1 Introduction

Due to higher coordination number and flexible geometries, lanthanide ions create fascinating structures which have potential applications in material science.[1,2,3,4,5] Lanthanide coordination polymers (LnCPs) exhibit high luminescence efficiency, narrow bandwidth, and long luminescence lifetime.[6] Lanthanide metal itself acts as both photoluminescent and magnetic center that make these materials interesting over others. With these advantages, properties related to pore size and surface area, make these ideal materials for developing new multifunctional materials.[7,8,9,10,11,12,13,14,15] In comparison with transition metals, lanthanide ions have more flexible and higher coordination numbers varying from 6 to 12. This might lead to more diverse architectures and topologies.[16,17,18,19,20,21,22] The flexibility of the coordination geometry of lanthanides usually results in cluster-based secondary building units (SBUs), which is responsible for thermal robustness of these structures.[23,24,25] Lanthanide ions form stable architectures with carboxylate ligand due to high affinity of lanthanides for oxygen. The differences in emission, magnetic and optical properties of lanthanide ions provide a platform to precisely tune these properties.[26]

However, designing a practical method to prepare controlled geometries of LnCPs still remains challenging.[27] In addition, lanthanide contraction may also affect the coordination number resulting in different crystal structures.[28,29,30] Lanthanum metal ion has tendency to coordinate with solvent molecules when reacted with small rigid ligand. It was shown that, choice of organic linkers and effective synthetic strategy produce lanthanide frameworks with intense and long-lived fluorescent emission. Especially, ligands containing both N- and O donors can exhibit great adaptability while coordinated to metal center.[31,32,33,34,35,36] The coordination ability of the 4-hydroxy derivative of dipicolinic acid, that is, 4-hydroxypyridine-2,6-dicarboxylic acid (Chelidamic acid, CA), will be ideal to explore the structural diversity of LnCPs.[37,38,39,40,41]

In this work, we report synthesis, structure, optical band gap energy and magnetism of a series of chelidamic acid based lanthanide coordination polymers. Among these, compound 1 is completely new and other three compounds (2–4) are synthesized in similar conditions to compare their properties though they are reported in the literature.[22, 42, 43] Single crystal data analysis revealed that lanthanide polyhedral units are connected with CA ligand to form the layered structure. These layers are H-bonded to form the three-dimensional supramolecular structure. We have measured the optical band gap energy and magnetic properties and correlated with the corresponding structures.

2 Experimental

2.1 Materials

All chemicals were purchased from Sigma-Aldrich and used without further purification. \(\hbox {Eu}(\hbox {NO}_{3})_{3}\). \(\hbox {H}_{2}\hbox {O}, \hbox {Gd}(\hbox {NO}_{3})_{3}\). \(6\hbox { H}_{2}\hbox {O}\), \(\hbox {Tb}(\hbox {NO}_{3})_{3}\). \(5\hbox { H}_{2}\hbox {O}\), \(\hbox {Dy}(\hbox {NO}_{3})_{3}\). \(\hbox {H}_{2}\hbox {O}\), chelidamic acid (CA), N,N’ dimethyl formamide (DMF) and milli-Q water were used for the synthesis. The IR spectra were recorded from prepared KBr pellets, using IR prestige-21 (Shimadzu) spectrometer. The TGA was performed on SDT Q600 (Shimadzu) analyser. Powder X-ray diffraction data (Figure S1 in Supplementary Information) were collected using an X’pert PRO (PANalytics) powder diffractometer equipped with \(\hbox {Cu K}_{\upalpha }\) radiation (1.5405 Å).

2.2 Synthesis

2.2.1 Compound 1

\(\hbox {Eu}(\hbox {NO}_{3})_{3}.\hbox {H}_{2}\hbox {O}\) (0.1 mmol) was dissolved in 2 mL DMF. Chelidamic acid (CA) (0.1 mmol) was dissolved in 8 mL distilled water separately and pH of the solution was adjusted to 8 by using 1 M KOH solution. Both metal and ligand solutions were mixed together in a 20 mL glass vial then the vial was heated at \(100\,^{\circ }\hbox {C}\) for 3 days. Colourless block-shaped crystals were obtained. Then crystals were washed with DMF and dried in vacuum for further use. Yield: 60% based on chelidamic acid.

2.2.2 Compounds 2–4

Except the metal salt, synthetic procedure of compounds 2–4 were same as the synthesis of compound 1. The metal salts \(\hbox {Gd}(\hbox {NO}_{3})_{3}.6\hbox { H}_{2}\hbox {O}\) (or) \(\hbox {Tb}(\hbox {NO}_{3})_{3}.5\hbox { H}_{2}\hbox {O}\) (or) \(\hbox {Dy}(\hbox {NO}_{3})_{3}.\hbox {H}_{2}\hbox {O}\) were used instead of \(\hbox {Eu}(\hbox {NO}_{3})_{3}.\hbox {H}_{2}\hbox {O}\). In all the cases, colourless block-shaped crystals were formed. Crystals were washed with DMF and dried in vacuum for further use. Yield: 55% (Gd), 67% (Tb), 63% (Dy) - based on chelidamic acid. IR peaks for compounds 1-4 (KBr pellet) \(\upnu \) \((\hbox {cm}^{-1})\): 1711, 1420-1585, 1350 and 3200-3600.

2.3 Single crystal X-ray Diffraction

The single crystal diffraction data were collected on a Bruker AXS Smart Apex CCD diffractometer at 298 K. The X-ray generator was operated at 50 kV and 35 mA using \(\hbox {Mo K}_{\upalpha }(\uplambda =0.7107{\AA })\) radiation. A total of 606 frames were collected in three different settings of \(\upvarphi \,(0^{\circ },\,90^{\circ },\,180^{\circ })\) keeping detector position \((2\uptheta )\) fixed at \(-25^{\circ }\).

The data reduction and empirical absorption correction were done using SAINTPLUS [44] and SADABS program,[45] respectively. SHELXS2014 package[46] was used for the structure solution and refinement. All hydrogen atoms were placed in calculated positions during the final step of the refinement process. For the final refinement, the hydrogen atoms were placed in geometrically ideal positions and refined using the riding mode. The last cycles of the refinement included atomic positions, anisotropic thermal parameters for all the non-hydrogen atoms, and isotropic thermal parameters for all the hydrogen atoms. Full-matrix-least-squares structure refinement against \(\hbox {F}^{2}\) was carried out using the WINGX package of programs.[47] The crystallographic parameters for compounds 1–4 are provided in Table 1. CCDC 1495104 - 1495107 contain crystallographic information for compounds 1-4, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre (CCDC) viawww.ccdc.cam.ac.uk/data_request/cif

Table 1 Crystallographic parameters for compounds 1–4.
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2.4 Optical Band-Gap Measurements

The optical absorption spectra of all the compounds were collected on a Shimadzu UV/Vis/NIR spectrophotometer. We have ground the samples with \(\hbox {BaSO}_{4}\) (as reference material) and made a thin film on the glass surface. The films were made between two quartz plates with edges sealed by paraffin tape. Kubelka-Munk function was used to calculate the band gap energy from the optical absorption spectra.[48,49,50]

We have used a two-flux model to determine the absorption coefficient of the sample. For a crystalline solid with a band gap \((E_{\mathrm{bg}})\), the frequency \((\upnu )\) dependence of the absorption coefficient \((\kappa )\) can be approximated as,

$$\begin{aligned} \kappa (v)=\frac{B_T \left( {h\nu -E_{bg} } \right) ^{n}}{h\nu } \end{aligned}$$
(1)

Where, \(\hbox {B}_{\mathrm{T}}\) is a constant derived from the square of the averaged dipolar momentum matrix element and n is equal to 0.5 and 2 for direct and indirect band gap transitions, respectively. The absorption coefficient \((\upkappa )\) can be measured from the reflectance (R) and expressed as \(\upkappa =(1-\hbox {R})^{2}/(2\hbox {R})\). Using the above equation, the band gap of a material can be obtained by extrapolating to zero with the linear fit to a plot of \((\kappa \hbox {h}\upnu )^{1/\mathrm{n}}{ vs}\). \(\hbox {h}\upnu \). The lowest electronic transition between valence and conduction bands is the optical band gap of the material.

2.5 Magnetic Measurements

The magnetic properties were measured using a vibrating sample magnetometer (VSM) attachment in a commercial (Quantum Design) physical property measurement system (PPMS). Magnetic susceptibility \(\chi \) as a function of temperature (T) was measured at different applied fields and over a large T-range \((2\hbox { K }\le \hbox { T }\le 380\hbox { K})\). Magnetization isotherm (magnetization M vs. applied field H) was also measured at \(T=2.1\hbox { K}\) up to a maximum varying field of 9 T.

3 Results and Discussion

3.1 Structure description

Compounds 1–4 crystallize in the monoclinic crystal system with space group \(P2_{1}/c\). Asymmetric unit of each compound contains one Ln metal center, one chelidamic acid ligand, three coordinated water molecules and one lattice water molecule. All the compounds are isostructural and possess a two-dimensional architecture. In all the cases Ln ion has eight coordination and adopts distorted dodecahedron geometry. Four oxygen atoms and one nitrogen atom from the chelidamic acid ligand and three coordinated water molecules satisfy the eight coordination (Figure 1a). The Ln - O bond lengths are in the range of 2.293–2.501 Å, Ln - N bond lengths are in the range of 2.435–2.487 Å (Table 2). In the chelidamic acid one of the carboxylate group bonded with metal in \(\upmu _{2}:\upeta ^{1}:\upeta ^{1}\) mode while another carboxylate group has the \(\upmu _{1}:\upeta ^{1}\) mode (Figure S4a in Supplementary Information). This connectivity between lanthanide metal ion and chelidamic acid gives rise to two-dimensional structure (Figure 1b). Furthermore, the 2D frameworks are further assembled to form a 3D supramolecular network by the hydrogen bonds between water molecules and carboxylate O atoms or hydroxyl groups (Figure S4b in Supplementary Information). The extensive H-bonding between coordinated water and framework oxygen atoms, coordinated water and lattice water molecules play crucial role for the stability of the structure. Interestingly, coordinated and lattice water molecules H-bonded to form the tetrameric water cluster (Figure S5 in Supplementary Information). Topological analysis using TOPOS software identified that all the four structures have same topological network with uninodal net, 5-c net, which can be presented as a Schläfli symbol \(\{3^{3}.4^{3}.5^{4}\}\) (td10 = 294) (see Supplementary Information).[51]

Fig. 1
figure 1

(a) Distorted dodecahedron geometry of \(\hbox {LnO}_{7}\hbox {N}\) unit. (b) Two-dimensional layered architecture.

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Table 2 Selected bond distances (Å) of compounds 1–4.
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3.2 Band Gap Energy

The diffuse reflectance spectra of the powder samples of compounds 1–4 as well as Na-salt of the ligand were collected in order to measure the optical band gap energy \((\hbox {E}_{\mathrm{bg}})\). The optical band gap energy was confirmed as the intersection point between the energy axis and the line extrapolated from the linear portion of the absorption edge in a plot of Kubelka-Munk function vs. energy (Figure S6 in Supplementary Information). The band gap values are given in Table 1. Structural description of compounds 1–4 revealed that the metal nodes are well isolated and connected by an organic linker. The electronic structure calculations on these types of compounds predicted that the HOMO-LUMO gap of the organic ligand determines the band gap of these materials.[52,53,54] In this case, the band gap energy varies from 3.70 eV to 3.85 eV for compounds 1–4. The band gap energy of the Na-salt of the ligand was 3.84 eV. It was shown that the band gap energy decreases from the ligand molecule and the difference is negligible. This proved that the perturbation of the states related to conduction and valence bands are not significant. All the compounds have a similar structure with identical bonding connectivity of the ligand and metal ion. The only difference is the number of f-electrons in the corresponding metal ion, which may have little effect towards the band gap energy (Table 3).

Table 3 Experimentally measured band gap energy for compounds 1–4 and Na-salt of ligand, respectively.
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3.3 Magnetic properties

The temperature dependent magnetic susceptibilities of compounds 2–4 were measured from 2 K to 380 K in an external magnetic field of 1 T. The magnetic measurements were not performed for compound 1 as the effective momentum of the Eu-based compound is almost zero. In order to extract the magnetic parameters, the inverse magnetic susceptibility \((1/\upchi )\) data in the high-T regime were fitted by the Curie-Weiss (CW) equation:

$$\begin{aligned} \upchi = [{\upchi }_{0}+ \hbox {C}/ (T- {\uptheta }_{CW})] \end{aligned}$$
(2)

Where, \(\upchi _{0}\) is the T-independent susceptibility which includes core diamagnetic susceptibility \((\upchi _{\mathrm{core}})\) and Van-Vleck paramagnetic susceptibility \((\upchi _{\mathrm{VV}})\). In the second term in the CW eq., Curie constant \(C = [N_{A}g^{2}\mu _{\mathrm{B}}^{2}/3k_{B}] S (S + 1)\), where \(\hbox {N}_{\mathrm{A}}\) - Avogadro Number, g- Landé g-factor, \(\mu _{\mathrm{B}}\) - Bohr magneton, \(k_{\mathrm{B}}\) - Boltzmann constant, S- spin quantum number, T - Temperature, and \(\uptheta _{\mathrm{CW}}\) the characteristic Curie-Weiss temperature. The experimental effective moment \(\mu _{\mathrm{eff}}\) was calculated from the room temperature \({\chi }T\) value using the equation, \(\mu _{\mathrm{eff}=} 2.83\sqrt{\chi T}\).

For Compound 2, as shown in Figure 2a, \(\chi (T)\) increases with a decrease in temperature. Indication of magnetic ordering was not observed in the studied temperature range. \({\chi }T(T)\) is equal to \(7.92\hbox { cm}^{3}\hbox {K mol}^{-1}\) at 350 K, which is close to the value expected \((7.87\hbox { cm}^{3}\hbox {K mol}^{-1})\) for one isolated Gd(III) ion \((^{8}\hbox {S}_{7/2},\,\hbox {g} = 2)\). Upon cooling, \(\chi \hbox {T}\) remains almost constant down to 100 K and then decreases and reaches a minimum value of \(7.80\hbox { cm}^{3}\hbox {K mol}^{-1}\) at 11 K (Inset in Figure 2a). The decreasing trend of \(\upchi \hbox {T}\) upon cooling is due to the thermal depopulation of stark sublevels.[55, 56]

\(1/\chi \) data at higher temperatures \((\hbox {T}>150\hbox { K})\) were fitted using Eq. 1 (Figure 2b) yields \(\upchi _{0 }\approx 0.00285\hbox { cm}^{3}/\hbox {mol}\), \(C\approx 13\hbox { cm}^{3}\hbox { K }^{-1}\), and \(\uptheta _{CW}\approx -0.15\hbox { K}\). The negligible value of \(\theta _{CW}\) suggests that the interaction between the Gd spins is very weak. The experimental effective moment was calculated to be \(\mu _{\mathrm{eff}} \approx 7.96\,\mu _{\mathrm{B}}\) at room temperature, which is in excellent agreement with the expected theoretical value of \(\mu _{\mathrm{eff}}\approx 7.94\mu _{\mathrm{B}}\). Inset of Figure S7(b) (in Supplementary Information) shows the magnetization isotherm at 2.1 K measured up to \(H=9\) T. Magnetization (M) increases with increasing magnetic field (H) and reaches the saturation with a value of \(M_{\mathrm{sat}}\approx 7.036\mu _{\mathrm{B}}\) at 9 T. The observed value of \(M_{\mathrm{sat}}\) agrees reasonably with the value \(7\mu _{\mathrm{B}}\) anticipated for an isolated Gd(III) ion with \(S=7/2\), \(L=0\) and g = 2.0. The overall shape of the curve is similar to that expected for a paramagnet.

Fig. 2
figure 2

(a) \(\upchi \) vs T of compound 2 measured at \(H=1\) T. (b) \(1/\upchi \) vs. T and the solid line is the CW fit using Eq. 1 at high temperatures which is extrapolated down to 0 K. Inset: Magnetization isotherm measured at \(T= 2.1\hbox { K}\).

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For Compound 3, \(\chi (T)\) measured at 1 T is shown in Figure 3a. \(\chi (T)\) data increases gradually as the temperature decreases in a Curie–Weiss (CW) manner, reflecting the paramagnetic nature of the compound. No indication of any magnetic ordering was found down to \(2 \hbox {K}.{\chi }T(T)\) value is equal to \(11.63\hbox { cm}^{3}\hbox { K mol}^{-1}\) at 350 K, which is close to the value expected for one isolated Tb(III) ion \((11.82\hbox { cm}^{3}\hbox { K mol}^{-1},\,^{7}\hbox {F}_{6}, \hbox {J} = 6, \hbox {g} = 3/2)\). The CW fitting at high temperatures (150 K – 380 K) yields \({\upchi }_{0}\approx -0.001\hbox { cm}^{3}/\hbox {mol}\), \(C\approx 11.94\hbox { cm}^{3}\hbox { K }^{-1}\), and \({\uptheta }_{CW}-\approx 0.71\hbox { K}\). At room temperature, the experimental effective moment was calculated to be \(\mu _{\mathrm{eff}} \approx 9.65 \mu _{\mathrm{B}}\). This value is closely matching with the expected theoretical value of \(\mu _{\mathrm{eff}} \approx 9.72 \mu _{\mathrm{B}}\). Inset of Figure 3b shows the magnetization isotherm at 2.1 K measured up to \(H=9\) T. M increases with H and saturates at around 3 T.

Fig. 3
figure 3

(a) \(\upchi \) vs T of compound 3 measured at \(H=1\) T. (b) \(1/\upchi \) vs. T and the solid line is the CW fit using Eq. 1 at high temperatures which is extrapolated down to 0 K. Inset: Magnetization isotherm measured at \(T= 2.1\hbox { K}\).

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Fig. 4
figure 4

(a) \(\upchi \) vs T of compound 4 measured at \(H=1\) T. (b) \(1/\upchi \) vs. T and the solid line is the CW fit using Eq. 1 at high temperatures which is extrapolated down to 0 K. Inset: Magnetization isotherm measured at \(T=2.1\hbox { K}\).

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For Compound 4, as shown in Figure 4a, \(\upchi \)(T) data shows a CW behaviour, which is typical for a paramagnet. The CW fitting yields \({\upchi }_{0}\approx -0.0017\hbox { cm}^{3}/\hbox {mol}\), \(C \approx 12.73\hbox { cm}^{3}\hbox { K}^{-1}\), and \({\uptheta }_{CW}\approx 0.01\hbox { K}\). The \({\chi }T\) value is \(12.56\hbox { cm}^{3}\hbox { K mol}^{-1}\) at 350 K. From the room temperature \({\chi }T\) value, the effective moment was calculated to be \(\mu _{\mathrm{eff}} \approx 10.03 \mu _{\mathrm{B}}\) which is comparable to the expected theoretical value of \(\mu _{\mathrm{eff}} \approx 10.65\, \upmu _{\mathrm{B}}\) for Dy(III) ions \((14.17\hbox { cm}^{3}\hbox { K mol}^{-1}, ^{6}\hbox {H}_{15/2},\,\hbox {J} = 15/2, \hbox {g} = 4/3)\). Inset of Figure 4b shows the magnetization isotherm at 2.1 K measured up to \(H= 9\hbox { T}\). M increases with H and saturates at around 3 T.

3.4 IR spectra and Thermal studies

In the IR spectra, peaks at \(1711\hbox { cm}^{-1}\), 1420–1585\(\hbox { cm}^{-1}\), \(1350\hbox { cm}^{-1}\) due to stretching frequency of C = O, C = C, C – N, respectively, were observed. The broad peak between \(3200{-}3600\hbox { cm}^{-1}\) is due to stretching frequency of O – H from the hydrogen-bonded water molecules (Figure S2 in Supplementary Information). The TGA graphs show that the first weight loss starts at \(120\,^{\circ }\hbox {C}\) due to the loss of coordinated and lattice water molecules from the structures. Second weight loss up to 48–52% for the four compounds were observed due to the breakdown of the framework structure with the loss of chelidamic acid (Figure S3 in Supplementary Information). The calcined products were found to be poorly crystalline by PXRD and majority of the lines correspond to \(\hbox {La}_{2}\hbox {O}_{3}\).

3.5 Luminescence Properties

The solid-state luminescence properties of compounds 1–4 as well as the free ligand CA were investigated at room temperature (Figure S7 in Supplementary Information). Free ligand CA displays a main emission band centered at 500 nm on excitation at 359 nm. This emission peak may be attributed to the \(\uppi ^{*}\rightarrow \uppi \) or \(\uppi ^{*}\rightarrow \hbox { n}\) transition (reverse of the absorption bands) in the ligand. Emission bands at 543 nm and 555 nm for compounds 2 and 4, respectively, were observed. These emission bands are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature and could be assigned to a combination of the charge transfer process and intra ligand emission because similar emissions are also observed for the free ligand.

In compounds 1 and 3, luminescence bands were observed on excitation at 359 nm and the characteristic emission from CA ligand around 500 nm almost disappeared (Figure S8 in Supplementary Information). New bands were observed and that could be due to f–f electronic transition in \(\hbox {Ln}^{3+}\) ions. In compound 1 (Eu-CA), luminescence bands are due to various transitions that occur from the \(^{5}\hbox {D}_{0}\) to \(^{7}\hbox {F}\) states. The intense peaks at 590, 615 and 694 nm result from the transitions \(^{5}\hbox {D}_{0} \rightarrow ^{7}\hbox {F}_{1}\), \(^{5}\hbox {D}_{0} \rightarrow ^{7}\hbox {F}_{2}\) and, \(^{5}\hbox {D}_{0} \rightarrow ^{7}\hbox {F}_{4}\), respectively. In compound 3 (Tb-CA), luminescence bands at 491, 545, 586, and 621 nm are due to the transitions \(^{5}\hbox {D}_{4} \rightarrow ^{7}\hbox {F}_{6}\), \(^{5}\hbox {D}_{4} \rightarrow ^{7}\hbox {F}_{5,}^{5}\hbox {D}_{4} \rightarrow ^{7}\hbox {F}_{4}\) and \(^{5}\hbox {D}_{4} \rightarrow ^{7}\hbox {F}_{3}\), respectively. The most intense emission corresponds to the hypersensitive transition of \(^{5}\hbox {D}_{4} \rightarrow ^{7}\hbox {F}_{5}\) (545 nm) (Figure S8).[57,58,59]

4 Conclusions

A series of lanthanide(III) based coordination polymers using chelidamic acid (CA) were synthesized by solvothermal methods. The lanthanide ion adopts distorted dodecahedron geometry and linked with chelidamic acid to form two-dimensional layered structure. These layers are H-bonded to form the three-dimensional supramolecular structure. From a structural point of view, it was shown that metal nodes are well isolated by ligand. As a result, the band gap of these materials depends solely on the organic ligand and HOMO-LUMO gap was not much changed with increasing number of the \(f-\)electrons. Due to weak bonding connectivity between the metal nodes and ligands the perturbation of the states related to the band gap are insignificant. Magnetic moments are influenced by the \(f-\)electrons and the observed values are closer to theoretical values. The present results suggest that optical and electronic properties are dependent on both the metal ion and ligand in these coordination polymers.