Article pubs.acs.org/JACS
All-in-One: Achieving Robust, Strongly Luminescent and Highly Dispersible Hybrid Materials by Combining Ionic and Coordinate Bonds in Molecular Crystals Wei Liu,† Kun Zhu,† Simon J. Teat,‡ Gangotri Dey,† Zeqing Shen,†,§ Lu Wang,† Deirdre M. O’Carroll,†,§ and Jing Li*,† †
Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, United States Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States § Department of Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854, United States ‡
S Supporting Information *
ABSTRACT: Extensive research has been pursued to develop low-cost and highperformance functional inorganic−organic hybrid materials for clean/renewable energy related applications. While great progress has been made in the recent years, some key challenges remain to be tackled. One major issue is the generally poor stability of these materials, which originates from relatively fragile/weak bonds between inorganic and organic constituents. Herein, we report a unique “all-in-one” (AIO) approach in constructing robust structures with desired properties. Such approach allows formation of both ionic and coordinate bonds within a molecular cluster, which greatly enhances structural stability while maintaining the molecular identity of the cluster and its high luminescence. The novel AIO structures are composed of various anionic (CumIm+n)n− clusters and cationic N-ligands. They exhibit high luminescence efficiency, significantly improved chemical, thermal and moisture stability, and excellent solution processability. Both temperature dependent photoluminescence experiments and DFT calculations are performed to investigate the luminescence origin and emission mechanism of these materials, and their suitability as energy-saving LED lighting phosphors is assessed. This study offers a new material designing strategy that may be generalized to many other material classes.
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INTRODUCTION Crystalline inorganic−organic hybrid semiconductors are a very interesting and unique class of functional materials. One of the most attractive features of these compounds is their ability in blending the existing (good) properties inherent to either inorganic or organic component, as well as in creating new features that are not possible by either counterpart alone, but as a result of their bonding interactions at the nanoscaled interfaces.1−6 Many of these materials are of great importance due to their enormous promise for use in clean and/or renewable energy devices, including but not limited to photovoltaics and solid-state lighting.7−16 However, a critical drawback is their generally low chemical and thermal stability, which must be addressed before their commercialization can be realized.17−19 One of the well-explored hybrid material families consists of I−VII binary metal halides (e.g., CuI).20−23 Most of them are constructed by forming coordinate bonds between neutral inorganic modules of various dimensionality (e.g., clusters of dimer, trimer, and tetramer, chains and sheets) and neutral organic ligands (either aromatic or aliphatic) via an electron lone-pair of N, P or S atom. A wealth of many structure types have been reported over the past several decades, ranging from © 2017 American Chemical Society
molecular (0D) species to one-dimensional (1D) chains, and from two-dimensional (2D) layers to extended three-dimensional (3D) networks.22−24 However, it was not until very recently that their potential for use as lighting phosphors that are free of rare-earth elements (REEs) was fully recognized and systematically evaluated.24−27 These studies reveal that structures built on cluster-based CumIm modules (e.g., Cu2I2 rhomboid dimer or Cu4I4 cubane tetramer) typically have much higher luminescence efficiency compared to those made of inorganic modules of higher dimensions, such as 1D chain or 2D layer of (CuI)∞.25−27 Structures with strong emission and enhanced stability can be achieved by incorporating (a) highly emissive inorganic cores, and (b) functionalized ligands that form stronger Cu-L coordinate bonds to generate extended 1D, 2D and 3D networks. While these developments are applaudable, some serious limitations remain, for example low blue-light excitability and poor solution processability, the latter being the common problem for most crystalline inorganic materials having extended network structures. These problems Received: May 3, 2017 Published: June 17, 2017 9281
DOI: 10.1021/jacs.7b04550 J. Am. Chem. Soc. 2017, 139, 9281−9290
Article
Journal of the American Chemical Society
Figure 1. (a) Illustration of the structure designing strategy for “all-in-one” type structures. Top left: a conventional neutral CuI(L) molecular species with Cu-L dative bond only. Bottom left: an ionic CuI(L) composed of ionic bond only. Right: “all-in-one” structure possessing both bonding types. (b) Examples of inorganic anionic cluster motifs obtained for compounds 1−10. C and H atoms are omitted for clarity purpose. Color scheme: Cu, cyan; I, purple; N, blue; P, green; S, yellow. Design and Synthesis of Ligands. Selected cationic organic ligands were prepared according to reported procedures with some modifications.28−30 For Route I, alkylation was controlled in such a way that it took place at one of the two N atoms. For Route II, alkylation occurred at the aromatic N. For Route III, alkylation happened at the functional group of the ligand. Preparation of 1-Benzyl-1,4-diazabicyclo[2.2.2]octan-1-ium (bz-ted).28 Ted (1.12 g, 10 mmol) was first dissolved in acetone (50 mL) upon sonication and benzyl bromide (1.71 g, 10 mmol) was added dropwise into it under magnetic stirring. The solution remained clear after the mixing and white precipitate formed in a few hours. The precipitate was collected by filtration, washed with ethyl acetate, and dried under vacuum. The yield is 87%. Preparation of 1-(3-Chloropropyl)-1,4-diazabicyclo[2.2.2] octan-1-ium (3-Cl-pr-ted).28 Ted (1.12 g, 10 mmol) was dissolved in ethyl estate (40 mL) upon sonication and 1-chloro-3-iodopropane (2.04 g, 10 mmol) was added slowly into it under magnetic stirring. The solution was stirred for 2 h and the white precipitate formed was collected by filtration, washed with ethyl acetate, and dried under vacuum. The yield is 67%. Preparation of 1-Propyl-1,4-diazabicyclo[2.2.2]octan-1-ium (pr-ted).28 Ted (1.12g, 10 mmol) was first dissolved in acetone (50 mL) upon sonication and 1-bromopropane (1.23 g, 10 mmol) was added dropwise into it under magnetic stirring. The solution remained clear after the mixing and colorless oily precipitate formed in an hour. The precipitate was collected by centrifuge, washed with ethyl acetate, and dried under vacuum. The yield is 68%. Preparation of 1-(2-Bromoethyl)-1,4-diazabicyclo[2.2.2]octan-1-ium (2-Br-et-ted).28 1-Bromo-2-iodoethane (2.34 g, 10 mmol) was added dropwise into acetone (50 mL) containing ted (1.12 g, 10 mmol) under magnetic stirring. White precipitate formed in a few hours, and the precipitate was collected by centrifuge, washed with ethyl acetate, and dried under vacuum. The yield is 70%. Preparation of 1-Isopropyl-1,4-diazabicyclo[2.2.2]octan-1ium (i-pr-ted).28 Ted (1.12 g, 10 mmol) was first dissolved in
must be resolved before any of their practical uses can be considered. To tackle these issues, we design a unique type of CuI-based hybrid structures by combining ionic and covalent (coordinate) bonds within a single molecular cluster. This unprecedented “all-in-one” (AIO) approach allows the creation of compounds bearing all three desired features: (i) being chemically and thermally robust (due to enhanced binding strength through two types of bonds); (ii) possessing high luminescence efficiency (as a result of inclusion of highly emissive inorganic cluster core); and (iii) having excellent solution processability (because of their high solubility and/or easy dispersibility in common solvents inherent to their molecular identity). As shown in Figure 1a (right), a typical AIO structure contains an anionic inorganic module and a cationic organic ligand. The soformed “ionic pair” is also bonded directly via a coordinate/ dative bond between the metal (Cu) and ligand (L), generating an overall charge-neutral molecular complex.
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EXPERIMENTAL SECTION
Materials. CuI (98%, Alfa Aesar); bulk methanol (98%, Alfa Aesar); dichloromethane (99+%, Alfa Aesar); 4-mercaptopyridine (>97%, Alfa Aesar); N,N-dimethylethylamine (>98%, TCI); N,Ndimethylformamide (99.8%, Sigma-Aldrich); thionyl chloride (97%, Alfa Aesar); formaldehyde (37% in aq. soln., Alfa Aesar); tributylamine (>98%, Alfa Aesar); 4,4′-dipyridyl disulfide (98%, TCI); 1,4diazabicyclo[2.2.2]octane (>98%, TCI); benzotriazole (99%, Alfa Aesar); benzyl bromide (99%, Alfa Aesar); 1-bromopropane (99%, Alfa Aesar); 1-bromobutane (>98%, Alfa Aesar); 1-chloro-3iodopropane (>98%, Alfa Aesar); 1-bromo-2-iodoethane (98%, Sigma-Aldrich); 2-bromopropane (99%, Alfa Aesar); sodium salicylate (99%, Merck), BaMgAl10O17:Eu2+ and YAG:Ce3+ type 9800 (Global Tungsten & Powders Corp), PolyOx N750 (Dow Chemical). 9282
DOI: 10.1021/jacs.7b04550 J. Am. Chem. Soc. 2017, 139, 9281−9290
Article
Journal of the American Chemical Society
Synthesis of Cu6I8(bu-ted)2 (6). Compound 6 was synthesized by the method similar as that of 1. Plate-shaped colorless single crystals could be obtained in 1 day. Yield is 83%. Synthesis of Cu2I4(mtp)2 (7). Compound 7 was prepared by the solvothermal reaction of CuI, KI, 4-mercaptopyridine and methanol in a molar ratio of 1:6:2:1:300 at 150 °C for 48 h. pH of the reaction mixture were adjusted by 1 M HCl to 2. Large amount of yellowish crystalline powder of 7 with the yield of 71% was generated. Rodshaped crystals suitable for single-crystal X-ray analyses were together with the powder and were separated for structure determination. Synthesis of Cu4I6(tpp)2(bttmm)2 (8). 0D-Cu2I2(tpp)3 (0.1 g) was first well dispersed in a mixture of toluene (2 mL) and CH2Cl2 (2 mL), and then was mixed with the ligand (0.1 g) in methanol (2 mL). The mixture was heated at 80 °C for 2 days, which afforded greenish crystalline powder along with plate-shaped single crystals. Yield is 59%. Synthesis of Cu4I6(tpp)2(btmdme)2 (9). Yellowish rock shaped single crystals were obtained under the similar condition as that of 8. Yield is 52%. Synthesis of Cu6I8(btmdb)2 (10). Saturated KI solution (2 mL) containing CuI (0.19 g, 1 mmol) was mixed with methanol (2 mL) containing the ligand (8 mmol). Then acetone (5 mL) was added and the reaction mixture was kept in the open air for the solvent to slowly evaporate. Yellowish rhombohedral-shaped single crystals formed in 3 days and were collected by filtration. Yield is 65%. Single Crystal X-ray Diffraction (SCXRD). Single crystal data of 1−10 were collected on a D8 goniostat equipped with a Bruker PHOTON100 CMOS detector at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, using synchrotron radiation. The structures were solved by direct methods and refined by fullmatrix least-squares on F2 using the Bruker SHELXTL package. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. The structures were deposited in Cambridge Structural Database (CSD) with numbers 1498474, 1529809−1529817. Powder X-ray Diffraction (PXRD) Analysis. Powder X-ray diffraction (PXRD) analyses were carried out on a Rigaku Ultima-IV unit using Cu Kα radiation (λ = 1.5406 Å). The data were collected at room temperature in a 2θ range of 3−50° with a scan speed of 1°/min. The operating power was 40 kV/40 mA. Room-Temperature Photoluminescence Measurements. Photoluminescence (PL) measurements were carried out on a Varian Cary Eclipse spectrophotometer. Powder samples were evenly distributed and sandwiched between two glass slides (which do not have emission in the visible range) for room temperature measurements. Temperature Dependent Photoluminescence Spectroscopy and Lifetime Measurements. Pressed pellets of 1 to 1.5 mm thickness were prepared in a 10 mm diameter die with the pressure of approximately 3000 psi. The pellets were sandwiched between two pieces of cover glass using a nonfluorescent double-sided tape (Bazic Product, permanent double-sided tape), and were mounted on the sample holder of a cryostat (Cryo Industries of America, Inc., Model 6NSVT Variable Temperature Optical Top-Loading Nitrogen Cryostat). A Cryo-con 32B Temperature Controller was used to precisely control the temperature of the sample chamber. An excitation wavelength of 380 nm (Spectra-Physics MaiTai BB Ti:sapphire laser (pulse width
