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Crystal structural and thermochromic luminescence properties modulation by ion liquid cations in bromoplumbate perovskites
Inorganic Chemistry Communications 112 (2020) 107690
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Crystal structural and thermochromic luminescence properties modulation by ion liquid cations in bromoplumbate perovskites
T
Shan-Shan Yu, Shi-Xiong Jiang, Hui Zhang, Hai-Bao Duan Key Laboratory of Advanced Functional Materials of Nanjing, Nanjing Xiaozhuang University, Nanjing 211171, PR China
GRAPHICAL ABSTRACT
Three compounds exhibits fascinating thermochromic luminescent properties. An obvious color change of 3. Three compounds have very low melting point.
ARTICLE INFO
ABSTRACT
Keywords: Bromoplumbates perovskites Melting point Thermochromic luminescent
The present work deals with three one-dimensional (1D) bromoplumbate perovskites, namely [C4-Mim][PbBr3] (1), [C5-Mim][PbBr3] (2) and [C6-Mim][PbBr3] (3) (where C4-Mim+ = 1-butyl-3-methylimidazolium, C5Mim+ = 1-pentyl-3-methylimidazolium, C6-Mim+ = 1-hexyl-3-methylimidazolium). 1D inorganic chain is built from corner-sharing PbBr6 octahedron in three compounds. 1 crystallizes in chiral space group P212121 at 100 K. 2 and 3 crystallizes in the same monoclinic crystal system P21/n at 100 K. Three compounds have low melting point compared to reported haloplumbates organic-inorganic perovskites, and the melting point of is 98 °C for 1, 102 °C for 2, and 113 °C for 3. Interestingly, three compounds show multi-band emission and exhibits fascinating thermochromic luminescent properties.
Haloplumbates organic-inorganic perovskites have been widely investigated by their tunable structures and a wide range of novel physical properties in semiconductor, photocatalysis, luminescence, nonlinear optics, thermochromism, and the others [1–7]. Three-
dimensional (3D) perovskites have been successfully employed in photovoltaic and light emitting devices [8,9], and some two-dimensional (2D) perovskites have also shown some interesting physical properties [10,11] For example, the 3D CH3NH3PbI3-xClx perovskites
E-mail address: [email protected] (H.-B. Duan). https://doi.org/10.1016/j.inoche.2019.107690 Received 2 October 2019; Received in revised form 13 November 2019; Accepted 19 November 2019 Available online 09 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 112 (2020) 107690
S.-S. Yu, et al.
Fig. 1. (a) Molecular structure of 1 at 100 K, where all H atoms are omitted for clarity (the symmetric codes: A = 1.5+x, 0.5−y, 1.5−z); (b) packing diagram viewed approximately along the a-axis; (c) cation conformation of 2 at 100 K and (d) cation conformation of 3 at 100 K.
(2) and [C6-Mim][PbBr3] (3) (where C4-Mim+ = 1-butyl-3-methylimidazolium, C5-Mim+ = 1-pentyl-3-methylimidazolium, C6Mim+ = 1-hexyl-3-methylimidazolium) were reported. The white needle-shaped hybrid crystals 1, 2 and 3 were obtained by slow evaporation of the dimethylformamide (DMF) solution containing equimolar PbBr2 and corresponding ion liquid at room temperature over 20 days. The phase purity of three compounds were confirmed by powder X-ray diffraction (Figs. S1–S3). Hybrid 1 crystallizes in chiral space group P212121 in 100 K; an asymmetric unit is composed of one Pb2+ ions and three different Br− ions together with one C4-Mim+ cations (Fig. 1a); all atoms occupy general positions. The Pb2+ ions is coordinated with six Br− ions forming distorted PbBr6 octahedron, the PbeBr bond lengths change from 2.906 Å to 3.166 Å. Each Br− ions link two PbBr6 octahedron forming an infinite one dimension (1D) chain by μ2 connection model. In 1, the hydrocarbon chain of C4-Mim+ cations all adopt trans conformation, and the neighbouring cations shows face to face packing. The weak CeH…Br interaction was occurred between the organic and inorganic parts. Along the [1 0 0] direction, each 1D bromoplumbate chain is surrounded by six C4-Mim+ cations columns (Fig. 1b). Hybrid 2 and 3 crystallizes in the same monoclinic crystal system P21/n at 100 K, which belong to non chiral space group and is different from 1. The asymmetric unit and packing structure of three compounds are similar. The distinct structure difference are in the cations conformation. (1) In hybrid 2, hydrocarbon chains adopt trans conformation (Fig. 1c), however, the hydrocarbon chain shows mixed trans and gauche conformation in 3 (Fig. 1d). The C4eC5 bond length of gauche
have being discovered to show amazing bipolar and bistable resistive switching behavior with small on-off voltage of < 1.0 V. Room temperature ferroelectric phase has been reported in 2D (BA)2(MA)(n−1)PbnX(3n+1)-type perovskites (where BA = C4H9NH3+; MA = CH3NH3+; X = Br or Cl) [12,13]. 2D perovskites (EDBE)PbI4 (EDBE = 2,2-(ethylenedioxy) shows broadband emission due to structural distortions of the inorganic lattice [14]. However, most of researches related with organic-inorganic perovskites are mainly focused on small size cation (such as Cs+, CH3NH3+). Recently, some one-dimensional (1D) and zero-dimensional (0D) haloplumbates perovskites have been obtained using larger organic cations. Low-dimensional haloplumbates perovskites exhibit much higher stability under high humidity compared to 3D haloplumbates perovskites [15,16]. Futhermore, a subclass of 1D perovskites has recently attracted attention for their potential application in broadband solid state lighting due to the presence of highly stokes shifted light emission. Recently, some white-light emission 1D hybrid perovskite from broadband lighting acrossing the entire visible spectrum have been reported [17–19]. 1D structure [C4N2H14][PbBr4] enabled strong quantum confinement with the formation of self-trapped excited states and gave efficient bluish white light emission with PLQE of 20% for bulk single crystals. In our previous study, large size organic cations were chosen for self-assembly 1D haloplumbates perovskites with broadband emission properties [20–22]. Here, imidazole-based ion liquid were used to synthesis 1D haloplumbates perovskites. The crystal structure and luminescence properties of three new compounds [C4-Mim][PbBr3] (1), [C5-Mim][PbBr3] 2
Inorganic Chemistry Communications 112 (2020) 107690
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Fig. 2. DSC curves for compounds (a) 1; (b) 2 and (c) 3 in the heating/cooling process.
conformation (1.462 Å) in 3 is shorter than trans conformation in 2 (1.515 Å); (2) The C7 atoms of alkyl chain shows structurally disorder with two site in 3 and is structurally order in 2. The 1D [PbBr3]− inorganic chain also consisted of corner-sharing PbBr6 octahedron in 2 and 3. However, the PbeBr bond length are different from 1. The longer PbeBr bond length is 3.310 Å in 2 and 3.312 Å in 3. Thus the degree of distortion of PbBr6 octahedron is different. In general, the octahedral distortion of PbX6 can be estimated by using the parameter of Δoct [Eq. (1)] [23,24] oct
=
1 6
second step weight loss related to the residual PbBr2 begins sublimation at approximately 550 °C and this volatilization procedure was completed at 700 °C. Differential scanning calorimetry (DSC) is performed on the polycrystalline samples of three compounds confirming the thermal properties in the temperature ranges of −50 to 160 °C. As shown in Fig. 2, one endothermic peaks appeared in the first heating process for each compound and peaks are located at ca. 98 °C for 1, 102 °C for 2, and 113 °C for 3, which correspond to the melting process of three compounds and have been proved by melting point measurement. In the following cooling process, supercooling behavior were observed for the three compounds. No exothermic peaks occurred when the sample were cooled to −50 °C confirming the formation of supercooled liquids in the cooling cycle (Fig. 2), such type of supercooling thermal behavior often appears in a liquid crystal and ion liquid system and is thought to be related to the release of structural strain [21]. Interesting, it can be proved that the melting point of hybrids can be decreased by introducing imidazole-based ion liquid as cations, and to the best of our knowledge, this is a very rare example for the haloplumbate-based organic-inorganic hybrid with very low melting point. Three compounds are colorless and transparent under ambient suggesting little absorption in the visible light region. Fig. S5 shows the optical absorption spectra for the three compounds at room temperature. Three compounds have similar absorption curves containing one strong sharp absorption at ca. 210 nm and a broad absorption band below 400 nm. Different from 1 and 3, a weak shoulder peak ca.
6
[(di i=1
dm)/ dm ]2
(1)
In Eq. (1), dm and di represent the mean PbeX bond length and the six individual PbeX bond lengths, respectively. According to Eq. (1), the obtained Δoct parameter is 0.731 × 10−3 for 1, 2.505 × 10−3 for 2, 2.34 × 10−3 for 3. It can be found that the PbBr6 octahedron for 2 and 3 shows much larger parameters of Δoct, which are related to the distortion of the PbeBr bond lengths compared with 1, indicating that heavy distortion occurs in the PbBr6 coordinated octahedron of 2 and 3. The thermal stability was evaluated for three compounds by TG analysis under nitrogen atmosphere and the corresponding TG curves are depicted in Fig. S4. Three compounds show similar thermal weight loss behavior, and two steps weight loss was observed from 30 to 800 °C. For 1 as example, 1 is thermally stable below 320 °C and first step decomposing above 320 °C related to loss of organic cation. The 3
Inorganic Chemistry Communications 112 (2020) 107690
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Fig. 3. (a) Solid state emission spectra of 1 in the temperature range of 10–300 K; (b) solid state emission spectra of 1 at 40 and 100 K; (c) temperature dependent of solid state emission spectra of 2; (d) solid state emission spectra of 3 at selected temperature showing the variation of the intensity of two emission bands and (e) CIE chromaticity diagram of 3 in the temperature range of 10–300 K.
394 nm was observed in 2. Compared with other haloplumbate organicinorganic hybrid, we can see that the peak positions of this work (1D inorganic chain) exhibit a blue shift compared with 3D inorganic chain, which is consistent with the rule that the energy gap decrease as the dimensionality increases [21]. Three compounds show tunable emission from the temperature dependent photoluminescence spectra (Fig. 3a, c and S6). The room temperature photoluminescence spectra of 1 show abroad emission band, ranging from 500 to 750 nm (Fig. S7).
For 1, with decreasing temperature, the intensities of the abroad bands enhance rapidly, indicating that the thermal activation of non-radiative relaxation processes is efficiently suppressed at low temperature. From 100 to 55 K, a weak shoulder band occurred centered at ca. 451 nm (high energy emission band), however, the peak intensity of 610 nm is nearly not changed and 666 nm is enhance slightly (Fig. S8). When the temperature further decreased to 40 K, the intensity of shoulder band centered at ca. 451 nm enhance rapidly, and the center position of the 4
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band at 611 nm shows a red shift to 625 nm at 10 K (Fig. 3b). The thermochromic luminescence properties of 1 can be seen from CIE (x, y) chromaticity diagram coordinates. As shown in Fig. S9, the plot of CIE coordinate versus temperature displays a non-monotonic change, and the corresponding luminescence color changes from orange at 300 K (CIE is (0.4532, 0.4738)) to yellowish white at 10 K (CIE is (0.355, 0.2891)). Similar with 1, the temperature dependent photoluminescence spectra of 2 also shows two emission bands (Fig. 3c). Different from 1, the high energy emission band was occurred at 275 K. With the temperature decreased, the intensities of the two emission bands enhance and no anomalies were found. The rang of color change of 2 is narrow compared with 1 and 3. The CIE coordinate of 2 is (0.4606, 0.4461) at 300 K, and (0.4622, 0.3331) at 300 K, which are all in yellow region (Fig. S10). For 3, room temperature photoluminescence spectra is similar with 1 and 2, yellow emission was observed with CIE coordinate is (0.473, 0.4503) at 300 K. When the temperature decreased, the high energy emission band was occurred at 125 K. However, when the temperature decreased to 55 K, the intensity of two emission bands show different tendency towards temperature. The intensity of the high energy emission band centered at 447 nm enhance rapidly and low energy emission band slightly decreased (Fig. 3d), which leads to a great change in the color of 3. As shown in Fig. 3e, 3 is blue and CIE coordinate is (0.2506, 0.1042) at 10 K. The vibrational relaxation of excited state electrons to the ground state via nonradiative internal conversion is one of main fluorescence quenching mechanisms, and this process is suppressed in the low temperature, and thus, the emission generally enhances upon cooling. Especially for the organic component, dynamic motion of the imidazole ring resulted in the excited state relaxing to the ground state through the nonradiative internal conversion is frozen at low temperature. However, the dynamic motion of the organic cations and rigidity inorganic component with two different emission bands show different sensitivity to temperature. The relative intensities between two emission bands change with temperature, which leads to the thermochromic luminescence properties of 1–3. Furthermore, the emission quantum yields of 1–3 were investigated. At room temperature 1–3 have low quantum yields of 3.4%, 5.2% and 2.8%, respectively. In conclusion, three bromoplumbate organic-inorganic perovskites were synthesized and structurally characterized. The general crystal structure characteristic in this family is the formation of 1D [PbBr3]− chain. The hydrocarbon chain length in countercation finely modulates the packing structure of the three compounds. Three compounds have very low melting point compared to reported haloplumbate-based organic-inorganic perovskites, which can be ascribed to introducing of imidazole-based ion liquid. Three compounds show fascinating thermochromic luminescence properties owing to the different emission bands showing different temperature responses. Interestingly, 3 have wide range of color change from blue to yellow with the temperature increased. This work inspire us to further search for bromoplumbate organic-inorganic compounds and investigate the relationship of the crystal structure and optical properties.
Acknowledgements The authors thanks Natural Science Foundation of JiangSu Province and Natural Science Training Foundation of Nanjing Xiaozhuang University for their financial support (grant No: BK20171125, 2016NXY12 and 2017NXY15). Appendix A. Supplementary material CCDC 1956678 for 2, 1956685 for 3 at 100 K contains the supplementary crystallographic data. The data can be obtained free of charge via < http://www.ccdc.cam.ac.uk/conts/retrieving.html > , or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:(+44) 1223-336-033; or e-mail: deposit@ ccdc.cam.ac.uk. Supplementary data to this article can be found online at https://doi.org/10.1016/j.inoche.2019.107690. References [1] E.R. Dohner, E.T. Hoke, H.I. Karuadasa, J. Am. Chem. Soc. 136 (2014) 1718. [2] C. Wehrenfenging, M. Liu, H.J. Snaith, M.B. Johnston, L.M. Herz, J. Phys. Chem. Lett. 5 (2014) 1300. [3] J.I. Fujisawa, T. Ishihara, Phys. Rev. B 70 (2004) 113203. [4] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Science 348 (2015) 1234. [5] M. Liu, M.B. Johnston, H.J. Snaith, Nature 501 (2013) 395. [6] J. Lee, H. Kim, N. Park, Acc. Chem. Res. 49 (2016) 311–319. [7] J.S. Manser, M.I. Saidaminov, J.A. Christians, O.M. Bakr, P.V. Kamat, Acc. Chem. Res. 49 (2016) 330. [8] H. Cho, S.H. Jeong, M.H. Park, Y.H. Kim, C. Wolf, C.L. Lee, J.H. Heo, A. Sadhanala, N. Myoung, S. Yoo, S.H. Im, R.H. Friend, T.W. Lee, Science 350 (2015) 1222. [9] Y.H. Kim, H. Cho, T.W. Lee, Proc. Natl. Acad. Sci. U. S. A. 113 (2016) 11694. [10] J. Byun, H. Cho, C. Wolf, M. Jang, A. Sadhanala, R.H. Friend, H. Yang, T.W. Lee, Adv. Mater. 28 (2016) 7515. [11] H. Tsai, W. Nie, J.C. Blancon, C.C. Stoumpos, R. Asadpour, B. Harutyunyan, A.J. Neukirch, R. Verduzco, J.J. Crochet, S. Tretiak, L. Pedesseau, J. Even, M.A. Alam, G. Gupta, J. Lou, P.M. Ajayan, M.J. Bedzyk, M.G. Kanatzidis, A.D. Mohite, Nature 536 (2016) 312. [12] W.Q. Liao, Y. Zhang, C.L. Hu, J.G. Mao, H.Y. Ye, P.F. Li, S.D. Huang, R.G. Xiong, Nat. Commun. 6 (2015) 7338. [13] L. Li, X. Liu, Y. Li, Z. Xu, Z. Wu, S. Han, K. Tao, M. Hong, J. Luo, Z. Sun, J. Am. Chem. Soc. 141 (2019) 2623. [14] D. Cortecchia, S. Neutzner, A. Ram, S. Kandada, E. Mosconi, D. Meggiolaro, F. De Angelis, C. Soci, A. Petrozza, J. Am. Chem. Soc. 139 (2017) 39. [15] C. Xue, Z.Y. Yao, J. Zhang, W.L. Liu, J.L. Liu, X.M. Ren, Chem. Commun. 54 (2018) 4321. [16] S. Wang, Y. Yao, J. Kong, S. Zhao, Z. Sun, Z. Wu, L. Li, J.H. Luo, Chem. Commun. 54 (2018) 4053. [17] Z. Yuan, C. Zhou, Y. Tian, Y. Shu, J. Messier, J.C. Wang, L.J. van de Burgt, K. Kountouriotis, Y. Xin, E. Holt, K. Schanze, R. Clark, T. Siegrist, B. Ma, Nat. Commun. 8 (2017) 14051. [18] Y. Peng, Y.P. Yao, L. Li, Z.Y. Wu, S. Wang, J.H. Luo, J. Mater. Chem. C 6 (2018) 6033. [19] H. Barkaoui, H. Abid, A. Yangui, S. Triki, K. Boukheddaden, Y. Abid, J. Phys. Chem. C 122 (2018) 24253. [20] H.B. Duan, S.S. Yu, Y.B. Tong, H. Zhou, X.M. Ren, Dalton Trans. 45 (2016) 4810. [21] H.B. Duan, H.R. Zhao, X.M. Ren, H. Zhou, Z.F. Tian, W.Q. Jin, Dalton Trans. 40 (2011) 1672. [22] H.B. Duan, S.S. Yu, S.X. Liu, H. Zhang, Dalton Trans. 46 (2017) 2220. [23] N.W. Thomas, Acta Crystallogr. Sect. B 45 (1989) 337. [24] J.A. Alonso, M.J. Martinez-Lope, M.T. Casais, M.T. Fernandez-Diaz, Inorg. Chem. 39 (2000) 917.
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Crystal structural and thermochromic luminescence properties modulation by ion liquid cations in bromoplumbate perovskites
T
Shan-Shan Yu, Shi-Xiong Jiang, Hui Zhang, Hai-Bao Duan Key Laboratory of Advanced Functional Materials of Nanjing, Nanjing Xiaozhuang University, Nanjing 211171, PR China
GRAPHICAL ABSTRACT
Three compounds exhibits fascinating thermochromic luminescent properties. An obvious color change of 3. Three compounds have very low melting point.
ARTICLE INFO
ABSTRACT
Keywords: Bromoplumbates perovskites Melting point Thermochromic luminescent
The present work deals with three one-dimensional (1D) bromoplumbate perovskites, namely [C4-Mim][PbBr3] (1), [C5-Mim][PbBr3] (2) and [C6-Mim][PbBr3] (3) (where C4-Mim+ = 1-butyl-3-methylimidazolium, C5Mim+ = 1-pentyl-3-methylimidazolium, C6-Mim+ = 1-hexyl-3-methylimidazolium). 1D inorganic chain is built from corner-sharing PbBr6 octahedron in three compounds. 1 crystallizes in chiral space group P212121 at 100 K. 2 and 3 crystallizes in the same monoclinic crystal system P21/n at 100 K. Three compounds have low melting point compared to reported haloplumbates organic-inorganic perovskites, and the melting point of is 98 °C for 1, 102 °C for 2, and 113 °C for 3. Interestingly, three compounds show multi-band emission and exhibits fascinating thermochromic luminescent properties.
Haloplumbates organic-inorganic perovskites have been widely investigated by their tunable structures and a wide range of novel physical properties in semiconductor, photocatalysis, luminescence, nonlinear optics, thermochromism, and the others [1–7]. Three-
dimensional (3D) perovskites have been successfully employed in photovoltaic and light emitting devices [8,9], and some two-dimensional (2D) perovskites have also shown some interesting physical properties [10,11] For example, the 3D CH3NH3PbI3-xClx perovskites
E-mail address: [email protected] (H.-B. Duan). https://doi.org/10.1016/j.inoche.2019.107690 Received 2 October 2019; Received in revised form 13 November 2019; Accepted 19 November 2019 Available online 09 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 112 (2020) 107690
S.-S. Yu, et al.
Fig. 1. (a) Molecular structure of 1 at 100 K, where all H atoms are omitted for clarity (the symmetric codes: A = 1.5+x, 0.5−y, 1.5−z); (b) packing diagram viewed approximately along the a-axis; (c) cation conformation of 2 at 100 K and (d) cation conformation of 3 at 100 K.
(2) and [C6-Mim][PbBr3] (3) (where C4-Mim+ = 1-butyl-3-methylimidazolium, C5-Mim+ = 1-pentyl-3-methylimidazolium, C6Mim+ = 1-hexyl-3-methylimidazolium) were reported. The white needle-shaped hybrid crystals 1, 2 and 3 were obtained by slow evaporation of the dimethylformamide (DMF) solution containing equimolar PbBr2 and corresponding ion liquid at room temperature over 20 days. The phase purity of three compounds were confirmed by powder X-ray diffraction (Figs. S1–S3). Hybrid 1 crystallizes in chiral space group P212121 in 100 K; an asymmetric unit is composed of one Pb2+ ions and three different Br− ions together with one C4-Mim+ cations (Fig. 1a); all atoms occupy general positions. The Pb2+ ions is coordinated with six Br− ions forming distorted PbBr6 octahedron, the PbeBr bond lengths change from 2.906 Å to 3.166 Å. Each Br− ions link two PbBr6 octahedron forming an infinite one dimension (1D) chain by μ2 connection model. In 1, the hydrocarbon chain of C4-Mim+ cations all adopt trans conformation, and the neighbouring cations shows face to face packing. The weak CeH…Br interaction was occurred between the organic and inorganic parts. Along the [1 0 0] direction, each 1D bromoplumbate chain is surrounded by six C4-Mim+ cations columns (Fig. 1b). Hybrid 2 and 3 crystallizes in the same monoclinic crystal system P21/n at 100 K, which belong to non chiral space group and is different from 1. The asymmetric unit and packing structure of three compounds are similar. The distinct structure difference are in the cations conformation. (1) In hybrid 2, hydrocarbon chains adopt trans conformation (Fig. 1c), however, the hydrocarbon chain shows mixed trans and gauche conformation in 3 (Fig. 1d). The C4eC5 bond length of gauche
have being discovered to show amazing bipolar and bistable resistive switching behavior with small on-off voltage of < 1.0 V. Room temperature ferroelectric phase has been reported in 2D (BA)2(MA)(n−1)PbnX(3n+1)-type perovskites (where BA = C4H9NH3+; MA = CH3NH3+; X = Br or Cl) [12,13]. 2D perovskites (EDBE)PbI4 (EDBE = 2,2-(ethylenedioxy) shows broadband emission due to structural distortions of the inorganic lattice [14]. However, most of researches related with organic-inorganic perovskites are mainly focused on small size cation (such as Cs+, CH3NH3+). Recently, some one-dimensional (1D) and zero-dimensional (0D) haloplumbates perovskites have been obtained using larger organic cations. Low-dimensional haloplumbates perovskites exhibit much higher stability under high humidity compared to 3D haloplumbates perovskites [15,16]. Futhermore, a subclass of 1D perovskites has recently attracted attention for their potential application in broadband solid state lighting due to the presence of highly stokes shifted light emission. Recently, some white-light emission 1D hybrid perovskite from broadband lighting acrossing the entire visible spectrum have been reported [17–19]. 1D structure [C4N2H14][PbBr4] enabled strong quantum confinement with the formation of self-trapped excited states and gave efficient bluish white light emission with PLQE of 20% for bulk single crystals. In our previous study, large size organic cations were chosen for self-assembly 1D haloplumbates perovskites with broadband emission properties [20–22]. Here, imidazole-based ion liquid were used to synthesis 1D haloplumbates perovskites. The crystal structure and luminescence properties of three new compounds [C4-Mim][PbBr3] (1), [C5-Mim][PbBr3] 2
Inorganic Chemistry Communications 112 (2020) 107690
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Fig. 2. DSC curves for compounds (a) 1; (b) 2 and (c) 3 in the heating/cooling process.
conformation (1.462 Å) in 3 is shorter than trans conformation in 2 (1.515 Å); (2) The C7 atoms of alkyl chain shows structurally disorder with two site in 3 and is structurally order in 2. The 1D [PbBr3]− inorganic chain also consisted of corner-sharing PbBr6 octahedron in 2 and 3. However, the PbeBr bond length are different from 1. The longer PbeBr bond length is 3.310 Å in 2 and 3.312 Å in 3. Thus the degree of distortion of PbBr6 octahedron is different. In general, the octahedral distortion of PbX6 can be estimated by using the parameter of Δoct [Eq. (1)] [23,24] oct
=
1 6
second step weight loss related to the residual PbBr2 begins sublimation at approximately 550 °C and this volatilization procedure was completed at 700 °C. Differential scanning calorimetry (DSC) is performed on the polycrystalline samples of three compounds confirming the thermal properties in the temperature ranges of −50 to 160 °C. As shown in Fig. 2, one endothermic peaks appeared in the first heating process for each compound and peaks are located at ca. 98 °C for 1, 102 °C for 2, and 113 °C for 3, which correspond to the melting process of three compounds and have been proved by melting point measurement. In the following cooling process, supercooling behavior were observed for the three compounds. No exothermic peaks occurred when the sample were cooled to −50 °C confirming the formation of supercooled liquids in the cooling cycle (Fig. 2), such type of supercooling thermal behavior often appears in a liquid crystal and ion liquid system and is thought to be related to the release of structural strain [21]. Interesting, it can be proved that the melting point of hybrids can be decreased by introducing imidazole-based ion liquid as cations, and to the best of our knowledge, this is a very rare example for the haloplumbate-based organic-inorganic hybrid with very low melting point. Three compounds are colorless and transparent under ambient suggesting little absorption in the visible light region. Fig. S5 shows the optical absorption spectra for the three compounds at room temperature. Three compounds have similar absorption curves containing one strong sharp absorption at ca. 210 nm and a broad absorption band below 400 nm. Different from 1 and 3, a weak shoulder peak ca.
6
[(di i=1
dm)/ dm ]2
(1)
In Eq. (1), dm and di represent the mean PbeX bond length and the six individual PbeX bond lengths, respectively. According to Eq. (1), the obtained Δoct parameter is 0.731 × 10−3 for 1, 2.505 × 10−3 for 2, 2.34 × 10−3 for 3. It can be found that the PbBr6 octahedron for 2 and 3 shows much larger parameters of Δoct, which are related to the distortion of the PbeBr bond lengths compared with 1, indicating that heavy distortion occurs in the PbBr6 coordinated octahedron of 2 and 3. The thermal stability was evaluated for three compounds by TG analysis under nitrogen atmosphere and the corresponding TG curves are depicted in Fig. S4. Three compounds show similar thermal weight loss behavior, and two steps weight loss was observed from 30 to 800 °C. For 1 as example, 1 is thermally stable below 320 °C and first step decomposing above 320 °C related to loss of organic cation. The 3
Inorganic Chemistry Communications 112 (2020) 107690
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Fig. 3. (a) Solid state emission spectra of 1 in the temperature range of 10–300 K; (b) solid state emission spectra of 1 at 40 and 100 K; (c) temperature dependent of solid state emission spectra of 2; (d) solid state emission spectra of 3 at selected temperature showing the variation of the intensity of two emission bands and (e) CIE chromaticity diagram of 3 in the temperature range of 10–300 K.
394 nm was observed in 2. Compared with other haloplumbate organicinorganic hybrid, we can see that the peak positions of this work (1D inorganic chain) exhibit a blue shift compared with 3D inorganic chain, which is consistent with the rule that the energy gap decrease as the dimensionality increases [21]. Three compounds show tunable emission from the temperature dependent photoluminescence spectra (Fig. 3a, c and S6). The room temperature photoluminescence spectra of 1 show abroad emission band, ranging from 500 to 750 nm (Fig. S7).
For 1, with decreasing temperature, the intensities of the abroad bands enhance rapidly, indicating that the thermal activation of non-radiative relaxation processes is efficiently suppressed at low temperature. From 100 to 55 K, a weak shoulder band occurred centered at ca. 451 nm (high energy emission band), however, the peak intensity of 610 nm is nearly not changed and 666 nm is enhance slightly (Fig. S8). When the temperature further decreased to 40 K, the intensity of shoulder band centered at ca. 451 nm enhance rapidly, and the center position of the 4
Inorganic Chemistry Communications 112 (2020) 107690
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band at 611 nm shows a red shift to 625 nm at 10 K (Fig. 3b). The thermochromic luminescence properties of 1 can be seen from CIE (x, y) chromaticity diagram coordinates. As shown in Fig. S9, the plot of CIE coordinate versus temperature displays a non-monotonic change, and the corresponding luminescence color changes from orange at 300 K (CIE is (0.4532, 0.4738)) to yellowish white at 10 K (CIE is (0.355, 0.2891)). Similar with 1, the temperature dependent photoluminescence spectra of 2 also shows two emission bands (Fig. 3c). Different from 1, the high energy emission band was occurred at 275 K. With the temperature decreased, the intensities of the two emission bands enhance and no anomalies were found. The rang of color change of 2 is narrow compared with 1 and 3. The CIE coordinate of 2 is (0.4606, 0.4461) at 300 K, and (0.4622, 0.3331) at 300 K, which are all in yellow region (Fig. S10). For 3, room temperature photoluminescence spectra is similar with 1 and 2, yellow emission was observed with CIE coordinate is (0.473, 0.4503) at 300 K. When the temperature decreased, the high energy emission band was occurred at 125 K. However, when the temperature decreased to 55 K, the intensity of two emission bands show different tendency towards temperature. The intensity of the high energy emission band centered at 447 nm enhance rapidly and low energy emission band slightly decreased (Fig. 3d), which leads to a great change in the color of 3. As shown in Fig. 3e, 3 is blue and CIE coordinate is (0.2506, 0.1042) at 10 K. The vibrational relaxation of excited state electrons to the ground state via nonradiative internal conversion is one of main fluorescence quenching mechanisms, and this process is suppressed in the low temperature, and thus, the emission generally enhances upon cooling. Especially for the organic component, dynamic motion of the imidazole ring resulted in the excited state relaxing to the ground state through the nonradiative internal conversion is frozen at low temperature. However, the dynamic motion of the organic cations and rigidity inorganic component with two different emission bands show different sensitivity to temperature. The relative intensities between two emission bands change with temperature, which leads to the thermochromic luminescence properties of 1–3. Furthermore, the emission quantum yields of 1–3 were investigated. At room temperature 1–3 have low quantum yields of 3.4%, 5.2% and 2.8%, respectively. In conclusion, three bromoplumbate organic-inorganic perovskites were synthesized and structurally characterized. The general crystal structure characteristic in this family is the formation of 1D [PbBr3]− chain. The hydrocarbon chain length in countercation finely modulates the packing structure of the three compounds. Three compounds have very low melting point compared to reported haloplumbate-based organic-inorganic perovskites, which can be ascribed to introducing of imidazole-based ion liquid. Three compounds show fascinating thermochromic luminescence properties owing to the different emission bands showing different temperature responses. Interestingly, 3 have wide range of color change from blue to yellow with the temperature increased. This work inspire us to further search for bromoplumbate organic-inorganic compounds and investigate the relationship of the crystal structure and optical properties.
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