DOI: 10.1002/chem.201303991
Communication
& Luminescent Materials
Luminescent Mechanochromic Porous Coordination Polymers Tian Wen, Xiao-Ping Zhou, De-Xiang Zhang, and Dan Li*[a]
Chem. Eur. J. 2014, 20, 644 – 648
644
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication (G = toluene, 1 a; ethanol, 1 b; toluene, 2; 4-pt = 5-(4-pyridyl)tetrazole), as mechanochromic materials. Compounds 1 a, 1 b, and 2 were prepared by in situ click synthesis, an efficient and green approach documented by our group and others.[23–27] Reactions of Cu2O, 4-cyanopyridine, and NaN3 in different solvents (toluene for 1 a and 2, ethanol for 1 b, for details see the Experimental Section) under solvothermal conditions resulted in crystalline products. Compound 2 was formed as a concomitant of 1 a. 1 b can also be synthesized by the reaction of CuBr and 5-(4-pyridyl)tetrazole under similar conditions.[28] However, 1 a and 2 could not be obtained successfully through direct reactions of CuI and 4-pt. Although the color and shape of crystals 1 a and 2 are similar, the compounds can be distinguished easily under UV light (Figure S1 in the Supporting Information). 1 a, 1 b, and 2 were characterized by single-crystal X-ray diffraction at room temperature (Table 1) and feature similar gridlike 2D layer structures (Figure 1 a). 1 a crystallizes in the triclinic P1¯ space group. The layers are stacked layer-by-layer in an ABAB packing mode, and a similar case is also found for the structure of 1 b (Figure 1 b, c). In both structures, all CuI atoms adopt a triangular coordination geometry, coordinated by three nitrogen atoms of 4-pt. Two CuI atoms are bound by 4pt ligands and form a dimeric structure. The Cu–Cu distances between two adjacent layers (3.220 , 3.254 ) in 1 a are slightly longer than those in 1 b (3.205 , 3.201 ). Guest molecules (toluene in 1 a and ethanol in 1 b) occupy the cavities of the frameworks. They are highly disordered and cannot be determined by X-ray diffraction analysis at room temperature. Crystals of 2 crystallize in the monoclinic P21/c space group (Table 1) as a 2D grid-like framework (Figure 1 d). The layer structure of 2 is topologically identical with 1 a and 1 b except that the framework is slightly “squashed”. However, the layer packing in 2 is distinctively different. The whole structure is constructed from an identical layer-to-layer stacking repeated as an AAA packing mode (Figure 1 d). The shortest Cu–Cu distance between adjacent layers in 2 is 3.654 , longer than those in 1 a (3.220 , 3.254 ). 1 a and 2 could be classified as supramolecular isomers owing to having the same composition and different packing structures. Regarding the ligand, there are unsupported and short Cu–Cu distances between copper(I) centers located in adjacent Cu(4-pt) grid layers, and so there probably exist Cu···Cu interactions (also referred to as cuprophilicity) in 1 a, 1 b, and 2. The crystal structures of 1 a, 1 b, and 2 at a cryogenic temperature (100 K, Table 1) were also determined. Although the space group P1¯ is retained for 1 a at 100 K, all unit-cell dimensions including cell lengths and angles changed significantly, indicating a new phase was formed. When crystals of 1 a were cooled from 293 K to 100 K, sliding and slight squashing occurred in the framework, leading to an alternation of relative positions between two adjacent layers (Figure S2 in the Supporting Information). The crystal transformation between room and cryogenic temperatures is reversible. This phenomenon implies that the 2D Cu–4-pt network is flexible and dynamic enough to be modified by external stimulus (i.e., cooling). In
Abstract: Three 2D luminescent isomeric porous coordination polymers are synthesized and characterized. Their luminescence properties can be modified by grinding and they can act as mechanochromic materials and their properties are probably related to the weak interactions of cuprophilicity and p–p interactions.
Porous coordination polymers (or metal–organic frameworks, MOFs), constructed through coordination bonds between metal ions or metal clusters and organic linkers, have received increasing attention in recent years owing to their potential applications in gas storage and capture,[1, 2] separation,[3] chemical sensing,[4, 5] catalysis,[6] biomedical imaging, and drug delivery.[7] Compared with traditional inorganic porous materials, for example, zeolites, porous coordination polymers are less robust and more flexible owing to their formation from relatively weak coordination bonds. This structural flexibility may endow porous coordination polymers with some unique properties, for example, response to external stimuli, thus providing the possibility of forming dynamic stimuli-responsive materials (or smart materials).[8] These properties are fundamentally important in the applications of sensing and detection. Stimuliresponsive porous coordination polymers triggered by external factors such as the presence of guest molecules, photons, and heating have been extensively studied.[9–11] In contrast, mechanical stimulation remains unexplored for coordination polymers, although solid-state preparations for porous coordination polymers have been established.[12] By being treated with mechanical force (grinding), the luminescence or colors of some coordination oligomers, including gold(I),[13, 14] silver(I),[15] platinum(II),[16–19] and copper(I)[20] complexes, changed distinctively, showing stimuli-response properties. The structures of these complexes underwent modification upon grinding. For instant, some complexes lost crystallinity, sometimes accompanied by the alternation of supramolecular interactions (for example, metallophilicity). Compared with oligomeric metal complexes, coordination polymers feature infinite networks and are more rigid, making it harder to stimulate them by mechanic force.[21, 22] In view of the great challenge to tune rigid coordination polymer structures by mechanical force and the successful cases of mechano-responsive oligomers, we anticipated that low-dimension (1D and 2D) porous coordination polymers supported by weak interactions are probably more suitable to act as mechanical-force-responsive materials than 3D ones. Such low-dimension structures may be easily modified mechanically owing to their weak connections. Herein, we report three 2D luminescent isomeric porous coordination polymers, Cu(4-pt)·G [a] T. Wen, Prof. Dr. X.-P. Zhou, D.-X. Zhang, Prof. Dr. D. Li Department of Chemistry and Research Institute for Biomedical and Advanced Materials, Shantou University Guangdong 515063 (China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303991. Chem. Eur. J. 2014, 20, 644 – 648
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Communication Table 1. Summary of crystal data and refinement results. T [K] 1 a 293 100 1 b 293 100 2 293 100
a []
b []
c []
a [8]
b [8]
g [8]
7.1544(6) 7.5321(3) 7.2059(2) 7.0485(6) 3.6547(5) 3.5974(3)
15.3457(11) 10.6828(5) 15.3431(4) 15.2337(8) 17.127(2) 17.2471(11)
16.4009(13) 10.9671(6) 15.9791(5) 16.0392(8) 13.7926(18) 13.6263(8)
93.540(6) 104.631(4) 95.315(2) 95.618(4) 90.00 90.00
102.074(7) 89.973(4) 96.638(2) 94.709(6) 90.46 90.579(6)
95.765(6) 94.265(4) 98.272(2) 98.855(6) 90.00 90.00
Figure 1. a) Layer structure of the coordination polymers (red = Cu, blue = N, black = C); Packing structures of 1 a (b), 1 b (c), and 2 (d).
contrast, the unit-cell parameters for 1 b and 2 differ little at room temperature and 100 K. Solid samples of 1 a, 1 b, and 2 are emissive at room temperature with emission bands centered at 535, 552, and 615 nm, giving green, yellow, and orange colors, respectively, upon excitation at 395 nm (Figure 2 and Figure S1). The emissions of these complexes are likely from metal-to-ligand charge transfer [Cu!p*(tetrazolate)], and are probably involved with the Cu···Cu [3d!4s] cluster-centered (CC) excited states.[24, 25, 28, 29] Despite the fact that the structures of 1 a show differences at room and cryogenic temperatures, the emission bands at both temperatures do not shift significantly (Figure S3 in the Supporting Information) suggesting that the slight sliding in the 2D structure of 1 a does not affect the emission. The emission maxima of 1 b has a redshift of about 17 nm compared with that of 1 a, probably owing to the existence of the different guest solvent in the host framework.[28, 30] Singlecrystal X-ray study of 1 a at 100 K shows that there is no p–p interaction existing between toluene and ligand 4-pt (Figure S4 in the Supporting Information). Emission spectra of 1 a before and after removal of the guest solvent under vacuum at 120 8C Chem. Eur. J. 2014, 20, 644 – 648
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are almost identical (Figure S5 in the Supporting InformaSpace R1 WR2 V [3] tion), demonstrating that the group influence of the non-polar toluene on the structure and lu1745.6(2) P1¯ 0.0628 0.1502 851.33(7) P1¯ 0.0351 0.0713 minescence of the host 1725.80(9) P1¯ 0.0500 0.1497 framework is limited. In 1 b, 1685.22(19) P1¯ 0.0614 0.0969 as a polar molecule, ethanol 863.33(19) P21/c 0.1059 0.3016 creates hydrogen bonds with 845.40(10) P21/c 0.0728 0.1971 the nitrogen atoms of the tetrazole group in 4-pt (Figure S6 in the Supporting Information). The interaction between the guest ethanol and the 4-pt of the host framework leads to the redshift of the emission band. There is a large redshift in the emission of 2 relative to those of 1 a (80 nm) and 1 b (63 nm). Careful examination finds that two structural factors from the AAA packing style may cause the redshift: i) Aggregation by Cu···Cu interactions. Although the shortest Cu···Cu separation of 3.654(4) in 2 is longer than those inside double-layers (AB) in 1 a and 1 b, all copper(I) atoms in 2 are involved in Cu···Cu interactions forming polymeric Cu···Cu chains. In contrast, the Cu···Cu interactions in 1 a and 1 b between two double-layers are much longer (Figures S7 and S8 in the Supporting Information). It has been reported that Cu···Cu interactions play an important role in influencing the luminescence properties of copper(I) aggregates.[14, 20, 29, 31, 32] In a copper(I) cluster-centered (CC) excited state, when the Cu–Cu interactions become stronger, the emission energy becomes lower, resulting in an emission band with longer wavelength. ii) Strong p–p interactions in ligands between the adjacent layers. The 4-pt ligands are closely packed in 2 with large overlap and short distance (3.654 , Figure S8), indicating strong p–p interactions between the adjacent layers. However, 4-pt in 1 a and 1 b adopts a staggered arrangement with weaker interactions between adjacent layers than those in 2 (Figure 1 a, b, Figure S7). Experimental and theoretical results revealed that the strong p–p interactions of the
Figure 2. Emission spectra of 1 a (a, solid), 1 b (b, dot), and 2 (c, dash) in the crystalline state, and strongly ground samples of 1 a (a’, dash dot), 1 b (b’, short dot) and 2 (c’, short dash).
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Communication tween the Cu(4-pt) layers in the structures. The sliding should be accompanied with alterations of the supported weak interactions (Cu···Cu and p–p interactions), thus changing the photoluminescence properties of the compounds. It has been reported that the crystalline phase and emission properties of some ground mechanochromic oligomer metal complexes can be recovered upon exposure to solvent or heating.[13–20] PXRD patterns of ground samples of 1 a, 1 b, and 2 did not show prominent peaks after the samples were treated with organic solvents (toluene for 1 a, 2, and ethanol for 1 b) for two days. This is probably due to the fact that the coordination polymer samples cannot dissolve in organic solvents, and that recrystallization barely occurred, unlike the case for metal oligomers. Interestingly, ground solid of 1 b immediately showed bright yellow luminescence (in UV light) when one drop of ethanol was added (Figure S12 in the Supporting Information); however, similar phenomena were not observed in the cases of 1 a and 2 treated with toluene. Solidstate photoluminescence measurements found that ground solid of 1 b treated with ethanol showed a broad emission band centered at 562 nm upon excitation at 296 nm (Figure S13 in the Supporting Information). Thermogravimetric analysis (TGA) found that guest molecules of 1 b were lost during grinding (Figure S14 in the Supporting Information). The loss of guest molecules should leave voids for subsequent occupation by ethanol molecules, thus, inducing gliding of Cu(4-pt) layers and leading to luminescence change. When the sample was completely dry, the luminescence changed to dark orange. This transformation can be repeated reversibly at least three times. Thermogravimetric curves for samples 1 a and 2 before and after grinding do not show significant change (Figures S15 and S16 in the Supporting Information), revealing that the toluene guest molecules are only partially removed by grinding owing to the relatively higher boiling point of toluene than ethanol. To test the recovery of the crystalline form of the ground samples in organic solvents, we immersed each ground sample (1 a, 1 b, and 2) in ethanol, chloroform, and toluene, respectively, and found that the recovery in chloroform occurred after one month, an observation that was confirmed by powder X-ray diffraction studies (Figures S9, S10, and S11). The luminescence properties of the ground 1 a, 1 b, and 2 samples are also recovered entirely for the samples immersed in chloroform, the only notable change being that the spectral peaks become slightly broader (Figure S17 in the Supporting Information). These properties are probably due to the inherent self-restore function of coordination polymers under solvent stimuli. The phenomenon of transformation from amorphous to crystalline state has been well documented in reported coordination polymers and metal frameworks.[34–37] To highlight the significance of the luminescent mechanochromic functions of 1 a, 1 b, and 2, we also studied a series of reported classical luminescence coordination polymers or MOFs. Two reported iso-structural coordination polymers formulated as Cu(3-pt) (3-pt = 5-(3-pyridyl)tetrazole) feature a wavelike layer structure without obvious interactions between adjacent layers, and are emissive at room temperature
ligand should lower the LUMO energy level, leading to emissions of lower energy.[33] Based on the above discussion, the unique features of the compounds inspired us to explore the luminescence response gained upon treating with mechanical force stimuli. We anticipated that the interlayer interactions (Cu···Cu interactions, and p–p interactions) in the structures should change upon external force stimuli (e.g., cooling and grinding), causing modification of the emissions of the compounds. When crystalline samples were ground gently in an agate mortar with a pestle, the colors of 1 a, 1 b, and 2 under ambient light remained almost unchanged (Figure S1 and Figure 3 a–c). Upon irradiation with a UV lamp (365 nm), the ground samples of 1 a, 1 b, and 2 emitted in green, yellow, and orange, respectively (Figure 3 a’–c’). When the samples were treated with strong grinding, the luminescence of the samples was converted into relatively weak orange emissions (Figure 3 a“–c”), consistent with their corresponding emission spectra (Figure 2 a’–c’). The emissive lifetime measurements (see Table S1 in the Supporting Information) reveal that the lifetimes of the emissions (at ms scale, double-exponential decay) are very similar for all samples (gently ground or strongly ground) for 1 a, 1 b, and 2, a fact that indicates that grinding does not change the origin of their luminescence. The X-ray powder diffraction patterns of as-synthesized crystalline samples fit well with their corresponding simulated ones, indicating that their structures do not change under gentle grinding (Figures S9, S10, and S11 in the Supporting Information). However, the prominent peaks disappeared when the samples were treated with strong grinding, indicating the loss of crystalline form to an amorphous state (curves c in Figures S9, S10, and S11). The resulting amorphous state from strong grinding is likely caused by the disordered sliding be-
Figure 3. Photos showing the luminescence changes of 1 a, 1 b, and 2 upon gentle and strong grinding at room temperature. a–c) Gently ground samples of 1 a (a), 1 b (b), and 2 (c) under ambient light; a’–c’) samples of (a), (b), and (c) under a UV lamp (365 nm) at room temperature; a’’–c’’) samples of strongly ground 1 a, 1 b, and 2 under a UV lamp (365 nm). Chem. Eur. J. 2014, 20, 644 – 648
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Communication with emission bands centered at 501 and 533 nm, respectively.[38] After being treated with strong grinding, emissions of both isomers did not change (Figures S18 and S20 in the Supporting Information), their crystalline forms were also unchanged, and the X-ray powder patterns are in accord with those of gently ground samples (Figures S19 and S21 in the Supporting Information). Similar treatments were also carried out for rigid carboxylate-based luminescent compounds, for example, MOF-69C, MOF-75, and MOF-76.[39] No detectable mechanochromic phenomena were found (Figures S22–S27 in the Supporting Information) in these emissive MOFs. In summary, we have found that the luminescence of three Cu(4-pt) porous coordination polymers can be modified by grinding. The results indicate that the mechanochromic properties are related to the weak interactions of cuprophilicity and p–p interactions. Our results provide compelling evidence and enlightenment for designing and applying porous coordination polymers as mechanochromic materials. Although our ground amorphous samples can be recovered to crystalline phases, the recovery duration is quite long (about one month). Finding recoverable mechanochromic materials with a fast response remains a challenge for the potential applications of these kinds of materials.
Keywords: coordination polymers · cuprophilicity · grinding · luminescence · mechanochromism
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Experimental Section Syntheses of 1 a and 2 A mixture of Cu2O (0.0143 g, 0.1 mmol), 4-cyanopyridine (0.0105 g, 0.1 mmol), NaN3 (0.0325 g, 0.5 mmol), and toluene solvent (9.0 mL, mixing with a few drops of NH3·H2O) was sealed in a 13 mL Teflonlined stainless steel reactor, which was heated in an oven to 120 8C for 72 h and then cooled to room temperature (25 8C) at a rate of 3 8C·0.5 h1. Light yellow crystals of 1 a were obtained as the main product (0.0302 g, 65 %). The temperature of synthesis of 1 a is not critical; 1 a can also be synthesized at 140, 160, and 180 8C. Crystals of 2 are concomitants of 1 a, the yield is lower (< 10 %), and separation was obtained mechanically.
Synthesis of 1 b The synthesis of 1 b is similar to that of 1 a except the toluene solvent was replaced with ethanol. Light yellow crystals of 1 b were obtained (0.0279 g, 60 %). CCDC-922420, 922421, 922422, 922423, 922424, and 922425 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif
Acknowledgements This work was financially supported by the National Basic Research Program of China (973 Program, 2012CB821706 and 2013CB834803), the National Natural Science Foundation for Distinguished Young Scholars of China (20825102), the National Natural Science Foundation of China (91222202, 21171114, 21101103), Natural Science Foundation of Guangdong Province (S201140004334) and Shantou University.
Chem. Eur. J. 2014, 20, 644 – 648
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Received: October 11, 2013
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Communication
& Luminescent Materials
Luminescent Mechanochromic Porous Coordination Polymers Tian Wen, Xiao-Ping Zhou, De-Xiang Zhang, and Dan Li*[a]
Chem. Eur. J. 2014, 20, 644 – 648
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Communication (G = toluene, 1 a; ethanol, 1 b; toluene, 2; 4-pt = 5-(4-pyridyl)tetrazole), as mechanochromic materials. Compounds 1 a, 1 b, and 2 were prepared by in situ click synthesis, an efficient and green approach documented by our group and others.[23–27] Reactions of Cu2O, 4-cyanopyridine, and NaN3 in different solvents (toluene for 1 a and 2, ethanol for 1 b, for details see the Experimental Section) under solvothermal conditions resulted in crystalline products. Compound 2 was formed as a concomitant of 1 a. 1 b can also be synthesized by the reaction of CuBr and 5-(4-pyridyl)tetrazole under similar conditions.[28] However, 1 a and 2 could not be obtained successfully through direct reactions of CuI and 4-pt. Although the color and shape of crystals 1 a and 2 are similar, the compounds can be distinguished easily under UV light (Figure S1 in the Supporting Information). 1 a, 1 b, and 2 were characterized by single-crystal X-ray diffraction at room temperature (Table 1) and feature similar gridlike 2D layer structures (Figure 1 a). 1 a crystallizes in the triclinic P1¯ space group. The layers are stacked layer-by-layer in an ABAB packing mode, and a similar case is also found for the structure of 1 b (Figure 1 b, c). In both structures, all CuI atoms adopt a triangular coordination geometry, coordinated by three nitrogen atoms of 4-pt. Two CuI atoms are bound by 4pt ligands and form a dimeric structure. The Cu–Cu distances between two adjacent layers (3.220 , 3.254 ) in 1 a are slightly longer than those in 1 b (3.205 , 3.201 ). Guest molecules (toluene in 1 a and ethanol in 1 b) occupy the cavities of the frameworks. They are highly disordered and cannot be determined by X-ray diffraction analysis at room temperature. Crystals of 2 crystallize in the monoclinic P21/c space group (Table 1) as a 2D grid-like framework (Figure 1 d). The layer structure of 2 is topologically identical with 1 a and 1 b except that the framework is slightly “squashed”. However, the layer packing in 2 is distinctively different. The whole structure is constructed from an identical layer-to-layer stacking repeated as an AAA packing mode (Figure 1 d). The shortest Cu–Cu distance between adjacent layers in 2 is 3.654 , longer than those in 1 a (3.220 , 3.254 ). 1 a and 2 could be classified as supramolecular isomers owing to having the same composition and different packing structures. Regarding the ligand, there are unsupported and short Cu–Cu distances between copper(I) centers located in adjacent Cu(4-pt) grid layers, and so there probably exist Cu···Cu interactions (also referred to as cuprophilicity) in 1 a, 1 b, and 2. The crystal structures of 1 a, 1 b, and 2 at a cryogenic temperature (100 K, Table 1) were also determined. Although the space group P1¯ is retained for 1 a at 100 K, all unit-cell dimensions including cell lengths and angles changed significantly, indicating a new phase was formed. When crystals of 1 a were cooled from 293 K to 100 K, sliding and slight squashing occurred in the framework, leading to an alternation of relative positions between two adjacent layers (Figure S2 in the Supporting Information). The crystal transformation between room and cryogenic temperatures is reversible. This phenomenon implies that the 2D Cu–4-pt network is flexible and dynamic enough to be modified by external stimulus (i.e., cooling). In
Abstract: Three 2D luminescent isomeric porous coordination polymers are synthesized and characterized. Their luminescence properties can be modified by grinding and they can act as mechanochromic materials and their properties are probably related to the weak interactions of cuprophilicity and p–p interactions.
Porous coordination polymers (or metal–organic frameworks, MOFs), constructed through coordination bonds between metal ions or metal clusters and organic linkers, have received increasing attention in recent years owing to their potential applications in gas storage and capture,[1, 2] separation,[3] chemical sensing,[4, 5] catalysis,[6] biomedical imaging, and drug delivery.[7] Compared with traditional inorganic porous materials, for example, zeolites, porous coordination polymers are less robust and more flexible owing to their formation from relatively weak coordination bonds. This structural flexibility may endow porous coordination polymers with some unique properties, for example, response to external stimuli, thus providing the possibility of forming dynamic stimuli-responsive materials (or smart materials).[8] These properties are fundamentally important in the applications of sensing and detection. Stimuliresponsive porous coordination polymers triggered by external factors such as the presence of guest molecules, photons, and heating have been extensively studied.[9–11] In contrast, mechanical stimulation remains unexplored for coordination polymers, although solid-state preparations for porous coordination polymers have been established.[12] By being treated with mechanical force (grinding), the luminescence or colors of some coordination oligomers, including gold(I),[13, 14] silver(I),[15] platinum(II),[16–19] and copper(I)[20] complexes, changed distinctively, showing stimuli-response properties. The structures of these complexes underwent modification upon grinding. For instant, some complexes lost crystallinity, sometimes accompanied by the alternation of supramolecular interactions (for example, metallophilicity). Compared with oligomeric metal complexes, coordination polymers feature infinite networks and are more rigid, making it harder to stimulate them by mechanic force.[21, 22] In view of the great challenge to tune rigid coordination polymer structures by mechanical force and the successful cases of mechano-responsive oligomers, we anticipated that low-dimension (1D and 2D) porous coordination polymers supported by weak interactions are probably more suitable to act as mechanical-force-responsive materials than 3D ones. Such low-dimension structures may be easily modified mechanically owing to their weak connections. Herein, we report three 2D luminescent isomeric porous coordination polymers, Cu(4-pt)·G [a] T. Wen, Prof. Dr. X.-P. Zhou, D.-X. Zhang, Prof. Dr. D. Li Department of Chemistry and Research Institute for Biomedical and Advanced Materials, Shantou University Guangdong 515063 (China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303991. Chem. Eur. J. 2014, 20, 644 – 648
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Communication Table 1. Summary of crystal data and refinement results. T [K] 1 a 293 100 1 b 293 100 2 293 100
a []
b []
c []
a [8]
b [8]
g [8]
7.1544(6) 7.5321(3) 7.2059(2) 7.0485(6) 3.6547(5) 3.5974(3)
15.3457(11) 10.6828(5) 15.3431(4) 15.2337(8) 17.127(2) 17.2471(11)
16.4009(13) 10.9671(6) 15.9791(5) 16.0392(8) 13.7926(18) 13.6263(8)
93.540(6) 104.631(4) 95.315(2) 95.618(4) 90.00 90.00
102.074(7) 89.973(4) 96.638(2) 94.709(6) 90.46 90.579(6)
95.765(6) 94.265(4) 98.272(2) 98.855(6) 90.00 90.00
Figure 1. a) Layer structure of the coordination polymers (red = Cu, blue = N, black = C); Packing structures of 1 a (b), 1 b (c), and 2 (d).
contrast, the unit-cell parameters for 1 b and 2 differ little at room temperature and 100 K. Solid samples of 1 a, 1 b, and 2 are emissive at room temperature with emission bands centered at 535, 552, and 615 nm, giving green, yellow, and orange colors, respectively, upon excitation at 395 nm (Figure 2 and Figure S1). The emissions of these complexes are likely from metal-to-ligand charge transfer [Cu!p*(tetrazolate)], and are probably involved with the Cu···Cu [3d!4s] cluster-centered (CC) excited states.[24, 25, 28, 29] Despite the fact that the structures of 1 a show differences at room and cryogenic temperatures, the emission bands at both temperatures do not shift significantly (Figure S3 in the Supporting Information) suggesting that the slight sliding in the 2D structure of 1 a does not affect the emission. The emission maxima of 1 b has a redshift of about 17 nm compared with that of 1 a, probably owing to the existence of the different guest solvent in the host framework.[28, 30] Singlecrystal X-ray study of 1 a at 100 K shows that there is no p–p interaction existing between toluene and ligand 4-pt (Figure S4 in the Supporting Information). Emission spectra of 1 a before and after removal of the guest solvent under vacuum at 120 8C Chem. Eur. J. 2014, 20, 644 – 648
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are almost identical (Figure S5 in the Supporting InformaSpace R1 WR2 V [3] tion), demonstrating that the group influence of the non-polar toluene on the structure and lu1745.6(2) P1¯ 0.0628 0.1502 851.33(7) P1¯ 0.0351 0.0713 minescence of the host 1725.80(9) P1¯ 0.0500 0.1497 framework is limited. In 1 b, 1685.22(19) P1¯ 0.0614 0.0969 as a polar molecule, ethanol 863.33(19) P21/c 0.1059 0.3016 creates hydrogen bonds with 845.40(10) P21/c 0.0728 0.1971 the nitrogen atoms of the tetrazole group in 4-pt (Figure S6 in the Supporting Information). The interaction between the guest ethanol and the 4-pt of the host framework leads to the redshift of the emission band. There is a large redshift in the emission of 2 relative to those of 1 a (80 nm) and 1 b (63 nm). Careful examination finds that two structural factors from the AAA packing style may cause the redshift: i) Aggregation by Cu···Cu interactions. Although the shortest Cu···Cu separation of 3.654(4) in 2 is longer than those inside double-layers (AB) in 1 a and 1 b, all copper(I) atoms in 2 are involved in Cu···Cu interactions forming polymeric Cu···Cu chains. In contrast, the Cu···Cu interactions in 1 a and 1 b between two double-layers are much longer (Figures S7 and S8 in the Supporting Information). It has been reported that Cu···Cu interactions play an important role in influencing the luminescence properties of copper(I) aggregates.[14, 20, 29, 31, 32] In a copper(I) cluster-centered (CC) excited state, when the Cu–Cu interactions become stronger, the emission energy becomes lower, resulting in an emission band with longer wavelength. ii) Strong p–p interactions in ligands between the adjacent layers. The 4-pt ligands are closely packed in 2 with large overlap and short distance (3.654 , Figure S8), indicating strong p–p interactions between the adjacent layers. However, 4-pt in 1 a and 1 b adopts a staggered arrangement with weaker interactions between adjacent layers than those in 2 (Figure 1 a, b, Figure S7). Experimental and theoretical results revealed that the strong p–p interactions of the
Figure 2. Emission spectra of 1 a (a, solid), 1 b (b, dot), and 2 (c, dash) in the crystalline state, and strongly ground samples of 1 a (a’, dash dot), 1 b (b’, short dot) and 2 (c’, short dash).
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Communication tween the Cu(4-pt) layers in the structures. The sliding should be accompanied with alterations of the supported weak interactions (Cu···Cu and p–p interactions), thus changing the photoluminescence properties of the compounds. It has been reported that the crystalline phase and emission properties of some ground mechanochromic oligomer metal complexes can be recovered upon exposure to solvent or heating.[13–20] PXRD patterns of ground samples of 1 a, 1 b, and 2 did not show prominent peaks after the samples were treated with organic solvents (toluene for 1 a, 2, and ethanol for 1 b) for two days. This is probably due to the fact that the coordination polymer samples cannot dissolve in organic solvents, and that recrystallization barely occurred, unlike the case for metal oligomers. Interestingly, ground solid of 1 b immediately showed bright yellow luminescence (in UV light) when one drop of ethanol was added (Figure S12 in the Supporting Information); however, similar phenomena were not observed in the cases of 1 a and 2 treated with toluene. Solidstate photoluminescence measurements found that ground solid of 1 b treated with ethanol showed a broad emission band centered at 562 nm upon excitation at 296 nm (Figure S13 in the Supporting Information). Thermogravimetric analysis (TGA) found that guest molecules of 1 b were lost during grinding (Figure S14 in the Supporting Information). The loss of guest molecules should leave voids for subsequent occupation by ethanol molecules, thus, inducing gliding of Cu(4-pt) layers and leading to luminescence change. When the sample was completely dry, the luminescence changed to dark orange. This transformation can be repeated reversibly at least three times. Thermogravimetric curves for samples 1 a and 2 before and after grinding do not show significant change (Figures S15 and S16 in the Supporting Information), revealing that the toluene guest molecules are only partially removed by grinding owing to the relatively higher boiling point of toluene than ethanol. To test the recovery of the crystalline form of the ground samples in organic solvents, we immersed each ground sample (1 a, 1 b, and 2) in ethanol, chloroform, and toluene, respectively, and found that the recovery in chloroform occurred after one month, an observation that was confirmed by powder X-ray diffraction studies (Figures S9, S10, and S11). The luminescence properties of the ground 1 a, 1 b, and 2 samples are also recovered entirely for the samples immersed in chloroform, the only notable change being that the spectral peaks become slightly broader (Figure S17 in the Supporting Information). These properties are probably due to the inherent self-restore function of coordination polymers under solvent stimuli. The phenomenon of transformation from amorphous to crystalline state has been well documented in reported coordination polymers and metal frameworks.[34–37] To highlight the significance of the luminescent mechanochromic functions of 1 a, 1 b, and 2, we also studied a series of reported classical luminescence coordination polymers or MOFs. Two reported iso-structural coordination polymers formulated as Cu(3-pt) (3-pt = 5-(3-pyridyl)tetrazole) feature a wavelike layer structure without obvious interactions between adjacent layers, and are emissive at room temperature
ligand should lower the LUMO energy level, leading to emissions of lower energy.[33] Based on the above discussion, the unique features of the compounds inspired us to explore the luminescence response gained upon treating with mechanical force stimuli. We anticipated that the interlayer interactions (Cu···Cu interactions, and p–p interactions) in the structures should change upon external force stimuli (e.g., cooling and grinding), causing modification of the emissions of the compounds. When crystalline samples were ground gently in an agate mortar with a pestle, the colors of 1 a, 1 b, and 2 under ambient light remained almost unchanged (Figure S1 and Figure 3 a–c). Upon irradiation with a UV lamp (365 nm), the ground samples of 1 a, 1 b, and 2 emitted in green, yellow, and orange, respectively (Figure 3 a’–c’). When the samples were treated with strong grinding, the luminescence of the samples was converted into relatively weak orange emissions (Figure 3 a“–c”), consistent with their corresponding emission spectra (Figure 2 a’–c’). The emissive lifetime measurements (see Table S1 in the Supporting Information) reveal that the lifetimes of the emissions (at ms scale, double-exponential decay) are very similar for all samples (gently ground or strongly ground) for 1 a, 1 b, and 2, a fact that indicates that grinding does not change the origin of their luminescence. The X-ray powder diffraction patterns of as-synthesized crystalline samples fit well with their corresponding simulated ones, indicating that their structures do not change under gentle grinding (Figures S9, S10, and S11 in the Supporting Information). However, the prominent peaks disappeared when the samples were treated with strong grinding, indicating the loss of crystalline form to an amorphous state (curves c in Figures S9, S10, and S11). The resulting amorphous state from strong grinding is likely caused by the disordered sliding be-
Figure 3. Photos showing the luminescence changes of 1 a, 1 b, and 2 upon gentle and strong grinding at room temperature. a–c) Gently ground samples of 1 a (a), 1 b (b), and 2 (c) under ambient light; a’–c’) samples of (a), (b), and (c) under a UV lamp (365 nm) at room temperature; a’’–c’’) samples of strongly ground 1 a, 1 b, and 2 under a UV lamp (365 nm). Chem. Eur. J. 2014, 20, 644 – 648
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Communication with emission bands centered at 501 and 533 nm, respectively.[38] After being treated with strong grinding, emissions of both isomers did not change (Figures S18 and S20 in the Supporting Information), their crystalline forms were also unchanged, and the X-ray powder patterns are in accord with those of gently ground samples (Figures S19 and S21 in the Supporting Information). Similar treatments were also carried out for rigid carboxylate-based luminescent compounds, for example, MOF-69C, MOF-75, and MOF-76.[39] No detectable mechanochromic phenomena were found (Figures S22–S27 in the Supporting Information) in these emissive MOFs. In summary, we have found that the luminescence of three Cu(4-pt) porous coordination polymers can be modified by grinding. The results indicate that the mechanochromic properties are related to the weak interactions of cuprophilicity and p–p interactions. Our results provide compelling evidence and enlightenment for designing and applying porous coordination polymers as mechanochromic materials. Although our ground amorphous samples can be recovered to crystalline phases, the recovery duration is quite long (about one month). Finding recoverable mechanochromic materials with a fast response remains a challenge for the potential applications of these kinds of materials.
Keywords: coordination polymers · cuprophilicity · grinding · luminescence · mechanochromism
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Experimental Section Syntheses of 1 a and 2 A mixture of Cu2O (0.0143 g, 0.1 mmol), 4-cyanopyridine (0.0105 g, 0.1 mmol), NaN3 (0.0325 g, 0.5 mmol), and toluene solvent (9.0 mL, mixing with a few drops of NH3·H2O) was sealed in a 13 mL Teflonlined stainless steel reactor, which was heated in an oven to 120 8C for 72 h and then cooled to room temperature (25 8C) at a rate of 3 8C·0.5 h1. Light yellow crystals of 1 a were obtained as the main product (0.0302 g, 65 %). The temperature of synthesis of 1 a is not critical; 1 a can also be synthesized at 140, 160, and 180 8C. Crystals of 2 are concomitants of 1 a, the yield is lower (< 10 %), and separation was obtained mechanically.
Synthesis of 1 b The synthesis of 1 b is similar to that of 1 a except the toluene solvent was replaced with ethanol. Light yellow crystals of 1 b were obtained (0.0279 g, 60 %). CCDC-922420, 922421, 922422, 922423, 922424, and 922425 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif
Acknowledgements This work was financially supported by the National Basic Research Program of China (973 Program, 2012CB821706 and 2013CB834803), the National Natural Science Foundation for Distinguished Young Scholars of China (20825102), the National Natural Science Foundation of China (91222202, 21171114, 21101103), Natural Science Foundation of Guangdong Province (S201140004334) and Shantou University.
Chem. Eur. J. 2014, 20, 644 – 648
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Received: October 11, 2013
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