& 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 � 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim644

CommunicationDOI: 10.1002/chem.201303991

Abstract: Three 2D luminescent isomeric porous coordina-tion polymers are synthesized and characterized. Their lu-minescence properties can be modified by grinding andthey can act as mechanochromic materials and their prop-erties are probably related to the weak interactions of cu-prophilicity and p–p interactions.

Porous coordination polymers (or metal–organic frameworks,MOFs), constructed through coordination bonds betweenmetal ions or metal clusters and organic linkers, have receivedincreasing attention in recent years owing to their potentialapplications in gas storage and capture,[1, 2] separation,[3] chemi-cal sensing,[4, 5] catalysis,[6] biomedical imaging, and drug deliv-ery.[7] Compared with traditional inorganic porous materials, forexample, zeolites, porous coordination polymers are lessrobust and more flexible owing to their formation from rela-tively weak coordination bonds. This structural flexibility mayendow porous coordination polymers with some unique prop-erties, for example, response to external stimuli, thus providingthe possibility of forming dynamic stimuli-responsive materials(or smart materials).[8] These properties are fundamentally im-portant in the applications of sensing and detection. Stimuli-responsive porous coordination polymers triggered by externalfactors such as the presence of guest molecules, photons, andheating have been extensively studied.[9–11] In contrast, me-chanical stimulation remains unexplored for coordination poly-mers, although solid-state preparations for porous coordina-tion polymers have been established.[12]

By being treated with mechanical force (grinding), the lumi-nescence or colors of some coordination oligomers, includinggold(I),[13, 14] silver(I),[15] platinum(II),[16–19] and copper(I)[20] com-plexes, changed distinctively, showing stimuli-response proper-ties. The structures of these complexes underwent modifica-tion upon grinding. For instant, some complexes lost crystallin-ity, sometimes accompanied by the alternation of supramolec-ular interactions (for example, metallophilicity). Compared witholigomeric metal complexes, coordination polymers feature in-finite networks and are more rigid, making it harder to stimu-late them by mechanic force.[21, 22]

In view of the great challenge to tune rigid coordinationpolymer structures by mechanical force and the successfulcases of mechano-responsive oligomers, we anticipated thatlow-dimension (1D and 2D) porous coordination polymers sup-ported by weak interactions are probably more suitable to actas mechanical-force-responsive materials than 3D ones. Suchlow-dimension structures may be easily modified mechanicallyowing to their weak connections. Herein, we report three 2Dluminescent isomeric porous coordination polymers, Cu(4-pt)·G

(G = toluene, 1 a ; ethanol, 1 b ; toluene, 2 ; 4-pt = 5-(4-pyridyl)te-trazole), as mechanochromic materials.

Compounds 1 a, 1 b, and 2 were prepared by in situ clicksynthesis, an efficient and green approach documented by ourgroup and others.[23–27] Reactions of Cu2O, 4-cyanopyridine, andNaN3 in different solvents (toluene for 1 a and 2, ethanol for1 b, for details see the Experimental Section) under solvother-mal conditions resulted in crystalline products. Compound 2was formed as a concomitant of 1 a. 1 b can also be synthe-sized by the reaction of CuBr and 5-(4-pyridyl)tetrazole undersimilar conditions.[28] However, 1 a and 2 could not be obtainedsuccessfully through direct reactions of CuI and 4-pt. Althoughthe color and shape of crystals 1 a and 2 are similar, the com-pounds can be distinguished easily under UV light (Figure S1in the Supporting Information).

1 a, 1 b, and 2 were characterized by single-crystal X-ray dif-fraction at room temperature (Table 1) and feature similar grid-like 2D layer structures (Figure 1 a). 1 a crystallizes in the triclin-ic P1̄ space group. The layers are stacked layer-by-layer in anABAB packing mode, and a similar case is also found for thestructure of 1 b (Figure 1 b, c). In both structures, all CuI atomsadopt a triangular coordination geometry, coordinated bythree nitrogen atoms of 4-pt. Two CuI atoms are bound by 4-pt ligands and form a dimeric structure. The Cu–Cu distancesbetween two adjacent layers (3.220 �, 3.254 �) in 1 a are slight-ly 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 theframeworks. They are highly disordered and cannot be deter-mined 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 layerstructure of 2 is topologically identical with 1 a and 1 b exceptthat the framework is slightly “squashed”. However, the layerpacking in 2 is distinctively different. The whole structure isconstructed from an identical layer-to-layer stacking repeatedas an AAA packing mode (Figure 1 d). The shortest Cu–Cu dis-tance between adjacent layers in 2 is 3.654 �, longer thanthose in 1 a (3.220 �, 3.254 �). 1 a and 2 could be classified assupramolecular isomers owing to having the same composi-tion and different packing structures. Regarding the ligand,there are unsupported and short Cu–Cu distances betweencopper(I) centers located in adjacent Cu(4-pt) grid layers, andso there probably exist Cu···Cu interactions (also referred to ascuprophilicity) in 1 a, 1 b, and 2.

The crystal structures of 1 a, 1 b, and 2 at a cryogenic tem-perature (100 K, Table 1) were also determined. Although thespace group P1̄ is retained for 1 a at 100 K, all unit-cell dimen-sions including cell lengths and angles changed significantly,indicating a new phase was formed. When crystals of 1 a werecooled from 293 K to 100 K, sliding and slight squashing oc-curred in the framework, leading to an alternation of relativepositions between two adjacent layers (Figure S2 in the Sup-porting Information). The crystal transformation between roomand cryogenic temperatures is reversible. This phenomenonimplies that the 2D Cu–4-pt network is flexible and dynamicenough to be modified by external stimulus (i.e. , cooling). In

[a] T. Wen, Prof. Dr. X.-P. Zhou, D.-X. Zhang, Prof. Dr. D. LiDepartment of Chemistry and Research Institute forBiomedical and Advanced Materials, Shantou UniversityGuangdong 515063 (China)E-mail : dli@stu.edu.cn

Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201303991.

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contrast, the unit-cell parameters for 1 b and 2 differ little atroom temperature and 100 K.

Solid samples of 1 a, 1 b, and 2 are emissive at room temper-ature with emission bands centered at 535, 552, and 615 nm,giving green, yellow, and orange colors, respectively, upon ex-citation at 395 nm (Figure 2 and Figure S1). The emissions ofthese complexes are likely from metal-to-ligand charge transfer[Cu!p*(tetrazolate)] , and are probably involved with theCu···Cu [3d!4s] cluster-centered (CC) excited states.[24, 25, 28, 29]

Despite the fact that the structures of 1 a show differences atroom and cryogenic temperatures, the emission bands at bothtemperatures do not shift significantly (Figure S3 in the Sup-porting Information) suggesting that the slight sliding in the2D structure of 1 a does not affect the emission.

The emission maxima of 1 b has a redshift of about 17 nmcompared with that of 1 a, probably owing to the existence ofthe different guest solvent in the host framework.[28, 30] Single-crystal 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 S4in the Supporting Information). Emission spectra of 1 a beforeand after removal of the guest solvent under vacuum at 120 8C

are almost identical (Figure S5in the Supporting Informa-tion), demonstrating that theinfluence of the non-polar tol-uene on the structure and lu-minescence of the hostframework is limited. In 1 b,as a polar molecule, ethanolcreates hydrogen bonds withthe nitrogen atoms of the tet-razole group in 4-pt (Fig-

ure S6 in the Supporting Information). The interaction betweenthe guest ethanol and the 4-pt of the host framework leads tothe redshift of the emission band.

There is a large redshift in the emission of 2 relative tothose of 1 a (80 nm) and 1 b (63 nm). Careful examination findsthat two structural factors from the AAA packing style maycause the redshift : i) Aggregation by Cu···Cu interactions. Al-though the shortest Cu···Cu separation of 3.654(4) � in 2 islonger than those inside double-layers (AB) in 1 a and 1 b, allcopper(I) atoms in 2 are involved in Cu···Cu interactions form-ing polymeric Cu···Cu chains. In contrast, the Cu···Cu interac-tions in 1 a and 1 b between two double-layers are muchlonger (Figures S7 and S8 in the Supporting Information). Ithas been reported that Cu···Cu interactions play an importantrole in influencing the luminescence properties of copper(I) ag-gregates.[14, 20, 29, 31, 32] In a copper(I) cluster-centered (CC) excitedstate, when the Cu–Cu interactions become stronger, the emis-sion energy becomes lower, resulting in an emission band withlonger wavelength. ii) Strong p–p interactions in ligands be-tween the adjacent layers. The 4-pt ligands are closely packedin 2 with large overlap and short distance (3.654 �, Figure S8),indicating strong p–p interactions between the adjacentlayers. However, 4-pt in 1 a and 1 b adopts a staggered ar-rangement with weaker interactions between adjacent layersthan those in 2 (Figure 1 a, b, Figure S7). Experimental and the-oretical results revealed that the strong p–p interactions of the

Table 1. Summary of crystal data and refinement results.

T[K]

a [�] b [�] c [�] a [8] b [8] g [8] V [�3] Spacegroup

R1 WR2

1 a 293 7.1544(6) 15.3457(11) 16.4009(13) 93.540(6) 102.074(7) 95.765(6) 1745.6(2) P1̄ 0.0628 0.1502100 7.5321(3) 10.6828(5) 10.9671(6) 104.631(4) 89.973(4) 94.265(4) 851.33(7) P1̄ 0.0351 0.0713

1 b 293 7.2059(2) 15.3431(4) 15.9791(5) 95.315(2) 96.638(2) 98.272(2) 1725.80(9) P1̄ 0.0500 0.1497100 7.0485(6) 15.2337(8) 16.0392(8) 95.618(4) 94.709(6) 98.855(6) 1685.22(19) P1̄ 0.0614 0.0969

2 293 3.6547(5) 17.127(2) 13.7926(18) 90.00 90.46 90.00 863.33(19) P21/c 0.1059 0.3016100 3.5974(3) 17.2471(11) 13.6263(8) 90.00 90.579(6) 90.00 845.40(10) P21/c 0.0728 0.1971

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).

Figure 2. Emission spectra of 1 a (a, solid), 1 b (b, dot), and 2 (c, dash) in thecrystalline 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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ligand should lower the LUMO energy level, leading to emis-sions of lower energy.[33]

Based on the above discussion, the unique features of thecompounds inspired us to explore the luminescence responsegained upon treating with mechanical force stimuli. We antici-pated that the interlayer interactions (Cu···Cu interactions, andp–p interactions) in the structures should change upon exter-nal force stimuli (e.g. , cooling and grinding), causing modifica-tion of the emissions of the compounds.

When crystalline samples were ground gently in an agatemortar with a pestle, the colors of 1 a, 1 b, and 2 under ambi-ent light remained almost unchanged (Figure S1 and Fig-ure 3 a–c). Upon irradiation with a UV lamp (365 nm), theground samples of 1 a, 1 b, and 2 emitted in green, yellow, andorange, respectively (Figure 3 a’–c’). When the samples weretreated with strong grinding, the luminescence of the sampleswas converted into relatively weak orange emissions (Fig-ure 3 a“–c”), consistent with their corresponding emission spec-tra (Figure 2 a’–c’). The emissive lifetime measurements (seeTable S1 in the Supporting Information) reveal that the life-times of the emissions (at ms scale, double-exponential decay)are very similar for all samples (gently ground or stronglyground) for 1 a, 1 b, and 2, a fact that indicates that grindingdoes not change the origin of their luminescence.

The X-ray powder diffraction patterns of as-synthesized crys-talline samples fit well with their corresponding simulatedones, indicating that their structures do not change undergentle grinding (Figures S9, S10, and S11 in the Supporting In-formation). However, the prominent peaks disappeared whenthe samples were treated with strong grinding, indicating theloss of crystalline form to an amorphous state (curves c in Fig-ures S9, S10, and S11). The resulting amorphous state fromstrong grinding is likely caused by the disordered sliding be-

tween the Cu(4-pt) layers in the structures. The sliding shouldbe accompanied with alterations of the supported weak inter-actions (Cu···Cu and p–p interactions), thus changing the pho-toluminescence properties of the compounds.

It has been reported that the crystalline phase and emissionproperties of some ground mechanochromic oligomer metalcomplexes can be recovered upon exposure to solvent orheating.[13–20] PXRD patterns of ground samples of 1 a, 1 b, and2 did not show prominent peaks after the samples were treat-ed with organic solvents (toluene for 1 a, 2, and ethanol for1 b) for two days. This is probably due to the fact that the co-ordination polymer samples cannot dissolve in organic sol-vents, and that recrystallization barely occurred, unlike thecase for metal oligomers. Interestingly, ground solid of 1 b im-mediately showed bright yellow luminescence (in UV light)when one drop of ethanol was added (Figure S12 in the Sup-porting Information); however, similar phenomena were notobserved in the cases of 1 a and 2 treated with toluene. Solid-state photoluminescence measurements found that groundsolid of 1 b treated with ethanol showed a broad emissionband centered at 562 nm upon excitation at 296 nm (Fig-ure S13 in the Supporting Information). Thermogravimetricanalysis (TGA) found that guest molecules of 1 b were lostduring grinding (Figure S14 in the Supporting Information).The loss of guest molecules should leave voids for subsequentoccupation by ethanol molecules, thus, inducing gliding ofCu(4-pt) layers and leading to luminescence change. When thesample was completely dry, the luminescence changed to darkorange. This transformation can be repeated reversibly at leastthree times. Thermogravimetric curves for samples 1 a and 2before and after grinding do not show significant change (Fig-ures S15 and S16 in the Supporting Information), revealingthat the toluene guest molecules are only partially removed bygrinding owing to the relatively higher boiling point of toluenethan ethanol.

To test the recovery of the crystalline form of the groundsamples in organic solvents, we immersed each groundsample (1 a, 1 b, and 2) in ethanol, chloroform, and toluene, re-spectively, and found that the recovery in chloroform occurredafter one month, an observation that was confirmed bypowder X-ray diffraction studies (Figures S9, S10, and S11). Theluminescence properties of the ground 1 a, 1 b, and 2 samplesare also recovered entirely for the samples immersed in chloro-form, the only notable change being that the spectral peaksbecome slightly broader (Figure S17 in the Supporting Infor-mation). These properties are probably due to the inherentself-restore function of coordination polymers under solventstimuli. The phenomenon of transformation from amorphousto crystalline state has been well documented in reported co-ordination polymers and metal frameworks.[34–37]

To highlight the significance of the luminescent mechano-chromic functions of 1 a, 1 b, and 2, we also studied a series ofreported classical luminescence coordination polymers orMOFs. Two reported iso-structural coordination polymers for-mulated as Cu(3-pt) (3-pt = 5-(3-pyridyl)tetrazole) featurea wavelike layer structure without obvious interactions be-tween adjacent layers, and are emissive at room temperature

Figure 3. Photos showing the luminescence changes of 1 a, 1 b, and 2 upongentle and strong grinding at room temperature. a–c) Gently ground sam-ples 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’’) samplesof strongly ground 1 a, 1 b, and 2 under a UV lamp (365 nm).

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with emission bands centered at 501 and 533 nm, respective-ly.[38] After being treated with strong grinding, emissions ofboth isomers did not change (Figures S18 and S20 in the Sup-porting Information), their crystalline forms were also un-changed, and the X-ray powder patterns are in accord withthose of gently ground samples (Figures S19 and S21 in theSupporting Information). Similar treatments were also carriedout for rigid carboxylate-based luminescent compounds, forexample, MOF-69C, MOF-75, and MOF-76.[39] No detectable me-chanochromic phenomena were found (Figures S22–S27 in theSupporting Information) in these emissive MOFs.

In summary, we have found that the luminescence of threeCu(4-pt) porous coordination polymers can be modified bygrinding. The results indicate that the mechanochromic prop-erties are related to the weak interactions of cuprophilicity andp–p interactions. Our results provide compelling evidence andenlightenment for designing and applying porous coordinationpolymers as mechanochromic materials. Although our groundamorphous samples can be recovered to crystalline phases,the recovery duration is quite long (about one month). Findingrecoverable mechanochromic materials with a fast response re-mains a challenge for the potential applications of these kindsof materials.

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 Teflon-lined stainless steel reactor, which was heated in an oven to 120 8Cfor 72 h and then cooled to room temperature (25 8C) at a rate of3 8C·0.5 h�1. Light yellow crystals of 1 a were obtained as the mainproduct (0.0302 g, 65 %). The temperature of synthesis of 1 a is notcritical ; 1 a can also be synthesized at 140, 160, and 180 8C. Crystalsof 2 are concomitants of 1 a, the yield is lower (<10 %), and sepa-ration was obtained mechanically.

Synthesis of 1 b

The synthesis of 1 b is similar to that of 1 a except the toluene sol-vent was replaced with ethanol. Light yellow crystals of 1 b wereobtained (0.0279 g, 60 %).

CCDC-922420, 922421, 922422, 922423, 922424, and 922425 con-tain the supplementary crystallographic data for this paper. Thesedata can be obtained free of charge from The Cambridge Crystallo-graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

Acknowledgements

This work was financially supported by the National Basic Re-search Program of China (973 Program, 2012CB821706 and2013CB834803), the National Natural Science Foundation forDistinguished Young Scholars of China (20825102), the Nation-al Natural Science Foundation of China (91222202, 21171114,21101103), Natural Science Foundation of Guangdong Province(S201140004334) and Shantou University.

Keywords: coordination polymers · cuprophilicity · grinding ·luminescence · mechanochromism

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Received: October 11, 2013

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