A Volcano-Group-like Halogeno(cyano)cuprate with Efficient Green Luminescence ,†
‡
Xi Liu* and Guo-Cong Guo
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 776–778
College of Chemistry, Chongqing Normal UniVersity, Chongqing 400047, P. R. China, and State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ReceiVed October 7, 2007; ReVised Manuscript ReceiVed December 17, 2007
ABSTRACT: The first two-dimensional halogeno(cyano)cuprate, [(Me4N)(H3O)][(Cu4Cl4)][Cu(CN)2]2, with an unprecedented volcanogroup-like structure possessing peculiar 4-fold Cu4Cl4 crowns has been solvothermally synthesized, which exhibits efficient green luminescence. The emission mechanism was studied in detail to elucidate the relationship of the luminescent properties and crystal structures, which is helpful for the design and synthesis of more efficient luminescent materials. Great interest is presently being focused on the controllable preparation of molecular functional materials using cyanometallate1 and cyanide2 as building blocks and simultaneously employing ancillary ligands with different geometric constraints to afford supramolecular frameworks with diverse structures.3 Among these functional coordination polymers, mixed halogeno(cyano)cuprates have been shown to be long-lived and highly luminescent materials with abundant optical transitions according to others4,5 and our6 recent studies. To design and synthesize more efficient luminescent halogeno(cyano)cuprates, on the one hand, the solid-state emission mechanism of these complexes has been studied by us to elucidate the relationship of the luminescent properties and crystal structures.6 The previous results indicate that in these complexes, the small cyanide groups have rigid linear structures with extended conjugate π electron systems, possessing good potential ability to enhance photoelectron transfer and nonlinear optical reactivity, while the association of halide ions with copper cyanide species can greatly enhance the lifetime and quantum yield of luminescence.4 On the other hand, different synthetic routes are introduced to this system to obtain the halogeno(cyano)cuprates with different compositions and frameworks. One practical method to synthesize halogeno(cyano)cuprates is the reactions of copper cyanide and corresponding halogenide compounds;5,6 another useful method is utilizing the precursor of cyanide, such as RC≡NR′ (R, R′ ) Li, Na, SiMe3, etc.) to react with the copper halogenide accompanied by appropriate ancillary ligands by an organic synthetic route. Thus far, all halogeno(cyano)cuprates were synthesized by the former route,5,6 which may limit further research in this area. The latter route, as a new synthetic route—syntheses using an organic route—is worth exploring, for it may lead to the assembly and construction of novel types of halogeno(cyano)cuprates possessing different compositions and frameworks. The reaction7 of copper(I) chloride, tetramethylammonium chloride, and Me3SiC≡NLi in tetrahydrofuran (THF) under solvothermal conditions produces the first two-dimensional (2D) halogeno(cyano)cuprate, [(Me4N)(H3O)][(Cu4Cl4)][Cu(CN)2]2 (1), which is characterized by X-ray single-crystal structural analysis,8 elemental analysis, and IR spectroscopy.9 The structure of 1 exhibits an unprecedented volcano-group-like structure with peculiar 4-fold Cu4Cl4 crowns. As shown in Figure 1, there are two unique Cu(I) ions in the asymmetric unit. The threecoordinate Cu1 ion is in a distorted trigonal-planar coordination environment and coordinated by two µ-Cl and one cyanide N atom, while the two-coordinate Cu2 is linearly connected by two cyanide * To whom correspondence should be addressed. † Chongqing Normal University. ‡ Chinese Academy of Sciences.
C atoms to form the subunit of [Cu(CN)2]- with the C-Cu-C bond angle of 180°. Four adjacent Cu1 ions are connected by four µ-Cl to form a peculiar eight-membered Cu4Cl4 crown with 4-fold symmetry, which remains unknown in metal-halogen complexes, although several complexes containing distorted eight-membered M4X4 (M ) metal, X ) halogen) rings with low symmetry were reported in literature.10 Each Cu4Cl4 crowns is further quadruple bridged by [Cu(CN)2]- subunits with a Cl-Cu-N bond angle of 127.30(4)° to form a fluctuate volcano-group-like layer along the a and b directions (Figure 2). Within each volcano-like repeating units, the shortest distances between the diametrically opposed atoms (Cu · · · Cu) in the two openings are 4.4689(3) and 12.9254(8) Å, respectively. These fluctuate layers vertically stack together along the c direction to form a three-dimensional (3D) structure merely by van der Waals interactions. The discrete hydronium ions and [Me4N]+ cations locate between these layers and have weak O-H · · · Cl hydrogen bonding and ionic interactions with these layers, respectively (Figure S1, Supporting Information). Compound 1 displays a strong green luminescent emission band in the solid-state at 506 nm upon photoexcitation at 278 nm (Figure 3), and its lifetime was measured to be 22.1 µs, suggesting it to be a good candidate for luminescent material. Density functional theory (DFT) calculations of the electronic band structure of 1 along with density of states (DOS) were carried out with the CASTEP code.11 The results indicate that the top of valence bands (VBs) are mostly formed by Cu-3d state mixing with a small amount of Cl-3p, Ccyanide-2p, and Ncyanide-2p states, while the bottom of the conduction bands (CBs) are almost a contribution from the hybridizations of Ccyanide-2p and Ncyanide-2p states mixing with a small amount of Cu4s states (Figure S2, Supporting Information and its explanation). The population analysis and DOS show that hybridization between Cu-3d and Cl-3p, Cu-3d and Ccyanide-2p, Cu-3d and Ncyanide-2p states takes place and covalent bond character appears between Cu and Cl atoms, Cu and cyanide groups. The calculated strongest absorption peak (also first absorption peak) localizes at about 4.15 eV (299 nm) with polycrystalline geometry (Figure S3, Supporting Information and its explanation), which is comparable to the strongest photoexcitation peak at 278 nm. Accordingly, the origin of the luminescent emission band may mainly ascribe to metal-toligand charge transfer (MLCT) where the electrons are transferred from the copper (Cu-3d states, VBs) to unoccupied π* orbitals of cyanide groups (Ccyanide-2p and Ncyanide-2p states, CBs). The ascription of the luminescence or electronic absorption is consistent with our previous investigation on the solid-state luminescence of other halogeno(cyano)cuprates.6 Combining with our previous time-dependent DFT calculations on halogeno(cyano)cuprates using the Gaussian2003 program, we can find generally,
10.1021/cg7009786 CCC: $40.75 2008 American Chemical Society Published on Web 02/12/2008
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Crystal Growth & Design, Vol. 8, No. 3, 2008 777
Figure 1. ORTEP representation of a repeating unit in 1 with 50% thermal ellipsoids. The part in the red dashed circle represents a peculiar Cu4Cl4 crown with 4-fold symmetry, and its dexter diagram represents the side view of the Cu4Cl4 crown. Selected bond distances (Å) and angles (°): Cu(1)-N(1) 1.902(5), Cu(2)-C(1) 1.840(5), N(1)-C(1) 1.128(7), Cu(1)-Cl(1) 2.301(1), C(1)–N(1)-Cu(1) 170.8(5), N(1)-C(1)-Cu(2) 177.5(5), Cu(1)-Cl(1)-Cu(1B) 86.74(6), Cl(1)-Cu(1)-Cl(1A) 105.31(7). Symmetry codes A: y, 1/2 - x, z; B: 1/2 - y, x, z; C: 1/2 - x, 1/2 - y, z.
Figure 2. A view of 2D fluctuate volcano-group-like layer in 1 along the a and b directions. The orange and green eight-numbered rings represent the Cu4Cl4 crowns upward and downward, respectively.
excitation for halogeno(cyano)cuprates is mainly dominated by the HOMO f LUMO transition, that is, the transition between the Cu(I) centered dπ ground-state and the cyanide group π* state (Figure S4, Supporting Information). This transition is both Laporte and spin allowed, and usually has a big molar extinction coefficient, with the potential ability to emit strong luminescence. The π-acceptor cyanide group presents a low-lying π* orbital and at the same time stabilizes the dπ orbital centered on the Cu(I) by retro-coordination, while the coordination of halide ions with Cu(I) allows the tuning of the Cu(I) centered dπ ground state, resulting in the influence on the lifetime and quantum yield of luminescence. In a word, the nature and energy of the HOMO and LUMO are especially important in understanding the long-lived and high luminescence of halogeno(cyano)cuprates since they are the orbitals involved in the lowest energy excited state.
Figure 3. Solid-state electronic excitation (λex ) 278 nm) and emission (λem ) 506 nm) of 1 at room temperature.
in each halogeno(cyano)cuprate, the highest occupied molecular orbital (HOMO) is mainly composed of the dπ orbital of Cu(I), while the lowest unoccupied molecular orbital (LUMO) mostly consists of the π* orbital of cyanide group, and the lowest singlet
In summary, we have synthesized the first 2D halogeno(cyano)cuprate with an unprecedented volcano-group-like structure by an organic synthetic route. The results are significant not only for producing a novel type of halogeno(cyano)cuprate and for further exploration of halogeno(cyano)cuprates by introducing a new synthetic route, but also for elucidating the relationship of the luminescent properties and crystal structures in halogeno(cyano)cuprates, which may be helpful for the design and synthesis of more efficient luminescent materials.
778 Crystal Growth & Design, Vol. 8, No. 3, 2008 Supporting Information Available: X-ray crystallographic files in CIF format; theoretical approach methodology and corresponding calculated results; lifetime of the luminescence. This material is available free of charge via the Internet at http://pubs.acs.org.
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Acknowledgment. We gratefully acknowledge financial support of the NSF of China (20701041), the NSF of CQ (CSTC2007BB4234) and Scientific project of Chongqing Municipal Education Commission (KJ070812).
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under autogenous pressure and heated at 160 °C for 3 days, followed by cooling at 0.1 °C/min to room temperature. The result orange crystals were collected with ca. 50% yield (based on CuCl). Crystal data of 1: Tetragonal, P4/nmm, a ) b ) 12.300(1), c ) 6.856(1) Å, V ) 1037.1(2) Å3, Z ) 2, Mr ) 720.29, ñ ) 2.306 g cm-3, F(000) ) 696, µ(MoKR) ) 6.561 mm-1, θmax ) 25.00°, 6513 measured reflections, 543 independent reflections. R1 (wR1) ) 0.0326 (0.0959) for 538 reflections (I > 2σ(I)) and 424 parameters. GOF ) 1.005. Crystal dimensions: 0.15 × 0.12 × 0.10 mm3. CCDC-657003. Data collection for 1 was performed on a Rigaku Mercury CCD diffractometer equipped with a graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) at room temperature. The intensity data set was collected with the ω scan technique and reduced by CrystalClear software. The structure was solved by the direct methods and refined by full-matrix least-squares techniques. Non-hydrogen atoms were located by difference Fourier maps and subjected to anisotropic refinement. The methyl H atoms were added according to the theoretical models, while the hydronium H atoms were not included in the compound. All of the calculations were performed by the Siemens SHELXTL version 5 package of crystallographic software. Since the C and N atoms in the bridging CN groups cannot distinguished crystallographically, assignments based on d(Cu-C) < d(Cu-N) have been made for the present compounds. Anal. Calcd for 1, C8H15Cl4Cu6N5O: C, 13.34; H, 2.10; N, 9.72. Found: C, 13.50; H, 2.22; N, 9.83. FT-IR (KBr, cm-1): 3447(w), 3332(w), 3268(w), 3124(w), 3031(w), 2814(w), 2141(vs), 1655(w), 1596(w), 1481(s), 1412(s), 1389(m), 1105(w), 987(w), 948(s), 851(w), 596(w), 515(w), 480(w), 451(w). ν (CN): 2141(vs). (a) For examples, see Mak, T. C. W.; Wong, H. N. C.; Sze, K. H.; Book, L. J. Organomet. Chem. 1983, 255, 123. (b) Xu, W.; Vittal, J. J.; Puddephatt, R. J. J. Am. Chem. Soc. 1995, 117, 8362. (c) Sugimoto, K.; Kuroda-Sowa, T.; Maekawa, M.; Munakata, M. Chem. Commun. 1999, 455. (d) Sullivan, R. M.; Liu, H.; Smith, D. S.; Hanson, J. C.; Osterhout, D.; Ciraolo, M.; Grey, C. P.; Martin, J. D. J. Am. Chem. Soc. 2003, 125, 11065. (a) Segall, M.; Linda, P.; Probert, M.; Pickard, C.; Hasnip, P.; Clark, S.; Payne, M. Materials Studio CASTEP, version 2.2; Accelrys: San Diego, CA, 2002. (b) Segall, M.; Linda, P.; Probert, M.; Pickard, C.; Hasnip, P.; Clark, S.; Payne, M. J. Phys.: Condens. Matter 2002, 14, 2717.
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