Open AccessCCS ChemistryCOMMUNICATION7 Dec 2022Influence of Wallach’s Rule on Chiral AIE Systems and Its Application in Cryptographic Information Storage Qiu Chen Peng, Xi Ming Luo, Yu Jing Qin, Ting Wang, Bing Bai, Xi Long Wei, Kai Li and Shuang Quan Zang Qiu Chen Peng Henan Key Laboratory of Crystalline Molecular Functional Materials, Henan International Joint Laboratory of Tumor Theranostical Cluster Materials, Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Xi Ming Luo Henan Key Laboratory of Crystalline Molecular Functional Materials, Henan International Joint Laboratory of Tumor Theranostical Cluster Materials, Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Yu Jing Qin Henan Key Laboratory of Crystalline Molecular Functional Materials, Henan International Joint Laboratory of Tumor Theranostical Cluster Materials, Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Ting Wang Henan Key Laboratory of Crystalline Molecular Functional Materials, Henan International Joint Laboratory of Tumor Theranostical Cluster Materials, Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Bing Bai Henan Key Laboratory of Crystalline Molecular Functional Materials, Henan International Joint Laboratory of Tumor Theranostical Cluster Materials, Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Xi Long Wei Henan Key Laboratory of Crystalline Molecular Functional Materials, Henan International Joint Laboratory of Tumor Theranostical Cluster Materials, Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Kai Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Henan Key Laboratory of Crystalline Molecular Functional Materials, Henan International Joint Laboratory of Tumor Theranostical Cluster Materials, Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author and Shuang Quan Zang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Henan Key Laboratory of Crystalline Molecular Functional Materials, Henan International Joint Laboratory of Tumor Theranostical Cluster Materials, Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201998 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Aggregation-induced emission luminogens (AIEgens) exhibit excellent luminescence properties in the aggregated state. For chiral AIEgens, a combination of right- and left-handed objects can be packed more tightly than homochiral objects due to Wallach’s rule. However, the manner in which Wallach’s rule can affect the luminescence properties of chiral AIEgens remains unclear. We report that, in contrast to their homochiral counterparts, racemic AIEgens obtained using binaphthyl-2,2′-diamine salicylaldehyde Schiff-bases ( BINAPAS) exhibit racemism-enhanced emission. It was found that the intramolecular motion of racemic BINAPAS is more restricted than that of chiral BINAPAS due to its denser packing mode, which significantly enhances the emission intensity of racemic BINAPAS. Based on this finding, a novel single-molecule cryptographic information storage system was constructed. This work offers a new perspective for understanding the influence of Wallach’s rule on the aggregation-induced emission properties of organic molecules, and more importantly, extends Wallach’s rule to the fundamental difference in chiral-photophysics. Download figure Download PowerPoint Introduction Aggregation-induced emission (AIE) is a luminescence phenomenon in which molecules exhibit a much stronger emission in the aggregated state than in the single-molecule state, which provides an effective strategy for the development of multifunctional luminescent solid materials.1–5 Emission properties, such as the wavelengths and lifetimes of AIE molecules, are significantly affected by the packing modes.6–9 Thus, to guide the development of new luminescent solid materials, it is very important to understand the effect of the packing modes of molecules on their emission properties. For chiral AIE molecules, the packing mode is described by the long-standing Wallach’s rule, which states that for chiral molecules, the single crystal of the racemic form is always denser than its chiral counterpart.10–19 Thus, racemic aggregation-induced emission luminogens (AIEgens) and their homochiral enantiomers should exhibit different emission properties in theory. However, the manner in which Wallach’s rule affects the AIE properties of chiral luminogens remains unclear. To solve this problem, we herein present chiral AIEgens obtained using binaphthyl-2,2′-diamine salicylaldehyde Schiff-bases ( BINAPAS). In contrast to their homochiral counterparts, the obtained racemic AIEgens exhibit racemism-enhanced emission (REE). BINAPAS AIEgens were prepared through a simple one-step reaction. The cross packing between alternating R- and S-forms in rac- BINAPAS crystals significantly restricted their intramolecular motions, and hence, suppressed the non-radiative pathway more effectively, thereby boosting the emission intensity. In contrast, the packing of homochiral BINAPAS, which is looser than that of the racemic BINAPAS, is affected by Wallach’s rule. As a result, the more active intramolecular motion provided an efficient non-radiative pathway, quenching the emission remarkably. These unique properties of BINAPAS were successfully used for the construction of a novel single-molecule cryptographic information storage system. Additionally, the different packing modes of the racemic and homochiral forms of BINAPAS endowed them with different solubilities, leading to a striking difference in their AIE curves. Being a chiral AIE system characterized by a simple synthesis and an excellent luminescence performance, BINAPAS provides a reference system to understand the influence of Wallach’s rule on the AIE properties of organic molecules and extends Wallach’s rule to the fundamental difference in chiral-photophysics. Results and Discussion The synthetic route of BINAPAS is shown in Scheme 1. Using (R)-1,1′-binaphthyl-2,2′-diamine or (S)-1,1′-binaphthyl-2,2′-diamine ((R)- 1/(S)- 1) and different salicylaldehyde derivatives as reagents, homochiral BINAPAS derivatives ((R)- 2/(S)- 2–(R)- 5/(S)- 5) were prepared using a simple one-step synthesis with favorable yields (72%–80%). The structures of BINAPAS were confirmed via 1H NMR spectroscopy, 13C NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis, and the phase purity of BINAPAS was characterized by powder X-ray diffraction. The detailed characterization data can be found in the Supporting Information Figures S17–S48. Racemic BINAPAS ((rac)- 2–(rac)- 5) were obtained by mixing the enantiomers in dichloromethane in a 1∶1 ratio, and the solvents were then removed through distillation under vacuum. Scheme 1 | Synthetic route of BINAPAS AIEgens. Download figure Download PowerPoint BINAPAS exhibited an intense emission in the aggregated state, which was investigated first (Figure 1a). For convenience, unless stated otherwise, BINAPAS ((R)- 2–(R)- 5) with the rectus configuration will be used as a representative example for homochiral BINAPAS AIEgens in the following discussions. For example, the luminescence quantum yield of (R)- 2 in the solid state was 4.0%, which is much higher than that in the EtOH solution, indicating a typical AIE characteristic. Normally, the AIE curve is used to evaluate the AIE features of molecules, showing that the emission intensity varies with the proportion of good to poor solvents.20 As shown in Figure 1b, a limited emission occurred for (R)- 2 and (rac)- 2 in EtOH, while intense emissions were observed in solutions with a high water fraction (fw) due to the formation of aggregates, which was confirmed by dynamic light scattering measurements ( Supporting Information Figures S1 and S2). As shown in Supporting Information Figure S3, the Stocks shift of BINAPAS reached >160 nm, which can be attributed to the characteristic excited-state intramolecular proton transfer process.21 It was noted that the AIE curves of (rac)- 2–(rac)- 5 were distinct from those of (R)- 2–(R)- 5 (Figure 1c, this aspect will be discussed later). In addition to the axial chirality inherent in individual BINAPAS molecules, (R)- 2–(R)- 5 and (S)- 2–(S)- 5 also have chiral helical arrangements in their packing structures ( Supporting Information Figure S4). The chiral structure and intense luminescence endow (R)- 2 and (S)- 2 with potential circularly polarized luminescence (CPL) properties.22–24 Thus, the circular dichroism (CD) spectra and CPL spectra of (R)- 2 and (S)- 2 were recorded. As shown in Figure 1d, (R)- 2 and (S)- 2 exhibited an excellent mirror-image relationship in the CD spectra. The absorption anisotropy factor (gCD) of the maximum peak was on the order of 10−4. The CPL spectra of the (R)- 2 and (S)- 2 enantiomers exhibit a wide peak around the maximum emission wavelength with a remarkable luminescence anisotropy factor (glum) on the order of 10−3 (Figure 1e and Supporting Information Table S1). Similarly, the AIE and CPL characteristics were also observed for (R)- 3/(S)- 3–(R)- 5/(S)- 5, and the corresponding data are shown in Supporting Information Figures S5–S7. Figure 1 | (a) Photograph of (R)-2 and (rac)-2 in solid state and different solutions under UV light irradiation. (b) Fluorescence spectra of (R)-2 and (rac)-2 in H2O/EtOH mixtures with different fw values. (c) Fluorescence intensity of (R)-2 at 537 nm and (rac)-2 at 534 nm as a function of fw. (d) CD spectra of (R)-2/(S)-2 in solid state. (e) CPL spectra of (R)-2/(S)-2 in solid state. Download figure Download PowerPoint As shown in Figure 2a and Supporting Information Figures S8 and S9, the respective fluorescence spectra and ultraviolet diffuse reflectance spectra of (rac)- 2–(rac)- 5 were similar to those of (R)- 2–(R)- 5 in the solid state. However, the luminescence quantum yields of (rac)- 2–(rac)- 5 were 1.5 to 6.6 times higher than those of (R)- 2–(R)- 5, respectively, while the lifetimes of (rac)- 2–(rac)- 5 were 1.6 to 2.9 times longer than those of (R)- 2–(R)- 5, respectively (Table 1). As racemic BINAPAS and homochiral BINAPAS have the same molecule structure, the occurrence of the REE can only be attributed to the different packing structures. This is described by the long-standing Wallach’s rule, which states that for chiral molecules, the single crystal of the racemic form is always denser than that of the chiral form. Next, the crystal structures of (rac)- 2–(rac)- 5 and (R)- 2–(R)- 5 were investigated via single-crystal X-ray diffraction ( Supporting Information Figures S4, S10–S14, and Tables S2–S5). As shown in Table 1, compared with the spiral arrangement of (R)- 2–(R)- 5 and (S)- 2–(S)- 5, (rac)- 2–(rac)- 5 showed tighter racemic arrangement, the respective crystal densities of (rac)- 2–(rac)- 5 were higher than those of (R)- 2–(R)- 5, which is in good agreement with Wallach’s rule. According to the literature, restricted intramolecular motion (RIM), which limits the non-radiative transition of the excited-state electrons toward the ground state, is one of the most important pathways for enhancing the emission of AIEgens in the aggregated state, resulting in a high luminescence quantum yield.25–28 For salicylaldehyde Schiff-base AIEgens, the C–N single bond is a crucial element for the AIE properties. Indeed, the rotation of this bond significantly influences the emission intensity.29,30 We propose a possible mechanism for the different emission properties of (rac)- 2–(rac)- 5 and (R)- 2–(R)- 5 (Figure 2b): the closer packing endows (rac)- 2–(rac)- 5 with stronger intermolecular interactions, thus effectively restricting the intramolecular motions and enhancing emission intensity. Figure 2 | (a) A comparison of fluorescence spectra between (R)-2 and (rac)-2 in solid state. (b) A possible mechanism for the different emissions of (R)-2 and (rac)-2. (c) Fluorescence spectra of (R)-2 and (rac)-2 in EtOH and glycerol, respectively. (d) Photograph of (R)-2 and (rac)-2 in solid state at different temperature. (e) Fluorescence spectra of (R)-1 in H2O/EtOH mixtures with different fw values. (f) A comparison of fluorescence spectra between (R)-1 and (rac)-1 in solid state. Download figure Download PowerPoint Table 1 | Physicochemical Parameters of the Compounds ρ (g cm−3) m.p. (°C) τ (ns) φ (%) (rac)- 2 1.32 287.0–289.7 2.58 16.9 (R)- 2 1.27 241.8–243.0 1.14 4.0 (rac)- 3 1.34 290.5–292.1 2.58 15.1 (R)- 3 1.25 274.9–275.7 0.91 2.3 (rac)- 4 1.39 272.6–273.8 2.26 9.0 (R)- 4 1.19 205.6–206.7 1.16 1.8 (rac)- 5 1.57 281.2–281.7 1.78 3.4 (R)- 5 1.51 176.6–178.9 1.13 2.2 Note: ρ, density of the crystal; m.p., melting point; τ, luminescence lifetime; φ, luminescence quantum yields. To confirm this hypothesis, a series of experiments were carried out. First, the fluorescence spectra of (rac)- 2–(rac)- 5 and (R)- 2–(R)- 5 were recorded in glycerol, a solvent with high viscosity (η = 1180 cP at 20 °C), which significantly restricts the intramolecular motion of solute molecules.31,32 As shown in Figure 2c and Supporting Information Figure S15, the fluorescence spectra of (rac)- 2–(rac)- 5 and (R)- 2–(R)- 5 in glycerol are almost the same, suggesting that these forms have the same emission properties in the dispersed state. Additionally, their emission intensity in glycerol was much higher than that in EtOH (η = 1.2 cP at 20 °C), which indicates that the enhanced emission intensity of (rac)- 2–(rac)- 5 and (R)- 2–(R)- 5 originates from the RIM process. Interestingly, the infrared spectra of (rac)- 2–(rac)- 5 and (R)- 2–(R)- 5 are almost the same; the only difference appears in the region around 1350–1400 cm−1, which is assigned to the bending vibration frequency of the C–N single bonds ( Supporting Information Figure S16). These results indicate that the C–N single bonds in (rac)- 2–(rac)- 5 and (R)- 2–(R)- 5 have different vibrational modes, supporting the proposed mechanism (Figure 2b). The intermolecular contacts of (rac)- 2–(rac)- 5 and (R)- 2–(R)- 5 in the solid state were further investigated. As shown in Table 1 and Figure 2d, the respective melting points of (rac)- 2–(rac)- 5 are significantly higher than those of (R)- 2–(R)- 5, indicating that the intermolecular contacts of (R)- 2–(R)- 5 are much weaker than those of (rac)- 2–(rac)- 5, which is in good agreement with Wallach’s rule. The stronger respective intermolecular interactions of (rac)- 2–(rac)- 5 compared with those of (R)- 2–(R)- 5 endow the former with a lower solubility than the latter. As a result, the respective AIE curves of (rac)- 2–(rac)- 5 are different from those of (R)- 2∼(R)- 5. The emission enhancement of (rac)- 2–(rac)- 5 was observed at a lower fw (Figure 1b and Supporting Information Figures S5b–S7b, indicated by a purple shadow). Furthermore, the chiral aggregation-caused quenching (ACQ) molecule of (R)- 1 was used as a control compound to understand the different influences of Wallach’s rule on the emission properties of luminescent molecules. As shown in Figure 2e, (R)- 1 exhibited an intense blue emission in the EtOH solution but a weak emission in a water-dominated solution, which is a typical ACQ feature. Interestingly, in contrast to the AIE molecules, (rac)- 1 exhibited a weaker emission than (R)- 1 in the solid state, which we attribute to the compact π–π stacking in racemic compounds, providing a non-radiative pathway for the excited-state electrons (Figure 2f).33–35 Inspired by the REE of BINAPAS, a novel single-molecule cryptographic information storage system was constructed. The operating principle of the system is shown in Figure 3a. Cross and circular patterns were recorded on a paper with (R)- 2 and (S)- 2, respectively. After spraying the (S)- 2 solution, the circular pattern was invisible, and the cross pattern appeared due to the formation of (rac)- 2, which exhibits a more intense emission than (R)- 2 and (S)- 2. In contrast, if the (R)- 2 solution was sprayed on the paper, the cross pattern was invisible, but the circular pattern appeared. Due to the same molecule structure of the enantiomers, hidden information is only displayed upon the addition of the right enantiomer (the wrong enantiomer would provide opposite information). Such a characteristic renders this single-molecule cryptographic information storage system much safer than the classic invisible ink. Figure 3 | (a) Schematic diagram of single-molecule cryptographic information storage system based on chiral AIE molecules. (b) An example of the cryptographic information storage using (R)-2 and (S)-2. Download figure Download PowerPoint Based on this operating principle, a “123” cryptographic information was stored on the paper (Figure 3b and Video 1). Three “8” letters were printed on paper with (R)- 2 and (S)- 2, and the “123” characters were recorded using (S)- 2. As (R)- 2 and (S)- 2 have the same emission properties, the “123” characters were completely invisible under daylight or UV light. After spraying the (R)- 2 solution on the paper, the hidden “123” characters immediately appeared under UV light. Conclusion The influence of Wallach’s rule on the AIE properties of organic molecules was investigated in this work using BINAPAS chiral AIEgens. BINAPAS exhibited unique REE properties in the aggregated state. The intramolecular motion of racemic BINAPAS was found to be more restricted than that of homochiral BINAPAS due to its denser packing mode. As a result, the emission intensity of racemic BINAPAS was much higher than that of homochiral BINAPAS. Based on the observed REE property, a novel single-molecule cryptographic information storage system was successfully constructed, which exhibits a higher security performance than the classic invisible ink. This work offers a new perspective for understanding the influence of Wallach’s rule on the AIE properties of organic molecules, and more importantly, extends Wallach’s rule to the fundamental difference in chiral-photophysics. Supporting Information Supporting Information is available and includes experimental details, selected spectra and data referred to in the paper ( Figures S1–S48 and Tables S1–S5), the video caption, and crystal data. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (grant nos. U1904172, 92061201, 21825106, and 21788102), the Program for Science & Technology Innovation Talents in Universities of Henan Province (grant nos. 164100510005 and 22HASTIT002), and the Excellent Young Scientist Foundation of Henan Province (grant no. 202300410374).