL powders before and after UV irradiation. (c) Visible and fluorescent pictures of TPE-FUL powder under UV and visible light irradiation. (d) Reversible structural isomerization between O-TPE-FUL and C-TPE-FUL under UV and visible light irradiation. Download figure Download PowerPoint To investigate the photoswitching properties of TPE-FUL, we studied the absorption and emission response of the TPE-FUL film over different UV irradiation times. The absorption spectra of the TPE-FUL film under different irradiation times are shown in Figure 2a. With increasing UV irradiation time, the absorption at 570 nm gradually increased, indicating the gradual conversion of O-TPE-FUL to C-TPE-FUL. After 40 min of UV irradiation, there was no further increase in absorption, which means the transformation (O-TPE-FUL to C-TPE-FUL) reached PSS. The C-TPE-FUL film was then irradiated with 524 nm-wavelength visible light. The absorption peak of TPE-FUL decreased gradually and returned to its original state after 15 min of irradiation. This indicated that C-TPE-FUL was converted to O-TPE-FUL under visible light irradiation (Figure 2b). The insets in Figures 2a and 2b show the TPE-FUL absorbance under UV light at 570 nm with different irradiation times and visible light at 524 nm, demonstrating the absorption response of TPE-FUL film under different light irradiations. We also explored the fluorescence changes under different irradiation times. As shown in Figure 2c, the O-TPE-FUL showed an emission peak at 465 nm. The fluorescence intensity gradually decreased and was almost quenched after 35 min of irradiation at 365 nm, indicating the gradual conversion of O-TPE-FUL to C-TPE-FUL. Then, the C-TPE-FUL film was irradiated with visible light at 524 nm. The fluorescence of the TPE-FUL film gradually increased and recovered to its initial intensity after 50 min of irradiation (Figure 2d), indicating that the conversion of C-TPE-FUL to O-TPE-FUL occurs under visible light irradiation. The graphs of TPE-FUL emission at 465 nm with different irradiation times under UV light and 524 nm visible light are shown in the insets of Figures 2c and 2d, suggesting a change in the fluorescence of the TPE-FUL film under different light irradiation. Thus, TPE-FUL exhibits a solid-state visual color and fluorescent switch property under alternating UV and visible light treatments. Figure 2 | (a) Absorption spectra of TPE-FUL film under UV irradiation and (b) visible light irradiation (Insert: the absorption of TPE-FUL film at 570 nm). (c) PL spectra of TPE-FUL film under UV irradiation and (d) visible light irradiation (Insert: the PL intensity of TPE-FUL film at 465 nm; wavelength/intensity of UV and visible light: 365 nm/2.8 mW cm−2 and 524 nm/8.3 mW cm−2). Download figure Download PowerPoint To gain further insight into the fluorescence change behavior before and after UV irradiation, the electronic density distribution of molecular orbitals (MOs) of TPE-FUL was calculated by time-dependent density functional theory (TD-DFT) calculations. As shown in Figure 3a, the electronic clouds on the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO)+1 of the O-TPE-FUL molecule were mostly concentrated in the TPE moiety, while the electronic cloud on the LUMO was predominantly distributed in the FUL unit. The lowest energy (HOMO → LUMO) transition is impossible because of the weak transition oscillator strength (f = 0.0188), while the higher energy (HOMO → LUMO+1) transition is symmetry-allowed because of the relatively strong transition oscillator strength (f = 0.7926). Consequently, the S0 → S1 (HOMO → LUMO) transition is impossible, whereas the S0 → S2 (HOMO → LUMO+1) transition is symmetry-allowed, indicating that the allowed transition of O-TPE-FUL is considered to be the LE state of the TPE moiety. The strong emission from TPE in the solid state leads to the strong solid-state fluorescence of O-TPE-FUL. In case of the electronic cloud distribution of the MOs of C-TPE-FUL, the electronic density of C-TPE-FUL on the HOMO is located along the TPE and FUL units, but the electronic density of the LUMO is mainly concentrated in the FUL unit (Figure 3b), indicating the possibility of ICT from TPE to FUL.51 TD-DFT calculation data showed that the higher energy transition (HOMO → LUMO+1) with a weak transition oscillator strength of 0.0364 is impossible, while the lowest energy transition (HOMO → LUMO) with a transition oscillator strength of 0.4950 is symmetry-allowed. Therefore, the S0 → S1 (HOMO → LUMO) transition of C-TPE-FUL is symmetry-allowed, whereas the S0 → S2 (HOMO → LUMO+1) transition is impossible, suggesting that the allowed transition of C-TPE-FUL is considered to be ICT from the TPE to the FUL moiety.52 Nonemissive FUL moieties in both solution and solid states result in fluorescence quenching of C-TPE-FUL. Figure 3 | Energy diagrams and frontier orbitals contribution of TPE-FUL and their energy transitions estimated by TD-DFT calculations at the B3LYP/6-31G(d,p) level. Download figure Download PowerPoint Photoswitching properties of TPE-FUL motivated us to explore its application in anticounterfeiting ink. We sprayed TPE-FUL solution onto filter paper and evaporated the solvent to obtain a light-yellow-colored filter paper that was evenly immersed in TPE-FUL solution (Figure 4a). Next, we placed an engraved mask on the filter paper and exposed it to 365 nm UV light. The color of the four-leaf clover pattern immediately turned to clear purple, which could be easily distinguished by the naked eye. In addition, by exposing the purple four-leaf clover pattern to visible light at 524 nm, the color of the purple pattern faded and it returned to the original colorless state. The reversibility of the color could be successfully achieved many times, and the purple butterfly pattern could be obtained. In addition, we explored the fluorescence pattern changes in TPE-FUL. First, we exposed the filter paper to 365 nm UV light to quench the fluorescence of TPE-FUL (Figure 4b). Subsequently, we placed the engraved mask on the filter paper and irradiated it with visible light at 524 nm. The fluorescence of the four-leaf clover pattern gradually recovered, and a cyan-fluorescent four-leaf clover pattern was obtained. When the filter paper was again exposed to UV light at 365 nm, the fluorescence once again disappeared. Fluorescent butterfly patterns were also obtained, which provided high contrast and clear signals. The reflective UV–vis spectra and the corresponding photoluminescence (PL) spectra of the filter paper printed with TPE-FUL before and after UV irradiation at 365 nm are shown in Figures 4c and 4d. The reversibility of color and fluorescence could be achieved many times (Figures 4e and 4f), indicating that TPE-FUL is a suitable anticounterfeiting ink. Figure 4 | Illustration of TPE-FUL printed on filter paper as an anticounterfeiting ink. The visual pictures taken under visible light and UV irradiation for 5 min (a) and their fluorescent images (b). (c) The reflective and (d) PL spectra of the filter paper printed with TPE-FUL before and after 365 nm UV irradiation. (e) Broken line graph of reflectance at 570 nm wavelength and (f) peak intensity of fluorescence emission at 465 nm through 10 cycles of 365 nm UV light and 524 nm visible light. Download figure Download PowerPoint Encouraged by its excellent reversible fluorescent switching behavior, we embedded TPE-FUL into a polymer matrix to obtain a photosensitive film. Morphological photos of polyacrylonitrile (PAN) films are shown in Supporting Information Figures S11 and S12. The absorption and emission spectra of electrospun PAN white fiber films embedded with TPE-FUL are shown in Figures 5a and 5b; these films can be further used as a controlled fluorescent writing and erasing material. We exposed the PAN film to UV light to quench the fluorescence. Then, we placed a steel plate engraved with the pattern “88” on top of the film, which was partially irradiated with visible light at 524 nm. After removing the steel plate, the original cyan fluorescence was restored in the exposed part of the film, while the part covered by the plate exhibited no fluorescence. Consequently, precise and high-contrast patterns of “55” were accurately obtained on the film with an obvious change in fluorescence. Next, we selectively irradiated the other parts and engraved different numbers. A pattern of “68” was obtained on the film under visible light irradiation at 524 nm (Figure 5c). The PAN film could be restored to its original fluorescent state under visible light irradiation. It is essential that multiple cycles of optical fluorescent writing and erasing can be realized without significant fatigue by employing different patterns. Figure 5 | (a) UV–vis absorption spectra and (b) PL spectra of TPE-FUL embedded electrospun PAN white fiber film before and after UV irradiation. (c) Illustration of TPE-FUL in fluorescent writing and erasing on PAN film. (d) Illustration of TPE-FUL in optical information encryption. The visible images and their corresponding fluorescent images taken after 5 min UV irradiation under visible light and in the dark. Download figure Download PowerPoint Photoswitching properties and photoinduced fluorescence changes of TPE-FUL also endow it with great potential for applications in optical information encryption. We placed the mask carved with the pattern “88” on a filter paper and sprayed TPE-FUL dissolved in methylene chloride on the filter paper. Under visible light irradiation, the digital pattern “88” was obtained as shown in Figure 5d, and it exhibited cyan fluorescence under UV light. Irradiation of part of the pattern “88” resulted in a purple-colored pattern “11” under visible light, and under UV light, a pattern “33” with cyan fluorescence was obtained. We also irradiated other parts with UV light, and obtained a purple-colored pattern “66,” which was visible to the naked eye; a cyan fluorescent pattern “11” appeared under UV light. The color and fluorescence returned to the initial state under visible light treatment at 524 nm. Through the above simple operations, we have demonstrated the excellent potential of TPE-FUL in the field of information encryption. Conclusions We fabricated a photoswitch based on a TPE-FUL, which exhibited excellent reversible changes in visual color and fluorescence with thermal stability and fatigue resistance under alternating UV and visible light treatments. The fluorescent switch showed fluorescence changes based on FUL in the solid state. TD-DFT calculations and analysis showed that the luminescence of O-TPE-FUL is attributed to the LE state of the TPE moiety, while the fluorescence quenching of C-TPE-FUL is due to ICT from TPE to FUL. Finally, we successfully demonstrated applications of TPE-FUL as an anticounterfeiting ink for optical erasing and information encryption. We believe that this fluorescence switch has important implications for the application expansion of advanced solid-state photoswitches. Supporting Information Supporting Information is available and includes synthesis and characterization of TPE-FUL, preparation of TPE-FUL electrospun PAN fiber film, some photophysical properties of TPE-FUL, Scheme S1, and Figures S1–S12. Conflict of Interest There is no conflict of interest to report. Funding Information This work was financially supported by the National Natural Science Foundation of China (nos. 21835001, 51773080, 21674041, and 52073116), Program for Changbaishan Scholars of Jilin Province, and the “Talents Cultivation Program” of Jilin University.