A Host Material for Deep-Blue Electrophosphorescence Based on a

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A Host Material for Deep-Blue Electrophosphorescence Based on a Cuprous Metal-Organic Framework Zhensheng You, Heng Li, Lijun Zhang, Bing Yu, Jin Zhang, and Xiaoming Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07627 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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The Journal of Physical Chemistry

A Host Material for Deep-Blue Electrophosphorescence Based on a Cuprous Metal-Organic Framework Zhensheng You,1 Heng Li,1 Lijun Zhang,1,* Bing Yu,1 Jin Zhang,1 and Xiaoming Wu2,*

1

Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, 300384 Tianjin, P.R. China. E-mail: [email protected]

2

Key Laboratory of Display Materials and Photoelectric Devices, Ministry of Education, School of

Materials Science and Engineering, Tianjin University of Technology, 300384 Tianjin, P.R. China. E-mail: [email protected]

*Corresponding Authors Requests for materials should be addressed to Lijun Zhang.

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Abstract Although metal-organic frameworks (MOFs) have promising applications in various fields, efforts to utilize MOFs as emissive layers in electroluminescent devices have not been progressed well due to their intrinsic nature. Fortunately, by using fluorene derivatives and cuprous ions, a novel MOF (CuP6) has been synthesized for blue electrophosphorescence. Topological structure of Cu-P6 displays a 3-connected unimodal net with a cross-stacking pattern. Such a structure allowed phosphorescent CuP6 to exhibit a good heat resistance and luminous efficiency (Φ = 0.62) at crystalline state. By containing both donor and acceptor in the structure, Cu-P6 possesses a balanced electron and hole mobility. Especially, electrical-driven deep-blue emission of a MOF was reported for the first time. Overall, this work may open new perspectives on developing highly efficient electrophosphorescent devices based on MOFs. The utilization of earth-abundant cuprous ions is another attractive feature of Cu-P6.

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Introduction Metal-organic framework (MOFs) are a type of tunable porous materials developing from coordination chemistry. Generally, various combinations of linkers and metal ions result in MOFs exhibiting many extraordinary properties. Due to these properties, MOFs have attracted increasing attentions in recent years.1-3 Thus far, their applications in medicine,4 catalysis,5-7 gas storage 8-10 and other fields

11-12

have been deeply investigated by scientists. Especially, it is well known that many

MOFs exhibit photoluminescent properties and thermal stabilities under various conditions, which suggests a certain prospective in electroluminescent devices. Hence, scientists never desisted from trying to explore the electroluminescent properties of MOFs. However, many MOFs behave like insulators and insoluble in most solvents, forcibly dissolving MOFs may cause decomposition or irreversible damage to their structures. Since vulnerable structures and insoluble properties of MOFs have prevented themselves from traditional coating method like thermal evaporation and spin coating, it is a challenge to make a MOFs-containing electroluminescent device. Therefore, electroluminescent behaviors of MOFs have hardly been reported.13-14 As organic light-emitting diodes (OLEDs) have been serving increasingly important roles in both scientific research and daily life, 15-17 it is an urgent issue to develop MOFs-containing electroluminescent devices. Late in 2015, Wang’s group constructed a novel MOF NNU-27 using a carboxylate ligand derived from anthracene.18 Electrical conductive NNU-27 exhibited an orange red emission at 575nm. Solventinduced electroluminescent behaviors of MOF-5 were also reported by Asadi’s and Garcia’s group respectively.19-20 MOF-5 exhibited red electroluminescence when it was excited by either AC or DC voltage source. Although only phosphorescent materials have the possibility to achieve 100% of internal quantum efficiency,21 electrophosphorescent behaviors of MOFs are still remaining unknown. 3 / 25



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Especially, as one of the three-primary colors, a new MOF-based host material for blue electrophosphorescnece with high color purity must be developed. As a blue-emission chromophore with a high charge mobility, fluorene derivatives have attracted attention from researchers for a long period. However, fluorene derivatives are generally suffering from relatively low-energy triplet exciton levels and singlet emission levels.22-24 Fortunately, these problems can be solved by introducing phosphine oxide groups into the backbone.25-26 Sapochak’s group reported an OLED device using 2,7-bis(diphenylphosphine oxide)-9,9-dimethylfluorene (PO6) as a host material.27 A sky-blue emission with a luminous power efficiency of 25.1lm/W was exhibited by doping PO6 with iridium(III)bis(4,6-(difluophenyl)pyridinato-N,C2)picolinate (FIrpic). Three years later, Chou’s group substituted both C9-substituents and created a 2,7-bis(diphenylphosphoryl)9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF)-based device by doping with FIrpic.28 This blue-emission device exhibited a significant enhancement in the luminous power efficiency (36.7lm/W). In this work, to improve the coordination ability of the ligands, trivalent phosphines connected to a fluorene, 2,7-bis(diphenylphosphino)-9,9-dimethylfluorene (P6), was selected as a linker in our original MOF. Based on a previous report of Thompson, we have realized that a key to achieve efficient electrophosphorescnece is to reduce the phosphorescence lifetime.29 Hence, we chose nonluminous cuprous ions as acceptors to transfer energies from donors. Herein, we report the first example of the deep-blue electrophosphorescent behavior exhibited by an original metal-organic framework whose composition was determined to be [Cu2(C39H32P2)2]n (CuP6). Its crystal structure as well as thermal, photoluminescent and electroluminescent properties are presented. As expected, Cu-P6 crystals possess a good photoluminescent efficiency (Φ = 0.62) and an electrical-driven deep-blue emission centered at 459nm. 4 / 25


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Experimental Section General Information. All reagents are commercially available except for additional descriptions. 2,7dibromo-9,9-dimethylfluorene (BrDMeF) was further purified through a short column chromatography using silica gel as stationary phase. The concentration of n-BuLi in hexanes was determined through a standard titration procedure using diphenylacetic acid as an indicator before every formal reactions. Synthesis of 2,7-Bis(diphenylphosphino)-9,9-dimethylfluorene. P6 was synthesized through a typical halogen-lithium exchange reaction. To begin with, 19.97g (56.7mmol, 1.00equiv) of purified BrDMeF were added into a three-necked flask before which nitrogen was filled in the flask following the standard schlenk-line technique. Then, the solution was stirred vigorously for 10min after a total of 200mL anhydrous THF had been added into the flask. EA-LN2 bath was utilized to cool the temperature to -84℃, 57.5mL of n-BuLi (2.121M in hexanes, 122.0mmol, 2.15equiv) were added dropwise under this temperature. After stirring for 3h, 22.5mL (125.0mmol, 2.20equiv) of chlorodiphenylphosphine were added. The temperature was kept at -84℃ for a further 2h then slowly heated to room temperature. The reaction was not allowed to be quenched with 30mL degassed methanol until continuing stirring for another 1h. Solvent was removed under the vacuum. Then the solid was washed by DCM for many times after which the organic phase was washed by saturated NaCl aqueous solution. DCM was removed under the vacuum. The resulted solid was purified by column chromatography (silica gel, Rf = 0.31, DCM:PE = 1:3) to give 24.18g of P6 in chemical pure (79%). A blue fluorescence can be observed when the product was exposed under the 365nm UV lamps. M.P. = 198-200℃. FT-IR (cm-1): 2921, 1958, 1885, 1816, 1731, 1663, 1578, 1428 (C-P), 999, 742, 694. 1H NMR (400MHz, CDCl3, TMS): δ 7.644 (d, J = 7.565 Hz, 2H), δ 7.455 (d, J = 8.197 Hz, 5 / 25



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Scheme 1. Synthesis of the Cu-P6 framework.

2H), δ 7.325 (s, 20H), δ 7.205 (m, 2H), δ 1.366 (s, 6H). 13C NMR (100MHz, CDCl3, TMS): δ 153.941 (d, J = 8.598 Hz), δ 139.407 (s), δ 137.465 (d, J = 11.012 Hz), δ 136.411 (d, J = 11.005 Hz), δ 133.628 (d, J = 19.380 Hz), δ 132.555 (d, J = 15.538 Hz), δ 128.515 (m), δ 120.212 (d, J = 6.378 Hz), δ 46.860 (s), δ 26.866 (s). 31P NMR (161MHz, CDCl3, relative to 85% H3PO4): δ -4.212 (s). Preparation of the Cu-P6 Framework. A solvothermal synthesis method was utilized in this preparation procedure. To begin with, 151.9mg (0.270mmol) of ligand P6, 13.5mg (0.136mmol) of CuCl and 4mL of the mixed solvent (CH3CN:THF = 6:17) were added into a Teflon liner. After vibrated in supersonic wave for 30min, the autoclave was packed tightly. The autoclave was cooled down to room temperature at a cooling rate of 10℃/h after it was heated at 150℃ for 3d. Clear yellowish crystalline plates were obtained after the autoclave was stewing for another one night. These crystals were then washed by DCM for three times and dried under vacuum for 6h. A bright aqua-blue color can be observed under 365nm UV lamps. Tf (℃): 243.8℃ (determined by DSC method during the second scanning). FT-IR (cm-1): 3048, 2957, 2856, 1960, 1898, 1818, 1584, 1476, 1432 (C-P), 1095, 744, 697, 508. General Characterization. Single-crystal X-ray diffraction was performed on a Bruker SMART CCD area-detector at room temperature. Olex2 v1.2.5 and Platon v1.17 were utilized in refinement process. Powder patterns were simulated by Mercury v3.9 at a wavelength of 1.54056nm. Measured powder 6 / 25


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patterns were performed on Rigaku Ultima IV within a constant range of 5°≤2θ≤50°, measured sample was made through a standard backpressure method. NMR data were recorded using Bruker400MHz with no nitrogen or argon atmosphere. Differential scanning calorimetry (DSC) data were collected by Netzsch DSC 200 F3 with a heating rate of 10K/min and a nitrogen flow, the sample was heated to 250℃ for 20min, then cooled to room temperature at a cooling rate of 50K/min, the glass transition temperature Tg was measured during the second scanning. Thermogravimetric analysis (TGA) were performed on a Netzsch TG 209 F3 with a heating rate of 10K/min and a nitrogen flow. FT-IR data were recorded by Bruker Vertex 70 using KBr disks. Solid-state UV-vis absorption spectrum were recorded by Hitachi 3900 using BaSO4 as references. Scanning electron microscope (SEM) measurements were performed on ZEISS Merlin Compact. Pictures in this report were all taken by a Nikon D610 DSLR camera equipped with a Sigma Art 50mm lens, original pictures were in NEF format. Photoluminescence Characterization. Solid-state fluorescence (FL) spectra and phosphorescence lifetime data were recorded on a Hitachi F-7000 FL spectrophotometer with no filters. Phosphorescence spectra was determined by the same instrument with a detector delay. Furthermore, all PL spectra were obtained at room temperature. Internal photoluminescence quantum yield (Φ) was determined using Edinburgh FLS 920P with an integrating sphere following the steps as description. First, Al2O3 was filled in a sample holder and set in the integrating sphere, then the photoluminescence emission spectrum was measured. The observed peak area was defined as A. Second, the sample was filled in another sample holder and set in the integrating sphere. The obtained excitation peak area was determined as B, emission peak area was determined as C. Hence, Φ can be calculated as bellow: 𝛷 = 𝐶⁄(𝐴 − 𝐵) 7 / 25



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Computational Methods. According to the time-dependent density functional theory (TD-DFT), electronic structure calculations were performed with GAUSSIAN software at the B3LYP/6-31G* level. The starting geometry of Cu-P6 was obtained from the single crystallographic data. The geometry of P6 was simulated by MATERIALS STUDIO software. Electroluminescence Characterization. Several ITO/Cu-P6/ITO sandwiches were made before formal tests. To begin with, commercially available glass substrate covered by ITO (sheet resistance ≤ 10 Ohm) was cut into pieces in the size of approximately 10mm×20mm and washed by toluene, acetone, ethanol and deionized water in sequence then dried under an infrared lamp with continuous nitrogen flow. Next, several pieces of good crystals obtained from solvothermal method were tiled on the conductive side of ITO glass and covered by another ITO glass (conductive side was in contact with crystals). The sandwich was fastened by two plastic clothespins. Zhaoxin PS-6005D was utilized as a DC voltage source. A tandem resistance (50 Ohm) was added into the circuitry to avoid the electrical short. Current and voltage were measured by two multimeters (Yongqiang DT890B) respectively. Electroluminescence spectra were recorded by Hitachi F-7000 FL spectrophotometer. Data Availability. The single crystal X-ray crystallographic structure reported in this article has been deposited at the Cambridge Crystallographic Data Center (CCDC), under deposition number CCDC 1545515.

The

data

can

be

obtained

free

of

charge

from

the

CCDC

through

www.ccdc.cam.ac.uk/structures. All other data and materials are available from the authors on a reasonable request.

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Results and Discussion Crystal Structure. The structure of the Cu-P6 framework was characterized through single-crystal X-ray diffraction, and the crystallographic details are summarized in Table S1. It has been calculated by PLATON software that Cu-P6 has a solvent accessible volume of 1061.2Å3 (14.8% in 7188.2Å3) and a SQUEEZE process was utilized to handle those voids.30 The comparison of powder X-ray diffraction (PXRD) patterns gave us a total agreement with our refined structure (Figure S1).

Figure 1. Asymmetric unit of the Cu-P6 framework. All hydrogens are omitted for clarity convenience.

The X-ray crystallography indicates that Cu-P6 crystallizes in the triclinic space group P -1. The asymmetric unit of Cu-P6 contains four P6 ligands, four copper(I) ions and four chloride ions (Figure 1). One P6 ligand is acting as a μ2-bridge and linking two copper(I) ions by using two terminal Pdonors, the distances between two copper(I) ions are ranged from 12.673Å to 12.825Å. Furthermore, P-C (fluorene rings) distances are in the range of 1.813-1.852Å. Two types of phosphine were observed, as evidenced by measuring the dihedral angles between two phenyl groups connected to the same phosphine. C-P-C (phenyls) angles in these two types of phosphine are in ranges of 59.462°-67.631° and 83.532°-87.347°, respectively. These two different types of phosphine are connecting to the same cuprous ions, P-Cu-P angles are in the range of 115.824°-117.001°, the bond angles of Cu-P-C 9 / 25



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(fluorene rings) are ranged from 114.502°to 116.568°. The copper(I) ions are four coordinated by two phosphine from two different P6 groups and two chloride ions, distances of seven Cu-P coordination bonds in one asymmetric unit are all ranged from 2.271Å to 2.304Å. As for Cu-Cl distances, they are in the range of 2.372-2.471Å. It indicates that the bond strength of Cu-P are not much stronger than Cu-Cl and lower than C-P. Crystal structure of Cu-P6 shows that the coordination bonds have prevented neighboring fluorene from

forming

face-to-face

π-π

interactions.

Besides

coordination

bonds,

bis(diphenylphosphine)fluorene linkers are fixed in the crystal structure by C-H…Cl close contacts

Figure 2. (a) Three dimensional architecture of the Cu-P6 framework. (b) Simplified topological structure of Cu-P6. (c) View of the topological structure along c axis. (d) & (e) View of the cross-stacking between neighboring networks along c and b axis, respectively. (f) & (g) Slipping of fluorene rings in the same network from different views. All hydrogens are omitted for clarity convenience.

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(Table S2). Three dimensional architecture of Cu-P6 can be simplified to a novel uninodal 3connected topology with a point symbol (42·6) by using TOPOS software (Figure 2a and b).31-32 To our surprise, these networks exhibit a cross-stacking pattern along the c axis (Figure 2c). P6 linkers, which come from neighboring networks, are rotating relatively to each other by an angle of 73° (average level, Figure 2d). This rotation should be attributed to the huge steric hindrance brought by dpp (dpp = diphenylphosphino) groups. Furthermore, neighborhoods kept away from each other in a distance of 5.213Å, no face-to-face π-π interactions can form in such a long distance (Figure 2e). Fluorene rings are slightly slipping from one another in a distance of 2.566Å, this slipping is continuing along the network and forming an S-shaped tortuosity in the network (Figure 2f and g). Although the networks are not growing along a straight line, they still keep paralleling to each other (Figure 2b). According to former reports, such a fantastic cross-stacking pattern should promote a high luminous efficiency at crystalline state.33

Figure 3. DSC (blue line) and TGA (red line) traces of Cu-P6 (with a heating rate of 10K/min, nitrogen flow).

Thermal Properties. The Thermogravimetric analysis (TGA) trace (Figure 3, red line) have revealed that the Cu-P6 framework is thermally stable till 388.7℃. The glass transition temperature Tg of CuP6 was determined from the crossover point of the two tangents to flat and steep lines at the beginning of the endothermic peak through the differential scanning calorimetry (DSC) method during the second 11 / 25



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scanning (Figure 3, blue line). The fusion temperature Tf of Cu-P6 was estimated from the highest point of the endothermic peak. Measured values of Tg and Tf were 214.2℃ and 243.8℃, respectively. Such high values of Tg and Tf should allow Cu-P6 to exhibit a good heat resistant performance during some extreme operating conditions as a host material in electroluminescent devices.

Figure 4. Solid-state photoluminescence (PL) spectra of (a) P6 (fluorescence (FL) emission spectrum was excited at 280nm, λem of FL excitation spectrum was at 465nm, phosphorescence (PH) emission spectrum was excited at 365nm) and (b) CuP6 (FL emission spectrum was excited at 375nm, λem of FL excitation spectrum was at 465nm, PH emission spectrum was excited at 350nm) at room temperature. The inserted pictures are showing the samples excited by 365nm UV lamps.

Photoluminescent Properties. UV-vis absorption spectra of P6 and Cu-P6 are illustrated in Figure 4a and b (red lines), respectively. The P6 ligand exhibited two absorption peaks at 266nm and 322nm, which are assigned to π-π* and n-π* electronic transitions, respectively. The first absorption peak has red-shifted to 290nm upon the formation of the Cu-P6 framework. This is because the conjugated 12 / 25


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system was expanded by the formation of complex polymer and thus to reduce the energy of π* orbit. A large red shift (322nm → 371nm) of the second absorption peak appeared after the formation of the Cu-P6 framework. This is not surprising, as the phosphorous atom is an electronic donor in cuprous complexes. Electron transportation leads to a redshift of the absorption peak caused by n-π* transition. The Cu-P6 framework exhibited an additional absorption peak at 229nm, the appearing of this absorption peak can be attributed to a ligand-to-metal charge transfer (LMCT) after the introduction of cuprous acceptor. A large Stoke’s shift was observed by comparing the absorption spectra between P6 and Cu-P6. Generally, a larger Stoke’s shift can be observed in complexes due to their complex energy levels. The optical energy gaps Eg of P6 and Cu-P6 were estimated by extrapolating the lowestenergy absorption peak to zero absorbance (see Figure S2). Following this method, Eg of P6 and CuP6 were determined to be 3.22eV and 2.83eV. This Eg value of the Cu-P6 framework is comparable lower than that of the well-known blue emitting material GaN (Eg = 3.42±0.02eV).34 Furthermore, since the intrinsic luminescent wavelength of the Cu-P6 framework can be calculated to be ~438nm (pure blue), this MOF exhibits a huge prospective to be utilized as emissive layers in blue electroluminescent devices. The solid-state fluorescence (FL) spectra of P6 and Cu-P6 are shown in Figure 4a and b (green lines), respectively. The FL excitation spectra of Cu-P6 exhibited the same shape as that of ligand P6, which indicates that these materials have the same electronic-transition pattern described in UV-vis absorption spectra. In this report, the singlet energies ES were determined by making the tangent line to the highest energy peak in FL spectra meet at the x axis.35 Hence, ES of P6 and Cu-P6 were estimated to be 3.24eV and 3.08eV, respectively. The solid-state phosphorescence (PH) spectra of P6 and Cu-P6 were determined at room temperature 13 / 25



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and illustrated in Figure 4a and b (blue lines). Compared to the FL emission spectrum, the PH emission spectrum of Cu-P6 was not significantly redshifted and exhibited two emission peaks at 468nm and 501nm. Triplet energies ET were indicated by the same way as ES. P6 and Cu-P6 exhibited a similar peak corresponding to an ET ~2.78eV. Hence, singlet-triplet energy gaps △EST of P6 and Cu-P6 can be calculated to be 0.46eV and 0.30eV, respectively. Compared to the ligand P6, △EST of Cu-P6 has decreased by 0.16eV.

Figure 5. Room temperature phosphorescence lifetime diagram of Cu-P6 (λem=468nm, λex=398nm) and P6 (λem=543nm, λex=371nm).

The room temperature PH lifetimes of Cu-P6 and P6 were examined and illustrated in Figure 5. The reported MOF lost nearly 75% of its PH intensity within the first millisecond. At approximately 10ms after excitation, nearly no PH was detected. In contrast, the decreasing of the P6 PH intensity was significantly slower than that of Cu-P6. To determine the PH lifetime τ of Cu-P6, the examined curve was fitted to the following double exponential function: 14 / 25


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𝑦 = 𝐴1 𝑒

−𝑥⁄ 𝑡1

+ 𝐴2 𝑒

−𝑥⁄ 𝑡2

+ 𝑦0

Where A1 = 0.6857, t1 = 0.6857, A2 = 0.42987, t2 = 1.84361, y0 = 0.00645, and R2 = 0.99957. The τ value was calculated as a weighted average in the following manner: 𝜏=

(𝐴1𝑡1 + 𝐴2 𝑡2 ) ⁄(𝐴 + 𝐴 ) 1 2

Hence, τCu-P6 = 0.867ms. τP6 was determined using the same method as Cu-P6. The equation parameters of the fitted curve were listed as following: A1 = 0.4241, t1 = 8.1613, A2 = 0.3022, t2 = 0.7724, y0 = 0.2751, and R2 = 0.98068. Hence, τP6 = 5.087ms. Compared to the ligand, the PH lifetime of Cu-P6 has decreased by 4.22ms (83%), which indicates that a large amount of energies has been transferred from the P6 donor to the cuprous acceptor. As Thompson previously reported, an appropriate host material with a short PH lifetime is a key to achieve the efficient electrophosphorescence.29 The sharply decreasing of PH lifetime and △ EST prove that the introduction of cuprous ions improves the potential applications of Cu-P6 in electrophosphorescent devices. Notably, the internal PL quantum yield Φ of Cu-P6 was determined to be 0.62 based on the absolute method using an integrating sphere at room temperature in the crystalline state. Such a high PL efficiency originates from its unique cross-stacking pattern in the crystalline state. Computational Results. According to the time-dependent density functional theory (TD-DFT) calculations, the spatial distributions have revealed that the full separation of highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) in Cu-P6 reduced the energy gap between singlet and triplet effectively (Figure 6).35-36 This should be assigned to the introduction of cuprous ions and indicates a more efficient phosphorescent emission of the Cu-P6 framework. Generally, the introduction of other expensive and rare heavy atoms like iridium are chosen in this context.37 The utilization of earth-abundant cuprous ions is another attractive feature of Cu-P6. 15 / 25



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Figure 6. Calculated spatial distributions of HOMOs and LUMOs of P6 and Cu-P6.

The electrochemical parameters of P6 and Cu-P6 are summarized in Table 1. The HOMOs and the LUMOs energies of Cu-P6 were calculated to be -4.42eV and -1.30eV respectively (see the Experimental Section), which implied a more balanced electron and hole mobility of the Cu-P6 frameworks by comparing to EHOMO and ELUMO of P6, PO6 and POAPF.27-28 This is not surprising, as both donor and acceptor are contained in the structure of Cu-P6. Table 1. Electrochemical parameters of the reported chemicals. EHOMO (eV) ELUMO (eV) Eg (eV) △EST (eV) P6 -5.60 -1.97 3.22,a 3.63b 0.46 Cu-P6 -4.42 -1.30 2.83,a 3.13b 0.30 a Obtaining from UV-Vis absorption spectra. b Calculating from computational methods.

Electroluminescent Properties. We performed a simple test to further evaluate the potential of CuP6 as a host material in blue-emitting electrophosphorescent devices following a reported procedure.18 An ITO/Cu-P6/ITO (ITO = indium tin oxide, Cu-P6 was utilized in crystalline state directly obtained from the solvothermal method, see Experimental Section for more details) sandwich was assembled as a simple electroluminescent device. To record the electroluminescent behavior, this device was connected to a direct-current (DC) voltage source. Semiconducting and electroluminescent behaviors of this sandwich device were recorded as our expectation (Figure 7). Cu-P6 exhibited a deep-blue 16 / 25


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Figure 7. (a) I-V behavior ,(b) electroluminescent spectrum (excited at 14V) and photoluminescence spectrum (excited at 398nm) as well as (c) the CIE coordinate (marked as a five-pointed star, based on its EL spectrum) of the simple ITO/CuP6/ITO sandwich device.

electroluminescence centered at 459nm. Compared to its PL spectrum, the maximum EL emission peak of Cu-P6 has blue-shifted by 10nm. The Commission Internationale de L’Eclairage (CIE) coordinate was calculated based on Figure 7b and the result is illustrated in Figure 7c (x = 0.1343, y = 0.1089). This was the first time that blue electroluminescence of a MOF was observed in such a high color purity. However, similar to Wang’s result, this device exhibited a much higher driving voltage than those of typical electroluminescent devices.18 As Lu and Chen previously reported, the thickness of the emissive layer in an electroluminescent device based on a MOF could influence the efficiency of the device (the best thickness is around 1μm).13 In this simple device, the average thickness of the emissive layer was determined to be a much higher value of 83μm by using SEM measurements. Hence, we concluded this phenomenon was caused by the inefficient thickness control of the sovolthermal method.

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Conclusions In summary, we designed, synthesized and characterized an original cuprous MOF Cu-P6. Cu-P6 was synthesized by using a typical halogen-lithium exchange reaction following a solvothermal reaction. A rigid coordination network connected by copper(I) ions and P6 linkers was characterized by single-crystal X-ray diffraction. This network can be simplified to a novel uninodal 3-connected topology with a point symbol (42∙6) and exhibit a cross-stacking pattern between neighboring nets along the c axis, which is considered to be beneficial to the luminous ability. The rigid structure provided Cu-P6 with a good heat resistant performance during DSC and TGA tests. Cu-P6 exhibited a high photoluminescent efficiency at room temperature, which originates from its fantastic crossstacking pattern and the introduction of cuprous ions. Cu-P6 possessed a balanced electron and hole mobility by containing both donor and acceptor in the structure. Most importantly, the deep-blue electrophosphorescent behavior of a MOF was observed for the first time by assembling a simple ITO/Cu-P6/ITO sandwich device. The thickness of the crystalline layer was regarded as a huge barrier to exhibit highly efficient electroluminescence. Since the luminescent behavior of Cu-P6 was related to its crystal structure, a creative coating method, which is not damaging to the structure, should be further investigated. We are currently pursuing the growth of crystals on the surface of ITO glass, and other further studies are under way.

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Supporting Information Measured and simulated powder patterns of Cu-P6 (Figure S1). Determination of optical energy gaps for P6 and Cu-P6 (Figure S2). Crystal data and structure refinement for Cu-P6 framework (Table S1). Close contacts due to C-H…Cl interactions in Cu-P6 (Table S2). Geometric parameters for Cu-P6 (Table S3). NMR spectra.

Notes The authors declare no competing financial interest.

Acknowledgement The authors acknowledge the financial support provided by Tianjin Municipal Science and Technology Commission, and also grateful for Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion for providing research facilitation and assistance.

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Scheme 1. Synthesis of the Cu-P6 framework. 63x17mm (300 x 300 DPI)


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Figure 1. Asymmetric unit of the Cu-P6 framework. All hydrogens are omitted for clarity convenience. 58x39mm (300 x 300 DPI)



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Figure 2. (a) Three dimensional architecture of the Cu-P6 framework. (b) Simplified topological structure of Cu-P6. (c) View of the topological structure along c axis. (d) & (e) View of the cross-stacking between neighboring networks along c and b axis, respectively. (f) & (g) Slipping of fluorene rings in the same network from different views. All hydrogens are omitted for clarity convenience. 148x104mm (300 x 300 DPI)


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Figure 3. DSC (blue line) and TGA (red line) traces of Cu-P6 (with a heating rate of 10K/min, nitrogen flow). 52x32mm (300 x 300 DPI)



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Figure 4. Solid-state photoluminescence (PL) spectra of (a) P6 (fluorescence (FL) emission spectrum was excited at 280nm, λem of FL excitation spectrum was at 465nm, phosphorescence (PH) emission spectrum was excited at 365nm) and (b) Cu-P6 (FL emission spectrum was excited at 375nm, λem of FL excitation spectrum was at 465nm, PH emission spectrum was excited at 350nm) at room temperature. The inserted pictures are showing the samples excited by 365nm UV lamps. 116x159mm (300 x 300 DPI)


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Figure 5. Room temperature phosphorescence lifetime diagram of Cu-P6 (λem=468nm, λex=398nm) and P6 (λem=543nm, λex=371nm). 101x121mm (300 x 300 DPI)



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Figure 6. Calculated spatial distributions of HOMOs and LUMOs of P6 and Cu-P6. 71x59mm (300 x 300 DPI)


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Figure 7. (a) I-V behavior ,(b) electroluminescent spectrum (excited at 14V) and photoluminescence spectrum (excited at 398nm) as well as (c) the CIE coordinate (marked as a five-pointed star, based on its EL spectrum) of the simple ITO/Cu-P6/ITO sandwich device. 70x20mm (300 x 300 DPI)



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The TOC file 44x31mm (300 x 300 DPI)


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A Host Material for Deep-Blue Electrophosphorescence Based on a

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