Multicolor Circularly Polarized Luminescence of a Single-Component System Revealing Multiple Information Encryption

Open AccessCCS ChemistryRESEARCH ARTICLES3 Feb 2024Multicolor Circularly Polarized Luminescence of a Single-Component System Revealing Multiple Information Encryption Ying Hu†, Zizhao Huang†, Itamar Willner and Xiang Ma Ying Hu† Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 , Zizhao Huang† Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 , Itamar Willner Institute of Chemistry, The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904 and Xiang Ma *Corresponding author: E-mail Address: [email protected] Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 https://doi.org/10.31635/ccschem.023.202302904 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Metal-free materials with multicolor tunable circularly polarized luminescence (CPL) are attractive because of their potential applications in information storage and encryption. Here, we designed two enantiomers composed of chiral dialkyl glutamides and achiral vibration-induced emission (VIE) moiety, which can switch on CPL after a simple gelation process. It is noteworthy that the CPL colors vary in different solvents, and this is attributed to various self-assembly-induced microstructures, in which the VIE moiety is restrained to different degrees. Accordingly, a multidimensional code system composed of a quick response code, a ultraviolet (UV) light-activated color code, and a CPL information figure was constructed. To our satisfaction, the system possesses multiple information-storage functions. The orthogonal anticounterfeiting and CPL-enhanced encryption functions also improve the system information encryption ability. In brief, this study provides a practical example of CPL applied to information security and an effective approach to obtain a single-component color-tunable CPL material with multiple information storage and encryption functions as well. Download figure Download PowerPoint Introduction Chirality can be seen everywhere in nature. Photochemistry has received a lot of attention in recent years,1–3 and circularly polarized luminescence (CPL) is an intuitive manifestation of molecular chirality in the excited state, which is of great significance in photochemistry. Materials and molecules with CPL have received ever-increasing attention due to their numerous applications in photoelectric devices such as the organic light-emitting diode,4–8 quantitative detection,9 asymmetric catalysis,10 information encryption and decryption,11 quantum computation,12 chiral recognition,13–15 noninvasive cancer diagnosis,16 and more. In recent years, substantial research efforts have been diverted to explore CPL, and significant developments in the field have been achieved.17–24 Self-assembly without demanding design and tedious synthesis of complex molecules with multiple functional groups is promising and has become an important method to design CPL materials,25–27 functional components including complex systems,28–30 host–guest systems,31–34 chiral polymer systems,35,36 liquid crystal systems,37–39 hydrophilic and hydrophobic systems,40,41 and metal–organic framework systems42 have consequently been developed to expand self-assembly-inducing systems. Recently, Tian's group elaborately explored a class of intriguing N,N′-disubstituted-dihydrophenazine derivatives, which exhibit unique environmentally sensitive dual fluorescence emission, namely vibration-induced emission (VIE). Diverse research activities stemming from the special properties of this kind of molecules were developped.43–46 As shown in Figure 1, the molecules exhibited a considerable steric hindrance in the molecular structure that resulted in a saddle-shaped configuration. In solution, the molecular configuration tended to change from bent-shape to planar-shape through the intramolecular vibrations, leading to the orange-red fluorescence upon excitation. In the solid or aggregated state, the rigidification blocked the vibration of the structure in the excited state, resulting in an emission of intrinsic blue light. In addition, the different assemblies of the VIE derivatives were found to have distinct optical and morphological characteristics. The regulated VIE molecules-based emission has also been reported to monitor the self-assembly process. When the VIE moiety was in relatively rigid assemblies, the vibrational motions of the VIE moiety were restricted, and thus the blue emission gradually emerged. While in a relatively soft assembly, the occurrence of molecular vibrations led to the emission of red light. Inspired by synthetic flexibility to modify VIE at the molecular level, we reasoned that noncovalent interactions between VIE molecules in suitable supramolecular assemblies can be utilized to fabricate multicolor CPL materials. Figure 1 | Structures of DG/LG-DPAC and the mechanism of the VIE. Download figure Download PowerPoint Here, by proper molecular modification, we exerted multiple asymmetric fields to the VIE scaffold to induce them to self-assemble into luminescent supramolecular assemblies with adjustable emission properties. The supramolecular organogels, based on the self-assembly of building blocks with diverse noncovalent interactions and physical functions orthogonally integrating at the molecular scale and synergistically organizing into supramolecular assemblies, offer a facile way to adjust the properties of the gels and endow the gels with switchable emission functions. Hence, the VIE moiety and the dialkyl glutamides were combined to obtain a VIE-based supramolecular gelator DG/LG-DPAC which exhibited favorable gelation ability in universal organic solvents ranging from polar to nonpolar environments. The gelation process enabled the switch of CPL. No CPL was generated in the system before the gelation process, but after the gelation process, significant CPL signals appeared in the system. Interestingly, CPL colors varied in different solvents, and distinct morphological characteristics were associated with the color transitions. The noncovalent bond-driven self-assembly regulated the excited-state deformation of VIE molecules as a structural element in the preparation of controllable emission. Inspired by the multicolor CPL property and a three-dimensional (3D) color code smart application, multidimensional information storage and encryption were realized by multiple codes. Two-dimensional (2D) quick response (QR) code, 3D color code, and four-dimensional (4D) CPL information figure stored large levels of information, resulting in an information folder. It is worth noticing that the orthogonal light-response anticounterfeiting property between the QR code and the color code strengthens the multiple information encryption functions. Further, the invisible CPL signals hidden in the color code worked as a secret lock to encrypt 2D and 3D information. Then only the CPL information figure obtained by verifying that the CPL signal is correct and can open the secret lock. It also provides an example of the CPL signal applied in information security. Experimental Methods Reagents and solvents All reagents were purchased from Sigma-Aldrich (Germany) or TCI Chemicals (Japan) and used without further purification. Solvents were purified according to standard laboratory methods. The molecular structures of DG/LG-DPAC were confirmed using 1H NMR, 13C NMR, and high-resolution electronic spray ionization (ESI) mass spectroscopy or matrix-assisted laser desorption ionization time-of-flight mass spectroscopy. Equipment and measurements Thin-layer chromatography was used to monitor the reactions, and silica gel (300–400 mesh) was used in column chromatography. Molecular masses were determined by a Waters LCT premier XE spectrometer (Waters, United States). The UV–vis absorption spectra and photoluminescence (PL) spectra were performed on a Varian Cary 60 spectrophotometer (Varian, United States) and a Horiba Fluoromax-4 spectrometer (Horiba, Japan) at 25 °C, respectively. Phosphorescence and the lifetime of delayed emission spectra were recorded on an Agilent Cary Eclipse spectrophotometer (Agilent, United States) (phosphorescence mode; delay time = 0.1 ms; gate time = 2.0 ms). Quantum yields were measured by using an integrating sphere on a Hamamatsu Quantaurus-QY C11347-11 (Hamamatsu, Japan). Powder X-ray diffraction (XRD) was performed on a D/max2550V (Rigaku, Japan). CPL spectra were acquired using the JASCO CPL-200 spectrofluoropolarimeter (JASCO, Japan). Circular dichroism (CD) spectra were acquired using the JASCO J815 spectrophotometer (JASCO, Japan). Scanning electron microscopy (SEM) images were obtained by using an S-3400N (Hitachi, Japan) (droplets of the sample solution (5*10−5 M) were applied to a silicon slice, dried in air at room temperature, and then coated with nano Au in a vacuum). Infrared spectral dates were obtained by Fourier transform infrared (FT-IR) spectrometer 7800∼350/cm 0.01/cm/6700 (Thermo Nicolet, United States). Rheological properties were measured by using a TA Discovery HR-3 rheometer (TA Instruments, United States). Photographs were taken with a digital camera. Preparation of gels The preparation of supramolecular gels was carried out under ambient temperature conditions. A weighed sample of LG-DPAC or DG-DPAC (5 mg) was mixed with corresponding solvents (1 mL) in a vial and heated until the solid was dissolved. Then the sample vial was cooled to room temperature. If no flow was observed when inverting the vial, a stable gel had been formed. The light source used was UV lamp (λ = 365 nm, 5 W), and the distance of UV source-sample used in the irradiation experiments was 10 cm with a light intensity of 35 mW cm−2 in the dark. Preparation and scanning app of 3D color code The preparation of supramolecular gels was carried out under ambient temperature conditions. According to the color distribution of the designed 3D color code, weighed samples of LG-DPAC or DG-DPAC (1 mg) were mixed with various solvents (0.2 mL) in the vials to obtain corresponding colors. Vials with LG-DPAC or DG-DPAC were orange units in toluene (TOL) under UV light, white in ethyl acetate (EA), and blue in methanol (MeOH). What's more, empty vials were identified as black units due to the presence of black background plates. After formation of stable gels, the vials were arranged according to the color code image. The 3D code and corresponding scan software used in the present study was COLORCODE® ( http://colorzip.com). At the time of publication, this information-reading application was available free of charge for use in code scanning but not, to the authors' knowledge, for any other purpose. Results and Discussion Optical properties Compounds DG/LG-DPAC were prepared according to the synthetic route presented in Supporting Information Figure S1. 1H NMR, 13C NMR, and ESI high-solution mass spectroscopy analyses were employed to confirm their structures (for detailed characterization data, see Supporting Information Figures S2–S7). The VIE-based gelator DG/LG-DPAC were composed of achiral VIE moiety and the chiral dialkyl glutamides group, which was linked by the covalent amide linkage. As shown in Supporting Information Figures S8 and S9, the introduction of the chiral dialkyl glutamides group did not affect the VIE property unambiguously.47 In tetrahydrofuran, we observed that the absorption bands were almost all in the UV absorbance region. Meanwhile in the emission spectrum the long-wavelength emission was located at around 600 nm, while the short-wavelength emission was at around 450 nm. This was consistent with the absorption and emission spectra of the VIE molecules. Rheological tests were performed on organogels to investigate their mechanical properties ( Supporting Information Figure S10). As shown in the strain sweep measurement, the values of the storage modulus (G′) were distinctly larger than the loss modulus (G″), confirming the formation of the organogels and good mechanical properties. Furthermore, we unexpectedly found that the formation of supramolecular organogels proceeded in diverse organic solvents, as shown in Supporting Information Table S1, suggesting the excellent capacity for the formation of organogels. To explore the optical properties of DG/LG-DPAC before and after the gelation process, three representative solvents TOL, EA, and MeOH were selected from low polarity to high polarity for a series of tests. As shown in Supporting Information Figure S11, in different solvents and in the solid state the fluorescence emission spectra of DG/LG-DPAC were not the same; they had different intensity ratios at the blue and red bands. After the gelation process, the organogels also displayed different colors. As Figure 2a showed, there were two bands at 460 and 600 nm in the fluorescence emission spectrum of organogel formed in TOL. A bright orange-red light was produced due to the combination of the two emission bands. The gel formed in EA produced a near-white emission because the ratio of the band around 460 nm and the band around 600 nm increased. In the organogel formed in MeOH, the band located at around 600 nm almost disappeared, and then the gel emitted blue light. We also noticed that the fluorescence quantum yield (FQY) in the MeOH was 12.7%, while in the EA it reached 23.1%, and in the TOL the FQY was 28%. The 1931 CIE coordinate points are displayed in Figure 2b and the color transitions can be seen clearly. Meanwhile, as shown in Figure 2c–e, by comparing before and after the gelation process, the luminescence intensity was strengthened after the gel-formation process. It is noteworthy that the enhancement in the band intensity was mainly at the blue band. We believe that the nonradiative transition of the DG/LG-gel DPAC was restricted, and the vibrations of the DPAC moiety were blocked by the gelation process. The blue band was intensified to a greater extent, indicating that the bent-to-planar transition was suppressed to a certain extent. Consequently, more DG/LG-gel DPAC molecules stayed in the bent state after the gelation process. Figure 2 | (a) Fluorescence emission (solid line) and absorption (dotted line) spectra of the DG/LG-DPAC gels in various organic solvents (top: TOL, mid: EA, and bottom: MeOH). (b) The corresponding 1931 CIE-coordinated diagram. The fluorescence emission spectral changes of DG-DPAC gel in (c) TOL, (d) EA, and (e) MeOH upon cooling (5 K/min, from 333 to 263 K), the red lines are the emission spectra of the solution state, and the blue lines correspond to the emission spectra of the gel state. The excitation wavelength is 365 nm. Download figure Download PowerPoint Chiral properties CD and CPL were employed to investigate the chirality of the organogels. As shown in Supporting Information Figure S12, there were no CD signals in solution. Yet, after the gelation process, the three pairs of chiral gels all showed relatively good symmetric CD spectra, and the Cotton Effects matched the absorption spectra ( Supporting Information Figures S13–S18) well. The LG-DPAC exhibited positive signals, and the DG-DPAC exhibited negative signals ( Supporting Information Figure S19). In Figure 3a–d, as in the CD spectra, there was no CPL in the solution state ( Supporting Information Figures S20–S22). Yet, in the gel state the CPL was switched on, and the CPL of different colors was obtained. Further, the CPL spectra corresponded well with fluorescence emission spectra. Luminescent dissymmetry factor (Glum) is an important parameter of CPL properties, and the Glum values corresponded to 7.3*10−4, 6.7*10−4, and 7.7*10−4, respectively. Figure 3 | The CPL spectra of DG/LG-gel DPAC in various organic solvents (a) TOL, (b) EA, (c) MeOH, and (d) Glum spectra in different solvents (top: TOL, mid: EA, and bottom: MeOH), the excitation wavelength is 365 nm. Download figure Download PowerPoint Meanwhile, the CPL signals were consistent with CD, the signals of LG-DPAC were positive signals, and the signals of DG-DPAC were negative signals. It is believed that the gelation process allowed molecules to assemble into supramolecular microstructures. The chiral induction between the chiral dialkyl glutamides group and the achiral VIE moiety was therefore achieved. In this case, regulated multicolor CPL was realized in the achiral VIE moiety. Microstructures and morphologies Upon exploring the reasons for the different CPL emissions of the organogels formed in the various solutions, we found that the microstructures of the gels changed concomitantly with the luminescence transition. We firstly got xerogels from organogels in diverse solvents by a simple natural volatilization process, and the nanostructures of the xerogels were characterized by SEM. Interestingly, the diverse self-assembled nanostructures were clearly observed in SEM ( Supporting Information Figure S27). In the case of organogels in TOL, several uniform nanorods were obtained with helical conformation, which suggested the organization in one dimension. The tightly wrapped nanofibers were observed from EA xerogels. Meanwhile, in the xerogels constructed in MeOH, we observed that these spirals formed a twisted flaky structure which resembled a flower in its entirety. Hence, these microscopic investigations demonstrated that the polarity of organic solvent dramatically influenced the spatial alignment of DG-DPAC gels and then induced a broad range of morphological variation from nanofibers to nanorods to twisted flaky structures. To further demonstrate that CPL emission derives from self-assembly, DG-DPAC was chosen as the representative to be doped into polymethyl methacrylate (PMMA) to obtain a flexible film ( Supporting Information Figure S23). The emergence of phosphorescence ( Supporting Information Figures S25 and S26) was attributed to the attenuation of nonradiative transitions and vibrations triggered by the rigid environment generated by PMMA ( Supporting Information Figure S24). Nevertheless, the self-assembly process of DG-DPAC was destroyed by the rigid environment generated by PMMA, resulting in the destruction of the chiral structure and the disappearance of the CPL signal. Moreover, we have measured the XRD patterns and FT-IR spectra of DG-DPAC in xerogels to understand the packing in the supramolecular organogels. As shown in Supporting Information Figure S28, organogels formed in these three solutions had almost identical band shapes, indicating that multicolor is not the effect of molecular changes and different groups in the molecule. Meanwhile, in Supporting Information Figure S29, the XRD patterns revealed that the xerogel formed in TOL had three bands that appeared at 2θ values of 1.84°, 3.68°, and 5.52°, respectively. Thus, it was suggested that the DG-DPAC molecule self-assembled into an orderly structure. We also noticed that in EA the XRD band shape changed, and there was only one band that situated at 2θ = 1.84°. Meanwhile, the xerogel formed in MeOH did not show any bands in the XRD patterns. These results confirmed that the multicolor tunability of organogels was related to the morphology. Multidimensional code system Information storage and anticounterfeiting have attracted growing interest with the rise of the information age. Nonetheless, there is a lack of systems that combine multiple information encryption with multiple information storage. Herein, the commonly used 2D QR code, the widely valued 3D color code48,49 and the promising CPL signal were combined to form a multifunctional information storage and information encryption system, called the multidimensional code system. Small vials containing organogels formed in various solvents were arranged as a 5*5 square matrix on a black background board. Based on their multicolor tunable property, orange-red, near-white, and blue were introduced into the square matrix as element colors to fabricate a color code. Moreover, by adjusting the arrangement, various color codes were obtained to get diverse information. As illustrated in Figure 4a, the 3D information was obtained by scanning with a smartphone application of COLORCODE under UV light (365 nm) but could not operate under natural light. Meanwhile, the color code on the right was scanned to access information of "VIE" after being excited by UV light, while the color code on the left was scanned to access information of "CPL." It is noteworthy that the combination of QR code with color code can be orthogonal. As shown in Supporting Information Videos S1–S6, under natural light through scanning, the 2D information turned on and the QR code output "ECUST," but the 3D information turned off. In contrast, under UV light the opposite phenomenon was observed, where the 3D information turned on and the color code output "CPL" or "VIE," but the 2D information turned off (Figure 4b). Figure 4 | (a) 3D color-code scanning illustration (left), color-code photos and scanning images under natural and UV irradiation (right). (b) The orthogonal scanning illustration of 3D color code and 2D QR code (left), the orthogonal property photos under natural light and UV light as well as the orthogonal scanning images of color code and QR code (right). Download figure Download PowerPoint Most importantly, CPL was introduced to expand the scope of information storage and strengthen information encryption. As shown in Figure 5a, the CPL signal test was performed on each vial in the color code, and positive signals, negative signals, and no signal were obtained based on the chirality of gels. Among them, the CPL vials with positive signal were marked as black, the CPL vials with negative signal as light gray, and the CPL vials with no CPL signal as dark gray, and then a CPL information picture could be formed. In Figure 5b, four CPL information pictures are provided. They correspond to the same color code, and the difference between them is the chirality of the gels that make up the color code. Meanwhile, for each 3D color code, this 4D CPL information picture has 325 combinations and arrangements. Consequently, the CPL information figure can be regarded as a secret lock to protect 2D and 3D information. First, we defined only the information figure "212" in Figure 5c as key, and then the gels of color code were tested to output a CPL information figure. If the output figure was exactly "212," the 2D and 3D information were unlocked and considered true and correct; however, if the output figure was not "212," the 2D and 3D information were locked and considered untrue and false. That is, the CPL signals encrypted the QR code and the color code. Further, when used for information storage, the CPL information figure could be regarded as a 4D CPL information folder to broaden the information storage. Figure 5 | (a) Color-code CPL signal decoding process illustration. (b) Four CPL information figures decoded from the same color code, named as "212," "101," "213," and "42," respectively. (c) If the color code was decoded to be "212," the 2D and 3D information was unlocked (left). If the color code was decoded to be "101" or another CPL information figure, it could not be unlocked (right). Download figure Download PowerPoint Conclusions In summary, we obtained a supramolecular organogel by combining the VIE moiety and chiral dialkyl glutamides. The DG/LG-DPAC molecules exhibited diverse optical phenomena, and they showed fluorescence of different colors in different solvents without CD and CPL signals. However, after the gelation process, the CPL and CD switched on, and CD signals were obtained in the gels formed in the three tested solvents TOL, EA, and MeOH. Meanwhile, obvious CPL signals were obtained in these gels, and their emission colors were different. The gel formed in TOL exhibited orange-red CPL, EA near-white CPL and MeOH blue CPL. It is noteworthy that these colors corresponded perfectly to the double emission of the VIE moiety. It is reasonable to infer that different CPL emissions originated from the fact that the VIE unit existed in different microenvironments. Concomitant to the color changes of the gels formed in different solutions, SEM confirmed that the gels with VIE properties possessed different microstructures in various solutions. Based on the multicolor CPL property of DG/LG-DPAC, a multidimensional code system composed of 2D QR code, 3D color code, and 4D CPL information figure was constructed. The color code under UV light can be scanned to output information. Furthermore, the combination of color codes with 2D QR code possessed an orthogonal encryption function. The UV light simultaneously turns on the 3D information and turns off the 2D information . Interestingly, the 4D CPL information picture based on the CPL signal can be designed as a secret lock of 2D and 3D information; only the right CPL information figure can open the secret lock. Above all, this work provides a single-component, color-tunable CPL material that can be used in information storage and anticounterfeiting. It also provides basic concepts for the design of single-component CPL systems and the application of CPL. Supporting Information Supporting Information is available and includes general procedures, characterization of products, and other spectroscopic data. Conflict of Interest The authors declare no other competing interests. Funding Information We gratefully acknowledge the financial support from the National Key Research and Development Program of China (grant no. 2022YFB3203500), the National Natural Science Foundation of China (grant nos. 21788102, 22125803, and 22020102006), project support by the Shanghai Municipal Science and Technology Major Project (grant no. 2018SHZDZX03), the Program of Shanghai Academic/Technology Research Leader (grant no. 20XD1421300), and the Fundamental Research Funds for the Central Universities. Acknowledgments The authors thank Dr. B. Ding and Y. Zhao for helpful discussions. References 1. Wang J.; Dang Q.; Gong Y.; Liao Q.; Song G.; Li Q.; Li Z.Precise Regulation of Distance Between Associated Pyrene Units and Control of Emission Energy and Kinetics in Solid State.CCS Chem.2021, 3, 274–286. Link, Google Scholar 2. 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