Syntheses of ferrocene-functionalized indium-based metal–organic frameworks for third order nonlinear optical application

Abstract

We propose a rigid–flexible synergistic strategy for the syntheses of ferrocene-functionalized indium-based metal–organic frameworks (In-MOFs) through introducing rigid carboxylic acids as auxiliary ligands, where the large cations are used to induce flexible 1,1′-ferrocene dicarboxylic acid (H₂FcDCA) in a controllable coordination mode to construct target frameworks. As an illustration of this strategy, a series of In-MOFs based on the H₂FcDCA ligand have been successfully synthesized. The introduction of rigid OX²⁻ ligands and large cation template leads to the syntheses of various In-MOFs. Furthermore, introducing functional cations and a ferrocene unit endows them with good third order nonlinear optical properties and redox activity.

---

Introduction

Metal–organic frameworks (MOFs) have received much attention not only because of their wide applications but also because of their rational design and synthesis.^1–11 The design synthesis of 4-connected frameworks is quite important due to their ultrahigh porosity and exceptional stability.^6,12 Typically, for example, zeolite imidazole frameworks (ZIFs) are composed of divalent tetrahedral metal centers and imidazole bridged ligands,^12–14 and boron imidazolate frameworks (BIFs) are based on monovalent/divalent tetrahedral metal centers, B(imidazole)₄ ligands,^15,16etc.^13,17,18 The interactions between ligands, that is, link–link interaction play an important role in the framework's diversity. In addition, some high-valence metal centers (such as In³⁺,Ti⁴⁺, Zr⁴⁺ or Hf⁴⁺), especially In³⁺, have been reported as efficient catalysts in multicomponent reactions,¹⁹ and interestingly, they can also act as tetrahedral units (such as InN₂(CO₂)₄, InN₄(CO₂)₄, InN₄(CO₂)₂ or In(CO₂)₄) by chelating coordination (two or more coordination atoms and forms a chelating ring with a central atom or ion) of organic ligands.^20–28 Some indium-based MOFs (In-MOFs) with 4-connected topologies can be reasonably designed and synthesized based on the In³⁺ tetrahedral center with various ligands.^29–33 Considering the local and global charge balance, these frameworks are usually anionic and require cations as a guest, endowing them with ion exchange properties.^34–36 At the same time, cations can act as structure direct agents (SDAs) in In-MOFs similar to the organic amines in the formation of zeolites, and they can also act as active units to improve their performance.^37,38 Similarly, the introduction of functional units on the ligand can also broaden their applications, for instance, homochiral In-MOFs have been developed by using chiral ligands.^39,40 However, considering the diversity of ligands, there is still great potential to construct In-MOFs based on new functional ligands.

1,1′-Ferrocene dicarboxylic acid (H₂FcDCA) has been widely studied as a functional ligand due to its excellent reduction–oxidation and photoelectric activity.^41–49 Nevertheless, limited by its flexible conformation and uncontrollable coordination mode, a large number of low-dimensional structures and clusters are usually obtained instead of MOFs.^50–53

In this work, we propose a rigid–flexible synergistic strategy for the syntheses of In-MOFs, that is, introducing rigid carboxylic acids as auxiliary ligands as well as large cations to induce FcDCA²⁻ to construct target frameworks in a controllable coordination mode. As an illustration of this strategy, a series of In-MOFs based on the H₂FcDCA ligand have been successfully synthesized, namely, (TPP)⁺₂[In₂(FcDCA)₃(OX²⁻)]·5H₂O (1, TPP = tetraphenylphosphine; OX²⁻ = oxalate); (TPP)⁺[In(FcDCA)₂]·5H₂O (2); (Hbpy)⁺[In(FcDCA)(OX²⁻)]·H₂O (3, bpy = 4,4′-bipyridine); (MB)⁺[InCl(FcDCA)(OX²⁻)_0.5] (4, MB = methylene blue); (TPP)⁺[InCl(FcDCA)(OX²⁻)_0.5] (5); (TPP)⁺[InCl₂(FcDCA)(H₂O)]·H₂O (6); and [In(FcDCA)(Im)(H₂O)] (7, HIm = imidazole). The introduction of rigid OX²⁻ ligands and a large cation template leads to the syntheses of various frameworks. Furthermore, the presence of functional cations and a ferrocene unit endows them with good third order nonlinear optical properties and redox activity.

Experimental

Materials and instruments

All chemicals were commercially purchased and directly used without further purification. PXRD data were collected on a Rigaku Mini Flex II diffractometer using Cu-Kα radiation (λ = 1.54056 Å) under ambient conditions. The UV-vis diffuse reflection data were recorded at room temperature using a powder sample with BaSO₄ as a standard (100% reflectance) on a PerkinElmer Lamda-950 UV spectrophotometer and scanned at 200–800 nm. FT-IR spectra were recorded on KBr pellets on a Bruker VERTEX70 FT-IR spectrometer with the range of 400–4000 cm⁻¹. Thermal stability studies were carried out on a Netschz STA449F3 thermal analysis system with a heating rate of 10 K min⁻¹ under a N₂ atmosphere. The gas adsorption isotherms were measured by using ASAP-2020 volumetric adsorption equipment. Fluorescence spectra (FL) and the luminescence decay curves were performed on an Edinburgh FLS1000 fluorescence spectrometer. ¹H NMR spectra were recorded on a Bruker AVANCE III spectrometer (400 MHz).

Synthesis of (TPP)⁺₂[In₂(FcDCA)₃(OX²⁻)]·5H₂O (1)

A mixture of oxalic acid (9.0 mg, 0.10 mmol), TPPBr (42.0 mg, 0.10 mmol), imidazole (7.0 mg, 0.10 mmol), H₂FcDCA (27.0 mg, 0.10 mmol), In(NO)₃·xH₂O (60.0 mg, 0.20 mmol), NaOH (8.0 mg, 0.20 mmol), and H₂O (5.0 ml) was added to 23 ml glass vials, respectively, sealed with ultrasound for 5 minutes, and heated in a100 °C oven for 1 day to generate yellow crystals (yield: 8.0%). Elem anal. found: C, 55.3; H, 3.8. Calc. for C₈₆H₇₄Fe₃In₂O₂₁P₂: C, 54.2; H, 3.9%. FTIR (KBr, cm⁻¹): 3480 (v), 3079 (m), 2360 (m), 1646 (s), 1581 (s), 1476 (m), 1453 (m), 1342 (s), 1308 (s), 1187 (s), 1104 (s), 1031 (s), 979 (v), 844 (m), 797 (w), 720 (s), 690 (s), 524 (m) (Fig. S1†). The crystals are decomposed in trifluoroacetic acid and then the test was carried out: ¹H NMR (400 MHz, DMSO-d₆) δ 7.88 (t, J = 7.0 Hz, 3H, TPP⁺), 7.74–7.65 (m, 12H, TPP⁺), 4.66 (s, 4H, FcDCA²⁻), 4.38 (s, 4H, FcDCA²⁻) (Fig. S2–S4†).

Synthesis of (TPP)⁺[In(FcDCA)₂]·5H₂O (2)

A mixture of L-tartaric acid (15.0 mg, 0.10 mmol), TPPBr (42.0 mg, 0.10 mmol), H₂FcDCA (27.0 mg, 0.10 mmol), In(NO)₃·xH₂O (60.0 mg, 0.20 mmol), NaOH (8.0 mg, 0.20 mmol), and H₂O (5 ml) was added to 23 ml glass vials, respectively, sealed with ultrasound for 5 minutes, and heated in a 100 °C oven for 1 day and yellow crystals were obtained (yield: 25.0%). Elem anal. found: C, 53.1; H, 4.2. Calc. for C₄₈H₄₆Fe₂InO₁₃P: C, 52.9; H, 4.4%. FTIR (KBr, cm⁻¹): 3450 (v), 3084 (m), 2356 (m), 1650 (s), 1576 (s), 1490 (m), 1447 (m), 1373 (m), 1338 (s), 1182 (s), 1091 (s), 1013 (s), 983 (v), 820 (m), 736 (s), 723 (s), 680 (s), 515 (m). ¹H NMR (400 MHz, DMSO-d₆) δ 7.86 (t, J = 7.0 Hz, 3H, TPP⁺), 7.73–7.64 (m, 12H, TPP⁺), 4.65 (s, 4H, FcDCA²⁻), 4.37 (s, 4H, FcDCA²⁻) (Fig. S5†).

Synthesis of (Hbpy)⁺[In(FcDCA)(OX²⁻)]·H₂O (3)

A mixture of oxalic acid (9.0 mg, 0.1 mmol), 4,4-bipyridine (bpy, 16.0 mg, 0.10 mmol), H₂FcDCA (27.0 mg, 0.10 mmol), In(NO)₃·xH₂O (60.0 mg, 0.20 mmol), NaOH (8.0 mg, 0.20 mmol), and H₂O (5 ml) was added to 23 ml glass vials, respectively, sealed with ultrasound for 5 minutes, and heated in a 100 °C oven for 1 day and yellow crystals were obtained (yield: 16.0%). Elem anal. found: C, 43.9; H, 3.5; N, 4.1. Calc. for C₂₄FeInN₂O₉H₁₉: C, 44.3; H, 3.2; N, 4.3%. FTIR (KBr, cm⁻¹): 3482 (v), 3105 (v), 2343 (m), 1685 (s), 1620 (v), 1567 (w), 1473 (s), 1382 (m), 1317 (s), 1178 (s), 1087 (s), 1005 (v), 800 (s), 693 (s), 576 (s), 490 (s). ¹H NMR (400 MHz, DMSO-d₆) δ 9.13 (s, 6H, bpy), 8.55 (s, 6H, bpy), 4.65 (s, 4H, FcDCA²⁻), 4.37 (s, 4H, FcDCA²⁻) (Fig. S6†).

Synthesis of (MB)⁺[InCl(FcDCA)(OX²⁻)_0.5] (4)

A mixture of oxalic acid (9.0 mg, 0.10 mmol), MB (C₁₆H₁₈ClN₃S, 32.0 mg, 0.10 mmol), H₂FcDCA (27.0 mg, 0.10 mmol), In(NO)₃·xH₂O (60.0 mg, 0.20 mmol), NaOH (8.0 mg, 0.20 mmol), and H₂O (5 ml) was added to 23 ml glass vials, respectively, sealed with ultrasound for 5 minutes, and heated in a 100 °C oven for 3 days and yellow crystals were obtained (yield: 31.0%). Elem anal. found: C, 45.9; H, 3.8; N, 5.2. Calc. for C₂₉H₂₆ClFeInN₃O₆S: C, 46.1; H, 3.6; N, 5.3%. FTIR (KBr, cm⁻¹): 2356 (m), 1646 (v), 1529 (s), 1476 (s), 1430 (m), 1384 (m), 1325 (s), 1234 (s), 1135 (s), 1040 (s), 940 (m), 879 (w), 797 (m), 670 (s), 494 (s), 430 (s). Methylene blue is an alkaline dye that may decompose in trifluoroacetic acid. There is no characteristic peak of methylene blue in ¹H NMR but there is a characteristic peak of the FcDCA²⁻ ligand. ¹H NMR (400 MHz, DMSO-d₆) δ 4.66 (s, 4H, FcDCA²⁻), 4.38 (s, 4H, FcDCA²⁻) (Fig. S7†).

Synthesis of (TPP)⁺[InCl(FcDCA)(OX²⁻)0.5] (5)

A mixture of oxalic acid (OX; 2.0 mg, 0.02 mmol), TPPBr (42.0 mg, 0.10 mmol), H₂FcDCA (27.0 mg, 0.10 mmol), InCl₃·4H₂O (60 mg, 0.20 mmol), NaOH (16.0 mg, 0.40 mmol), and H₂O (4 ml) was added to 23 ml glass vials, respectively, sealed with ultrasound for 5 minutes, and heated in a 100 °C oven for 7 days to generate several yellow plate crystals of 5 and a large amount of unidentified precipitate. Their crystal structures are described below.

Synthesis of (TPP)⁺[InCl₂(FcDCA)(H₂O)]·H₂O (6)

A mixture of TPPBr (42.0 mg, 0.10 mmol), H₂FcDCA (27.0 mg, 0.10 mmol), InCl₃·4H₂O (60 mg, 0.20 mmol), NaOH (16.0 mg, 0.40 mmol), and H₂O (4 ml) was added to 23 ml glass vials respectively, sealed with ultrasound for 5 minutes, and heated in a 100 °C oven for 7 days to generate several yellow plate crystals of 6 and a large amount of unidentified precipitate. Their crystal structures are described below.

Synthesis of [In(FcDCA)(Im)(H₂O)] (7)

A mixture of TPPBr (42.0 mg, 0.10 mmol), imidazole (7.0 mg, 0.10 mmol), H₂FcDCA (27.0 mg, 0.10 mmol), In(NO)₃·xH₂O (60.0 mg, 0.20 mmol), 5 ml of formamide and polyethylene glycol (v : v, 1 : 1) was added to 23 ml glass vials, respectively, sealed with ultrasound for 5 minutes, and heated in a 100 °C oven for 3 days and yellow crystals were obtained (yield: 20%). Elem anal. found: C, 37.6; H, 2.5; N, 6.1. Calc. for C₁₅H₁₃FeInN₂O₅: C, 37.4; H, 2.7; N, 5.9%. FTIR (KBr, cm⁻¹): 3460 (v), 3310 (s), 3150 (m), 1675 (s), 1496 (m), 1393 (s), 1347 (s), 1238 (s), 1195 (m), 1078 (s), 1031 (m), 944 (s), 823 (m), 801 (m), 654 (s), 597 (m), 550 (s), 494 (s). ¹H NMR (400 MHz, DMSO-d₆) δ 7.76 (s, 1H, Im⁻), 7.26 (s, 2H, Im⁻), 4.64 (s, 4H, FcDCA²⁻), 4.36 (s, 4H, FcDCA²⁻) (Fig. S8†).

X-Ray crystallographic analysis

Single crystal X-ray diffraction data of porous materials were collected using a Hybrid Pixel Array detector equipped with Ga-Kα radiation (λ = 1.3405 Å) at about 298 K and 100 K. The structures were solved by dual-direct methods using ShelXT and refined by the full-matrix least-squares technique based on F² using SHELXL.⁵⁴ Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added theoretically, riding on the concerned atoms and refined with fixed thermal factors. All absorption corrections were performed using the multi-scan program.

Z-scan measurements

The Z-scan technique was applied to investigate the third order nonlinear optical (NLO) behaviour of the samples with an output wavelength of 532 nm. A nanosecond light source irradiated by a PL2250 laser (EKSPLA) was a Q-switched Nd:YAG pulsed laser system with a repetition frequency of 10 HZ. For the practical application of NLO, the hybrid transparent and flexible films were prepared by ultrasonic stripping and dispersing crystal materials into the polydimethylsiloxane (PDMS) matrix by the literature reported method.⁵⁵

Results and discussion

Single crystal X-ray diffraction analysis shows that compound 1 crystallizes in the monoclinic C2/c space group. The asymmetric unit of 1 contains two In(III) ions, and three FcDCA²⁻, one oxalate, two cationic guest TPP⁺ and five guest water molecules. Each In(III) ion is 8-coordinated with eight carboxyl oxygen atoms of three FcDCA²⁻ ligands and one OX²⁻ ligand (Fig. 1a). The In–O bond distance ranges from 2.154 Å to 2.572 Å (Table S2†). Among the three FcDCA²⁻ ligands, carboxyl torsion angle of two FcDCA²⁻ ligands is 152.306°, and the other one is 180° (Fig. 1b). Two oxygen atoms of the same carboxyl group of the three FcDCA²⁻ ligands are coordinated to the indium metal in the form of chelate coordination. The OX²⁻ ligand also acts as a bridging ligand to coordinate on two In atoms.

Fig. 1 (a) Coordination environment of the In atom in 1; (b) the OX²⁻ ligand and FcDCA²⁻ ligands with different torsion angles are simplified as linkers; (c) the 2D layer is constructed from In atoms and FcDCA²⁻ ligands; (d) the 3D pillar-layered framework of 1; (e) the dia topology of 1; and (f) π⋯π interaction of two TPP⁺ in one dia cage.

The main structural feature of 1 is its zeolitic framework. Firstly, each In atoms is linked by three FcDCA²⁻ ligands to form a 2D honeycomb-like layer along the bc plane (Fig. 1c). Then these layers are pillared by OX²⁻ ligands to form a pillar-layered 3D framework (Fig. 1d). As expected, each In atom acts as a 4-connected node, and the whole framework can be simplified into the dia topology (Fig. 1e).⁵⁶ Each dia cage of 1 is filled with two TPP⁺ with weak π⋯π interactions to balance the charge (Fig. 1f), making it nonporous.

2 has been synthesized by using flexible L-tartaric acid to replace rigid OX²⁻ under similar conditions. Only FcDCA²⁻ ligands are observed in 2 which may be attributed to the weak coordination ability of L-tartaric acid. Similarly, each In(III) ion of 2 is coordinated by four FcDCA²⁻ ligands as a 4-connected node (Fig. 2a), but it is connected by four FcDCA²⁻ linkers to form a 2D layer (Fig. 2b) with sql topology. These layers are stacked in the ABAB mode (Fig. 2c) and the porosity of 2 is 10.8%. TPP⁺ cations are located between layers. There are weak π⋯π interactions between TPP⁺ cations (Fig. 2d). Comparing these two compounds, we found that in the presence of large TPP⁺ cations, flexible FcDCA²⁻ ligands and rigid OX ligands cooperate to form a 3D framework (1), while flexible FcDCA²⁻ ligand tends to form the 2D layer (2) instead of the 3D framework, which is similar to the layer previously obtained with protonated 4,4-bipyridine cations.⁵¹ This is may be attributed to the difference in the torsion angle. Two carboxyl groups of FcDCA²⁻ ligands form torsion angles of 143.535° and 157.248° in 2, which is smaller than that of 1.

Fig. 2 (a) Coordination environment of In atom in 2; (b) the 2D layer of 2 based on In atoms and FcDCA²⁻ ligands; (c) packing structure of 2; and (d) π⋯π interaction of TPP⁺ in 2.

To test the universality of this rigid–flexible synergy strategy, compounds 3 to 7 have been synthesized through changing cations and rigid ligands.

As mentioned above, we previously reported the FcDCA²⁻ based 2D layer with protonated 4,4-bipyridine cations in the presence of 4,4-bipyridine.⁵¹ In this work, compound 3 with a 3D framework is obtained by using 4,4-bipyridine to replace TPPBr in 1. The asymmetric unit of 3 contains an indium(III) ion, one FcDCA²⁻, one OX²⁻, one protonated Hbpy⁺ cation and one guest water molecule. Each In(III) ion is 8-coordinated by two FcDCA²⁻ ligands and two OX²⁻ ligands (Fig. 3a). The In–O bond length ranges from 2.201 Å to 2.582 Å (Table S4†). The torsion angle between two carboxyl groups of the FcDCA²⁻ is 143.419° which is close to that in 1.

Fig. 3 (a) Coordination environment of the In atom in 3; (b) 3D framework of 3; (c) the dia topology of 3, FcDCA²⁻ linker: green; OX²⁻ linker: pink; and (d) the π⋯π and hydrogen-bonding interactions of the bipyridine cations in 3.

In 3, both ligands act as bridging ligands to connect In atoms to form a dia-type 3D framework (Fig. 3b and c) containing 1D channel along the b-axis, which is filled with Hbpy⁺ cations. Interestingly, the Hbpy⁺ cations formed a 1D chain through H-bond interactions (Fig. 3d). In addition, strong π⋯π interaction between adjacent pyridine rings is also observed. Each Hbpy⁺ has a certain degree of torsion, and the torsion angle is 134.465°. Compared with 1, the ratio of the FcDCA²⁻ ligand to oxalic acid changes from 3 : 1 to 2 : 2, and the presence of smaller Hbpy⁺ cations improves the porosity of 3 (20%).

When larger methylene blue is introduced into the reaction system, 1D ladder-like 4 is obtained, which indicates that the appropriate template size is also important. In 4, each In atom is coordinated by two FcDCA²⁻, one OX²⁻ and one Cl anion (Fig. 4a). The In atom is connected by FcDCA²⁻ to form a 1D chain, which is linked by OX²⁻ to form a ladder-like structure. The Cl anions are pendent on the ladder which hinders the formation of the framework (Fig. 4b). The guest MB⁺ of 4 is stuck in the middle of each ladderlike chain with a strong π⋯π interaction (Fig. 4c and d). In addition, methylene blue with a strong absorption chromophore makes 4 fluorescent (Fig. S9†).

Fig. 4 (a) Coordination environment of In atom in 4; (b) the 1D ladder of 4; (c) packing structure of 4; and (d) π⋯π interaction between MB in 4.

It should be noted that the introduction of a Cl anion from MB is not conducive to the formation of the framework. The employment of InCl₃·4H₂O to replace In(NO)₃·xH₂O also leads to similar results, 5 and 6. The coordination environment and structure of 5 is the same as 4 (Fig. 5a, c and e). The difference is that the guest of 5 is TPP⁺ with a weak π⋯π interaction (Fig. 5g). In 6, one OX²⁻ is replaced by Cl⁻ and one coordinated water (Fig. 5b). Therefore, each In atom is connected by FcDCA²⁻ to form a 1D chain (Fig. 5d). Hydrogen bonds (O4–H4B⋯O6, the angle of the O–H⋯O is 144.5°, the distance of O⋯O is 2.773 Å (Table S8†)) between coordinating water and carboxylate oxygen of the FcDCA²⁻ ligand are observed, which link adjacent chains to form a 2D layer (Fig. 5f). TPP⁺ cations lie between these layers with weak π⋯π interactions (Fig. 5h).

Fig. 5 Structure comparison of 5 and 6: coordination environment of In atoms in 5 (a) and 6 (b); the 1D ladder of 5 (c); and hydrogen bonded 2D layer of 6 (d); packing structure of 5 (e) and 6 (f); and the π⋯π interactions of TPP⁺ in 5 (g) and 6 (h).

Besides OX²⁻, imidazole commonly used in ZIFs is also chosen as a rigid auxiliary ligand to check the possibility. By adding imidazole molecules to the solvothermal system, 7 is synthesized. The In atom of 7 is 7-coordinated by four oxygen atoms from two FcDCA²⁻ ligands, two nitrogen atoms from two imidazole ligands and one coordinated water (Fig. 6a). Two carboxylates of the FcDCA²⁻ ligand form a small torsion angle of 50.423° and two twisted FcDCA²⁻ ligands coordinate on two In atoms to generate one In₂(FcDCA)₂ unit (Fig. 6b), which is similar to the former results.⁵¹ Differently, the In₂(FcDCA)₂ unit is bridged by four imidazole ligands to form a 2D layer with sql topology (Fig. 6c and d). The porosity of 7 is 24.7%. The permanent porosity of 2, 3 and 7 was confirmed by the reversible CO₂-adsorption measurement at 195 K (Fig. S10†). Tentatively, we think that the huge differences between the imidazole ligand and carboxylic acid make them mismatched and the target framework cannot be obtained.

Fig. 6 (a) Coordination environment of In atom in 7; (b) the In₂(FcDCA)₂ unit as 4-connected node (yellow) and im ligands as the linker (blue); (c) 2D layer of 7; and (d) the sql topology of 7.

Here we make a brief summary of the synthesis and structure:

(1) the rigid–flexible synergy strategy is effective for the synthesis of ferrocene-functionalized In-MOFs;

(2) rigid carboxylic acids will be good auxiliary ligands than that of the flexible one and azolate ligands;

(3) cations of suitable size can have positive effects;

(4) compared with chloride, indium nitrate may be more suitable for the synthesis of In-MOFs.

Based on these findings, more exploratory synthesis toward In-MOFs with 4-connected zeolite topologies is under way.

The phase purity of samples 1 to 4 and 7 can be confirmed by powder X-ray diffraction (PXRD) (Fig. S11†). In order to study the thermal stability of these complexes, we carried out thermal analysis (TGA) of compounds 1 to 4 and 7. TGA curve shows that both 1 and 2 have two obvious loss steps: the weight loss between room temperature and ∼110 °C corresponds to the removal of free water molecules in the crystal. The crystal material between ∼110 and ∼310 °C is stable and then starts to decompose (Fig. S12†). Thermogravimetric analysis of 3 exhibits that the first weight loss ratio from room temperature to 100 °C is 8.31%, which may be the loss of water molecules. The crystal material between 100 and 210 °C is stable. After 210 °C, 3 starts to decompose and the OX²⁻ may be removed first (6.77%). The thermal stability of 4 is not very good, which decomposes from the beginning of heating and the first continuous weight loss may occur for OX²⁻. Thermogravimetric analysis of 7 exhibits that the first continuous weight loss ratio from room temperature to 160 °C is 2.40%, which may be ascribed to the loss of free high boiling point solvent guest. And then 7 starts to decompose, in which Im⁻ is removed first (13.60%).

Third order nonlinear optical measurement

It is well-known that the ferrocene unit with an electron rich ring conjugation system and large cations with π⋯π stacking may exhibit nonlinear optical effects.^57–62 Therefore we further studied the third order nonlinear optical (NLO) properties of compounds 1 to 4 and 7. In order to ensure structural stability, the crystals of these materials were dispersed in the PDMS thin film. By using Z-scan technology and a 532 nm nanosecond laser, PDMS thin films were found to have no detectable NLO response. Under the same test conditions, the minimum normalized transmittance (T_min) of 1 to 4 and 7 at the focal point (Z = 0) showed the following order: 1 > 2 > 4 > 3 > 7 (Fig. 7a and b). Compounds 1 to 4 and 7 exhibited significant nonlinear optical response and reverse saturated absorption (RSA).⁶³ We believe that the clear RSA response of these crystal materials is mainly attributed to the FcDCA²⁻ ligands with nonlinear optical properties and the π⋯π stacking between the guests. Compared with 4 and 7, the RSA response of 1 and 2 is slightly enhanced, the reason may be that the number of FcDCA²⁻ ligands in each unit cell is the largest. Compared with 7, 3 and 4 has a slightly stronger response, which may be due to the strong π⋯π interaction of guest stacking that increases the electron cloud density, which is conducive to the excited transition of electrons. The performance of 4 is better than 3 because the π⋯π distance of the MB⁺ stacking is closer and the π⋯π. interaction is slightly stronger (Fig. 3d and 4d). The plots of output fluence versus input fluence calculated from the corresponding Z-scan curves reveal a typical optical limiting (OL) behaviour (Fig. 7c). To evaluate the NLO responses quantitatively, nonlinear absorption coefficients (β) of 1 to 4 and 7 were calculated by fitting nonlinear related equations, and the corresponding values of β were estimated to be ∼2.8 × 10⁻⁹, ∼2.1 × 10⁻⁹, ∼1.7 × 10⁻⁹, ∼0.7 × 10⁹, and ∼0.3 × 10⁻¹⁰ m W⁻¹ (Fig. 7d), respectively, where the positive values imply the RSA behaviour of the samples.

Fig. 7 (a) number of FcDCA ligands in each unit cell; (b) the open-aperture Z-scan spectrum of 1 to 4, 7 and PDMS; (c) the curves of output fluence versus input fluence of 1 to 4, 7 and PDMS; (d) comparison of nonlinear transmittance and nonlinear absorption coefficients (β); nonlinear refractive response of 1 (e), 2 (f) and 4 (h) shows valley peak trend indicating their self-focusing behavior; 3 (g) and 7 (i) show a clear peak-to-trough trend, indicating their self-defocusing behavior.

In addition, we performed closed-aperture Z-scan measurement of these MOFs (Fig. 7e–i). χ values were calculated by the formula⁵⁵ and the order is from ∼2.03 × 10⁻¹⁰ to ∼6.77 × 10⁻¹⁰ esu (Table 1). The results indicate the relatively low T_min (∼0.41) and third-order NLO susceptibility of 1 in comparison with other MOF-based materials (Table 2), thus making this MOF a particularly attractive third-order NLO material.

Table 1 Linear and NLO data of 1 to 4 and 7. T_min: linear transmittance; β: nonlinear coefficient; n₂: nonlinear refractive coefficients; Im χ(3): the imaginary parts of the third-order susceptibility; Re χ(3): the real parts of the third-order susceptibility; χ(3): the absolute value of the third-order susceptibility

Samples	T min	β (×10^−9 m W^−−1)	n 2 (×10^−16 m^2 W^−1)	Im χ^(3) (×10^−11 esu)	Re χ^(3) (×10^−10 esu)	χ ^(3) (×10^−10 esu)
1	∼0.41	∼2.80	∼4.40	∼2.51	∼6.77	∼6.77
2	∼0.61	∼2.10	∼4.10	∼2.34	∼5.08	∼5.59
3	∼0.85	∼1.70	∼−3.10	∼−1.77	∼−4.11	∼4.47
4	∼0.69	∼2.00	∼3.72	∼2.12	∼4.84	∼5.28
7	∼0.90	∼0.70	∼−2.00	∼−1.14	∼−1.69	∼2.03

---

Table 2 β value and normalized transmittance (T_min) of some related crystal materials

Compound	T min	β (×10^−9 m W^−−1)	χ ^(3) (×10^−10 esu)	Ref.
ZnTPyP(Cu)	∼0.61	∼5.7 × 10^3	∼5.43 × 10^3	55
ZnTPyP(Ni)	∼0.81	∼1.8 × 10^3	∼4.07 × 10^3	55
[(Tp*WS3Cu3)2(Btta)3]	0.71	2 × 10^−4	2 × 10^−9	66
{[(Tp*WS3Cu3)2(Petta)3]BF4}	0.67	3 × 10^−4	2 × 10^−9	66
[Tp*W(μ3-S)3(μ3-S′)Cu3]4	0.8	1 × 10^−3	1.8 × 10^−9	66
[Et4N]{(Tp*WS3Cu3)[Cu2(CN)4.5]}2·2PhCH2CN	0.64	6 × 10^−6	2.2 × 10^−9	66
[NH4][WS4Cu4(dpypy)Cu(CN)4]·DMF	0.8	0.08	3.4 × 10^−18	67
[NH4][WS4Cu4(tpb)Cu2(CN)5]·DMF	0.92	0.012	7.4 × 10^−19	67
1	∼0.41	∼2.8	∼6.77	This work
2	∼0.61	∼2.1	∼5.59	This work
3	∼0.85	∼1.7	∼4.47	This work
4	∼0.69	∼2.0	∼5.28	This work
7	∼0.9	∼0.7	∼2.03	This work

---

Photocurrent measurement

Besides the optical limiting response, the presence of a ferrocene unit drives us to study its photo- and redox-activity. The UV-visible diffuse reflectance results show that compounds 1 to 4 and 7 have strong light absorption in the visible light region and the ultraviolet light region, which are consistent with their colours (Fig. S13–S17†). Their photocurrent responses are studied under visible light irradiation (Fig. S18†). In order to eliminate errors caused by uneven film thickness, a back illumination method is used. During the test, on–off cycle irradiation from a xenon lamp was used (with an interval of 10 s) with a manual shield, and the voltage was maintained at 0.2 V. Obvious photocurrent directions were observed for compounds 1 to 4 and 7. On the one hand, the photocurrent generated rapidly and remained stable, and the intensity did not decrease significantly, indicating that they exhibit a good photoelectric response and high photophysical stability. On the other hand, we find that the response effects exhibit the following tendency: 4 > 2 > 7 > 1 ≈ 3. The difference in the photocurrent response intensity of compounds 1, 2, 3 and 7 does not exceed 0.1 μA cm⁻⁻², which may be the result of the combined action of the molecular structure, light absorption and effective charge transfer.^64,65 The relatively strong photocurrent response of compound 4 may be due to the introduction of a strong chromophore methylene blue guest, which enhances the light absorption of compound 4 and reduces the band gap (Fig. S16†).

Redox activity

Their redox activity is also studied. In the reversible redox process, the ferrocene building block ([FeII(Cp)₂]) is oxidized to ferrocenium ion ([FeIII(Cp)₂]⁺) and subsequently reduced to [FeII((Cp)₂]. Compounds are coated on FTO conductive glass (20th cycle). Different compounds have different signal intensities. 1 to 4 and 7 can detect a reversible redox process (the 20th cycle), with an oxidation peak of 0.505 V–0.534 V and a reduction peak of 0.404 V–0.410 V (Fig. S19†). This redox process can be assigned to the reversible oxidation [FeII(Cp)₂]/[FeIII(Cp)₂]⁺ of FcDCA²⁻.

Conclusions

In this work, we demonstrate the rigid–flexible synergistic strategy for the syntheses of ferrocene-functionalized indium-based MOFs (In-MOFs) through the introduction of rigid carboxylic acids as auxiliary ligands as well as large cations to induce flexible 1,1′-ferrocene dicarboxylic acid (H₂FcDCA) to construct target frameworks. The target compounds exhibit various frameworks and good third order nonlinear optical properties and redox activity. Therefore, the present work offers a valuable guidance for the development of In-MOFs exhibiting optical properties and redox activity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from NSFC (21971241 and 21871050).

References

1. J.-B. Lin, T. T. T. Nguyen, R. Vaidhyanathan, J. Burner, J. M. Taylor, H. Durekova, F. Akhtar, R. K. Mah, O. Ghaffari-Nik, S. Marx, N. Fylstra, S. S. Iremonger, K. W. Dawson, P. Sarkar, P. Hovington, A. Rajendran, T. K. Woo and G. K. H. Shimizu, A scalable metal-organic framework as a durable physisorbent for carbon dioxide capture, Science, 2021, 374, 1464–1469.
2. Y. Su, K. I. Otake, J. J. Zheng, S. Horike, S. Kitagawa and C. Gu, Separating water isotopologues using diffusion-regulatory porous materials, Nature, 2022, 611, 289–294.
3. N. Li, Z. Chang, M. Zhong, Z.-X. Fu, J. Luo, Y.-F. Zhao, G.-B. Li and X.-H. Bu, Functionalizing MOF with redox-active tetrazine moiety for improving the performance as cathode of Li–O₂ batteries, CCS Chem., 2021, 3, 1297–1305.
4. G. Cai, P. Yan, L. Zhang, H.-C. Zhou and H.-L. Jiang, Metal-organic framework-based hierarchically porous materials: Synthesis and applications, Chem. Rev., 2021, 121, 12278–12326.
5. T. He, X.-J. Kong, Z.-X. Bian, Y.-Z. Zhang, G.-R. Si, L.-H. Xie, X.-Q. Wu, H. Huang, Z. Chang, X.-H. Bu, M. J. Zaworotko, Z.-R. Nie and J.-R. Li, Trace removal of benzene vapour using double-walled metal-dipyrazolate frameworks, Nat. Mater., 2022, 21, 689–695.
6. J. Yang, Y.-B. Zhang, Q. Liu, C. A. Trickett, E. Gutierrez-Puebla, M. A. Monge, H. Cong, A. Aldossary, H. Deng and O. M. Yaghi, Principles of designing extra-large pore openings and cages in zeolitic imidazolate frameworks, J. Am. Chem. Soc., 2017, 139, 6448–6455.
7. Y.-X. Tan, F. Wang and J. Zhang, Design and synthesis of multifunctional metal-organic zeolites, Chem. Soc. Rev., 2018, 47, 2130–2144.
8. S. Krause, N. Hosono and S. Kitagawa, Chemistry of soft porous crystals: structural dynamics and gas adsorption properties, Angew. Chem., Int. Ed., 2020, 59, 15325–15341.
9. Y. Wang, S. Wang, Z.-L. Ma, L.-T. Yan, X.-B. Zhao, Y.-Y. Xue, J.-M. Huo, X. Yuan, S.-N. Li and Q.-G. Zhai, Competitive coordination-Oriented monodispersed ruthenium sites in conductive MOF/LDH hetero-nanotree catalysts for efficient overall water splitting in alkaline media, Adv. Mater., 2022, 34, 2107488.
10. Y. Zhu, J. Gu, X. Yu, B. Zhang, G. Li, J. Li and Y. Liu, The multifunctional design of metal-organic framework by applying linker desymmetrization strategy: synergistic catalysis for high CO2-epoxide conversion, Inorg. Chem. Front., 2021, 8, 4990–4997.
11. X.-X. Wu, J.-X. Wei, T. Zhang, Y. Yang, Q. Liu, X. Yan and Y. Tang, Novel synthesis of in situ CeOx nanoparticles decorated on CoP nanosheets for highly efficient electrocatalytic oxygen evolution, Inorg. Chem. Front., 2021, 8, 4440–4447.
12. H. Wang, X. Pei, M. J. Kalmutzki, J. Yang and O. M. Yaghi, Large cages of zeolitic imidazolate frameworks, Acc. Chem. Res., 2022, 55, 707–721.
13. J.-P. Zhang, Y.-B. Zhang, J.-B. Lin and X.-M. Chen, Metal azolate frameworks: from crystal engineering to functional materials, Chem. Rev., 2012, 112, 1001–1033.
14. S. Guo, H.-Z. Li, Z.-W. Wang, Z.-Y. Zhu, S.-H. Zhang, F. Wang and J. Zhang, Syntheses of new zeolitic imidazolate frameworks in dimethyl sulfoxide, Inorg. Chem. Front., 2022, 9, 2011–2015.
15. H.-X. Zhang, M. Liu, T. Wen and J. Zhang, Synthetic design of functional boron imidazolate frameworks, Coord. Chem. Rev., 2016, 307, 255–266.
16. H.-X. Zhang, Q.-L. Hong, J. Li, F. Wang, X. Huang, S. Chen, W. Tu, D. Yu, R. Xu, T. Zhou and J. Zhang, Isolated square-planar copper center in boron imidazolate nanocages for photocatalytic reduction of CO₂ to CO, Angew. Chem., Int. Ed., 2019, 58, 11752–11756.
17. M. Eddaoudi, D. F. Sava, J. F. Eubank, K. Adil and V. Guillerm, Zeolite-like metal-organic frameworks (ZMOFs): Design, synthesis, and properties, Chem. Soc. Rev., 2015, 44, 228–249.
18. Y. Sun, C. Zhuo, F. Wang and J. Zhang, Synthesis of an interrupted AST–Type zeolitic imidazolate framework, Chin. J. Chem., 2020, 38, 449–452.
19. P. Brandao, A. J. Burke and M. Pineiro, A decade of indium-catalyzed multicomponent reactions (MCRs), Eur. J. Org. Chem., 2020, 5501–5513.
20. Z.-Z. Lin, F.-L. Jiang, L. Chen, C.-Y. Yue, D.-Q. Yuan, A.-J. Lan and M.-C. Hong, A highly symmetric porous framework with multi-intersecting open channels, Cryst. Growth Des., 2007, 7, 1712–1715.
21. Y. Liu, V. C. Kravtsov, D. A. Beauchamp, J. F. Eubank and M. Eddaoudi, 4-Connected metal–organic assemblies mediated via heterochelation and bridging of single metal Ions: Kagomé Lattice and the M₆L₁₂ Octahedron, J. Am. Chem. Soc., 2005, 127, 7266–7267.
22. J. A. Brant, Y. Liu, D. F. Sava, D. Beauchamp and M. Eddaoudi, Single-metal-ion-based molecular building blocks (MBBs) approach to the design and synthesis of metal–organic assemblies, J. Mol. Struct., 2006, 796, 160–164.
23. Y.-L. Liu, V. Kravtsov, R. Larsen and M. Eddaoudi, Molecular building blocks approach to the assembly of zeolite-like metal-organic frameworks (ZMOFs) with extra-large cavities, Chem. Commun., 2006, 1488–1490.
24. Y.-X. Tan, Y.-P. He, M. Wang and J. Zhang, A water-stable zeolite-like metal-organic framework for selective separation of organic dyes, RSC Adv., 2014, 4, 1480–1483.
25. Y. Yuan, J. Li, X. Sun, G. Li, Y. Liu, G. Verma and S. Ma, Indium–organic frameworks based on dual secondary building units featuring halogen-decorated channels for highly effective CO₂ fixation, Chem. Mater., 2019, 31, 1084–1091.
26. S.-L. Hou, J. Dong, X.-L. Jiang, Z.-H. Jiao and B. Zhao, A noble-metal-free metal-organic framework (MOF) catalyst for the highly efficient conversion of CO₂ with propargylic alcohols, Angew. Chem., Int. Ed., 2019, 58, 577–581.
27. S. E. Springer, J. J. Mihaly, N. Amirmokhtari, A. B. Crom, M. Zeller, J. I. Feldblyum and D. T. Genna, Framework isomerism in a series of btb-containing In-derived metal-organic frameworks, Cryst. Growth Des., 2019, 19, 3124–3129.
28. Y. Han, H. Zheng, K. Liu, H. Wang, H. Huang, L.-H. Xie, L. Wang and J.-R. Li, In situ ligand formation-driven preparation of a heterometallic metal-organic framework for highly selective separation of light hydrocarbons and efficient mercury adsorption, ACS Appl. Mater. Interfaces, 2016, 8, 23331–23337.
29. Y. Liu, V. Kravtsov and M. Eddaoudi, Template-directed assembly of zeolite-like metal-organic frameworks (ZMOFs): a usf-ZMOF with an unprecedented zeolite topology, Angew. Chem., Int. Ed., 2008, 47, 8446–8449.
30. S. Chen, J. Zhang, T. Wu, P. Feng and X. Bu, Multiroute synthesis of porous anionic frameworks and size-tunable extraframework organic cation-controlled gas sorption properties, J. Am. Chem. Soc., 2009, 131, 16027–16029.
31. S.-T. Zheng, J.-T. Bu, Y. Li, T. Wu, F. Zuo, P. Feng and X. Bu, Pore space partition and charge separation in cage-within-cage indium-organic frameworks with high CO2 uptake, J. Am. Chem. Soc., 2010, 132, 17062–17064.
32. S.-T. Zheng, F. Zuo, T. Wu, B. Irfanoglu, C. Chou, R. A. Nieto, P. Feng and X. Bu, Cooperative assembly of three-ring-based zeolite-type metal-organic frameworks and Johnson-type dodecahedra, Angew. Chem., Int. Ed., 2011, 50, 1849–1852.
33. S.-T. Zheng, C. Mao, T. Wu, S. Lee, P. Feng and X. Bu, Generalized synthesis of zeolite-type metal-organic frameworks encapsulating immobilized transition-metal clusters, J. Am. Chem. Soc., 2012, 134, 11936–11939.
34. J. Yu, Y. Cui, C. Wu, Y. Yang, Z. Wang, M. O'Keeffe, B. Chen and G. Qian, Second-order nonlinear optical activity induced by ordered dipolar chromophores confined in the pores of an anionic metal-organic framework, Angew. Chem., Int. Ed., 2012, 51, 10542–10545.
35. J. E. Farid Nouar, J. F. Eubank, P. Forster and M. Eddaoudi, Zeolite-like metal-organic frameworks (ZMOFs) as hydrogen storage platform: lithium and magnesium Ion-exchange and H₂-(rho-ZMOF) interaction studies, J. Am. Chem. Soc., 2009, 131, 2864–2870.
36. Q. Li, D.-X. Xue, Y.-F. Zhang, Z.-H. Zhang, Z. Gao and J. Bai, A dual-functional indium-organic framework towards organic pollutant decontamination via physically selective adsorption and chemical photodegradation, J. Mater. Chem. A, 2017, 5, 14182–14189.
37. J. Zhang, S. Chen and X. Bu, Multiple functions of ionic liquids in the synthesis of three-dimensional low-connectivity homochiral and achiral frameworks, Angew. Chem., Int. Ed., 2008, 47, 5434–5437.
38. Y. L. Mohamed, H. Alkordi, R. W. Larsen, J. F. Eubank and M. Eddaoudi, Zeolite-like metal-organic frameworks as platforms for applications: On metalloporphyrin-based catalysts, J. Am. Chem. Soc., 2008, 130, 12639–12641.
39. J. Zhang, S. Chen, T. Wu, P. Feng and X. Bu, Homochiral crystallization of microporous framework materials from achiral precursors by chiral catalysis, J. Am. Chem. Soc., 2008, 130, 12882–12883.
40. X. Chen, H. Jiang, X. Li, B. Hou, W. Gong, X. Wu, X. Han, F. Zheng, Y. Liu, J. Jiang and Y. Cui, Chiral phosphoric acids in metal-organic frameworks with enhanced acidity and tunable catalytic selectivity, Angew. Chem., Int. Ed., 2019, 58, 14748–14757.
41. Z. Deng, C. Fang, X. Ma, X. Li, Y.-J. Zeng and X. Peng, One stone two birds: Zr-Fc metal-organic framework nanosheet for synergistic photothermal and chemodynamic cancer therapy, ACS Appl. Mater. Interfaces, 2020, 12, 20321–20330.
42. Z. Deng, H. Yu, L. Wang, J. Liu and K. J. Shea, Ferrocene-based metal-organic framework nanosheets loaded with palladium as a super-high active hydrogenation catalyst, J. Mater. Chem. A, 2019, 7, 15975–15980.
43. X.-D. Du, W. Zheng, X.-H. Yi, J.-P. Zhao, P. Wang and C.-C. Wang, General strategy for lanthanide coordination polymers constructed from 1,1′-ferrocenedicarboxylic acid under hydrothermal conditions, CrystEngComm, 2018, 20, 2608–2616.
44. R. Rajak, M. Saraf, A. Mohammad and S. M. Mobin, Design and construction of a ferrocene based inclined polycatenated Co-MOF for supercapacitor and dye adsorption applications, J. Mater. Chem. A, 2017, 5, 17998–18011.
45. K. Hirai, H. Uehara, S. Kitagawa and S. Furukawa, Redox reaction in two-dimensional porous coordination polymers based on ferrocenedicarboxylates, Dalton Trans., 2012, 41, 3924–3927.
46. J. Huo, L. Wang, E. Irran, H. Yu, J. Gao, D. Fan, B. Li, J. Wang, W. Ding, A. M. Amin, C. Li and L. Ma, Hollow ferrocenyl coordination polymer microspheres with micropores in shells prepared by Ostwald Ripening, Angew. Chem., Int. Ed., 2010, 49, 9237–9241.
47. X.-R. Meng, G. Li, H.-W. Hou, H.-Y. Han, Y.-T. Fan, Y. Zhu and C.-X. Du, A series of novel metal-ferrocenedicarboxylate coordination polymers: crystal structures, magnetic and luminescence properties, J. Organomet. Chem., 2003, 679, 153–161.
48. D. Guo, Y.-T. Li, C.-Y. Duan, H. Mo and Q.-J. Meng, Heterodimetallic compounds assembled from ferrocenedicarboxylato and ferrocenecarboxylato ligands, Inorg. Chem., 2003, 42, 2519–2530.
49. G. Li, H.-W. Hou, L.-K. Li, X.-R. Meng, Y.-T. Fan and Y. Zhu, Novel Pb(II), Zn(II), and Cd(II) coordination polymers constructed from ferrocenyl-substituted carboxylate and bipyridine-based ligands, Inorg. Chem., 2003, 42, 4995–5004.
50. J. Benecke, E. S. Grape, A. Fuss, S. Wohlbrandt, T. A. Engesser, A. K. Inge, N. Stock and H. Reinsch, Polymorphous Indium metal-organic frameworks based on a ferrocene linker: redox activity, porosity, and structural diversity, Inorg. Chem., 2020, 59, 9969–9978.
51. R. Gao, S.-M. Chen, F. Wang and J. Zhang, Single-crystal syntheses and properties of indium-organic frameworks based on 1,1′-Ferrocenedicarboxylic Acid, Inorg. Chem., 2021, 60, 239–245.
52. Z. Huang, H. Yu, L. Wang, X. Liu, T. Lin, F. Haq, S. Z. Vatsadze and D. A. Lemenovskiy, Ferrocene-contained metal organic frameworks: From synthesis to applications, Coord. Chem. Rev., 2021, 430, 213737.
53. J. Benecke, S. Mangelsen, T. A. Engesser, T. Weyrich, J. Junge, N. Stock and H. Reinsch, A porous and redox active ferrocenedicarboxylic acid based aluminium MOF with a MIL-53 architecture, Dalton Trans., 2019, 48, 16737–16743.
54. G. M. Sheldrick, Crystal structure refinement with SHELXL, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3–8.
55. D.-J. Li, Q.-H. Li, Z.-R. Wang, Z.-Z. Ma, Z.-G. Gu and J. Zhang, Interpenetrated metal-porphyrinic framework for enhanced nonlinear optical limiting, J. Am. Chem. Soc., 2021, 143, 17162–17169.
56. V. A. Blatov, A. P. Shevchenko and D. M. Proserpio, Applied topological analysis of crystal structures with the program package topospro, Cryst. Growth Des., 2014, 14, 3576–3586.
57. N. J. Long, Organometallic Compounds for nonlinear optics-the search for en-light-enment!, Angew. Chem., Int. Ed. Engl., 1995, 34, 21–38.
58. K. Senthilkumar, K. Thirumoorthy, G. Vinitha, K. Soni, N. S. P. Bhuvanesh and N. Palanisami, Synthesis and characterization of d₁₀ metal complexes of 3-Me-5-FcPz: Structural, theoretical and third order nonlinear optical properties, J. Mol. Struct., 2017, 1128, 36–43.
59. C. Gao, J. Zhou, M. Cui, D. Chen, L. Zhou, F. Li and X.-L. Li, Distinct nonlinear optical responses in three pairs of 2D homochiral Ag(i) enantiomers modulated by dicarboxylic acid ligands, Inorg. Chem. Front., 2022, 9, 284–293.
60. X.-L. Li, A. Wang, M. Cui, C. Gao, X. Yu, B. Su, L. Zhou, C.-M. Liu, H.-P. Xiao and Y.-Q. Zhang, Modulating two pairs of chiral Dy(III) enantiomers by distinct beta-diketone ligands to show giant differences in single-ion magnet performance and nonlinear optical response, Inorg. Chem., 2022, 61, 9283–9294.
61. M. Cui, L. Yang, F. Li, L. Zhou, Y. Song, S.-M. Fang, C.-M. Liu and X.-L. Li, Multifunctional Dy(III) enantiomeric pairs showing enhanced photoluminescences and third-harmonic generation responses through the coordination role of homochiral tridentate N,N,N-pincer ligands, Inorg. Chem., 2021, 60, 13366–13375.
62. G. Li, Y. Song, H. Hou, L. Li, Y. Fan, Y. Zhu, X. Meng and L. Mi, Synthesis, Crystal structures, and third-order nonlinear optical properties of a series of ferrocenyl organometallics, Inorg. Chem., 2003, 42, 913–920.
63. M. Sheik-Bahae, A. A. Said, T.-H. Wei, D. J. Hagan and E. W. Van-Stryland, Sensitive measurement of optical nonlinearities using a single beam, IEEE J. Quantum Electron., 1990, 26, 760–769.
64. X. Liu, M. Kozlowska, T. Okkali, D. Wagner, T. Higashino, G. Brenner-Weiss, S. M. Marschner, Z. Fu, Q. Zhang, H. Imahori, S. Brase, W. Wenzel, C. Woll and L. Heinke, Photoconductivity in metal-organic framework (MOF) thin films, Angew. Chem., Int. Ed., 2019, 58, 9590–9595.
65. C. Wang, S. Wang, F. Kong and N. Chen, Ferrocene-sensitized titanium-oxo clusters with effective visible light absorption and excellent photoelectrochemical activity, Inorg. Chem. Front., 2022, 9, 959–967.
66. Q.-F. Chen, X. Zhao, Q. Liu, J.-D. Jia, X.-T. Qiu, Y.-L. Song, D.-J. Young, W.-H. Zhang and J.-P. Lang, Tungsten(VI)-copper(I)-sulfur cluster-supported metal-organic frameworks bridged by in situ click-formed tetrazolate ligands, Inorg. Chem., 2017, 56, 5669–5679.
67. J. Zhang, Q. Xiang, Y. Zhu, J. Yang, Y. Song and C. Zhang, Two W/S/Cu-cluster-containing metal–organic frameworks fabricated by multidentate organic ligands: New topologies, strong NLO properties, and efficient luminescent detection, Cryst. Growth Des., 2021, 21, 3225–3233.

---

Footnote

† Electronic supplementary information (ESI) available: Additional experimental details, crystallographic studies, additional figures, general characterization, fluorescence spectra, photocurrent, cyclic voltammetric, and adsorption characterization. CCDC 2204897–2204903. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi01949c

---