Crystal structures and luminescent properties of new lanthanide(III) complexes derived from 2-phenyl-4-pyrimidinecarboxylate

Abstract

In this work, five novel lanthanide(III) coordination polymers derived from 2-phenylpyrimidine-4-carboxylic acid (Hppmc), namely, [Ln(ppmc)₃(H₂O)₂]·2H₂O [Ln = Eu (1), Tb (2)] and [Ln(ppmc)₃(H₂O)₂] [Ln = Eu (3), Gd (4), Tb (5)] were successfully synthesized by a facile solution method and characterized by single-crystal X-ray diffraction, power X-ray diffraction (PXRD), infrared (IR) spectroscopy, elemental analysis, and thermogravimetric analysis (TGA). It was found that subtly different reaction conditions result in disparate structural characteristics. For example, by combining Hppmc with lanthanide(III) ions at room temperature, compounds 1 and 2 featuring a carboxylate-bridging chain structure, in which the carboxylates adopt both chelating and bridging modes, are isolated. However, the reaction at 35 °C generates three isostructural compounds 3–5 with a distinct chain structure, in which the lanthanide ions are connected by carboxylates via syn–syn and syn–anti modes. Photoluminescent studies of the Eu³⁺ and Tb³⁺ complexes reveal that the Hppmc ligand is a better sensitizer for Tb³⁺ ion than for Eu³⁺ ion. The investigation of the relationship between the crystal structures and the photoluminescence properties indicate that the coordination environments of lanthanide ions and the arrangement of the ligands are the dominating factors that affect the luminescence behaviors of the solid samples.

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Introduction

Current interest in the design and construction of lanthanide-based coordination polymers has led to increasing attention in this area not only because of their aesthetically appealing architectures but also because of their intriguing photoluminescence properties.¹ Their pure emission colors, sharp emission lines, and long excited-state lifetimes² have received much attention and inspired numerous applications in various technological and research areas such as electroluminescent devices,³ analytical tools,⁴ and medical imaging devices.⁵ The luminescence properties displayed by almost all lanthanide ions are due to the f–f transitions. However, the transition of the 4f electron into the 4f* excited state is forbidden by the selection rules, leading to weak or non-existent emission bands. Thus, the use of organic chromophores as “antenna” ligands is often necessary and desirable. The ligands absorb energy in the ultraviolet (UV) region of spectrum, transferring it to stimulate Ln³⁺ emission in a process known as the antenna effect.⁶ The energy match for the intramolecular energy migration efficiency from the lowest triplet energy level of the ligand to the resonant energy level of the lanthanide ion plays the most important role.⁷ Furthermore, the photoluminescent properties can be affected by other factors, including the coordination environment of the central lanthanide,⁸ the arrangement of chromophoric units,⁹ the oscillation of –OH, –NH, C–H bonds¹⁰ and so on. So far, a lot of efforts have been made to improve the fluorescence of lanthanide coordination polymers, such as selecting organic ligands with conjugated motifs,¹¹ synthesizing mixed-lanthanide¹² or mixed lanthanide-transition-metal compounds,¹³ and regulating the structures and the photoluminescent properties through postsynthetic methods.¹⁴ Though the final structure of lanthanide compounds can be influenced by reaction conditions, such as solvents,¹⁵ reaction time,¹⁶ and temperature,¹⁷ the lanthanide compounds have a degree of structural predictability and tunability which provide a new platform for studying the relationship between the crystal structures and the photoluminescent properties. In our initial reports, 2-phenylpyrimidine-4-carboxylic acid (Hppmc) proved suitable for the construction of lanthanide coordination polymers with interesting magnetic and photoluminescent properties.^18,19 In order to further study the relationship between the crystal structures and the photoluminescent properties, we explore in the present study the potential for improving the photoluminescence properties of the lanthanide coordination polymers with different structures by changing the synthetic conditions. Herein, we report a series of new lanthanide coordination polymers obtained under different synthetic conditions, which exemplify two unique structural types: [Ln(ppmc)₃(H₂O)₂]·2H₂O [Ln = Eu (1), Tb (2)] for type I and [Ln(ppmc)₃(H₂O)₂] [Ln = Eu (3), Gd (4), Tb (5)] for type II. These compounds were characterized by infrared (IR) spectroscopy, elemental analysis (EA), thermogravimetric analysis (TGA), and X-ray single-crystal and power structural analyses. Furthermore, the fluorescence properties of the compounds were studied in detail. The distinct characteristics of the two structural types of these new compounds provided an excellent opportunity for probing the influence factors of the luminescence properties of the solid samples, including the type and the coordination environment of the metal centers and the arrangement of lumophores.

Experimental section

Synthesis

All the starting materials were commercially available reagents of analytical grade and used without further purification. The Hppmc ligand was prepared according to literature methods.²⁰

[Eu(ppmc)₃(H₂O)₂]·2H₂O (1). Eu(Ac)₃·6H₂O (32.9 mg, 0.1 mmol) was added to a 3 mL aqueous solution containing Hppmc (60.0 mg, 0.3 mmol) and NaOH (3 mL, 0.1 mol L⁻¹). The resulting solution was stirred for another 10 min to give white precipitate. After adding water/ethanol (1 : 5, 15 mL), the resulting mixture was heated for another 5 min to give a clear solution. Upon filtration, the resulting colorless solution was left to evaporate at room temperature for several days before giving rise to colorless columnar crystals of complex 1, which were filtered off and air-dried. Yield, 32.0 mg (38.4% based on Eu³⁺). Elemental analysis for C₃₃H₃₉EuN₆O₁₀ (%): C, 48.24; H, 3.56; N, 10.23. Found (%): C, 47.93; H, 3.70; N, 9.76. Selected IR (KBr, cm⁻¹): 3412(w), 1602(s), 1552(s), 1462(s), 1407(s), 743(s).

[Tb(ppmc)₃(H₂O)₂]·2H₂O (2). Compound 2 was synthesized following a procedure similar to the synthesis of 1, with the only exception that Tb(Ac)₃·6H₂O was used instead of Eu(Ac)₃·6H₂O. Yield, 33.0 mg (38.5% based on Tb³⁺). Elemental analysis for C₃₃H₃₉TbN₆O₁₀ (%): C, 47.84; H, 3.53; N, 10.14. Found (%): C, 47.86; H, 3.47; N, 9.95. Selected IR (KBr, cm⁻¹): 3412(w), 1600(s), 1549(s), 1460(m), 1408(s), 741(s).

[Ln(ppmc)₃(H₂O)₂] [Ln = Eu (3), Gd (4), Tb (5)]. Ln(NO₃)₃·6H₂O (0.1 mmol) was added to a 3 mL aqueous solution containing Hppmc (60.0 mg, 0.3 mmol) and NaOH (3 mL, 0.1 mol L⁻¹). The resulting solution was stirred for another 10 min, water/ethanol (1 : 1, 18 mL) was added, and the mixture was heated until the precipitate disappeared. Upon filtration, the resulting solution was kept at 35 °C in the oven for several days before giving rise to colorless needle-shaped crystals of complexes 3–5, which were filtered off and air-dried. Yield, 50.7%, 50.7% and 47.8% based on lanthanide ions, for 3, 4 and 5, respectively. Elemental analysis for 3 (%): C, 50.46; H, 3.21; N, 10.70. Found (%): C, 50.42; H, 3.13; N, 10.51. Selected IR for 3 (KBr, cm⁻¹): 3398(w), 1702(m), 1632(s), 1548(s), 1459(m), 1407(s), 743(s). Elemental analysis for 4 (%): C, 50.12; H, 3.19; N, 10.63. Found (%): C, 50.06; H, 3.16; N, 10.43. Selected IR for 4 (KBr, cm⁻¹): 3399(w), 1703(m), 1632(s), 1548(s), 1459(m), 1408(s), 743(s). Elemental analysis for 5 (%): C, 50.01; H, 3.18; N, 10.60. Found (%): C, 49.97; H, 3.11; N, 10.36. Selected IR for 5 (KBr, cm⁻¹): 3399(w), 1706(m), 1633(s), 1548(s), 1459(m), 1407(s), 743(s).

X-ray crystallography and physical measurement

Single-crystal X-ray diffraction data of compounds 1–5 were collected at 293 K using a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). All of the structures were solved by direct methods and refined with the full-matrix least-squares technique based on F² using the SHELXL program.²¹ All non-hydrogen atoms in the complexes were refined with anisotropic displacement parameters. Hydrogen atoms of the water molecules were located from the difference Fourier maps and refined with restraint of the O–H and H⋯H distances (0.96 and 1.52 Å, respectively). Other hydrogen atoms were placed at the calculation positions. A summary of the crystallographic data and details of the structure refinements are given in Table 1. Selected bond lengths and bond angles are listed in Tables S1 and S2.† Power X-ray diffraction data for all samples were collected at room temperature on bulk samples with Cu Kα radiation (λ = 1.54059 Å).

Table 1 Crystal data and structure refinement parameters for complexes 1–5

Compound	1	2	3	4	5
Formula	C33H29EuN6O10	C33H29TbN6O10	C33H25EuN6O8	C33H25GdN6O8	C33H25TbN6O8
F w	821.58	828.54	785.55	790.84	792.51
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P21/n	P21/n	P21/c	P21/c	P21/c
a (Å)	13.329(1)	13.376(1)	15.873(1)	15.896(1)	15.860(2)
b (Å)	10.348(1)	10.305(1)	22.954(2)	22.952(2)	22.849(3)
c (Å)	23.849(2)	23.900(2)	8.926(1)	8.930(1)	8.889(1)
α (°)	90	90	90	90	90
β (°)	92.199(2)	92.146(1)	104.803(1)	104.757(1)	104.730(2)
γ (°)	90	90	90	90	90
V (Å^3)	3287.0(6)	3291.9(4)	3144.2(4)	3150.9(5)	3115.4(6)
Z	4	4	4	4	4
µ (mm^−1)	1.976	2.216	2.057	2.167	2.333
F(000)	1648	1656	1568	1572	1576
GOF	0.965	1.007	1.024	1.064	1.039
Data collected	19 708	19 630	19 003	19 100	18 881
Unique	7537	7489	7230	7211	7200
R int	0.0656	0.0514	0.0317	0.0350	0.0360
R 1 [I > 2σ(I)]	0.0482	0.0472	0.0280	0.0279	0.0294
wR2 [I > 2σ(I)]	0.0994	0.1064	0.0280	0.0657	0.0625
R 1 [all data]	0.0982	0.0828	0.0371	0.0429	0.0424
wR2 [all data]	0.1182	0.1228	0.0693	0.0811	0.0669

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Elemental analyses of carbon, hydrogen and nitrogen were performed on an ElementarVario El analyzer. The FTIR spectra were recorded with a Nicolet Magna 750 FT/IR spectrometer with pressed KBr pellets in the 4000–400 cm⁻¹ regions. Thermogravimetric analyses (TGA) were carried out using a Mettler-Toledo TGA/DSC1. Fluorescence spectra were recorded on an RF-5301PC spectrophotometer, phosphorescence spectra were measured with F-7000 and the emission quantum yields (Φ_QY) were measured with an Edinburgh Instrument fls920 spectrometer.

Result and discussion

Crystal structures

Structure of [Ln(ppmc)₃(H₂O)₂]·2H₂O [Ln = Eu (1), Tb (2)]. X-ray structure analyses reveal that 1 and 2 are isostructural and crystallize in the monoclinic space group P2₁/n. As a representative example, the crystal structure of 1 is described in detail. As shown in Fig. 1a, the asymmetric unit of 1 contains one crystallographically independent Eu³⁺ ion, three ppmc⁻ ligands, two coordinating water molecules, and two solvating water molecules. Each Eu³⁺ ion is eight-coordinated by two water molecules and six carboxylate oxygen atoms from five different ppmc⁻ ligands. The coordination geometry around Eu³⁺ ion is a slightly distorted square-antiprism (Fig. S1a†). The Eu–O bond lengths range from 2.325 to 2.505 Å and the bond angles of O–Eu–O are in the range of 65.65–160.0°, comparable to those in other Eu³⁺ carboxylate-based complexes observed in previous reports.²² Owing to the effect of lanthanide contraction,²³ the Tb–O bonds in 2 are slightly shorter than the corresponding Eu–O bonds in 1. The ppmc⁻ ligands are divided into two types, one acting as a chelating terminal ligand while the other serving as a bridging one. Two bridging ppmc⁻ ligands connect adjacent Eu³⁺ ions in a syn–anti mode, forming a one-dimensional (1D) chain running along the b direction with a Ln⋯Ln distance of 5.530 and 5.503 Å for 1 and 2, respectively (Fig. 1b). The adjacent chains are connected to form a two-dimensional (2D) layer through π–π interactions between the bridging ppmc⁻ ligands with a centroid⋯centroid distance of 3.656 and 3.660 Å for 1 and 2. Moreover, neighboring layers are further linked, also by π–π stacking between the terminal ppmc⁻ ligands, and they feature a centroid⋯centroid separation of 4.172 and 4.161 Å for 1 and 2, thus producing a 3D supramolecular structure (Fig. S1b†).

Fig. 1 Coordination environment of Eu³⁺ (a) and the 1D chain connected by ppmc⁻ (b) in 1 (symmetry code: a: 3/2 − x, y − 1/2, 3/2 − z; b: 3/2 − x, y + 1/2, 3/2 − z; the hydrogen atoms are omitted for clarity).

Structure of [Ln(ppmc)₃(H₂O)₂] [Ln = Eu (3), Gd (4), Tb (5)]. Compounds 3–5 are isostructural and crystallize in the monoclinic P2₁/c space group. Here, the crystal structure of the compound 3 is described as an example. The asymmetric unit of 3 is composed of one lanthanide ion, two coordination water molecules and three ppmc⁻ ligands (Fig. 2a). Similar to 1 and 2, the lanthanide ions in 3 are also eight-coordinated by eight oxygen atoms (six from six ppmc⁻ ligands and two from water molecules), resulting in a distorted square antiprism coordination geometry (Fig. S2a†). The Eu–O bond lengths range from 2.330 to 2.546 Å, which are typical for Eu³⁺ complexes.²⁴ The mean Ln–O bond lengths in 3–5 follow the order of Eu–O (2.410 Å) > Gd–O (2.400 Å) > Tb–O (2.379 Å), consistent with the lanthanide contraction effect (Table S2†).²³ In contrast to those found in compounds 1 and 2, all the ppmc⁻ ligands in 3–5 adopt bidentate bridging mode. Three ppmc⁻ ligands (one in syn–anti mode and the other two in syn–syn mode) link lanthanide ions to form a 1D chain extending along the c-axis direction (Fig. 2b). The distances between the neighboring Ln³⁺ ions are 4.592 and 4.953 Å for 3 and 4, respectively, which are significantly shorter than the Ln⋯Ln separation in complexes 1 and 2. This discrepancy is probably due to the different amounts of bridging ligands between two mental ions.²⁵ The more bridging ligands, the shorter the metal–metal distance, since the ligands are expected to reduce cation–cation repulsion.²⁵ The adjacent chains are linked to each other via hydrogen bond between the nitrogen atom of pyrimidine ring and coordination water molecules (N4⋯H7C = 1.951(0) Å, O7⋯N4 = 2.868(0) Å, ∠O7–H7C⋯N4 = 166.4(3)°, 1 − x, y + 1/2, 3/2 − z) to yield a slab extending along the ab plane (Fig. S2b†).

Fig. 2 Coordination environment of Eu³⁺ (a) and the 1D chain connected by ppmc⁻ (b) in 3 (symmetry code: a: x, 1/2 − y, z + 1/2; b: x, 1/2 − y, z − 1/2; the hydrogen atoms, nitrogen atoms and partial carbon atoms are omitted for clarity).

Powder XRD pattern, TGA data and IR

The simulated and experimental PXRD patterns of compounds 1–5 obtained at room temperature are presented in Fig. 3. Their peak positions correspond well with the simulated PXRD patterns calculated based on single crystal diffraction data, indicating that all of the samples were in a pure phase. The differences in intensity may be due to the preferred orientation of the power samples.²⁶

Fig. 3 Powder X-ray diffraction profiles of 1–5 together with the corresponding patterns derived from the single crystal data.

The thermal stability of 1–3 and 5 was examined by TGA in a N₂ atmosphere using single-phase polycrystalline samples at a heating rate of 5 °C min⁻¹. As shown in Fig. S3,† all samples showed two obvious steps of weight loss from 25 °C to 800 °C. Compounds 1 and 2 are isomorphous and exhibit similar thermal behavior. They gradually lost their lattice water molecules at about 60 °C. After the loss of the lattice water molecules, the coordinating water molecules begin to be liberated. A total weight loss of 8.8% was observed for 1 and 2 (calculated 8.8% for 1 and 8.7% for 2). In contrast, for compounds 3 and 5, the weight loss associated with the coordinating water molecules occurs between 75 °C and 155 °C (weight loss 4.7%, calculated 4.6%). The weight loss above 270 °C is attributed to the thermal decomposition of the organic components. The remaining weight of 21.1–22.6% indicates that the final products are Eu₂O₃ for 1 and 3 and Tb₄O₇ for 2 and 5 (calculated 21.4%, 22.6%, 22.4%, 23.6% for 1, 2, 3 and 5, respectively).

The IR spectra of 1–5 are quite similar. They all show strong and broad absorption bands around 3400 cm⁻¹, which may be assigned to the characteristic peaks of OH stretching vibration. The strong vibrations located at around 1630, 1550 and 1407 cm⁻¹ correspond to the asymmetric and symmetric stretching vibrations of the carboxylate group of ppmc⁻ ligands, respectively.

Photoluminescence properties

It is well-known that the luminescence of Ln³⁺ ions is greatly influenced by the intramolecular energy transfer from the triplet state of ligands to the resonance level of Ln³⁺ ions.²⁷ Herein, the triplet-state energy of Hppmc ligands of the Eu³⁺ and Tb³⁺ complexes was estimated from the Gd³⁺ complex by measuring the phosphorescence spectra at the low temperature (77 K).²⁸ The phosphorescence spectrum of 4 was measured and shown in Fig. S4.† Judging from the onset of the emission bands (433 nm),²⁵ the energy of lowest triplet state can be determined to be 23 094 cm⁻¹ for Hppmc. This energy is higher than the resonant energy level of Eu³⁺ (18 674 cm⁻¹) and Tb³⁺ (20 500 cm⁻¹), confirming the suitability of the ligands as sensitizers for the excitation of Eu³⁺ and Tb³⁺. The four compounds examined show characteristic red or green emission attributed to Eu³⁺ and Tb³⁺ under the ultraviolet light.

The solid-state luminescent property of 1–3 and 5 were investigated at room temperature. Fig. 4 and S5† show the normalized excitation and emission spectra registered at room temperature. The broad excitation band in the 250–450 nm range can be attributed to the π–π* transition of the conjugated organic ligand and possible ligand-to-metal charge transfer (LMCT) state.^18,29 When 1 and 3 were excited at 354 nm and 332 nm, respectively, their solid-state emission spectra exhibited typical emission patterns characteristic of Eu³⁺ ions with narrow, sharp and well-separated bands which are assigned to the intra-⁴f₆ electron transitions of Eu³⁺. The emissions that appeared in the range of 450–800 nm can be ascribed to the ⁵D₀ → ⁷F_n (n = 1, 2, 3, 4) transitions of the Eu³⁺ ion. The ⁵D₀ → ⁷F₁ (580 nm) transition is a magnetic dipole transition, the intensity of which is insensitive to the coordination environment. The ⁵D₀ → ⁷F₂ (616 nm) is an electric dipole transition, which is extremely sensitive to the coordination environment.³⁰ The integrated intensity ratio of the ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transitions is 4.0 and 2.3 for 1 and 3 respectively, indicating that the Eu³⁺ ions occupy a low-symmetry site without an inversion center,³¹ which is in good agreement with the crystallographic analysis.

Fig. 4 Solid-state excitation/emission spectra and emission decay patterns (insets) of 1 (a) and 2 (b).

Under excitation of 334 nm, complexes 2 and 5 exhibit spectra characteristic of Tb³⁺ ion from the emitting level (⁵D₄) to the ground multiplet (⁷F_6–2). The emission positions of 2 and 5 are similar except for slight differences in the photoluminescence intensities. The emission spectrum shows five emission bands at 490, 543, 585, 619 and 646 nm, respectively. The most intense emission is dominated by the ⁵D₄ → ⁷F₅ transition at 543 nm, which results in green fluorescence.

To better understand the luminescent properties of the four compounds, the lifetime and quantum yield were determined from the luminescence decay profiles. For all compounds, the decay curves were well fitted to a single-exponential function (see the insets of Fig. 4 and S5†), indicating the preference of a single chemical environment around the Ln³⁺ ions. The result fits well with the X-ray structure analyses. Moreover, as listed in Table 2, the lifetimes for Tb³⁺-containing complexes 2 and 5 are longer than those found for Eu³⁺-containing compounds 1 and 3. The result of lifetime measurements appeared to follow the same trend as the quantum yield, which can be attributed to the energy difference between the triplet energy level of ppmc⁻ ligands and the resonance level of Ln³⁺ ions. The energy difference for Eu³⁺ is significantly larger (4420 cm⁻¹) than that of Tb³⁺ ion (2594 cm⁻¹),³² indicating that ppmc⁻ ligand is more suitable for tuning the photoluminescence of Tb³⁺ than Eu³⁺. It is worth noting that this result is in sharp contrast to the prediction we previously made,¹⁹ which omitted the role that the secondary ligand plays in the intramolecular energy transfer process. Since the triplet state of ppmc⁻ is higher than that of 1,10-phenanthroline (phen) (22 132 cm⁻¹), the secondary ligand used in our earlier study, it is reasonable to presume that the phen molecule may act as an energy channel to transfer the energy from the triplet state of ppmc⁻ to the resonance level of Ln³⁺ ions.³³ The lowest triplet energy level of phen ligand matches better the resonance levels of Eu³⁺ than those of Tb³⁺.

Table 2 Lifetimes and quantum yields of complexes 1–3 and 5

	Φ QY	λ ex (nm)	τ (ms)
1	7.5%	354	0.34(3)
2	36.1%	334	0.51(1)
3	6.6%	332	0.40(2)
5	19.5%	334	1.03(2)

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Interestingly, compounds 5 has a much longer lifetime than 2, which can be attributed to the distinct coordination environments of the metal centers in these two compounds. The Tb³⁺ ions in 5 coordinate to six ppmc⁻ ligands and two water molecules, while the asymmetric unit of 2 comprises five ppmc⁻ ligands, two coordination water molecules, and two guest water molecules. Thus the Tb³⁺ in 5 has more “antenna” ligands that can absorb more light energy. In addition, there is no guest water molecular in the structure of 5 that virtually decreases the nonradiative decay rate and increases the life time of the sample.³⁴ Therefore, we reason that a higher content of “antenna” ligands and less deactivation, allowing 5 to exhibit a longer lifetime than compound 2.^25,35 However, it is surprising that complex 2 has a lower content of “antenna” ligands and more water molecules, and yet shows a relatively higher quantum yield than 5. The result is also supported by the photographs of the compounds taken under the same UV lamp (Fig. S6†). This seemingly contradicting result might be due to the π–π interactions between interdigitating ppmc⁻ ligands in 2, which bring the luminophores closer and enable energy transition more effectively.³⁶ In addition, the phenyl substituent of the ligands might be locked into fixed positions, which reduces the elevation of the nonradiative transition.¹⁹ These are also likely reasons for compound 1 having a shorter lifetime and higher quantum yield than 3.

Conclusions

In summary, by virtue of fine-tuning the reaction condition, we have employed 2-phenylpyrimidine-4-carboxylic acid as an antenna ligand to successfully synthesize five Ln³⁺ coordination polymers featuring distinct structural motifs. The carboxylate groups of the Hppmc ligand in these compounds adopt various coordination or bridging mode. The photophysical properties, including the excitation and emission spectra, low temperature phosphorescence spectra, lifetime, and quantum yield of these compounds have been thoroughly studied. It is worth highlighting that the different photoluminescent properties are attributed to the different structural characteristics of the compounds, such as the number of the “antenna” and water molecules and their supramolecular architectures. These factors affect the luminescence behavior of lanthanide coordination polymers in a manner that is amenable to synthetic manipulations. This work thus provides important insight into understanding the relationship between structure and luminescence behavior of these systems. We are currently applying these principles to design new generations of compounds with enhanced photoluminescent properties that may lead to a range of practical applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21171023), National Key Basic Research Program of China (2013CB933402), and Beijing Higher Education Young Elite Teacher Project. Z. W. acknowledges the financial support of the University of South Dakota and South Dakota EPSCoR.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The detail of the selected bond lengths and detail of hydrogen bonds, coordination environment, 3D supramolecular structure and the photoluminescence data, and X-ray crystallographic data in CIF format of compounds 1–5. CCDC 1420293–1420297. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra21607a

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