Lanthanide coordination compounds with 1H-benzimidazole-2-carboxylic acid: syntheses, structures and spectroscopic properties

Abstract

A flexible multidentate ligand, 1H-benzimidazole-2-carboxylic acid, was synthesized to construct a series of lanthanide coordination polymers [Ln(HBIC)₃]_n (Ln = Eu (1), Tb (2), Gd (3), Pr (4), Nd (5); H₂BIC = 1H-benzimidazole-2-carboxylic acid) under hydrothermal conditions. All the compounds were fully characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction, thermal analysis and various spectroscopic techniques. Structural analyses reveal that they are isostructural and feature a 2D wave-like layer structure with distorted grids, in which the adjacent Ln³⁺ centers are bridged by the HBIC⁻ ligands with two kinds of new coordination modes and the adjacent HBIC⁻ ligands are tightly bound by two types of distinct intra-layer hydrogen bonds. The adjacent 2D layers are further interconnected by strong inter-layer hydrogen bond ring motifs R₂²(10) to generate a 3D supramolecular architecture. Optical studies indicate that the compounds 1, 2, 4 and 5 exhibit characteristic luminescence emission bands of the corresponding lanthanide ions in the visible or near-infrared regions at room temperature. In particular, compound 2 displays bright green luminescence in the solid state with a satisfactory ⁵D₄ lifetime of 1.2 ms and a high overall quantum yield of 31%, due to an ideal energy gap between the lowest triplet state energy level of H₂BIC ligand and the ⁵D₄ state energy level of Tb³⁺. The energy transfer mechanisms in compounds 1 and 2 were also described and discussed.

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Introduction

Over the past decade, the rational design and synthesis of lanthanide-based metal–organic frameworks (Ln-MOFs) have provoked great interest for their fascinating architectures and potential technological application as functional materials.¹ Especially, owing to their unique photophysical properties (characteristic sharp emission and high color purity, long excited-state luminescence lifetimes up to milliseconds) as a result of transitions within the partially-filled 4f shells of the trivalent lanthanide ions, Ln-MOFs are excellent candidates for the development of optical devices.² However, the lanthanide ions usually give weak luminescence because of the low molar absorption coefficient (less than 10 M⁻¹ cm⁻¹) of the Laporte forbidden f–f transitions.³ An effective approach to circumvent this problem is to efficiently transfer the energy from the ligand-centered triplet excited states of an adjacent strongly absorbing chromophore to the lanthanide metal centers.⁴ Therefore, searching for suitable organic antenna chromophores with high absorption in the UV/near-UV spectral region is an attractive task.

Given that lanthanide ions have high affinities for hard donor atoms such as the oxygen of carboxylic groups⁵ and a delocalized π-electric system could provide a strongly absorbing chromophore.⁶ Aromatic multidentate ligands with oxygen or hybrid oxygen–nitrogen atoms might be a good sensitizer to stimulate lanthanide ion luminescence. Recently, we have focused on a multidentate ligand, 1H-benzimidazole-2-carboxylic acid (H₂BIC), in which the large π-conjugated system of the benzimidazole moiety is expected as an effective chromophore for lanthanide luminescence. The H₂BIC ligand possesses four potential metal binding sites and shows flexible and various coordination modes (Scheme 1), wherein the oxygen atoms of the carboxylic group could coordinate with the lanthanide ions in terminal monodentate or diverse bridging motifs, or chelate Ln³⁺ with the help of the N atoms from the benzimidazole group. Meanwhile, depending on the degree of deprotonation, the H₂BIC ligand can act as a hydrogen bond acceptor and donor to benefit the formation of stable supramolecular structures. On the other hand, it was found that the presence of water molecules in the first coordination sphere of the lanthanide ion is prone to quench the emission due to the activation of nonradiative decay pathways.⁷ So ancillary ligands with excellent chelating ability such as oxalate are widely used to prevent solvent and water molecules from entering the lanthanide coordination sphere.⁸ For the deprotonated HBIC⁻ or BIC²⁻ ligands, the sufficiently strong chelate ability like C₂O₄²⁻ makes them excellent candidates to form stable Ln-MOFs without coordinated water or solvent molecules.⁹

Scheme 1 Diverse coordination modes of H₂BIC (M = metal).

It is worth noting that lanthanide coordination polymers based on 1H-benzimidazole-2-carboxylic acid ligands have not been explored.¹⁰ In this contribution, five new two-dimensional Ln-MOFs based on H₂BIC ligands, [Ln(HBIC)₃]_n (Ln = Eu (1), Tb(2), Gd(3), Pr(4) and Nd(5)), were hydrothermally synthesized and structurally characterized. The photophysical properties and thermal stability of them were also investigated. To the best of our knowledge, 1–5 are the first instances of Ln-MOFs constructed with H₂BIC ligands.

Experimental

Materials and analytical methods

The 1H-benzimidazole-2-carboxylic acid (H₂BIC) ligand was prepared using o-phenylenediamine as the starting material by oxidizing 2-(hydroxymethyl)benzimidazole in alkaline conditions (Scheme 2). Other chemicals and reagents were obtained from commercial sources and used without further purification. Elemental analyses for C, H, and N were performed on an Elementar Vario EL III analyzer. IR spectra were recorded on a Tensor 27 spectrometer (Bruker Optics, Ettlingen, Germany) as KBr pellets in the range of 4000–400 cm⁻¹. ¹H NMR spectrum was recorded on an INOVA-400 Varian 400 MHz instrument in DMSO-d₆ solution at room temperature. Chemical shifts are reported in ppm relative to TMS. TG-DTG-DSC experiments were carried out in a simulated air atmosphere (N₂: 80 mL min⁻¹, O₂: 20 mL min⁻¹) using a NETZSCH STA 449F3 equipment at a heating rate of 10 °C min⁻¹ from 30 to 1000 °C, an empty Al₂O₃ crucible was used as a reference. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). Optical diffuse reflectance and UV absorption studies were carried out with a Shimadzu UV-2450 spectrophotometer. The solid-state photoluminescence analyses and lifetime measurements were performed on an Edinburgh FLSP920 fluorescence spectrometer.

Scheme 2 Synthetic procedures for the ligand 1H-benzimidazole-2-carboxylic acid.

Synthesis of H₂BIC

2-(Hydroxymethyl)benzimidazole. A mixture of o-phenylenediamine (16.2 g, 0.15 mol), sodium glycolate (35.2 g, 0.6 mol) in 300 mL HCl (4 mol L⁻¹) was stirred at 120 °C for 2 h. The reaction mixture was cooled to room temperature and filtered. The filtrate was adjusted to pH ≈ 7 by saturated sodium hydroxide solution and finally filtered to give a large amount of white microcrystals of 2-(hydroxymethyl)benzimidazole. Yield: 19.0 g (85.7%). M.p.: 131–132 °C. Anal. Calc. For C₈H₈N₂O (M_r = 148.16): C, 64.85 H, 5.44 N, 18.91. Found: C, 64.22 H, 5.88 N, 19.01%. ¹H NMR (400 MHz, DMSO-d₆): δ, ppm: 12.344 (s, 1 H), 7.484–7.491 (d, 2 H), 7.110–7.151 (m, 2 H), 5.744 (s, 1 H), 4.705 (s, 2H) (see Fig. S1, ESI†). IR (KBr, v/cm⁻¹): 3270(m), 2918(s), 2850(s), 1623(w), 1580(w), 1490(m), 1452(s), 1439(s), 1380(m), 1351(m), 1274(m), 1207(m), 1106(w), 1055(s), 1034(s), 1002(s), 995(m), 958(w), 871(w), 828(w), 769(w), 750(s), 740(s), 639(w), 451(w).

1H-Benzimidazole-2-carboxylic acid (H₂BIC). A mixture of 2-(hydroxymethyl)benzimidazole (7.40 g, 0.05 mol) and sodium hydroxide (4.00 g, 0.1 mol) in H₂O (180 mL) was allowed to be stirred at 100 °C for 2 h, to which potassium permanganate (11.9 g, 0.075 mol) was added while kept stirring at 100 °C for 10 h. Then the reaction mixture was cooled to room temperature and filtered, and the filtrate was acidified with dilute hydrochloric acid to pH = 5. Upon being filtered off, washed with water, and air-dried, a large quantity of yellow solid was obtained. Yield: 7.15 g (64.9%, based on o-phenylenediamine). M.p.: 166–167 °C. Anal. Calc. For C₈H₆N₂O₂ (M_r = 162.15): C, 59.26 H, 3.73 N, 17.28. Found: C, 60.01 H, 3.81 N, 17.03%. ¹H NMR (400 MHz, DMSO-d₆): δ, ppm: 7.66–7.62 (m, 2 H), 7.37–7.34 (m, 2 H) (see Fig. S2, ESI†). IR (KBr, v/cm⁻¹): 2923(m), 2842(m), 1653(s), 1530(m), 1495(s), 1388(s), 1352(s), 1279(w), 1151(w), 1020(w), 905(m), 847(m), 802(w), 766(m), 731(m), 635(m), 585(m). After several days, colorless crystals of dihydrated 1H-benzimidazol-2-carboxylate were obtained by slow evaporation of an ethanol solution of H₂BIC, which was determined by single X-ray diffraction analysis (Fig. 1 and Table 1).^10d

Fig. 1 (a) Perspective view of H₂BIC·2H₂O. (b) The intermolecular H-bonding interactions in H₂BIC·2H₂O.

Table 1 Crystal data and structure refinement summary for H₂BIC·2H₂O and compounds 1–5

	H2BIC·2H2O	1	2	3	4	5
a R 1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w[(Fo^2)^2]^1/2.
Empirical formula	C8H10N2O4	C24H15N6O6Eu	C24H15N6O6Tb	C24H15N6O6Gd	C24H15N6O6Pr	C24H15N6O6Nd
Formula weight	198.18	635.38	642.34	640.67	624.33	627.66
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c	P21/n	P21/n	P21/n	P21/n	P21/n
a (Å)	6.860(2)	9.7575(10)	9.7089(14)	9.7434(11)	9.8425(7)	9.853(2)
b (Å)	7.368(2)	8.9776(9)	8.9276(13)	8.9656(10)	9.0698(7)	9.073(2
c (Å)	18.967(5)	26.005(3)	25.930(4)	26.000(3)	26.084(2)	26.152(6)
α (°)	90	90	90	90	90	90
β (°)	109.738(9)	91.113(1)	91.090(2)	91.179(2)	91.216(1)	91.153(4)
γ (°)	90	90	90	90	90	90
V (Å^3)	902.4(4)	2277.5(4)	2247.1(6)	2270.8(4)	2327.9(3)	2337.3(10)
Z	4	4	4	4	4	4
D c (g cm^−3)	1.459	1.853	1.899	1.874	1.781	1.784
T (K)	296(2)	296(2)	296(2)	296(2)	296(2)	296(2)
μ (mm^−1)	0.119	2.810	3.204	2.977	2.147	2.276
F(000)	416	1248	1256	1252	1232	1236
Reflections collected/unique	4373/1596	11 759/4461	10 687/3983	11 088/4012	11 382/4131	11 228/4141
R(int)	0.0750	0.0316	0.0452	0.0535	0.0329	0.0716
Data/restraints/parameters	1596/0/127	4461/0/334	3983/0/334	4012/0/334	4131/0/338	4141/0/334
GOF on F^2	1.015	1.097	1.066	1.033	1.037	1.009
R 1 [I > 2σ(I)]	0.0696	0.0250	0.0410	0.0373	0.0290	0.0549
wR2^b (all data)	0.2105	0.0785	0.1021	0.0774	0.0661	0.1249

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Synthesis of [Eu(HBIC)₃]_n (1)

A mixture of EuCl₃·6H₂O (0.0366 g, 0.1 mmol), H₂BIC (0.0162 g, 0.1 mmol), NaOH (0.0040 g, 0.1 mmol) and NaN₃ (0.0065 g, 0.1 mmol) was dissolved in 6 mL distilled water. The resulting solution was stirred for about 30 min at room temperature, sealed in a 10 mL Teflon-lined stainless steel autoclave, and heated at 110 °C for 3 days under autogenous pressure. The reaction system was then cooled to room temperature at a rate of 5 °C h⁻¹. Colorless flaky crystals of 1 were collected in 48% yield (based on Eu). Anal. Calcd. for C₂₄H₁₅EuN₆O₆ (M_r = 635.38): C, 45.37 H, 2.38 N, 13.23. Found: C, 45.81 H, 2.64 N, 12.99%. IR (KBr, v/cm⁻¹): 3141(br, m), 1652(s), 1616(s), 1521(m), 1497(m), 1466(s), 1398(s), 1331(s), 1299(m), 1228(w), 1027(w), 988(w), 849(m), 821(m), 779(w), 739(s), 631(m), 580(m).

Synthesis of [Tb(HBIC)₃]_n (2)

An identical procedure with 1 was followed to prepare 2 except EuCl₃·6H₂O was replaced by TbCl₃·6H₂O (0.0374 g, 0.1 mmol) or Tb₂(SO₄)₃·6H₂O (0.0357 g, 0.05 mmol). Colorless flaky crystals of 2 were collected in 54% yield (based on Tb). Anal. Calcd. for C₂₄H₁₅TbN₆O₆ (M_r = 642.35): C, 44.88 H, 2.35 N, 13.08. Found: C, 44.61 H, 2.58 N, 13.01%. IR (KBr, v/cm⁻¹): 3128(br, m), 1650(s), 1611(s), 1528(m), 1499(m), 1467(s), 1399(s), 1335(s), 1308(m), 1235(w), 1025(w), 987(w), 860(m), 825(m), 777(w), 748(s), 638(m), 584(m).

Synthesis of [Gd(HBIC)₃]_n (3)

An identical procedure with 1 was followed to prepare 3 except EuCl₃·6H₂O was replaced by GdCl₃·6H₂O (0.0372 g, 0.1 mmol). Colorless flaky crystals of 3 were collected in 61% yield (based on Gd). Anal. Calcd. for C₂₄H₁₅GdN₆O₆ (M_r = 640.67): C, 44.99 H, 2.36 N, 13.12. Found: C, 45.21 H, 2.88 N, 12.92%. IR (KBr, v/cm⁻¹): 3137(br, m), 1653(s), 1606(s), 1525(m), 1491(m), 1461(s), 1398(s), 1329(s), 1300(m), 1231(w), 1020(w), 986(w), 858(m), 820(m), 776(w), 743(s), 633(m), 585(m).

Synthesis of [Pr(HBIC)₃]_n (4)

An identical procedure with 1 was followed to prepare 4 except EuCl₃·6H₂O was replaced by Pr(NO₃)₃·6H₂O (0.0435 g, 0.1 mmol) or PrCl₃·6H₂O (0.0355 g, 0.1 mmol). Light-green flaky crystals of 4 were collected in 33% yield (based on Pr). Anal. Calcd. for C₂₄H₁₅PrN₆O₆ (M_r = 624.33): C, 46.17 H, 2.42 N, 13.46. Found: C, 46.67 H, 2.50 N, 13.98%. IR (KBr, v/cm⁻¹): 3133(br, m), 1657(s), 1609(s), 1531(m), 1500(m), 1463(s), 1401(s), 1325(s), 1302(m), 1238(w), 1027(w), 990(w), 854(m), 822(m), 779(w), 741(s), 635(m), 581(m).

Synthesis of [Nd(HBIC)₃]_n (5)

An identical procedure with 1 was followed to prepare 5 except EuCl₃·6H₂O was replaced by Nd(NO₃)₃·6H₂O (0.0438 g, 0.1 mmol). Light-purple flaky crystals of 5 were collected in 39% yield (based on Nd). Anal. Calcd. for C₂₄H₁₅NdN₆O₆ (M_r = 627.66): C, 45.93 H, 2.41 N, 13.39. Found: C, 45.51 H, 2.44 N, 13.11%. IR (KBr, v/cm⁻¹): 3132(br, m), 1652(s), 1604(s), 1528(m), 1489(m), 1466(s), 1402(s), 1330(s), 1303(m), 1238(w), 1029(w), 988(w), 851(m), 823(m), 779(w), 741(s), 634(m), 580(m).

X-ray crystallography

Single-crystal X-ray diffraction data for H₂BIC·2H₂O and compounds 1–5 were collected on a Bruker SMART APEXII CCD diffractometer, equipped with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) using ω and φ scan mode. All the structures were solved by direct methods and refined with full-matrix least-squares refinements based on F² using SHELXS-97 and SHELXL-97.¹¹ All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in geometrically calculated positions. The detailed crystallographic data and the structure refinement parameters of H₂BIC·2H₂O and1–5 are listed in Table 1. Selected bond distances, angles and hydrogen bonds are given in Tables S1 and S2 (see ESI†).

Results and discussion

Synthesis

As well known, the hydro/solvothermal method used to be a non-ignorable “hero” in the preparation of metal–organic coordination frameworks with fascinating architectures and function. It gives a more convenient and effective route to synthesize high-quality single crystals of MOFs over other methods.¹² However, because of the limitation of unintuitive reaction phenomenon and hard-to-monitor reaction process, the research on the complex mechanism of reaction process in the reaction vessel is still in the primary exploration phase at present, and many possible mechanisms could only be deduced by the intuitive results. Therefore, according to the reaction conditions and results of isolating 1–5, it is not difficult to find that the different anions of lanthanide salts (Cl⁻, SO₄²⁻ or NO₃⁻) have no effects on the formation of 1–5. Moreover, it should be noted that the presence of a molar equivalent of NaN₃ as the structure-directing agent was a prerequisite for the preparation of 1–5.

Structural description

Fig. 1a depicts that the ligand 1H-benzimidazole-2-carboxylic acid crystallizes as the dihydrate, and the organic molecule exists in a zwitterionic form with the carboxyl group deprotonated and the benzimidazole group protonated,^10d which has a significant advantage for the formation of abundant hydrogen bonds. As shown in Fig. 1b, both the benzimidazole N atoms are involved in forming hydrogen bonds as donors: one gives N2–H2⋯O1 hydrogen bonds to connect two parallel adjacent 1H-benzimidazole-2-carboxylate molecules, yielding a centro-symmetric hydrogen-bond motif R₂²(10); and the other N atom (N1) is attracted by O3 from a lattice water molecule to give a intermolecular N1–H1⋯O3 hydrogen bond. Notably, the carboxyl O atom (O2) exhibits a three-connected hydrogen-bond motif, in which O2 is employed as accepter to form three kinds of different hydrogen bonds with lattice water molecules (Table S2, ESI†). In the crystal, molecules are packed into a three-dimensional network (Fig. 2a) via these hydrogen bonds and two distinct π–π stacking interactions between the benzimidazole groups of adjacent molecules [centroid–centroid distances = 3.7602(10) and 3.6135(10) Å] (Fig. 2b). It is of interest to note that a helical chain generated by the hydrogen-bond interactions between carboxyl oxygen atoms (O2) and lattice water molecules is observed (Fig. 2c).

Fig. 2 (a) 3D supramolecular network of H₂BIC·2H₂O formed by intermolecular hydrogen-bonding and π–π stacking interactions. (b) The two distinct π–π stacking interactions in H₂BIC·2H₂O. (c) Helical chain structure generated by the hydrogen-bond interactions between carboxyl oxygen atoms and lattice water molecules.

X-ray analyses performed on single crystals of compounds 1–5 reveal an isostructural mode possessing one 2D wave-like layer structure with distorted grids. Hence, only the structure of 1 is described in detail. Compound 1 crystallizes in the monoclinic space group P2₁/n, and the asymmetric unit consists of one crystallographically independent Eu³⁺ ion and three HBIC⁻ ligands. As illustrated in Fig. 3a, the central Eu³⁺ cation is eight-coordinated by five carboxyl oxygen atoms (O1, O3–O6) and three nitrogen atoms (N1, N3 and N5) from five HBIC⁻ ligands, resulting in a distorted bicapped triangle prism coordination geometry (Fig. 3b). The Eu–O bond distances range from 2.338(3) to 2.487(3) Å with the mean of 2.415(3) Å, which is slightly shorter than the average Eu–N distance, 2.549(3) Å. The average O–Eu–O bond angle is 105.56(10)°, similar to that of N–Eu–N [105.43(10)°], but much lager than the mean O–Eu–N bond angle value, 94.87(11)°. All of the bond lengths and angles in 1 are within normal ranges and comparable with those observed in other reported Eu³⁺-carboxylates (Table S1, ESI†).¹³

Fig. 3 (a) Coordination environment of Eu³⁺ in 1, H atoms are omitted for clarity. Symmetry code: #1, 1/2 − x, −1/2 + y, 1/2 − z; #2, 3/2 − x, −1/2 + y, 1/2 − z. (b) Coordination polyhedron geometry of Ln³⁺ in 1–5.

Due to the effect of lanthanide contraction, the Ln³⁺ ions radii are very close [Shannon's ionic radii of Ln³⁺ in 1–5: 1.126 Å (Pr³⁺), 1.109 Å (Nd³⁺), 1.066 Å (Eu³⁺), 1.053 Å (Gd³⁺), and 1.04 Å (Tb³⁺)]. Comparing the geometries of isomorphic 1–5 (Table 2), it is evident to find out that the differences on the Ln–O/N bond lengths and correlative bond angles are very subtle, and the variation of some geometry in 1–5 shows dependency on the respective Shannon's effective ionic radii. For example, the average length of Ln–O decreases along with reducing of the Ln³⁺ radii (Ln–O: Pr³⁺ > Nd³⁺ > Eu³⁺ > Gd³⁺ > Tb³⁺), and the average bond angle of N–Ln–N increases accordantly with the Ln³⁺ radii increasing (∠_N–Ln–N: Pr³⁺ < Nd³⁺ < Eu³⁺ < Gd³⁺ < Tb³⁺). However, because the final formation of lanthanide compounds may be influenced by many complicated factors, not all the geometry variations in 1–5 are in line with the consistent tendency, there are also some exceptions. The average lengths of Ln–N in these compounds are 2.618(7), 2.549(3), 2.541(5) and 2.526(5) Å for compounds 5, 1, 3 and 2, steadily decreasing from Nd³⁺ to Tb³⁺, respectively. Yet the average Pr–N distance [2.612(3) Å] is shorter than the average Nd–N length, which is not in good agreement with the radii of the two Ln³⁺ ions. Meanwhile, the situation of average O–Ln–N bond angle is similar: ∠_O–Pr–N [94.78(9)°] < ∠_O–Nd–N [94.79(6)°] < ∠_O–Eu–N [94.87(11)°] < ∠_O–Tb–N [94.87(16)°], just ∠_O–Gd–N [94.90(13)°] > ∠_O–Tb–N [94.87(16)°], while the average O–Ln–O bond angles in 1–5 seemingly look random in the changing tendency: ∠_O–Nd–O [105.66(5)°] > ∠_O–Pr–O [105.65(8)°] > ∠_O–Tb–O [105.57(15)°] > ∠_O–Eu–O [105.56(10)°] > ∠_O–Gd–O [105.51(12)°].

Table 2 The geometry comparison for compounds 1–5

	1 (Eu^3+)	2 (Tb^3+)	3 (Gd^3+)	4 (Pr^3+)	5 (Nd^3+)
a Range of bond length or bond angle. b Average bond length or bond angle.
Ln–O^a (Å)	2.338(3)–2.487(3)	2.318(4)–2.467(4)	2.326(4)–2.480(4)	2.376(3)–2.524(3)	2.370(5)–2.522(5)
Ln–O^b (Å)	2.415(3)	2.394(4)	2.405(4)	2.455(2)	2.451(5)
Ln–N^a (Å)	2.519(3)–2.567(3)	2.493(6)–2.546(5)	2.514(5)–2.557(5)	2.593(3)–2.631(3)	2.586(7)–2.656(7)
Ln–N^b (Å)	2.549(3)	2.526(5)	2.541(5)	2.612(3)	2.618(7)
O–Ln–O^a (°)	70.63(9)–157.00(10)	70.36(15)–157.36(16)	70.70(12)–156.80(14)	70.64(8)–157.05(9)	70.48(19)–157.2(2)
O–Ln–O^b (°)	105.56(10)	105.57(15)	105.51(12)	105.65(8)	105.66(5)
O–Ln–N^a (°)	63.63(9)–152.20(11)	63.99(16)–152.26(16)	64.09(13)–152.23(13)	63.14(9)–152.29(9)	62.97(19)–152.5(2)
O–Ln–N^b (°)	94.87(11)	94.87(16)	94.90(13)	94.78(9)	94.79(6)
N–Ln–N^a (°)	70.80(11)–152.32(11)	71.00(18)–152.56(18)	70.67(16)–152.53(15)	70.02(10)–151.46(10)	70.2(2)–151.7(2)
N–Ln–N^b (°)	105.43(10)	105.67(17)	105.60(15)	104.53(10)	104.76(6)
Grid size (Å × Å)	6.9561(5) × 6.7012(5)	6.9219(8) × 6.6662(8)	6.9443(7) × 6.6949(6)	7.0167(4) × 6.7600(4)	7.0234(12) × 6.7663(11)

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In compound 1, all H₂BIC ligands are present in the mono-deprotonated form, HBIC⁻. Two kinds of new coordination modes of HBIC⁻ (Modes I and II in Scheme 1) are first observed. One is the HBIC⁻ ligand chelating one Eu³⁺ center with one carboxyl oxygen atom and one imidazole nitrogen atom first, and the other carboxyl oxygen atom further bridges another Eu³⁺ ion, leading to a chelating-bridging μ₂-κN, O: κO coordination fashion (Mode I); the other is the HBIC⁻ ligand chelating the Eu³⁺ center with a simple μ₁-κN, O coordination fashion (Mode II). The adjacent Eu³⁺ centers are bridged by the HBIC⁻ ligands in Mode I into 1D infinite chains (Fig. 4), and these chains are further interweaved each other, generating a 2D wave-like layered structure in the ab plane (Fig. 5a). In this 2D network, each Eu³⁺ center is coordinated by four HBIC⁻ ligands in Mode I, and the remaining two sites are occupied by another HBIC⁻ ligand in Mode II. From the viewpoint of network topology, the structure of 1 can be simplified as a four-connected 2D (4,4) network with the Eu³⁺ ions as nodes (Fig. 5b), in which each distorted grid has a size of 6.7012(5) Å × 6.9561(5) Å, slight larger than those in compounds 2 [6.9219(8) × 6.6662(8)] and 3 [6.9443(7) × 6.6949(6)], and smaller than those in compounds 4 [7.0167(4) × 6.7600(4)] and 5 [7.0234(12) × 6.7663(11)] (Table 2).

Fig. 4 1D infinite chain structure in compound 1.

Fig. 5 (a) A wave-like 2D network in compound 1, H atoms are omitted for clarity. (b) A schematic view of the topology framework of 1.

It is worth mentioning that the deprotonated carboxylate and protonated benzimidazole N atom of the HBIC⁻ ligand make the amphoteric HBIC⁻ ligand a good candidate to be involved in the construction of strong hydrogen bonds as a donor or acceptor. As can be seen from Fig. 6a, each grid in the 2D layer is formed by four HBIC⁻ ligands bridging four Eu³⁺ centers with Mode I, and the grid is further twisted by two types of distinct intra-layer hydrogen bonds [N2–H2⋯O5^#1 = 2.718(4) Å and N6–H6⋯O1^#2 = 2.782(4) Å. Symmetry code: #1, −x + 3/2, y + 1/2,−z + 1/2; #2 −x + 1/2, y + 1/2, −z + 1/2], where the adjacent HBIC⁻ ligands are tightly bound together like the bis(chelating)-bridging oxalate, thus resulting in a wave-like 2D layer. Moreover, the HBIC⁻ ligands with Mode II are arranged face to face between two adjacent layers, and the exposed carboxylic O atoms and H atoms participate in constructing the strong inter-layer hydrogen bond R₂²(10) ring motifs [N4–H4⋯O2^#3 = 2.824(5) Å, #3: −x + 1, −y + 1, −z] (Fig. 6b), which finally bridges the adjacent layers into a 3D supramolecular architecture (Fig. 6c).

Fig. 6 (a) Intra-layer hydrogen-bonding interactions in 1, the benzene rings of the benzimidazole groups are omitted for clarity. (b) Inter-layer hydrogen-bonding interactions in 1. (c) 3D supramolecular architecture assembled via intra-layer and inter-layer hydrogen bonds in 1, where pink and green dashed lines denote intra-layer and inter-layer hydrogen-bonding interactions, respectively.

Thermal analysis

To examine the thermal stability of 1–5, the thermogravimetric analyses for crystal samples of 1–5 were performed under a simulated air atmosphere with a heating rate of 10 °C min⁻¹ from ambient temperature up to 1000 °C. As shown in Fig. S3, ESI,† TG analyses indicate that all these compounds have high thermal stability and exhibit similar thermal behavior due to their isostructural nature. Herein, only the thermal stability of 1 was taken as a representative example for discussion. The TG curve depicted in Fig. 7 reveals that 1 remains stable up to 354.7 °C, which could be ascribed to the abundant intra-layer and inter-layer hydrogen bonds occurring in framework 1. Then the compound experiences almost one-step weight loss of 71.85% from 354.7 to 528.3 °C, which is attributed to the thermal decomposition of the organic components. The final mass of the residue is 28.15% of the initial weight, which agrees well with that expected value for the formation of a stoichiometric quantity of Eu₂O₃ (calcd. 27.69%). The end product was identified as the body-centered cubic Eu₂O₃ by PXRD (Fig. S4, ESI;† JCPDS NO. 34-0392).

Fig. 7 TG-DTG-DSC curves of compound 1 with a heating rate of 10 °C min⁻¹.

The DTG curve for 1 shows that the whole process of weight loss consists of two sequential thermal decomposition processes, and the sharp peaks located at 391.1 °C and 462.7 °C, respectively, represent the fastest decomposition of the two processes, which may caused by the two distinct binding fashions between Eu³⁺ ions with ligands in 1. In addition, the broad exothermic peak of the DSC curve shows that the compound decomposes with a large heat of 7002 J g⁻¹.

The DSC experiments were also carried out from room temperature to 1000 °C under a simulated air atmosphere to investigate the heat variation of the lanthanide compound system. As illustrated in Fig. 8, the DSC curves of 1–5 are similar, and each of them shows a broad downward band with two consecutive exothermic peaks, corresponding to the two sequential thermal decomposition processes. There are also some significant differences on the heat release and the decomposition peak temperature. As can be seen from Fig. 8, all the five compounds display high heats, the maximum 2 (7697 J g⁻¹) outputs about 1330 J g⁻¹ more than the minimum 5 (6361 J g⁻¹). Correspondingly, the greatest decomposition peak temperature occurred in 2 (480.6 °C) is approximately 25 °C higher than that in 5 (454.4 °C). It should be noted that the variation tendency of both heat release and decomposition peak temperature are consistent with that of the average Ln–N/O lengths, that is, the decomposition heat and peak temperature decrease with the increase of average Ln–N/O lengths. This finding confirms that the shorter distance between Ln³⁺ ion and O/N atom, the greater the heat yielded by thermo-decomposition.

Fig. 8 DSC plots of compounds 1–5 with a heating rate of 10 °C min⁻¹.

UV-vis spectra

The UV-vis absorption spectra of the free ligand H₂BIC and its corresponding Ln³⁺ compounds 1–5 were recorded in DMSO solution (c = 2 × 10⁻⁵ M), as shown in Fig. 9. The identical trends in absorption spectra of compounds 1–5 and the free ligand, demonstrate that the coordination of the Ln³⁺ ion does not have a significant influence on the singlet excited state of the free ligand. Nevertheless, compared with the H₂BIC ligand, a slight blue shift is discernible in the absorption maximum of the five compounds, which could be attributable to the perturbation induced by the metal ion coordination. The H₂BIC ligand displays three absorption bands at 269, 275, and 281 nm, which are assigned to singlet–singlet ¹π–π* absorptions of the aromatic rings. The molar absorption coefficient values for compounds 1–5 at about 273 nm of 3.35 × 10⁴, 3.55 × 10⁴, 3.02 × 10⁴, 3.49 × 10⁴ and 3.44 × 10⁴ L mol⁻¹ cm⁻¹, respectively, are approximately three times as high as that of the ligand (1.11 × 10⁴ at 275 nm), indicating the presence of three ligands in each compound, this is also in good agreement with the crystallographic analysis. The aforementioned features suggest that the H₂BIC ligand has a strong ability to absorb light.

Fig. 9 UV-vis absorption spectra of H₂BIC and compounds 1–5 in DMSO solution (2 × 10⁻⁵ M).

Energy transfer between the H₂BIC ligand and Ln³⁺ (Ln = Eu, Tb)

For lanthanide coordination compounds, the trivalent lanthanide ion luminescence generally needs to be stimulated by a strong chromophore ligand via the “antenna effect”, and the details can be described as the following steps: the chromophore ligand is first excited from ground state into the first excited singlet state due to the UV absorption, immediately nonradiative intersystem crossing of the ligand from the singlet to the triplet state occurs; and the intramolecular energy is further transferred from the ligand-centered triplet state to the excited 4f states of lanthanide ion, followed by internal conversion to the lower emitting states simultaneously; finally, the characteristic lanthanide emission is generated by the radiative transition from the lanthanide ion emissive states to the lower different energy states.¹⁴ So it can be seen that the intramolecular energy migration efficiency from the organic ligand to the central Ln³⁺ ions plays a key role in the luminescence property of the lanthanide coordination compound.

The choice of a suitable sensitizer is very important. According to Reinhoudt's empirical rule, the intersystem crossing process becomes effective when energy gap ΔE [singlet energy level (¹ππ*) – triplet energy level (³ππ*)] is at least 5000 cm⁻¹.¹⁵ Thus, the singlet and triplet energy levels of the H₂BIC ligand should be firstly determined. The ¹ππ* of H₂BIC is estimated by reference to the wavelength of the UV-vis higher absorption edge of Gd³⁺ compound 3, and the pertinent value is 286 nm (34 965 cm⁻¹) (see Fig. 9). Because the first excited state of Gd³⁺ (about 32 000 cm⁻¹), ⁶P_7/2, is too high to accept any energy from the first excited triplet state of the ligand through intramolecular ligand-to-metal energy transfer, the phosphorescence spectrum of the corresponding Gd³⁺ compound could actually reveal the triplet energy level of the ligand.¹⁶ As shown in Fig. S5, ESI,† the triplet energy level (³ππ* = 24 630 cm⁻¹) of H₂BIC is determined by the lower wavelength emission edge (406 nm) from the low-temperature phosphorescence spectrum of the Gd³⁺ compound 3.

Fig. 10 shows the diverse energy-level states of H₂BIC, Eu³⁺ and Tb³⁺ and possible energy transfer pathways among them. The energy gap for the H₂BIC ligand ΔE (¹ππ* − ³ππ* = 10 335 cm⁻¹) is significantly higher than 5000 cm⁻¹, which suggests that the intersystem crossing process is effective for the H₂BIC ligand.^15,17 Further, the triplet energy level of H₂BIC is also obviously higher than the ⁵D₀ level of Eu³⁺ (17 300 cm⁻¹) and ⁵D₄ level of Tb³⁺ (20 500 cm⁻¹), giving rise to an advantageous energy-transfer condition in 1 or 2. Of course, this does not mean that the higher the better, and the energy gap between the lowest triplet state of the H₂BIC ligand and the ⁵D₀ state (Eu³⁺) or the ⁵D₄ state (Tb³⁺) must be appropriate. The gap may be too large or too small to get satisfactory luminescence efficiency; a large gap always yields an inefficient energy transfer and a small gap is beneficial to a back-energy transfer.¹⁸ Latva's empirical rule states that an optimal ligand-to-metal energy transfer process for Ln³⁺ requires ΔE (³ππ* − ⁵D_J) = 2500–4000 cm⁻¹ for Eu³⁺ and 2500–4500 cm⁻¹ for Tb³⁺.¹⁹ Given that the energy gap between the triplet state and ⁵D₀ state of Eu³⁺ is 7330 cm⁻¹, the H₂BIC ligand is not in an ideal situation for the sensitization of Eu³⁺ luminescence. Whereas the energy gap between the triplet state and ⁵D₄ state of Tb³⁺ is 4130 cm⁻¹, suggesting that the H₂BIC ligand could strongly sensitize Tb³⁺ emission in 2.

Fig. 10 Schematic energy level diagrams and energy transfer mechanisms for compounds 1 and 2. S₁ represents the first excited singlet state and T₁ represents the first excited triplet state.

Photophysical property

Photoluminescence properties of 1 and 2

Considering that the H₂BIC ligand is an adequate light-harvesting chromophore, and expected to be used for sensitizing lanthanide ion luminescence. The solid-state luminescence properties of 1–2 were studied at room temperature. The solid-state excitation and emission spectra of the Eu³⁺ compound 1 recorded at room temperature are displayed in Fig. 11. The excitation spectrum (Fig. 11a) monitored around the intense ⁵D₀ → ⁷F₂ transition of the Eu³⁺ exhibits a broad and relatively weak band between 250 and 350 nm (λ_max = 331 nm), which can be assigned to the π–π* electronic transition of the H₂BIC ligand. A series of sharp characteristic f–f transitions of the Eu³⁺ at 361 (⁷F₀ → ⁵G₆), 380 (⁷F₀ → ⁵H₄) and 394 nm (⁷F_0,1 → ⁵L₆) are also evident, which can be reasonably assigned to the transitions between the ⁷F_0,1 and the ⁵L₆ and ⁵D_2,1 levels.²⁰ The more intense 4f absorption of Eu³⁺ suggests that the luminescence sensitization via excitation of the H₂BIC ligand is not very promising in 1.

Fig. 11 Room-temperature excitation (a) and emission (b) spectra for 1 (λ_ex = 394 nm) with emissions monitored at approximately 613 nm.

Under excitation at 394 nm, the ambient temperature emission spectrum of 1 displays characteristic sharp emission bands of Eu³⁺ in the spectral range of 570–750 nm. As shown in Fig. 11b, the five anticipated transitions from the excited ⁵D₀ state to the ⁷F_J ground-state multiplet are well resolved, that is, ⁵D₀ → ⁷F₀ at 579 nm, ⁵D₀ → ⁷F₁ at 593 nm, ⁵D₀ → ⁷F₂ at 613 nm, ⁵D₀ → ⁷F₃ at 654 nm, and ⁵D₀ → ⁷F₄ at 700 nm.^20a,20b,21 Compared with others, the maximum intensities at 579 and 654 nm are much weaker, because their corresponding ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₃ transitions are both strictly forbidden in magnetic and electric dipole schemes.^14c,21c The ⁵D₀ → ⁷F₁ transition is relatively stronger, which is a magnetic dipole. The ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₁ transitions consist of only one peak at 579 nm and three split peaks at 593 nm, respectively, which suggests the existence of a single chemical environment around the Eu³⁺ ion in 1.^16a,22 It is clear that the hypersensitive ⁵D₀ → ⁷F₂ transition (electric dipole) shows the highest intensity at 613 nm, pointing to a highly polarizable chemical environment around the Eu³⁺ ion. The luminescent intensity ratio of the (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) is 11.53, which indicates that compound 1 exhibits bright red light emission and the Eu³⁺ ion in it is not located in a site with inversion center symmetry.²³

Fig. 12 summarizes the room-temperature excitation and emission spectra of Tb³⁺ compound 2 in the solid state. The excitation spectrum of 2 (Fig. 12a) monitored around the hypersensitive ⁵D₄ → ⁷F₅ transition (545 nm) of Tb³⁺ reveals a broad band between 280 and 350 nm with a maximum at approximately 333 nm, which can be attributable to the π–π* electronic transition of the H₂BIC ligand. Because this broad band overlaps the absorption spectrum of the H₂BIC ligand in the region of 280–350 nm, the energy transfer from the ligand to Tb³⁺ is operative. In addition, a series of faint f–f transitions ⁷F₅ → ⁵L₉ (359 nm), ⁷F₅ → ⁵L₁₀ (369 nm) and ⁷F₅ → ⁵G₆ (378 nm) are also observed in the excitation spectrum of 2,²⁴ which are much weaker than the absorptions owing to the organic ligands. Apparently, luminescence sensitization via ligand excitation is significantly more efficient than the direct excitation of the Tb³⁺ absorption level.

Fig. 12 Room-temperature excitation (a) and emission (b) spectra for 2 (λ_ex = 333 nm) with emissions monitored at approximately 545 nm.

Being excited by UV light (λ_ex = 333 nm), the solid-state emission spectrum of 2 recorded at room temperature (Fig. 12b) exhibits the characteristic narrow emission bands of Tb³⁺, which results from deactivation of the ⁵D₄ excited state to the corresponding ground state ⁷F_J (J = 6, 5, 4, 3) of the Tb³⁺ ion centered at 489, 545, 584, and 621 nm.^20a–d The most intense emission centered at 545 nm represents the hypersensitive ⁵D₄ → ⁷F₅ transition, which shows the strongest green emission of 2. Furthermore, the emission band from the H₂BIC ligand is not detected, implying efficient energy transfer from the ligand to the central Tb³⁺ ion in 2, which is further confirmed by the very high emission intensity of Tb³⁺.

For a deeper research on the photophysical behaviors of the compounds 1–2, the lifetime values for the ⁵D₀ level of 1 and ⁵D₄ level of 2 (τ_obs) were measured at ambient temperature based on the corresponding luminescent decay profiles by fitting with monoexponential functions (Fig. S6 and S7, ESI†), and the relevant values are summarized in Table 3. The typical decay profiles indicate that single chemical environments exist around the emitting Eu³⁺/Tb³⁺ ions. The ideal luminescence lifetimes for 1 (0.952 ms) and 2 (1.207 ms) could be due to the strong chelating coordination ability and rational steric stabilization of the HBIC⁻ ligand, which effectively prevented the solvent guest molecules from entering the coordination sphere of the lanthanide ion and greatly reduced the energy loss caused by the O–H vibrations.

Table 3 The radiative (A_RAD) and nonradiative (A_NR) decay rates, ⁵D₀/⁵D₄ lifetime (τ_obs), intrinsic quantum yield (Φ_Ln), radiative lifetimes (τ_RAD), energy transfer efficiency (Φ_sen), and overall quantum yield (Φ_overall) for compounds 1–2 in the solid state

Photophysical parameters	Compound 1 (Eu^3+)	Compound 2 (Tb^3+)
A RAD (s^−1)	343	—
A NR (s^−1)	706	—
τ obs (μs)	952 ± 1	1207 ± 1
τ RAD (μs)	2911 ± 1	1380 ± 1
Φ Ln (%)	33	87
Φ sen (%)	8	36
Φ overall (%)	2 ± 1	31 ± 3

---

In addition, to clearly explain the sensitization efficiency of the H₂BIC ligand (Φ_sen), it should be necessary to determine the overall luminescence quantum yields (Φ_overall) of 1 and 2 first. In compliance with the method described by Bril et al.,^18b,25 the overall quantum yields of 1 and 2 were respectively estimated to be 2% and 31% through the following expression:

where r_x and r_st represent the diffuse reflectance of the compounds (with respect to a fixed wavelength) and the standard phosphor, respectively (Fig. S8, ESI†), and BaSO₄ was used as a reflecting standard to acquire absolute intensity values. A_x and A_st are the areas under the compounds and the standard emission spectra, respectively. Φ_st is the quantum yield of the standard phosphor, peprylene (purchased from Aldrich), whose emission spectrum consists of an intense broad band with a maximum at around 580 nm and a constant Φ value (98%) for excitation wavelength at 436 nm.²⁶ The final calculated Φ_overall value for each sample was taken from the average of three measurement values, and the corresponding test errors were estimated to be ±10%.^25b,c

As is well-known, when the internal process in the system (usually treated as a “black box”) is not considered explicitly, the overall quantum yield (Φ_overall) for a lanthanide compound can be defined as the product of the intrinsic quantum yield of Ln³⁺ ion (Φ_Ln) and the efficiency of energy transfer from the ligand to Ln³⁺ ion (Φ_sen),^18a,27 which can be depicted as:

Φ_overall = Φ_senΦ_Ln (2)

Hence, Φ_Ln can be used for calculating the sensitization efficiency of the ligand (Φ_sen). Eqn (3) shows the calculating formula of Φ_Ln, in which A_RAD, A_NR, τ_obs and τ_RAD correspond to the radiative decay rate, nonradiative decay rate, ⁵D₀/⁵D₄ lifetimes and radiative lifetimes, respectively.

For Eu³⁺ compound 1, the A_RAD, A_NR and τ_RAD are finally calculated to be 343 s⁻¹, 706 s⁻¹ and 2911 μs viaeqn (4), here, the energy of the ⁵D₀ → ⁷F₁ transition (MD) and its oscillator strength are assumed to be constant, and A_MD,0 (14.65 s⁻¹) represents the spontaneous emission probability of the ⁵D₀ → ⁷F₁ transition in vacuo,^6c,28n (1.5) is the average refractive index of the medium,^14bI_TOT/I_MD represents the ratio of the total area of the corrected Eu³⁺ emission spectrum to the area of the ⁵D₀ → ⁷F₁ band. Combined with the measured ⁵D₀ lifetime value (952 μs), Φ_Eu of compound 1 is finally determined to be 33%.

In comparison with the complicated measurement and calculation of Eu³⁺ compound 1, the intrinsic quantum yield of Tb³⁺ (Φ_Tb) in 2 can be directly estimated by means of eqn (5) using the assumption that the decay process at 77 K in deuterated solvents is purely radiative (Fig. S9, ESI†).²⁹

The correlative photophysical parameters of compounds 1–2 are presented in Table 3, the lower sensitization efficiency of the ligand (η_sen = 8%) in 1 indicates that the energy transfer from the first excited triplet state of ligand to the excited ⁵D₀ level of Eu³⁺ is not an optimal process. It is also evident from Table 3 that the overall luminescence quantum yield of 2 is much higher than that of 1, though both of them have satisfactory luminescence lifetime. This could be due to the weak sensitization efficiency of the H₂BIC ligand and smaller intrinsic quantum yield of the Eu³⁺ ion in 1. On the other hand, because of the smaller energy gap between the ligand triplet and Tb³⁺ ion excited states (4130 cm⁻¹), the Tb³⁺ compound 2 exhibits good luminescence efficiency with a overall quantum yield of 31% in the solid state, thus rendering it as a promising candidate for use in various photonic applications.

In summary, the coordinated solvent molecules, the conjugation effect of chromophores and/or co-ligands, and the energy gap between the ligand triplet state and the lowest-lying excited state of Ln³⁺ ion play cooperative roles in the photoluminescence properties of Ln-MOFs, as evidenced by some monomeric Ln-MOFs (Table S3, ESI†).

Photoluminescence properties of 4 and 5

Compared with the extensively studied europium and terbium compounds, less effort has been devoted to the investigation of Nd³⁺/Pr³⁺-containing compounds. Actually, they also exhibit good luminescent properties in both visible and near-infrared (800–1400 nm) regions, and especially the advantages of signal transmittance of near-infrared radiation enable widespread application in areas such as laser systems, medical diagnosis and telecommunications.^2c,30 Therefore, the photoluminescence properties of 4 and 5 were investigated in the solid state at room temperature. As shown in Fig. 13, compound 5 displays the characteristic emission bands of Nd³⁺ ion in the near-infrared region upon 354 nm radiation excitation: two moderate-intensity emission bands at 869 and 893 nm (⁴F_3/2 → ⁴I_9/2), a very strong emission band at 1060 nm (⁴F_3/2 → ⁴I_11/2), and a series of weak bands around 1329 nm (⁴F_3/2 → ⁴I_13/2). The splitting of the ⁴F_3/2 → ⁴I_9/2 transition band is mainly due to the “crystal field effect” of the Nd³⁺ ion located at a C₁ site.³¹ The decay curves are well fitted with a monoexponential function. The lifetime of the excited ⁴F_3/2 state of Nd³⁺ ion at room temperature for 5 was measured to be 1.21 μs (Fig. S11, ESI†). Because Pr³⁺ can show emission lines originating from three different levels (³P₀, ¹D₂ and ¹G₄) which span the visible and NIR regions, the luminescence spectrum of Pr³⁺ compound is much more complex in comparison to that of other Ln³⁺ systems, leading to far fewer studies of Pr³⁺ emission spectra.³²Fig. 14 depicts the room temperature emission spectrum of 4. Unlike 5, no characteristic emission bands of Pr³⁺ in the NIR region were detected, whereas compound 4 exhibits a series of characteristic emission bands for Pr³⁺ ion in the visible region with a 399 nm excitation source. The emissions at 462 and 479 nm can be respectively assigned to ¹I₆ → ³H₄ and ³P₁ → ³H₄ transitions. A series of emission bands centered at approximately 499, 545, and 614 nm correspond to the ³P₀ → ³H_J (J = 4, 5 and 6, respectively) transitions. The two peaks observed at 597 and 604 nm are attributed to the ¹D₂ → ³H₄ transition, and the weak signal with a maxima at 643 nm can be referred to the ³P₀ → ³F₂ transition.³³ All the observed transitions in Fig. 14 suggest that the triplet state of the H₂BIC ligand is located in a suitable level and there is enough intramolecular energy transferred from the ligand-centered triplet state to the excited ¹D₂, ³P₀, even the higher ³P₁ and ¹I₆ levels. Therefore, the energy transfer from the H₂BIC ligands to the Pr³⁺ ions is effective.

Fig. 13 Solid-state emission spectrum for 5 (λ_ex = 354 nm) in the near-infrared region at room temperature.

Fig. 14 Solid-state emission spectrum for 4 (λ_ex = 399 nm) at room temperature.

Conclusions

In this paper, a series of lanthanide frameworks involving 1H-benzimidazole-2-carboxylic acid ligands (1–5) have been hydrothermally synthesized and structurally characterized for the first time. The isostructural compounds 1–5 exhibit 2D (4,4) networks with distorted grids constructed by zwitterionic HBIC⁻ ligands with two kinds of unprecedented coordination modes. The thermal analyses indicate that their frameworks exhibit good thermal stability and release a large amount of heat during the decompositions. The detailed optical properties investigations reveal that: 1 and 2 exhibit remarkable lanthanide-centered luminescence emissions in the visible region showing luminescence lifetimes at millisecond order and intrinsic quantum yields of 33% and 87%, respectively; compound 5 emits strong characteristic luminescence in the near-infrared region. The results suggest that the strong chelating-bridging ligand HBIC⁻, which could efficiently occupy the first coordination sphere of the lanthanide ion, is an excellent chromophore for constructing luminescence materials. In addition, the more efficient energy migration [ΔE (³ππ* − ⁵D₄) = 4130 cm⁻¹] in 2 gives rise to a larger overall quantum yield of 31% over a poor value of 2% in 1, demonstrating that the suitable energy gap between the ligand triplet state and the lowest-lying excited state of Ln³⁺ ion plays a crucial role in the efficient sensitization of Ln³⁺ ion emission, and the HBIC⁻ ligand is preferable for the sensitization of Tb³⁺-centered luminescence.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 21073142, 21173168 and 21127004), the Natural Science Foundation of the Department of Education of Shaanxi Province (Nos. 2010JK882, 2010JQ2007, 11JK0578 and 11JS110).

References

1. (a) F. N. Shi, L. Cunha-Silva, R. A. S. Ferreira, L. Mafra, T. Trindade, L. D. Carlos, F. A. A. Paz and J. Rocha, J. Am. Chem. Soc., 2008, 130, 150; (b) Z. Chen, B. Zhao, Y. Zhang, W. Shi and P. Cheng, Cryst. Growth Des., 2008, 8, 2291; (c) S. V. Eliseeva and J. C. G. Bünzli, Chem. Soc. Rev., 2010, 39, 189; (d) A. de Bettencourt-Dias, Dalton Trans., 2007, 2229; (e) V. Chandrasekhar, B. M. Pandian, R. Azhakar, J. J. Vittal and R. Clérac, Inorg. Chem., 2007, 46, 5140; (f) D. Parker, Coord. Chem. Rev., 2000, 205, 109; (g) M. N. Bochkarev, Chem. Rev., 2002, 102, 2089.
2. (a) R. A. Poole, G. Bobba, M. J. Cann, J. C. Frias, D. Parker and R. D. Peacock, Org. Biomol. Chem., 2003, 1, 905; (b) J. Kido and Y. Okamoto, Chem. Rev., 2002, 102, 2357; (c) J. C. G. Bünzli, Chem. Rev., 2010, 110, 2729; (d) D. Y. Liu, K. Z. Tang, W. S. Liu, C. Y. Su, X. H. Yan, M. Y. Tan and Y. Tang, Dalton Trans., 2010, 39, 9763; (e) B. Chen, L. Wang, Y. Xiao, F. R. Fronczek, M. Xue, Y. Cui and G. Qian, Angew. Chem., Int. Ed., 2009, 48, 500.
3. (a) S. I. Klink, G. A. Hebbink, L. Grave, P. G. B. Oude, F. C. J. M. van Veggel and M. H. V. Werts, J. Phys. Chem. A, 2002, 106, 3681; (b) Lanthanide Probes in Life, Chemical and Earth Sciences: Theory and Practice, ed. J. C. G. Bünzli and G. R. Choppin, Elsevier, Amsterdam, 1989.
4. (a) C. Yang, L. M. Fu, Y. Wang, J. P. Zhang, W. T. Wong, X. C. Ai, Y. F. Qiao, B. S. Zou and L. L. Gui, Angew. Chem., Int. Ed., 2004, 43, 5010; (b) R. V. Deun, P. Nockemann, P. Fias, K. V. Hecke, L. V. Meervelt and K. Binnemans, Chem. Commun., 2005, 590; (c) A. D'Aléo, J. Xu, E. G. Moore, C. J. Jocher and K. N. Raymond, Inorg. Chem., 2008, 47, 6109; (d) S. Petoud, S. M. Cohen, J. C. G. Bünzli and K. N. Raymond, J. Am. Chem. Soc., 2003, 125, 13324; (e) M. D. Allendorf, C. A. Bauer, R. K. Bhakta and R. J. T. Houk, Chem. Soc. Rev., 2009, 38, 1330; (f) D. Parker, R. S. Dickins, H. Puschmann, C. Crossland and J. A. K. Howard, Chem. Rev., 2002, 102, 1977; (g) J. C. G. Bünzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048.
5. (a) S. K. Ghosh and P. K. Bharadwaj, Inorg. Chem., 2005, 44, 3156; (b) S. Q. Su, W. Chen, C. Qin, S. Y. Song, Z. Y. Guo, G. H. Li, X. Z. Song, M. Zhu, S. Wang, Z. M. Hao and H. J. Zhang, Cryst. Growth Des., 2012, 12, 1808.
6. (a) X. J. Wang, Z. M. Cen, Q. L. Ni, X. F. Jiang, H. C. Lian, L. C. Gui, H. H. Zuo and Z. Y. Wang, Cryst. Growth Des., 2010, 10, 2960; (b) A. de Bettencourt-Dias and S. Viswanathan, Dalton Trans., 2006, 4093; (c) S. Viswanathan and A. de Bettencourt-Dias, Inorg. Chem., 2006, 45, 10138.
7. (a) B. Zhao, L. Yi, Y. Dai, X. Y. Chen, P. Cheng, D. Z. Liao, S. P. Yan and Z. H. Jiang, Inorg. Chem., 2005, 44, 911; (b) Y. L. Lu, J. Y. Wu, M. C. Chan, S. M. Huang, C. S. Lin, T. W. Chiu, Y. H. Liu, Y. S. Wen, C. H. Ueng, T. M. Chin, C. H. Hung and K. L. Lu, Inorg. Chem., 2006, 45, 2430.
8. (a) J. Xu, J. W. Cheng, W. P. Su and M. C. Hong, Cryst. Growth Des., 2011, 11, 2294; (b) X. Feng, J. S. Zhao, B. Liu, L. Y. Wang, S. W. Ng, G. Zhang, J. G. Wang, X. G. Shi and Y. Y. Liu, Cryst. Growth Des., 2010, 10, 1399; (c) X. P. Zhou, W. X. Ni, S. Z. Zhan, D. Li and Y. G. Yin, Inorg. Chem., 2007, 46, 2345; (d) J. G. Mao, Coord. Chem. Rev., 2007, 251, 1493.
9. S. R. Zheng, S. L. Cai, J. Fan, T. T. Xiao and W. G. Zhang, Inorg. Chem. Commun., 2011, 14, 818.
10. (a) J. Fan, S. L. Cai, S. R. Zheng and W. G. Zhang, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2011, 67, m346; (b) F. Sączewski, E. Dziemidowicz-Borys, P. J. Bednarski, R. Grünert, M. Gdaniec and P. Tabin, J. Inorg. Biochem., 2006, 100, 1389; (c) L. L. Di, Y. Wang, G. W. Lin and T. Lu, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2010, 66, m610; (d) X. J. Yao and Q. Yuan, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2011, 67, o1399.
11. (a) G. M. Sheldrick, SHELXS-97, Program for Solution of Crystal Structures, University of Göttingen, Germany, 1997; (b) G. M. Sheldrick, SHELXL-97, Program for Refinement of Crystal Structures, University of Göttingen, Germany, 1997.
12. (a) G. X. Liu, K. Zhu, H. M. Xu, S. Nishihara, R. Y. Huang and X. M. Ren, CrystEngComm, 2010, 12, 1175; (b) D. S. Li, Y. P. Wu, P. Zhang, M. Du, J. Zhao, C. P. Li and Y. Y. Wang, Cryst. Growth Des., 2010, 10, 2037.
13. (a) H. B. Zhang, N. Li, C. B. Tian, T. F. Liu, F. L. Du, P. Lin, Z. H. Li and S. W. Du, Cryst. Growth Des., 2012, 12, 670; (b) H. Wang, S. J. Liu, D. Tian, J. M. Jia and T. L. Hu, Cryst. Growth Des., 2012, 12, 3263; (c) F. N. Dai, D. Sun and D. F. Sun, Cryst. Growth Des., 2011, 11, 5670.
14. (a) P. J. M. Lehn, Angew. Chem., Int. Ed. Engl., 1990, 29, 1304; (b) G. F. de Sá, O. L. Malta, C. de Mello Donegá, A. M. Simas, R. L. Longo, P. A. Santa-Cruz and E. F. da Silva Jr, Coord. Chem. Rev., 2000, 196, 165; (c) D. B. Ambili Raj, S. Biju and M. L. P. Reddy, Inorg. Chem., 2008, 47, 8091.
15. F. J. Steemers, W. Verboom, D. N. Reinhoudt, E. B. van der Tol and J. W. Verhoeven, J. Am. Chem. Soc., 1995, 117, 9408.
16. (a) M. Shi, F. Y. Li, T. Yi, D. Q. Zhang, H. M. Hu and C. H. Huang, Inorg. Chem., 2005, 44, 8929; (b) H. Xin, M. Shi, X. C. Gao, Y. Y. Huang, Z. L. Gong, D. B. Nie, H. Cao, Z. Q. Bian, F. Y. Li and C. H. Huang, J. Phys. Chem. B, 2004, 108, 10796.
17. J. C. G. Bünzli and C. Piguet, in Encylopedia of Materials: Science and Technology, ed. K. H. J. Buschow, R. W. Cahn, M. C. B. Flemings, B. Ilschner, E. J. Kramer and S. Mahajan, Elsevier Science Ltd, Oxford, 2001. ch. 1.10.4, p. 4465.
18. (a) S. Biju, M. L. P. Reddy, A. H. Cowley and K. V. Vasudevan, Cryst. Growth Des., 2009, 9, 3562; (b) D. B. Ambili Raj, S. Biju and M. L. P. Reddy, Dalton Trans., 2009, 7519.
19. M. Latva, H. Takalo, V. M. Mukkala, C. Matachescu, J. C. Rodriguez-Ubis and J. Kanakare, J. Lumin., 1997, 75, 149.
20. (a) W. T. Carnall, in Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner and L. Eyring, Elsevier, Amsterdam, The Netherlands, 1987, ch. 24, vol. 3, p. 171; (b) G. H. Dieke, Spectra and Energy levels of Rare Earth Ions in Crystals, Interscience, New York, 1968; (c) G. Zucchi, O. Maury, P. Thuéry, F. Gumy, J. C. G. Bünzli and M. Ephritikhine, Chem.–Eur. J., 2009, 15, 9686; (d) J. Xia, B. Zhao, H. S. Wang, W. Shi, Y. Ma, H. B. Song, P. Cheng, D. Z. Liao and S. P. Yan, Inorg. Chem., 2007, 46, 3450.
21. (a) J. Q. Chen, Y. P. Cai, H. C. Fang, Z. Y. Zhou, X. L. Zhan, G. Zhao and Z. Zhang, Cryst. Growth Des., 2009, 9, 1605; (b) L. Fu, R. A. Sá Ferreira, N. J. O. Silva, A. J. Fernandes, P. Ribeiro-Carlo, I. S. Gonalves, V. de Zea Bermudez and L. D. Carlos, J. Mater. Chem., 2005, 15, 3117; (c) M. H. V. Werts, R. T. F. Jukes and J. W. Verhoeven, Phys. Chem. Chem. Phys., 2002, 4, 1542; (d) S. Biju, D. B. Ambili Raj, M. L. P. Reddy and B. M. Kariuki, Inorg. Chem., 2006, 45, 10651.
22. (a) H. Xin, F. Y. Li, M. Shi, Z. Q. Bian and C. H. Huang, J. Am. Chem. Soc., 2003, 125, 7166; (b) J. Kai, D. F. Parra and H. F. Brito, J. Mater. Chem., 2008, 18, 4549; (c) M. S. Liu, Q. Y. Yu, Y. P. Cai, C. Y. Su, X. M. Lin, X. X. Zhou and J. W. Cai, Cryst. Growth Des., 2008, 8, 4083.
23. D. B. Ambili Raj, B. Francis, M. L. P. Reddy, R. R. Butorac, V. M. Lynch and A. H. Cowley, Inorg. Chem., 2010, 49, 9055.
24. S. Raphael, M. L. P. Reddy, A. H. Cowley and M. Findlater, Eur. J. Inorg. Chem., 2008, 4387.
25. (a) A. Bril and A. W. De Jager-Veenis, J. Electrochem. Soc., 1976, 123, 396; (b) C. De Mello Donegá, S. Alves Jr and G. F. De Sá, Chem. Commun., 1996, 1199; (c) L. D. Carlos, C. De Mello Donegá, R. Q. Albuquerque, S. Alves Jr, J. F. S. Menezes and O. L. Malta, Mol. Phys., 2003, 101, 1037.
26. W. H. Melhuish, J. Opt. Soc. Am., 1964, 54, 183.
27. (a) S. Comby, D. Imbert, C. Anne-Sophie, J. C. G. Bünzli, L. J. Charbonnière and R. F. Ziessel, Inorg. Chem., 2004, 43, 7369; (b) S. Quici, M. Cavazzini, G. Marzanni, G. Accorsi, N. Armaroli, B. Ventura and F. Barigelletti, Inorg. Chem., 2005, 44, 529; (c) M. Xiao and P. R. Selvin, J. Am. Chem. Soc., 2001, 123, 7067.
28. Y. H. Kim, N. S. Baek and H. K. Kim, ChemPhysChem, 2006, 7, 213.
29. (a) N. Sabbatini, M. Guardigli and J. M. Lehn, Coord. Chem. Rev., 1993, 123, 201; (b) I. Nasso, S. Bedel, C. Galaup and C. Picard, Eur. J. Inorg. Chem., 2008, 2064.
30. (a) J. Rocha, L. D. Carlos, F. A. Almeida Paz and D. Ananias, Chem. Soc. Rev., 2011, 40, 926; (b) J. C. G. Bünzli and S. V. Eliseeva, J. Rare Earths, 2010, 28, 824; (c) M. H. V. Werts, J. W. Hofstraat, F. A. J. Geurts and J. W. Verhoeven, Chem. Phys. Lett., 1997, 276, 196; (d) Y. Hasegawa, T. Ohktbo, K. Sogabe, Y. Kawamura, Y. Wada, H. Nakashima and S. Yanagida, Angew. Chem., Int. Ed., 2000, 39, 357; (e) K. Driesen, R. V. Deun, R. C. Görller-Walrand and K. Binnemans, Chem. Mater., 2004, 16, 1531; (f) M. D. Ward, Coord. Chem. Rev., 2007, 251, 1663.
31. (a) J. L. Song, C. Lei and J. G. Mao, Inorg. Chem., 2004, 43, 5630; (b) G. A. Hebbink, L. Grave, L. A. Woldering, D. N. Reinhoudt and F. C. J. M. van Veggel, J. Phys. Chem. A, 2003, 107, 2483; (c) W. K. Wong, H. Liang, J. Guo, W. Y. Wong, W. K. Lo, K. F. Li, K. W. Cheah, Z. Zhou and W. T. Wong, Eur. J. Inorg. Chem., 2004, 829; (d) Y. F. Yuan, T. Cardinaels, K. Lunstroot, K. V. Hecke, L. V. Meervelt, C. Görller-Walrand, K. Binnemans and P. Nockemann, Inorg. Chem., 2007, 46, 5302; (e) S. Faulkner and S. J. A. Pope, J. Am. Chem. Soc., 2003, 125, 10526; (f) S. J. A. Pope, A. M. Kenwright, V. A. Boote and S. Faulkner, Dalton Trans., 2003, 3780; (g) E. Galdecka, Z. Galdecki, P. Gawryszewska and J. Legendziewicz, New J. Chem., 2000, 24, 387.
32. (a) G. M. Davies, R. J. Aarons, G. R. Motson, J. C. Jeffery, H. Adams, S. Faulkner and M. D. Ward, Dalton Trans., 2004, 1136; (b) A. I. Voloshin, N. M. Shavaleev and V. P. Kazakov, J. Lumin., 2001, 93, 199.
33. (a) Y. X. Chi, S. Y. Niu and J. Jin, Inorg. Chim. Acta, 2009, 362, 3821; (b) Y. L. Huang, M. Y. Huang, T. H. Chan, B. C. Chang and K. H. Lii, Chem. Mater., 2007, 19, 3232.

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR spectra of ligand, TGA and PXRD patterns, crystallographic details and additional photophysical data. CCDC 874290 (H₂BIC·2H₂O), 874292 (1), 874291 (2), 874299 (3), 874293 (4), 874296 (5). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ce26120k

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