X = Clâ, Brâ, or I - ACS Publications - American Chemical Society

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Diimidazolium Halobismuthates [Dim]2[Bi2X10] (X = Cl−, Br−, or I−): A New Class of Thermochromic and Photoluminescent Materials Alberto García-Fernández,*,† Ismael Marcos-Cives,† Carlos Platas-Iglesias,‡ Socorro Castro-García,*,† Digna Vázquez-García,† Alberto Fernández,† and Manuel Sánchez-Andújar† †

QuiMolMat Group, Departamento de Química, Facultade de Ciencias & Centro de Investigacións Científicas Avanzadas (CICA), Universidade da Coruña, 15071 A Coruña, Spain ‡ React! Group, Departamento de Química, Facultade de Ciencias & Centro de Investigacións Científicas Avanzadas (CICA), Universidade da Coruña, 15071 A Coruña, Spain S Supporting Information *

ABSTRACT: We present a novel family of polyhalide salts of Bi(III) with the general formula [Dim]2[Bi2X10], where Dim2+ is the diimidazolium cation (C9H14N4)2+ and X is Cl−, Br−, or I−. Single-phase materials are easily obtained by means of a mild solution chemistry method performed at room temperature. This [Dim]2[Bi2X10] family exhibits a crystal structure based on halobismuthate [Bi2X10]4− dimers, built by distorted {BiX6} octahedra interconnected by edge sharing, and sandwiched between two diimidazolium cations. The optical band gaps displayed by these materials (1.9−3.2 eV) allow their classification as semiconductors. Additionally, the three halides display photoluminescence with emission in the visible range. The behavior of [Dim]2 [Bi 2 I10 ] is particularly interesting, as it shows an optical band gap of 1.9 eV, a broad band photoluminescence emission, and a relatively long emission lifetime of 190 ns. Moreover, the iodide and bromide compounds also exhibit a reversible solid state thermochromism, being the first example of a bromobismuthate with this property. The diimidazolium cations play an important structural role by stabilizing the crystal structure and balancing the charges of the [Bi2X10]4− dimers. Furthermore, density functional theory calculations suggest that they play a key role in the thermochromic behavior. Therefore, compounds [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) represent a very versatile family in which the optical band gap can be tuned by changing the halide or temperature. This makes them promising new materials for different optoelectronic applications, in particular for obtaining new solar absorbers.

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humidity.6−8 In the search for materials beyond MAPbI3, attempts to substitute Pb(II) with Sn(II) or Ge(II) have produced compounds that oxidize rapidly to Sn(IV) and Ge(IV), respectively, in air.9 More recently, the similar chemistries of isoelectronic 6s2 cations Pb(II) and Bi(III) have aroused interest in developing Bi(III) halide-based semiconductors for this purpose.10,11 The family of Bi(III) halide organic−inorganic hybrids has attracted a great deal of attention over the past few decades because of the interesting optical and electronic properties that they often exhibit12−14 (i.e., semiconductivity, nonlinear optical behavior, luminescence,15,16 photochromism,17−20 solvochromism, thermochromism,21−24 ferroelectricity,25 etc.). The importance of these Bi(III) compounds is obviously related to their amazing variety of architectures. Indeed, a great structural diversity of polyhalide salts of Bi(III) can be found in the literature,13 ranging from discrete anions, binuclear to

INTRODUCTION In recent years, hybrid organic−inorganic materials have arisen as a singular family of materials with functional properties that are of great interest for energy, environmental, and technological applications, among others.1,2 These compounds integrate organic and inorganic building blocks in their structure, where the chemical diversity and multiple possible combinations give rise to a great structural richness, including several thermally driven structural transitions, a high degree of flexibility, and (multi)functional properties. In this context, a family of hybrid halide perovskites have attracted a great deal of attention as very promising materials for low-cost and highefficiency photovoltaic applications,3,4 reaching with the wellknown methylammonium lead iodide (MAPbI3) an efficiency of 22%,5 a value that surpasses those of dye-sensitized, quantum dot, organic and amorphous silicon solar cells and other emerging photovoltaic (PV) technologies. However, perovskite solar cells have to overcome several inconveniences in terms of their commercial viability, such as the toxicity of lead, their bad long-term stability, or their instability in the presence of © XXXX American Chemical Society

Received: March 9, 2018

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DOI: 10.1021/acs.inorgchem.8b00629 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry octanuclear complexes or the rare example of the [Bi10I20]10+ cation,26 to two-dimensional polymeric structures. The stoichiometry of reagents and their concentrations, temperature, solvent, and organic cation are factors that strongly influence the resulting crystal structure. In particular, the organic cation plays an important role in the arrangements of the Bi−X network depending on its volume, shape, charge, ability to build hydrogen bonds, etc. In this context, a large variety of organic cations were combined with bismuth halides to obtain different structures and physical properties. However, most of these studies focused on viologen cations (N-alkylated derivatives of 4,4′-bipyridine), which allow us to obtain materials with interesting optical properties, such as photochromism, thermochromism, luminescence, and solvochromism.27−29 In the search for new organic cations to obtain hybrid polyhalobismuthates, we have focused on five-membered heterocyclic rings such as imidazole and tri-, tetra-, and pyrazole. These types of ligands has been widely used in organometallic chemistry because of their versatile coordination modes.30−32 Imidazolium cations possess preorganized structures promoted mainly through H-bonds that induce structural directionality, in contrast to classical salts in which the aggregates are mainly formed through ionic interactions. Moreover, aromatic stacking can also occur as a secondary attractive interaction (i.e., imidazolium cations are electronpoor rings, so the interaction between the π-electrons is usually weak).33 Additionally, 2,2′-biimidazole (C6H6N4) has also proven to be a very useful building unit in the construction of new coordination compounds because of the presence of four nitrogen atoms that allow coordination to different metal centers.34−36 Some of them show interesting optical properties such as photoluminescence and fluorescence.37,38 In this work, we have chosen the divalent [1,1′-dimethyl-3,3′methylene-diimidazolium]2+ cation (Dim2+) with the formula [C9H14N4]2+. This cation can be described as two rigid Nmethylimidazolium units linked by a flexible methylene spacer (see Scheme 1) that can experience a certain torsion, helping its

thermochromic properties of hybrid organic−inorganic halobismuthates.

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Materials and Methods. Starting reagents CH2Cl2 (≥99.5%), CH2Br2 (≥98.5%), and CH2I2 (≥99.0%) were provided by Fluka. NMethylimidazole (99%) was provided by Acros Organics. Bismuth halides BiCl3 (≥98%), BiBr3 (≥98%), and BiI3 (99%) were provided by Sigma-Aldrich. All of them were used without further treatment. Synthesis. DimX2: (C9H14N4)X2 (X = Cl−, Br−, or I−). The diimidazolium halide salts, DimX2, were easily prepared by direct reaction between N-methylimidazole (C4H6N2) and the appropriate dihalomethane (CH2X2, where X = Cl−, Br−, or I−) (see Scheme 1), using a slight modification of a method that was previously published by Scherg et al.39 In particular, the starting reagents were refluxed under the conditions summarized in Table S1. After cooling to room temperature, the resulting white solid was triturated with diethyl ether and washed several times with acetone and diethyl ether. The diimidazolium salts are hygroscopic and must be stored in a vacuum desiccator to prevent the reaction with atmospheric moisture and the formation of a DimX2·yH2O hydrate, which could alter the stoichiometry of the next reactions. [Dim]2[Bi2X10]: [C9H14N4]2[Bi2X10] (X = Cl−, Br−, or I−). Powder samples of [Dim]2[Bi2X10] were prepared by means of a mixed solution method performed at room temperature. For this purpose, equimolar amounts of BiX3 and DimX2 were dissolved in 2 mL of N,N-dimethylformamide (DMF) and water, respectively. The two 1 M solutions were mixed by observing the instantaneous precipitation of the desired compound. Visually homogeneous orange (I), yellow (Br), or white (Cl) powders were isolated after filtration. Single crystals of [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) were also prepared by a slow diffusion method at room temperature. In a typical experiment, 2 mL of an aqueous solution containing 0.2 mol of DimX2 (X = Cl−, Br−, or I−) was placed at the bottom of a glass tube. This solution was slowly layered with 2 mL of DMF, in a first step, and then 2 mL of a 0.1 M BiX3 (X = Cl−, Br−, or I−) solution in DMF. For the optimal solution of the bismuth salts, 100 μL of the corresponding hydrogen halide was added to the corresponding solution. After a few days, single crystals of the three compounds were obtained (orange needles for I, yellow needles for Br, and colorless needles for Cl). They were collected by filtration and washed several times with ethanol. Elemental Analysis. Elemental chemical analyses (C, N, and H) were performed using a FLASHEA1112 (Thermo Finnigan) analyzer. The results demonstrate the formation of the desired compounds, showing experimental data in concordance with the theoretical ones. Elemental composition calculated for C18H28Bi2Cl10N8: C, 19.15; H, 2.50; N, 9.93. Found: C, 16.48; H, 1.85; N, 8.10. Elemental composition calculated for C18H28Bi2Br10N8: C, 13.74; H, 1.79; N, 7.12. Found: C, 13.66; H, 1.51; N, 6.71. Elemental composition calculated for C18H28Bi2I10N8: C, 10.58; H, 1.38; N, 5.48. Found: C, 10.93; H, 1.19; N, 5.23. Thermogravimetric Analyses (TGA). Thermogravimetric analyses of the obtained polycrystalline powders were performed in TGADTA thermal analysis SDT2960 equipment. For these experiments, approximately 27 mg of each sample was heated at a rate of 5 K/min from 300 to 1200 K using corundum crucibles under a flow of dry nitrogen. Powder X-ray Diffraction (PXRD). Powder X-ray diffraction patterns from the obtained polycrystalline powder samples were collected at room temperature in a Siemens D-5000 diffractometer using Cu Kα radiation. They were compared with the profiles obtained from the single-crystal structures at room temperature, which were generated by using Mercury version 3.5.1.40 Single-Crystal X-ray Diffraction (SCXRD). Three-dimensional X-ray data were collected on a Bruker X8 Apex diffractometer using graphite-monochromatic Mo Kα radiation. All the measured reflections were corrected for Lorentz and polarization effects and for absorption by semiempirical methods based on symmetryequivalent and repeated reflections. The structures were determined

Scheme 1. General Synthetic Route of the Diimidazolium Halidesa

a

EXPERIMENTAL SECTION

The solvent (toluene) is not needed when X = Br−.

incorporation into versatile crystalline structures.33 We present a very easy synthetic route for preparing new halobismuthates with the formula [Dim]2[Bi2X10] using three halides X = Cl−, Br−, and I−. We hypothesized that the electron-deficient imidazolium cation could facilitate the transfer of charge from the electron-donor iodobismuthate entities, resulting in materials with thermochromic behavior. A detailed structural characterization was accomplished by means of powder and single-crystal X-ray diffraction. The optical properties and their thermochromic behavior were studied by ultraviolet−visible (UV−vis) spectroscopy and photoluminescence. Additionally, we have gained information about the electronic structure and optical properties of these new compounds by density functional theory (DFT) calculations. These theoretical studies provide unprecedented insight into the origin of the B


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Inorganic Chemistry by direct methods and refined by full matrix least squares on F2 with allowance for thermal anisotropy of all non-hydrogen atoms. The structure solution and refinement were performed using the program package SHELX-14.41 Additionally, the presence of voids in the obtained crystal structure was studied using the SQUEEZE/PLATON tool.42 In the case of the iodide compound, we have observed four voids within the unit cell, each with a volume of ∼28 Å3. These voids are partially occupied (∼50%) by water molecules. Ultraviolet−Visible (UV−vis) Spectroscopy. Optical diffuse reflectance measurements of the polycrystalline powder samples were performed at room temperature using a Jasco V-730 UV−vis doublebeam spectrophotometer with a single monochromator, operating from 200 to 900 nm. BaSO4 was used as a non-absorbing reflectance reference. The generated reflectance versus wavelength data were used to estimate the band gap of the material by converting reflectance to absorbance data according to the Kubelka−Munk equation:43 F(R) = α = (1 − R)2/2R, where R is the reflectance data and α values are the absorption coefficients. Solid State Fluorescence. Emission spectra were recorded on a Horiba FluoroMax Plus-P spectrofluorometer equipped with a 150 W ozone-free xenon arc lamp and a R928P photon counting emission detector, as well as a photodiode reference detector for monitoring lamp output. Samples were excited using a 150 W xenon arc lamp at 250 nm, and then the emission was measured from 300 to 750 nm. Temperature-dependent luminescence measurements and thermochromic images were recorded with a cryo-stage, Linkam LTS 420 hotstage, and Linkam THMS-LNP95 cooling system. Time-resolved photoluminescence decays were measured on a Horiba FluoroMax Plus-P spectrofluorometer working in the timecorrelated single-photon counting (TCSPC) lifetime spectroscopy mode, using a 370 nm nanoled as the excitation source. Density Functional Theory (DFT) Calculations. The [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) systems were investigated by using DFT calculations within the hybrid-meta GGA approximation with the TPSSh functional44 and the Gaussian 09 package.45 The geometries were taken from the corresponding single-crystal X-ray structures and were not optimized. In these calculations, we employed the relativistic effective core potential of Dolg (ECP60MDF)46 for Bi, which includes 60 electrons in the core. The valence space (5s5p5d6s6p) was described by the cc-pVTZ basis set, which presents a (12s11p8d1f)/[5s4p3d1f] contraction scheme.47 The remaining atoms were described by using the standard 6-311+G(d,p) basis set [6311G(d,p) for I].48 Calculations used the default SCF energy convergence threshold (10−8 hartree) and an ultrafine grid (99 radial shells and 590 angular points). Density-of-states (DOS) plots were obtained from Mulliken population analysis49 with the aid of Multiwfn version 3.2,50 using Gaussian functions with half-widths at half-height of 0.02 au.

Figure 1. Thermogravimetric analysis (TGA) of [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) under a nitrogen atmosphere upon heating at a rate of 5 K/min.

Crystal Structures Determined by SCXRD. Single-crystal X-ray diffraction analysis shows that the three [Dim]2[Bi2X10] compounds (X = Cl−, Br−, and I−) are isomorphous and display a monoclinic symmetry (space group P21/c) at room temperature. The crystallographic data of the three compounds are summarized in Tables 1 and S2−S4 and Figure 2. Table 1. Selected Bond Lengths (angstroms) Observed for [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) at 293 K bond

X = Cl−

X = Br−

X = I−

Bi(1)−X(1) Bi(1)−X(2) Bi(1)−X(3) Bi(1)−X(3) Bi(1)−X(4) Bi(1)−X(5)

2.563(1) 2.737(1) 2.914(1) 2.845(1) 2.659(1) 2.623(1)

2.7339(9) 2.8042(9) 3.0140(8) 3.0175(8) 2.8919(9) 2.7633(8)

2.9827(6) 3.0260(6) 3.1757(5) 3.2574(5) 3.1176(6) 2.9821(6)

Bi(2)−X(6) Bi(2)−X(7) Bi(2)−X(7) Bi(2)−X(8) Bi(2)−X(9) Bi(2)−X(10)

2.700(1) 2.885(1) 2.988(1) 2.686(1) 2.579(1) 2.545(1)

2.8461(8) 3.0260(8) 3.0896(9) 2.8439(8) 2.7290(8) 2.697(1)

3.0721(6) 3.2491(5) 3.2497(5) 3.0655(6) 2.9433(5) 2.9249(7)

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RESULTS AND DISCUSSION Synthesis and Basic Characterization. The novel family of diimidazolium polyhalobismuthates was easily obtained by means of a mild solution chemistry method performed at room temperature. For the synthesis, two solutions of BiX3 and DimX2 were mixed at room temperature with instantaneous precipitation of the desired compound. The obtained polycrystalline powder samples were singlephase materials, as confirmed by comparison between the experimental PXRD patterns and the profiles calculated from the single-crystal structures obtained at room temperature (see Figure S1). It is worth noting that this family of compounds is stable in air at room temperature for several months, as confirmed by PXRD. Additionally, their thermal stability has been evaluated by TGA. The results (Figure 1) indicate that the chloride and bromide compounds are stable until ∼560 K while the iodide derivative is stable until ∼525 K.

Figure 2. Crystal structure of [Dim]2[Bi2X10] (X = Cl−, Br−, and I−) compounds at room temperature. C


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Inorganic Chemistry The crystal structures of [Dim]2[Bi2X10] compounds contain two ionic species: diimidazolium Dim2+ cations and halobismuthate [Bi2X10]4− anions. These latter anions are [Bi2X10]4− dimers formed by two edge-sharing {BiX6} octahedra with two μ2-X ligands (Figures 2, S2, and S3), as commonly found in other bismuthates.51−53 As described previously for other bismuthates based on [Bi2X10]4− anions,13,54−57 the main feature is the irregular octahedral environment of Bi(III) cations. Each Bi(III) cation is linked to two bridging and four terminal halide ligands, which results in six different Bi−X distances ranging from ∼2.54 to ∼2.99 Å for X = Cl−, from ∼2.69 to ∼3.08 Å for X = Br−, and from ∼2.92 to ∼3.26 Å for X = I−. The Bi−Xterminal bond lengths are significantly shorter than the Bi−Xbridging ones. Selected bond lengths and structural details are listed in Table 1 (see also Tables S3 and S4). The distortion of the octahedral coordination is also evidenced in the values of the X−Bi−X angles (see Table S3), which deviate significantly from the ideal values of 90° and 180°. It is worth noting that the bismuth cations are not positioned at the center of the octahedra; in fact, they move away from the shared octahedral edge to minimize the Bi(III)−Bi(III) interaction.58 This off-center displacement is likely related to the stereochemical activity of the Bi(III) lone pair (6s2 orbital) and can be explained by Brown’s model.59 In these new structures, each [Bi2X10]4− dimer is sandwiched between two diimidazolium cations [C9H14N4]2+, as shown in Figure 2, and also in Figures S2 and S3. Those diimidazolium cations can be described as rigid N-methyl-imidazolium units linked by a methylene spacer, with structural parameters similar to those reported for the related 1,1′-dimethyl-3,3′-methylenediimidazolium dibromide compound.33,60 UV−vis Diffuse Reflectance Spectroscopy. The optical spectra of the three halobismuthates, measured at room temperature using the diffuse reflection mode by UV−vis spectrophotometry, are presented in Figure 3. The spectra show absorption cutoff wavelengths of ∼390, ∼460, and ∼620 nm for the chloride, bromide, and iodide compounds, respectively. The corresponding optical band gaps, determined using the Kubelka−Munk equation,42 are 3.2, 2.7, and 1.9 eV (for chloride, bromide, and iodide compounds, respectively). These band gaps fall within the range of typical semiconductor materials, being comparable to the values found for other bismuth halides studied as potential solar absorbers. Relevant examples include (CH3NH3)BiI4,61 A3Bi2I9 (A = K+, Rb+, Cs+, NH4, or CH3NH3),62,63 Cs2AgBiX6 (X = Cl− or Br−),64 (CH 3 NH 3 ) 2 KBiCl 6 , 6 5 and p-phenylenediammonium [C6H4(NH3)2]2Bi2I10,66 which present band gaps from 1.90 to 3.04 eV. In agreement with the literature results, our compounds show a regular red shift with the halide, from Cl− to I−. Very interestingly, the optical band gap of the iodide compound is lower than the optical band gap found in other related bismuth iodides.66−68 In this context, it is worth highlighting that this band gap value makes the iodide material a promising new solar absorber. Photoluminescence Studies. The steady state photoluminescence (PL) spectra obtained at room temperature for the [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) compounds are shown in Figure 4. All these spectra show a broad photoluminescence peak with maxima at 435 nm for X = Cl−, 450 nm for X = Br−, and 615 nm for X = I−, which are redshifted with respect to the optical absorption onset. The timeresolved photoluminescence decay, shown in Figures 5 and S4, was fitted with a double-exponential decay giving fast

Figure 3. (a) Absorption UV−vis spectra and (b) Tauc plots for the [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) compounds at room temperature.

Figure 4. Steady state photoluminescence spectra for the [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) compounds at room temperature.

component lifetimes of 2.41(3), 2.98(3), and 1.79(2) ns and slow component lifetimes of 27.0(1), 27.6(1), and 190(3) ns for the chloride, bromide, and iodide compounds, respectively. It is worth highlighting the relatively long lifetime exhibited by the iodide compound, similar to those reported for BiI3 films and single crystals (180−240 ns) that have been considered as candidates to produce high-performance PV devices.69 We tentatively assign the shorter component of the emission decay to the fluorescence of the Dim2+ cations, while the longer lifetime likely corresponds to the charge-transfer excited state of the [Bi2X10]4− entities (see DFT studies below). D


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It is worth noting that several iodobismuthate compounds had been reported as thermochromic materials, in all cases with color changes from (dark) red at room temperature to orange upon cooling to liquid nitrogen temperature.13,22,70 Nevertheless, to the best of our knowledge, the compound [Dim]2[Bi2Br10] represents the first reported example of a thermochromic bromobismuthate. The steady state photoluminescence properties of this bromobismuthate have been characterized at different temperatures. Figure 6b shows a remarkable displacement of the emission band of this compound with temperature, from 425 nm (2.92 eV) at 153 K to 460 nm (2.69 eV) at 393 K, related to the thermal modification of the color. We have carefully looked for potential structural modifications that allow us to understand and rationalize the mechanisms involved in the observed thermochromism. In this context, and with reference to the origin of the thermochromism, solid state inorganic materials have been divided into two main classes: materials exhibiting an abrupt color change at a specific temperature caused by a change in the crystal structure and materials exhibiting a gradual color change, which is more common, and related to a slight lattice expansion or contraction.21,53,71 Crystallographic data were collected at 100 and 293 K on a single crystal of the iodide compound, as the halobismuthate exhibits the most important color changes (Figure S5 and Table S2). Interestingly, this compound presents the same crystal structure at both temperatures and does not present any phase transition upon being heated or cooled. We have carefully analyzed the bond lengths (I−I, Bi−I, and Bi−Bi) at both temperatures (Table S4), finding that the largest changes affecting Bi−Bi and Bi−I distances are approximately 0.06 Å. Those changes are very small in comparison with those of other related iodobismuthate thermochromic materials, which experience changes of ∼0.21 Å.22,53 Therefore, these results indicate that the origin of thermochromism in our materials is not related to structural modifications. DFT Calculations. To gain information about the electronic structure of [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) materials and, consequently, their absorption, emission, and thermochromism, DFT calculations were performed for the three compounds. It was especially interesting to test if the changes in structure with temperature are sufficient to cause subtle changes in orbital energies associated with the electronic transitions that contribute to the visible crystal color. The calculations were performed on the [Dim]2[Bi2X10] unit represented in Figure S2, constituted by a [Bi2X10]4− dimeric entity sandwiched by two Dim2+ cations. Following DFT calculations, we obtained DOS plots with the aid of Mulliken population analysis to assess the contributions of the [Bi2X10]4− and Dim2+ entities to the overall DOS (Figures 7−9). The band gaps calculated for the [Bi2X10]4− entities match the experimental trend in that the band gap decreases on descending the halogen group, with estimated values of 3.6, 2.9, and 2.1 eV for Cl−, Br−, and I−, respectively (Table 2 and Figure 7). This decreasing band gap is the result of a stabilization of the conduction band, as the valence bands of the three [Bi2X10]4− systems present very similar energies and shapes (Figure 7). It is worth noting that there is excellent agreement between the band gaps obtained from optical absorption experiments for the three [Dim]2[Bi2X10] compounds with these values obtained from DFT calculations for the [Bi2X10]4− units (Table 2). Specifically, the band gaps

Figure 5. Time-resolved photoluminescence decay for the [Dim]2[Bi2I10] compound at room temperature.

Thermochromism. In addition to their photoluminescent emission, the bromide and iodide bismuthates exhibit a completely reversible chromatic variation induced by temperature, showing that both are thermochromic materials. As shown in Figure 6a, the iodide compound is bright red at around room temperature; when it cools to 153 K, the color changes to orange, and when it is heated to 393 K, the powder becomes dark red. In the case of the bromide compound, the color changes from uncolored at 153 K to bright yellow at 393 K.

Figure 6. (a) [Dim]2[Bi2X10] (X = Br− or I−) powders at different temperatures showing the thermochromic behavior of these compounds. (b) Steady state photoluminescence spectra of [Dim]2[Bi2Br10] at 153, 295, and 393 K. E


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Figure 7. Overlay of the density-of-states plots calculated for the three [Bi2X10]4− fragments (X = Cl−, Br−, or I−) (structures determined at 293 K). The energies of the HOMOs are indicated by dashed vertical lines.

Figure 8. Density-of-states plot calculated for the [Bi2I10]4− entity and contributions from Bi(III) 6s and 6p orbitals and I 5p orbitals (structures determined at 293 K).

estimated by DFT calculations are slightly larger than those obtained by optical absorption experiments. This is in line with previous results, which showed that hybrid functionals tend to provide band gaps that are larger than those of the corresponding nonhybrid counterparts.72 The nature of the valence and conduction bands of the [Bi2X10]4− entities of the [Dim]2[Bi2X10] systems was further analyzed using Mulliken population analysis. The results (Figures 8 and S6) show that the valence band presents a dominant contribution of halide p orbitals (5p orbitals for [Bi2I10]4−) but contains some minor contributions from the Bi(III) 6s lone pair and Bi(III) 6p orbitals. On the other hand, the conduction band contains a major contribution from Bi(III) 6p orbitals. Thus, the absorption bands observed in the visible region of the spectrum for [Dim]2[Bi2X10] compounds are associated with charge-transfer Bi(6p) ← X(np) transitions, with n = 3, 4, or 5 for X = Cl−, Br−, or I−, respectively. Conversely, the emission bands observed in the corresponding emission spectra can be assigned as Bi(6p) → X(np) chargetransfer bands. A recent DFT study reached the same conclusion for a series of iodobismuthates that contain {BiI6} octahedra sharing two opposite edges.69 Therefore, the experimentally observed absorption and emission spectra of the three [Dim]2[Bi2X10] units (X = Cl−, Br−, or I−) at room

Figure 9. Density-of-states plots obtained from DFT calculations for the [Dim]2[Bi2X10] systems (structures determined at 293 K): (a) X = Cl−, (b) X = Br−, and (c) X = I−. The energies of the HOMOs are indicated by dashed vertical lines.

Table 2. Band Gaps Obtained from Optical Absorption Experiments and DFT Calculations for the [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) Compounds

[Dim]2[Bi2Cl10] [Dim]2[Bi2Br10] [Dim]2[Bi2I10]

optical absorption experiments (eV)

DFT calculations (eV)

3.2 2.7 1.9

3.6 2.9 2.1

temperature are related to electron transitions within the [Bi2X10]4− anions. On the other hand, to elucidate the origin of the observed thermochromism, DFT calculations of the iodobismuthate derivative, [Dim]2[Bi2I10], were performed using the structures F


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Inorganic Chemistry determined at 100 and 293 K, following the procedure described above. Figure S7 shows the obtained DOS plots calculated for the structures measured at 100 and 293 K. The valence and conduction bands present virtually identical energies and shapes at both temperatures. Therefore, we cannot attribute the reversible thermochromism to an electronic structural change related to a decrease and/or increase in X−X, Bi−X, and/or Bi−Bi interatomic distances. Hence, while the electronic structure of the [Bi2X10]4− unit allows settling the observed change in the band gap energy with halide composition, the origin of the thermochromism cannot be justified taking into account only the anions. Panels a−c of Figure 9 present the DOS plots for the three [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) halides, calculated at room temperature, overlaying the states of the [Bi2X10]4− units and the two Dim2+ cations. They show that the occupied orbitals of the Dim2+ units possess an energy (less than −8 eV) lower than the valence band of the [Bi2X10]4− anions, which expands from approximately −7 to −3 eV. More interestingly, it can also be observed that the valence bands of the [Bi2X10]4− units overlap with the conduction band of the Dim2+ cations, resulting in HOMO−LUMO gaps of only ∼0.5 eV. This overlap can favor a thermal excitation of the electrons from the valence band of halide p orbitals to the conduction band of Dim2+ cations. Besides, this charge transfer may be thermally induced so that the populations of electrons in the conduction band of the Dim2+ cations increase upon heating. This allows us to rationalize the observed decreases in the band gaps in our iodide and bromide compounds with an increase in temperature. This assumption is also supported by the observation of relatively short distances [3.326(5)− 3.584(7) Å] between the rings of the Dim2+ cations and halogen atoms of the [Bi2X10]4− units (see Figure S3). In this context, we have to note that the charge transfer between the halide and organic cations has been previously reported to explain the photochromism in related compounds, such as bismuthate halides with various viologen cations.19,27 Accordingly, the Dim2+ cations play a structural role in balancing the charges and stabilizing the crystal structure, and very importantly, they are key to the thermochromic response of these halobismuthates.

with emission in the visible range. Among the three halides, we have to highlight the optoelectronic properties of [Dim]2[Bi2I10], with an optical band gap of 1.9 eV, broad photoluminescence emission at 615 nm, and a relatively long lifetime of 190 ns. Very interestingly, iodide and bromide compounds exhibit a reversible solid state thermochromism. The iodobismuthate changes gradually from dark red at 393 K to orange at 153 K, while the bromobismuthate changes from bright yellow at 393 K to uncolored at 153 K. To the best of our knowledge, the [Dim]2[BiBr10] compound reported here is the first example of a thermochromic bromobismuthate. With respect to the origin of the observed thermochromism, crystal structure analysis and DFT calculations point to a mechanism different from those previously reported for similar halobismuthates (a phase transition and/or thermal expansion). In the new [Dim]2[Bi2X10] compounds, the overlap between the valence band of the [Bi2X10]4− dimers and the empty conduction band of the Dim2+ cations allows an easy increase in the population of these conduction band with temperature. Therefore, diimidazolium cations play an important structural role in balancing the charges and stabilizing the crystal structure in those compounds, while favoring their thermochromic behavior. In summary, the [Dim]2[Bi2X10] (X = Cl−, Br−, or I−) compounds constitute a new family of versatile optical materials, with interesting absorption, emission, and thermochromic properties, where the optical band gap can be tuned by changing the halide and temperature.

CONCLUSIONS We have synthesized a novel hybrid organic−inorganic family of diimidazolium polyhalobismuthates, [Dim]2[Bi2X10] (X = Cl−, Br−, or I−), by means of a mild solution chemistry method performed at room temperature, leading to the easy formation of single-phase materials. The crystal structures of this new family are based on halobismuthate dimers [Bi 2 X 10 ] 4− sandwiched between two diimidazolium cations. This is the first example of halobismuthates in which the Dim2+ cations are used as countercations (and not as ligands) to obtain organic− inorganic hybrid compounds. All three halides are semiconductors, exhibiting optical band gaps of 3.2, 2.7, and 1.9 eV (for chloride, bromide, and iodide, respectively), comparable to the values found for other halobismuthates that were investigated as potential solar absorbers. The experimental values match those calculated by DFT for the [Bi2X10]4− units. All the [Dim]2[Bi2X10] halides exhibit absorption bands in the visible region of the spectrum, associated with charge-transfer Bi(6p) ← X(np) transitions, where n = 3, 4, or 5 for X = Cl−, Br−, or I−, respectively. Additionally, all these compounds display photoluminescence

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00629. PXRD patterns of polycrystalline powder samples, crystal structure details and additional tables, time-resolved photoluminescence decay at room temperature for bromide and chloride compounds, single-crystal thermochromic pictures, and additional DOS plots obtained via DFT (PDF)

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CCDC 1824881−1824884 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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Alberto García-Fernández: 0000-0003-1671-9979 Carlos Platas-Iglesias: 0000-0002-6989-9654 Socorro Castro-García: 0000-0003-1501-2410 Alberto Fernández: 0000-0003-2504-6016 Manuel Sánchez-Andújar: 0000-0002-3441-0994 Notes

The authors declare no competing financial interest. G


Article

Inorganic Chemistry

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Interactions and Size Effect of Inorganic Oligomer. Inorg. Chem. 2014, 53, 5538−5545. (18) Lin, R.; Xu, G.; Wang, M.; Lu, G.; Li, P.; Guo, G. Improved Photochromic Properties on Viologen-Based Inorganic − Organic Hybrids by Using π - Conjugated Substituents as Electron donors and stabilizers. Inorg. Chem. 2013, 52, 1199−1205. (19) Xu, G.; Guo, G. C.; Wang, M. S.; Zhang, Z. J.; Chen, W. T.; Huang, J. S. Photochromism of a Methyl Viologen bismuth(III) Chloride: Structural Variation before and after UV. Angew. Chem., Int. Ed. 2007, 46, 3249−3251. (20) Leblanc, N.; Allain, M.; Mercier, N.; Sanguinet, L. Stable Photoinduced Separated Charge State in Viologen Halometallates: Some Key Parameters. Cryst. Growth Des. 2011, 11, 2064−2069. (21) Pandey, S.; Andrews, A. P.; Venugopal, A. Manifestation of Helicity in One-Dimensional Iodobismuthate. Dalt. Trans. 2016, 45, 8705−8707. (22) Goforth, A. M.; Tershansy, M. A.; Smith, M. D.; Peterson, L.; Kelley, J. G.; Debenedetti, W. J. I.; zur Loye, H.-C. Structural Diversity and Thermochromic Properties of Iodobismuthate Materials Containing D-Metal Coordination Cations: Observation of a High Symmetry [ Bi3I11]2-Anion and of Isolated I - Anions. J. Am. Chem. Soc. 2011, 133, 603−612. (23) Adonin, S. A.; Sokolov, M. N.; Abramov, P. A.; Kozlova, S. G.; Pishchur, D. P.; Sheludyakova, L. A.; Fedin, V. P. Inorganica Chimica Acta Thermochromic Behavior and Phase Transition of New Octanuclear Polyiodobismuth (III) Ate. Inorganica Chim. Inorg. Chim. Acta 2014, 419, 19−25. (24) Li, G.; Hao, P.; Shen, J.; Yu, T.; Li, H.; Fu, Y. Bipyridyltriazolium Chlorobismuthate with Thermo-/Photochromic and Photoluminescent Switching Behaviors Based on ET and CT. Inorg. Chem. 2016, 55, 11342−11347. (25) Leblanc, N.; Mercier, N.; Zorina, L.; Simonov, S.; AubanSenzier, P.; Pasquier, C. Large Spontaneous Polarization and Clear Hysteresis Loop of a Room-Temperature Hybrid Ferroelectric Based on Mixed-Halide [BiI3Cl2] Polar Chains and Methylviologen Dication. J. Am. Chem. Soc. 2011, 133, 14924−14927. (26) Liu, Y.; Hu, C.; Comotti, A.; Ward, M. D. Supramolecular Archimedean Cages Assembled with 72 Hydrogen Bonds. Science 2011, 333, 436−440. (27) Mercier, N. The Templating Effect and Photochemistry of Viologens in Halometalate Hybrid Crystals. Eur. J. Inorg. Chem. 2013, 2013, 19−31. (28) Zhu, Y.; Yu, T.; Hao, P.; Shen, J.; Fu, Y. Halogen-Dependent Thermochromic Properties in Three Methyl-Viologen/Haloargentate Charge Transfer (CT) Salts. J. Cluster Sci. 2016, 27, 1283−1291. (29) Bi, W.; Leblanc, N.; Mercier, N.; Auban-Senzier, P.; Pasquier, C. Thermally Induced Bi(III) Lone Pair Stereoactivity: Ferroelectric Phase Transition and Semiconducting Properties of (MV)BiBr5 (MV = Methylviologen). Chem. Mater. 2009, 21, 4099−4101. (30) Ezugwu, C. I.; Kabir, N. A.; Yusubov, M.; Verpoort, F. Metal − Organic Frameworks Containing N-Heterocyclic Carbenes and Their Precursors. Coord. Chem. Rev. 2016, 307, 188−210. (31) Marcos, I.; Domarco, O.; Peinador, C.; Fenández, A.; Fernández, J. J.; Vázquez-García, D.; García, M. D. Self-Assembly of Dinuclear Pd(II)/Pt(II) Metallacyclic Receptors Incorporating NHeterocyclic Carbene Complexes as Corners. Dalt. Trans. 2017, 46, 4182−4190. (32) Naya, L.; Vázquez-García, D.; Fernández, A.; Lõpez-Torres, M.; Ojea, V.; Marcos, I.; Vila, J. M.; Fernández, J. J. Preparation of Imidazol-2-Ylidene Carbene Palladacycles with Bi- and Tridentate Schiff Bases - Analyses of the Spectroscopic, Molecular Structure, and DFT Calculation Data. Eur. J. Inorg. Chem. 2016, 2016, 422−431. (33) Leclercq, L.; Schmitzer, A. R. Influence of the Flexibility of the Diimidazolium Cations on Their Organization into Crystalline Materials. Cryst. Growth Des. 2011, 11, 3828−3836. (34) Chen, W. Synthesis and Characterization of a Novel CadmiumBiimidazole Compound With a 1-D Chain-Like Motif Synthesis and Characterization of a Novel Cadmium- Biimidazole Compound With a

ACKNOWLEDGMENTS The authors acknowledge financial support of Ministerio de Economiá y Competitividad (MINECO) and EU-FEDER (ENE2014-56237-C4-4-R) and Xunta de Galicia (GRC2014/ 042). C.-P.-I. thanks Centro de Supercomputación de Galicia (CESGA) for providing the computer facilities. I. M. thanks fundación Segundo Gil Davila for financial support.

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