Article pubs.acs.org/cm
Cation-Induced Strategy toward an Hourglass-Shaped Cu6I7− Cluster and Its Color-Tunable Luminescence Muxin Yu,†,‡ Lian Chen,*,† Feilong Jiang,† Kang Zhou,† Caiping Liu,† Cai Sun,†,‡ Xingjun Li,† Yan Yang,†,‡ and Maochun Hong*,† †
State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, China 350002 ‡ University of the Chinese Academy of Sciences, Beijing, China 100049 S Supporting Information *
ABSTRACT: We have designed and synthesized a series of twodimensional materials featuring with a (3,6)-connected kgd layer, in which an unprecedented anionic Cu6I7− cluster was first trapped through a cation-induced synthetic strategy. The emission colors of these cluster-based luminophores gradually shift from blue to yellow as the monovalent cations (Li+, Na+, NH4+, K+, TEA+) located between the neighboring layers changed. SCXRD analyses discover that the variation of the emission may be attributed to the transformation of the hourglass-shaped Cu6I7− cluster. The bright, tunable, and broad luminescent emissions make them promising candidates as phosphors for light-emitting diodes (LEDs). Particularly, compound 1-TEA emitting intensive yellow light with high luminescence quantum efficiency (QY = 79.9%) shows extremely high thermal, pH, organic solvent, and mechanical photostabilities. By employing it as a yellow phosphor, we fabricate a series of white lighting materials with high color rendering index (CRI).
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INTRODUCTION Metal ions with d8/d10 electronic configuration often aggregate to polynuclear metal clusters through noncovalent metal··· metal bonding (called metallophilicity), which have attracted considerable attention for their diverse structures and exotic optical functionalities.1−6 The interesting optical, electrical, and chemical properties endow polynuclear metal aggregates with attractive applications in optical sensing,7−9 biological labeling,10 catalysis,11,12 and photoelectric devices.13−15 Recently, particular interests are paid to copper(I)−halide clusters (mostly copper iodides) not only due to their rich photophysical and photochemical properties but also to the advantages of the abundant reserves and low cost of the copper element.16−19 The classical formula of the polynuclear copper(I)−iodide compounds can be expressed as [(CuxIy)x−yLz]n. Experimental and theoretical studies show that the luminescence properties of cluster-based compounds largely depend on the compositions, geometries, nuclear numbers, and sizes of the cluster cores as well as metal··· metal interactions in them.20−24 However, the preparation of polynuclear metal clusters is a long-standing challenge since the assembly involves complicated self-organization processes which are hard to control. Very limited species of copper(I)− iodide clusters, especially those of high nuclearities (x ≥ 6), are unveiled despite a variety of ligands employed, which seriously constrains the further understanding and applications of this kind of materials.25,26 On the other hand, as the neutral © 2017 American Chemical Society
nitrogen or phosphine ligands are commonly used, most of the reported copper(I)−iodide clusters are uncharged (x = y), in which Cu 4 I 4 and Cu 2 I 2 are most stable and usually isolated.27−31 The charged (anionic and cationic) clusters are still underdeveloped due to lack of efficient synthetic strategies.32−34 Herein, we introduce a cation-induced strategy for controllable assembly of anionic copper(I)−iodide clusters, by which a family of coordination compounds (CPs) containing an unprecedented hourglass-shaped Cu6I7− cluster has been systematically synthesized. The luminescence properties and structure−property relationships of these compounds are investigated in detail. It is interesting to find that the cations can not only induce anionic clusters but also participate in the modulation of the luminescence colors of the cluster-based CPs.
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RESULTS AND DISCUSSION Synthetic Strategy. As promising photofunctional materials for solid-state lighting, displaying, sensing, and solar energy harvesting applications, the d10/d8 metal aggregates suffer from poor stability and low room-temperature quantum yields. Assembling metal clusters to coordination polymers has proved an efficient way to improve their stability and quantum Received: May 2, 2017 Revised: September 20, 2017 Published: September 20, 2017 8093
DOI: 10.1021/acs.chemmater.7b01790 Chem. Mater. 2017, 29, 8093−8099
Article
Chemistry of Materials efficiency.35,36 A tripodal ligand tppa (tppa = N,N′,N″-tris(3pyridinyl)phosphoric triamide) was employed to construct extended cluster-based networks for promoting their performance. As mentioned above, uncharged clusters tend to be isolated by using this kind of neutral ligand. Therefore, the addition of monovalent iodide salts was used to change this situation, which was based on the following considerations: (1) positively charged ions may induce and stabilize the anion clusters with novel structures and (2) the extra iodine source may facilitate the generation of CuxIy clusters with x < y. As expected, an extended layer structure with an unprecedented anionic Cu6I7− cluster showing excellent stability and high luminescence efficiency has been obtained. The synthetic experiments were systematically conducted by variation of iodide salts with different cation sizes (LiI, NaI, NH4I, KI, TEAI, TPAI, TEAI = tetraethylammonium iodide and TPAI = tetrapropylammonium iodide). A series of coordination polymers, {Li(H2O)(EtOH)3[Cu6I7(tppa)2]}n (1-Li), {Na(H 2 O)(EtOH) 3 [Cu 6 I 7 (tppa) 2 ]} n (1-Na), {NH 4 (H 2 O)(EtOH)3[Cu6I7(tppa)2]}n (1-NH4), {K(H2O)2(MeOH)2[Cu6I7(tppa)2]·MeCN}n (1-K), {[N(Et)4][Cu6I7(tppa)2]}n (1-TEA), and {[N(Pr)4][Cu6I7(tppa)2]}n (1-TPA) with similar 2D structures based on the same clusters were prepared, suggesting that the synthetic method has a certain degree of universality. The employment of tetrabutylammonium iodide (TBAI) did not get the similar structure, instead, a chain-like compound, [Cu2I2(tppa)2]n (2), with neutral rhombohedral cluster Cu2I2 was obtained, whose structure and properties are shown in Figures S1−S4. We assume that the TBA+ cation is too big to participate in constructing the 2D [Cu6I7(tppa)2]n structures like other six cations. Details of the syntheses are described in the Supporting Information. Phase purities of the CPs were confirmed by elemental analysis and powder X-ray diffraction (PXRD) determination (see the Experimental Section and Figure S5 in the SI). Structure Description. Single-crystal X-ray diffraction (SCXRD) analyses reveal that the five compounds 1-Li, 1Na, 1-NH4, 1-K, and 1-TEA contain similar layer structures based on the same copper halide clusters. And, therefore, only 1-K is discussed in detail as a representative. The compound 1K crystallizes in R3̅c space group, featuring an anionic hexanuclear copper halide cluster Cu6I7−. As Figure 1a shows, six equivalence Cu1 atoms are connected by one center μ6bridging I1 atom and six μ2-bridging I2 atoms, constructing the hourglass-shaped Cu6I7− cluster with two quadrihedrons. Viewed along the c axis, the cluster looks like a double-deck hexagram (Figure 1b). Each Cu atom in the cluster is tetrahedrally coordinated to three iodine atoms and one nitrogen atom from the organic ligand tppa (Figure S6). Consequently, the cluster acts as a 6-connected node and can be further linked by tripodal organic ligands tppa from six directions (three from the upper quadrihedron and the other three from the nether one) to form a layer structure with a binodal (3,6)-connected kagomé dual (kgd) net topology with the Schläfli symbol of (43)2(46·66·83) (Figure 1d), which packs in an ABCA model along the c axis. The main difference of the five compounds is that these negatively charged layers are balanced by different monovalent cations (Li+, Na+, NH4+, K+, and TEA+) which occupy the space between the neighboring layers (Figure 1e). SCXRD reveals the positions of the cations, and the solvents are somewhat disordered, which cannot be completely determined. Nevertheless, it clearly shows that they
Figure 1. Structure presentation of Cu6I7− cluster viewed from side (a) and top (b) as well as its sandglass shape (c), the (3,6)-connected kgd layer structure of the compounds (d), and schematic stacking diagram with different monovalent cations occupying the interlayer space (e). Symmetry code: A (1 − x + y, 2 − x, z), B (2 − y, 1 + x − y, z), C (2 − x, 2 − y, 1 − z), D (y, 1 − x + y, 1 − z), E (1 + x − y, x, 1 − z).
are slightly different in the ways that the cations balance the negatively charged layers for alkali metal and ammonium cations (Figures S7 and S8). Alkali metal cations (Li+, Na+, K+), which possess high affinity to oxygen ions, are located near the oxygen atoms of PO groups from the tppa ligand and their positions are between the opposite PO groups from the neighboring layers, generating alkali metal···oxygen interactions (e.g., the distance of K···O is ca. 2.485 Å). The oxygencontaining solvents (H2O and EtOH or MeOH) are located around the alkali metal ions to complete the coordination spheres. In the absence of this kind of interaction, ammonium cations (NH4+ and TEA+) are situated more randomly in the interlayer space. No hydrogen interactions are found between ammonium cations and the layers. Although the high-quality single crystal of 1-TPA was not obtained for SCXRD, the results of PXRD, elemental analysis, and TGA reveal that 1TPA has a similar structure with the other five compounds (Figures S5 and S9). Photoluminescence. Solid-state photoluminescence measurements were performed on both free ligand and the six compounds at room temperature (Figures 2, S14, and S15). The free tppa exhibits a weak emission band with a maximum peak around 457 nm under excitation at 365 nm. The emissions of these cluster-based CPs have two characteristic features. First, all the compounds display strong and broad single-band emissions with similar shapes. Compared to the free ligand, the luminescences of the CPs red shift more or less and the intensities are significantly enhanced (almost 40-fold above). It is worth noting that the broad emissions are nearly 300 nm fullwidth, giving the materials a great advantage in fabricating WLEDs with high color-rendering index (CRI) values.37 Second, it is interesting that the emission peaks of the CPs red shift regularly along with the variation of the countercations embedded in the layers. When excited at 350 nm, 360 nm, 350 nm, 375 nm, 400 nm, and 395 nm, the peaks of 1-Li, 1-Na, 1NH4, 1-K, 1-TEA, and 1-TPA are 485 nm, 510 nm, 532 nm, 8094
DOI: 10.1021/acs.chemmater.7b01790 Chem. Mater. 2017, 29, 8093−8099
Article
Chemistry of Materials
here may come from the low energy (LE) transfer, which is assigned to a combination of halide-to-metal charge transfers (XMCT) and copper-centered d to s, p transitions. This emission is called “Cluster Centered” (CC) as it is principally based on the cluster core and is essentially independent of the nature of ligands.42 The attribution is evidenced by a high energy (HE) emission arising from the luminescence spectra at low temperature (Figure S23). The different origins of excitation (M/XLCT) and emission (CC) imply the intramolecular energy transfer may occur, which is verified by the relatively long fluorescence emission lifetime (Table 1). Since SCXRD is a powerful tool for elucidating structure− property relationships in highly crystalline materials, detailed single crystal structure analyses are performed to decipher how the countercations affect the luminescent emissions of these CPs. As we know, metallophilic interactions play an important role on the luminescence of d8/d10 polynuclear metal clusters.13,44,45 Since the overall structures of the five CPs are quite similar, we speculate that different countercations affect cuprophilicity in five compounds, thus leading to the variation of the emission peaks. There are two kinds of intracluster Cu··· Cu distances, nominated as Dx and Dy, which determine the width and the height of the cluster, respectively. As Scheme 1 shows, the three Cu(I) ions in the same quadrihedron constitute a regular triangle in parallel with the cluster-based layer and (a, b) plane, in which the Cu···Cu distance is defined as Dx. Whereas, Dy is the distance of Cu(I) ions in the opposite quadrihedrons whose direction is perpendicular to the layer and along the c axis. The selected parameters including Dx, Dy, and the layer distance (determined by the opposite P atoms from the neighboring layers) of five CPs are listed in Table 1. Dx values of the five CPs range from 2.884 to 2.703 Å, which are shorter than or comparable to the sum of the van der Waals radii of Cu(I) (2.800 Å), indicating Cu···Cu interactions may occur between the Cu(I) ions in the same quadrihedron.46,47 Dy is in the range of 4.899 Å to 5.070 Å and much longer than Dx, which suggests there is no metallophilic interactions between the Cu(I) ions in the opposite quadrihedrons and Dx may be primarily responsible for the change of the luminescent colors. When the countercations range from Li+ to TEA+ ions, that is, their sizes tend to expand, the layer distance becomes larger and Dy turns longer accordingly. Meanwhile, the shrink of Dx is observed, leading to the stronger cuprophilic interactions. Previous theoretical and experimental works in the CuI cluster have demonstrated that a change in the Cu···Cu distance is greatly responsible for luminescence variations, especially those of the LE emission bands.40,42,48−50 According to these works, the Cu−Cu bonds in the excited state are of bonding character. As the Cu···Cu distances
Figure 2. Photoluminescence of the five compounds: (a) excitation and emission spectra; (b) CIE-1931 chromaticity diagram; (c) solidstate luminescence image under the 365 nm UV light; (d) LED lamps displayed at working condition.
550 nm, 575 nm, and 580 nm, respectively. The CIE-1931 emission profile shows that the footprints of the luminescent colors range across cyan, green, green-yellow, and finally yellow. The above features in the luminescence spectra suggest that (1) the origin of emissions of the CPs is similar and (2) the luminescences of cluster-based CPs can be tuned by exclusively changing the monovalent cations between the layers. Besides, the photoluminescence spectra of these compounds dispersed in polar solvent EtOH and nonpolar solvent cyclohexane were also measured (Figures S16 and S17), which are almost same with their solid-state spectra. The results of DOS (densities of states) calculations (Figures S18−S22) show that the five compounds exhibit similar valence bands (VBs) and conduction bands (CBs), which is corresponding to their isomorphic structures. Take 1-K for example: the VBs near the Fermi level are principally dominated by the 3d orbitals of the copper ions, mixed partly with 5p orbitals of iodide ions and p−π* orbitals of the ligands. Whereas, the CBs are composed mainly of the ligands p−π* antibonding orbitals and a little amount of the potassium cations 4s orbitals and copperions 4s, 4p orbitals.38,39 Thus, the excitation could be attributed to the energy transfers consisting of metal-to-ligand charge transfers (MLCT) and halide-toligand charge transfers (XLCT) primarily.40,41 The luminescence emission of copper(I)−halide clusters commonly has two bands, high energy (HE) and low energy (LE).21 According to the literature,42,43 the emissions of Cu6I7− cluster-based CPs
Table 1. Selected Structure Parameters and Photoluminescence Data of the Compounds with Different Cations cation Li layer distance (Å) Dy (Å) Dx (Å) emission peaks (nm) CIE QY (%) τ298K (μs) τ77K (μs)
+
9.031 4.899 2.884 485 0.21, 0.33 27.8 ± 0.5 8.09 14.8
Na
+
9.474 4.935 2.822 510 0.24, 0.41 29.8 ± 0.2 2.88 6.56
NH4+
K+
TEA+
TPA+
9.678 4.954 2.791 532 0.31, 0.52 24.2 ± 0.3 5.90 6.30
9.941 4.977 2.779 550 0.40, 0.53 51.2 ± 1.0 7.51 8.60
10.224 5.070 2.703 575 0.46, 0.51 79.9 ± 0.9 7.45 8.08
580 0.48, 0.50 80.6 ± 0.5 16.5 16.6
8095
DOI: 10.1021/acs.chemmater.7b01790 Chem. Mater. 2017, 29, 8093−8099
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Chemistry of Materials Scheme 1. Illustration of the Variation Tendency of the Cu6I7− Clusters in the Five CPs with Different Cations
become shorter, the bonding character increases, and thus the energy level is lowered and the energy gap between the excited states and the ground state is narrowed, finally resulting in the red shifts of the emission peaks (Scheme S1).21,51−55 The relationship between the Cu···Cu interactions and the emissions in our study is in accord with the previous observations. The transformation of the Cu6I7− cluster is illustrated in Scheme 1, showing the flexibility of the hourglassshaped Cu6I7− cluster. With the expansion of the cation size, higher but thinner clusters are obtained, which is reflected in the decrease of the a (b) axis and the increase of the c axis. It is worth mentioning that Cu6I7− is a rare example of copper(I)− iodide clusters that shows flexibility in two dimensions (height and width). And as shown in Scheme S2, the structure parameters and emission wavelengths can be linearly stimulated, which further confirms the inherent relationship between their luminescence emissions and cation-tuned structures. Consequently, we not only realize anionic cluster synthesis through cation injection but also achieve PL color tunability by adjustment of cation sizes. Different from previous investigations in which emission colors are tuned by altering the main structures, this color modulation strategy through simply tuning the cations (guests) between the layered structures (hosts) is more convenient and controllable. Photostability. The quantum yields (QYs) of the six CPs are listed in Table 1, in which 1-TEA and 1-TPA show extremely high QYs with values up to 79.9 ± 0.9% and 80.6 ± 0.5%, respectively. The high QYs of 1-TEA and 1-TPA may be ascribed to the absence of solvents in their structures which reduces nonirradiation transitions derived from the thermal vibrations of the solvents. TGA analyses suggest that the thermal decomposition temperature (300 °C) of 1-TEA is much higher than the other CPs (Figure S9). The superiority of 1-TEA in the luminescence efficiency and thermostability inspires us to further investigate its photostability which is a key requirement for a material in practical applications. A series of photostability tests were conducted on 1-TEA to evaluate its suitability for lighting devices. PL intensities of five CPs after heating at 120 °C in the air without any protection are shown in Figure 3a. The result shows that luminescence of 1-TEA is
Figure 3. Photostability tests of 1-TEA: PL intensities of the five compounds after heating at 120 °C for 48 h (a), PL intensities of 1TEA after immersing in pH 1−14 buffer solutions for 24 h (b) and after soaking in various solvents for 35 days (c), and emission spectra of 1-TEA ground before and after ball milling for 30 min (d).
more stable than that of the other four CPs and has a neglectable change in its intensity after heating at 120 °C for 48 h, suggesting its high photostability to heat. Chemical photostability is assessed by soaking the fresh samples in the aqueous solutions of pH = 1−14 for 24 h, followed by photoluminescence test (Figure 3b). The result suggests that the emission intensity of 1-TEA can bear the acidic/basic conditions in a wide pH range (pH = 2−12). The photostability to solvents is also evaluated by comparing the PL intensities of 1-TEA before and after soaking in a given solvent for 35 days. The result shows that the emissions of the compound possess high stabilities in diethyl ether (DEE), hexane (Hex), dichloromethane (DCM), methanol (MeOH), water (H2O), and ethanol (EtOH) (Figure 3c). After ball milling for 30 min, PL intensity is even a little stronger compared to the original sample (Figure 3d). Furthermore, 1TEA can keep its PL intensity unchanged after exposure to the open air for six months (Figure S26). Generally speaking, 1TEA has a supremely high photostability toward thermal, air, 8096
DOI: 10.1021/acs.chemmater.7b01790 Chem. Mater. 2017, 29, 8093−8099
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Chemistry of Materials
The series of two-component white phosphors were then made into LEDs. The corresponding electro-luminescence characteristics are shown in Figure S32, and the important numerical data are listed in Table S1. Among the five white LEDs devices, the best performance was observed in the LED 3 with 15.9 cd/A current efficiency (CE) and 6.6% external quantum efficiency (EQE). Besides, the CRIs of the five LEDs are all above 75, varying from 76.1 to 86.5, which further verifies that the 1-TEA is a great candidate as a yellow phosphor for high CRI values.
acidic/basic condition, organic solvents, and mechanical force. The PXRD patterns show the framework of 1-TEA remains unchanged after the above photostability tests, indicating its supreme framework stability (Figures S27−S31). The excellent photostability and high quantum efficiency make 1-TEA a good candidate as RE-free and noble-metal-free lighting phosphors. White Phosphors Fabrication. To evaluate the potential application of 1-TEA as a yellow phosphor, a series of twocomponent white phosphors were fabricated by mechanically blended 1-TEA (YP) with commercial blue phosphor BAM: Eu2+ (BP). The photoluminescence results for the phosphors with different contents of 1-TEA are shown Figure 4, and the
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CONCLUSIONS Monovalent iodide salts with different cations are introduced to systematically prepare a family of two-dimensional materials with an unprecedented anionic Cu6I7− cluster. It is interesting that the hourglass-shaped Cu6I7− cluster is flexible in two dimensions leading to gradient photoluminescence colors according to the cations employed. The cations play an important role in this study: (1) as inducers creating a positive atmosphere to induce novel anion Cu6I7− clusters; (2) as guests intercalating into the layers to control the layer distances and bond lengths; and (3) as modulators systematically adjusting the emission colors by changing the metallophilic interactions. The unprecedented Cu6I7− series can be considered as a class of promising phosphors attributed to their RE-free, low-cost, and environmentally friendly synthetic procedures as well as conveniently tunable luminescent properties. Moreover, the superb photostability and high quantum efficiency of 1-TEA make it an excellent candidate for solid-state lighting. The design strategy opens up a new avenue for the exploration of photofunctional copper(I)-cluster-based materials with novel structures and high performances.
Figure 4. Luminescence spectra (a), CIE coordinations (b), and photoluminescence image under 365 nm UV light (c) of the BP (BAM:Eu2+), YP (1-TEA), and the two-component phosphors with different weight percentages of YP.
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S Supporting Information *
Table 2. CIE, CRI, CCT, and QY of the Prepared White Phosphors (YP = yellow phosphor) YP amount (wt %)
excitation (nm)
0
360
1.7
370
2.6
380
4.1
380
7.5
380
8.0
360
CIE 0.14, 0.06 0.30, 0.29 0.32, 0.32 0.33, 0.33 0.34, 0.35 0.36, 0.37
CRI
CCT (K)
-
-
77.6 ± 0.7
87.4
8189
43.7 ± 0.9
82.4
5994
40.3 ± 0.6
81.3
5711
46.9 ± 0.5
77.4
5177
34.6 ± 0.3
75.1
4708
34.4 ± 1.2
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01790. Experimental details on methods and syntheses, further characterization results, and DFT calculations (PDF) Crystallographic data of 1-Li, 1-Na, 1-NH4, 1-K, 1-TEA, and 2 (CCDC 1537508-1537512 and 1561914) (CIF)
QY (%)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Lian Chen). *E-mail:
[email protected] (Maochun Hong). ORCID
Maochun Hong: 0000-0002-1347-6046 important parameters are summarized in Table 2. The weight percentages of YP in the white phosphors are always less than 8%, implying that 1-TEA is really efficient as YP. In addition, the white phosphors are well-tuned, with the correlated color temperature (CCT) decreasing from 8189 to 4708 K and the color rendering index (CRI) increasing from 75.1 to 87.4, meeting diverse requirements for lighting use. In comparison with commercial white LEDs made by yellow phosphor YAG:Ce3+, which usually has a low CRI of
