An Azole-Based MetalâOrganic Framework toward Direct White-Light

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An Azole-Based Metal-Organic Framework toward Direct White-Light Emissions by the Synergism of LigandCentered Charge Transfer and Interligand #–# Interactions Rong Li, Shuai-Hua Wang, Zhi-Fa Liu, Xuxing Chen, Yu Xiao, Fa-Kun Zheng, and Guo-Cong Guo Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00528 • Publication Date (Web): 17 May 2016 Downloaded from http://pubs.acs.org on May 19, 2016

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Crystal Growth & Design

An Azole-Based Metal−Organic Framework toward Direct White-Light Emissions by the Synergism of Ligand-Centered Charge Transfer and Interligand π– π Interactions Rong Li, Shuai-Hua Wang, Zhi-Fa Liu, Xu-Xing Chen, Yu Xiao, Fa-Kun Zheng* and Guo-Cong Guo* 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

1 Environment ACS Paragon Plus


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ABSTRACT:

An

azole-based

metal−organic

framework

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[Cd(tzphtpy)2]n·6.5nH2O,

hydrothermally synthesized from the reaction mixture of a π–electron rich azole ligand 4(tetrazol-5-yl)phenyl-2,2’:6’,2”-terpyridine (Htzphtpy) and CdBr2⋅4H2O, displays direct whitelight emissions at a wide range of excitation wavelengths (286–386 nm) through modulating the relative intensity of complementary yellow and blue colors. Photoluminescence experiments, and density of states (DOS) and the calculations of time-dependent density functional theory (TDDFT) demonstrate that the higher-energy blue emission is attributed to intraligand charge transfer and the lower-energy yellow emission is related to interligand π–π interactions. The synergism of ligand-centered charge transfer and interligand π–π interactions is first found in white-light emitting azole-based MOFs. Our work provides a new synthetic strategy for azolebased or other systems white-light emitting materials.

KEYWORDS. azole, metal−organic framework, π–π interaction, white-light emission, tunable luminescence.


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INTRODUCTION White-light emitting materials have drawn extensive attention due to their wide applications in lighting and display systems.1 Traditional methods used to generate white light include exciting multi-phosphors by a UV LED,2 mixing a blue LED with a yellow phosphor,3 or blending multiLEDs.4 An alternative new approach is to excite single-component phosphors by a UV LED, which has many advantages such as intrinsic color balance, excellent stability, better reproducibility and simpler fabrication processes.5 The single-component materials should emit complementary yellow and blue colors or three primary colors (red, green and blue) in suitable intensities to realize white-light emissions. 6 Although a lot of single-component white light materials have been found in organic molecules or polymers,7 glass ceramics,8 metal-doped or hybrid inorganic materials,9 nanomaterials,10 and metal complexes,11 it remains a challenge to exactly balance intrinsic complementary colors for white-light emissions. Metal–organic frameworks (MOFs), as a new family of porous coordination polymers, were first reported with luminescence in 2002.1e,

12

Benefiting from their unique advantages of

incorporating rich luminescent emitting sources and allowing efficient modulation of luminescent characteristics, MOFs materials have become promising candidates for luminescence-based applications.13 In order to achieve the necessary complementary colors for white-light emissions, various methods have been adopted to prepare the single-component white-light-emitting materials. Up to date, the most commonly used ones are doping with rare earth elements (e.g. yellow-Dy3+, green-Tb3+, red-Eu3+)

14

at various concentrations,

incorporating transition metals (e.g., Ag+1, Au+1, Pd2+)15 based on ligand-to-metal charge transfer (LMCT) or metal-to-ligand charge transfer (MLCT) mechanism or changing the guest molecules of MOFs.1f, 16 The other effective method to obtain white light is based on ligand-centered



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emissions via intra- and/or intermolecular charge transfer (CT). This approach has been successfully applied in white-light emitting organic materials 17 and has rarely been used in MOFs. 18 As is well-known, π–π stacking interactions affect optical properties and play an important role in the construction of functional materials. 19 The rational utilization of π–π stacking interactions may be a useful strategy for obtaining luminescent MOF materials. Herein, we employ a new ligand Htzphtpy (4-(tetrazol-5-yl)phenyl-2,2':6',2''-terpyridine, Chart 1), consisting of delocalized electrons π–conjugated terpyridine and tetrazole groups, to assemble a Cd(II) MOF. It has been reported that terpyridine and its derivatives have abundant photoelectric properties 20 , and tetrazole group is an excellent chromophore reported in our previous work.21 Furthermore, the π–conjugated frame can easily form intra/intermolecular π–π interactions through self-assembly, which results in adjustable photoluminescence.17, 19 By the usage

of

this

azole

ligand,

we

hydrothermally

synthesized

a

new

1D

MOF

[Cd(tzphtpy)2]n·6.5nH2O 1 with white-light emissions at a wide range of excitation wavelengths (286–386 nm) through altering the relative intensity of two complementary blue and yellow colors. The white-light emission falls in the 1931 CIE coordinate of (0.33, 0.36) approaching the ideal value (0.33, 0.33) excited at 326 nm. A "structure-CT-photoluminescence" relationship demonstrates that the solid-state higher-energy (HE) emission originates from the intraligand charge transfer and the lower-energy (LE) emission roots in interligand charge transfer associated with the interligand π–π interactions. To the best of our knowledge, here we first report an azole-based MOF with direct white-light emissions via π–π interactions.


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Chart 1. The molecular structure of Htzphtpy. EXPERIMENTAL SECTION Materials and instruments. The commercially available reagents were analytical grade except methanol. Methanol was of spectral grade. Rigaku Miniﬂex II diffractometer was used to obtain the experimental powdered X-ray diffraction (PXRD) data of 5° ≤ 2θ ≤ 65° at 40 mA and 40 kV using a Cu-Kα radiation (λ = 1.540598 Å). The Mercury Version 1.4 software (http://www.ccdc.cam.ac.uk/products/mercury) was utilized to achieve simulated PXRD patterns dependent on the X-ray crystallographic structure. The experiments of thermogravimetric analysis (TGA) were performed by a METTLER TOLEDO thermogravimetric analyzer, and the heating rate of samples in Al2O3 crucible was maintained with 10 K min–1 in N2 atmosphere. An Elementar Vario EL III microanalyzer was used to proceed elemental analyses. A Perkin-Elmer Spectrum was applied to record FT-IR spectra with KBr disks. A Perkin-Elmer Lambda 950 spectrophotometer was

utilized

to

achieve

UV-Vis

spectra.

The determination

of

photoluminescence (PL) data including PL spectra, lifetime and quantum yield was done on Edinburgh FL920 fluorescence spectrometer. The CIE caculator-version 3 software was used to calculate the CIE coordinates, and CRI and CCT values.



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Calculation of the density of states (DOS). The density functional theory (DFT) equipped with one of the three non-local gradient corrected exchange–correlation functional (GGA-PBE) was utilized to calculate the density of states (DOS) for 1 on the basis of the X-ray crystallographic data. The CASTEP code was adopted to calculate the DOS with the Materials Studio V5.5 software package.22 The valence electrons and core electrons were based on a plane wave basis and normconserving pseudopotential, respectively.23 The number of included plane waves was decided by the cutoff energy of 300.0 eV in the basis. For the purpose of calculating the optical characteristics of the synthesized compounds accurately, a 1 × 1 × 1 Monkhorst–Pack k-point sampling was used to perform the numerical integration of the Brillouin zone. The CASTEP code default values were determined by other parameters in the calculations. Quantum chemical calculations. Density functional theory calculations were conducted by the Gaussian 09 suite of programs.24 Electronic structure calculation for Htzphtpy monomer and dimer in the Htzphtpy crystal structure, and Htzphtpy dimer in 1 were performed by employing the B3LYP functional in conjunction with the all-electron 6-31+G(D) basis set for all atoms. All the structural models were not optimized to keep the real geometry. Time-dependent density functional theory (TD-DFT) calculations were conducted to assign the transient absorption spectra at the B3LYP/6-31G(d) level. The hydrogen atoms of Htzphtpy in 1 were retained in order to balance the charge and form a neutral molecule. Crystallization of 4-(tetrazol-5-yl)phenyl-2,2':6',2''-terpyridine (Htzphtpy). With the purpose of obtain the crystal structure of Htzphtpy, the commercially purchased Htzphtpy (0.1 mmol) was kept heating for 3 days at 150 οC in a 25 mL Teflon-lined stainless steel vessel with 7.0 mL of distilled water. The yellow prismatic crystals of Htzphtpy were achieved after a cooling rate of 5 οC·h–1 to room temperature, and then washed by distilled water. Anal. Calcd for


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C22H15N7 (Htzphtpy): C, 70.01, H, 4.01, N, 25.99%. Found: C, 69.84, H, 4.02, N, 25.31%. IR (KBr pellet, cm–1): 3437 b, 3063 w, 1583 s, 1547 m, 1465 s, 1393 s, 1264 w, 1213 w, 1172 w, 1013 m, 843 m, 793 s, 761 s, 659 m, 617 w, 520 w. Synthesis of [Cd(tzphtpy)2]n·6.5nH2O (1). The reaction mixture of Htzphtpy (0.2 mmol), CdBr2·4H2O (0.1 mmol) and NaOH (0.2 mmol) was added into a 25 mL Teflon-lined stainless steel vessel with 7.0 ml of distilled water, and kept heating for 3 days at 140 οC. The yellow block crystals of 1 were isolated after cooling to room temperature, and then washed by distilled water. Yield: 50% (based on Htzphtpy) for 1. Anal. Calcd for C44H41N14CdO6.5 1: C, 53.80, H, 4.21, N, 19.97%. Found: C, 54.27, H, 4.44, N, 20.15%. IR (KBr pellet, cm–1): 3422 b, 3068 w, 1604 s, 1552 s, 1475 m, 1393 m, 1239 w, 1146 w, 1013 m, 864 m, 782 s, 766 w, 653 w, 617 w, 587 w. Single crystal structures determination. A Rigaku Pilatus CCD diffractometer and Rigaku Saturn-724 CCD diffractometer both using Mo-Kα (λ = 0.71073 Å) radiation source were used to collect X-ray crystallographic data of Htzphtpy and 1, respectively. The ω scan technique was applied so as to collect a set of complete diffraction data. The SHELXTL version 5 package was performed to solve the structures directly. 25 The non-hydrogen atoms were located by the subsequent successive difference Fourier syntheses. A full-matrix least-squares refinement on F2 was conducted to refine the final structures. All atoms except hydrogen atoms were performed through the anisotropic refinement. The hydrogen atoms of ligands were inserted geometrically and refined by the riding mode. All of the calculations were carried out by the SHELXTL-2014 program package of crystallographic software. 26 Crystallographic parameters and structural refinement details for Htzphtpy and 1 are listed in Table 1. The selected bond distances and angles are given in Table S1.



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Table 1. Crystallographic parameters and structure refinement detais for Htzphtpy and 1.

a

Compound

Htzphtpy

1

Formula

C22H15N7

C44H41N14O6.5Cd

Formula mass

377.41

981.31

Space group

P21/c

C2/c

a/Å

10.937(6)

28.831(8)

b/Å

7.216(4)

17.947(4)

c/Å

22.837(11)

19.693(5)

β/º

92.782(13)

112.991(4)

V/Å3

1800.2(17)

9380(4)

Z

4

8

Dc /g cm–3

1.393

1.391

µ/mm–1

0.089

0.529

F(000)

784

4024

reflns collected

15910

10650

unique reflns

3336

10650

Rint

0.0318

0.0280

GOF

1.027

1.020

R1 a[I > 2σ(I)]

0.0492

0.0544

wR2b (all data)

0.1433

0.1688

CCDC No.

1439466

1439468

R1 = Σ||Fo| – |Fc||/Σ|Fo|, bwR2 = Σ[(w(Fo2 – Fc2)2)/Σ[w(Fo2)2)]1/2


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RESULTS AND DISCUSSION Synthesis and crystal structure. As far as we know, the crystal structure of Htzphtpy is first documented in this work, and no coordination compounds based on Htzphtpy have been described to date. Single crystals of Htzphtpy were successfully obtained by the hydrothermal synthesis method. The ligand Htzphtpy can slightly soluble in polar organic solvents such as DMF and CH3OH. While in most common organic solvents, the crystalline product of 1 is insoluble. The PXRD measurement and elemental analyses affirm the phase purity of Htzphtpy and 1. The experimental PXRD patterns of the samples show good agreement with their simulated ones based on the crystal structures (Figure S1). The weight loss of 7.16% in 1 at about 97 οC should correspond to the loss of six and a half H2O guest molecules for per asymmetric unit (calcd: 6.79%). No weight loss occurs until heating up to approximate 300 οC. (Figure S2a). The disappearance of PXRD peaks of 1 at 100 οC (Figure S3) denotes the framework collapse of 1 upon the loss of the solvent water molecules, which has existed in other reported MOFs. 27 The ligand Htzphtpy has no obvious weightlessness platform and shows thermal instability (Figure S2b). Htzphtpy has only one molecule in each asymmetric unit based on the structural analysis from the single-crystal X-ray diffraction (Figure 1a). The Htzphtpy molecules stack in a chiasmatic mode in its crystal structure (Figure 1b).28 Compound 1 shows a 1D double-chained structure, built up by tzphtpy− and Cd2+ ions. There are one crystallographically independent Cd2+ atom, two tzphtpy– ligands and six and a half lattice water molecules in per asymmetric unit of 1 (Figure 1c). The Cd2+ atom is coordinated by four N atoms (N12A, N23, N25B and N15) with a distorted tetrahedral coordination geometry (τ4 = 0.76) 29 , and the N–Cd–N angles vary from 93.38(7) to 150.29(7)°. The Cd–N distances in the range of 2.277(2)–2.305(7) Å are coincident



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with those reported tetrahedrally coordinated Cd2+ complexes.30 The tzphtpy− ligands in 1 stack into a Y-shaped configuration, different from the chiasmatic mode of the free ligand (Figures 1b and 1d). The tzphtpy− ligand uses two N atoms (one from tetrazole and the other from terpyridine) to coordinate two Cd2+ ions to generate a 1D pearl-necklace-like chain (Figure 2a). The supramolecular 3D framework is formed through interligand π–π interactions between adjacent 1D chains with a total solvent-accessible volume about 10.0% calculated by PLATON software (Figure 2b).31 The larger hole is generated through eight 1D chains via π–π interactions, while the smaller one is formed through four 1D chains (Figure 2b). The average dimensions of two different holes are ~8.0 and ~2.0 Å, respectively, when considering the van der Waals radius of nearest atoms (Figure S4).


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Figure 1. (a) The molecular structure of the free ligand Htzphtpy; (b) The chiasmatic packing mode in Htzphtpy; (c) The asymmetric unit in compound 1; (d) The Y-styled packing mode of tzphtpy− ligand in 1 with the Cd2+ atoms omitted for clarity. Symmetry codes: (A) –x + 1.5, –y + 1.5, –z; (B) –x + 1.5, –y + 1.5, –z + 1. Hydrogen atoms and lattice water molecules were omitted for clarity.

Figure 2. For 1: (a) 1D pearl-necklace-like chain; (b) 3D supramolecular framework with lattice water molecules filling in the holes. The red balls in the holes are O atoms of lattice water molecules. It is well known that π–π interactions generally exist in overlapping of aromatic rings with approximately parallel ring planes defined by interplanar distances of about 3.3–3.8 Å,32 and advance intra/intermolecular charge transfer affecting optical characteristic.19 The strong π–π interactions in Htzphtpy crystals occur between pyridine ring (B) and tetrazole ring (A), and



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pyridine ring (C) and its symmetry-related one (C), with the centroid–centroid (Cg to Cg) distances of 3.68 and 3.54 Å, and the dihedral angles of 2.77 and 0°, respectively (Figure 3a, Table S2). For 1, π–π interactions exist not only in 1D intra-chain, but also between the two adjacent 1D chains (Figure 3b, Table S2). The intra-chained strong π–π interactions occur between pyridine ring (B) and pyridine ring (E), and pyridine ring (C) and benzene ring (G), with the Cg to Cg distances of 3.54 and 3.63 Å, and dihedral angles of 6.54 and 6.85ο, respectively. The Cg to Cg distances between pyridine ring (A) and pyridine ring (D), and pyridine ring (F) and benzene ring (H) are 3.90 and 4.10 Å, with dihedral angles of 10.41 and 19.26ο, respectively, which demonstrates the existence of comparably weak π–π interactions. The inter-chained π–π interactions are also found between the adjacent pyridine ring (B) and pyridine ring (D) with the Cg to Cg distance of 3.78 Å and the dihedral angle of 4.62°.


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Figure 3. (a) The sketch of π–π interactions between aromatic rings in Htzphtpy crystal stacking structure; (b) The intra- and inter-chained π–π interactions in 1. Photoluminescence properties. The UV-Vis absorption spectra in the solid-state indicate that Htzphtpy and 1 reveal an intense absorption in the 200–400 nm of UV region and a weak absorption band of visible light (400–750 nm) (Figure S5a). The photoluminescence (PL) spectra of Htzphtpy and 1 were investigated at room temperature (Figures 4 and S6). The crystalline sample of Htzphtpy displays a broad strong lower-energy (LE) yellow emission band centered at 554 nm and a higher-energy (HE) one around at 435 nm when excited at 345 nm, with their lifetime values of 5.39 and 3.11 ns, respectively (Figure S7, Table S3). The PL emission spectra of 1 exhibit two emission peaks at 567 and 454 nm at the excitation wavelength of 386 nm in the solid state, with lifetime values of 8.26 and 9.89 ns, respectively (Figure S8, Table S4). The PL lifetime values in the ns scale for Htzphtpy and 1 are indicative of fluorescence characteristics.

Figure 4. The emission spectra of crystalline powders of Htzphtpy and 1 at room temperature excited at 345 and 386 nm, respectively. The dashed line represents the emission spectrum of Htzphtpy in the CH3OH solution (6 × 10−5 mol/L) excited at 284 nm.



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The main emission bands of Htzphtpy and compound 1 fall in the blue and yellow regions, which provides the possibility of white-light emissions from the mixture of the two complementary colors. The variation of the excitation wavelengths can adjust the relative intensities of the dual emissions.18c, 33 With this in mind, the emission spectra of Htzphtpy and compound 1 have been monitored at different excitation wavelengths. The LE and HE emission intensities of Htzphtpy are enhanced at the same time with excitation wavelengths increasing from 286 to 396 nm (Figure S9). Nevertheless, the intensity of LE and HE bands could not mutually match to generate white light, and the overall emission of Htzphtpy falls in the yellowlight region (Table S5, Figure S10). The PL quantum yield (PLQY) of Htzphtpy crystal samples has not been detected within the fluorescence spectrometer error range at the excitation wavelengths of 345 and 396 nm. However, unlike Htzphtpy, the dual emission intensities of 1 are comparable when modulating the excitation wavelengths between 286 and 386 nm, and white-light emissions are obtained (Figure 5). The white-light emission is achieved with the 1931 CIE coordinate of (0.33, 0.36), a proper CRI value of 77, a favorable CCT magnitude of 5328 K and the PLQY of 2.3% when excited at 326 nm. The comparative higher PLQY is 8.9% upon excitation at 386 nm. Upon the excitation wavelength is above 386 nm, 1 shows a yellowdominative light emission for the intensity of LE emission increases more rapidly (Table S6, Figure 5 inset). In other words, the color-adjustable ability of 1 is better than that of the free ligand Htzphtpy through altering the excitation wavelengths, and the PLQY of 1 has also been improved when the free ligand Htzphtpy is coordinated to metal atoms to form a rigid CdMOF.3b,34


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Figure 5. The solid-state emission spectra of 1 under different excitation wavelengths at room temperature. Inset: The CIE-1931 chromaticity diagram and optical images of 1 in the powdered sample with the excitation wavelengths 1 to 8 being 286, 326, 356, 386, 396, 413, 440 and 460 nm, respectively. As mentioned above, the dual emissions of Htzphtpy and 1 feature excitation-wavelength dependence (Figure 6). In order to clearly illustrate the relationship of dual emissions of Htzphtpy, the relative intensities of each emission peak have been compared at various excitation wavelengths (Figure S11). As shown in Figure 6a, the HE emission with λmax = 435 nm exhibits the highest relative intensity under the excitation of 306 nm (compared to the intensity of corresponding LE emission), while the LE emission centered at 554nm displays continuously increasing relative intensity (compared to the intensity of corresponding HE emission) with the increasing excitation wavelengths. The same experimental phenomena also occur in 1 (Figures 6b and S12). This observation of excitation-wavelength dependent emission suggests that the dual emissions of Htzphtpy and 1 are mutually isolated.32 That is to say, the dual emissions originate from different luminous centers.



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Figure 6. The relative intensity of dual emissions of Htzphtpy (a) and 1 (b) with excitation wavelengths being 286, 306, 326, 356 and 386 nm, respectively. In view of the similar UV-Vis absorption and PL emission profiles of Htzphtpy and 1 (Figures 4 and S5a), we speculate that the emissions of 1 originate from ligand-centered charge transition with the aid of DOS calculation. As illustrated in Figure 7, Cd2+ ions make little or no contribution to the top of the valence bands (VBs) or the bottom of the conduction bands (CBs). The p–π orbitals of tzphtpy–are mainly contributors to the VBs between the energy –4.0 eV and Fermi level (0.0 eV), while the CBs between the energy 1.5 and 3.5 eV mostly result from the p–


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π* antibonding orbitals of the tzphtpy– ligand. The DOS calculation results confirm that the emissions of 1 stem mainly from ligand-centered charge transition.

Figure 7. The partial and total DOS of 1 with set the position of 0 eV for the Ferm level. For the purpose of elucidating the essence of dual emissions, the emission spectrum of Htzphtpy in the dilute CH3OH solution has been measured (the red dashed line in Figure 4). There is only one HE emission centered at 422 nm and the LE emission is absent. Therefore, we believe that the HE emission in the solid state originates from π–π* and/or n–π* intraligand charge transfer and the LE emission in the solid state roots in interligand charge transfer associated with interligand π–π interactions.35 Based on the above analysis of crystal structures, the interligand π–π interactions promote the formation of π–dimer between ligands in Htzphtpy crystals and 1. In order to further clarify the mechanism of LE emission and “structure-CT-PL property” relationship, TD-DFT calculations have been performed on the non-optimized structures of the monomer and dimer of Htzphtpy. The simulated absorption spectra of monomer and dimer are almost coincident with the experimental spectra in the dilute CH3OH solution and solid state, reflecting the reasonableness of our selected calculation modes (Table S7, Figure 8).



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By comparing the main absorption peaks of monomer and dimer, the CT routes can be deduced as follows. In monomer of Htzphtpy, the excited electron transfers from the HOMO-1 (97) to LUMO (99) orbits (Figure S13b) and contributes to π–π* and/or n–π* intraligand CT. In dimer, the main UV-Vis absorption peak is derived from the electron transfer from the HOMO-3 (193) to LUMO (197) and HOMO-2 (194) to LUMO+1 (198) orbits (Figure 9). The overlaps of intermolecular electron clouds of pyridine rings via π–π interactions exist in the LUMO (197) orbit, which is consistent with the Htzphtpy dimer crystal structure. When the excited electrons transfer from the HOMO-3 (193) to LUMO (197) orbits, the interligand CT generates and results in the LE emission via π–π interactions. Furthermore, the simulated UV-Vis absorption spectra of 1 have also been calculated by TD-DFT utilizing Htzphtpy dimer cutting from the structure of 1 (Figure S14). Similarly by analysing the highest absorption peak of Htzphtpy dimer in 1, it clearly shows overlaps of interligand electron clouds between dimer (Figure S15). The interligand CT via π–π interactions occurs from the HOMO-3 (193) to LUMO+1 (198) and HOMO-2 (194) to LUMO+2 (199) orbits, and the similar LE emission has been obtained in 1. The above analyses and discussions allow us to verify that the photoluminescence of Htzphtpy and its complex 1 derives from ligand-centered CT and the LE emission is closely related to the interligand π–π interactions.


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Figure 8. The absorption spectra of the Htzphtpy monomer (a) and dimer (b) calculated by TDDFT (red curves) and from experimental results (black curves) along with the calculated oscillator strengths (f, blue bars).



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Figure 9. The corresponding frontier molecular orbitals using the Htzphtpy dimer geometry in Htzphtpy crystal structure when the absorption peak is 297.6 nm. CONCLUSIONS In summary, direct white-light emissions of azole-based MOFs by the synergism of ligandcentered charge transfer and interligand π–π interactions have first been reported. The mixture of an electron donor conjugated azole ligand 4-(tetrazol-5-yl)phenyl-2,2’:6’,2”-terpyridine (Htzphtpy)

and

CdBr2⋅4H2O

under

hydrothermal

reaction

condition

to

yield

[Cd(tzphtpy)2]n·6.5nH2O 1, which features a 1D pearl-necklace-like chain. The photoluminescent emissions of 1 could be tunable from white to yellow light by varying the excitation wavelengths and direct white-light emission can be achieved at a wide range (286–386 nm). The white-light emission is attained with the 1931 CIE coordinate of (0.33, 0.36), a CRI value of 77, a CCT magnitude of 5328 K and a PLQY of 2.3% when excited at 326 nm. The molecular stacking structure, ligand-centered charge transfer and interligand π–π interactions, theoretical calculations and corresponding PL properties of Htzphtpy and 1 are comprehensively investigated to establish a “structure-CT-property” relationship. The experimental and theoretical results indicate that both HE and LE emissions are originally from ligand-centered charge transfer and the LE emission is closely relevant to the interligand π–π interactions. This work gives an insight into photoluminescence mechanism and provides a new approach in designing white-light emitting material candidates with more excellent optical performances.

ASSOCIATED CONTENT Supporting Information This material is available free of charge on the ACS Publications website at DOI: xxxxxxxx.


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Supplementary tables, additional figures, PXRD patterns, IR spectra and TGA curves (PDF) Crystallographic information (CIF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by 973 Program (2011CBA00505) and National Nature Science Foundation of China (21371170).

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Table of Contents Synopsis

Direct white-light emissions of azole-based MOFs by the synergism of ligand-centered charge transfer and interligand π–π interactions have first been reported. White-light emissions can be realized at a wide range of excitation wavelengths (286–386 nm). Our work provides a new synthetic strategy for azole-based or other systems white-light emitting materials.


An Azole-Based MetalâOrganic Framework toward Direct White-Light

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An Azole-Based MetalâOrganic Framework toward Direct White-Light