Constructing multi-cluster copper(I) halides using conformationally flexible ligands

Muxin Yu ab, Caiping Liu b, Shengchang Li b, Yunfang Zhao b, Jiangquan Lv a, Zhu Zhuo bc, Feilong Jiang b, Lian Chen *b, Yunlong Yu *a and Maochun Hong b
aOrganic Optoelectronics Engineering Research Centre of Fujian's Universities, College of Electronics and Information Science, Fujian Jiangxia University, Fuzhou, Fujian 350108, China. E-mail:
bState Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. E-mail:;
cXiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021, China

Received 6th April 2020 , Accepted 23rd May 2020

First published on 26th May 2020

Three unprecedented multi-cluster copper(I) halides (MCCHs) have been assembled using conformationally flexible ligands. Further explorations demonstrate that the conformational compliance of the ligands may be the key to trap and stabilize the various copper(I)-halide clusters in one system, which opens a new way for the construction of multi-Cu(I)-cluster complexes. Moreover, the MCCHs show distinctive temperature-dependent photoluminescence.

The transition metal ions Cu(I)/Ag(I)/Au(I) with a d10 electronic configuration often aggregate into clusters with diverse structures and fascinating properties.1 Among them, Cu(I) clusters have attracted extensive attention due to their high abundance, low cost, low toxicity, and high-quality luminescence.2 Recently, a new and significant branch of Cu(I) clusters containing two or more kinds of Cu(I) clusters in one system has emerged.3 This novel species of Cu(I) clusters will not only enrich structural diversity but also promote the development of multifunctional materials. For instance, Li's group has incorporated Cu2I2 and Cu6Pz6 to obtain a self-calibrated luminescent molecular thermometer.3b Our group has also reported a multi-Cu(I)-cluster MOF with Cu4I4 and Cu6S6 as the functional groups exhibiting both the thermochromic and near-IR characters.3c However, very limited species of the multi-Cu(I)-cluster complexes have been unveiled and only one effective method has been discovered to date: using bi- or multi-functional ligands to connect different clusters.3 The lack of available synthetic strategies seriously constrains the further understanding and exploration of this kind of complex. Hence, developing new synthetic methods for multi-Cu(I)-cluster complexes remains an urgent and critical challenge.

Recent studies have revealed that Cu(I)-halide clusters have attractive potential in photoelectric devices,4 optical sensing5 and photocatalysis.6 However, multi-Cu(I)-cluster complexes combining different types of Cu(I)-halide clusters, named multi-cluster Cu(I) halides (MCCHs), are very rare.7 Herein, we report two kinds of MCCHs assembled using a conformationally flexible ligand TPPA (Fig. S1, ESI). MCCH-1 contains neutral Cu4I4 and Cu7I7 clusters simultaneously, while MCCH-2 is composed of cationic Cu6I5+ and anionic Cu6I7 clusters. The structural analyses and theoretical calculations reveal that the multiple conformations of the flexible ligand may offer more opportunities to trap and stabilize various Cu(I)-halide clusters, suggesting that using a flexible ligand as a linker could be a new clue to construct complexes with more than one kind of Cu(I) cluster. A similar flexible ligand was then used to further prove this concept. Remarkably, these obtained MCCHs exhibit exceptional temperature-dependent photoluminescence properties.

Light yellow crystalline MCCH-1 was obtained from the reaction of TPPA and CuI in a mixed solvent of MeCN/DMF at 85 °C through a solvothermal method. SCXRD reveals that it crystallizes in the non-centrosymmetric trigonal group P31c. The most striking feature of MCCH-1 in the structure is that it incorporates two kinds of neutral Cu(I) halide clusters simultaneously: tetranuclear cubane-type Cu4I4 clusters and heptanuclear pinwheel-type Cu7I7 clusters. As shown in Fig. 1a, Cu4I4 presents a classical cubane-type.8 Four Cu(I) ions in the cluster are all four-coordinated, in which, besides three μ2-I ions, one (Cu4) is coordinated with a MeCN molecule while the other three (Cu5) are coordinated with the pyridines of the ligands. Thus, each Cu4I4 cluster is only connected by the three linkers (TPPA ligands). These 3-connected Cu4I4 clusters and TPPA ligands are interconnected and lead to a 2D cluster-based honeycomb (hcb)9 layer (layer A). In the nonclassical pinwheel-type heptanuclear Cu(I)-halide cluster of Cu7I7,10 there are three crystallographically independent Cu and I ions. Cu3 and I1 locate at the crystallographic C3 axis and act as an axle of the pinwheel. The wings are made up of three outer rhomboidal Cu2I2 units constituted by Cu1, Cu2, I2 and I3 ions (Fig. 1b). Each pinwheel-shaped Cu7I7 cluster is then six-connected by tridentate TPPA ligands generating a 2D cluster-based layer with a kagomé dual (kgd) topology9 (layer B). In space, the hcb layer made by Cu4I4 and the kgd net built by the Cu7I7 array arrange in an ABA′B′ mode along the c axis (Fig. 1c). To the best of our knowledge, this kind of Cu(I)-halide complex containing topologically different layers based on two kinds of clusters is reported here for the first time.

image file: d0cc02472d-f1.tif
Fig. 1 Representation of the structures of MCCH-1 (a–c) and MCCH-2 (d–f). (a) Cu4I4 cluster and Cu4I4-cluster-contained layer; (b) Cu7I7 cluster and Cu7I7-cluster-contained layer; (c) packing mode of MCCH-1 viewed from the b axis. (d) Cu6I5+ cluster and Cu6I5+-cluster-contained layer; (e) Cu6I7 cluster and Cu6I7-cluster-contained layer; (f) packing mode of MCCH-2 viewed from the b axis. Colour codes: Cu in blue, I in orange and all H atoms and solvents are omitted for clarity.

Different from MCCH-1, MCCH-2 was obtained in the MeCN/DEF solvent at 120 °C and crystallized in the centrosymmetric group P63/m. MCCH-2 contains cationic rugby-ball-shaped Cu6I5+ clusters and anionic sandglass-shaped Cu6I7 clusters simultaneously. In a Cu6I5+ cluster, a trigonal prismatic core is constructed by six Cu atoms, which is then tethered by three μ4-bridging I atoms (I2) at the prism side faces and two μ3-capping I ions (I1) at the opposite triangular faces (Fig. 1d). The cluster cores are connected to the six ligands of mirror symmetry resulting in a double-layered hcb network (layer C).11 Besides the cationic Cu6I5+ cluster, MCCH-2 contains another type of Cu(I)-halide cluster: the sandglass-shaped anionic Cu6I7 cluster,12 in which, six Cu atoms of two regular triangles are connected by one centre μ6-bridging I atom and six μ2-bridging I ions. The Cu6I7 core is six-connected by the TPPA ligands, leading to a layer with kgd topology (layer D, Fig. 1e). Similar to MCCH-1, the layers based on different clusters arrange alternately in a CDC’D’ mode (Fig. 1f).

In a previous study, the multi-cluster frameworks were achieved by bi- or multi-functional ligands with multiple donor atoms or ligating modes.3 Our method is totally different since the TPPA ligand has only one kind of N donor site and one coordination mode. We suppose that the flexible ligand TPPA can adopt various conformations that can meet the coordinated requirements of different clusters.13 Through carefully analysing the structures, we find that there are five conformations of TPPA in the two MCCHs (three in MCCH-1, named I, II, III, and two in MCCH-2, named IV, V), verifying the conformational diversity of TPPA. The structure images and the detailed geometric parameters of the five conformations are presented in Fig. 2 and Table S1 (ESI). Among the seven variates listed in Table S1 and Fig. S2 (ESI), θ1 and θ2 define the tension of the central O[double bond, length as m-dash]P–(NH)3 tripod in the ligand, and θ3, θ4, ω, φ and d reflect the elasticity of the dentate branches covering the P–NH–pyridine arms and the pyridine ring, which influence the orientation of the donor N atom. The changes of θ1 and θ2 are within 2°, indicating that the central O[double bond, length as m-dash]P–(NH)3 tripod of the TPPA ligand is quite robust. Nevertheless, the θ3, θ4, ω, φ and d vary dramatically with changes up to 4.18°, 26.94°, 112.41°, 20.63° and 0.589 Å, respectively. This information illustrates that the flexibility of TPPA mainly comes from the dentate branches especially in the stretch of the P–NH–pyridine arms and the rotation of the pyridine ring, which results in the multiple steric types of the coordinated N donor.

image file: d0cc02472d-f2.tif
Fig. 2 Presentation of the ligand TPPA conformations in MCCH-1 (a) and 2 (b), and the calculated relative energies of the five TPPA conformations (c).

Performed using the Gaussian 09 simulation package,14 the relative energies of the different conformations of the TPPA ligand were calculated at the B3LYP/6-311G(d,p) level of theory.15 As shown in Fig. 2c, the relative energy differences among the conformations are small, within 24.4 kJ mol−1 in the same MCCH, while those in different MCCHs are significantly greater and more than 82.2 kJ mol−1, being about at least 3.4 times larger than those of the same MCCH.

Based on the above analyses, we assume that for a flexible ligand, several conformations can coexist under a certain environment and their structures can be elaborately tuned by experimental variables such as solvents and temperatures. These conformations may easily change into each other and cannot be separated since their energy levels are so close.16 The ligands with multiple conformations could match the coordination requirements of different Cu(I)-halide clusters and could trap and stabilize them to generate the framework with more than one kind of cluster.17 Thus, the use of conformationally flexible ligands may be a promising route towards MCCHs.

The proof of concept study was carried out using another flexible ligand TPTA (Fig. S1, ESI). As expected, a new MCCH named MCCH-3 was obtained successfully, which exhibits a similar dual-cluster (Cu4I4 and Cu7I7) structure to MCCH-1. The structure of MCCH-3 is presented in Fig. S3 (ESI) and the geometric parameters, as well as the calculated relative energies, of the TPTA conformations that existed in MCCH-3 are listed in Table S2 (ESI). In addition, the conformationally flexible ligand Tbmiz can be used to fabricate an MCCH with the mixed clusters of Cu6I7 and Cu3I2+.18 All these results confirm the potential of the proposed strategy.

Then, the photoluminescence (PL) properties of the as-synthesized MCCHs were studied. As shown in Fig. 3a, at room temperature, the three MCCHs display one similar broad emission band with peaks at 510, 570 and 600 nm, respectively, when excited at their optimal excitation wavelengths. These bands can be ascribed to the low-energy (LE) emissions of copper-halide clusters, originating from the cluster centred excited state called 3CC.19 The samples can give out bright green, yellow and orange light when exposed to UV light at 365 nm (Fig. 3b). When the temperature is lowered, the emission peak of MCCH-1 exhibits a blue shift from 510 nm to 492 nm (Fig. 3c). It is interesting that the PL intensity of MCCH-1 shows an abnormal temperature dependence as it is almost unchanged from 10 to 150 K. For MCCH-2, as the temperature decreases to 180 K, high-energy (HE) emission (485 nm) arises (Fig. 3d), which could be attributed to the halide/metal-to-ligand charge-transfer (X/MLCT).20 The PL decay behaviours of the two emissions are different, proving that they stem from two distinct origins (Fig. S6, ESI). As the temperature rises, the intensity of HE (IHE) decreases normally; however, that of LE (ILE) shows a negative thermal quenching (NTQ) with a 22% increase from 10 to 77 K and then decreases monotonously (Fig. 3d inset). The temperature-dependent luminescent behaviour of MCCH-3 (Fig. S7, ESI) is similar to that of MCCH-1. Its intensity drops slightly (less than 7%) as the temperature rises from 10 to 150 K. The above-mentioned intensity variations are in accordance with the results of the PL lifetime measurement (Fig. S9 and S10, ESI).

image file: d0cc02472d-f3.tif
Fig. 3 (a) Room temperature excitation and emission spectra and (b) the photos of the samples under UV (365 nm) lamp illumination and CIE coordinates of MCCH-1, 2, and 3. The temperature-dependent PL spectra of MCCH-1 (c) and MCCH-2 (d), inset: IPLT curves. (e) The IPL − 1/T correlation of MCCH-1 and 3 fitted using the Arrhenius equation (solid line) and that of MCCH-2 characterized using a rewritten Arrhenius equation with an activation energy (Eq) for the NTQ phenomenon (dash line).

To further understand the anomalous temperature dependence of PL intensity (IPL) in the three MCCHs, the IPLT correlations are fitted using the Arrhenius equation (Fig. 3e) and the thermal activation energies (ΔE) are obtained.21 The result (Table S3, ESI) shows that all three MCCHs exhibit high ΔE with 1707 cm−1, 1349 cm−1 and 1138 cm−1 for MCCH-1, MCCH-2 (LE) and MCCH-3, respectively, which are much larger than the reported values of copper halide complexes.3b,22 The high thermal activation energies give them good PL thermal stabilities in a certain temperature range. As the temperature rises, the LE of MCCH-2 even increases by 22% showing an obvious NTQ effect (Eq ≈ 75 cm−1). We suppose that, besides the high ΔE which may inhibit the non-radiative transitions, the thermal activated population transfer from the X/MLCT state (HE) to the 3CC state (LE) may be responsible for it.23 The configurational-coordinate models with Franck–Condon analysis in Fig. 4 are proposed for better interpretations.21b

image file: d0cc02472d-f4.tif
Fig. 4 The schematic illustration of the proposed configurational-coordinate model for MCCH-1, 2, and 3 presenting emission and excitation transitions, the Stokes shift |RiR0|, the thermal activation energies ΔEi (i = 1, 2, 3), the Eq and the attribution of NTQ.

Luminescent materials with high thermal activation energies are significant since a stable PL intensity in a wide range of temperatures is highly desirable in many applications, such as photoelectric devices, biological labelling, industrial coating, and optical sensing.24 Cu(I)-halide complexes with high thermal activation energies have seldom been reported before. We assume that the abnormal temperature-dependent behaviours and the high thermal activation energies of MCCH-1, 2, and 3 may be associated with their mixed-cluster structures. The di-exponential decay profiles (Table S4, ESI) demonstrate that there are more than one 3CC or X/MLCT excited states in these mixed-cluster complexes25 and the multiple excited states probably offer more chances for radiative transitions so that the non-radiative transitions are restricted under a certain temperature.

Due to the distinctive temperature-dependent PL behaviours, MCCH-1, 2, and 3 show great promise for optical temperature sensors. The presence of temperature-induced spectral red-shifts is observed in MCCH-1 and 3, suggesting that they could be used for spectral PL thermometry.26 The low temperature-dependent dual-emitting behaviour makes MCCH-2 a promising self-calibrated cryogenic thermometer.27 To assess the thermometric performance of MCCH-2, the emission spectra have been measured in a narrow temperature interval (10 K) (Fig. S11, ESI), and the working curves and equations (ln(IHE/ILE)–T) are given in Fig. S12 (ESI). Great linear correlations can be found in the range of 10–150 K with R2 up to 0.99915.

In conclusion, three multi-cluster copper halides have been prepared by taking advantage of conformationally flexible ligands. These complexes possess high thermal activation energies and exhibit anomalous temperature-dependent photoluminescence. We anticipate the present study will shed light on the synthesis of multi-Cu(I)-cluster complexes and stimulate the explorations of the Cu(I) clusters.

This work was supported by the National Key R&D Program of China (2018YFA0704502, 2017YFA0206800), the Strategic Priority Research Program of CAS (XDB20000000), the Key Research Program of Frontier Science, CAS (QYZDY-SSW-SLH025), NSFC (21671189, 21731006, 21901242), the Youth Science Research Program of Fujian Jiangxia University (JXZ2018003) and the Youth Innovation Promotion Association CAS.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Electronic supplementary information (ESI) available. CCDC 1971744–1971746. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0cc02472d

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