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• Metal–organic cages (MOCs) as a special subset of supramolecular cages, have been attracted extensive attentions from scientists [1-4]. Although the components and assembly principles of MOCs are similar to metal–organic frameworks (MOFs) [5-8], the molecular nature of MOCs bestows them with easy solution-processability unattainable with traditional framework materials, and their inherently modular nature allows for atomically precise structure and composition through self-assembly. The chemical reactions involving pre-assembled MOCs, either achieved on the surface or in the cavity, or the transformation of the post-assembled MOCs into multidimensional materials, are all defined as post-synthetic modification (PSM) of MOCs. Over the past ten years, a few stable pre-fabricated MOCs have been used for PSM, enabling unexpected functional applications [9-15]. For example, through stepwise PSM of Zr-MOC, Yuan et al. synthesized heterometal-based coordination cages, which have been shown excellent photocatalytic activity in H₂ production and CO₂ reduction [16-18]. Interestingly, the amine-functionalized NH₂-Zr-MOC reacted with acyl chloride or aromatic dialdehydes (condensation reaction) to form hyper-crosslinked MOCs, which have been shown to be useful in seawater desalination and antimicrobial studies [19,20].

  Many tetrahedral M₄L₆ cages constructed from octahedral metal ions and achiral bidentate ligands, forms intrinsically chiral structure with all metal centers having the same chiral configuration (Δ or Λ) [21-25]. However, such M₄L₆ cages usually exist as racemic mixtures in solution or in the solid- state. In most cases, these cages have only been applied as hosts in storage/encapsulation, recognition/separation, catalysis, etc. [26,27]. To explore the chiral applications, it is important to obtain homochiral MOCs. However, little attention has been paid to the resolution of MOCs because resolution of these cages is very difficult and examples of such resolution are very rare. So far only several racemic M₄L₆ cages have been successfully resolved by guest cations or enantiopure organic molecules [28-30], but little research has been done on their chiral applications especially in circularly polarized luminescence (CPL) field.

  In 2017, by using the edge-bridged strategy [31], our group successfully constructed an anionic Ti₄L₆ or Zr₄L₆ cage (L = embonate) with calixarene-like coordination-active vertices [32], which further features high solubility and stability in common solvents, providing an opportunity for PSM. By using the Ti₄L₆ or Zr₄L₆ cage as a precursor to react with metal ions or organic ligands, a series of functionally-modified cage materials were synthesized by two-step reaction [33-38]. Fortunately, we have achieved the chiral resolution of Ti₄L₆ cages, and discovered that the separated homochiral Ti₄L₆ cage shows strong CD signal peaks [39,40].

  CPL reflecting chirality in the excited states of materials requires bright luminescence and sufficiently strong chiral signals for measurement. Herein we successfully separated the racemic Zr₄L₆ cages and modified the π-conjugated coordination silver cations onto the homochiral cage, so as to realize its CPL application. Through introducing chiral organic ligands 2,2′-bis(diphenylphosphino)−1,1′-binaphthyl (R/S-BINAP) and 1,2-diphenyl-1,2-ethanediamine (1S,2S/1R,2R-DPEN), the anionic ΔΔΔΔ-Zr₄L₆ and ΛΛΛΛ-Zr₄L₆ cages were completely resolved, respectively, and the cations (such as [Ag₂(DPPM)₂]²⁺, chiral [Ag₂(PPh₃)₂(DPEN)]²⁺ and [Ag(PPh₃)(DPEN)]⁺; DPPM = bis(diphenylphosphino)methane; PPh₃ = triphenylphosphine) were modified onto the cage (Scheme 1). Accordingly, two pair of pure enantiomers (TC-375(Δ/Λ) and PTC-376(Δ/Λ)) were synthesized and structurally characterized. Remarkably, the decorated homochiral hetero-metallic cages present interesting CPL response. This work provides a new resolution strategy for racemic M₄L₆ cages and expands their chiral application.

  Scheme 1

  

  Scheme 1.  Schematic diagram of the chiral resolution and post-synthetic modification of tetrahedral Zr₄L₆ cage.

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  Though the method reported in our previous work, the Zr₄L₆ cage (PTC-101(Δ,Λ)) was synthesized in large quantities (Scheme S1 in Supporting information) [32,38]. Although Zr₄L₆ cage is a chiral one, both ΔΔΔΔ-Zr₄L₆ and ΛΛΛΛ-Zr₄L₆ isomers exist in the achiral crystal structure of PTC-101(Δ,Λ). In order to obtain the homochiral Zr₄L₆ cages, the chiral R/S-BINAP ligand was introduced into the assembly system, and DPPM ligand was also added. Both of two ligands are the ones who are large π-conjugated molecules. Accordingly, R/S-BINAP, DPPM and Ag⁺ ions were dissolved into the 1,4-dioxane/MeCN/H₂O solution of PTC-101(Δ,Λ) for solvothermal reaction, and yellowish crystals of PTC-375(Λ) and PTC-375(Δ) were obtained, respectively. Single-crystal X-ray diffraction analysis revealed both PTC-375(Λ) and PTC-375(Δ) crystallize in chiral space group P2₁ (Table S1 in Supporting information). Herein only the structure of PTC-375(Δ) will be described in detail. As shown in Fig. 1a and Fig. S1 (Supporting information), the asymmetric unit consists of five crystallographically independent Ag⁺ ions. The Ag1 and Ag4 are three-coordinated to one carboxyl O atom from the cage and two P atoms from two DPPM ligands. The Ag2 and Ag3 are four-coordinated to two carboxyl O atoms from the cage and two P atoms from two DPPM ligands. The Ag5 is four-coordinated to four P atoms from two S-BINAP ligands. There are two types of in-situ formed coordination silver cations. One is the mononuclear [Ag(S-BINAP)₂]⁺ ion, in which the Ag(I) center is P,P-chelated by two S-BINAP ligands. Another is the dinuclear [Ag₂(DPPM)₂]²⁺ ion, in which two Ag(I) atoms are bridged by two DPPM. Interestingly enough, the anionic ΔΔΔΔ-Zr₄L₆ cage captures two [Ag₂(DPPM)₂]²⁺ cations via the Ag−O coordination bonds, in which the carboxyl O atoms of the tetrahedron coordinate to the Ag atom of the [Ag₂(DPPM)₂]²⁺ moieties. The Ag−O bond lengths are in the range of 2.33−2.64 Å, which is within the normal bond length range. And then, these decorated cages are co-crystallized with chiral [Ag(S-BINAP)₂]⁺ ions by the weak interactions to form a homochiral ordered supramolecular framework (Fig. 1c and Fig. S2 in Supporting information). Expectedly, the opposite [Ag(R-BINAP)₂]⁺ units and ΛΛΛΛ-Zr₄L₆ cages can be observed in PTC-375(Λ) (Fig. S3 in Supporting information).

  Figure 1

  

  Figure 1.  (a, b) X-ray crystal molecular structures of the decorated homochiral cage enantiomers in PTC-375(Δ/Λ) and PTC-376(Δ/Λ) and (c, d) their 3D packed supramecular frameworks. Atom color code: purple, Ag; olive, Zr; pink, P; red, O; gray, C.

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  When the chiral BINAP ligand is replaced by DPEN, and the achiral DPPM is replaced by PPh₃, another pair of enantiomers (PTC-376(Λ) and PTC-376(Δ)) is synthesized. Both PTC-376(Λ) and PTC-376(Δ) crystallize in orthorhombic chiral space group P2₁2₁2₁. In the asymmetric unit of PTC-376(Δ), there are six independent Ag⁺ ions. The Ag1 and Ag3 are three-coordinated to one carboxyl O atom from the cage, one P atom from one PPh₃ ligand and one N atom from one 1S,2S-DPEN ligand. The Ag2, Ag4 and Ag5 are four-coordinated to two carboxyl O atoms from the cage, one P atom from one PPh₃ ligand and one N atom from one 1S,2S-DPEN ligand. The Ag6 is four-coordinated to four MeCN N atoms. Structure analysis reveals that each ΔΔΔΔ-Zr₄L₆ cage in PTC-376(Δ) catches one [Ag(PPh₃)(1S,2S-DPEN)]⁺ and two [Ag₂(PPh₃)₂(1S,2S-DPEN)]²⁺ cations (Fig. 1b). There are also two types of in-situ formed coordination silver cations in PTC-376(Δ), and both of them are all modified to the cage via the Ag−O coordination bonds. As shown in Fig. S4 (Supporting information), these Ag(I) centers are three- and four-coordinated. In addition, an in-situ formed [Ag(MeCN)₄]⁺ cation is firmly locked in the center of each ΔΔΔΔ-Zr₄L₆ cage, in which all the hydrogen atoms of the encapsulated [Ag(MeCN)₄]⁺ are involved in C–H···π interactions with the central aryl rings of the L ligands, and the MeCN moieties are inserted to the windows of the cage, reflecting its suitable size and shape complementarity for such tetrahedral cavity. While these decorated cages exhibit different supramolecular accumulation from PTC-375(Δ) (Fig. 1d and Fig. S5 in Supporting information). The mirror images can also be observed in the chiral structures of PTC-376(Λ) and PTC-376(Δ) (Figs. S4 and S6 in Supporting information).

  Obviously, both N,N-chelated and P,P-chelated type ligands readily coordinate with Ag⁺ ions to form cations, which are easy to assembly with anionic Zr₄L₆ cages through supramolecular interactions or coordination bonds, revealing that the Zr₄L₆ cage is highly modifiable. On the other hand, the stepwise transfer of homochirality is presented in the synthesis process. The origin of achieved homochirality is from the enantiopure ligand (R/S-BINAP or R/S-DPEN). Next, the related chiral coordination cations carried out their enantioselectivity towards anionic Zr₄L₆ cages through suitable weak interactions or coordination bonds, making successful resolution of Zr₄L₆ cages and leading to a homochiral structure with only ΛΛΛΛ-Zr₄L₆ isomers in PTC-375(Λ) or PTC-376(Λ), and ΔΔΔΔ-Zr₄L₆ isomers in PTC-375(Δ) or PTC-376(Δ). Experimental Zr:Ag molar ratios of 1:1.21 (calcd. 1:1.25) for PTC-375(Δ) and 1:1.46 (calcd. 1:1.50) for PTC-376(Δ) were measured by ICP-AES, which are identical to their formulas, respectively. The EDS spectra analyses and powder X-ray diffraction patterns (PXRD) of these compounds confirmed the phase purity of the large amount crystals (Figs. S7, S8, S11 and S12 in Supporting information). PXRD proved that their crystals are stable in air. The thermogravimetric analysis (TGA) curves and UV–vis absorption spectra are also provided (Figs. S9, S10, S24 and S25 in Supporting information).

  To demonstrate the chirality of these obtained materials, liquid-state CD spectra of PTC-375(Δ/Λ) and PTC-376(Δ/Λ) were carried out at room temperature. As shown in Figs. 2a and b, their CD peaks show perfect mirror images, indicating their absolute configuration and enantiomeric nature. In addition, we chose about 15 different single crystals of PTC-375(Λ) or PTC-376(Λ) from the same reactive bottle and tested their CD spectra (Figs. S17 and S18 in Supporting information). The similar CD peaks confirmed the complete resolution of Zr₄L₆ cages in the process of synthesizing these compounds. For PTC-375(Δ) and PTC-375(Λ), there are four obvious CD peaks around 390, 340, 290 and 265 nm, respectively. Most of CD peaks are similar to those of spontaneously resolved PTC-102(Δ) and PTC-102(Λ) with only ΔΔΔΔ-Zr₄L₆ and ΛΛΛΛ-Zr₄L₆ isomers in its chiral structure (Fig. S16 in Supporting information). Compared to single-component ΔΔΔΔ-Zr₄L₆ and ΛΛΛΛ-Zr₄L₆ cages, the only prominent CD peak in PTC-375(Δ) and PTC-375(Λ) is the one at 340 nm, which is attributed to the phenyl π–π* transition of the chiral BINAP ligand, as demonstrated by its CD spectra (Fig. S14 in Supporting information). The CD peak at 340 nm is exactly missing in both PTC-376(Δ) and PTC-376(Λ).

  Figure 2

  

  Figure 2.  (a, b) Liquid-state CD spectra of PTC-375(Δ/Λ) and PTC-376(Δ/Λ) in DMF. (c) Luminescence spectra of Zr₄L₆, PTC-375(Δ) and PTC-376(Δ) in the solid state (inset: their crystal photos at ultraviolet light (365 nm)). (d) CPL spectra of PTC-375(Δ/Λ) (λ_ex = 370 nm). (e) CPL spectra of PTC-376(Δ/Λ) (λ_ex = 370 nm). (f) Dissymmetry factors (glum).

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  The solid-state emission spectra of Zr₄L₆ cage raw material (PTC-101(Δ/Λ)), PTC-375(Δ) and PTC-376(Δ) were measured in air at room temperature. Upon excitation at 430 nm, the Zr₄L₆ cage shows an obvious emission peak at 480 nm (Fig. 2c), which may belong to the π–π* transition of the L ligands in Zr₄L₆ cage (Fig. S27 in Supporting information). Under the same excitation wavelength and test condition, PTC-375(Δ) and PTC-376(Δ) display a similar emission peak, and their emission peak can be observed at 475 and 480 nm, respectively. The excitation spectra of them are shown in Fig. S26 (Supporting information). Remarkably, both PTC-375(Δ) and PTC-376(Δ) display significantly stronger photoluminescence compared to the Zr₄L₆ cage, with emission intensities approximately twice as high. Both chiral ligands (R/S-BINAP and 1S,2S/1R,2R-DPEN) and phenylphosphine-type ligands (PPh₃ and DPPM) are highly conjugated organic ligands. Thus, modifying these π-conjugated coordination Ag cations onto the Ti₄L₆ cage obviously improved the photoluminescent property of the cage. Their PL photos also confirm the above results. Since these crystals crystallize in chiral space group, and they all belong to non-centrosymmetric structures, so their second-harmonic generation (SHG) properties were also studied. The result shows that both of them display weak powder SHG efficiencies (Fig. S28 in Supporting information).

  It is well known that CPL, which reflects the chirality of a material's excited state, requires a strong enough chirality signal and bright luminescence to be measured. Thus, the strong CD signal peaks and enhanced PL properties of PTC-375(Δ/Λ) and PTC-376(Δ/Λ) encouraged us to further study their CPL performance. Although both materials are supramolecular packed framework, they can be stable in air, as demonstrated by PXRD (Figs. S11 and S12 in Supporting information). In order to avoid the influence of solvents on the materials and ensure their structural stabilities, we tested their solid state CPL properties (gently grind about 3 mg of the sample and fix it in the middle of the sample hole in the solid sample holder). The degree of CPL can be evaluated by the dissymmetry factors (g_lum), which is defined as g_lum = 2(I_L - I_R)/(I_L + I_R), where I_L and I_R represent the luminescence intensities of left- and right-handed polarized light, respectively. As shown in Figs. 2d and e, there are obvious CPL emission bands of PTC-375(Δ/Λ) and PTC-376(Δ/Λ) enantiomers. The CPL extremum of PTC-375(Δ/Λ) can be observed at 480 nm with the g_lum of −0.41×10⁻² and 0.51×10⁻², and the CPL extremum of PTC-372(Λ/Δ) can be observed at 500 nm with the g_lum of −1.36×10⁻² and 1.36×10⁻², respectively. Prominently, the g_lum values of PTC-376 enantiomers in the excited state are about 3 times that of PTC-375 enantiomers (Fig. 2f). As can be seen from Fig. 2c, the luminescence intensity of PTC-375 and PTC-376 is not much different, but the location and intensity of their CD peaks are significantly different (Figs. 2a and b), which may be caused by their structural differences (such as ligands, coordination environments, spatial stacking patterns, supramolecular interactions), thus resulting in differences in CPL performance. In contrast, the g_lum values of PTC-376 enantiomers are comparable to those of the reported Ag nanoclusters [41-44]. The above results well prove that the introduction of chiral organic ligands can effectively separate Zr₄L₆ cage, and the perfect combination of luminescent Ag units and Zr₄L₆ cages can produce synergistic CPL effect, which realizes chiral synergistic amplification and exhibits an enhanced CPL response in practical testing. In short, by purposeful modification on the homochiral Zr₄L₆ cage, we can obtain cage-based CPL materials that cannot be obtained by direct synthesis.

  In this work, by modifying π-conjugated coordination silver cation onto homochiral Zr₄L₆ cages, two pair of pure enantiomers (PTC-375(Δ/Λ) and PTC-376(Δ/Λ)) have been successfully synthesized and structurally characterized. Interestingly, the decorated homochiral hetero-metallic cages present interesting CPL response, and PTC-376 enantiomers show a CPL output with g_lum values up to ~1.4 × 10⁻². These results indicate the potential application of such homochiral cages in CPL field.

  Declaration of competing interest

  There is no conflict of interest to report.

  CRediT authorship contribution statement

  Xin Meng: Software, Investigation, Formal analysis, Data curation. Xin-Ya Cai: Software. Qing-Rong Ding: Methodology. Shan-Shan Chen: Software. Shu-Mei Chen: Writing – review & editing. Yan-Ping He: Writing – review & editing, Writing – original draft, Funding acquisition, Conceptualization. Jian Zhang: Conceptualization.

  Acknowledgments

  This work is supported by the National Natural Science Foundation of China (Nos. 92261108 and 2022YFA1503303) and the STS Project of Fujian-CAS (No. 2023T3054).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110402.

  
  
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  Scheme 1  Schematic diagram of the chiral resolution and post-synthetic modification of tetrahedral Zr₄L₆ cage.

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  Figure 1  (a, b) X-ray crystal molecular structures of the decorated homochiral cage enantiomers in PTC-375(Δ/Λ) and PTC-376(Δ/Λ) and (c, d) their 3D packed supramecular frameworks. Atom color code: purple, Ag; olive, Zr; pink, P; red, O; gray, C.

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  Figure 2  (a, b) Liquid-state CD spectra of PTC-375(Δ/Λ) and PTC-376(Δ/Λ) in DMF. (c) Luminescence spectra of Zr₄L₆, PTC-375(Δ) and PTC-376(Δ) in the solid state (inset: their crystal photos at ultraviolet light (365 nm)). (d) CPL spectra of PTC-375(Δ/Λ) (λ_ex = 370 nm). (e) CPL spectra of PTC-376(Δ/Λ) (λ_ex = 370 nm). (f) Dissymmetry factors (glum).

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