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Cite this: Chem. Commun., 2014, 50, 15956
Mechanical grinding of a single-crystalline metal–organic framework triggered emission with tunable violet-to-orange luminescence†
Received 21st October 2014, Accepted 31st October 2014
Jian-Ke Sun, Cheng Chen, Li-Xuan Cai, Cai-Xia Ren, Bin Tan and Jie Zhang*
DOI: 10.1039/c4cc08316d www.rsc.org/chemcomm
A metal–organic framework (MOF) featuring intriguing Borromean entanglement exhibits a unique mechanochromic luminescence with on–off switching. The concomitant excitation wavelengthdependent emission behavior can be utilized to tune the emission color from violet to orange.
Mechanochromic luminescent (piezochromic) material is a kind of ‘‘smart’’ or ‘‘intelligent’’ materials whose emission behavior changes in response to external pressure or other mechanical forces.1 It shows great potential for applications in information storage, display devices and extremely significant pressure sensors in various fields.2 One of the promising approaches to realize the mechanochromic luminescence is to control the molecular packing in the solid state via an external pressure stimulus.3 Different from conventional chemical reaction-induced luminescence switching,4 this approach allows achievement of high reversibility and reproducibility of piezochromic transition under low pressure, which makes it more attractive in practical applications. Despite the advance in material synthesis, the rational design of such material is still a great challenge. Besides, a high visual contrast is a prerequisite for practical use. However, the conventional two-color luminescence switching is difficult to achieve this purpose. To date, only a few mechanochromic materials with on–off switching have been reported, and all of them are based on purely organic molecules.5 Recently, an intriguing strategy has been suggested by Luo et al. through directly blending a fluorescent donor (D) and a non-emissive acceptor (A) to form a binary complex, which exhibits an on–off mechanochromic luminescence via external pressure-induced phase separation.5a Although such an approach is promising to achieve a high contrast luminescence, the less predicable molecular packing as well as the loss of illustration of structure transformation greatly limit the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details, characterization and additional structural data. CCDC 993232. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc08316d
15956 | Chem. Commun., 2014, 50, 15956--15959
precise control of fluorescence emission. Thus, the development of a new kind of material and design principle is highly desirable. Metal–organic framework (MOF) as a new kind of hybrid material has received much attention over the last few years.6 The modular synthesis process makes this kind of material highly designable and controllable at the molecular scale.7 Besides, the material is also attractive owing to its unique softness, which may be potentially used for developing stimuli-responsive devices.8 To date, although numerous luminescent MOF materials have been reported,9 the exploration of mechanochromic MOFs is still quite scare.10 We are interested in constructing functional materials based on pyridinium derivatives. Recently, we have synthesized a series of pyridinium molecules and discovered the improved luminescence behavior of this family owing to the simultaneous incorporation of both electron-rich (benzoate) and -deficient units (pyridinium ring) in a single molecular skeleton.11 Inspired by the recently reported donor–acceptor charge transfer (CT) interaction-controlled luminescence, together with the high controllability of the intermolecular distance and angle in MOF materials,12 it can be conceived that the utilization of such pyridinium ligands to construct metal–organic framework materials would be conducive to define the molecular packing of organic chromophores and thus manipulate the intermolecular CT interactions between donor and acceptor units. We anticipate that the strength of intermolecular CT interactions in the framework can be tuned by external force, which in turn leads to a change in its luminescent behavior. Herein, a zwitterionic pyridinium ligand Bpebc featuring a semi-rigid skeleton is designed (Scheme S1, ESI†) and assembled with metal ions to yield a highly entangled MOF material with a Borromean topology {[Eu(Bpebc)1.5(SO4)(H2O)2](OH)16.5H2O}n 1 (H2BpebcCl2 = 1,1 0 -bis-(4-carboxybenzyl)-trans-1,2-bis(4-pyridinium)ethylene dichloride). Interestingly, the single-crystalline MOF is highly sensitive to an external pressure stimulus and exhibits unique on–off mechanochromic luminescent switching. Compared with conventional organic mechanochromic luminescent materials, MOFs featuring high crystallinity and ‘softness’ provide an opportunity to detect the structure change after an external
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pressure stimulus, which has more advantages for illustrating the structure–property relationship at a molecular scale. The present work is the first example that the MOF material displays on–off mechanochromic luminescent switching without the introduction of additional stimuli. The highly visual contrast (102) can be easily detected by naked eyes. Attractively, the ground sample g-1 exhibits an excitation wavelength-dependent emission behavior with the color ranging from violet to orange, which has not yet been observed in mechanochromic luminescence materials. The self-assembly of neutral Bpebc with Eu2(SO4)3 gives compound 1. X-ray single crystal diffraction reveals that 1 crystallizes in a trigonal space group R3% and possesses a 2D - 3D n-fold Borromean entangled topology. The Bpebc ligand with a C2 symmetry bridges two Eu(III) ions through two terminal carboxylate groups, and each metal ion serves as a 3-connection node in the framework, thus creating an infinite 2D honeycomb-like (6, 3) network that contains large edge-sharing pseudo-hexagonal rings with a metal ion at each corner and a Bpebc molecule at each edge. The Bpebc ligands adopt trans configurations to form non-planar hexagonal rings with chair conformations (Fig. 1a and b). The SO42 anions graft on the metal centers in chelating coordination modes. The corresponding inner-diameter of the ring is 29.69 Å, as measured between opposite metal corners. One intriguing feature of the present structure is the formation of a Borromean entanglement. As shown in Fig. 1c, the adjacent three layers are fully entangled with each other but none of them are interlocked. One can clarify this point by observing that the red ring is above the green one, the blue ring above the red one, but the green ring lies above the blue one. Such a non-interpenetrated group is a typical characteristic of Borromean topology. The adjacent three layers arrange in an – ABCABC – manner (Fig. 1d and Fig. S1 in ESI†), leading to a so-called n-fold Borromean linkage (2D - 3D). Despite the formation of entanglement in the present framework, a large void with an irregular butterfly-like pore window is still available in 1 (Fig. 1e). PLATON calculation reveals that the free volume of pore space is 31.9%, which is occupied by charge-balancing anions as well as water molecules.
Fig. 1 (a) Top and (b) side view of the pseudo-hexagonal ring in 1. The SO42 anions on the metal centers are omitted for clarity. (c) Space-filling representation of the Borromean link of three separate hexagonal rings (shown as red, blue and green). (d) Topological view of six adjacent honeycomb (hcb) nets in n-fold Borromean entanglement. Three thick hcb nets and three thin hcb nets form two Borromean weaves, which further interlock each other between thick green and thin blue nets around the pink circles. (e) The 1D open channel with a butterfly-like shape formed by Borromean entanglements.
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Fig. 2 Photos showing luminescence changes upon grinding single crystals at room temperature. (a and d) Single crystals and ground samples of 1 under ambient light. (b and e) Single crystals and ground samples of 1 under UV light. (c and f) Recrystallized single crystals and the corresponding ground samples shaped as ‘‘F’’ with a spatula under UV light.
The single crystal of 1 is non-luminescent under UV light (l = 365 nm) as shown in Fig. 2. Surprisingly, upon grinding 1 with a spatula on a piece of filter paper, the powdered sample g-1 exhibits a purplish blue luminescence. The change in luminescent intensity is large enough to be easily distinguished by naked eyes. Such luminescence sensitivity to external pressure is rarely reported in crystalline MOF materials. Upon dissolution of the crushed powder in hot water and subsequent solvent evaporation, the compound reverts to the initial non-emissive state (Fig. 2c). Fig. 2f shows a photographic image of the ‘F’ shape after grinding the recrystallized single crystals, indicating a reversible mechanochromism in 1. Spectral measurements reveal that the ground sample g-1 exhibits an interesting excitation wavelength-dependent emission behavior (Fig. 3). Upon excitation at 380 nm, an intense emission profile with a peak centered at 425 nm is observed (quantum yield: 3.3%). Besides, two small peaks with weak intensities appear at 594 and 618 nm, respectively. Upon increasing the excitation wavelength to 410 nm, a broad profile in the region of 500–750 nm appears. Moreover, the emission band shows slight red-shift with a further increase in
Fig. 3 (a) The excitation wavelength-dependent emission behaviors of ground sample g-1. (b) CIE 1931 chromaticity diagram. The black dots signify the luminescent color coordinates for the corresponding states: (a) (0.23, 0.16), lex = 290 nm, (b) (0.17, 0.09), lex = 380 nm, (c) (0.49, 0.48), lex = 410 nm, (d) (0.53, 0.45), lex = 465 nm, and (e) (0.59, 0.41), lex = 520 nm.
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excitation wavelengths. Finally, an emission centered at 620 nm is observed upon excitation at 520 nm. The tunability of the emission color is further revealed by the Commission Internationale de I’Eclairage (CIE) calculation. As shown in Fig. 3b, the emission color can be tuned from violet (0.23, 0.16) to orange (0.59, 0.41), with blue (0.17, 0.09), yellow (0.49, 0.48) and yellow orange (0.53, 0.45) in between. It should be noted that such a unique emission behavior has never been reported in mechanochromic luminescent materials. To investigate the origin of the luminescence turn-on process, we first check the luminescence behavior of the H2BpebcCl2 ligand since the luminescence of MOF materials often stems from the ligand transition.9 The H2BpebcCl2 ligand is nonluminescent in the solid state under UV light, but exhibits a dual-emission with peaks centered at 500 and 625 nm in aqueous solution (lex = 375 nm). Further excitation at 505 nm gives a single emission with a peak centered at 640 nm (Fig. S2, ESI†). Such luminescence behavior is similar to that of cationic stilbazolium derivatives containing both the electron donor and acceptor in solution.11a,13 It is noteworthy that the excitation at 380 nm leads to almost white light emission with CIE coordinate of (0.35, 0.37) which is very close to the coordinate (0.33, 0.33) of the standard white-light illumination (Fig. S2 and S3, ESI†). Comparing the emission behavior of g-1 with that of the H2BpebcCl2 ligand in solution, it can be envisaged that the present mechanochromism is likely related to the ligand emission. So the questions arise: why did the luminescence dramatically decrease in a single-crystalline MOF? Why an external pressure stimulus could effectively turn on the luminescence? What happened in these processes? Since the H2BpebcCl2 ligand displays an obvious luminescence in the solution state, the quenched luminescence in a single-crystalline state should be related to its packing geometry. Detailed structural analysis reveals that the conjugated pyridinium moiety is sandwiched between adjacent benzoate groups, with an interplanar distance of 3.5 Å. Such a specific supramolecular arrangement between electron-rich delocalized units (benzoate) and electron-deficient units (pyridinium ring) has significant consequences for the intermolecular CT interaction.14 It is reported that the formation of the strong intermolecular CT state will effectively depopulate the emitting state, resulting in dramatic luminescence quenching.12a,b,15 The calculation of charge population reveals that the natural charge on a single pyridinium ring is +0.37, while the natural charge on a carboxylate group is 0.59 (Table S1, ESI†). Both values are much lower than their apparent charges of +1 and 1, respectively, indicating that their ground states should be associated with a strong intermolecular interaction.16 Moreover, the strong intermolecular CT interactioninduced quenching effect could also be demonstrated by guestdependent luminescence behaviors. Since the host framework g-1 contains electron-deficient pyridinium units on the pore wall, a series of guest molecules possessing different electron-donating abilities are introduced into porous space to evaluate the effect of intermolecular CT interaction on the luminescence of g-1. After immersing g-1 in the solution containing the guest with strong electron-donating ability such as phloroglucinol, the color of g-1
15958 | Chem. Commun., 2014, 50, 15956--15959
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quickly turns to red accompanied with quenched luminescence (Fig. S4, ESI†), and the quenched luminescence cannot recover after succeeding grinding of the guest-containing sample. Such a distinct color change of g-1 after incorporation of electrondonating molecules is similar to the observations reported in some host–guest compounds containing bipyridinium units (e.g., methylviolegen, MV2+) and electron donors,14b,17 suggesting the occurrence of a CT interaction between the electrondeficient pyridinium moiety and phloroglucinol, which can be verified by the appearance of a strong and broad shoulder band in the UV-vis spectrum (Fig. S5, ESI†). The elemental analysis also confirms the existence of phloroglucinol in the host framework (ESI†). In fact, the host g-1 can even detect the trinitrophenol (PA) molecule through quenched luminescence (Fig. S4, ESI†), although the electron-donating ability of the hydroxyl group in PA decreases due to the electron-withdrawing effect of nitro groups around it. In comparison, the incorporation of a guest molecule such as nitrobenzene only containing nitro groups almost has no effect on the original color of g-1 as well as its luminescence (Fig. S4, ESI†). As the strong intermolecular CT interactions are prone to quench the luminescence in 1, the external pressure-induced emission may be relevant to a weakening of such interactions to some extent, which liberates the luminescence of the framework in g-1. To further demonstrate our hypothesis, the powder X-ray diffraction (PXRD) patterns were recorded for both 1 and g-1 (Fig. S6, ESI†). Compared with the calculated PXRD pattern from the single crystal structure data, the position shift of some peaks can be observed after grinding, suggesting that a phase transformation occurs in this process. It is noteworthy that the elemental analysis of 1 and g-1 gives the same composition (Experimental section in ESI†), indicating that no chemical reaction related to adsorption or removal of guest molecules is induced by grinding. To reveal the exact movement of the components in the framework, the subtle structural transformation that involves the lattice plane movement was analyzed based on the shift of PXRD patterns. The peaks for (210) at 10.021 (d210 spacing of 4.438 Å) and (213) at 13.171 (d213 spacing of 3.389 Å), which correspond to the lattice faces intercalated between adjacent interchain benzoate and pyridinium moieties, move to 9.851 (d210 spacing of 4.516 Å) and 13.041 (d213 spacing of 3.426 Å), respectively, indicating the expansion of the corresponding lattice by 0.088 and 0.037 Å, respectively. Such expansions along the stacking direction of the benzoate and pyridinium moieties would directly decrease the strength of intermolecular CT interactions. Moreover, the PXRD pattern of ground sample g-1 shows expansion movements of the (312) plane at 16.361 (d312 spacing of 2.741 Å), and the (300) plane at 17.411 (d300 spacing of 2.581 Å) to 16.151 (d312 spacing of 2.776 Å) and 17.071 (d300 spacing of 2.631 Å), respectively, which correspond to the sliding of adjacent chains to further weaken the intermolecular CT interactions between the electro-rich and -deficient species (Fig. S6, ESI†). Such specific structural transformations liberate the suppressed fluorescence of the original framework, thereby resulting in highly visual contrast upon an external pressure stimulus.
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Although the external pressure stimulus liberates the fluorescence of the framework, it is noteworthy that the luminescence behavior of g-1 is slightly different from that of the ligand in solution. As shown in Fig. 3a, only a series of sharp lines with weak intensity are observed in a region of 500–750 nm upon excitation at 290 or 380 nm. Since g-1 contains Eu(III) ion in the framework, the emissions in the region of 500–750 nm can be ascribed to the influence of the metal ion. Upon excitation at 290 nm, the emission bands centered at 545, 590, 615 and 690 nm are quite similar to the characteristic emissions of Eu(III), which can be attributed to the f–f 5D0 - 7Fj ( j = 1–4) transitions, respectively. The low intensity of these bands suggests that the ligand is not an efficient sensitizer for the Eu(III) ion. We have employed the Latva’s empirical rule18 to assess the antenna efficiency of the ligand. It is well recognized that the energylevel match between the triplet state of the ligand to the 5Dj state of the Ln(III) cation is one of the key factors that governs the luminescence efficiency of Ln(III) complexes.19 Latva’s empirical rule states that an optimal ligand-to-metal energy transfer process for Eu(III) requires an energy gap DE in gate 2500–4000 cm1. However, in the present case, the ligand (500 nm: 2.65 104 cm1) is not a good sensitizer due to a higher DE value (9500 cm1), leading to weak emissions of Eu(III) in the region of 500–750 nm upon excitation at 290 or 380 nm. In conclusion, a new MOF material with a Borromean entanglement has been synthesized. Mechanical grinding of a single-crystalline MOF triggers an emission, leading to high-contrast luminescence switching that can be easily detected by naked eyes. This process is attributed to the external pressure-induced shift of the lattice faces, which weakens the intermolecular CT interactions and liberates the emission in a ground sample. Moreover, the ground sample exhibits a rarely reported excitation wavelength-dependent emission behavior with the color ranging from violet to orange. The present work may enlighten an avenue to rationally design and develop functional mechanochromic switching materials in future. This work was supported by the NNSF of China (Grant No. 21271173/20973171).
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Notes and references 1 Y. Sagara and T. Kato, Nat. Chem., 2009, 1, 605. 2 Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu and J. Xu, Chem. Soc. Rev., 2012, 41, 3878. 3 (a) S. J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M. G. Choi, D. Kim and S. Y. Park, J. Am. Chem. Soc., 2010, 132, 13675; (b) G. Q. Zhang, J. W. Lu, M. Sabat and C. L. Fraser, J. Am. Chem. Soc., 2010, 132, 2160; (c) M. J. Teng, X. R. Jia, X. F. Chen and Y. Wei,
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Angew. Chem., Int. Ed., 2012, 51, 6398; (d) S. Perruchas, X. F. Le Goff, S. Maron, I. Maurin, F. Guillen, A. Garcia, T. Gacoin and J. P. Boilot, J. Am. Chem. Soc., 2010, 132, 10967; (e) V. N. Kozhevnikov, B. Donnio and D. W. Bruce, Angew. Chem., Int. Ed., 2008, 47, 6286; ( f ) Y. Sagara and T. Kato, Angew. Chem., Int. Ed., 2008, 47, 5175; (g) H. Ito, T. Saito, N. Oshima, N. Kitamura, S. Ishizaka, Y. Hinatsu, M. Wakeshima, M. Kato, K. Tsuge and M. Sawamura, J. Am. Chem. Soc., 2008, 130, 10044; (h) K. Nagura, S. Saito, H. Yusa, H. Yamawaki, H. Fujihisa, H. Sato, Y. Shimoikeda and S. Yamaguchi, J. Am. Chem. Soc., 2013, 135, 10322. D. A. Davis, A. Hamilton, J. Yang, L. D. Cremar, D. V. Gough, S. L. Potisek, M. T. Ong, P. V. Braun, T. J. Martı´nez, S. R. White, J. S. Moore and N. R. Sottos, Nature, 2009, 459, 68. (a) J. Luo, L. Y. Li, Y. L. Song and J. Pei, Chem. – Eur. J., 2011, 17, 10515; (b) M. S. Kwon, J. Gierschner, S. J. Yoon and S. Y. Park, Adv. Mater., 2012, 24, 5487; (c) M. J. Teng, X. R. Jia, S. Yang, X. F. Chen and Y. Wei, Adv. Mater., 2012, 24, 1255; (d) X. L. Luo, J. N. Li, C. H. Li, L. P. Heng, Y. Q. Dong, Z. P. Liu, Z. S. Bo and B. Z. Tang, Adv. Mater., 2011, 23, 3261. H. Li, M. Eddaoudi, M. O’Keeffe and O. M. Yaghi, Nature, 1999, 402, 276. M. Eddaoudi, J. Kim, N. L. Rosi, D. T. Vodak, J. Wachter, M. O’Keeffe and O. M. Yaghi, Science, 2002, 295, 469. S. Horike, S. Shimomura and S. Kitagawa, Nat. Chem., 2009, 1, 695. (a) Y. J. Cui, Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev., 2012, 112, 1126; (b) M. D. Allendorf, C. A. Bauer, R. K. Bhakta and R. J. T. Houk, Chem. Soc. Rev., 2009, 38, 1330. (a) B. Tzeng, T. Chang and H. Sheu, Chem. – Eur. J., 2010, 16, 9990; (b) T. Wen, D. X. Zhang, J. Liu, R. Lin and J. Zhang, Chem. Commun., 2013, 49, 5660; (c) T. Wen, X. P. Zhou, D. X. Zhang and D. Li, Chem. – Eur. J., 2014, 20, 644. (a) J. K. Sun, W. Li, C. Chen, C. X. Ren, D. M. Pan and J. Zhang, Angew. Chem., Int. Ed., 2013, 52, 6653; (b) X. H. Jin, J. Wang, J. K. Sun, H. X. Zhang and J. Zhang, Angew. Chem., Int. Ed., 2011, 50, 1149; (c) J. K. Sun, X. H. Jin, L. X. Cai and J. Zhang, J. Mater. Chem., 2011, 21, 17667. (a) Y. Takashima, V. M. Martinez, S. Furukawa, M. Kondo, S. Shimomura, H. Uehara, M. Nakahama, K. Sugimoto and S. Kitagawa, Nat. Commun., 2011, 2, 168; (b) B. D. McCarthy, E. R. Hontz, S. R. Yost, T. Van Voorhis ˘, J. Phys. Chem. Lett., 2013, 4, 453; (c) P. L. Feng, K. Leong and M. Dinca and M. D. Allendorf, Dalton Trans., 2012, 41, 8869. B. Wandelt, P. Turkewitsch, B. R. Stranix and G. D. Darling, J. Chem. Soc., Faraday Trans., 1995, 91, 4199. (a) H. Yoshikawa and S. Nishikiori, Dalton Trans., 2005, 3056; (b) Q. X. Yao, Z. F. Ju, X. H. Jin and J. Zhang, Inorg. Chem., 2009, 48, 1266; (c) P. W. Carter, S. G. DiMagno, J. D. Porter and A. Streitwieser, J. Phys. Chem., 1993, 97, 1085; (d) J. H. Kim, S. V. Lindeman and J. K. Kochi, J. Am. Chem. Soc., 2001, 123, 4951. W. R. Ware and H. P. Richter, J. Chem. Phys., 1968, 48, 1595. Z. W. Tang, X. W. Chen, H. Chen, L. M. Wu and X. B. Yu, Angew. Chem., Int. Ed., 2013, 52, 5832. (a) A. S. Jalilov, S. Patwardhan, A. Singh, T. Simeon, A. A. Sarjeant, G. C. Schatz and F. D. Lewis, J. Phys. Chem. B, 2014, 118, 125; (b) Q. X. Yao, L. Pan, X. H. Jin, J. Li, Z. F. Ju and J. Zhang, Chem. – Eur. J., 2009, 15, 11890; (c) J. W. Lee, K. Kim, S. W. Choi, Y. Ho Ko, S. Sakamoto, K. Yamaguchi and K. Kim, Chem. Commun., 2002, 2692; (d) H. Yoshikawa, S. Nishikiori, K. Suwinska, R. Luboradzki and J. Lipkowski, Chem. Commun., 2001, 1398. (a) M. Latva, H. Takalo, V. M. Mukkala, C. Matachescu, J. C. Rodrı´guezUbis and J. Kankare, J. Lumin., 1997, 75, 149; (b) N. Arnaud and J. Georges, Spectrochim. Acta, Part A, 2003, 59, 1829. ¨nzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048. J. C. G. Bu
Chem. Commun., 2014, 50, 15956--15959 | 15959
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COMMUNICATION
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Cite this: Chem. Commun., 2014, 50, 15956
Mechanical grinding of a single-crystalline metal–organic framework triggered emission with tunable violet-to-orange luminescence†
Received 21st October 2014, Accepted 31st October 2014
Jian-Ke Sun, Cheng Chen, Li-Xuan Cai, Cai-Xia Ren, Bin Tan and Jie Zhang*
DOI: 10.1039/c4cc08316d www.rsc.org/chemcomm
A metal–organic framework (MOF) featuring intriguing Borromean entanglement exhibits a unique mechanochromic luminescence with on–off switching. The concomitant excitation wavelengthdependent emission behavior can be utilized to tune the emission color from violet to orange.
Mechanochromic luminescent (piezochromic) material is a kind of ‘‘smart’’ or ‘‘intelligent’’ materials whose emission behavior changes in response to external pressure or other mechanical forces.1 It shows great potential for applications in information storage, display devices and extremely significant pressure sensors in various fields.2 One of the promising approaches to realize the mechanochromic luminescence is to control the molecular packing in the solid state via an external pressure stimulus.3 Different from conventional chemical reaction-induced luminescence switching,4 this approach allows achievement of high reversibility and reproducibility of piezochromic transition under low pressure, which makes it more attractive in practical applications. Despite the advance in material synthesis, the rational design of such material is still a great challenge. Besides, a high visual contrast is a prerequisite for practical use. However, the conventional two-color luminescence switching is difficult to achieve this purpose. To date, only a few mechanochromic materials with on–off switching have been reported, and all of them are based on purely organic molecules.5 Recently, an intriguing strategy has been suggested by Luo et al. through directly blending a fluorescent donor (D) and a non-emissive acceptor (A) to form a binary complex, which exhibits an on–off mechanochromic luminescence via external pressure-induced phase separation.5a Although such an approach is promising to achieve a high contrast luminescence, the less predicable molecular packing as well as the loss of illustration of structure transformation greatly limit the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details, characterization and additional structural data. CCDC 993232. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc08316d
15956 | Chem. Commun., 2014, 50, 15956--15959
precise control of fluorescence emission. Thus, the development of a new kind of material and design principle is highly desirable. Metal–organic framework (MOF) as a new kind of hybrid material has received much attention over the last few years.6 The modular synthesis process makes this kind of material highly designable and controllable at the molecular scale.7 Besides, the material is also attractive owing to its unique softness, which may be potentially used for developing stimuli-responsive devices.8 To date, although numerous luminescent MOF materials have been reported,9 the exploration of mechanochromic MOFs is still quite scare.10 We are interested in constructing functional materials based on pyridinium derivatives. Recently, we have synthesized a series of pyridinium molecules and discovered the improved luminescence behavior of this family owing to the simultaneous incorporation of both electron-rich (benzoate) and -deficient units (pyridinium ring) in a single molecular skeleton.11 Inspired by the recently reported donor–acceptor charge transfer (CT) interaction-controlled luminescence, together with the high controllability of the intermolecular distance and angle in MOF materials,12 it can be conceived that the utilization of such pyridinium ligands to construct metal–organic framework materials would be conducive to define the molecular packing of organic chromophores and thus manipulate the intermolecular CT interactions between donor and acceptor units. We anticipate that the strength of intermolecular CT interactions in the framework can be tuned by external force, which in turn leads to a change in its luminescent behavior. Herein, a zwitterionic pyridinium ligand Bpebc featuring a semi-rigid skeleton is designed (Scheme S1, ESI†) and assembled with metal ions to yield a highly entangled MOF material with a Borromean topology {[Eu(Bpebc)1.5(SO4)(H2O)2](OH)16.5H2O}n 1 (H2BpebcCl2 = 1,1 0 -bis-(4-carboxybenzyl)-trans-1,2-bis(4-pyridinium)ethylene dichloride). Interestingly, the single-crystalline MOF is highly sensitive to an external pressure stimulus and exhibits unique on–off mechanochromic luminescent switching. Compared with conventional organic mechanochromic luminescent materials, MOFs featuring high crystallinity and ‘softness’ provide an opportunity to detect the structure change after an external
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pressure stimulus, which has more advantages for illustrating the structure–property relationship at a molecular scale. The present work is the first example that the MOF material displays on–off mechanochromic luminescent switching without the introduction of additional stimuli. The highly visual contrast (102) can be easily detected by naked eyes. Attractively, the ground sample g-1 exhibits an excitation wavelength-dependent emission behavior with the color ranging from violet to orange, which has not yet been observed in mechanochromic luminescence materials. The self-assembly of neutral Bpebc with Eu2(SO4)3 gives compound 1. X-ray single crystal diffraction reveals that 1 crystallizes in a trigonal space group R3% and possesses a 2D - 3D n-fold Borromean entangled topology. The Bpebc ligand with a C2 symmetry bridges two Eu(III) ions through two terminal carboxylate groups, and each metal ion serves as a 3-connection node in the framework, thus creating an infinite 2D honeycomb-like (6, 3) network that contains large edge-sharing pseudo-hexagonal rings with a metal ion at each corner and a Bpebc molecule at each edge. The Bpebc ligands adopt trans configurations to form non-planar hexagonal rings with chair conformations (Fig. 1a and b). The SO42 anions graft on the metal centers in chelating coordination modes. The corresponding inner-diameter of the ring is 29.69 Å, as measured between opposite metal corners. One intriguing feature of the present structure is the formation of a Borromean entanglement. As shown in Fig. 1c, the adjacent three layers are fully entangled with each other but none of them are interlocked. One can clarify this point by observing that the red ring is above the green one, the blue ring above the red one, but the green ring lies above the blue one. Such a non-interpenetrated group is a typical characteristic of Borromean topology. The adjacent three layers arrange in an – ABCABC – manner (Fig. 1d and Fig. S1 in ESI†), leading to a so-called n-fold Borromean linkage (2D - 3D). Despite the formation of entanglement in the present framework, a large void with an irregular butterfly-like pore window is still available in 1 (Fig. 1e). PLATON calculation reveals that the free volume of pore space is 31.9%, which is occupied by charge-balancing anions as well as water molecules.
Fig. 1 (a) Top and (b) side view of the pseudo-hexagonal ring in 1. The SO42 anions on the metal centers are omitted for clarity. (c) Space-filling representation of the Borromean link of three separate hexagonal rings (shown as red, blue and green). (d) Topological view of six adjacent honeycomb (hcb) nets in n-fold Borromean entanglement. Three thick hcb nets and three thin hcb nets form two Borromean weaves, which further interlock each other between thick green and thin blue nets around the pink circles. (e) The 1D open channel with a butterfly-like shape formed by Borromean entanglements.
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Fig. 2 Photos showing luminescence changes upon grinding single crystals at room temperature. (a and d) Single crystals and ground samples of 1 under ambient light. (b and e) Single crystals and ground samples of 1 under UV light. (c and f) Recrystallized single crystals and the corresponding ground samples shaped as ‘‘F’’ with a spatula under UV light.
The single crystal of 1 is non-luminescent under UV light (l = 365 nm) as shown in Fig. 2. Surprisingly, upon grinding 1 with a spatula on a piece of filter paper, the powdered sample g-1 exhibits a purplish blue luminescence. The change in luminescent intensity is large enough to be easily distinguished by naked eyes. Such luminescence sensitivity to external pressure is rarely reported in crystalline MOF materials. Upon dissolution of the crushed powder in hot water and subsequent solvent evaporation, the compound reverts to the initial non-emissive state (Fig. 2c). Fig. 2f shows a photographic image of the ‘F’ shape after grinding the recrystallized single crystals, indicating a reversible mechanochromism in 1. Spectral measurements reveal that the ground sample g-1 exhibits an interesting excitation wavelength-dependent emission behavior (Fig. 3). Upon excitation at 380 nm, an intense emission profile with a peak centered at 425 nm is observed (quantum yield: 3.3%). Besides, two small peaks with weak intensities appear at 594 and 618 nm, respectively. Upon increasing the excitation wavelength to 410 nm, a broad profile in the region of 500–750 nm appears. Moreover, the emission band shows slight red-shift with a further increase in
Fig. 3 (a) The excitation wavelength-dependent emission behaviors of ground sample g-1. (b) CIE 1931 chromaticity diagram. The black dots signify the luminescent color coordinates for the corresponding states: (a) (0.23, 0.16), lex = 290 nm, (b) (0.17, 0.09), lex = 380 nm, (c) (0.49, 0.48), lex = 410 nm, (d) (0.53, 0.45), lex = 465 nm, and (e) (0.59, 0.41), lex = 520 nm.
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excitation wavelengths. Finally, an emission centered at 620 nm is observed upon excitation at 520 nm. The tunability of the emission color is further revealed by the Commission Internationale de I’Eclairage (CIE) calculation. As shown in Fig. 3b, the emission color can be tuned from violet (0.23, 0.16) to orange (0.59, 0.41), with blue (0.17, 0.09), yellow (0.49, 0.48) and yellow orange (0.53, 0.45) in between. It should be noted that such a unique emission behavior has never been reported in mechanochromic luminescent materials. To investigate the origin of the luminescence turn-on process, we first check the luminescence behavior of the H2BpebcCl2 ligand since the luminescence of MOF materials often stems from the ligand transition.9 The H2BpebcCl2 ligand is nonluminescent in the solid state under UV light, but exhibits a dual-emission with peaks centered at 500 and 625 nm in aqueous solution (lex = 375 nm). Further excitation at 505 nm gives a single emission with a peak centered at 640 nm (Fig. S2, ESI†). Such luminescence behavior is similar to that of cationic stilbazolium derivatives containing both the electron donor and acceptor in solution.11a,13 It is noteworthy that the excitation at 380 nm leads to almost white light emission with CIE coordinate of (0.35, 0.37) which is very close to the coordinate (0.33, 0.33) of the standard white-light illumination (Fig. S2 and S3, ESI†). Comparing the emission behavior of g-1 with that of the H2BpebcCl2 ligand in solution, it can be envisaged that the present mechanochromism is likely related to the ligand emission. So the questions arise: why did the luminescence dramatically decrease in a single-crystalline MOF? Why an external pressure stimulus could effectively turn on the luminescence? What happened in these processes? Since the H2BpebcCl2 ligand displays an obvious luminescence in the solution state, the quenched luminescence in a single-crystalline state should be related to its packing geometry. Detailed structural analysis reveals that the conjugated pyridinium moiety is sandwiched between adjacent benzoate groups, with an interplanar distance of 3.5 Å. Such a specific supramolecular arrangement between electron-rich delocalized units (benzoate) and electron-deficient units (pyridinium ring) has significant consequences for the intermolecular CT interaction.14 It is reported that the formation of the strong intermolecular CT state will effectively depopulate the emitting state, resulting in dramatic luminescence quenching.12a,b,15 The calculation of charge population reveals that the natural charge on a single pyridinium ring is +0.37, while the natural charge on a carboxylate group is 0.59 (Table S1, ESI†). Both values are much lower than their apparent charges of +1 and 1, respectively, indicating that their ground states should be associated with a strong intermolecular interaction.16 Moreover, the strong intermolecular CT interactioninduced quenching effect could also be demonstrated by guestdependent luminescence behaviors. Since the host framework g-1 contains electron-deficient pyridinium units on the pore wall, a series of guest molecules possessing different electron-donating abilities are introduced into porous space to evaluate the effect of intermolecular CT interaction on the luminescence of g-1. After immersing g-1 in the solution containing the guest with strong electron-donating ability such as phloroglucinol, the color of g-1
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quickly turns to red accompanied with quenched luminescence (Fig. S4, ESI†), and the quenched luminescence cannot recover after succeeding grinding of the guest-containing sample. Such a distinct color change of g-1 after incorporation of electrondonating molecules is similar to the observations reported in some host–guest compounds containing bipyridinium units (e.g., methylviolegen, MV2+) and electron donors,14b,17 suggesting the occurrence of a CT interaction between the electrondeficient pyridinium moiety and phloroglucinol, which can be verified by the appearance of a strong and broad shoulder band in the UV-vis spectrum (Fig. S5, ESI†). The elemental analysis also confirms the existence of phloroglucinol in the host framework (ESI†). In fact, the host g-1 can even detect the trinitrophenol (PA) molecule through quenched luminescence (Fig. S4, ESI†), although the electron-donating ability of the hydroxyl group in PA decreases due to the electron-withdrawing effect of nitro groups around it. In comparison, the incorporation of a guest molecule such as nitrobenzene only containing nitro groups almost has no effect on the original color of g-1 as well as its luminescence (Fig. S4, ESI†). As the strong intermolecular CT interactions are prone to quench the luminescence in 1, the external pressure-induced emission may be relevant to a weakening of such interactions to some extent, which liberates the luminescence of the framework in g-1. To further demonstrate our hypothesis, the powder X-ray diffraction (PXRD) patterns were recorded for both 1 and g-1 (Fig. S6, ESI†). Compared with the calculated PXRD pattern from the single crystal structure data, the position shift of some peaks can be observed after grinding, suggesting that a phase transformation occurs in this process. It is noteworthy that the elemental analysis of 1 and g-1 gives the same composition (Experimental section in ESI†), indicating that no chemical reaction related to adsorption or removal of guest molecules is induced by grinding. To reveal the exact movement of the components in the framework, the subtle structural transformation that involves the lattice plane movement was analyzed based on the shift of PXRD patterns. The peaks for (210) at 10.021 (d210 spacing of 4.438 Å) and (213) at 13.171 (d213 spacing of 3.389 Å), which correspond to the lattice faces intercalated between adjacent interchain benzoate and pyridinium moieties, move to 9.851 (d210 spacing of 4.516 Å) and 13.041 (d213 spacing of 3.426 Å), respectively, indicating the expansion of the corresponding lattice by 0.088 and 0.037 Å, respectively. Such expansions along the stacking direction of the benzoate and pyridinium moieties would directly decrease the strength of intermolecular CT interactions. Moreover, the PXRD pattern of ground sample g-1 shows expansion movements of the (312) plane at 16.361 (d312 spacing of 2.741 Å), and the (300) plane at 17.411 (d300 spacing of 2.581 Å) to 16.151 (d312 spacing of 2.776 Å) and 17.071 (d300 spacing of 2.631 Å), respectively, which correspond to the sliding of adjacent chains to further weaken the intermolecular CT interactions between the electro-rich and -deficient species (Fig. S6, ESI†). Such specific structural transformations liberate the suppressed fluorescence of the original framework, thereby resulting in highly visual contrast upon an external pressure stimulus.
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Although the external pressure stimulus liberates the fluorescence of the framework, it is noteworthy that the luminescence behavior of g-1 is slightly different from that of the ligand in solution. As shown in Fig. 3a, only a series of sharp lines with weak intensity are observed in a region of 500–750 nm upon excitation at 290 or 380 nm. Since g-1 contains Eu(III) ion in the framework, the emissions in the region of 500–750 nm can be ascribed to the influence of the metal ion. Upon excitation at 290 nm, the emission bands centered at 545, 590, 615 and 690 nm are quite similar to the characteristic emissions of Eu(III), which can be attributed to the f–f 5D0 - 7Fj ( j = 1–4) transitions, respectively. The low intensity of these bands suggests that the ligand is not an efficient sensitizer for the Eu(III) ion. We have employed the Latva’s empirical rule18 to assess the antenna efficiency of the ligand. It is well recognized that the energylevel match between the triplet state of the ligand to the 5Dj state of the Ln(III) cation is one of the key factors that governs the luminescence efficiency of Ln(III) complexes.19 Latva’s empirical rule states that an optimal ligand-to-metal energy transfer process for Eu(III) requires an energy gap DE in gate 2500–4000 cm1. However, in the present case, the ligand (500 nm: 2.65 104 cm1) is not a good sensitizer due to a higher DE value (9500 cm1), leading to weak emissions of Eu(III) in the region of 500–750 nm upon excitation at 290 or 380 nm. In conclusion, a new MOF material with a Borromean entanglement has been synthesized. Mechanical grinding of a single-crystalline MOF triggers an emission, leading to high-contrast luminescence switching that can be easily detected by naked eyes. This process is attributed to the external pressure-induced shift of the lattice faces, which weakens the intermolecular CT interactions and liberates the emission in a ground sample. Moreover, the ground sample exhibits a rarely reported excitation wavelength-dependent emission behavior with the color ranging from violet to orange. The present work may enlighten an avenue to rationally design and develop functional mechanochromic switching materials in future. This work was supported by the NNSF of China (Grant No. 21271173/20973171).
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