Amide-Templated Iodoplumbates: Extending Lead-Iodide Based

Article pubs.acs.org/crystal

Amide-Templated Iodoplumbates: Extending Lead-Iodide Based Hybrid Semiconductors Sagi Eppel,*,† Natalia Fridman,‡ and Gitti Frey*,† †

Department of Materials Science and Engineering and ‡Schulich Faculty of Chemistry, Technion − Israel Institute of Technology, Haifa 32000, Israel S Supporting Information *

ABSTRACT: Lead iodide−organic hybrids (iodoplumbates) have emerged as a class of materials with promising electronic and optical properties, and potential applications in photovoltaics and electronic devices. Hybrid iodoplumbates are composed of organic cations and lead iodide anions that exhibit diverse morphologies which determine the optical and electronic properties of the crystal. However, the diversity of the iodoplumbates is limited by the types of organic cations amenable for integration into the structure. Amides represent one of the largest groups of organic molecules, yet no examples of iodoplumbates based on protonated amide cations have been demonstrated so far. In this work, we show that it is possible to consistently grow iodoplumbates from amides following two distinct pathways. The first pathway involves growing iodoplumbates using amidium (protonated amides) as the organic cation in the crystal, which occurs for tertiary amides and urea. The second pathway involves growing iodoplumbates from primary and secondary amides, resulting in crystals containing the ammonium hydrolysis product of the amide. This path also leads to an interesting case of ring opening crystallization. The lead iodide one-dimensional chain motif composes most of the resulting structures. The large number of available amide molecules suggests that this method considerably expands the range of possible iodoplumbate structures.

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INTRODUCTION Lead(II) iodide−organic hybrid materials (iodoplumbates) have been extensively explored owing to their diverse range of structural motifs which induce a variety of optical and electronic properties. Iodoplumbate optical properties include photoluminescence,1,2 photochromic,3,4 and nonlinear optics,5−9 while the electronic features include semiconductivity4,5,10−12 and dielectric13 behaviors. These properties make iodoplumbate materials promising candidates for various applications including solar cell,14−16 light emitting diodes,17 and dielectric media.13 The optical and electrical properties of iodoplumbates are mainly determined by the topology of the inorganic lead(II) iodide component.12 The overall crystal structure of iodoplumbates is determined by the interplay between the negatively charged lead(II) iodide octahedral complex and the positively charged organic cations.4,12 The geometry and topology of the lead(II) iodide component are, therefore, controlled by the type of organic cation used to form the crystals. Namely, the organic cation acts as a template, which controls the topology, dimensionality, and geometry of the inorganic lead-iodide complex and by this controls the optical and electronic properties of these materials. It is, therefore, possible to control and tune the material properties by changing the type of organic cations used for the crystallization.4,12 It has been demonstrated that depending © XXXX American Chemical Society

on the type of organic cation, the topology of the lead(II) iodide complex can obtain a range of structural motifs with various dimensionalities including one-dimensional (1D) chains, two-dimensional (2D) layers, and three-dimensional (3D) perovskite networks.1,3−5,12,15,18,19 To date, the organic cations that have been used for such crystals were based mainly on ammonium and amidinium cations.12,14 However, the limited set of organic groups that can be used as cations for creating iodoplumbates poses a major limitation to the diversity of both the structure of iodoplumbates and their preparation modes. In this study we demonstrate a significant extension of the iodoplumbates family by offering an alternative group of organic cations that can be used in iodoplumbates: amides. Being one of the largest families of organic molecules, amides can enable the formation of a large number of new iodoplumbate crystals. Creating iodoplumbate crystals based on protonated amides is not trivial, giving the very low basicity of amides, which often have negative pKa values (for amideconjugated acids).20 As result of their low basicity amides rarely crystallized in protonated form. According to a recent survey, only 81 crystal structures, which represent 0.15% of amideReceived: May 11, 2015 Revised: July 28, 2015

A

DOI: 10.1021/acs.cgd.5b00655 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Hydrolysis of amides under acidic conditions and the two suggested crystallization pathways. Pathway 1: The protonated amide (amidium) crystallizes with lead iodide. Pathway 2: The ammonium hydrolysis product crystallizes with the lead iodide.

Table 1. Crystallographic, Optical and Thermal Data for Structures 1−6 1

2

3

4

5

6

C3.1I1.6N0.5O0.8H7.7Pb0.5 368.06 r.t. orthorhombic Fdd2 21.1940(2) 40.1610(6) 8.0230(2) 90.00 90.00 90.00 6829.0(14) 31 15.285 4665.3 1360 1360 [0.0000]

DMF hydrolysed to dimethylamine C2NH8Pb1I3 634.00 r.t. orthorhombic Cmc21 8.7560(8) 15.1670(1) 8.1830(4) 90.00 90.00 90.00 1086.72(1) 4 23.977 1024.0 736 736 [0.0000]

caprolactam hydrolysed to aminohexanoic acid C6I3H14NO2Pb 705.96 −73(2) monoclinic C2 22.299(5) 7.964(5) 8.8500(17) 90.00 113.377(10) 90.00 1442.7(9) 4 18.090 1200.0 1138 1138 [ 0.0560]

146 1.138 0.0360 0.0911 0.0468, 0.1055 needle yellow 2.70

71 1.033 0.0374 0.1017 0.0402, 0.1035 needle yellow 2.85

31 1.031 0.0616 0.1542 0.0660, 0.1591 hexagonal yellow 2.76

137 1.093 0.0203 0.0475 0.0205, 0.0476 block/plate yellow 2.83

r.t, 123

r.t, 125

230,310

200

organic component

urea

NMP

dimethylacetamide

diethylacetamide

formula M temperature (°C) crystal system space group a/Å b/Å c/Å α/° β/° γ/° volume/Å3 Z μ/mm−1 F(000) reflections collected independent reflections [Rint] parameters goodness-of-fit on F2 R1 [I ≥ 2σ (I)] wR2 [I ≥ 2σ (I)] R1, wR2 [all data] crystal habit color band gap (eV)

C2I3N4H9O2Pb 709.03 −73(2) hexaclinic P1̅ 4.6370(2) 11.1120(9) 13.4600(10) 71.462(3) 80.868(5) 84.288(5) 648.33(8) 2 20.131 594.0 1880 1880 [0.0980]

C30I9N6O6Pb3H57 2361.55 r.t. orthorhombic P212121 11.553(6) 19.825(3) 25.184(4) 90.00 90.00 90.00 5768.1(3) 4 13.589 3918.3 8042 5639 [0.0000]

C8H18I3N2O2Pb 762.13 −73(2) monoclinic P21/c 15.9250(3) 15.3020(9) 7.8990(9) 90.00 104.071(3) 90.00 1867.1(2) 4 13.989 1348.0 2772 2772 [0.0910]

110 1.093 0.0593 0.1543 0.0608, 0.1564 needle yellow 2.78 (phase B: 2.83)a 150, 260

276 1.051 0.0759 0.1901 0.0826, 0.1954 needle yellow 2.86 125

decomposition temperature °C (TGA) a

Structure 1 phase B is discussed in the TGA section. and urea were purchased from Merck. Hydriodic acid 55%−58% was purchased from Alfa Aesar. N,N-Diethylacetamide A.R was purchased from Kodak. N,N-Dimethylformamide was purchased from Frutarom Ltd. ε-Caprolactam, lead iodide, N,N-dimethylacetamide, and Nmethylpyrrolidinone were purchased from Sigma-Aldrich. A Cary 5000 UV−Vis-NIR spectrophotometer (Agilent Technologies) with a DRA2500 integrating sphere attachment was used in diffuse reflectance mode over a range of 250−1000 nm for optical band gap measurements. A Varian Cary Eclipse spectrofluorometer was used to measure the photoluminescence spectra, with excitation wavelength 380 nm. Q 5000 IR TGA device was used from R.T. up to 500 °C with heating rate of 10 °C/min under medical air. Elemental analysis (CHNS) was done using a Thermo Scientific CHNS analyzer (Flash2000). 1H NMR (200 MHz) spectra were recorded on Bruker AVANCE-200 spectrometers. X-ray crystallographic measurements. Xray data was collected using a CCD diffractometer at room temperature. Data collection was performed using monochromatized

containing structures in the Cambridge Structural Database (CSD), contained protonated amides.21 In this work, we show that iodoplumbate crystals containing protonated amide cations can easily be grown when using tertiary amides and urea. On the other hand, growing iodoplumbates from secondary and primary amides can lead to iodoplumbate crystals containing the ammonium product of the amide hydrolysis reaction (Figure 1). This is an interesting case of reaction-driven crystallization which, for cyclic amides, leads to a unique ring opening crystallization.

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EXPERIMENTAL SECTION

Materials and Instrumentation. All materials were purchased in purity of 99% or more and used without further purification. All experiments were performed under air. Dimethylammonium chrloride B



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hydriodic acid (1 mL; 55% by weight in water). Caprolactam (0.8 g, 71 mmol) was added, and the solution was stirred for 2 h under mild heating (60 °C). The solution was left in an open vessel at room temperature for a few days, after which large yellow plate-like crystals appeared. For measurement of optical absorbance, photoluminescence and elemental analysis and mass spectroscopy, the crystals were washed with diethyl ether and dried under a vacuum. Mass spectrometry in DMF/THF (1:1): Found m/z: 132.2 (100%). Calc. for protonated aminohexanoic acid (C6N1O2H14+), m/z: 132.2 (Figure S1 in Supporting Information). Elemental analysis (CHNS). Found: C, 10.0%; N, 1.8%; H, 1.8%. Calc. for PbI3C6O2N1H14: C, 10.0%; N, 1.9%; H, 1.9%. Hydrolysis and Crystallization of Primary Amides to Ammonium [(NH4+)(H2O)2PbI3−]n. The procedure is given for acetamide, but similar procedures for other primary amides followed the same path and led to the same structure. PbI2 (50 mg, 0.11 mmol) was completely dissolved in hydriodic acid (0.5 mL; 55% by weight in water). Acetamide (0.1 g) was added and the solution was stirred for 10 min under mild heating (50 °C). The solution was left in an open vessel at room temperature for a few days, after which large yellow needle-like crystals appeared. For measurement of optical absorbance photoluminescence and elemental analysis, the crystals were washed with chloroform. The X-ray measurement showed cell parameters identical to the previously reported structure of NH4(H2O)Pb1I3.28 Elemental analysis (CHNS). Found: C, 0.09%; N, 2.3%; H, 0.9%. Calc. for NH4(H2O)Pb1I3: C, 0%; N, 2.2%; H, 1.2%.

Mo Kalpha radiation using omega scans and phi scans to cover the Ewald sphere.22 Accurate cell parameters were obtained with the amount of indicated reflections (Table 1).23 The structure was solved by SHELXS97 direct methods24 and refined by SHELXL97 program package25 and Olex2 version 1.2.6.26 The atoms were refined anisotropically. Mercury Software was used for molecular graphics.27 Data are presented in Table 1. Experimental Procedures. Structure 1: Urea Lead-Iodide [(CON2H4)2H+PbI3−]n. PbI2 (0.2 g, 0.44 mmol) and urea (2.0 g 33.3 mmol) were dissolved in hydriodic acid (1 mL; 55−58% by weight in water). The solution was stirred at 60 °C for several minutes until all solids completely dissolved. The solution was left for a few days in an open vessel at room temperature, after which needle-shaped crystals appeared. For measurement of optical absorbance, photoluminescence, and elemental analysis, the crystals were washed with chloroform and diethyl ether and dried under a vacuum for 30 min. Elemental analysis (CHNS). Found: C, 3.5%; N, 7.7%; H, 1.3%. Calc. for C2N4O2H9Pb1I3: C, 3.4%; N, 7.9%; H, 1.3%. Structure 2: N-Methyl-2-pyrrolidone Lead-Iodide [(C5ONH9)2H+PbI3−]n. PbI2 (0.6 g, 1.3 mmol) was dissolved in hydriodic acid (1.65 mL; 55−58% by weight in water). N-Methyl-2pyrrolidone (7 mL, 72.6 mmol) was added to the solution. The solution was stirred at about 50 °C for several minutes until all solids completely dissolved. After a few days in an open vessel at room temperature, large yellow needle-shaped crystals appeared. For measurement of optical absorbance photoluminescence and elemental analysis, the crystals were washed with diethyl ether and dried under vacuum for 30 min. Elemental analysis (CHNS). Found: C, 15.4%; N, 3.5%; H, 2.4%. Calc. for C10N2O2H19Pb1I3: C, 15.2%; N, 3.5%; H, 2.4%. Structure 3: N,N-Dimethylacetamide Lead(II) Iodide [(C4ONH9)2H+PbI3−]n. PbI2 (0.1 g, 0.22 mmol) was dissolved in hydriodic acid (0.5 mL; 55−58% by weight in water). N,NDimethylacetamide (3.3 mL, 35.5 mmol) was added to the solution. The solution was stirred at about 55 °C for several minutes until all solids completely dissolved. After a few days in an open vessel at room temperature, large yellow needle-shaped crystals appeared. For measurement of optical absorbance photoluminescence and elemental analysis, the crystals were washed with chloroform and dried under a vacuum for 30 min. After a few days outside the solution the crystals turned dark. Elemental analysis (CHNS) (of the yellow crystals). Found: C, 12.2%; N, 3.5%; H, 2.3%. Calc. for C8O2N2H19Pb1I3: C, 12.5%; N, 3.6%; H, 2.5%. Structure 4: N,N-Diethylacetamide Lead(II) Iodide Hydrate [(C6ONH14+)(H2O)PbI3−]n. PbI2 (1 g, 2.2 mmol) was dissolved in hydriodic acid (8.4 mL; 55−58% by weight in water). N,NDiethylacetamide (3 mL, 24.1 mmol) was added to the solution. The solution was stirred at about 50 °C for several minutes until it turned completely clear. The solution was left for a few days in an open vessel at room temperature, after which large yellow needle-like crystals appeared. For measurement of optical absorbance photoluminescence and elemental analysis, the crystals were first washed with chloroform and dried under a vacuum. After a few days outside the solution the crystals turn black. Elemental analysis (CHNS) (of the yellow crystals). Found: C, 10.1%; N, 1.9%; H, 2.1%.: Calc. for C6N1O1.5Pb1I3: C, 10.0%; N, 1.9%; H, 2.1%. Structure 5: DMF/Dimethylamine Lead(II) Iodide [(NC2H8+)PbI3−]n. PbI2 (0.4 g, 0.87 mmol) was dissolved in hydriodic acid (1.1 mL; 55−58% by weight in water). Dimethylformamide (DMF; 3 mL, 39 mmol) was added, and the solution was stirred for 30 min under mild heating (60 °C). The solution was left in an open vessel at room temperature for a few days, after which large hexagonal crystals appeared. The crystals were filtered and washed with chloroform and diethyl ether. 1H NMR: δH (200 MHz; D2O/DCl; structure 5): δ = 1.34 (m, 6H, Me), 6.3−5.8(t, 2H,NH2). 1H NMR reference: δH (200 MHz; D2O/DCl; (CH3)2NH2+Cl−): δ = 1.34 (m, 6H, Me), 6.3−5.8 (t, 2H,NH2). Elemental analysis (CHNS). Found: C, 3.8%; N, 2.2%; H, 1.3%. Calc. for N1C2H8PbI3: C, 3.8%; N, 2.2%; H, 1.3%. Structure 6: Caprolactam/Aminohexanoic Acid Lead(II) Iodide [(NC6O2H8+)PbI3−]n. PbI2 (0.1 g, 0.22 mmol) was dissolved in

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RESULTS AND DISCUSSION Growing iodoplumbates crystals is often achieved by mixing PbI2 and an amine in a solution of concentrated hydriodic acid, followed by either cooling or evaporation of the solvent. Such conditions are both strongly acidic and hydrolytic (due the water content of the hydriodic acid).29−32 When amides are exposed to such conditions they can either undergo protonation to form the amidium cation, or they can completely hydrolyze to form the ammonium cation (Figure 1). The mechanism for amide hydrolysis under acidic conditions (Figure 1) suggests the formation of an initial amidium intermediate, followed by a nucleophilic attack of water molecules to form the ammonium cation.29−31 However, amides are very weak bases, owing to the presence of the CO bond (pKa < 0 of conjugated acid).20 As such, amides rarely appear in their protonated form because once formed the intermediate is instantly hydrolyzed or deprotonated. According to a survey from 2013, only 81 structures, which represent approximately 0.15% of the structures containing amides in the CSD, contained protonated amides.21 A survey performed by us showed that there were no examples of haloplumbates (PbXn, X = Cl, Br, I) containing protonated amides reported in the CSD. The crystallization conditions used in this study involve a solution of hydriodic acid, an amide, and PbI2. Given the very low basicity of iodide (pKa ≈ −10) and water (pKa ≈ −2), the amides (−1 < pKa ≈ < 1) are the strongest base in the solution.20 As a result, the protonated amide might take the role of the cation in the crystallization of iodoplumbates. Whether the amidium intermediate will crystallize with the PbI3− or hydrolyze to form the ammonium product depends on its stability. We speculate that when the protonated amide in the intermediate state is stabilized, for example, by electron donating groups or steric effects,30,31 the amidium intermediate could crystallize with the PbI3− ions, as suggested by pathway 1 in Figure 1. On the other hand, when the intermediate state is not stabilized the hydrolysis is fast and the ammonium product will crystallize with the PbI3− ions, as indicated by pathway 2 in Figure 1. C



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Protonated Amide Dimer Motif. In three out of the four structures containing the protonated amide (structures 1−3, Figure 3a−c), the protonated amide cations are stabilized in the crystal by sharing protons between two amides in a dimer hydrogen-bond motif as shown in Figure 4. This motif is common for crystals of protonated tertiary amides.21 Owing to the heavy-atom effect common for iodoplumbates, the hydrogen atoms could not be identified in the X-ray diffraction pattern. However, the arrangement of the amides in the crystals is unambiguously that of a protonated amide dimer (Figure 4), with an O···O distance of 2.43−2.45 Å, characteristic of such hydrogen bonds (Figure 4).21 The molar ratio of two amides to one PbI3− fragment implies one proton (positive charge) for every pair of amides, which further supports this structure assignment. The only case in which the dimer motif does not appear is structure 4 (Figure 3d). In this case, the amide to PbI3− ratio is 1:1, implying one proton/charge per amide group. The protonated amide in structure 4 is stabilized by interactions with several negatively charged iodides and the electron-rich oxygen of the water molecule, as shown in Figure 5. The iodides and the oxygen form an electron-rich, negatively charged pocket surrounding the positively charged amidium cation. The location of the proton cannot be accurately determined due to the heavy atom effect. However, on the basis of known structures of protonated amides, the proton is positioned in proximity to the amide oxygen in the center of the “negative” cavity.21,30,31,33,34

IODOPLUMBATES BASED ON PROTONATED AMIDES The hydrolysis rates of amides are strongly affected by both steric hindrance and electron-repelling groups and are much slower for amides surrounded by carbon groups than primary amides.30,31 Therefore, to check the feasibility of the intermediate amidium cation to crystallize with PbI3− we select urea and ternary amides, as shown in Figure 2. Although urea

Figure 2. Amides used in this study for growing iodoplumbates based on amidium ions.

does not experience steric hindrance, it also has a low hydrolysis rate due to the relative stability of the protonated state.33,34 Tertiary amides are mostly liquid in room temperature, and hence the crystallization experiment was performed using excess of the amide as the main solvent. In the case of urea, solid urea was dissolved in excess in small amounts of hydriodic acid, and the mixture was used as the solvent for the crystallization. All crystallization experiments resulted in large needle-like crystals after periods ranging from a few hours to a few days. The resulting crystal structures are shown in Figure 3, and their crystallographic data summarized in Table 1.

Figure 3. Structures of iodoplumbates based on protonated amides: (a) structure 1 based on protonated urea; (b) structure 2 based on protonated N-methyl-2-pyrrolidone; (c) structure 3 based on protonated dimethylacetamide; (d) structure 4 based on protonated diethylacetamide. The crystallographic data for these structures are listed in Table 1. D



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Figure 4. Hydrogen motif of protonated amide dimer motif for structures 1 (d), 2 (c), and 3 (b).

Figure 5. Protonated amide in structure 4 stabilized in a negatively charged cavity. The distances between the oxygen of the amidium to the neighboring iodides and water oxygen are given in angstroms.

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dimethylammonium cations (Figure 6b),32 which crystallize in the new structure shown in Figure 7a. Three different X-ray measurements of this structure all gave solutions with low R factors (6.1%, 3.55%, and 8.1%) and the clear lead iodide rod structure shown in Figure 7a. However, the exact structure of the organic fragment could not be accurately determined in any of these three structures. This suggests some level of disorder in the orientation (but not in the location) of the dimethylammonium in the crystal. The identity of the dimethylammonium in these crystals was established by dissolving the crystals in D2O and DCl and comparing the 1H NMR to that of commercial dimethylammonium (see Supporting Information Figures S2−S5) and by elemental analysis of the crystal. Ring-Opening Crystallization of Caprolactam. A specifically interesting case is the growth of iodoplumbates from the hydrolysis of caprolactam, which led to a new type of dynamic ring-opening crystallization (Figure 6c). The acidic conditions of the crystallization experiment enable three equilibrium states for the organic molecule: ring (caprolactam), polymer (nylon), and chain (1-aminohexanoic acid), as shown in Figure 6c.35 The resulting crystals contained the protonated 1-aminohexanoic acid (the chain) as the organic cation, as shown in Figure 7b (structure 6). This new crystal structure is held together by the combination of the lead iodide 1D chain motif (Figure 7b) and hydrogen bonded sheets of the organic cations (Figure 8). Notably, some of the crystallization experiments of this case have led to the gelation of the solution prior to the formation of crystals, probably due to the formation of nylon polymer.

IODOPLUMBATES FROM AMIDE HYDROLYSIS The crystallization of iodoplumbates from primary amides as well as from DMF and caprolactam (Figure 6) have led to the

Figure 6. Hydrolysis pathways for the formation of iodoplumbates from amides.

formation of crystals containing the ammonium hydrolysis product of the amide as the organic cation, as shown in Figure 6.29−32 For primary amides (CONH2), the hydrolysis product formed under acidic conditions is the ammonium ion (NH4+; Figure 6a), which crystallizes in the already-known iodoplumbate hydrate structure.28 For DMF, the hydrolysis products are E



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Figure 7. Iodoplumbate structures based on the hydrolysis pathway of the amides of (a) DMF, structure 5; and (b) caprolactam, structure 6. The crystallographic data for these structures is listed in Table 1.

formation could be related to the symmetry elements of the crystal (Table 2, Figure 10).38,39 Indeed, in four out of five of Table 2. Symmetry and Abundance of the 1D Lead-Iodide Rod Motif (Figure 9b) alla

inversion (on Pb) glide plane on rod 3-fold rotation axis on rod 2-fold screw axis on rod 6-fold screw axis on rod 2-fold rotation axis normal to rod (on Pb) mirror plane normal to rod (on iodide) mirror plane on rod no symmetry structures containing the rod motif

Figure 8. Hydrogen-bond motif in structure 6 (aminohexanoic acid). The hydrogen bonds are marked as dashed lines.

The Lead-Iodide 1D Chain Motif and Its Symmetry. The crystals based on protonated urea (structure 1) showed a double-row lead(II) iodide inorganic rod structure shown in Figure 9a. Survey of the CSD show at least three occurrences of similar motifs.36,37 The inorganic lead iodide complex in all other crystals of protonated tertiary amides and hydrolyzed amides (structures 2−6) showed the 1D chain motif ([PbI3]∞, Figure 9b), which is common in iodoplumbates based on organic cations.5,11,28 This chain motif (Figure 9b) appears in 44 other iodoplumbate crystals in the CSD (CSD-2014) and is likely the most common lead-iodide motif for hybrid crystals. This motif has a high level of symmetry, which implies that its

solved hereb

# of structures

fraction (%)

# of structures

fraction (%)

19 18 8 7 5 3

41 39 17 15 11 7

0 2 0 4 0 0

0 40 0 80 0 0

2

4

0

0

2 3 49

4 7 100

1 0 5

20 0 100

a

All 44 structures containing this motif in the 2014 version of the Cambridge structural database (CSD) and the five structures (2−6) solved here. bStructures 2−6 solved here.

the structures solved here, this motif occupies a 2-fold screw axis (Figure 10c), and in two out of five cases it occupies a glide

Figure 9. (a) Lead-iodide double-rod motif (structure 1). (b) Lead-iodide rod motif (structures 2−6). F



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Figure 10. Symmetry positions of the [PbI3−]∞ 1D rod motif: (a) inversion center on Pb (yellow dot); (b) mirror or glide plane on the rod; (c) rotation or screw axis on the rod; (d) 2-fold rotation axis normal to the rod; and (e) mirror plane normal to rod. The abundance of each symmetry element in the motif appears in Table 2.

Figure 11. Optical properties of structures 1−6 (structure 1 phase B is discussed in the TGA section): (a) Absorption spectra (normalized); (b) photoluminescence for excitation wavelength of 380 nm (3.27 eV). All photoluminescence measurements was done under identical conditions (Ex. and Em. slits 10 and 20 nm wide, respectively).

motif.1,5,11,13,6,7,33,40 All structures show broad photoluminescence bands in the range of 1.9−2.9 eV for excitation at 3.27 eV (Figure 11b).41 These transitions are consistent with previous reports concerning similar structures and are attributed to excitons that are self-trapped6,7 due to the strong exciton-lattice interactions7,9,42 in the 1D lead iodide chain motif.40,43 In general, all structures have similar optical behavior associated with the lead iodide rod motif which acts as a 1D semiconductor wire surrounded by a dielectric media composed of the organic components.6 Such an arrangement induces strong exciton binding energy that can lead to photoluminescence and other nonlinear optical properties.40,42,43 The results reported here are in good agreement with earlier reports showing that the organic component has only a minor effect on the optical properties.44 Attempts to measure the quantum yield were unsuccessful due to low signals; however, the photoluminescence is still distinctively higher in comparison to that of PbI2 (Figure 11b).

plane (Figure 10b). Surveying all 44 crystals containing this motif in the CSD shows that this motif appears, almost conclusively, on some of the special symmetry positions of the crystal (Table 2). The most common symmetry positions for such a motif to occupy are (a) an inversion center on the lead atom (41%, Figure 10a), (b) a glide plane on the rod axis (39%, Figure 10b), or (c) a 3-fold rotation axis, again on the rod axis (17%, Figure 10c). Only in three structures (7%) does this motif not occupy a symmetry position. This strongly implies that the formation of the lead(II) iodide rod motif (Figure 9b) affects and is affected by the symmetry of the crystal.

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ABSORPTION SPECTRA AND PHOTOLUMINESCENCE The absorption spectra of all structures prepared in this study (Figure 11a) show a semiconductor-type behavior with band gaps in the range of 2.7−2.8 eV (Table 1). The similar values of band gaps implies little effect of the organic moiety on the optical properties.9,40 These band gaps are consistent with those previously reported for the [PbI3]∞ and correspond to the valence to conduction band transition of the lead iodide G



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THERMOGRAVIMETRIC ANALYSIS (TGA) Thermogravimetric analysis (TGA) results for all the structures discussed here are presented in Figure 12. The thermogram of

precursors for the simple preparation of functional hybrid iodoplumbates. As in the case of structure 1, the change of color from yellow to red after cooling is reversible even after several heating and cooling cycles. The fact that both of these structures (1 and 5) contain relatively small molecules (urea and dimethyl amine) might help explain their similar thermochromic behavior.

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CONCLUSION The current work demonstrates two new pathways for growing iodoplumbate crystals from amides. The first pathway is based on protonated amides as the organic cation in the crystals, while the second includes the ammonium product of amide hydrolysis as the organic cation. The latter is possible by performing the hydrolysis concurrently with the crystallization process and provides a simple and alternative method for growing large crystals of ammonium-based iodoplumbates. We conclude that amides with a relatively stable protonated state (due to steric effects and/or electron donating group) crystallize in their protonated form (first pathway). On the other hand, amides with an unstable protonated state and fast hydrolysis rates, crystallized in their (hydrolyzed) ammonium form. A specifically interesting case is that of the ring-opening crystallization of caprolactam, in which the crystallization appears to be a dynamic interplay between the three equilibrium processes of ring opening/closing, polymerization, and crystallization. All crystals show semiconductor behavior with a band gap of 2.7−2.8 eV and photoluminescence.

Figure 12. Thermogravimetric analysis (TGA) of structures 1−6.

structure 1, the iodoplumbate based on protonated urea, shows a phase transition starting at around 150 °C, which involves a mass loss equivalent to the release of half of the urea molecules in the crystal (leaving beyond one urea per proton). In the temperature range of this transition, the color of the material turns from yellow to red. However, unlike the mass loss, the color change is reversible (the material turns yellow when cooled). The red phase shows high thermal stability and decomposes at 260−300 °C, releasing all the remaining urea molecules and equivalent amount of iodides, to leave beyond PbI2. The urea crystal after the loss of half of the urea molecules and cooling is referred to as structure 1 Phase B. Further analysis of structure 1 phase B shows that it maintains some level of crystallinity (see XRD pattern in Figure S6 in the Supporting Information) but loses the single crystal structure of the original phase. Interestingly, the yellow-to-red color transformation is reversible during several heating and cooling cycles. The rest of the amide-based structures (2−4) show low thermal stability and completely decompose at around 120− 130 °C. This decomposition involves the release of all amide molecules and the equivalent amount of iodide anion, to leave beyond PbI2. The thermograms of structures 3 and 4 (iodoplumbates based on protonated dimethylacetamide and diethylacetamide, respectively) indicate that these structure start to decompose at room temperature after a few hours. The decomposition process involves a color change from yellow to black and a small mass loss. The PbI2 that results from the decomposition of structures 1−4 and 6 starts to melt at 400 °C (in agreement with literature values) following mass loss, due the release of the remaining iodides. In contrast to all other structures reported here, structure 5 (dimethylammonium) shows high thermal stability and reaches a temperature of ∼310 °C before any mass losses occur. However, there is a reversible color change from yellow to red at around 230 °C, which may imply a mass-conserving phase-transition; this color change is reversed upon cooling. A fast mass loss for this structure (5) occurs at 330−360 °C and corresponds to the loss of all organic and iodide fragments, leaving beyond lead. The resulting lead phase (Pb) shows no further mass loss (Figure 12). The only iodoplumbates, we know of, that show a similar decomposition pattern is the methylammonium PbI3 perovskite,15 and hence we believe that hybrid iodoplumbates prepared from amides, such as those prepared in this study, might be used as

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00655. Mass spectra and H1 NMR for structures 5−6 and commercial reference material, as well as XRD for structure 1 Phase B (PDF) Crystallographic information files (ZIP) Crystallographic information file for structure 1 (CIF) Crystallographic information file for structure 2 (CIF) Crystallographic information file for structure 3 (CIF) Crystallographic information file for structure 4 (CIF) Crystallographic information file for structure 5 (CIF) Crystallographic information file for structure 6 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

(S.E.) *E-mail: [email protected]. (G.F.) *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the Israeli Nanotechnology Focal Technology Area project on “Nanophotonics and Detection” and the Grand Technion Energy Program (GTEP), and comprises part of The Leona M. and Harry B. Helmsley Charitable Trust reports on Alternative Energy series of the Technion, Israel Institute of Technology, and the Weizmann Institute of Science.

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REFERENCES

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Amide-Templated Iodoplumbates: Extending Lead-Iodide Based

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