Reversible photoluminescence switching behavior and luminescence thermochromism of copper(I) halide cluster coordination polymers

Abstract

Two novel coordination polymers [Cu₃I₄(C₁₆H₃₂N₄)]·ClO₄ (1) and [Cu₆I₆Br₂(C₁₆H₃₂N₄)(CH₃CN)₂] (2) (C₁₆H₃₂N₄·2Br⁻ = 4-Aza-1-azoniabicyclo-[2.2.2]octane, 1,1′-(1,4-butanediyl)bis-dibromide) have been synthesized under solvothermal conditions. At room temperature, compounds 1 (Φ = 46.2%) and 2 (Φ = 26.62%) show strong photoluminescence in solid state. At low temperature, compound 1 displays obvious thermochromic luminescence while compound 2 has no obvious change which can be observed by naked eye. However, compound 2 displays a interesting phenomenon. The luminescence of compound 2 is reversible when the coordinated CH₃CN molecules in structure are removed by heat and recovered from acetonitrile solvent.

---

Introduction

Coordination polymers consisting of copper(I) halide clusters have attracted wide interest in the past decades.¹ These compounds exhibit rich photoluminescence with potential applications as organic light-emitting diodes.² Recently, these compounds have become hotspot in inorganic chemistry because they respond to exterior stimuli such as thermal treatment (thermochromism), and mechanical grinding (piezochromism).³ Meanwhile, The influence of coordinated solvent or guest molecules is an intriguing and important subject of coordination polymer now, since new frameworks or properties can be detected during the removal/introduction of small coordinated molecules or guest molecules.^4,5 Many chemists have devoted to the study of the framework properties upon guest removal/introduction.⁶ Interestingly, similar research has been reported in copper(I) halide cluster coordination polymers although they are limited.⁷ Furthermore, the photoluminescence behavior of copper(I) halide cluster coordination polymers is largely dependent on their structure. Consequently, their luminescence is likely to change accompanying the transformation of structure.

Herein we report two coordination polymers, namely, [Cu₃I₄(C₁₆H₃₂N₄)]·ClO₄ (1) and [Cu₆I₆Br₂(C₁₆H₃₂N₄)(CH₃CN)₂] (2) prepared by solvothermal reactions. Although the copper source of two compounds is different, two similar copper(I) halide cluster are obtained. The organic ligand (L) C₁₆H₃₂N₄·2Br⁻ = 4-Aza-1-azoniabicyclo-[2.2.2]-octane, 1,1′-(1,4-butanediyl)bis-dibromide is shown in Fig. 1. The L connects the copper(I) halide cluster to form two coordination polymers with different frameworks. Compound 1 features a three-dimensional (3D) cation-framework which contains anion ClO₄⁻ to balance the charge. Compound 2 exhibits a two-dimensional (2D) zigzag layer. In this work, we mainly investigate the luminescent properties of compound 1 and 2.

Fig. 1 Structure of the ligand (L).

Experimental section

Materials and methods

All reagents and solvents used in the syntheses are obtained from commercial sources without any purification. The luminescence spectrum and the internal quantum yields are performed with an Edinburgh Instruments FLS920 spectrofluorometer. Element analyses are measured on a Perkin-Elmer 2400 elemental analyzer. The infrared (IR) spectra are recorded on a Nicolet Impact 410 FTIR spectrometer with KBr pellets. Thermogravimetric experiments are carried out by a TGA Q500 V20.10 Build 36 from room temperature to 800 °C at a heating rate of 10 °C min⁻¹. The X-ray powder diffraction (XRPD) experiments perform in a Rigaku D/max 2550 X-ray powder diffractometer with Cu Kα radiation.

Synthesis

[Cu₃I₄(C₁₆H₃₂N₄)]·ClO₄ (1). Cu(NO₃)₂·3H₂O (0.2 mmol, 0.048 g), L (0.044 g, 0.1 mmol), HIO₄·2H₂O (0.091 g, 0.4 mmol), NaClO₄ (0.056 g, 0.4 mmol) and NaHCO₃ (0.025 g, 0.3 mmol) were placed in 5 mL H₂O/C₄H₈O₂ (1,4-dioxane, v : v = 2 : 3) solution and stirred a few hours, then transferred and sealed in a Teflon-lined steel autoclave. The autoclave was kept in a 140 °C oven for 4 days. After cooling to room temperature, colorless block crystals were collected by filtration, washed with water, ethanol and air dried (yield 52%). Elemental analysis calcd (%) for 1 (C₁₆H₃₂Cu₃I₄N₄ClO₄): C 17.82, H 2.99, N 5.20; found: C 17.59, H 2.76, N 4.97. Selected IR peak (cm⁻¹): 2942, 2888, 2012, 1458, 1083, 845.

[Cu₆I₆Br₂(C₁₆H₃₂N₄)(CH₃CN)₂] (2). CuI (0.076 g, 0.4 mmol), L (0.044 g, 0.1 mmol) were placed in 5.0 mL H₂O/CH₃CN (v : v = 2 : 3) solution, then 50 μL KOH (1.0 mol L⁻¹) aqueous solution was added and stirred for a few hours. Finally, transferred and sealed in a Teflon-lined steel autoclave. The autoclave was kept in a 140 °C oven for 4 days. After cooling to room temperature, colorless pillar crystals were harvested by filtration, washed with water, ethanol and air dried (yield: 38%). Elemental analysis calcd (38%) for 2 (C₂₀H₃₈Br₂Cu₆I₆N₆): C 15.45, H 2.30, N 5.05; found: C 15.31, H 2.45, N 5.12. Selected IR peak (cm⁻¹): 3486, 2954, 2298 (ν_CN) 2265 (ν_CN), 2080, 1459, 709.

X-ray crystallography

Single crystal diffraction data for complexes 1 and 2 were collected on a Rigaku RAXIS-RAPID equipped with a narrow-focus, 5.4 kW sealed tube X-ray source (graphite-monochromated Mo Kα radiation, λ = 0.71073 Å). Data processing was accomplished with the PROCESS-AUTO processing program. Direct methods were used to solve the structure using the SHELXL crystallographic software package. All non-hydrogen atoms were easily found from the difference Fourier map. All non-hydrogen atoms were refined anisotropically. The crystallographic information and structure refinement details are summarized in Table 1.

Table 1 Crystal data collection and structure refinement for 1 and 2

Compound	1	1	2	2
a R1 = ∑‖Fo| − |Fc‖/∑|Fo|.b wR2 = {∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)]^2}^1/2.
Temperature/K	298 K	120 K	298 K	120 K
Formula	C16H32ClCu3I4N4O4	C16H32ClCu3I4N4O4	C20H38Br2Cu6I6N6	C20H38Br2Cu6I6N6
fw (g mol^−1)	1078.13	1078.13	1665.02	1665.02
Crystal system	Orthorhombic	Orthorhombic	Orthorhombic	Orthorhombic
Space group	Ama2	Ama2	Pbca	Pbca
a(Å)	28.800(6)	28.536(6)	14.357(3)	14.027(3)
b(Å)	10.330(2)	10.276(2)	13.713(3)	13.637(3)
c(Å)	9.2214(18)	9.2102(18)	20.207(4)	20.343(4)
V(Å^3)	2743.4(10)	2700.7(9)	3978.2(14)	3891.2(13)
Z	4	4	4	4
F(000)	2016	2016	3048	3048
μ(mm^−1)	6.933	7.043	9.837	10.057
Dcalcd(g cm^−3)	2.610	2.652	2.780	2.842
R1^a	0.0315	0.0282	0.0841	0.0692
wR2^b	0.0874	0.0729	0.1672	0.1499

---

Results and discussion

Crystal structures

X-ray crystal structure analysis reveals that compounds 1 and 2 crystallize in the orthorhombic Ama2 and Pbca space group, respectively. In compound 1, trinuclear Cu₃I₄ is the smallest repeating unit whose structure feature is similar to the incomplete cubane-like M₃S₄ (M = Mo, Fe) clusters. The adjacent two Cu₃I₄ clusters are connected each other through a μ₃-I atom to form a 1-D chain (Fig. 2a) which extends along two different directions in space, one along the [0 1 1] axis, and the other along [0 1 1] axis. The chains parallelly arrange in the bc plane and form two kinds of virtual inorganic layers, A and B. The adjacent layers alternately pack in ABAB fashion along the a axis (Fig. 2). Organic ligands (L) act as linkers, connecting chains in the adjacent two layers to form a 3-D cation-framework which contains anion ClO₄⁻ to balance the charge. In compound 2, its unit (Cu₃I₃Br) is almost the same as that of compound 1 (Cu₃I₄). Two Cu₃I₃Br clusters are connected to form a 1-D chain by the μ₃-Br atom (Fig. 3a). These chains are parallel to the b axis. Notably, because one coordination site in the unit is occupied by acetonitrile molecule, organic ligands connect the chains to form a 2-D zigzag layer which packs along the c axis (Fig. 3).

Fig. 2 (a) Cu₃I₄ chain of 1. (b) 3-D structure of 1. (c) Schematic view of the 3-D framework of 1.

Fig. 3 (a) Cu₃I₃Br chain of 2. (b) 2-D layer (right) and schematic view of the 2-D structure of 2. (c) 2-D layers of 2 packing along the c axis.

Thermogravimetric analysis, XRPD pattern and IR spectrum

Measurements of the thermal behavior of compound 1 and 2 are performed in air from room temperature to 800 °C at 10 °C min⁻¹. The TG curves show that compound 1 can be stable up to ca. 300 °C. For the compound 2, the first sharp weight loss in the temperature range 190–240 °C corresponds to one CH₃CN molecule mass loss. The mass loss is about 4.42% (calcd 4.93%). The IR spectrum show the CH₃CN molecule has removed by heat and recovered from acetonitrile solvent. The XRPD patterns confirm the framework of compound 2 has slightly changed accompany the removal and recovery of the CH₃CN molecule.

Luminescence properties

Compound 1 and 2 display excellent solid-sate photoluminescence properties at room temperature. The luminescent test results show that under optimal excitation (λ_1Ex = 345 nm, λ_2Ex = 296 nm), the photoluminescence with emission maxima is at 507 nm and 497 nm, respectively (Fig. 6 and 7). The internal quantum yields of the compounds are measured at room temperature (λ_Ex = 300 nm) and the values are as high as 46.2% and 26.62%. According to analysis of the structures of compounds 1, 2 and studies of the luminescence of copper(I) halides, their emission light should be attributed to the similar cluster-centered excited state and halide-to-metal charge transfer character.^7e Interestingly, at low temperature, sample 1 emit bright yellow light which exhibit bright green light at room temperature. Unfortunately, the emission of sample 2 has no obvious change that can be observed (Fig. 5). The variable temperature luminescence spectrum of 1 and 2 is shown in Fig. 4. Their emission maximum appears to have shifted from 507 nm to 542 nm and 497 nm to 502 nm when the temperature decreased from 298 K to 30 K. The thermochromic luminescence for compound 1 is reversible, since warming progressively up to room temperature, the green emission is recovered. Studies on thermochromic luminescence phenomenon of copper(I) halides clusters, suggest that it is often very dependent on the Cu⋯Cu distances in copper(I) halide cluster. Because the energy of cluster-centered excited state is largely dependent on the Cu⋯Cu interactions.^8,9 Here we have analyzed X-ray crystal structure on the same single crystals of 1 and 2 at low temperature. The structural data shows that the Cu⋯Cu distances of 1 and 2 have changed as the temperature decreases. They decrease from 2.7641 Å, 2.7641 Å, 2.5747 Å (298 K) to 2.7384 Å, 2.7384 Å, 2.5624 Å (120 K) and 2.621 Å, 2.689 Å, 2.672 Å (298 K) to 2.587 Å, 2.652 Å, 2.654 Å (120 K) for 1 and 2, respectively. Because the Cu⋯Cu distances become shorter at low temperature, the bonding character increases and the energy level becomes lower. Thus, the difference in energy level between the ground state and the excited state becomes smaller. Therefore, the emission lights have red-shift accompanying the temperature decrease.^7c,8 Unexpectedly, the Cu⋯Cu distances have decreased at low temperature, but the emission of compound 2 has no obvious shifted. The single crystal data show that the cell parameter of compound 2 has increased along c axis as the temperature decreases (Table 1). Comparing the bond length of the copper(I) halide cluster at 298 K and 120 K, we find that some copper–halide bonds have slight increased in compound 2 while all the bonds have slight decreased in compound 1 (Table S2, ESI†). To best of our knowledge, the bond lengths of the copper(I) halide cluster usually decrease at low temperature, this abnormal phenomenon has not reported in copper(I) halide cluster coordination polymers. Because part of the copper–halide bonds has increased, the progress of halide-to-metal charge transfer and the energy cluster-centered excited state would be affected. These may be the reason why the compound 2 displays a small red-shift.

Fig. 4 Temperature dependence of solid-state emission spectra of 1 (a) and 2 (b).

Fig. 5 Corresponding calculated CIE coordinate of 1 (a) and 2 (b) at different temperature.

Though the emission of sample 2 has no obvious change that can be observed at low temperature, it displays a more interesting phenomenon. The CH₃CN molecules in structure can be removed by heat and recovered from CH₃CN solvent. When sample 2 is heated at 240 °C for 2 h (the TG curve shows that the first sharp weight loss at about 240 °C corresponds to one CH₃CN molecule mass loss) (Fig. S3, ESI†), the acetonitrile-free sample 2a is obtained. The infrared spectrum of 2 and 2a proves that the coordinated acetonitriles are removed completely (Fig. S5, ESI†). By analyzing the XRPD pattern of samples 2 and 2a, we find that there are evident differences between them, indicating the framework structure of the crystal has transformed. The results of indexing the XRPD pattern of 2a are showed in Table S3 (see ESI†). More importantly, after 2a is immersed in acetonitrile 24 h, the powder of XRD pattern and infrared spectrum became the same as that for the original sample of 2 (Fig. S2 and S5 ESI†). The conversion of XRPD and infrared spectrum confirms that the solvent molecules are recoordinated. Most interestingly, in this progress the luminescence of sample 2 correspondingly changes. The acetonitrile-free sample 2a (Φ = 18.32%) exhibits bright yellow at λ_2aEm = 537 nm that completely different to sample 2 (λ_2Em = 497 nm) and the optimal excitation of 2a is at 310 nm (Fig. 7). After being immersed in acetonitrile 24 h, the emission light recovers. Consequently, CH₃CN molecules in the structure play the role of an unusual reversible photoluminescence switch. Because of lacking available single crystal data, we cannot clearly point out what specific change happened. The results of indexing the XRPD have confirmed the structure has changed. We speculate the electron cloud density of the copper(I) which connects with the acetonitrile molecule may change after removing the acetonitrile molecule. The Cu 2p X-ray photoelectron spectroscopy (XPS) peak at 932.8 (2p_3/2) and 952.6 (2p_1/2) in sample 2 is shifted to 932.5 (2p_3/2) and 952.2 (2p_1/2) in sample 2a, indicating our speculation is right (Fig. 8). Analyzing the change of microenvironment by XPS spectra in Coordination polymers has been reported.¹⁰ According to previous literatures report, the Cu⋯Cu bonding character which mainly affects the energy of cluster-centered excited state is formed by the electron density moving from copper d orbitals to copper s orbitals.^8c In other words, the removal the acetonitrile molecule has changed the electron cloud density of the copper(I), which results in the energy of cluster-centered excited state changing and the optical signal correspondingly change. Similar research in our group has reported that the coordinated water molecule in a classic lanthanide coordination polymer is reversible and its effect on magnetism of the coordination polymer has been analyzed.¹¹

Fig. 6 Solid-state luminescence spectra of compound 1.

Fig. 7 Solid-state luminescence spectra of sample 2 (blue) and 2a (red).

Fig. 8 The Cu 2p XPS spectra of sample 2 (black) and 2a (red).

Conclusions

In conclusion, two novel compounds which respectively incorporate Cu₃I₄ and Cu₃I₃Br clusters in different frameworks have synthesised under solvothermal conditions. They display strong photoluminescence properties. Interestingly, compound 1 exhibits obvious luminescence thermochromic phenomenon. At low temperature, the emission maximum appears to shift from 507 nm (at room temperature) to 542 nm. Compound 2 displays an important reversible photoluminescence switching behavior. Acetonitrile molecules in the structure can be removed and recovered. The luminescence correspondingly changes accompanying the removal and recoordination of acetonitrile molecules. Therefore, acetonitrile molecules can serve as an unusual photoluminescence switch perfectly modulating the luminescence. Based on these excellent properties, these materials inspire us that copper(I) halide cluster coordination polymers are promising for research in sensor application.

Acknowledgements

This work was supported by the Foundation of the National Natural Science Foundation of China (no. 21371069, 21241010), Specialized Research Fund for the Doctoral Program of Higher Education (no. 20110061110015) and National High Technology Research and Develop Program (863 program) of China (no. 2013AA031702).

Notes and references

1. (a) M. Vitale and P. C. Ford, Coord. Chem. Rev., 2001, 219, 3; (b) R. Peng, M. Li and D. Li, Coord. Chem. Rev., 2010, 254, 1; (c) P. C. Ford, Coord. Chem. Rev., 1994, 132, 129; (d) S. Perruchas, C. Tard, X. F. L. Goff, A. Fargues, A. Garia, S. Kahal, J. Saillard, T. Gacion and J. Boilot, Inorg. Chem., 2011, 50, 10682; (e) X. Zhang, W. Liu, G. Z. Wei, D. Banerjee, Z. Hu and J. Li, J. Am. Chem. Soc., 2014, 136, 14230; (f) Y. Kang, F. Wang, J. Zhang and X. Bu, J. Am. Chem. Soc., 2012, 134, 17881; (g) X. Tan, L. Li, J. Zhang, X. Han, L. Jiang, F. Li and C. Su, Chem. Mater., 2012, 24, 480; (h) W. J. Gee and S. R. Batten, Cryst. Growth Des., 2013, 13, 2335.
2. (a) Z. Liu, M. F. Qayyum, C. Wu, M. T. Whited, P. I. Djurovich, K. O. Hodgson, B. Hedman, E. I. Solomon and M. E. Thompson, J. Am. Chem. Soc., 2011, 133, 3700; (b) D. Volz, D. M. Zink, T. Bocksrocker, J. Friedrichs, M. Nieger, T. Baumann, U. Lemmer and S. Bräse, Chem. Mater., 2013, 25, 3414.
3. (a) T. H. Kim, Y. W. Shin, J. H. Jung, J. S. Kim and J. Kim, Angew. Chem., Int. Ed., 2008, 47, 685; (b) D. Braga, L. Maini, P. P. Mazzeo and B. Ventura, Chem. –Eur. J., 2010, 16, 1553; (c) H. Kitagawa, Y. Ozawa and K. Toriumi, Chem. Commun., 2010, 46, 6302; (d) S. Perruchas, X. F. L. Goff, S. Maron, I. Maurin, F. Guillen, A. Garcia, T. Gacoin and J. Boilot, J. Am. Chem. Soc., 2010, 132, 10967; (e) K. M. Henline, C. Wang and R. D. Pike, Cryst. Growth Des., 2014, 14, 1449; (f) X. Shan, H. Zhang, L. Chen, M. Wu, F. Jiang and M. Hong, Cryst. Growth Des., 2013, 13, 1377; (g) Q. Benito, X. F. L. Goff, S. Maron, A. Fargues, A. Garia, C. Martineau, F. Taulelle, S. Kahlal, T. Gacion, J. P. Boilot and S. Perruchas, J. Am. Chem. Soc., 2014, 136, 11311.
4. (a) G. Férey and C. Serre, Chem. Soc. Rev., 2009, 38, 1380; (b) S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334.
5. (a) W. M. Bloch, R. Babarao, M. R. Hill, C. J. Doonan and C. J. Sumby, J. Am. Chem. Soc., 2013, 135, 10441; (b) J. Li, D. J. Timmons and H. Zhou, J. Am. Chem. Soc., 2009, 131, 6368; (c) Y. Zhang, T. Liu, S. Kanegawa and O. Sato, J. Am. Chem. Soc., 2009, 131, 7942; (d) J. Li, P. Huang, X. Wu, J. Tao, R. Huang and L. Zheng, Chem. Sci., 2013, 4, 3232; (e) W. M. Bloch and C. J. Sumby, Chem. Commun., 2012, 48, 2534.
6. (a) S. K. Ghosh, J. Zhang and S. Kitagawa, Angew. Chem., Int. Ed., 2007, 46, 7965; (b) S. K. Ghosh, W. Kaneko, D. Kiriya, M. Ohba and S. Kitagawa, Angew. Chem., Int. Ed., 2008, 47, 8843; (c) R. Medishetty, L. L. Koh, G. K. Kole and J. J. Vittal, Angew. Chem., Int. Ed., 2011, 50, 10949; (d) L. Wen, P. Cheng and W. Lin, Chem. Commun., 2012, 48, 2846.
7. (a) X. Xue, X. Wang, R. Xiong, X. You, B. F. Abrahams, C. Che and H. Ju, Angew. Chem., Int. Ed., 2002, 114, 2944; (b) H. Kitagawa, H. Ohtsu and M. Kawano, Angew. Chem., Int. Ed., 2013, 52, 12395; (c) T. H. Kim, Y. W. Shin, J. H. Jung, J. S. Kim and J. Kim, Angew. Chem., Int. Ed., 2008, 47, 685; (d) J. Y. Lee, S. Y. Lee, W. Sim, K. Park, J. Kim and S. S. Lee, J. Am. Chem. Soc., 2008, 130, 6902; (e) J. Y. Lee, H. J. Kim, J. H. Jung, W. Sim and S. S. Lee, J. Am. Chem. Soc., 2008, 130, 13838.
8. (a) F. D. Angelis, S. Fantacci, A. Sgamellotti, E. Cariati, R. Ugo and P. C. Ford, Inorg. Chem., 2006, 45, 10576; (b) K. R. Kyle, C. K. Ryu, J. A. DiBenedetto and P. C. Ford, J. Am. Chem. Soc., 1991, 113, 2954; (c) D. Sun, S. Yuan, H. Wang, H. Lu, S. Feng and D. Sun, Chem. Commun., 2013, 49, 6152.
9. (a) P. C. Ford, E. Cariati and J. Bourassa, Chem. Rev., 1999, 99, 3625; (b) C. K. Ryu, M. Vitale and P. C. Ford, Inorg. Chem., 1993, 32, 869; (c) M. Vitalle, C. K. Ryu, W. E. Palke and P. C. Ford, Inorg. Chem., 1994, 33, 561.
10. (a) L. Li, Z. Chen, H. Zhong and R. Wang, Chem. –Eur. J., 2014, 20, 3050; (b) X. Song, S. Song, S. Zhao, Z. Hao, M. Zhu, X. Meng, L. Wu and H. Zhang, Adv. Funct. Mater., 2014, 24, 4034; (c) B. Chen, L. Wang, Y. Xiao, F. R. Fronczek, M. Xue, Y. Cui and G. Qian, Angew. Chem., Int. Ed., 2009, 48, 500.
11. Q. Zhou, F. Yang, B. Xin, G. Zeng, X. Zhou, K. Liu, D. Ma, G. Li, Z. Shi and S. Feng, Chem. Commun., 2013, 49, 8244.

---

Footnote

† Electronic supplementary information (ESI) available: Crystal data, synthetic method of ligand, IR spectra, XRPD patterns, pattern indexing results. CCDC [1037335–1037338]. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra05157f

---