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Hierarchical Assembly and Aggregation-Induced Enhanced Emission of a Pair of Isostructural Zn14 Clusters Xue-Li Chen,†,§ Hai-Bing Xu,†,§ Xing-Xing Shi,‡ Yuexing Zhang,† Tao Yang,† Mohamedally Kurmoo,⊥ and Ming-Hua Zeng*,†,‡

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Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules & College of Chemistry & Chemical Engineering, Hubei University, Wuhan 430062, P. R. China ‡ Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, Department of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541001, P. R. China ⊥ Institut de Chimie de Strasbourg, CNRS-UMR 7177, Université de Strasbourg, 4 rue Blaise Pascal, 67070 Strasbourg, France S Supporting Information *

ABSTRACT: Information of solid-state and solution structures is crucial in the characterization of molecular clusters and in advancing the understanding of their diverse properties. [Et3NH]2[Zn14(hmq)8(OH)4X10] [X = Cl and Br; H2hmq = 2(hydroxymethyl)quinolin-8-ol] consist of a peanut-shaped Zn10O12 core, in which the Zn atoms occupy the faces and corners of an octahedron and are protected by bonded halogen atoms and bulky organic ligands. Observation of the [Zn14(hmq)8(OH)4X10]2− fragment in electrospray ionization mass spectrometry (ESI-MS) suggests that the cluster is stable in solution. ESI-MS analyses from dissolved crystals and mother liquors reveal that Zn(hmq) self-assembles to Zn5(hmq)4Cl, then dimerizes through four [OH]− bridges to Zn10(hmq)8(OH)4Cl2, and progressively captures four ZnCl2 one-by-one to [Zn14(hmq)8(OH)4Cl10]2−. Because the supramolecular interactions between the anion and cation in the solid suppress the rotation/vibration of the halogen atoms and confine the movable organic ligands on the rigid Zn−O core, both crystal phases exhibit intense photoluminescence, much stronger than that in solution. This is the first coordination cluster to exhibit “aggregation-induced enhanced emission”. In addition, preliminary tests indicate that these coordination clusters are promising for organic-light-emitting-diode applications.

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formation process of POM clusters,5 metal−organic macrocycles,15 and 3d metal clusters.6−11 Recently, we reported the assembly process of [Zn5(H2Ln)6](NO3)4]·8H2O·2CH3OH [H3Ln = 1,2-bis(benzo[d]imidazol-2-yl)ethenol], which has potential as a pH-responsive vis/near-IR fluorescent bioprobe.11 With such information in hand, we were encouraged to extend the study to larger zinc clusters. On the other hand, evidence of “aggregation-induced enhanced emission” (AIEE) was the major impetus of the present study.16 AIEE-active supramolecular frameworks,17 metallic complexes,18,19 metal− organic frameworks,20 and metallacycles and metallacages21 are promising contemporary materials for large-area thin solid films and devices.22 However, to the best of our knowledge, AIEE activity in photoluminescence is yet to be reported for a highnuclear coordination metal cluster (Table S1). Herein we report the syntheses, structures, hierarchical assembly, and luminescence of [Et3NH]2[Zn14(hmq)8(OH)4-

INTRODUCTION

The study of polynuclear transition-metal clusters is a hot issue owing to the structure dependence of the physical properties.1−10 Clusters have been fascinating scientists by their structural beauty and versatile functions, such that the design of metal clusters with potential applications such as luminescence and bioactivity,11 catalysis,12 and magnetism6−10 is constantly being pursued. Tracking and controlling the assembly process during their syntheses under hydro- or solvothermal conditions are tedious tasks because the processes are highly dependent on the reaction conditions and several physical parameters.6−10,13 Therefore, understanding the underlying principles of the formation process will provide guidelines for designs and synthetic strategies to eventually deliver materials with desired integrated functional properties. Although quite complex, some advances have been made in the case of coordination cages,4,14 polyoxometalates (POMs),3 and 3d metal coordination clusters.6−11 It has been demonstrated that crystallography and mass spectrometry (MS) are complementary in unveiling the © 2017 American Chemical Society

Received: August 28, 2017 Published: October 30, 2017 14069

DOI: 10.1021/acs.inorgchem.7b02210 Inorg. Chem. 2017, 56, 14069−14076

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Inorganic Chemistry

octahedral Zn6, forming the Zn−O core (Table S5), which is further encapsulated by the organic outer layer, forming a threeshell organic−inorganic hybrid. Alternatively, the core, which can be viewed as a peanut-like Zn10(hmq)8(OH)4Cl2, has four decorating ZnCl2 groups around the belt. Zn10(hmq)8(OH)4Cl2 is composed of two Zn5(hmq)4Cl parts, which are fused symmetrically by four μ2-OH groups. The cluster is constructed from eight {Zn(hmq)}, four {OH−}, two {ZnCl+}, and four {ZnCl2}. The three donor atoms of the doubly charged hmq2− chelates Zn2+ along a meridian to form Zn(hmq) (Figure 1a). The Zn center then has either three open sites to adopt six-coordination or two for five-coordination. In addition, the two O atoms (methoxy and phenoxy) can further propagate laterally, giving it five potential divergent coordination possibilities as a metalloligand. The O(phenoxo) atom with sp2 hybridization can be extended via a μ2 bridge because the distance between two neighboring O(phenoxo) atoms is within 2.938−2.996 Å (Zn−O = 1.96− 1.98 Å), which favors for ZnCl2 insertion between the two O atoms. Zn2+ is extendable via the sp3-hybridized O(methoxy) atom of the other hmq2−, forming a Zn4O4 eight-membered ring of Zn4(hmq)4 (Figure 1b) with ∠ZnOZn in the range of 121.13−124.04° (Table S6b). The O atoms at the vertex of the ∠ZnOZn angle point to one side of the Zn4O4 eight-membered ring, and the distance between the p-O(methoxy) atoms is 3.97 Å (Zn−O = 2.09−2.12 Å), which are beneficial for Zn2+ coordination, thus forming Zn5 (hmq)4Cl by Zn4(hmq)4 capturing ZnCl. Zn2+ is also extendable via O from μ2-OH, which connects two Zn4(hmq)4 or Zn5(hmq)4Cl together to form Zn8(hmq)8(OH)4 or Zn10(hmq)8(OH)4Cl2, respectively. Finally, the latter captures four ZnCl2 one-by-one to form the integral anionic cluster. The cluster structure of 1-Cl is different from known low- and moderate-oxidation-state transition metal−oxo clusters; it is structurally analogous to the highvalent POM [W10O32]4− (Figure 2), which is composed of two W5 units fused by four μ2-O atoms.23

X 10 ] [X = Cl (1-Cl) and Br (1-Br); H 2 hmq = 2(hydroxymethyl)quinolin-8-ol], in which 1-Cl displays a high quantum yield of 23.5%, ranked eighth among the reported zinc quinolone derivatives (Table S2). Complementary results of electrospray ionization mass spectrometry (ESI-MS) and density functional theory (DFT) calculations reveal that Zn(hmq) is initially formed and self-assembled to Zn5(hmq)4Cl, then dimerized through four [OH]− bridges to Zn10(hmq)8(OH)4Cl2, and finally led to [Zn14(hmq)8(OH)4Cl10]2− by the capture of four {ZnCl2} fragments one-by-one. It is worth noting that only nine zinc clusters are known with a nuclearity of ≥14 (Figure S1 and Table S3), and none of them possess AIEE activity. In the present case, because interactions between the anion and cation in the crystal suppress the rotation/vibration of the halogen atoms, the emission is effective, but their freedom in solution reduces the efficiency. Therefore, the two compounds are the first coordination clusters to exhibit AIEE. In addition, we fabricated a solid-state light-emitting diode with 1-Cl as the active component.

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RESULTS AND DISCUSSION Crystal Structures of 1-Cl and 1-Br. Because the two compounds form an isostructural pair, we will only describe the structure of 1-Cl (Figure 1 and Table S4). X-ray diffraction data

Figure 1. (a) Basic building unit Zn(hmq) and the structure of 1-Cl. The arrows show possible extension sites. The Zn8 cube shares the faces of the Zn6 octahedron (gray, C; red, O; blue, N; turquoise, Zn). (b) Construction steps: Zn(hmq) → Zn4(hmq)4 → Zn5(hmq)4Cl (or Zn8(hmq)8(OH)4) → Zn10(hmq)8(OH)4Cl2 → Zn14(hmq)8(OH)4Cl10.

Figure 2. (a) “Peanut”-like Zn10 cluster. (b) Crystal structure of the “peanut”-shaped [W10O32]4− cluster. Color scheme: W, blue; O, red.

reveal that 1-Cl crystallizes in the monoclinic space group C2/c. The asymmetric unit consists of half of the cluster, that is, seven Zn2+, four deprotonated hmq2−, two μ2-OH−, five Cl−, and a protonated amine [Et3NH]+. 1-Cl contains a Zn−O core with eight Zn atoms occupying the faces of an octahedron into a cube and six Zn atoms at the apexes and an organic outer layer (Figure 1a). The cubane-like Zn8 is surrounded by the

The powder X-ray diffraction (PXRD) patterns of 1-Cl and 1-Br match the simulated ones using the single-crystal structure data (Figure S3a). These results confirm the single-phase nature of the bulk. The thermal stability of the compounds was investigated at a heating rate of 5 °C/min over the temperature range from 30 to 800 °C in flowing N2. Thermogravimetric analysis (TGA; Figure S3b) reveals that they undergo weight 14070

DOI: 10.1021/acs.inorgchem.7b02210 Inorg. Chem. 2017, 56, 14069−14076

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Figure 3. (a) Negative-mode ESI-MS spectra of dissolved crystals of 1-Cl for different in-source energies. (b) Details of major species assigned in the ESI-MS spectrum. (c) Possible destruction pathways. Solvents are omitted for clarity.

Figure 4. (a) Negative-mode ESI-MS spectra of solutions under different conditions: stirring 30 min at room temperature in the absence (1) or presence (2) of Et3N; treated at 140 °C for 1 h (3), 9 h (4), and 24 h (5). (b) Proposed construction. Solvents are omitted for clarity.

losses of 7.3% below 320 °C for 1-Cl and 5.6% below 315 °C for 1-Br, corresponding to the departure of two [(Et)3NH]+ (calcd 7.0% for 1-Cl and 6.0% for 1-Br). Then they are gradually decomposed with increasing temperature, indicating their stability up to 300 °C. ESI-MS of 1-Cl and 1-Br. The destruction processes of the clusters monitored by ESI-MS at variable in-source energies were performed on solutions of the crystals in dimethyl sulfoxide (DMSO) and diluted with acetonitrile (CH3CN).6−8,11 When the in-source energy is 0 eV, two single series of peaks in the negative-mode spectra at m/z 1361.53 ([Zn14(hmq)8(OH)4Cl10]2−) and 2688.09 ([Zn14(hmq)8(OH)4Cl9]−) for 1-Cl (Figures 3a and S4 and Table S8) and m/z 1600.25 and 1640.72, where Solv = (H2O)2 and (CH3CN)2(CH3OH), respectively, in [Zn14(hmq)8(OH)4Br10(Solv)]2− for 1-Br (Figures S5 and Table S9) are observed, suggesting that the integrity of the cores of 1-Cl and 1-Br is stable. As for 1-Cl, upon an increase in the in-source energy stepwise to 20 eV, no change was observed, suggesting stable

clusters. Moreover, when it reaches between 70 and 100 eV, new weak peaks of lower mass appear at m/z 831.92 ([Zn 3 (hmq) 3 Cl(CH 3 CN) 2 ] − ), 911.77 ([Zn 4 (hmq) 3 Cl 2 (CH3O)(CH3OH)]−), 1206.65 ([Zn5(hmq)4(OH)Cl2(H2O)(CH3CN)2]−), 2092.20 ([Zn8(hmq)8(OH)4(H2O)3(CH3CN) − 3H]−), and m/z 2357.16 and 2395.14, where Solv = (CH3CN)4(H2O)(CH3OH) and (CH3CN)4(H2O)5), respectively, in [Zn10(hmq)8(OH)4Cl(Solv)]− (Figure S4 and Table S8). We can infer the progressive degradation process: [Zn14]2− → [Zn14]− → [Zn10]− → [Zn8]−. Alternatively, [Zn10]− may split into two [Zn5]− (Figure 3c). Further confirmation was the values from the calculated Mayer bond order using DFT, giving the destruction process as follows: Zn14(hmq)8(OH)4Cl10 → Zn10(hmq)8(OH)4Cl2 → Zn5(hmq)4Cl (Table S7). To further illustrate the structural assembly of 1-Cl, we want to make use of tandem mass spectrometry (MS/MS), which could clearly provide sensitive and selective analysis of complex mixtures. Unfortunately, the parent ion peaks have all broken up into ligand peaks or mononuclear fragments. 14071

DOI: 10.1021/acs.inorgchem.7b02210 Inorg. Chem. 2017, 56, 14069−14076

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Inorganic Chemistry In turn, five solutions of 1-Cl were prepared to investigate the construction process (Figures S6). In the absence of Et3N (1 in Figure 4a), only two prominent peaks were observed at m/z 309.92 ([Zn(Hhmq)Cl2]−) and 442.56 ([Zn2(hmq)Cl3(H2O)2]−), suggesting that Zn(hmq) is the basic building unit for construction of the cluster. In the presence of Et3N (2 in Figure 4a), new peaks were observed at m/z 991.71 ([Zn4(hmq) 3 Cl 3(CH3 OH) 2(CH3 CN)]−), 1266.67 ([Zn5(hmq)4(OH)Cl2 + (ZnCl)(OH)(CH3CN)]−), and 1403.78 ([Zn6(hmq)5(OH)Cl2(CH3CN)(H2O)]−), together with some other weak peaks. Upon treatment at 140 °C (3 and 4 in Figure 4a), further progression of the peaks was observed at m/z 1541.64 ([Zn7(hmq)5(OH)2Cl3(CH3CN)]−), 2227.86 ([Zn10(hmq) 8 (OH) 4 Cl(CH 3 CN)(CH 3 OH)(H 2 O)] − ), 2365.71 ([Zn 11 (hmq) 8 (OH) 4 Cl 3 (CH 3 CN)(CH 3 OH)(H 2 O)] − ), 2501.60 ([Zn 1 2 (hmq) 8 (OH) 4 Cl 5 (CH 3 CN)(CH 3 OH)(H 2 O)] − ), 2639.95 ([Zn 13 (hmq) 8 (OH) 4 Cl 7 (CH 3 CN)(CH3OH)(H2O)]−), and 2775.16 ([Zn14(hmq)8(OH)4Cl9(CH3CN)(CH3OH)(H2O)]−). However, after 24 h (5 in Figure 4a), the related peaks for [Zn11]−, [Zn12]−, [Zn13]−, and [Zn14′]− disappeared, indicating that all of 1-Cl separated out from the solution by crystallization (Figures S7−S9 and Tables S10−S12). Although at first sight there are five possible extensions for Zn(hmq) that can complicate the self-assembly process, the observed fragments suggest possibly orientational recognization assembly. For instances, it indicates two [Zn5′]− fragments fused by four μ2-OH groups to form [Zn10′]−, followed by the capture of four ZnCl2 one-by-one to form [Zn11]−, [Zn12]−, [Zn13]−, and [Zn14′]− (Figure 4b). Unlike the construction of POM from an identical basic unit ([XMyOz]q− (X = H+, firstrow transition metal),5 choosing different ligands and metal ions can achieve different metalloligands as the basic building units for the construction of coordination clusters. Thus, elucidating the formation process of 1-Cl will thrive in the design and synthesis of coordination clusters, control of their structures, and finally their physical and chemical properties. Optical Properties of 1-Cl and 1-Br. Both 1-Cl and 1-Br exhibit cyan luminescence (Figure S11) with quantum yield (Φem) increasing with concentration, Φem = 2.52% in dilute DMSO (10−5 M) and 23.5% in the solid state for 1-Cl, and from 1.65% to 18.9% for 1-Br (Table S14). To address this discrepancy, the quenching effect of the movable components, including the four ZnX2 motifs located at the outer cage, the two apexes ZnX at the cage caps and the [Et3NH]+, should be considered. Because any molecular motion consumes energy, in the structure of 1-Cl, 10 Cl atoms are attached to one core (Figure 5a); the former can rotate/vibrate against the latter via the single-bond axes. The restricted rotation/vibration of the Cl atoms in the crystals is most likely responsible for enhancement of the emission with respect to the solution. In solution, cations and anions are present as solvent-separated ion pairs; thus, [Et3NH]+ cannot influence the rotation/vibration of the Cl, which is the source soaking up the energy, thus weakening the luminescence. The Cl···C distances of 3.460−3.665 Å and ∠CHCl angles of 124.30−145.69° in the crystal indicate weak interactions (Table S6 and Figure S2), which can prevent the rotation/vibration of Cl atoms, but in solution, they are free to rotate/vibrate. The rotation/vibration will consume the energies such that Zn(hmq) is depleted of any for luminescence to occur from a dilute solution. Because Br− (1.96 Å, 79.9) is bigger and heavier than Cl− (1.81 Å, 35.5), their rotation/

Figure 5. (a) Cluster unit showing the luminophore and quencher components of the structure. (b) View along the c axis of the spacefilling representation of [Et3NH]+ surrounding a cluster.

vibration of Br is slower and requires more energy than that of Cl, thus narrowing the gap of Φem between the isolation and aggregation state in 1-Br (Table S14). Additionally, there are six cavities in their structures of 1-Cl and 1-Br, locating the peripheral counterions [Et3NH]+. Equatorially, two of them connect to ZnX2 by an intermolecular three-center hydrogenbond N−H···(X)2, and the other two connect to ZnX2 through two hydrogen-bond C−H···X···H−C. Perpendicularly, another two are packed between two clusters near the ZnX motif. Consequently, the six flexible [Et3NH]+ ions not only fill the void space of the lattice in crystal but also form hydrogen bonds with {ZnXn} in Zn14X10 cluster, preventing the rotation/ vibration of the halogen (Figures 5b and S2). It is the first example where the [Et3NH]+ groups are tightly packed around the cluster, leaving little room for the halogens to wobble and helping in the restriction of the halogen movements in the solid state, inducing high emission in the solid state. The more restriction of the halogen rotation/ vibration, the less dissipation of excitation energies and the more emission from Zn(hmq). According to previous reports, such a phenomenon is known as “aggregation-induced enhanced emission” (AIEE).16 Besides the dependence on the concentration, the luminescence intensity is also dependent on the solvent and temperature (Figures S13−S17).16 Because 1-Cl and 1-Br are more soluble in DMSO than in CH3CN, their emission intensities are largely improved by increasing the poor solubility solvent CH3CN content in the mixed solvents (DMSO/ CH3CN). When CH3CN in 1-Cl reached 99%, the mixture out turned to be a suspension of 300−550 nm particles, as confirmed by dynamic light scattering (DLS). With larger size and high population of the nanoparticles, the area of the emission profiles in 99% CH3CN is ca. 10 and 6 times that in DMSO for 1-Cl and 1-Br (Figures 6 and S14−S16), respectively. Upon cooling, their emission intensities in CH3CN (freezing point = 225 K) increase gradually, and the areas of the emission profiles at 228 K are ca. 2.5 times that at 298 K for 1-Cl and ca. 1.5 times for 1-Br under the same conditions (Figure S17). All of these results further confirm that the AIEE for 1-Br is weaker than that for 1-Cl because Br− is bigger than Cl−. Furthermore, because pressurization brings the anion and cation closer, the reduced distance between the anion and cation increases the interactions among them, further inducing enhanced light emission (Figure S18). Moreover, the PL intensity increases “linearly” with the viscosity when the ethylene glycol fraction is below 50% in CH3CN/ethylene glycol mixtures. If the ethylene glycol contents surpass 50%, the 14072

DOI: 10.1021/acs.inorgchem.7b02210 Inorg. Chem. 2017, 56, 14069−14076

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electroluminescent intensities of organic light-emitting diodes (OLEDs) also improve with increasing concentration of 1-Cl (5, 10, and 20%) in the host materials. Because of the solubility limitation, the devices were only optimized by modifying the percentage of 1-Cl dopant in the host; other parameters were not optimized. As depicted in Table S16, although the current and external quantum efficiencies are acceptable, the devices possess ca. 3.4 V turn-on voltage and 1000−1390 cd/m2 brightness.

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CONCLUSION We successfully obtained high-nuclear 1-Cl and 1-Br clusters by organic−inorganic hybrid and utilized crystallography, ESIMS, and theoretical calculations to unveil the hierarchical assembly process. It is the first example of a metallic cluster to show a bottom-up assembly including a multistage one by one attachment using time-dependent ESI-MS. Furthermore, Zn14X10 is the first AIEE-active metallic cluster in which the anion and cation are mutually suppressed through a synergistic effect to decrease the dissipation excited energies, rendering 1Cl a high-intensity emitter in the solid state. Such an approach could broaden the ways for achieving AIEE-active emission and avoid significant synthetic effort on a functional AIE-active group. Also, the AIEE-active emission metallic clusters would be promising in OLED applications.

Figure 6. Emission spectra (λex = 375 nm) of 1-Cl (2 × 10−5 M) in DMSO/CH3CN mixtures containing different volume fractions of CH3CN at 298 K.

PL intensity mounts rapidly (Figure S19). Simultaneously, the absorption spectral profiles of 1-Cl experience small red shifts, and the absorption tails extend well into the long-wavelength region with high ethylene glycol contents (>50%; Figure S20). In a word, photoluminescence of 1-Cl is dependent on the concentration, solvent, and temperature, it also exhibits viscochromism and piezochromism, and all of the photophysical properties of 1-Cl are consistent with the AIEE characteristics. Traditionally, the strategy on the design of an AIEE-active metallic complex is covalently anchored AIEE-type chromophores, which requires significant synthetic effort, to metal ions, such as the well-known tetraphenylethylene (TPE) or its derivatives (Table S15);16−22 the so-called AIEE-active emission mainly originates from that of TPE by obstructing the rotation of the phenyl groups and shutting down nonradiative relaxation pathways, thus enabling emission with a hundred-to-thousand-fold increase in the peak emission intensity upon aggregation. As a complementary AIEE, a small scale of the AIEE activity within 10 times enhancement is first observed in the metallic cluster within an inorganic−organic hybrid, in which the organic fragment acts as a luminophore and the movable halogen as a quencher. Also, the AIEE luminescent performance stems from different building blocks. Such an approach could broaden the ways for achieving the AIEE activity and avoid tedious modification of the AIEE-active chromophores like the traditional method. Electroluminescence of 1-Cl was investigated using the device configuration, as shown in Figures 7 and S21.21 Φ increases from 5.0 to 7.9 to 17.8 to 23.5% with the doping contents of 5, 10, 20, and 100%, respectively. Additionally, the

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EXPERIMENTAL SECTION

Synthesis of 1-Cl and 1-Br. CH3CN solutions (12 mL) containing ZnCl2·4H2O (68 mg, 0.5 mmol) or ZnBr2·4H2O (113 mg, 0.5 mmol), H2hmq (27 mg, 0.15 mmol), and N(C2H5)3 (0.15 mL) in 25 mL Teflon-lined steel autoclaves were heated at 140 °C for 48 h. The autoclaves were then cooled to room temperature at a rate of 10 °C/h. Yellow acicular crystals of 1-Cl (22.4 mg, 41.8% based on H2hmq) or block crystals of 1-Br (20.9 mg, 33%) were collected, washed with CH3CN, and dried in air. Elem anal. Calcd for [(CH3CH2)3NH]2[Zn14(C10H7NO2)8(OH)4Cl10]: C, 37.74; H, 3.17; N, 4.78. Found: C, 38.13; H, 3.65; N, 4.45. Calcd for [(CH3CH2)3NH]2[Zn14(C10H7NO2)8(OH)4Br10]: C, 33.38; H, 2.86; N, 4.87. Found: C, 32.21; H, 3.01; N, 4.95. IR data for 1-Cl (KBr, cm−1): ν 3405(s), 1620(s), 1391(s), 1468(m), 1310(m), 815(m), 716(m). IR data for 1-Br (KBr, cm−1): ν 3405(s), 1620(s), 1391(s), 1468(m), 1310(m), 815(m), 716(m). Materials and Methods. All starting materials were of reagent grade from commercial sources and were used as received. The C, H, and N microanalyses were carried out with a Vario Micro Cube elemental analyzer. The Fourier transform infrared (FTIR) spectra were recorded from KBr pellets containing ca. 0.1 mg of compounds in the range of 4000−400 cm−1 on a PerkinElmer One FTIR spectrophotometer. UV−vis absorption spectra were measured on a PerkinElmer Lambda 35 UV−vis spectrophotometer. PXRD intensities were measured at 293 K on a Rigaku D/max-IIIA diffractometer (Cu Kα). The samples were prepared by crushing the single crystals and placed on a grooved aluminum plate, and the patterns were recorded from 3° to 50° at a rate of 5°/min. Calculated diffraction patterns for the compounds were generated by Mercury 3.8 from the single-crystal data. The thermal properties were measured using a gravimetric analyzer (Labsys evo TG-DTA) under a constant flow of dry nitrogen gas at a rate of 5 °C/min. DLS results were obtained on a Zetasizer (Nano ZS, Malvern Instruments). X-ray Crystallography. Single-crystal X-ray diffraction data for 1Cl and 1-Br were collected on a Rigaku R-AXIS SPIDER IP diffractometer employing graphite-monochromated Cu Kα radiation (λ = 1.54184 Å) using the θ−ω scan technique at 150 K. Their structures were solved by direct methods using ShelXS and refined using a full-matrix least-squares technique within ShelXL2015 and OLEX.2.24 All non-H atoms were refined with anisotropic thermal

Figure 7. Current density−voltage−luminance (J−V−L) characteristics of OLEDs using 20%, 10%, and 5% 1-Cl as the luminescent reagent (CD = current density; LM = brightness). 14073

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Inorganic Chemistry

(OH)4Cl10, and (2) the remaining Zn10(hmq)8(OH)4Cl2 is broken into two Zn5(hmq)4Cl fragments by the departure of four OH−. Optical Properties. The two compounds display similar absorption spectra consisting of an intense absorption band centered at 375 nm in DMSO, which is an intraligand transition of hmq2−,27 because the corresponding band is found at 310 nm for the free H2hmq ligand. The band is red-shifted from 310 to 375 nm due to deprotonation (Figure S10). This shift and their independence on the halogen confirm that it is due to intraligand [π−π*(H2hmq)] transition, which is also evidenced by the time-dependent DFT calculation results. Upon irradiation at λex = 310 nm, free H2hmq emits bright-blue luminescence centered at ca. 400 nm in both the solid state (τ = 1 ns) and solution (τ = 1.4 ns) (Figure S12). Once deprotonated and coordinated, they emit bright-cyan luminescence at ca. 500 nm (λex = 375 nm) in both the solid state and DMSO, with the lifetime ranging from 11 to 15 ns. Thus, luminescence originates from the same intraligand [π−π*(q)] levels.27 Emission and excitation spectra and emission lifetimes were measured using the same slit and iris. Ethylene glycol is a viscous liquid, whose viscosity at 25 °C is 25.66 cP, ∼69.35 times higher than that of CH3CN (0.37 cP). The solid-state quantum yields of powder samples in sealed quartz cuvettes and as films spin-coated on quartz substrates were measured using the integrating sphere (142 mm diameter) of an Edinburgh FLS980 spectrofluorophotometer (a signal-to-noise ratio of ca. 6000:1 using the Raman peak of water) with the same slit (1.9980 mm) and iris (10 mm, the largest one is 100 mm) in our experiments. The solid-state quantum yields of powder samples in quartz substrates and the liquid quantum yields in sealed quartz cuvettes were determined by the integrating sphere (142 mm diameter) using an Edinburgh FLS980 spectrofluorophotometer. Device Fabrication and Characterization. OLEDs were fabricated on patterned indium−tin oxide (ITO)-coated glass substrates using reported processes. ITO substrates were cleaned by sonication in deionized water, acetone, and isopropyl alcohol, followed by UV-ozone treatment for 15 min. Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was filtered through a 0.22 μm filter, spin-coated (at 4800 rpm) on the cleaned ITO glass substrates, and baked at 140 °C for 10 min in air to give a film of 30 nm thickness. The emitting layer was then overlaid by spin-coating (at 2100 rpm) using a filtered chlorobenzene solution (3.75 mg/mL) with mixed host materials and 1-Cl. Subsequently, 100 nm of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi), 1 nm of LiF, and 70 nm of Al were thermally deposited in an inert chamber at a base pressure of less than 4 × 10−4 Pa. For device configuration, ITO/PEDOT:PSS (30 nm/30 nm)/TCTA ((100 − x)/2%):OXD-7 ((100 − x)/2%):1-Cl (x %, 50 nm)/TPBi (50 nm)/ LiF (1 nm)/Al (100 nm) [TATC = tris[4-(9H-carbazol-9-yl)phenyl]amine; OXD-7 = 1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene], TPBi dissolved in diethyl sulfide at a concentration of 10 mg/mL was spin-coated (at 4400 rpm) onto the PEDOT:PSS holeinjecting layer and then annealed at 140 °C for 10 min to achieve a 50nm-thick film. The quantum yields were measured by an equal weight ratio of TATC and OXD-7 as a mixture doping different amounts of 1Cl. The electroluminescence (EL) spectra were recorded on a Horiba Jobin-Yvon FluoroMax-4 spectrometer. The current density−voltage− brightness (I−V−B) curves of the devices were recorded on a Keithley 2400/2000 sourcemeter and a calibrated silicon photodiode. All measurements of the devices were carried out at room temperature under ambient conditions.28

factors. Crystallographic data have been deposited at the Cambridge Crystallographic Data Center. CCDC reference numbers are 1540490 (1-Cl) and 1543214 (1-Br). The crystallographic data can be found in the Supporting Information or can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://summary.ccdc. cam.ac.uk/structure-summary-form. Crystal data for [Et3NH]2[Zn14(C10H7NO2)8(OH)4Cl10] (1-Cl): C2/c, a = 23.8137(7) Å, b = 17.6571(3) Å, c = 27.1621(8) Å, β = 116.239(4)°, V = 10244.3(5) Å3, Mr = 2927.43, Dc = 1.898 g/cm3, Z = 4, R1 = 0.0335 [I > 2σ(I)], wR2 = 0.1018 (all data), S = 1.04. Crystal data for [Et3NH]2[Zn14(C10H7NO2)8(OH)4Br10] (1-Br): C2/c, a = 23.9853(7) Å, b = 17.7549(4) Å, c = 27.3645(8) Å, β = 115.566(3)°, V = 10512.4(5) Å3, Mr = 3372.04, Dc = 2.131 g/cm3, Z = 4, R1 = 0.0486 [I > 2σ(I)], wR2 = 0.1395 (all data), S = 1.073. ESI-MS. ESI-MS measurements were conducted at a capillary temperature of 275 °C. Aliquots of the solution were injected into the device at 0.3 mL/h. The mass spectrometer used for the measurements was a Thermo Exactive Plus, and the data were collected in positive- and negative-ion modes. The spectrometer was calibrated with the standard tune mix to give a precision of ca. 2 ppm in the region of m/z 200−3000. The capillary voltage was 50 V, the tube lens voltage was 150 V, and the skimmer voltage was 25 V. The in-source energy was set to the range of 0−100 eV with a gas-flow rate at 15% of the maximum. The reactants were first stirred at 30 min at room temperature, followed by heating at 140 °C in Teflon-lined steel bombs. The experimental results indicated that the color of the reaction solution and sediment changed from bright yellow to brown yellow as the storage time increased. As time evolved, yellow crystals of 1-Cl were apparent to the naked eye and became larger and more abundant, and the sediment gradually increased. To investigate the self-assembly processes leading to the final crystalline product, preliminary ESI-MS was used to probe the integrity and behavior of the cluster in solution and detect trace intermediates of timedependent (0, 1, 3, 9, and 24 h) reaction solutions and sediments. Because of the solubility limitation in CH3CN, CH3OH, and H2O, the crystals and sediments of 1-Cl were dissolved in DMSO and diluted with CH3CN for ESI-MS at 275 °C (positive mode) and 310 °C (negative mode). A similar conclusion was reached by tracking the self-assembly process of 1-Br and probing the integrity and behavior of the 1-Br cluster in solution. DFT Calculations. DFT calculations were carried out using the B3LYP functional and adding the D3 version of Grimme’s dispersion with Becke-Johnson damping, SDD effective core potential for Zn, and 6-31G(d) basis sets for other elements using Gaussian 09 software.25 Bond orders were analyzed using Multiwf n software.26 The geometry of [Zn14(hmq)8(OH)4X10]2− (X = Cl and Br) was fully optimized, assuming C4 symmetry, and the Mayer bond order was then calculated based on the optimized geometry (Table S6). Attempts to calculate the complexation energies of different possible fragments based on single-crystal geometry failed because of a self-consistent-field convergence problem. As a result, we analyze and explain the different deconstruction processes of [Zn14]2− by summing the calculated bond order for bonds that need to be broken during the deconstructing process (demonstrated as SBO). Electronic absorption spectra of 1-Cl were calculated and simulated with the TD-B3LYP-D3 method and SDD/6-31G(d) basis set with the IEFPCM solvent model in DMSO based on the optimized C4-symmetry geometry. Electronic absorption spectra of H2hmq and Zn(hmq)Cl2 were also calculated for a comparison with the same method. The deviation of {ZnCl2} needs to break two Zn−O bonds and three possible Cl···H hydrogen bonds, resulting in a SBO value of 0.85 (2 × 0.377 + 2 × 0.0475). In comparison, removing one center OH− group breaks two Zn−O bonds and two H−O···H hydrogen bonds with a SBO value of 1.04 (0.329 + 0.330 + 0.395 × 2). These results indicate that the deviation of {ZnCl2} is much easier than removal of OH−. Removing {ZnCl+} on the top or bottom will break bonds with a SBO value of 1.37 (4 × 0.341). In conclusion, the possible deconstruction process of [Zn14]2− would be as follows: Zn14(hmq)8(OH)4Cl10 → Zn10(hmq)8(OH)4Cl2 → Zn5(hmq)4Cl. That is, (1) one to four [ZnCl2] leave Zn14(hmq)8-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02210. Experimental details, OLED fabrication, elemental analyses, IR, TGA, PXRD, X-ray crystallography 14074

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Inorganic Chemistry

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information, ESI-MS, DFT computations, UV−vis and fluorescence spectra, and EL performance (PDF) Accession Codes

CCDC 1540490 and 1543214 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hai-Bing Xu: 0000-0003-3909-414X Yuexing Zhang: 0000-0002-8510-5033 Tao Yang: 0000-0003-1864-8545 Mohamedally Kurmoo: 0000-0002-5205-8410 Ming-Hua Zeng: 0000-0002-7227-7688 Author Contributions §

X.-L.C. and H.-B.X. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation for Distinguished Young Scholars of China (Grant 21525101), NSFC (Grant 21571165), the NSF of China and Guangxi Province (Grants 91122032 and 2014GXNSFFA118003), the BAGUI scholar program (Grant 2014A001), and the Project of Talents Highland of Guangxi Province. M.K. is supported by the CNRS-France.

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