Open AccessCCS ChemistryRESEARCH ARTICLE1 Sep 2021

Secondary Metal Coordination Using a Tetranuclear Complex as Ligand Leading to Hexanuclear Complexes with Enhanced Thermal Barriers for Electron Transfer

    Postsynthesis of the paramagnetic square-shaped complex {[(Tp*Me)Fe(μ-CN)2(CN)][Co(dmbpy)2]}2(BPh4)2·6MeCN·H2O [ 1, Tp*Me = tris(3,4,5-trimethylpyrazole)-borate; dmbpy = 4,4′-dimethyl-2,2′-bipyridine)] by grafting transition metal(II) thiocyanates via its terminal cyano groups afforded three hexanuclear [Fe2Co2M2] clusters (M = Zn, 2; Co, 3; Cd, 4). The peripheral metal-complex units serving as excellent electron acceptors were found to help stabilize the low-temperature state of FeII,LS–CoIII,LS within the complex core. As a result, the desolvated complexes 2 4 underwent reversible and sharp thermally induced electron-transfer behavior with the transition temperatures (T1/2) up to 312, 296, and 365 K, respectively, demonstrating an effective means of manipulating thermal barriers of the celebrated cyano-bridged square core.


    The engineering of magnetically switchable molecular materials involving both thermally and photoinduced electronic spin transitions is a topical research area aimed at the development of next-generation materials for memory, display, switching, and spintronic devices.16 These materials may exhibit magnetic bistability or multistability with sensitive response to external stimuli, such as temperature, light, pressure, and so forth. In addition to the significant changes in magnetism, modulation on spin state may also endow materials with other synergizing functionalities, such as catalytic activity,710 negative thermal expansion,11,12 luminescence, and fluorescence.1316 Among the many complexes studied, cyanometallates featuring cyano-bridged metal centers of different kinds are most prominent; the small linear cyano ligand provides an intimate linkage, allowing for through-bond electron transfer (ET) to occur. Furthermore, using a building block approach, molecular precursors are allowed to self-assemble toward a common structural archetype, which may be further engineered readily and systematically at the atomic level.1720

    The research along this line started with the reports by Sato and co-workers2124 in which a three-dimensional (3D) Fe/Co analog of Prussian blue (PB) exhibited thermally and photoinduced ET between the paramagnetic FeIII,LS–CN–CoII,HS and the diamagnetic FeII,LS–CN–CoIII,LS configurations [high spin (HS) and low spin (LS)], accompanied by significant color changes. Dunbar and co-workers25,26 subsequently moved the research forward with a series of cyano-bridged trigonal bipyramidal {Fe2Co3} complexes. Later, Holmes and co-workers27 reported the first [Fe4Co4] cubic complex whose tunable magnetic properties mimic that of PB. Since then, with the use of molecular building blocks with judiciously disposed cyano ligand(s), a series of discrete cyanometallate complexes containing different Fe–CN–Co motifs2834 as well as one-dimensional (1D) chains were obtained,3538 with which interesting ET behaviors and the underlying electronic structures were thoroughly studied.3941

    Of particular interest amongst them are cyano-bridged [Fe2Co2] squares with Fe-bound terminal cyano ligands because their electronic structures can be systematically tuned either by modifying the coordination environment of individual metal centers or by regulating intermolecular interactions in the complexes. For example, equipping correct ligands with electron-donating functional groups around the Co sites or the Fe ones in the square complexes moved the transition to a higher or lower temperature.4249 Meanwhile, the intermolecular supramolecular interactions, such as π–π stacking, H-bonding interactions, and even solvent polarity have also been found to have profound effects on the stabilization of low-temperature (LT) states.5054 Specifically, the H-bonded lattice solvents acting as electron acceptors may pull down the charge density around Fe centers and stabilize the Fe(II) state to some degree, thus leading to a higher thermal barrier for ET. As an example, Oshio and co-workers55 introduced Brønsted acids into a square complex by cocrystallization, and the resulting H-bonded supramolecular assemblies showed multistepped spin transitions toward room temperature. The supramolecular approach, though effective, is inherently limited since such systems are not particularly thermally stable. For example, such systems generally contain a large number of solvent molecules that are readily lost upon heating, therefore eliminating their interesting ET behavior toward possible applications.5658

    In comparison with the supramolecular interactions, covalent bonds are more powerful and thermally stable. Given that a terminal cyano ligand has a free and coordination-capable N atom, postmodification by using the complex as ligand for the coordination of certain secondary (versus the primary metal ions in the parent complex) metal ions may be achieved, but remains very challenging due to the presence of electronic repulsion and steric hindrance.59 In this work, we report our findings along this line by first constructing a tetranuclear complex ( 1) that includes the well-known square-shaped cationic core of [Fe2Co2]2+, followed by its postsynthesis of a series of hexanuclear complexes [Fe2Co2M2] (M = Zn, 2; Co, 3; Cd, 4) with the secondary anionic metal(II) thiocyanates attached to the square core by way of the terminal cyano ligands (Scheme 1). In sharp contrast to 1, all the hexanuclear complexes underwent thermally induced reversible ET events at near room temperature—a key property for possible practical device applications of such materials. The profound electronic effects of postmodification by peripheral metal coordination are discussed below using complex 2 as representative. Related results obtained for complexes 3 and 4 are provided in the Supporting Information.

    Scheme 1

    Scheme 1 | Postsynthesis of square to hexanuclear complexes.

    Experimental Methods

    Materials and syntheses

    [NEt4][(Tp*Me)Fe(CN)3] [Tp*Me = tris(3,4,5-trimethylpyrazolyl)borate] was synthesized according to the literature.60 All other chemicals and reagents were commercially available and used without further purification.

    Synthesis of {[(Tp*Me)Fe(CN)3]2[Co(dmbpy)2]2[BPh4]2}·6MeCN·H2O (1)

    Treatment of CoCl2·6H2O (23.8 mg, 0.1 mmol), NaBPh4 (68.6 mg, 0.2 mmol), and 4,4′-dimethyl-2,2′-bipyridine (dmbpy, 36.8 mg, 0.2 mmol) in water (1 mL) and acetonitrile (5 mL) at room temperature afforded a clear orange solution that was stirred for 2 h. [NEt4][(Tp*Me)Fe(CN)3] (62.0 mg, 0.1 mmol) in MeCN (10 mL) and H2O (1 mL) were added into the above solution. After being stirred for 5 min, the dark red mixture was filtered and allowed to stand at room temperature for several days. Dark red crystals were obtained and isolated via filtration, then washed quickly with a cold 5∶1 (v∶v) MeCN/MeOH (1 mL) solution. Yield: 79 mg (64.5 %). Anal. Calcd for C150H164B4Co2Fe2N32O ( 1): C, 66.63; H, 6.11; N, 16.58. Found: C, 66.42; H, 5.96; N, 16.72. Infrared (IR) (KBr/Nujol, cm−1; 300 K): 3627 (w), 3424 (w), 2729 (w), 2538 (m), 2250 (w), 2152 (s), 2126 (w), 1637 (w), 1063 (s), 1598 (s), 1577 (m), 1566 (m), 1520 (m), 1490 (m), 1456 (vs), 1442 (vs), 1377 (vs), 1365 (vs), 1312 (s), 1243 (vs), 1173 (s), 1153 (s), 1101 (s), 1062 (m), 1032 (w), 1019 (s), 918 (w), 889 (w), 837 (m), 834 (m), 812 (w), 769 (vs), 735 (vs), 708 (vs), 667 (w), 651 (m), 625 (m), 613 (m), 607 (m).

    Synthesis of {[(Tp*Me)Fe(CN)3]2[Co(dmbpy)2]2[Zn(NCS)3]}·6DMF·2H2O (2)

    Treatment of 1 (67.5 mg, 0.025 mmol), Zn(ClO4)2·6H2O (18.5 mg, 0.05 mmol), and NaSCN (12.1 mg, 0.15 mmol) in acetonitrile (5 mL) at room temperature gave green precipitate. The precipitate was dissolved in dimethylformamide (DMF) (3 mL) which was layered by Et2O, and green crystals were obtained after 3 days. Yield: 42.1 mg (61.9%). Anal. Calcd for C114H150B2Co2Fe2N38O8S6Zn2 ( 2): C, 49.70; H, 5.49; N, 19.32; S, 6.98. Found: C, 49.92; H, 5.24; N, 19.15; S, 7.26. Anal. Calcd for the desolvated sample ( 2d, C96H104B2Co2Fe2N32S6Zn2): C, 50.56; H, 4.60; N, 19.65; S, 8.44. Found: C, 50.39; H, 4.46; N, 19.78; S, 8.32. IR (KBr/Nujol, cm−1; 300 K): 3132 (w), 2920 (m), 2860 (m), 2515 (m), 2360 (m), 2341 (m), 2111 (s), 2088 (vs), 2057 (vs), 1662 (vs), 1618 (s), 1558 (m), 1496 (m), 1434 (s), 1411 (m), 1386 (s), 1355 (m), 1323 (w), 1301 (m), 1280 (m), 1245 (s), 1220 (m), 1190 (w), 1172 (m), 1092 (s), 1037 (m), 1002 (m), 960 (w), 926 (m), 889 (w), 871 (m), 831 (s), 730 (m), 696 (m), 660 (m), 634 (m), 613 (w).

    Physical measurements

    X-ray crystallographic data

    The single-crystal data for complexes 1 4 at 100 K were collected on a Bruker D8 VENTURE diffractometer (Shenzhen, China) with monochromated Mo Kα radiation (λ = 0.71073 Å). Lorentz/polarization corrections were applied during data reduction, and the structures were solved by the direct method (SHELXS-2014).61 Refinements were performed by full-matrix least-squares (SHELXL-2014) on F2, and empirical absorption corrections (SADABS) were applied.62,63 Anisotropic thermal parameters were used for the nonhydrogen atoms. Hydrogen atoms were added geometrically and refined using a riding model. Weighted R factors (wR) and all the goodness-of-fit (S) values are based on F2; conventional R factors (R) are based on F, with F set to zero for negative F2. Notably, the lattice solvent molecules in 2 and 4 could not be properly modeled; therefore, their contributions were partially subtracted by the SQUEEZE program implemented in Olex2 (Durham University, UK). Data collection and structural refinement parameters are given in Supporting Information Table S1, and selected bond lengths and angles are given in Supporting Information Tables S2–S4. CCDC-1998180 ( 1), CCDC-1998181 ( 2), CCDC-1998178 ( 3), and CCDC-1998179 ( 4) contain the crystallographic data that can be obtained via (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223-336-033; or ).

    Powder X-ray diffraction

    Variable-temperature powder X-ray diffraction (PXRD) measurements were recorded using a Rigaku SmartLab X-ray diffractometer with Cu Kα radiation (45 kV, 200 mA) between 5° and 40° (2θ). The simulated patterns are calculated from the single-crystal data at 100 K.

    Thermogravimetric analysis

    The thermogravimetric analysis (TGA) measurements were carried out on freshly filtered crystals using the METTLER TOLEDO TGA2 instrument in an insert argon atmosphere over a temperature range of 30–500 °C with a heating rate of 10 °C /min.

    Elemental analysis

    Elemental analyses (EA) (C, H, N, S) were measured by a vario electroluminescence (EL) cube CHNOS Elemental Analyzer Elementar Analysensysteme GmbH.

    Fourier transform IR spectroscopy

    Fourier transform IR (FT-IR) spectra were recorded in the range 600–4000 cm−1 on a Bruker TENSOR II spectrophotometer. Variable temperature data were obtained by in situ heating and cooling in the sample holder.

    Magnetic and photomagnetic measurements

    Magnetic susceptibility data under 1 kOe direct current (dc) field were collected at temperatures between 2 and 400 K using a SQUID MPMS3 magnetometer. To prevent the loss of lattice solvents, fresh crystals were restrained in a minimum amount of mother liquor, which was loaded into the equipment and quickly cooled down to 200 K before the measurements. Solutions of 2 4 for magnetic measurements were prepared by dissolving the fresh crystals in DMF solvents. For the photomagnetic experiments, irradiation was performed on the fresh sample at 20 K, and light from a Diode Pumped Solid State laser (808 nm; 10 mW, MDL-III-808 nm, Changchun New Industries Optoelectronics Technology Co., Ltd laser) was guided via a flexible optical fiber (CNI fiber; 5 m length) into the SQUID magnetometer. Magnetic data were corrected for the diamagnetism of the sample holder and for the diamagnetism of the sample using Pascal’s constants.64

    Results and Discussion

    Crystal structure

    Single crystals of 1 were obtained by slow evaporation of a solution mixture of CoCl2·6H2O, NaBPh4, dmbpy, and [NEt4][(Tp*Me)Fe(CN)3] in acetonitrile at room temperature. Similar to other reported square analogues,40 the adjacent [(Tp*Me)Fe(CN)3] and [Co(dmbpy)2]2+ blocks in 1 that reside in the alternate corners are linked by cyano groups, giving a nearly square planar core of {Fe2(μ-CN)4Co2}2+ with two Fe-bound terminal cyano groups in anti-orientation ( Supporting Information Figures S1 and S2). Each Co ion is surrounded with six N atoms, two of which are from the bridging cyano ligands and the remaining four are from two chelating dmbpy ligands. The average Co–N bond length of 2.074 Å is consistent with the HS state of the CoII ion, thus giving a paramagnetic [FeIII,LS2CoII,HS2] configuration.

    Treatment of 1 with Zn(ClO4)2·6H2O and NaSCN, first in acetonitrile and then DMF/Et2O, afforded the hexanuclear complex 2. The single-crystal X-ray diffraction (SCXRD) study at 100 K revealed that 2 crystallized in the monoclinic space group P21/n with the cyano-bridged neutral hexanuclear structure [Fe2Co2Zn2] ( Supporting Information Table S1). In comparison with 1, the most salient feature for 2 is the two peripheral [Zn(NCS)3] fragments, which were grafted on the parent square core of 1 through the Fe-bond terminal cyano groups (Figure 1). As such, each of the Zn(II) centers is situated in a tetrahedron coordination sphere formed by four nitrogen atoms, three from thiocyanates and one from the cyano ligand. Although the surroundings of each Co center are similar to those in 1, the average Co–N bond length of 1.92 Å is significantly shorter (0.15 Å) than that in 1. This value is in good agreement with the LS state CoIII ion, and thus suggests the diamagnetic [FeII,LS2CoIII,LS2ZnII2] state. The Fe–C≡N–Co cyano linkages are nearly linear with the Fe–C≡N and Co–N≡C angles in the range of 175.6–179.1° and 174.2–175.9°, respectively, similar to those in 1. The individual complex units of 2 are fairly separated with the closest intercomplex M···M contacts of 9.8 Å ( Supporting Information Figure S3). No crystal data at above room temperature can be obtained due to the loss of lattice solvents. The TGA studies revealed a weight loss of 16.5% for 2 at below 100 °C ( Supporting Information Figure S8), corresponding to the release of six DMF and two H2O molecules (calculated 17.2%) per formula unit. It is followed by a broad region of thermal stability up to ca. 210 °C, indicating that the structural integrity may be maintained during the desolvation process. Accordingly, the validity of structural models obtained from SCXRD studies for the respective bulk samples was further investigated by means of the in situ variable temperature PXRD method (Figure 2). At 298 K, the PXRD patterns for 2 4 are in agreement with their SCXRD simulations, indicating the good purity of the samples. On the heating cycle between 298 and 390 K, the PXRD peaks for 2 and 3 underwent significant decrease in intensity at around 6° and 6.6°, accompanied by the enhancement of the peak at ca.7°, possibly due to the phase transition during the desolvation process (Figure 2 and Supporting Information Figures S11). Combined with the EA studies of the desolvated sample, the PXRD characteristics of 2 and 3 may be attributed to the changes in space group rather than the structural framework. For complex 4, the PXRD peaks at ca. 5.1° ({100} facets) disappeared with temperatures at above 320 K ( S12). Given that the {100} facets are located at the interfaces within the packing arrangements of 4 ( Supporting Information Figure S13), such behavior is primarily ascribed to the crystallographic disorders caused by the loss of lattice solvents. Moreover, all the desolvated analogues preserved similar PXRD patterns ( Supporting Information Figure S14). These results support the notion that the structural integrity is valid for the bulk samples of 2 4 both in solvated and desolvated forms. As the temperature was lowered, the diffraction patterns remained almost the same at 380–250 K, suggesting the excellent thermal stability of the desolvated phase ( 2d–4d). It should be noted that the small shift between the entire experimental and simulated PXRD patterns is ascribed to the differences in the measurement temperatures: the SCXRD data are collected at 100 K, whereas the PXRD patterns are measured at 298 K. And the decreased quality of the experimental PXRD patterns presented as the broadening of the peaks is mainly attributed to the significant vibrations of the powder that produced by the in situ thermal desolvation process in the PXRD instrument.

    Figure 1

    Figure 1 | Molecular structure of 2 at 100 K. Color codes: Fe, green; Zn, cyan; C, gray; N, blue; B, lavender; S, yellow. Hydrogen atoms and interstitial solvent molecules are omitted for clarity.

    Figure 2

    Figure 2 | In situ variable temperature PXRD for 2 (left, heating cycle) and 2d (right, cooling cycle). The simulated pattern is calculated based on the single-crystal data at 100 K.

    IR spectroscopy

    The intramolecular ET behavior of 2 was first investigated by variable temperature IR spectra in its solid state (Figure 3), which displayed temperature-dependent fingerprint signals of the cyano vibrations, reminiscent of other structurally related and thermally responsive cyano-bridged Fe/Co complexes.4248 At 298 K, the IR spectra of 2 exhibit absorptions at 2516 and 2112 cm−1, assignable to the νBH and νCN stretching for the [FeII,LS(μ-CN)CoIII,LS] units. The other two strong peaks at 2088 and 2062 cm−1 are assigned as the νCN stretchings of the overlap of NCS and [FeII,LS(μ-CN)CoIII,LS] linkage. Upon heating to 393 K, the νBH stretch shifts to higher energy of 2540 cm−1, but the νCN stretch at 2112 cm−1 disappeared, accompanied with the appearance of new νCN absorptions (2166 and 2156 cm−1), typical of the paramagnetic bridging [FeIII,LS(μ-CN)CoII,HS] species. When cooling back to room temperature, the IR spectra of the in situ desolvated sample ( 2d) showed a reversible variation and almost returned to its initial appearance, suggesting the occurrence of reversible thermal conversion between the paramagnetic [FeIII,LS(μ-CN)CoII,HS] phase and the diamagnetic [FeII,LS(μ-CN)CoIII,LS] phase.

    Figure 3

    Figure 3 | In situ variable temperature solid-state FT-IR spectra for 2 (left, heating mode) and 2d (right, cooling mode).

    Magnetic properties

    Further evidence that ET occurs in 2 was confirmed in a series of magnetic studies. Variable temperature magnetic susceptibility data for the fresh samples for 1 and 2 as well as the in situ desolvated analogue of 2d were collected in the applied dc field of 1 kOe. For 1, the χT product of 7.40 cm3 mol−1 K at 300 K is consistent with the expected value for the uncoupled paramagnetic [FeIII,LS2CoII,HS2] state (Figure 4). Upon cooling, the χT product increases continuously to a maximum of 8.93 cm3 mol−1 K at 10 K, suggesting intramolecular FeIII,LS–CoII,HS ferromagnetic couplings. It then decreases to 7.18 cm3 mol−1 K at 2 K, likely due to the magnetic anisotropy and/or intermolecular antiferromagnetic interactions. The isothermal magnetizations (M) as a function of the applied field (H) were collected at 2–5 K ( Supporting Information Figure S17), which increased quickly at small external fields and reached 6.79 Nβ at 2 K and 7 T, consistent with the [FeIII,LS2CoII,HS2] configuration and intramolecular ferromagnetic interaction. The high-field unsaturation is attributed to the considerable magnetic anisotropy of metal centers.65 No ET process was observed, indicating that 1 is thermally redox-inactive overall.

    Figure 4

    Figure 4 | Temperature-dependent magnetic susceptibility of 1 in the dark (1 kOe); 2 and 2d measured in the dark (1 kOe) and after light irradiation (808 nm, 10 mW) at 20 K (10 kOe).

    For 2, the χT product of <0.34 cm3 mol−1 K at below 300 K is in good agreement with the diamagnetic [FeII,LS2CoIII,LS2ZnII2] state. Upon heating at a sweeping rate of 2 K/min, the χT value increased gradually before a steep rise centered at T1/2 = 354 K occurred with a saturation value of 7.65 cm3 mol−1 K at 400 K, which is attributed to a complicated desolvated intermediate phase as evidenced by the variable temperature-PXRD and consistent with the paramagnetic [FeIII,LS2CoII,HS2ZnII2] configuration in desolvated forms (Figure 4). Given that the lattice solvents got lost in the same temperature range, the ET was supposed to be induced by the loss of solvent molecules. Such behavior is further supported by the results of solution studies for which the diamagnetic [FeIII,LS2CoII,LS2Zn2] state was maintained at 300–400 K ( Supporting Information Figure S18). Nevertheless, the sample of 2 was continuously maintained in the magnetometer under vacuum at 400 K for 1 h for the complete desolvated analogue ( 2d). Upon cooling, the χT value of 2d remains roughly constant down to 350 K, and then decreases abruptly at T1/2 = 312 K to a residual value of 0.85 cm3 mol−1 K at 280 K, which gradually decreases to 0.27 cm3 mol−1 K at 2 K. Furthermore, this phenomenon is reversible when heating the sample back to 400 K.

    To probe the possible photoresponse of the ET property, the magnetic properties were further investigated at low temperatures under light exposure. Under a selected laser light (808 nm, 10 mW) and at 20 K ( Supporting Information Figures S23 and S24), the χT product of both 2 and 2d gradually approaches a maximum of 3.07 and 4.69 cm3 mol−1 K after 10 h, respectively, characteristic for those Fe(II) → Co(III) metal-to-metal charge transfers (MMCT)4248 with the diamagnetic [FeII,LS(μ-CN)CoIII,LS] phase being partially converted (36–58%, in reference to the maximum χT value of 1) to the paramagnetic [FeIII,LS(μ-CN)CoII,HS] state. As shown in Figure 4, the photogenerated metastable states for both 2 and 2d exhibited a similar trend to that in 1 at below 60 K. They then readily relaxed back to the more stable thermodynamic diamagnetic [FeII,LS(μ-CN)CoIII,LS] state at near 108 K ( 2) and 114 K ( 2d), respectively. These results are comparable with those reported for cyano-bridged Fe/Co squares.4248

    Remarkably, this postsynthesis approach to functionalize the parent complex was successfully extended to the attachment of other secondary metal coordinations. Specifically, two other hexanuclear complexes {[(Tp*Me)Fe(μ-CN)3(M-NCS)][Co(dmbpy)2]}2·sol ( 3, M-NCS = [Co(NCS)2.2(SH)0.8], sol = 7.6DMF·2H2O; 4, M-NCS = [CdCl2(SH)], sol = 4DMF·Et2O) were obtained by adopting the same procedure for the synthesis of 2. It should be mentioned that NCS is usually supposed to be decomposed as CN and CH3S in either CH3CN or CH3OH,66,67 and only a few examples of NCS → SH formation68 have been reported. In this case, combined SCXRD and EA studies would more likely support the generation of SH rather than CN or CH3S. Given that the only source of S atoms is M-NCS, the unexpected SH would be generated from the S–CN bond dissociation that may have cooperated with the Co or Cd metal ions. The formation mechanism deserves further study. Nevertheless, both complexes 3 and 4 were structurally and magnetically characterized. Similar to 2, both complexes 3 and 4 and their desolvated analogues ( 3d and 4d) exhibited thermally and photoresponsively induced ET behaviors at different temperatures (Table 1). More detailed descriptions are seen in the Supporting Information.

    Table 1 | The Transition Temperatures (T1/2/K) for All the Complexes

    Compound 2 2d 3 3d 4 4d
    T1/2 354 312 358 296 368 365


    Three hexanuclear {Fe2Co2M2} complexes were obtained by postmodification of the well-known parent of the cyano-bridged {Fe2Co2} square by secondary metal coordination. The grafted metal complex units, by functioning as effective electron accepters, are capable of tuning the electronic structures of the resulting complexes with respect to their parent complex core, and therefore, modulating their ET behavior in a wide temperature range. In comparison with the electronic “perturbations” caused by supramolecular interactions, direct tuning of the electronic structure of the parent square unit via secondary metal coordination not only enables the system with inherent robustness, but also provides the opportunity to correlate the electronic structure, and hence, the modified redox properties with the peripheral modification. This work thus offers a simple yet efficient approach to fine-tuning the thermal energy barrier for ET of the celebrated {Fe2Co2} squares, leading to materials with bistability and multistability of potentially useful applications.

    Supporting Information

    Supporting Information is available.

    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 (nos. 21671095 and 21901108) and startup funds from SUSTech.


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