Planar Chiral [2.2]Paracyclophane-Based Zr(IV) Metal–Organic Frameworks

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES2 Sep 2022Planar Chiral [2.2]Paracyclophane-Based Zr(IV) Metal–Organic Frameworks Hong Jiang, Wenqiang Zhang, Bang Hou, Yan Liu and Yong Cui Hong Jiang School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 , Wenqiang Zhang School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 , Bang Hou School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 , Yan Liu School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 and Yong Cui *Corresponding author: E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 https://doi.org/10.31635/ccschem.022.202202285 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Self-assembly has been widely explored to improve the circularly polarized luminescence (CPL) activities of molecular chromophores, but it is hard to amplify CPL while maintaining strong emissions. Optically pure, planar-chiral [2.2]paracyclophane (pCp) is one of the most important sources of chirality for electronic and optoelectronic materials, but the photoluminescence of its derivatives is significantly quenched after forming aggregates. Here we demonstrate that reticulation of pCp chromophores into highly stable Zr(IV) metal–organic frameworks (MOFs) can simultaneously boost the dissymmetry factor (|glum|) and luminescence efficiency (ΦPL). Functionalization of pCp enables the synthesis of two tetracarboxylate linkers with different side arms, which are assembled into two highly stable Zr-MOFs with different topological structures. Geometrical constraints and segregated arrangements of the pCp chromophores imposed by the framework inhibit the aggregation-caused quenching effect and boost their CPL behaviors. Both Zr-MOFs display strong CPL emissions, affording |glum| and ΦPL values of up to 8.3 × 10−3 and 87%, respectively, which are amplified by ∼18- and 52-fold compared to the corresponding free ligands. This work highlights the potential of optimizing CPL performances of chiral chromophores by using MOFs as support structures. Download figure Download PowerPoint Introduction Considerable attention has been recently paid to circularly polarized luminescence (CPL) materials with high dissymmetry factor (|glum|) and luminescence efficiency (ΦPL) due to their potential applications in 3D displays and optical data storage devices.1–3 Metal–organic frameworks (MOFs) are highly promising as high-performance luminescent materials due to their modular and tunable features.4–7 By far, a large number of luminescent MOFs based on π-units such as tetraphenylethylene, benzothiazole, pyrene, and porphyrin have been explored for diverse applications.8–13 In contrast, chiral MOFs (CMOFs) capable of emitting CPL are less studied. Confining achiral dyes into CMOFs with suitable pore sizes and geometries (e.g. chiral zeolite imidazole framework-8 and γ-CD MOF) can induce CPL,14–16 but, in most cases, the emission efficiency of the included dyes was fairly low due to the aggregation-caused quenching (ACQ) effect. In contrast, the direct synthesis of CMOFs to generate CPL is more attractive.17–19 Inspiringly, Liu and coworkers recently integrated the chiral camphoric acid and an achiral luminogen with aggregation-induced emission (AIEgen) into a Cd-MOF, which emits CPL with high |glum| (0.012) and ΦPL (43%).20 However, it is difficult to harmonically integrate organic chromophores to control the CPL emission. Furthermore, the poor stability of the Cd-MOF toward humid and harsh reaction conditions limits its use in practical processes. Here we report the design and synthesis of two highly stable Zr-CMOFs based on planar-chiral building blocks and demonstrate their amplified CPL emissions and dissymmetry factors. It should be noted that self-assembly has been extensively explored to improve CPL activities of organic chromophores, but it is difficult to boost the |glum| values while maintaining intense emissions.21,22 The planar-chiral [2.2]paracyclophane (pCp) scaffold, which has a unique through-space π–π stacked conjugated structure, provides a conformationally stable chiral environment due to suppression of the rotation of phenylenes.23–25 pCp-derived organic molecules displayed excellent CPL properties in diluted solution, but their photoluminescence (PL) was significantly quenched after forming aggregates due to the ACQ effect. Interestingly, compared with the frequently used point-chiral and axially-chiral building blocks, the planar-chiral and helical-chiral molecules, which possess intrinsically fascinating electronic and optoelectronic properties, have not yet been explored for CMOF construction (Figure 1a–d).26–32 A chiral pCp-derived dicarboxylate acid was previously used for fabricating surface-mounted metal–organic structures for enantioselective adsorption.33 To make CPL-active Zr-CMOFs, four aromatic carboxylate acids were installed on the pCp scaffold to bind to metal clusters. This approach leads to two Zr-CMOFs with a (4,8)-connected flu or {414.612.82}{43.63}2 topology, which is controlled by the conformation of the pCp linkers. In Zr-MOFs, the chiral pCp chromophores form periodically ordered and segregated arrays that effectively inhibit the ACQ effect and rigidify their conformations, leading to an 18-fold amplification of the |glum| value (up to 8.3 × 10−3) and up to a 51-fold increase in the emission efficiency (up to 87%). Figure 1 | The four types of molecular chirality: (a) central, (b) axial, (c) planar, and (d) helical. Download figure Download PowerPoint Experimental Methods Synthesis of pCp-1 A mixture of ZrCl4 (10 mg, 0.03 mmol), H4 L 1 (6.9 mg, 0.01 mmol), dimethylformamide (DMF; 2.0 mL), and formic acid (1.0 mL) was sealed in a 15 mL vial with a screw cap and heated at 120 °C for 24 h. Colorless polyhedral crystals were collected, washed with DMF and acetone, and air-dried. Yield: 85%. Elemental analysis data of pCp-1: Calcd for C88H72O32Zr6: C, 48.29; H, 3.32. Found: C, 48.25; H, 3.50. Fourier-transform IR (FT-IR) (KBr, cm−1): 3383 (m), 2943 (w), 2867 (w), 1658 (w), 1599 (s), 1049 (s), 1183 (m), 937 (w), 904 (w), 868 (m), 843 (w), 788 (s), 761 (w), 733 (w), 714 (w), 655 (s), 477 (w). Synthesis of pCp-2 A mixture of ZrCl4 (10 mg, 0.03 mmol), H4 L 2 (7.8 mg, 0.01 mmol), DMF (2 mL), and trifluoroacetic acid (TFA; 0.8 mL) was sealed in a 15 mL vial with a screw cap and heated at 120 °C for 24 h. Yellow rod-like crystals were obtained, washed with DMF and acetone, and air-dried. Yield: 88%. Elemental analysis data of pCp-2: Calcd for C104H72O32Zr6: C, 52.46; H, 3.05. Found: C, 52.35; H, 2.97. FT-IR (KBr, cm−1): 3382 (m), 2933 (w), 2853 (w), 2206 (w), 1658 (s), 1604 (s), 1537 (m), 1479 (w), 1419 (s), 1310 (w), 1281 (w), 1204 (m), 1179 (w), 1151 (w), 1101 (w), 1017 (w), 906 (w), 858 (w), 780 (s), 726 (w), 696 (w), 656 (s), 523 (w), 485 (m). Results and Discussion The Suzuki cross-coupling reaction of enantiopure tetra-substituted pCp34 and 4-(methoxycarbonyl)phenylboronic acid produced the C2-symmetric dimethyl ester, which was subsequently hydrolyzed to produce the free acid H4L1 in an overall 78% yield. The elongated ligand H4L2 was synthesized (66% yield) by the Sonogashira coupling reaction followed by saponification. Considering the closed-shell electronic configuration of Zr(IV) ions and the extraordinary chemical and thermal stability of Zr-MOFs, H4 L1 and H4 L2 were used to react with Zr4+ to prepare chiral Zr-MOFs.35–41 It is well-known that acid modulators have significant effects on controlling both crystallinity and topology of Zr-MOFs. After screening types and concentration of modulators, we successfully obtained large single crystals of Zr-pCp suitable for single-crystal X-ray diffraction. Heating a mixture of H4L1 with ZrCl4 in DMF using formic acid as modulator resulted in the isolation of pCp-1 [Zr6O4(OH)8(H2O)4(L1)2] as colorless polyhedral crystals, whereas heating a mixture of H4L2 and ZrCl4 in DMF using TFA as modulator afforded pCp-2 [Zr6O4(OH)8(H2O)4(L2)2] as yellow rod-like crystals. Their formulas are confirmed by elemental analysis, FT-IR spectra ( Supporting Information Figure S9), and thermogravimetric analysis (TGA) ( Supporting Information Figure S11). Single-crystal structure analysis revealed that pCp-1 crystallizes in chiral space group I422 with the asymmetric unit consisting of a quarter of the formula ( Supporting Information Figure S3 and Table S1). Six Zr ions are held together with four μ3-OH− and four μ3-O2− to generate a D4h symmetric hexanuclear cluster Zr6(μ3-O)4(μ3-OH)4 with four pairs of OH/H2O as terminal groups ( Supporting Information Figure S4). Each pCp linker L1 with a distorted Td symmetry bridged four Zr6 clusters to form a (4,8)-connected flu framework (Figure 2a). Large octahedral interstitial cages constructed from six hexanuclear clusters and eight L1 linkers are thus generated with dimensions of 15.3 × 15.3 × 28.8 Å3 (Figure 3b). One cage was surrounded by eight neighboring cages by sharing faces to produce a 3D porous network with interconnecting 1D channels of 8.4 × 15.3 Å2 along the a/b directions (Figure 3c and Supporting Information Figure S5). Calculation by PLATON software indicated that 67.1% of the void space can be accessed by guest molecules.42 Figure 2 | (a) Construction of pCp-1 with (4,8)-connected flu topology from distorted tetrahedral symmetric H4L 1 and D4h symmetric Zr6 cluster; (b) construction of pCp-2 with (4,8)-connected {414.612.82}{43.63}2 network from planar H4L 2 and D2d symmetric Zr6 cluster. Download figure Download PowerPoint pCp-2 adopted a topological structure totally different from pCp-1. As indicated by single-crystal X-ray diffraction, pCp-2 crystallizes in the chiral hexagonal space group P6222 with an asymmetric unit consisting of a quarter of the formula ( Supporting Information Figure S3 and Table S1). Six Zr atoms are combined by eight μ3-O2−/OH− groups to form an irregular D2d symmetric Zr6(μ3-O)4(μ3-OH)4 cluster core, whereas eight μ3 oxygen atoms capped on the triangular faces form a highly distorted polyhedron ( Supporting Information Figure S4). This was distinct from the Zr6 cluster in pCp-1, in which the eight μ3-oxygens are arranged as an idealized cube. Two adjacent Zr6 clusters along the c direction are connected by four ligands to form a cage with diameter of 11.0 Å (Figure 3e). The cage extended in three directions creating a 3D network having triangular channels with the diameter of 6.5 Å along the c axis and rhombic channels with the dimension of 9.5 × 15.3 Å2 along the a/b directions (Figure 3f and Supporting Information Figure S5). The solvent accessible pore volume was 66.4%, as calculated by PLATON software.42 Topologically, the 4-connected linkers with a planar conformation are cross-linked by 8-connected Zr6 clusters to form a (4,8)-connected network with a point symbol of {414.612.82}{43.63}2 (Figure 2b). Note that the combination of 4-connected planar linker and 8-connected Zr6 cluster exclusively produced the sqc, scu, or csq topology in previous reports.43,44 Therefore, pCp-2 adopted a novel topology that has not been reported previously, which greatly enriches the library of Zr-MOFs. Figure 3 | Structures of L 1 (a) and L 2 (d) in pCp-1 and pCp-2; cages in pCp-1 (b) and pCp-2 (e); 3D porous structures of pCp-1 (c) and pCp-2 (f) viewed along the c-axis. The cavities are highlighted by yellow spheres. Zr6 cluster, blue polyhedron; C, grey; O, red. H was omitted for clarity. Download figure Download PowerPoint By further examining the crystal structures of pCp-1 and pCp-2, we observed that L 1 and L 2 adopt completely different conformations (Figure 3a,d). In L 1, four aromatic carboxylate side arms are not coplanar with the phenyl rings of the central pCp backbone; the torsion angle was approximately 142°, leading to a saddle-shaped conformation ( Supporting Information Figure S6). In contrast, the alkynyl benzoic carboxylate groups of L 2 are almost coplanar with the central phenyl rings, giving rise to a planar rectangular conformation ( Supporting Information Figure S6). The different conformations of L 1 and L 2 may be attributed to the alkynyl groups inserting into the vicinal phenyl rings, that drastically releasing tension of the whole molecule. Therefore, by the linker engineering strategy, we achieved the successful control of the flu and {414.612.82}{43.63}2 topologies of the two Zr-MOFs. The phase purity of bulk pCp-1 and pCp-2 was confirmed by comparison of their observed and simulated powder X-ray diffraction (PXRD) patterns (Figure 4a,b). The minor peak shift may originate from a certain degree of framework flexibility of pCp MOFs. Scanning electron microscopy images and optical microscopy of pCp-1 and pCp-2 showed a homogeneous morphology of polyhedral-shaped and rod-shaped crystals, respectively, further indicating the phase purity of the samples ( Supporting Information Figures S7 and S8). Both pCp-1 and pCp-2 are stable in air and common organic solvents. 1H NMR spectra of the digested CMOFs undoubtedly show all the peaks from linkers, indicating the ligands survive during the MOF synthesis ( Supporting Information Figures S1 and S2). The permanent porosities of pCp-1 and pCp-2 were investigated by N2 adsorption at 77 K. However, simple acetone solvent exchange and subsequent evacuation under vacuum led to surface adsorption, probably due to collapse of the frameworks. Therefore, we resorted to facilitating the solvent exchange process with lower surface tension n-hexane to prevent structure collapse. As a result, we successfully obtained a Brunauer–Emmett–Teller (BET) surface area of 1895 and 1473 m2/g for pCp-1 and pCp-2, respectively (Figure 4c,d). Figure 4 | PXRD patterns for pCp-1 (a) and pCp-2 (b) under different conditions. N2 adsorption isotherms for pCp-1 (c) and pCp-2 (d) under different conditions. Download figure Download PowerPoint The thermal stability of Zr-MOFs was evaluated by thermogravimetric analysis (TGA) under a nitrogen atmosphere. TGA indicated that pCp-1 and pCp-2 started to decompose at approximately 500 °C ( Supporting Information Figure S11). Such high thermal stability is comparable with those of other Zr-MOFs.44,45 We further investigated chemical stability of the two MOFs under different conditions, including boiling water, concentrated hydrochloric acid, and sodium hydroxide aqueous solutions (pH 12). After soaking the Zr-MOFs in these solvents for 48 h, the intensity of their PXRD patterns remained almost unchanged (Figure 4a,b). To further confirm the chemical stability, we conducted N2 adsorption tests on the MOFs after the aforementioned treatments. Both MOFs maintained their original BET surface areas without significant loss (Figure 4c,d). The high chemical stability of Zr-MOFs may be attributed to the strong bonding between the Zr(IV) and carboxylate groups and the rigidity of the pCp ligands.46,47 The photophysical properties of Zr-pCp MOFs were evaluated by PL and compared with the corresponding linkers as references. Upon irradiation, a strong blue luminescence peak at 445 nm was observed for pCp-1 (Figure 5a and Supporting Information Figure S13), which underwent a redshift of 30 nm compared with the dilute solution of the ligand H4 L 1. Also, the fluorescence spectra of pCp-2 showed an apparent bathochromic shift from 430 to 480 nm relative to H4 L 2 (Figure 5b and Supporting Information Figure S13). The 5s and 4d valence orbitals of Zr(IV) ions are empty, which can overlap with π orbitals of the ligand and extend the delocalized conjugated system, resulting in a redshifted emission. Then, we measured the photoluminescence quantum yields for the Zr-pCp MOFs and free ligands. In diluted solutions, the ΦPL of H4 L 1 and H4 L 2 were 76.3% and 71.1%, respectively, whereas, in the solid-state, ΦPL values were lower at 3.8% and 1.2%, as a result of the severe ACQ effect ( Supporting Information Figure S16 and Table S3).48,49 Surprisingly, when the ligands were assembled into MOFs, the ΦPL values drastically increased to 87.0% and 61.9% for pCp-1 and pCp-2 (in the solid-state), respectively (Figure 5c). Such ΦPL values for Zr-pCp MOFs are higher than most of the reported fluorescent Zr-MOFs, including the AIEgen-based Zr-MOFs ( Supporting Information Table S4).50–53 The significantly enhanced ΦPL can be attributed to the immobilization of the linkers into the MOF backbone efficiently prohibiting aggregation and thus suppressing the ACQ effect. To investigate the lifetimes of excitation states, the transient fluorescence decay spectra were recorded. Time-resolved fluorescence spectroscopy revealed that MOFs pCp-1 and pCp-2 have lifetimes (τ) of 2.87 and 2.11 ns, respectively, which are longer than lifetimes of 1.44 and 1.32 ns for the corresponding ligands ( Supporting Information Figure S17 and Table S3) because the formation of the rigid Zr-MOF effectively restrains the intramolecular torsional motion and increases the conformational rigidity of the linkers, giving rise to weakened vibrations of the ligands and reduced loss of excitation energy through a radiationless pathway. Figure 5 | (a,b) The fluorescence spectra for chiral ligands in diluted solutions and Zr-pCp MOFs in solid state. Inset: photographs of pCp-1 and pCp-2 under visible light (left) and UV irradiation (right). (c) Comparison of the fluorescence quantum yields for the Zr-pCp MOFs and ligands in the solid state. (d,e) CPL dissymmetric factor (glum) spectra of the Zr-pCp MOFs in the solid state and ligands in diluted solutions. (f) Comparison of the |glum| values for the Zr-pCp MOFs and free ligands. Download figure Download PowerPoint The chiroptical properties of Zr-pCp MOFs in the ground state were investigated by circular dichroism (CD) spectroscopy. As expected, mirror Cotton effects were observed in the absorption bands. The absorption peak was located at approximately 365 and 380 nm for pCp-1 and pCp-2, respectively, which can be assigned to the typical π–π* transition of the conjugated phenyl rings ( Supporting Information Figure S14). Moreover, compared with the ligands, the patterns of the Cotton effects for Zr-pCp MOFs have a redshift in all regions. This was also confirmed by the bathochromic shift of the solid UV–vis spectra for Zr-pCp MOFs with respect to the ligands ( Supporting Information Figure S12). Therefore, the global chirality of the 3D framework was generated via the chirality transfer from the planar-chiral linker. CPL spectra were measured to investigate the chiral properties of the excitation states. The dissymmetric factor glum, defined as glum = 2 × (IL − IR)/(IL + IR), is used to evaluate the degree of CPL, where IL and IR are the intensities of the left- and right-handed CPL, respectively. As shown in Figure 5d,e, both MOFs pCp-1 and pCp-2 showed obvious mirror-image CPL signals of around 445 and 480 nm, respectively, in accordance with the fluorescence spectra. The calculated |glum| values were 7.2 × 10−3 and 8.3 × 10−3 for pCp-1 and pCp-2, respectively. In contrast, the |glum| values for dilute solutions of H4 L 1 and H4 L 2 were 4.0 × 10−4 and 4.5 × 10−4, respectively. Moreover, the |glum| values for H4L1 and H4L2 in amorphous solid state were measured to be 3.5 × 10−4 and 9.0 × 10−4, respectively ( Supporting Information Figures S10 and S15). Obviously, the |glum| values of the pCp ligands were amplified by one order of magnitude upon incorporation into the frameworks (Figure 5f). In the molecular assemblies, the harmonic integration of chiral chromophores plays a key role in controlling the CPL emission, but it remains a substantial challenge and is far from being understood, especially for amorphous systems. In this study, we demonstrated that reticulation of chiral pCp chromophores into crystalline Zr-MOFs can significantly amplify both the |glum| and ΦPL values because of their rigidified conformation and periodically segregated arrangement, showing the great prospect of solving the trade-off issue between the dissymmetric factors and quantum yields for CPL luminophores by using MOFs as support structures (Figure 6). Notably, the |glum| and ΦPL values of Zr-pCp MOFs are higher than most of the reported planar-chiral pCp-derived molecules and assemblies.54–56 The present two Zr MOFs are comparable or favorably comparable to the most CPL-active MOFs and their hybrids regarding the dissymmetric factor and emission efficiency, but have the highest chemical stability ( Supporting Information Table S5 and Figure S18).14–20,57,58 Figure 6 | Schematic illustration of the enhance CPL performances of Zr-pCp MOFs from the ligands in the diluted solution and solid-state. Download figure Download PowerPoint Conclusion We have prepared two planar chiral pCp-based porous Zr-CMOFs with a flu or {414.612.82}{43.63}2 topology controlled by the pCp conformations. Both Zr-CMOFs exhibited high chemical stability toward water, acid, and base. In the frameworks, the chiral pCp chromophores have rigidified conformations forming well-ordered and segregated arrays, thereby inhibiting the ACQ effect and enhancing their CPL behaviors. In particular, compared with the free pCp linkers, the |glum| values are amplified by ∼18-fold and the ΦPL values are enhanced by ∼52-fold. To the best of our knowledge, this is the first report that uses reticulation of planar chiral luminophores into MOFs to fabricate CPL materials. We illustrated that direct incorporation of chiral chromophores into crystalline Zr-MOFs can simultaneously amplify the dissymmetry factor and enhance luminescence efficiency by rigidifying and the which is hard to in other and the design of more novel chiroptical materials. Supporting Information Supporting Information is and and data in and and and of all of is of to Information This work was by the Science of and Key of of Shanghai Shanghai and the Science 1. 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