Near-White Light Emission from Lead(II) Metal–Organic Frameworks

Aug 29, 2018 - The solid-state photoluminescence of 1–3 was investigated at room temperature. Interestingly 1, 2, and 3 achieved close to white ligh...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Near-White Light Emission from Lead(II) Metal−Organic Frameworks Abdul Malik P. Peedikakkal,*,† Hong Sheng Quah,§ Stacey Chia,‡ Almaz S. Jalilov,† Abdul Rajjak Shaikh,† Hasan Ali Al-Mohsin,† Khushboo Yadava,§ Wei Ji,*,‡ and Jagadese J. Vittal*,§ †

Department of Chemistry, King Fahd University of Petroleum and Minerals, P.O. Box 5048, Dhahran 31261, Saudi Arabia Department of Physics, National University of Singapore, 3 Science Drive 3, Singapore 117542 § Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 ‡

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S Supporting Information *

ABSTRACT: Reaction of bpy (bpy = 4,4′-bipyridine) with Pb(OAc)2·3H2O in DMF (DMF = dimethylformamide) afforded a metal−organic framework (MOF), [Pb2(μ-bpy)(μ-O2CCH3)2(μ-O2CCH3)2]·H2O (1). Reaction of bpy with Pb(O2CCF3)2 in a methanol and chloroform mixture furnished another MOF, [Pb(μ-bpy)(μ-O2CCF3)2]·1/2CHCl3 (2). However, the reaction of bpy with Pb(OAc)2·3H2O in the presence of trifluoroacetic acid in a similar reaction condition yielded a hydrogen-bonded zwitter-ionic complex of Pb(II), [Pb(bpy-H)2(O2CCF3)4] (3). All compounds have been characterized by single crystal X-ray crystallography, FTIR, and 1H NMR spectroscopies. Compound 1 forms four heptacoordinated Pb(II) joined by (OCCH3)-O− linkages, resulting in a 3D noninterpenetrated MOF net with a four-connected uninodal sra (SrAl2) topology. However, in 2, tetraconnected Pb4(O2CCF3)8 cluster units are linked further through eight bpy ligands to furnish a doubly interpenetrated MOF with a new topology but having the very similar connectivity of 1, whereas 3 forms a zigzag hydrogen-bonded chain structure. The variation of carboxylate anions, pH of the reaction medium, and the ratio of the reactants profoundly affected the final topological structure of the compounds synthesized. The solid-state photoluminescence of 1−3 was investigated at room temperature. Interestingly 1, 2, and 3 achieved close to white light emission when excited at 329, 376, and 330 nm, respectively. The systematic understanding of the photophysical properties of analogous Pb-based compounds may open new perspectives for developing single-phase white-light-emitting materials using Pb(II) based MOFs.

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toxicity.36 Further, Pb(II) has the ability to form remarkable and exceptional topologies of several framework and structures due to its variable geometries with wide coordination number (2−10) through the large ionic radius (1.20 Å).37,38 The presence of a lone pair having 6s2 on the external electronic configuration has inspired extensive awareness in photophysics, coordination chemistry, and photochemistry owing to its unique coordination preference and electronic intrinsic characteristics.39−42 Pb(II) MOFs have also presented interesting photophysical characteristics like luminescent and birefringence.43−45 A few Pb(II) MOFs have also been found to exhibit WLE.46−48 Pyridyl donor ligands are one of the most versatile linkers in the construction of MOFs, especially bpy (bpy = 4,4′bipyridine).49 Two-connecting linear linkers are the most regularly used nitrogen-based spacer ligands for the assembly of framework structures. However, the design of the desired topology is more challenging for Pb(II) based MOFs since the lone pair can interfere and obviate the desired structure. The

INTRODUCTION Materials with white light emission (WLE) have attracted immense consideration in present time due to their wide usage in displays and lightings.1,2 Conventionally, emission of white light is produced through exciting multiphosphors by UV LED,3 yellow phosphor mixing with blue LED,4 blending multi-LED.5 It is possible to overcome these problems by creating WLE starting from single-component materials. Single-component WLE is unusual but recognized to display by polymers,6 molecules of organic,7 ceramics,8 inorganic hybrid materials,9,10 and nanomaterials.11,12 Luminescent metal−organic frameworks (L-MOFs) have revealed to provide unlimited opportunities for fabricating single-component WLE.13−16 Luminescence is made to produce from both metal centers and organic ligands through metal−ligand/ ligand−metal charge transfer (MLCT/LMCT).17 Currently, majority studies are focused on lanthanides L-MOFs18−28 or dblock transition metal L-MOFs.29,30 WLE from intrinsic nonlanthanide MOFs is rather rare.31−35 On the contrary, Pb(II) compounds have displayed signs of success in applications like photovoltaic conversion, fluorescent sensors, organic lightemitting diodes, and electroluminescence, despite their © XXXX American Chemical Society

Received: March 15, 2018

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DOI: 10.1021/acs.inorgchem.8b00637 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data for 1−3 compounds

1

2

3

formula formula weight T/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg volume/Å3/Z ρ, Mg·cm−3 μ/mm−1 reflns collected independent reflns/Rint/GoF final R[I > 2σ(I)], R1/wR2

C18H22N2O9Pb2 824.75 223(2) triclinic P1̅ 14.051 (2) 14.653(2) 15.831(2) 69.806(2) 78.253(3) 75.840(2) 2940.7(7)/4 1.863 11.474 38087 13467/0.0511/1.034 0.0413/0.1106

C14.5H8.5N2O4Cl1.5F6Pb 649.10 100(2) monoclinic P21/c 16.190(2) 15.992(2) 15.968(2) 90 118.225(1) 90 3642.7(4)/8 2.367 9.568 25269 8358/0.0426/1.037 0.0339/0.0820

C28H18N4O8F12Pb 973.65 100(2) monoclinic C2/c 11.6498(7) 14.7333(9) 18.1933(11) 90 91.042(1) 90 3122.2(3)/4 2.071 5.533 10820 3591/0.0266/1.051 0.0261/0.0654

state were monitored on a Horiba Fluorolog-3 spectrometer. All the emission spectra were monitored with 2 nm slits. Computational Methods. All quantum chemical calculations were carried out using the Gaussian 09 suite of programs.54 Density functional theory (DFT) calculations were performed using the B3LYP hybrid functional; for metal Pb, the LANL2DZ basis set was used, while, for remaining atoms (C, H, N, O, F), the 6-31G(d) basis set was utilized. The experimentally obtained single crystal structure was used as an initial molecular structure for DFT calculations. Optimized geometries using DFT calculation were evaluated using frequency calculations in order to find a minimum energy structure with no imaginary frequency. Furthermore, the optimized geometry was used for single-point time-dependent DFT calculations (TDDFT) with the B3LYP functional in order to obtain absorption spectra. The Avogadro55 1.2.0 program was used for visualization of the molecular orbital structure (MO). The GaussSum56 3.0 program was used for extraction of the percent contributions of groups of atoms to each of the MOs. Synthetic Procedures. Caution! Lead compounds are potentially toxic and should only be used in small amounts and handled with caution. Synthesis of [Pb2(O2CCH3)4(bpy)]·H2O (1). A solution of Pb(O2CCH3)2·3H2O (189 mg, 0.5 mmol) dissolved in DMF (1 mL) was added to a solution of bpy (78 mg, 0.5 mmol) in DMF (0.5 mL). A pale yellow solution was observed, and the solution was kept for slow evaporation at room temperature. Colorless block-like single crystals of 1 were obtained after a day. The crystals were collected by filtration, washed with diethyl ether, and dried under vacuum. Yield: 14% Elemental Anal. for C18H22N2O9Pb2·0.4 DMF. Calcd C, 27.00; H, 2.93; N, 3.94%: Found: C, 27.06; H, 2.99; N, 3.90%. 1H NMR (DMSO-d6): δ 1.68 (s, 3H of −O2C−CH3), 7.84 (d, 4H of bpy), 8.73 (d, 4H of bpy). Selected IR (KBr, cm−1): 1649(m), 1596(s), 1558(s), 1411(s), 1218(s) 1066(m), 996(m), 805(s), 664(w), 618(s). Synthesis of [Pb(O2CCF3)2(bpy)]·1/2CHCl3 (2). Pb(O2CCF3)2 (129 mg, 0.3 mmol) was dissolved in 4 mL of CHCl3 and was layered using 2 mL of a methanolic solution of bpy (46.85 mg, 0.3 mmol). Colorless block crystals were obtained after 2 days. The crystals were harvested by filtration, washed with diethyl ether, and dried under vacuum. Yield: 56%. Elemental Anal. for C29H17N4O8Cl3F12Pb2· CHCl3. Calcd, C, 25.42; H, 1.28; N, 3.95%: Found: C, 25.28; H, 1.44; N, 3.95%. 1H NMR (DMSO-d6): δ 7.84 (d, 4H of bpy), 8.73 (d, 4H of bpy), 8.31 (s, −CH, solvent residual peak for CHCl3 in DMSO-d6). Selected IR (KBr, cm−1): 3443(br), 1684(s), 1597(m), 1412(w), 1208(s), 1134(s). Synthesis of [Pb(bpy-H)2(O2CCF3)4] (3). A solution of Pb(OAc)2 (152 mg, 0.4 mmol) with CF3COOH (89.6 mg, 0.8 mmol), both dissolved in CHCl3 (1 mL), was added to a solution of bpy (62.4 mg, 0.4 mmol) in MeOH (0.5 mL). A pale yellow solution was observed

topology of the frameworks can be again varied using monocarboxylate ligands with pyridyl donors, and they have been used to design photoreactive solids.50−52 Here, we describe two MOFs, namely, [Pb2(μ-bpy)(μO2CCH3)2(μ-O2CCH3)2]·H2O (1) and [Pb(μ-bpy)(μ-O2CCF3)2]·1/2CHCl3 (2). Compound 1 has been synthesized from the reaction of bpy with Pb(OAc)2·3H2O in DMF (DMF = dimethylformamide), whereas the reaction of bpy using Pb(O2CCF3)2 in methanol/chloroform mixture affords 2. The compound 1 has heptacoordinated Pb(II) coordinated by −(OCCH3)-O− linkages resulted in a noninterpenetrated MOF net having a four-connected uninodal sra (SrAl2) topology. The compound 2 has a tetra-connected Pb4(O2CCF3)8 cluster unit as the building block, and each cluster is linked further through eight bpy ligands to furnish an unprecedented doubly interpenetrated MOF net having the similar connectivity of the former structure. Furthermore, the reaction of bpy with Pb(OAc)2·3H2O and the mixing of trifluoroacetic acid (HTFA) in a similar reaction condition affords a hydrogen-bonded chain of zwitter-ionic complex of Pb(II), [Pb(bpy-H)2(O2CCF3)4] (3), which is isostructural to the compound reported earlier using a bpe ligand.53 We have investigated the photoluminescence (PL) properties of these Pb(II) compounds, 1−3. Interestingly, all of these solids show WLE. Computation calculations reveal that metal-centered (MC) transitions have a significant contribution to the white light emission. We have presented the details of our investigation below.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals were acquired from commercial sources and used as obtained. Reagent grade solvents were used. The yield of 1−3 was described with regard to the metal salts. 1H NMR spectra were monitored in a Bruker ACF 300 FTNMR spectrometer using TMS as an internal reference. The FT-IR spectra were recorded from KBr pellets (FTS 165 Bio-Rad FT-IR). Elemental analyses were completed in the Micro Analytical Laboratory, Department of Chemistry, National University of Singapore. UV−vis spectra were reordered at room temperature using a UV-2450 Shimadzu UV−visible spectrometer provided with an integrating sphere and reflecting reference of barium sulfate. Solidsate emission spectra of the samples of the compounds in the solid B

DOI: 10.1021/acs.inorgchem.8b00637 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and kept in a fridge for slow evaporation. Pale-yellow block crystals were obtained after 3 days. The crystals were harvested by filtration, washed with diethyl ether, and dried under vacuum. Yield: 61% Elemental Anal. for C28H18N4O8F12Pb·CHCl3. Calcd, C, 31.87; H, 1.75; N, 5.13%: Found: C, 31.92; H, 1.73; N, 5.15. 1H NMR (DMSO-d6): δ 7.89 (d, 4H of bpy), 8.76 (d, 4h of bpy). Selected IR (KBr, cm−1): 3421.38(br), 1671.74(m), 1641.72(m),1556.93(s), 1410.24(s), 1344.06(m), 1212.03(m). X-ray Crystallographic Analysis. Intensity data for 1−3 were collected on a Bruker APEX diffractometer equipped with a CCD detector and graphite-monochromated MoKα (λ = 0.71073 Å) radiation using a sealed tube (2.4 kW). Absorption corrections were achieved using the program SADABS,57 and the crystallographic package SHELXTL58 was used for all calculations. The crystal data, as well as details of data collection and refinement of 1−3, are summarized in Table 1. Selected bond lengths and angles are given in Tables S1 and S2. In 2, three of the CF3 groups were disordered. The F atoms of two CF3 groups were disordered into two positions with occupancy ratios refined to 0.57(2) and 0.46(2). But in the third CF3, all the atoms were disordered with the occupancy ratio refined to 0.58(2). In 3, O4 and one of the CF3 were disordered into two positions with occupancy ratio refined to 0.622(6). Restraints in bond length and thermal parameters were applied to the disordered part in 3. The hydrogen atom bonded to the nitrogen was located from the different Fourier map in 3.

2.646(4) Å; Pb(3)−O(10) 2.449(5) Å; Pb(3)−O(11) 2.405(7) Å; Pb(3)−O(12) 2.798(8) Å]. The significant lengthening of Pb1−O4 may be due to the poor overlap of the bridging oxygen atom with the valence orbital of Pb(II). The Pb4 is also chelating to two CH3CO2− anions [Pb(4)− O(13) 2.426(6) Å; Pb(4)−O(14) 2.652(9) Å; Pb(4)−O(15) 2.429(1) Å; Pb(4)−O(16) 2.816(8) Å] and further coordinated to the O6 atom [Pb(4)−O(6)h 2.737(1) Å (symmetry operator h: 1 − x, 1 − y, −z)] that is already chelated to Pb2. The O14 and O12 bridge Pb1, while O9 and O16 atoms bridge Pb2. Similarly, the O1 and O4 bridge Pb3, while the O6 and O8 atoms bridge Pb4. Two chelated CH3CO2− anions in each four Pb(II) metal ions are bridged further and make a tetra-connected Pb4(O2CCH3)8 cluster unit which forms a zigzag ladder structure (Figures S2 and S3). These ladders are further linked through four bpy ligands as shown in Figure 1a.

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RESULTS AND DISCUSSION Synthesis. MOFs 1 and 2 have been synthesized using two different monocarboxylate anions and reaction conditions. Colorless block crystals of 1 were obtained after slow evaporation of a pale-yellow DMF solution containing bpy and Pb(O2CCH3)2. However, colorless block crystals of 2 were furnished from a methanolic solution of bpy layered over a chloroform solution of Pb(O2CCF3)2. On the contrary, the reaction of Pb(O2CCH3)2 with bpy in the presence of HTFA in a similar solvent mixture yielded pale-yellow block crystals of 3. A similar hydrogen-bonded zwitter-ionic complex was reported for the bpe ligand.53,59 The influence of crystallization conditions and carboxylate anions on the final topology of the MOFs has been very well explored in the literature.49 Previously, several Pb(II) MOFs of bpe have been synthesized in this way.59 For example, from these Pb(II) MOFs, it is evident that the nature of the anions greatly influences the crystal growth and the resultant MOFs. Crystal Structures of Compounds 1−3. The single crystal X-ray diffraction studies reveal that the compound 1 crystallizes in space group P1̅ in a triclinic system with Z = 4. The asymmetric unit contains two [Pb(O2CCH3)4(bpy)] units and two water molecules (Figure S1). All the Pb(II) atoms reveal a highly distorted hemidirected seven-coordinate geometry affected by a stereochemically active Pb(II) lone pair. Pb1 and Pb2 are strongly coordinated to the nitrogen atoms of the one bpy ligand [Pb(1)−N(1) 2.616(2) Å and Pb(2)−N(2) 2.582(8) Å]. Further, Pb3 and Pb4 are strongly bridged by the nitrogen atoms of another bpy ligand [Pb(3)− N(3) 2.579(3) Å and Pb(4)−N(4) 2.627(1) Å]. The acetate ions act chelating as well as bridging ligands. The Pb1 and Pb2 are chelating to two CH3CO2− anions [Pb(1)−O(1) 2.647(6) Å; Pb(1)−O(2) 2.439(9) Å; Pb(1)−O(3) 2.409(7) Å; Pb(1)−O(4) 2.824(2) Å; Pb(2)−O(5) 2.429(2) Å; Pb(2)− O(6) 2.667(7) Å; Pb(2)−O(7) 2.419(7) Å; Pb(2)−O(8) 2.779(6) Å]. The O4 oxygen atom of the CH3CO2− anion involved in chelation to Pb1 is further bridged to Pb3 [Pb(3)− O(4)d 2.429(6) Å (symmetry operator d: 1 − x, 2 − y, 1 − z)] and Pb3 also chelating to two CH3CO2− anions [Pb(3)−O(9)

Figure 1. (a) Illustration of a perspective view of the packing structure of compound 1. (b) Illustration of the network topology representation sra in 1.

In addition, the four heptacoordinated Pb(II) are connected by −(OCCH3)-O− linkages, resulting in a noninterpenetrated MOF net with a four-connected uninodal sra (SrAl2) topology60−65 as shown in Figure 1b. Some of the bestreported structures MOF-69A,60 MOF-7161 and MIL-47,62 and MIL-5363 with sra topology also contain the similar structure of zigzag ladders via carboxylate secondary building units (SBUs). The Pb(II) metal ions are considered as the point of extension in 1. However, the metal SBU is composed of a rod of MO6 octahedra sharing reverse corners and the rods with the point of extension (carboxylate C atoms) connected into a zigzag ladder in MOF-71.61,64 Interestingly, the sra network in 1 contains a large void space which is partially C

DOI: 10.1021/acs.inorgchem.8b00637 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

and S6). This makes an unprecedented doubly interpenetrated MOF net with a four nodal net (point (Schlafli) symbol for net: {32;4}{33;43}{33;44;52;63;7;82}{34;47;52;64;7;83})65 in 2 as shown in Figure 2b. Although the connectivity of 2 looks very similar to the sra topology of the former structure, the topology analysis suggests that this compound has simple nets by the linking of these ladders. The linkages of ladders are able to generate several simple nets including sra, sra-c, irl, frl, fry, umr, and umu among others.64 However, the nets formed from the linking of the ladders in 2 were found as unprecedented compared to those of known nets. This doubly interpenetrated network 2 contains less void space compared 1 due to the interpenetration (total potential solvent area per unit cell is 12.2%).66 The voids are occupied by the chloroform molecules. Compound 3 crystallized in space group C2/c in monoclinic with Z = 4. This structure is isostructural to the compound [Pb(bpeH)2(O2CCF3)4] (where bpeH is monoprotonated 4,4′-bipyridylethylene) reported earlier with bpe as the spacer ligand and the bpy and fluorine atoms are disordered.58 Here also, the Pb(II) is coordinated to four CF3CO2− ligands and two monoprotonated bpy ligands to form highly distorted geometry with an O4N2 donor set (Figure S7). Of the four CF3CO2− anions with two are in chelating mode. The bond distances of Pb−O vary from 2.662(2) to 2.829(2) Å. The zwitter-ionic complexes are oriented through complementary N−H···O [N2−H2, 0.81(5) Å, N2···O1 (1/2 − x, 1/2 − y, 2 − z), 2.726(4) Å, and N2−H2···O1, 173°] hydrogen-bonding interactions among the protonated nitrogen atoms and O1 atoms of the monodentate trifluoroacetate ligand. A crystallographic two-fold axis is present at the Pb(II) metal center, while the center of the hydrogen-bonded bpy−H+ pair remains on an inversion center. Consequently, an inversion center is present between the adjacent bpy−H+ pairs that are assembled in parallel. The distance between the centroid of the pyridyl groups is 3.65 Å, indicating π···π interactions. The N−H···O hydrogen bonding furnishes a zigzag hydrogen-bonded polymer generating along the c-direction as shown in Figure 3. The bond angle N(1)−Pb(1)−N(1), 80°, contributes in

occupied by ordered water molecules. The total potential solvent accessible void is 872.2 Å which is 29.7% of the volume of the unit cell.66 Compound 2 crystallizes in space group P21/c in a monoclinic system with Z = 8. The asymmetric unit contains two [Pb(bpy)(O2CCF3)2] units and a chloroform (Figure S4). Two crystallographically independent Pb(II) centers are displayed. The Pb1 exhibits highly distorted hemidirected six-coordinate geometry, and Pb2 displays highly distorted hemidirected seven-coordinate geometry caused by the Pb(II) metal center having an active lone pair. The Pb1 and Pb2 are strongly coordinated to the nitrogen atoms of the two bpy ligands [Pb(1)−N(1) 2.531(3) Å and Pb(1)−N(3) 2.603(9) Å; Pb(2)−N(2) 2.535(6) Å and Pb(2)−N(4) 2.650(5) Å]. There are two types of trifluoroacetate anions, one acting as a bridging ligand, while the second chelates and bridges the Pb(II) atoms in a tridentate mode. The Pb2 is chelating to one CF3CO2− anion [Pb(2)−O(5) 2.503(3) Å; Pb(2)−O(6) 2.790(1) Å]. The O6 oxygen atom of the CF3CO2− anion is involved in chelation to Pb2 and further bridged Pb1 [Pb(1)− O(6)a 2.774(6) Å (symmetry operator a: x, y, −1 + z)]. Two more CF3CO2− anions bridge between Pb1 and Pb2 atoms [Pb(1)−O(1) 2.507(1) Å; Pb(1)−O(3) 2.678(7) Å; Pb(2)− O(2)b 2.777(1) Å; Pb(2)−O(4)e 2.609(2) Å; (symmetry operator b: x, y, 1 + z; e: 1 − x, 1 − y, −z)]. The O7 oxygen atom of one CF3CO2− anion bridges both Pb1 and Pb2 [Pb(1)−O(7)b 2.727(8) Å; Pb(2)−O(7) 2.623(3) Å; (symmetry operator b: −x + 1, −y + 2, −z)], and the other oxygen atom of the anion is uncoordinated. Due to the bridging and chelation mode of CF3CO2− anions between the Pb1 and Pb2, a tetra-connected Pb4(O2CCF3)8 cluster unit resulted and each cluster is linked further through eight bpy ligands as shown in Figure 2a. The Pb(II) metal ions act as the point of extension, and the tetra-connected Pb4(O2CCF3)8 form ladders. These ladders are interconnected (Figures S5

Figure 3. A portion of the zigzag hydrogen-bonded polymer 3 showing the π···π contacts. The hydrogen atoms have been omitted.

forming a zigzag conformation in 3 which is even less than the N(1)−Pb(1)−N(2) bond angle of 84° in the previously reported structure with bpe.58 Powder X-ray Diffraction. The bulk identities of 1−3 were verified by powder X-ray diffraction (PXRD) measurements (Figures S8−S10). The PXRD patterns for assynthesized samples match well with the simulated patterns from crystal data, representing that the crystal is typical of the pure bulk sample. Optical Properties. Due to their potential applications in photochemistry, chemical sensors, and electroluminescent displays, the solid-state photoluminescence properties of 1−3

Figure 2. (a) Illustration of a perspective view of the packing structure of a single net in compound 2. (b) Illustration of the interpenetrated network topology representation in 2. D

DOI: 10.1021/acs.inorgchem.8b00637 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Structurally, the compounds 1 and 2 only differ in chelating ligands, where 1 chelated with acetate ligands and 2 chelated with strongly electron-withdrawing trifluoroacetate ligands, vide supra. The fact that the electron-withdrawing groups decrease the overall molecular orbital energies suggests the probability of increased metal-centered emissions through s and p orbitals of the Pb(II) metal ions, which is evident from the characteristic emission of compound 2 at 574 nm, unlike structurally very similar compound 1. The Commission Internationale de l’Eclairage (CIE) chromaticity diagram as shown in Figure 6 contains the

were examined at room temperature. The solid-state UV−vis absorption spectral bands of 1−3 are shown in Figure S11. The compounds 1 and 2 exhibit the lowest energy absorption at ∼350 nm with a shoulder at ∼395 nm, while the compound 3 does not show features of the longer wavelength absorbance at 395 nm. The solids 1 and 2 show a bright white emission under UV light, and 3 shows WLE with a reddish tinge as shown in Figure 4. It is identified that the metal-free bpy ligand in DMF exhibits emission at 440 nm, attributable to n−π* and π → π* transitions.67

Figure 4. Photographs show the emission of compounds (a) 1, (b) 2, and (c) 3 solids exposed to UV light.

The compound 1 exhibits maximum emission in the solid state, situated at 502 nm on excitation at 329 nm as displayed in Figure 5. A red shift by about 62 nm was observed in the

Figure 6. Photograph of the CIE chromaticity diagram for compounds 1−3 (λex = 329 nm, λex = 376 nm, and λex = 330 nm for 1, 2, and 3, respectively).

quantified colors of the emission. The coordinates displayed in the chromaticity diagram of 1 are (0.24, 0.32) while excited at 329 nm light. The coordinates of 2 are displayed (0.33, 0.39) when excited at 376 nm light, and the coordinates of 3 are (0.26, 0.31) when excited at 330 nm light which are placed in the gamut, all three positioning near to the white gamut of the CIE-1931 color space chromaticity diagram.68,69 The compounds 1−3 illustrate rather remarkable emission which is very near to the white light region as shown in the chromaticity diagram. Although it is not easy to achieve high-quality white light using single-component materials which is required CIE (0.33, 0.33), the single-component white light emission observed before in Pb(II) based MOFs46−48 and coordination complexes.42 One of the Pb(II) complexes, [Pb4(Mq)6]· (ClO4)2, displays good quality white light emission with CIE (0.29, 0.30).42 Compound 3 shows the major ligand-centered emission peak at 450 nm with the shoulder peak at 550 nm that corresponds to the MLCT character that resulted in a white emission with a reddish tinge, with CIE coordinates (0.26, 0.31) as displayed in Figure 6. On the other hand, the compound 1 emits with the red-shifted major peak at 570 nm and the CIE coordinates (0.24, 0.32) as shown in Figure 6, giving close to a pure white emission. The compound 2 with the strongly electron-withdrawing chelating ligand, and the significant metal-centered emission, exhibits the highest quality white light with CIE coordinates (0.33, 0.39) and can be considered as a potential candidate for constructing singlecomponent WLE devices. Quantum chemical computations elucidate further insight into the photophysical properties of the complexes. We have selected [Pb(bpy)2(CF3COO)4]2− as a system to mimic the major contribution of the photophysical properties of 2, which

Figure 5. Luminescent emission spectra of compounds 1−3 (λex = 329 nm, λex = 376 nm, and λex = 330 nm for 1, 2, and 3, respectively) in the solid state at room temperature.

spectral emission peak of 1 in comparison to that of the ligand. The dual emission displayed compound 2 with maxima situated at 512 and 574 nm upon excitation at 376 nm as displayed in Figure 5. Red shifts by about 72 and 134 nm were observed in the spectral emission peak of 2 in comparison to that of the ligand. The maximum emission of compound 3 observed spectra was located at 467 nm upon excitation at 330 nm as shown in Figure 5. However, the maximum emission observed for 3 was at 467 nm upon excitation at both 300 and 330 nm, which closely resembles the ligand-centered emission. The origin of the red-shifted emission at 510 nm in both 1 and 2 is probably due to the significant contribution from the metal centers to the photoluminescence. Particularly, the emission of the compounds 1 and 2 is also ascribed to the ligand-to-metal charge transfer (LMCT) among the bipyridyl groups having delocalized π-bonds and the s and p orbitals of Pb(II) metal centers.67 Furthermore, compound 2 reveals the second emission at a long wavelength with a maximum at 574 nm. E

DOI: 10.1021/acs.inorgchem.8b00637 Inorg. Chem. XXXX, XXX, XXX−XXX

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using pure trifluoroacetic acid in the reaction mixture produced a hydrogen-bonded zwitter-ionic complex, 3. The coordination modes of the monocarboxylate anions influence the formation of two different MOFs using acetate and trifluoroacetate ligands. The formation of a hydrogen-bonded zwitter-ionic complex of bpy indicates that it is possible to generate a structure similar to the bpe analogue with the hydrogenbonded zigzag chain structures. Emission of 1−3 was monitored in the solid state at room temperature. Remarkably, all three compounds exhibit single-component near-white light emission owing to ligand-to-metal charge-transfer (LMCT) and metal-centered (MC) transitions. We hope that these studies would help us to have a better understanding of the solid-state lighting properties of the Pb(II) compound and to develop superior single-component WLE materials.

shows features of the MC transitions (Figure 7) as the component of the white-light-emitting photoluminescence.

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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.8b00637. X-ray crystallographic structures, selected bond distances, experimental and calculated PXRD patterns, UV/ vis spectra, orbital energies, XYZ coordinates, highest wavelength absorption data, coordinates for the B3LYP/ LANL2DZ/6-31G(d,p)-optimized structure, solid-state luminescent emission spectra (PDF) Accession Codes

CCDC 1047553−1047555 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.

Figure 7. Energy levels of the lower energy transitions and the frontier orbitals for the [Pb(bpy)2(CF3COO)4]2−.

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As shown in Figure 7, the highest occupied molecular orbital (HOMO) and HOMO-1 of the [Pb(bpy)2(CF3COO)4]2− contribute mainly from the d orbitals of Pb2+ ion and π orbitals of CF3CO2− anion. The lowest unoccupied molecular orbital (LUMO) and LUMO+1 orbitals are mainly contributed by the π and π* orbitals of the bpy ligands, which indicate that the main contributions of the lowest energy transitions involve transitions from metal to π* orbitals of the ligand (bpy), MLCT, and n orbitals of the anion CF3CO2− to π* orbitals of the ligand (bpy), LLCT. These calculated lowest ground state transition energies (HOMO → LUMO at 386 nm, and HOMO-1 → LUMO at 352 nm) compare closely with the experimental optical absorption energies of the compound 2 shoulder measured at 395 nm and the second lowest absorption peak at 340 nm; see Figure S11. Timedependent DFT calculations for the absorption spectra of compound [Pb(bpy)2(CF3COO)4]2− are in reasonable agreement with the experimental data (Figures S12 and S13). The most important lowest energy singlet excitations are presented in Table S2.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.M.P.P.). *E-mail: [email protected] (J.J.V.). *E-mail: [email protected] (W.J.). ORCID

Abdul Malik P. Peedikakkal: 0000-0002-4745-2843 Abdul Rajjak Shaikh: 0000-0003-4444-0684 Jagadese J. Vittal: 0000-0001-8302-0733 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M.P.P. would like to acknowledge the support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. IN131005. J.J.V. thanks the Ministry of Education, Singapore, for funding this project through NUS FRC Grant No. R-143-000-A12-114. We also thank Ms. Geok Kheng Tan and Ms. Hong Yimian for the collection of X-ray crystallographic data. We also thank Natalie S. Koh for her help in experiments.

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CONCLUSION Two MOFs have been isolated from the lead(II) ion and bpy ligand from two different monocarboxylate anions. Compound 1 displays a noninterpenetrated MOF with sra topology using acetate anions. However, compound 2 shows an unprecedented doubly interpenetrated topology where all the acetate ligands are replaced with trifluoroacetate ligands. However,

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