Synthesis and structure of color tunable and white-light emitting lanthanide metal–organic framework materials constructed from conjugated 1,1′-butadiynebenzene-3,3′,5,5′-tetracarboxylate ligand

Abstract

A series of novel lanthanide metal–organic frameworks, namely [Ln₂(BBTC)_1.5(DMF)₄]·2DMF·4H₂O [BBTC⁴⁻ = 1,1′-butadiynebenzene-3,3′,5,5′-tetracarboxylate, Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Er (6) and Yb (7)], have been prepared via solvothermal reaction. Single crystal X-ray analyses reveal that complexes 1–7 are isostructural and crystallize in the C2/c space group. They have the same three-dimensional (3D) architectures, topological analyses suggest that these frameworks possess the 3D (4,6)-connected network with Schläfli symbol of {4³·6³}₃·{4⁴·6⁷·8⁴}·{4⁸·6⁶·8}. Solid-state photoluminescent investigations at room temperature reveal that complex 2 and 4 emit the intensely red characteristic luminescence of Eu³⁺ ions and green luminescence emissions of Tb³⁺ ions, with a long lifetime up to 1.38 and 0.33 ms, respectively, during which the ligand emission of BBTC⁴⁻ was quenched by the Eu³⁺ and Tb³⁺ ions. Complexes 5, 6 and 7 also display characteristic f–f luminescence emissions in the near-infrared region, respectively. These results indicate that BBTC⁴⁻ is an ideal ligand with an “antenna effect” for Ln³⁺ ions, and an efficient energy transfer exists between them. Color tunable and white-light emitting materials were successfully achieved by carefully adjusting the doping concentration of Eu³⁺ and Tb³⁺ ions in the Gd³⁺ compound through trichromatic method.

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Introduction

Recently, the design and synthesis of lanthanide metal–organic frameworks (Ln–MOFs), self-assembled from lanthanide metal ions and organic linkers, have attracted increasing attention in material research due to their superior functional properties and actual or potential applications.^1–4 In comparison with transition metal ions, the lanthanide ions possess higher coordination numbers and more flexible coordination geometries, and thus lead to the formation of MOFs with versatile motifs.⁵ The lanthanide (4f) complexes usually show intense emission over narrow wavelength ranges and readily identifiable emission bands in both visible and NIR regions,^6–8 which are potentially applicable as fluorescent probes and electroluminescent devices.⁹ However, lanthanide ions are very poor at absorbing light directly because of the low extinction coefficients of the Laporte forbidden f–f transitions, resulting in inefficient luminescence. Alternatively, the emission of the lanthanides can be sensitized by a coordinated ligand via “antenna effect”.¹⁰ In such a case, excitation occurs at the singlet energy state of a suitable organic ligand, which subsequently relaxes to its triplet state and then transfers energy to the lanthanide ions, and usually the conjugated organic multicarboxylate molecules are good candidates for “antenna” ligands of lanthanide ions.

Up to now, Ln–MOFs have been extensively studied in sensing ions, vapors, explosives, etc.¹¹ And recently, tunable white-light emission of Ln³⁺ complexes has also been reported since white-light emitting materials and devices have potential applications in general lighting, low-cost back-lighting and fullcolor displays.^12,13 Qian et al. have reported a rational design strategy to construct white-light emitting materials by doping Eu³⁺ and Tb³⁺ ions into isostructural La³⁺ ions metal–organic framework.¹⁴ Zheng, Li and Zang et al. also achieved strong white-light emitters by varying the stoichiometric ratio of Eu³⁺, Tb³⁺ and La³⁺/Ga³⁺ ions within the mixed isostructural lanthanide metal–organic complexes.¹⁵ Moreover, single-phase Eu–Ag dual component white-light emitters have also been reported through assembly of d–f heterometallic MOFs.¹⁶ Generally, the white-light emitting materials are mainly produced through monochromatic, dichromatic, trichromatic, and tetrachromatic approaches.¹³ Among them, the solid state trichromatic white-light emitting materials have attracted much more attention in terms of their good color rendering properties and high luminous efficiency, and white-light emitting Ln–MOFs are candidates for light-emitting devices due to their structural diversity and ligand-dependent luminescence sensitization. But it is still very challenging and difficult to target white-light emitting Ln–MOF materials because different light emitters should compensate exactly through the trichromatic approaches. As is known, the Gd³⁺ complex sometimes may act as a blue emitter and shows ligand-centered visible emission in blue light region owing to the higher energy (32 150 cm⁻¹) of the Gd³⁺ lowest emitting level, and Eu³⁺ and Tb³⁺ complexes usually emit the intensely red characteristic luminescence of Eu³⁺ ions and green luminescence of Tb³⁺ ions. Therefore, Gd³⁺, Eu³⁺, and Tb³⁺ complexes might be incorporated into the resulting trichromatic white-light emitting materials.

Recently, our group and others have focused on developing and designing π-conjugated ligands with alkyne functionality, such as H₄EBTC (EBTC⁴⁻ = 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylate), H₄BBTC (BBTC⁴⁻ = 1,1′-butadiynebenzene-3,3′,5,5′-tetracarboxylate) and H₃CPEIP (CPEIP³⁻ = 5-[(4-carboxyphenyl)ethynyl]isophthalic acid).^17,18 The study on transition metal MOFs revealed that they are good ligands to construct porous MOFs with significant uptake of gases (H₂, CO₂, N₂, CH₄ or C₂H₂), especially for acetylene due to the incorporation of the triple C≡C bond into the organic backbone, and the investigation on lanthanide MOFs based on them indicated that they are efficient “antenna effect” ligands for enhancing Eu³⁺ or Tb³⁺ emission of f–f transitions. Even so, as far as we know, the systematic study on the Ln–MOFs' construction and property based on them are less reported, especially for H₄BBTC, and no work has been covered on white-light emitting materials of Ln–BBTC MOFs until now, thus we selected H₄BBTC as a ligand to design and synthesize Ln–BBTC MOFs so as to systematically investigate their photoluminescent properties and try to obtain color tunable and white-light emitting materials. Moreover, compared with EBTC⁴⁻, BBTC⁴⁻ is more flexible since the butadiyne group can make the two aromatic rings rotate more freely than those linked by the ethyne group, which may contribute to constructing MOFs with versatile motifs, and it has both aromatic ring and linear polyyne chain functionalities, which may potentially serve as specific sites for certain molecular recognition, unique gas sorption, or particular catalysis applications. All above considerations inspired us to explore new Ln–MOFs with rectangular-planar 4-connected tetracarboxylates ligand of H₄BBTC. In this contribution, we synthesized seven new Ln–MOFs [Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Er (6) and Yb (7)] based on it and systematically studied their crystal structures and photoluminescent properties. Moreover, we also prepared series of MOFs-based co-doped materials Eu_xGd_1−x–BBTC, Tb_yGd_1−y–BBTC and Eu_xTb_yGd_1−x−y–BBTC, and investigated the luminescence natures of them. And ultimately, we successfully obtained tunable white-light emitting materials through careful adjustment of the relative concentration of the doping of Eu³⁺ and Tb³⁺ ions in the Gd³⁺ complex.

Experimental section

Chemicals and reagents

1,1′-Butadiynebenzene-3,3′,5,5′-tetracarboxylate (H₄BBTC) was synthesized according to the method published before.^18a LnCl₃·nH₂O (Ln = Eu, Tb, Dy, Er and Yb) were prepared by the reaction of hydrochloric acid (6.0 M) with the corresponding lanthanide oxide, and then were evaporated at 80 °C. SmCl₃·6H₂O and GdCl₃·6H₂O were purchased from chemical company and used directly. N,N-Dimethyl formamide (DMF) was dried and distilled according to standard procedures. All commercially available chemicals were of analytical grade and used without further purification.

Physical measurements

Powder X-ray diffraction (PXRD) measurements were performed on a Bruker D8 Discover diffractometer with Cu Kα (λ = 1.54056 Å) radiation with a scan speed of 5° min⁻¹ and a step size of 0.02° in 2θ at room temperature. Elemental analyses for C, H and N were carried out on a PerkinElmer 240 elemental analyzer. Inductively coupled plasma (ICP) was performed on a PerkinElmer Optima 5300DV spectrometer. The morphology, elemental mapping images and composition of the samples were inspected using a scanning electron microscope (SEM, S-3400N II and S-4800, Hitachi) equipped with an energy-dispersive X-ray spectroscope (EDX, EX-250 and S 4800, HORIBA). The IR spectra were recorded on a NICOLET iS10 spectrometer using KBr pellets. Thermal gravimetric analyses (TGA) were measured using a DTA-TGA 2960 thermogravimetric analyzer with a heating rate of 20 °C min⁻¹ under nitrogen atmosphere. The UV/visible absorbance in the solid state at room temperature was performed on Shimadzu UV-3600 spectrometer with BaSO₄ as reflectance standard. The photoluminescence spectra in the solid state were recorded by F-7000 FL and RF-5301 PC spectrophotometer at room temperature, respectively. The luminescence decay curves were measured by an Edinburgh Instruments FLS920P spectrometer equipped with Xe900 xenon lamp.

Preparation and characterization of Ln–MOFs

[Sm₂(BBTC)_1.5(DMF)₄]·2DMF·4H₂O (1). A solution of H₄BBTC (10 mg, 0.026 mmol), SmCl₃·6H₂O (11 mg, 0.03 mmol), DMF (0.5 mL) and HNO₃ (0.06 mL, 1 M in DMF) were mixed and sealed in a 15 mL Teflon-lined autoclave and heated to 90 °C for 24 h. Light-yellow block-shaped crystals were obtained after gradually cooled to room temperature at a rate of 5 °C h⁻¹ (yield: 64% based on Sm). Anal. calcd for C₄₈H₅₉Sm₂N₆O₂₂: C, 42.00; H, 4.33; N, 6.12. Found: C, 41.98; H, 4.26; N, 6.24. Selected IR data (KBr pellet, cm⁻¹): 3390 (m), 3065 (w), 2933 (w), 1650 (s), 1550 (s), 1430 (s), 1378 (s), 1103 (s), 787 (m), 717 (m).

The analogous procedure above-mentioned was used for preparation of 2–7 (see ESI†), just replaced SmCl₃·6H₂O by other LnCl₃·6H₂O. The microanalysis and main IR bands are summarized in Table S1.† Moreover, the doped complexes Eu_xGd_1−x–BBTC, Tb_yGd_1−y–BBTC and Eu_xTb_yGd_1−x−y–BBTC were prepared by adopting the analogical method as for the isostructural Ln analogies, loading the corresponding LnCl₃·6H₂O (Ln³⁺ = Gd³⁺, Eu³⁺ or Tb³⁺) as the starting materials in different stoichiometric ratios. Their purities were confirmed by powder XRD (Fig. S1 and S2†) and elemental analysis of C, H and N (Tables S2 and S3†), energy dispersive X-ray (EDX) elemental mapping analysis for Eu_0.25Gd_0.75–BBTC, Tb_0.5Gd_0.5–BBTC and Eu_0.045Tb_0.055Gd_0.9–BBTC prove that each Eu_xGd_1−x–BBTC/Tb_yGd_1−y–BBTC/Eu_xTb_yGd_1−x−y–BBTC doped complex is not a simple mixture but a single phase coordination polymer (Fig. S3†), and inductively coupled plasma (ICP) analysis of these co-doped compounds further confirms that the amounts of Gd³⁺, Eu³⁺ and/or Tb³⁺ ions in them are almost identical with the concentration of the starting materials (Table S4†).

Crystallographic analyses

Single crystal X-ray analyses were conducted on a Bruker Smart Apex II CCD diffractometer using graphite monochromated Mo/Kα radiation (λ = 0.71073 Å) at 296 K. Data reductions and absorption corrections were performed using the SAINT and SADABS software packages,¹⁹ respectively. The structures were solved by direct methods with the SHELXL-97 software package.²⁰ All hydrogen atoms were placed at the calculated positions and refined riding on the parent atoms. All non-hydrogen atoms were refined anisotropically using the full-matrix least-squares method on F². In complex 1–7, solvent molecules in the structure were highly disordered and were impossible to refine using conventional discrete-atom models. To resolve this issue, the contribution of solvent electron density was removed by SQUEEZE routine in PLATON.²¹ It should be noted that the weak diffraction intensity of 1 and 3 with small single crystals result in the data with low completeness, which are shown in Table S5.† The main data of collection and refinement details of complexes 2, 4, 5, 6 and 7 are given in Table 1. Selected bond lengths and angles are listed in Tables S6 and S7.† The CCDC reference numbers are 1478788 for 2, 1478790 for 4, 1478791 for 5, 1478792 for 6 and 1478793 for 7, respectively.

Table 1 Crystallographic data and structural refinements for 2, 4, 5, 6 and 7^a

a R1 = Σ||F0| − |Fc||/Σ|F0|, wR2 = {Σ[w(F0^2 − Fc^2)^2]/Σ[w(F0^2)^2]}^1/2.
Compound	2	4	5	6	7
Formula	C48H59Eu2N6O22	C48H59Tb2N6O22	C48H59Dy2N6O22	C48H59Er2N6O22	C48H59Yb2N6O22
Formula weight	1376	1390	1397	1407	1418
Temperature (K)	296(2)	296(2)	296(2)	296(2)	296(2)
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	C2/c	C2/c	C2/c	C2/c	C2/c
a (Å)	30.0058(13)	29.9342(11)	29.9537(12)	29.9258(10)	29.8541(10)
b (Å)	21.2440(9)	21.1600(7)	21.1398(9)	21.0418(7)	20.9459(6)
c (Å)	19.5943(9)	19.5148(7)	19.4385(8)	19.4326(6)	19.3789(6)
α (deg)	90.00	90.00	90.00	90.00	90.00
β (deg)	93.870(1)	93.9350(1)	93.980(1)	94.000(1)	94.071(1)
γ (deg)	90.00	90.00	90.00	90.00	90.00
V (Å^3)	12 461.8(9)	12 331.7(8)	12 279.1(9)	12 206.8(7)	12 087.5(7)
Z	8	8	8	8	8
F(000)	4568	4600	4616	4648	4680
GOF	1.070	1.033	1.013	1.082	1.099
R1, wR2 [I > 2σ(I)]^a	0.0595, 0.1998	0.0508, 0.1564	0.0684, 0.2140	0.0660, 0.2122	0.0618, 0.1951

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Results and discussion

Crystal structures

Compounds 1–7 were obtained under the same conditions by the mild solvothermal reactions. Single crystal X-ray diffraction analyses reveal that all the compounds are isomorphous, crystallize in the monoclinic C2/c space group and exhibit a 3D framework. Powder X-ray diffraction (PXRD) of compounds 1–7 were recorded at room temperature. As shown in Fig. 1 and S4,† the peak positions of the theoretical and experimental PXRD patterns are in good agreement with each other, which clearly indicate the high purity and homogeneity of these samples. Besides this, the IR spectra of 1–7 are almost identical (Fig. S5†), and the microanalysis disclosed that complexes 1–7 have the analogous chemical components with a formula of [Ln₂(BBTC)_1.5(DMF)₄]·2DMF·4H₂O [Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Er (6) and Yb (7)]. In addition, the doped compounds Eu_xGd_1−x–BBTC, Tb_yGd_1−y–BBTC and Eu_xTb_yGd_1−x−y–BBTC are also isostructural with the parent complex 3, which are verified by powder X-ray diffraction (PXRD) analysis (Fig. S1 and S2†), as a result, only the structure of [Tb₂(BBTC)_1.5(DMF)₄]·2DMF·4H₂O (4) is representatively described in detail. The structures of 1, 2, 3, 5, 6 and 7 are shown in Fig. S6.†

Fig. 1 PXRD patterns of 1–7 at ambient temperature.

Crystal 4 belongs to the monoclinic system with space group of C2/c. As shown in Fig. 2a, its asymmetric unit cell contains two eight-coordinated Tb³⁺ ions, one and a half BBTC⁴⁻ and four coordinated DMF molecules. Two crystallographically independent Tb³⁺ ions (labeled as Tb1 and Tb2, respectively) are coordinated with eight oxygen atoms, exhibiting two different coordination geometries (Fig. 2b). The bond lengths of Tb–O bond distances range from 2.277(5) to 2.471(4) Å (Table S6†), and the O–Tb–O bond angles span from 48.6(7) to 156.6(6)° (Table S7†). These bond parameters are similar to the values of Ln–carboxylate complexes in literatures.²² The coordination geometry around Tb1 ions can be described as a distorted bicapped trigonal prism with its eight coordinated oxygen atoms coming from five BBTC⁴⁻ ligands (O1, O2, O7a, O8b, O11 and O12c) and two DMF molecules (O17 and O19). Among the five coordinated BBTC⁴⁻ ligands, one carboxylate from one BBTC⁴⁻ coordinates to Tb1 as a chelating mode, while the other four carboxylates from four different BBTC⁴⁻ ligands coordinate to two Tb1 centers in a bidentate bridging coordination mode to give a dinuclear Tb₂(CO₂)₄ (A) subunit (SBU) with Tb1⋯Tb1 distance of 5.537 Å within this SBU (Fig. 2c). As for Tb2 ions, they bond to six oxygen (O3, O4d, O5e, O6e, O9 and O10) from four BBTC⁴⁻ ligands and the other two oxygen (O18 and O20) from two DMF molecules, resulting in distorted trigonal dodecahedral geometries. Similarly, two Tb2 centers are linked by two carboxylates in a bidentate bridging coordination mode from two different BBTC⁴⁻ ligands to form a binuclear Tb₂(CO₂)₂ (B) subunit (SBU), with Tb2⋯Tb2 distance of 5.480 Å. Furthermore, two more carboxylates from another two BBTC⁴⁻ ligands coordinate to Tb2 as chelating mode. Ultimately, two kinds of BBTC⁴⁻ ligand (α and β form) with different steric configurations generate upon complexation. As shown in Fig. 2c, the torsion angle of the butadiyne group (C29–C30–C30j–C29j) is 4.289° and the dihedral angle between the two aromatic rings is 1.467° in α form, while those in β form are 6.427° (C9–C10–C11–C12) and 88.447°, respectively. That's to say, compared with EBTC⁴⁻ in constructing Ln–MOFs,^18b BBTC⁴⁻ is more flexible since the longer butadiyne chain can make the two aromatic rings rotate more freely than those linked by the ethyne group. As a result, the butadiyne chain in BBTC⁴⁻ is bent to two different degree, with one-half of the aromatic rings of the ligands rotated round it to be almost perpendicularly arranged so as to satisfy the coordination geometry of Tb³⁺ (β form). Moreover, in the α-BBTC⁴⁻ ligand, two carboxylates in the cis-positions adopt bidentate mode to bridge two Tb1 centers, and the other two adopt chelating mode to bind two Tb2 atoms. While in the β-BBTC⁴⁻ ligand, two opposite carboxylates in two phenyl rings adopt a bidentate bridging mode to link two Tb1 and two Tb2 ions, and the other two also in the para-positions adopt a chelating mode to coordinate one Tb1 and one Tb2 ion, respectively. Overall, each Tb₂(CO₂)₄ (A) and Tb₂(CO₂)₂ (B) SBU is surrounded by six BBTC⁴⁻ bridging ligands and each BBTC⁴⁻ ligand connects two Tb₂(CO₂)₄ (A) and two Tb₂(CO₂)₂ (B) SBUs, generating a three-dimensional (3-D) open framework. As shown in Fig. 2f, the coordinated DMF molecules, being disordered, locate in the channels, and there exist three types of one-dimensional channels along the c axis, which possess the open window of 10.3 × 6.3 Å (C), 7.9 × 5.8 Å (D) and 10.1 × 8.6 Å (E) when the van der Waals radii of the atoms at the wall of the channel are subtracted, respectively. Moreover, open channel systems also exist along the a and b axis, with an open window size of 6.7 × 6.3 Å and 6.3 × 3.9 Å, respectively (Fig. 2d and e). The total potential void volume is 63.4% calculated from PLATON/SOLV²¹ if the guest water, guest DMF and coordinated DMF molecules are removed (Fig. S7†). From the view point of topology, the 3-D structure of 4 can be described as a (4,6)-connected network with Schläfli symbol of {4³·6³}₃·{4⁴·6⁷·8⁴}·{4⁸·6⁶·8} by considering each Tb₂(CO₂)₄ (A) and Tb₂(CO₂)₂ (B) SBU as a 6-connected node and the organic ligand as a 4-connected node (Fig. 2g).

Fig. 2 (a) Coordination environments of Tb³⁺ ions with the H atoms omitted for clarity; symmetry codes: a = −x, −y, 1 − z; b = 0.5 + x, 0.5 + y, z; c = 0.5 − x, 0.5 − y, 1 − z; d = 0.5 − x, −0.5 − y, 1 − z; e = 0.5 + x, −0.5 − y, 0.5 + z; j = 1 − x, 1 + y, 1.5 − z; (b) coordination polyhedron of Tb³⁺ ions; (c) the local coordination environment of the Tb1 and Tb2 ions in 4. A and B represent Tb₂(CO₂)₄ and Tb₂(CO₂)₂ subunits, respectively; α and β represent BBTC⁴⁻ ligands with two different steric forms; (d–f) polyhedral representation of 4 seen from a, b, and c directions (the green and turquoise polyhedra correspond to [Tb1O₈] and [Tb2O₈] units, respectively); (g) (4,6)-connected network presented, orange represents the 4-connected node of organic ligand and blue represents the 6-connected node of SBU in 4.

It should be noted that the structures of Ln–BBTC MOFs in this work have some difference compared with our previous reported Eu–MOFs constructed from the same ligand, such as the torsion angle of the butadiyne group and the dihedral angle between the two aromatic rings in BBTC⁴⁻, even if their coordination modes of the ligand are similar (bidentate bridging and chelating mode). In Ln–BBTC of this work, the smaller torsion angle of the butadiyne group in both α and β form (4.289° and 6.427°) exhibits that the butadiyne has better linearity (the torsion angle of the butadiyne group in α and β form is 150.973° and 14.073° respectively in previous reported Eu–MOFs^18a); moreover, the smaller dihedral angle between the two aromatic rings in α form (1.467°) and the larger one in β form (88.447°) suggest that the two aromatic rings have better coplanarity or perpendicularity (the dihedral angle between the two aromatic rings in α and β form is 8.708° and 87.340° respectively in Eu–MOFs^18a). The different steric configuration of BBTC⁴⁻ ligand between Ln–BBTC and Eu–MOFs should be mainly ascribed to the different coordination environment of the lanthanide ions, since they have different coordinating solvents besides BBTC⁴⁻ ligand. And the different synthetic conditions, such as solvent and temperature, lead to the different coordination environment.

TG analyses

TGA measurements of compounds 1–7 were performed from room temperature to 750 °C under a nitrogen atmosphere to study the thermal stability of these lanthanide metal–organic frameworks. As shown in Fig. S8,† the isomorphism 1–7 exhibit similar thermal behavior with several continuous weight loss processes below 354 °C, and these weight loss processes correspond to the guest water, guest DMF molecules and coordinated DMF molecules releasing. As a representative example, compound 1 was discussed in detail. From the TG curve depicted in Fig. 3, compound 1 shows two main steps of sequential weight loss process with a percentage of 15.88% before 162 °C, corresponding to the release of four guest water molecules and two guest DMF molecules (calcd: 15.89%), and 21.29% in 162–354 °C temperature range, attributing to the loss of four coordinated DMF molecules (calcd: 21.27%). Above 354 °C, the whole framework gradually begins to break down upon further heating.

Fig. 3 TG curve of 1.

Photoluminescence properties

To investigate the luminescence properties of the lanthanide metal–organic frameworks, the UV-vis absorption spectrum for H₄BBTC was recorded in the solid state at room temperature (Fig. S9(a)†). It is observed that H₄BBTC exhibits intense broad bands centered at 340 nm, which can be attributed to π–π* transitions of the aromatic rings. And consequently, it gives a broad emission centered at 420 nm upon excitation at 340 nm due to π ← π* transitions (Fig. S9(b)†).

The solid-state luminescence properties of complexes 1–7 were investigated at room temperature. Upon excitation at 340 nm, compound 3 displays strong blue luminescence (ref. the CIE chromaticity diagram Fig. 6b) with a single broad emission band centered at 467 nm (Fig. 4, its excitation spectrum is given in Fig. S10(b)†), such a broad band emission can also be assigned to the intraligand π ← π* transition of BBTC⁴⁻. In comparison to the emission of H₄BBTC, compound 3 obviously shifts towards the lower energy side (red-shift), which may be attributed to the fact that the lowest excited state of Gd³⁺ (⁶P_7/2) is too high to accept energy from the ligand, making its characteristic 4f–4f transition invisible.^23,24 Therefore, the red-shift of the intense emission in 3 could be attributed to the coordination effect, which affects the HOMO and LUMO levels of the ligand to change the transition energy, and enables the rigidity of the aromatic backbones and reduces the loss of energy by radiationless decay of the intraligand emissions.²⁵

Fig. 4 Emission spectra of 2, 3 and 4 under excitation λ_ex = 340 nm at room temperature.

Compound 1, excited at 340 nm, displays characteristic orange luminescence of Sm³⁺ ions in the visible region (ref. the CIE chromaticity diagram Fig. 6b), and the typical emission bands centered at 563, 599 and 646 nm (Fig. S11(a)†) are assigned to the transitions from ⁴G_5/2 to ⁶H_J (J = 5/2, 7/2 and 9/2), respectively.

As shown in Fig. 4, compound 2 shows five characteristic emission bands of Eu³⁺ ions centered at 579, 592, 615, 653 and 701 nm with red luminescence upon excitation at 340 nm (ref. the CIE chromaticity diagram Fig. 6b). Five typical emission bands are corresponding to the ⁵D₀ → ⁷F_J (J = 0–4) transitions, respectively (its excitation spectrum is given in Fig. S10(a)†). The symmetric forbidden emission ⁵D₀ → ⁷F₀ at 579 nm is a weak band. The emission at 592 nm is corresponding to the magnetic dipole induced ⁵D₀ → ⁷F₁ transition, which is fairly insensitive to the environment of the Eu³⁺ ions, while the most intense emission at 615 nm is attributed to the electric dipole induced ⁵D₀ → ⁷F₂ transition, which is hypersensitive to the coordination environment of the Eu³⁺ ions. Moreover, the peak around 653 nm for the ⁵D₀ → ⁷F₃ transition is observed with weak intensity, and the electric dipole ⁵D₀ → ⁷F₄ transition around 701 nm is comprised of two peaks. It is well known that the intensity ratio between ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions is strongly relevant to the local symmetry of the Eu³⁺ ions. Specifically, the transition intensity of ⁵D₀ → ⁷F₂ increases significantly as the site symmetry of the Eu³⁺ ions are lowered, whereas the local coordination environment of Eu³⁺ ions have no clear impact on the ⁵D₀ → ⁷F₁ transition.²⁶ As for 2, the intensity ratio of I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁) is ca. 5, further indicating that the symmetry of Eu³⁺ ions is low and the Eu³⁺ ions are not located at the inversion center, which is in good agreement with our X-ray structural analyses. It is worth noting that the characteristic emission peaks from BBTC⁴⁻ ligand disappeared, which indicates the effective sensitization of Eu³⁺ ions from ligand excitation.

Upon sensitization of the Tb³⁺ ion by means of ligand excitation, compound 4 displays intense green luminescence when excited at 340 nm (ref. the CIE chromaticity diagram Fig. 6b), exhibiting four emission bands centered at 487, 542, 581 and 618 nm which are attributed to the ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions, respectively (Fig. 4, its excitation spectrum is shown in Fig. S10(c)†).²⁷ As expected, the ⁵D₄ → ⁷F₅ (sensitive to the nature of the surrounding atoms) is the strongest one according to its largest probability for both electric-dipole and magnetic-dipole-induced transitions.²⁸ Similar to that of compound 2, no ligand-based emission is observed, thereby also suggesting effective ligand to metal energy transfer.

When excited at 340 nm at room temperature, compound 5 displays photoluminescent behavior with typical Dy³⁺ emissions centered at 483 and 576 nm (Fig. S11(b),† its excitation spectrum is given in Fig. S10(d)†), which correspond to the characteristic emission of ⁴F_9/2 → ⁶H_J (J = 15/2 and 13/2) transitions of Dy³⁺ ions respectively.

We have also investigated the emission in the near-IR region of compound 5, which is a rarely described phenomenon.²⁹ Three emission bands centered at 950 (⁴F_9/2 → ⁶F_7/2), 976 (⁴F_9/2 → ⁶F_5/2) and 1147 nm (⁴F_9/2 → ⁶F_3/2) are observed for the Dy³⁺ complex (Fig. S11(c)†). Notably, the emission of Dy³⁺ complex is not a single sharp transition with the presence of well-split NIR emission peaks, which may be ascribed to the crystal-field splitting.³⁰

Upon excitation at 340 nm, compound 6 also gives rise to the characteristic NIR emission (Fig. S11(d)†). The emission band centered at 1533 nm covers large spectral range extending from 1450 to 1652 nm, ascribing to the typical ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions. Erbium-doped materials have been attracted much interest for many years, because the transition around 1540 nm is in the right position of the third telecommunication window.³¹

Compound 7 shows a broad band in the range of 920–1100 nm, comprised of a sharp peak at 977 nm when excited at 340 nm (Fig. S11(e)†), which is attributed to the ²F_5/2 → ²F_7/2 transition and broader vibronic components at the longer wavelength.³² The NIR emission of 7 is not a single sharp band but a broad band, this should be caused by the splitting of the energy levels of the Yb³⁺ ions as a consequence of ligand field effect.³³

The luminescence decay curves of 1–7 were obtained in the solid state at room temperature (Fig. 5 and S12†). The decay curves are well fitted into a single exponential function: y = y₀ + A₁ × exp(−t/τ), where y₀ and A₁ are two constants, and τ is the decay constant defined as the photoluminescence lifetime. The main emission peaks at 599, 615, 467, 542, 576, 1533 and 977 nm were monitored, respectively. The black dots are the raw experimental data and the red lines represent the fitting curves. The best fit of the experimental photoluminescence intensities to the above equation led to the following photoluminescence lifetimes: τ = 1.68 μs for 1, 1.38 ms for 2, 2.36 μs for 3, 0.33 ms for 4, 1.86 μs for 5, 3.34 μs for 6 and 2.91 μs for 7, respectively. In general, the f–f electronic transitions of lanthanide ions are forbidden, leading to long excited state decay time. In this work, it was found that the photoluminescence lifetimes of 2 and 4 are at millisecond magnitude order, falling in the range of lanthanide ion luminescence decay time 10–2000 μs.³⁴

Fig. 5 Luminescence decay profiles for 2 (a), 3 (b) and 4 (c).

Since the whole family are isostructural, it is expected that we can easily tune their emissions by doping methods. Among the isostructural Ln–BBTC MOFs, Eu–BBTC (2), Tb–BBTC (4) and Gd–BBTC (3) emitted intense red, green and blue emissions, respectively (Fig. 4), therefore, we engaged in the preparation of monodoped Eu_xGd_1−x–BBTC and Tb_yGd_1−y–BBTC compounds, and bidoped Eu_xTb_yGd_1−x−y–BBTC compounds, to investigate whether the different percent compositions would result in different colors and the white-light emission would be successfully achieved due to photoluminescence properties of the co-doped MOFs.

The solid-state emission spectra of the monodoped Eu_xGd_1−x–BBTC and Tb_yGd_1−y–BBTC compounds were investigated upon excitation at 340 nm (Fig. S13(a) and S14(a),† the solid-state excitation spectra of Eu_0.25Gd_0.75–BBTC and Tb_0.5Gd_0.5–BBTC are shown in Fig. S15(a) and (b)†). As expected, both exhibited characteristic emissions of Eu³⁺ and Tb³⁺ ions, or ligand-to-metal charge transfer, and along with the increase of Eu³⁺ and Tb³⁺ ions concentration, the luminescence intensity corresponding to the characteristic Eu³⁺ (615 nm) and Tb³⁺ (542 nm) ions almost linearly increased, while the luminescence intensity from BBTC⁴⁻ ligand (467 nm) slightly decreased, which could be stemmed by Ln³⁺ ions concentration effects.³⁵ It is of note that the Eu³⁺- or Tb³⁺-based emissions are systematically more intense than that of the BBTC⁴⁻ ligand in these monodoped compounds (Fig. S13(a) and S14(a)†), suggesting that partial energy transfers from Gd³⁺ to Eu³⁺ or Tb³⁺ ions and that the Ln³⁺-centered luminescence is significantly sensitized by the π-conjugated organic multicarboxylate ligands.^15a As a result, tunable colors could be readily generated from blue to pale pink, pale orange and red for Eu_xGd_1−x–BBTC compounds and from blue to pale blue and green for Tb_yGd_1−y–BBTC compounds, respectively (Fig. S13(b) and S14(b)†). The corresponding CIE chromaticity coordinates of these monodoped complexes are listed in Tables S8 and S9.†

The tunable colors from monodoped Eu_xGd_1−x–BBTC and Tb_yGd_1−y–BBTC compounds prompt us to investigate the photoluminescence properties of the bidoped Eu_xTb_yGd_1−x−y–BBTC compounds by compensating the red color from Eu³⁺ ions, green color from Tb³⁺ ions and blue color from BBTC⁴⁻ ligands so as to obtain the trichromatic white-light emission materials by adjusting the relative concentration of Eu³⁺, Tb³⁺ and Gd³⁺ ions in the co-doped Eu_xTb_yGd_1−x−y–BBTC MOFs. This would be of interest since white-light emitting materials have extensive applications in general lighting sources. Since white-light emission can be produced when the emission intensities around red, green and blue range are comparable,^15,36 and the energy transfer should be incomplete from the Gd³⁺ host to doped Ln³⁺ ions, which is the most significant feature to realize the white-light emission in the co-doped system.^15c We made an effort to optimize the relative concentration of Ln³⁺ dopants in Eu_xTb_yGd_1−x−y–BBTC MOFs, and the emission spectra of the doped compounds (A–E) which exhibit the typical ⁵D₀ → ⁷F₂ transition around 615 nm for Eu³⁺ in 2, ⁵D₄ → ⁷F₅ transition around 542 nm for Tb³⁺ in 4, and the broad band centered at 467 nm assigned to intraligand charge transfer of the ligands in 3 were obtained upon excitation at 340 nm (Fig. 6a, the solid-state excitation spectra of Eu_0.015Tb_0.035Gd_0.95–BBTC is given in Fig. S15(c)†). We can also see that the luminescence intensity of the Eu³⁺ and Tb³⁺ ions decreased significantly as the concentration of Eu³⁺ and Tb³⁺ ions was decreased due to the concentration effects (A–E),³⁵ while the emission intensity resulting from BBTC⁴⁻ ligands increased gradually, thus combination of the emissions from f–f transitions of Eu³⁺ and Tb³⁺ ions and those from BBTC⁴⁻ ligands can readily generate variations in the relative red, green and blue emission intensities when the doping ratios of Ln³⁺ dopants were changed, resulting in tunable colors (A–E) from pale yellow to pale yellow-green, pale green, near white, and white (Fig. 6b). The corresponding CIE chromaticity coordinates of these doped complexes are listed in Table S10.† All above reveals that the emission color of Eu_xTb_yGd_1−x−y–BBTC MOFs can be finely tuned through different combination of the amount of Ln³⁺ dopants, and ultimately, the white-light emission is readily produced when the red emission of Eu³⁺ ions, the green emission of Tb³⁺ ions, and the blue emission of BBTC⁴⁻ ligands occur simultaneously, and the emission peaks at 615, 542 and 467 nm are comparable in intensity (Fig. 6 and S16†), with the molar ratio of 1.5 : 3.5 : 95 (Eu³⁺/Tb³⁺/Gd³⁺) and the corresponding CIE coordinate (0.334, 0.336), which is very close to the coordinate for pure white-light (0.333, 0.333), thus ideal for white-light emission according to the 1931 CIE coordinate diagram (Fig. 6 and S16†).

Fig. 6 (a) Emission spectra and (b) CIE chromaticity diagram for 1, 2, 3, 4 and the doped Eu_xTb_yGd_1−x−y–BBTC (A–E) complexes excited at 340 nm. A–E (Eu, Tb, Gd%): A (4.5, 5.5, 90), B (4, 5, 91), C (3, 4.5, 92.5), D (2.5, 4, 93.5) and E (1.5, 3.5, 95).

Moreover, it should be noted that the luminescent emission intensity of Eu³⁺ at 615 nm (corresponding to the transition ⁵D₀ → ⁷F₂) is 2.3-fold lower in the co-doped Eu_0.015Tb_0.035Gd_0.95–BBTC complex than that in the pure complex Eu–BBTC (2), which is mainly due to the lower concentration of the Eu³⁺ ions (Fig. S17†). However, the emission intensity of Tb³⁺ at 542 nm (corresponding to the transition ⁵D₄ → ⁷F₅) decreases significantly and displays a 6.4-fold lower intensity compared to that in complex Tb–BBTC (4) (Fig. S17†), suggesting there is energy transfer from Tb³⁺ to Eu³⁺. Furthermore, the luminescent lifetimes of Eu³⁺ and Tb³⁺ emissions in this co-doped complex are 1.42 ms and 0.20 ms, respectively (Fig. S18†). Compared to the pure complex Eu–BBTC (2) and Tb–BBTC (4), the lifetime of Eu³⁺ in the co-doped complex is longer, while that of Tb³⁺ is shorter, further demonstrating that there exists intermetallic energy transfer from Tb³⁺ to Eu³⁺ centers in the co-doped complex, which is in agreement with others' reports.³⁷

Conclusion

In summary, seven isostructural lanthanide–MOFs, [Ln₂(BBTC)_1.5(DMF)₄]·2DMF·4H₂O [Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Er (6) and Yb (7)], based on the conjugated tetracarboxylate ligands of BBTC⁴⁻, were successfully synthesized under similar solvothermal conditions. Each lanthanide–MOFs exhibits a 3-D microporous network containing two different subunits of Ln₂(CO₂)₄ and Ln₂(CO₂)₂ connected by BBTC⁴⁻. Moreover, the Ln–MOFs (Ln = Sm, Eu, Tb, Dy, Er and Yb) displayed intense luminescence in solid state at room temperature and the emissions exhibit the typical f–f transitions of Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Er³⁺ and Yb³⁺ ions, respectively, further indicating that there exist efficient energy transfer from the conjugated tetracarboxylate ligands to the Ln³⁺ ions and the conjugated BBTC⁴⁻ ligand is a good “antenna effect” ligand for constructing luminescent lanthanide–MOFs. Compound 2, 3, and 4 acting as red-, blue-, and green-light emitting components are incorporated into the resulting tunable and white-light emitting materials by adjusting the relative concentration of Eu³⁺ and Tb³⁺ ions in the Gd³⁺ complex, suggesting that the identical coordination behavior of the different lanthanide ions allows in situ doping of various lanthanide ions into a parent MOFs, and the color of luminescence of the isostructural materials can be fine-tuned. This work provided the bright promise to exploit functional Ln–MOFs materials. It is expected that some more extensive endeavors to explore and develop the applications of solid state white-light emitting Ln–MOFs materials in lighting, imaging, agriculture, communication, and medicine will be carried out in the near future.

Acknowledgements

Authors thank National Nature Science Foundation of China for financial support (grant no. 21371091 and 21371150).

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Footnote

† Electronic supplementary information (ESI) available: The preparation of 2–7 is detailed in the supporting material. Elemental analysis and main IR bands for 1–7 are given in Table S1. PXRD patterns of Gd–BBTC, Eu_0.25Gd_0.75–BBTC and Tb_0.5Gd_0.5–BBTC are given in Fig. S1. PXRD patterns for complex 3 and co-doped compounds Eu_xTb_yGd_1−x−y–BBTC are shown in Fig. S2. Elemental analysis for Eu_xGd_1−x–BBTC and Tb_yGd_1−y–BBTC monodoped compounds are shown in Table S2. Elemental analysis for Eu_xTb_yGd_1−x−y–BBTC co-doped compounds are presented in Table S3. EDX-spectra of Eu_0.25Gd_0.75–BBTC, Tb_0.5Gd_0.5–BBTC and Eu_0.045Tb_0.055Gd_0.9–BBTC are given in Fig. S3. ICP analysis for Eu_xGd_1−x–BBTC, Tb_yGd_1−y–BBTC and Eu_xTb_yGd_1−x−y–BBTC co-doped compounds are shown in Table S4. The main data of collection and refinement details of complexes 1 and 3 are given in Table S5. Selected bond lengths and angles for 1–7 are given in Tables S6 and S7. PXRD patterns of 1–7 are plotted in Fig. S4. The IR spectra of 1–7 are presented in Fig. S5. The crystal structures of 1, 2, 3, 5, 6 and 7 are shown in Fig. S6. Space filled representation of the 3-D framework in 4 is shown in Fig. S7. The TG curves of 1–7 are presented in Fig. S8. The UV-vis absorption and emission spectra of ligand H₄BBTC are shown in Fig. S9. The excitation spectra of 2, 3, 4 and 5 are shown in Fig. S10. Emission spectra of 1 and 5, and NIR emission spectra of 5, 6 and 7 are shown in Fig. S11. Luminescence decay profiles for 1, 5, 6 and 7 are presented in Fig. S12. Emission spectra and CIE chromaticity diagram for Eu_xGd_1−x–BBTC are shown in Fig. S13. CIE coordinates of the Eu_xGd_1−x–BBTC are given in Table S8. Emission spectra and CIE chromaticity diagram for Tb_yGd_1−y–BBTC are given in Fig. S14. CIE coordinates of the Tb_yGd_1−y–BBTC are presented in Table S9. The excitation spectra of Eu_0.25Gd_0.75–BBTC, Tb_0.5Gd_0.5–BBTC, and Eu_0.015Tb_0.035Gd_0.95–BBTC are shown in Fig. S15. Emission spectra and CIE chromaticity diagram for Eu_0.015Tb_0.035Gd_0.95–BBTC are given in Fig. S16. CIE coordinates of the Eu_xTb_yGd_1−x−y–BBTC are presented in Table S10. Emission spectra of complexes 2, 4 and the doped Eu_0.015Tb_0.035Gd_0.95–BBTC complex are shown in Fig. S17. Luminescence decay profiles of Eu³⁺ and Tb³⁺ in the Eu_0.015Tb_0.035Gd_0.95–BBTC doped complex are presented in Fig. S18. CCDC 1478788, 1478790, 1478791, 1478792 and 1478793 are for 2, 4, 5, 6 and 7, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra22001k

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