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PAPER View Article Online

View Journal | View Issue

4422 | CrystEngComm, 2014, 16, 4422–4430 This journal is © The R

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory

for Carbon-based Functional Materials and Devices, and Collaborative Innovation

Center of Suzhou Nano Science and Technology, Soochow University, Jiangsu 215123,

China. E-mail: zhkang@suda.edu.cn; Fax: +86 512 65882846; Tel: +86 512 65880957

† Electronic supplementary information (ESI) available: Additional figures, TGcurves, PXRD patterns, IR spectra and analysis, detailed crystallographic dataand structural refinement parameters, selected bond distances and selectedhydrogen bond parameters for compounds 1–6, fluorescence decay curves.CCDC 973550–973555. See DOI: 10.1039/c3ce42566e

Cite this: CrystEngComm, 2014, 16,

4422

Received 17th December 2013,Accepted 21st February 2014

DOI: 10.1039/c3ce42566e

www.rsc.org/crystengcomm

Tuning luminescence via transition metal-directedstrategy in coordination polymers†

Fangfang Zhao, Huan Dong, BeiBei Liu, Guangju Zhang, Hui Huang, Hailiang Hu,*Yang Liu* and Zhenhui Kang*

Six coordination polymers of transition metals, namely, Cu(bpy)(H2L)2 (1), Co(bpy)(H2L)2(H2O)2 (2),

Mn(bpy)(H2L)2(H2O)2 (3), Ag(bpy)(H2L) (4), Cd(bpy)(HL)(H2O) (5), and [Zn(bpy)(HL)]·H2O (6) (bpy =

4,4′-bipyridine, H3L = 4-chloro-5-sulphamoylbenzoic acid), have been synthesized under hydrothermal

conditions. Single crystal X-ray diffraction analysis reveals that these compounds exhibit 1D linear chains

for 1–3 (2 and 3 are isomorphic), a 1D double chain for 4, a 2D square-grid sheet for 5 and a 3D

window-shaped net for 6. Photoluminescent investigation shows that compounds 2–6 exhibit distinct

tunable yellow-to-violet photoluminescence by varying the TMs. These five compounds show weak

Co(II)-based yellow, intense Mn(II)-based blue, Ag(I)-based green, Cd(II)-based violet and Zn(II)-based

violet luminescence upon irradiation with a standard UV lamp (λex = 254 nm) at room temperature.

However, the Cu(II)-based compound shows fluorescence quenching. Furthermore, the luminescent

quantum yields (QYs) and lifetimes under UV irradiation of 2–6 as well as the H3L and bpy ligands have

been obtained.

Introduction

Luminescent materials (phosphors) have been extensivelyexplored and realized for their diverse functionalities andapplications in lighting, display, sensing, and optical devices.1,2

To date, many luminescent materials, including inorganic(CdS/ZnS particles, rare earth oxide based luminescentmaterials) and organic (small organic molecules) luminescentmaterials have been commercially utilized. However, most ofthem still suffer from source limitation, unstability, environ-mentally problems and are rather expensive.

As novel luminescent materials,3–6 coordination polymers(CPs) have attracted much attention because they have adegree of structural predictability, in addition to well-definedenvironments for lumophores in crystalline form. Lumines-cence from CPs containing transition-metal ions (TMs) in theframework is typically centred on the linker rather than onthe metal, but can also involve charge transfer between themetal and linker. Recently, the amount of effort devoted to

the field of photoluminescence of CPs has been increasingand a large number of luminescent materials has beenreported, which include: (i) Noble (Au, Pt, etc.) and rare earth(La, Ce, etc.) metal-based luminescent materials. Noble andlanthanoid ions (Ln(III)) can emit sharp, but weak lumines-cence from transitions that are forbidden by electric dipoleselection rules. (ii) Linker-based luminescent materials withTMs. Among them, some novel interesting phenomena havebeen discovered, such as tunable UV-to-visible emission bycontrolling the guest molecules,3a tunable yellow-to-whiteemission or white-to-purplish blue by variation of the excita-tion wavelength,3b–e tunable color by changing the dopingconcentration,4a tunable visible-IR by changing the dopingelement,4b,c tunable emissions by controlling the type ofguest species,5a,b tunable UV-vis-NIR luminescent chiralLnMOFs5c and ion and small molecule sensing.6 In this field,TMs have shown the following three advantages:7 (i) Lack ofa quenching effect such as paramagnetic TMs and hightransparency in the UV region. (ii) Hard to oxidize/reduce.(iii) Permit a wide variety of geometries and coordinationnumbers and finally yield linker-based highly emissive mate-rials. However, to the best of our knowledge, studies focusingon the tunable luminescence of CPs via a TM-directed strategyare relatively scarce.

According to the above aspects, we chose TM ions as themetal centres, and H3L and bpy ligands as spacers. Herein, six newcoordination polymers, Cu(bpy)(H2L)2 (1), Co(bpy)(H2L)2(H2O)2 (2),Mn(bpy)(H2L)2(H2O)2 (3), Ag(bpy)(H2L) (4), Cd(bpy)(HL)(H2O) (5),

oyal Society of Chemistry 2014

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and [Zn(bpy)(HL)]·H2O (6) (bpy = 4,4′-bipyridine, H3L =4-chloro-5-sulphamoylbenzoic acid) have been successfullyprepared. Single crystal X-ray diffraction reveals that thesecompounds exhibit one dimensional (1D) linear chains for1–3, a 1D double chain for 4, a two dimensional (2D) square-grid sheet for 5 and a three dimensional (3D) window-shapednet for 6. Notably, with the addition of different metal salts,tunable yellow-to-violet emission was realized, and weakCo(II)-based yellow, intense Mn(II)-based blue, Ag(I)-basedgreen, Cd(II)- and Zn(II)-based violet luminescence was shownupon irradiation with a standard UV lamp (λex = 254 nm) atroom temperature, respectively (Scheme 1). However, theCu(II)-based compound exhibits no emission probably due tofluorescence quenching of Cu2+ by the H3L and bpy ligands.Furthermore, the luminescent quantum yields (QYs) and life-times under UV irradiation of 2–6 have been studied in detail.In this context, it is worth mentioning that compounds 5 and6 show intense violet short-wavelength luminescence with thehighest QY of 11.51%, and good photobleaching stability onexcitation with UV light for as long as twelve hours.

ExperimentalMaterials and general methods

All analytical reagents were purchased from commercialsources and used without further purification. Elementalanalyses of C, H, and N were performed using an EA1110elemental analyzer. The IR spectra were recorded in the range4000–400 cm−1 on a Nicolet 360 spectrometer with a pressedKBr pellet. The TG-DTA analysis was carried out by a UniversalAnalysis 2000 thermogravimetric analyzer (TGA) in N2 with aheating rate of 10 °C min−1. The crystal structures of the resul-tant products were characterized by X-ray powder diffraction(XRD) using an X'Pert-ProMPD (Holland) D/max-γA X-ray dif-fractometer with Cu Kα radiation (λ = 0.154178 nm). Crystaldata were collected on a Bruker X8 APEX II-CCD single crystalX-ray diffractometer with Mo Kα radiation (λ = 0.71073 Å).Luminescence spectra were collected on a Fluoromax-4 spec-trophotometer. The QY measurements were performed usinga Jobin-Yvon integrating sphere (Φ = (Ec − Ea)/(La − Lc) with Ec:emission spectrum of the sample, Ea: ‘blank’ emissionspectrum, La: ‘blank’ absorption and Lc: sample absorptionaround the excitation wavelength). The deconvoluted

This journal is © The Royal Society of Chemistry 2014

Scheme 1 Short-wavelength luminescence tuning via a transition metal-di

fluorescence decay curves, I(t), were analyzed by the nonlinearleast-squares method implemented into the IBH decay analysissoftware. In the measurements of emission and excitationspectra, the pass width is 5 nm for all compounds. All thefluorescent properties are obtained at room temperature.

Synthesis

Cu(bpy)(H2L)2 (1). A mixture of Cu(Ac)2·H2O (0.080 g,0.400 mmol), H3L (0.040 g, 0.170 mmol) and bpy (0.030 g,0.192 mmol) in 14 mL of H2O and one drop of NaOH(1.0 mol L−1) was sealed in an autoclave equipped with aTeflon liner (25 mL), and then heated at 120 °C for 4 daysunder autogenous pressure. After cooling to room temperature,blue block crystals formed and were washed with deionisedwater, and then dried in air. Yield: 47% (based on Cu) for 1.Anal. calc. for C24H18CuCl2N4O8S2: C, 41.80; H, 2.61; N, 8.13;Cu, 9.29%. Found: C, 41.49; H, 2.48; N, 8.51, Cu, 9.46%.

Co(bpy)(H2L)2(H2O)2 (2). Compound 2 was prepared in amanner similar to that for 1, except that Co(Ac)2·2H2O(0.100 g, 0.400 mmol) was used instead of Cu(Ac)2·H2O.After cooling to room temperature, pink block crystalsformed and were washed with deionised water, and thendried in air. Yield: 46% (based on Co) for 2. Anal. calc. forC24H18CoCl2N4O10S2: C, 40.20; H, 2.51; N, 7.82; Co, 15.69%.Found: C, 39.94; H, 2.75; N, 7.68, Co, 15.84%.

Mn(bpy)(H2L)2(H2O)2 (3). Compound 3 was prepared in amanner similar to that for 1, except that MnSO4·H2O(0.068 g, 0.400 mmol) was used instead of Cu(Ac)2·H2O. Aftercooling to room temperature, pale yellow block crystalsformed and were washed with deionised water, and thendried in air. Yield: 27% (based on Mn) for 3. Anal. calc. forC24H18Cl2MnN4O10S2: C, 40.43; H, 2.53; N, 7.86; Mn, 7.72%.Found: C, 41.02; H, 2.31; N, 7.59; Mn, 8.01%.

Ag(bpy)(H2L) (4). Compound 4 was prepared in a mannersimilar to that for 1, except that AgNO3 (0.067 g, 0.400 mmol)was used instead of Cu(Ac)2·H2O and two drops of NH3·H2O(1.0 mol L−1) replaced one drop of NaOH. After cooling toroom temperature, colorless block crystals formed and werewashed with deionised water, and then dried in air. Yield:16% (based on Ag) for 4. Anal. calc. for C17H13AgClN3O4S:C, 40.91; H, 2.61; N, 8.42; Ag, 21.63%. Found: C, 40.38;H, 2.93; N, 8.75; Ag, 21.44%.

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rected strategy.

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Cd(bpy)(HL)(H2O) (5). Compound 5 was prepared in amanner similar to that for 1, except that Cd(Ac)2·2H2O(0.107 g, 0.400 mmol) was used instead of Cu(Ac)2·H2O.After cooling to room temperature, colorless block crystalsformed and were washed with deionised water, and thendried in air. Yield: 59% (based on Cd) for 5. Anal. calc. forC17H12CdClN3O5S: C, 39.36; H, 2.32; N, 8.10; Cd, 21.69%.Found: C, 39.01; H, 2.79; N, 7.58, Cd, 21.96%.

[Zn(bpy)(HL)]·H2O (6). Compound 6 was prepared in amanner similar to that for 1, except that Zn(Ac)2·2H2O(0.088 g, 0.400 mmol) was used instead of Cu(Ac)2·H2O. Aftercooling to room temperature, pale yellow block crystalsformed and were washed with deionised water, and thendried in air. Yield: 57% (based on Zn) for 6. Anal. calc. forC17H12ZnClN3O5S: C, 43.34; H, 2.57; N, 8.92; Zn, 13.88%.Found: C, 43.79; H, 2.83; N, 9.29; Zn, 13.59%.

Crystallographic analyses

Diffraction measurements were performed at 150 K on aBruker Smart Apex CCD diffractometer using Mo Kα radia-tion (λ = 0.71073 Å). The structures were solved by directmethods using the program SHELXS-978 with anisotropicthermal parameters for all non-hydrogen atoms. All non-hydrogen atoms were refined anisotropically by full-matrixleast-squares methods in SHELXL-97.8 Hydrogen atoms ofthe carbon and nitrogen atoms were generated theoreticallyonto the specific atoms and refined isotropically with fixedthermal factors. In 2, 3, 5 and 6, hydrogen atoms attached towater molecules were not located and were included in themolecular formula directly. A summary of the crystallographicdata and structural refinement details is given in Table 1.Selected bond distances and angles are listed in Table S1.†

4424 | CrystEngComm, 2014, 16, 4422–4430

Table 1 Crystal data and structural refinements for compounds 1–6

1 2 3

Empirical formula C24H18Cl2CuN4O8S2 C24H18Cl2CoN4O10S2 C24H18Cl2Formula weight 689.01 716.39 712.40Crystal system Monoclinic Triclinic TriclinicSpace group C2/c P1̄ P1̄a (Å) 12.3032(16) 5.4158(6) 5.4158(6)b (Å) 11.0287(15) 11.4254(13) 11.4254(1c (Å) 19.618(3) 12.5590(14) 12.5590(1α (deg) 90 114.573(2) 114.573(2β (deg) 90.411(2) 90.156(2) 90.156(2)γ (deg) 90 95.928(2) 95.928(2)V (Å3) 2661.9(6) 702.06(14) 702.06(14Z 4 1 1Dc (g cm−3) 1.719 1.694 1.685μ (mm−1) 1.236 1.014 0.873F (000) 1396.0 363.0 361.0Collcd reflns 8746 3711 3713Unique reflns 3236 2420 2421Parameters 224 232 232Rint 0.0225 0.0111 0.0111GOF 1.046 1.026 1.018R1

a [I > 2σ(I)] 0.0251 0.0272 0.0346wR2

b (all data) 0.0677 0.0749 0.0924

a R1 =P

||F0| − |Fc||/P

|F0|.b wR2 =

P[w(F0

2 − Fc2)2]/

P[w(F0

2)2]1/2.

Hydrogen-bonding parameters are summarized in Table S2.†CCDC numbers are 973550–973555 for 1–6, respectively.

Results and discussion

Synthesis. The synthesis of the target six compounds wereall performed in 25 mL Teflon-lined stainless steel vesselswith the same mixed ligands H3L and bpy, the same molarratio of metal to ligand under comparable hydrothermal sys-tems. During the synthetic process, dilute sodium hydroxidesolution was used to deprotonate the carboxylate groups andthe amino-groups of H3L and promote the self-assembly.9

Remarkably, with almost the only change being the differentTM salts, the H3L ligand exhibits two coordinated modes.The carboxylate groups within all of the title compounds arecompletely deprotonated and coordinated to metal centreswith carboxylate O (Ocar) atoms via monodentate and/orchelating modes. Simultaneously, the sulphonylamino groupsalso respond to different metal centres actively. In 1–4, thesulphonylamino N (Nsul) atoms are uncoordinated with itsneutral state and act as potential sites for supramolecularinteractions. While in 5 and 6, the sulphonylamino groupsare deprotonated to coordinate with Cd2+ and Zn2+ ions viaNsul atoms, respectively. The potential coordinating groupsof chlorine atoms are found to be laid aside in all of thesix compounds.

Description of the crystal structures

Compounds 1–3. Compounds 1–3 possess similar 1Dlinear chains. Structural analysis revealed that the crystal ofcompound 1 conforms to the space group C2/c and those of2–3 conform to the space group P1̄. In all of the threecompounds, there is one crystallographically independent

This journal is © The Royal Society of Chemistry 2014

4 5 6

MnN4O10S2 C17H13AgClN3O4S C17H12CdClN3O5S C17H12ZnClN3O5S498.69 518.23 471.18Triclinic Triclinic MonoclinicP1̄ P1̄ C2/c7.4679(7) 10.1145(8) 15.746(4)

3) 10.8411(10) 10.2684(8) 10.962(3)4) 11.3461(11) 10.8689(9) 22.411(5)) 77.045(2) 74.219(1) 90

70.917(1) 68.085(1) 109.335(4)76.493(2) 64.291(1) 90

) 833.10(14) 935.39(13) 3650.1(16)2 2 81.988 1.840 1.7151.527 1.457 1.643496.0 512.0 1904.05739 6334 11 3914004 4497 4125292 297 2570.0142 0.0164 0.0281.011 1.026 1.010.0260 0.0241 0.03100.0616 0.0679 0.0811

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metal centre in the asymmetric unit, all of which are six-coordinated but in different coordination environments(Fig. 1a and S1a†). In compound 1, the Cu atom is coordi-nated by two pyridyl N (Nbpy) atoms from two bpy ligandsand four chelating Ocar atoms from two H2L

− ligands. In theisomorphic compounds 2 and 3, the Co/Mn site is coordi-nated by two Nbpy atoms, two Ocar atoms, and two coordi-nated water molecules. All the metal centres in the threecompounds are similarly coordinated by two H2L

− and twobpy ligands. The neighboring metal centres are linked by bpyligands to build up 1D chains, as shown in Fig. 1b and S1b.†The adjacent 1D coordination arrays are further parallelarranged via hydrogen interactions (N1–H⋯O2: 2.746 Å,N1–H⋯O4: 3.028 Å) to form a 2D supramolecular layer(Fig. 1c and S1c†). Moreover, the 2D arrays in the parallelstacking mode are further extended into a 3D supramoleculararrangement via hydrogen interactions.

Compound 4. Compound 4 crystallizes in the triclinic P1̄space group. The asymmetric unit of 4 contains oneindependent Ag atom, one H2L

− ligand and one bpy ligand.The Ag atom is three-coordinated by one Ocar atom and twoNbpy atoms, forming a distorted T-shaped geometry (Fig. 2a).

This journal is © The Royal Society of Chemistry 2014

Fig. 1 (a) Coordination environment of the Cu centre. (b) A drawingshowing the 1D chain of 1. (c) The 2D supramolecular arrangementformed via hydrogen interactions in 1. Hydrogen atoms are omitted forclarity. The dotted lines represent two kinds of hydrogen bonds.

In 4, the neighboring Ag ions are linked by bpy ligands tobuild up a 1D linear chain. Supported by π–π [3.657 Å] stackingbetween the pyridyl rings and the Ag–Ag [3.162 Å] interactions,each of the two isolated chains are merged into a polymericpair. Such a polymeric pair exhibits an infinite ladder-likestructure with the Ag–Ag bonds as rungs, as shown in Fig. 2b.The monodentate coordinated H2L

− ligands serve as side-armshanging on both sides of the ladder up and down (Fig. S2†).Through face-to-face π–π [3.611 Å] stacking interactionsbetween the benzene rings of the H3L molecules, these adja-cent polymeric pairs are further extended into a 2D supramo-lecular layer structure (Fig. 2c).

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Fig. 2 (a) Coordination environment of the Ag centre. (b) The 1Dpolymeric pairs exhibit an infinite ladder-like structure in 4. (c) The 2Dsupramolecular arrangement formed via π–π interactions in 4. The reddot lines show the face-to-face π–π interactions. Hydrogen atoms areomitted for clarity.

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Compound 5. Compound 5 crystallizes in the triclinic P1̄space group. The asymmetric unit of 5 contains oneindependent Cd atom, one HL2− ligand, two halves of bpyligands and one coordinated water molecule. As depicted inFig. 3a, the Cd centre possesses a distorted octahedralgeometry of [CdO3N3] which is completed by two chelatingOcar atoms, two Nbpy atoms, one Nsul atom and one watermolecule. In 5, two crystallographically equivalent Cd2+ ionsand two HL2− ligands construct a 16-membered dinuclear

4426 | CrystEngComm, 2014, 16, 4422–4430

Fig. 3 (a) Coordination environment of the Cd centre. Hydrogenatoms are omitted for clarity. (b) The 2D stair-like layer in 5. (c) Adjacentgrid sheets with a stagger-peaked fashion.

metallic macrocycle via Ocar and Nsul atoms. These dinuclearmacrocycles, which serve as the secondary building units, arefurther linked by bpy ligands to form a 2D square-grid sheet(Fig. 3b). Remarkably, the chelating carboxylate groups ofcoordinated HL2− molecules occupy the coordinate sites ofthe central metal ions, which interrupts the compound inconstructing high-dimensional architectures. If the dinuclearmacrocycle is considered as a 4-connected node, and eachbpy is defined as a linker, then the whole structure can besimplified as an sql net with a Schläfli symbol of 44·62. More-over, these square-grid sheets are stagger-peaked in an ABABfashion, which may decrease the molecular repulsion andstabilize the whole structure (Fig. 3c).

Compound 6. Compound 6 represents a 3D frameworkand crystallizes in the monoclinic space group C2/c. In theasymmetric unit, there is one crystallographically independentZn atom, one HL2− ligand and two halves of bpy ligands. Asdepicted in Fig. 4a, the Zn centre exhibits a distorted four-coordinated environment provided by one bridging Ocar atom,one Nsul atom and two Nbpy atoms. In 6, each HL2− ligand linkstwo Zn atoms via Nsul and Ocar atoms, forming a 1D wave-likechain (Fig. S3†). The two bpy ligands adopt similar linkingmodes but play different roles in forming the final 3D hybridframework, which can be understood in the following manner.Firstly, the N3-containing bpy ligands extend adjacent 1Dchains into a 2D layer structure with a Zn–Zn distance of11.223(9) Å, as shown in Fig. 4b. Secondly, the N4-containingbpy ligands serve as pillars to expand the 2D nets into a 3Dframework with a Zn–Zn distance of 11.293(11) Å. In 6, each Znatom is connected to the other four Zn atoms by two bpy andtwo HL2− ligands, respectively. From the topological point ofview, each Zn atom can be regarded as a 4-connected node,and each bpy and HL2− ligand can be regarded as bridginglinkers. Thus, the whole framework of 6 can be classified as awindow-shaped net with 65·8 topology (Fig. 4c).

Powder X-ray diffraction. To confirm the phase purity ofthe bulk samples in the solid state, their PXRD patterns wererecorded. For compounds 1–6, the measured XRD patternsclosely match the simulated patterns generated from theresults of the single-crystal diffraction data (Fig. S4†), indicativeof pure products. The dissimilarities in intensity may be due tothe preferred orientation of the crystalline powder samples.

FT-IR spectroscopy. IR spectroscopic studies of 1–6 werecarried out in the range 400–4000 cm−1 using the KBr pelletmethod. The typical peaks correspond to the carboxylate group,aromatic CH etc., with minor variations among the compounds(Fig. S5†). The observed bands are 3600–3200 cm−1 = ν(H2O),1610–1630 cm−1 = νas(COO), 1570–1300 cm−1 = ν(COO),1170–850 cm−1 = δ(CH), and 770–740 cm−1 = δ(COO).

TG analysis. To study the thermal stability, TG analyseswere performed in a N2 atmosphere at a heating rate of10 °C min−1 from 30 to 800 °C (Fig. S6†). The curves show thatcompounds 1–3 and 6 have two identifiable weight losssteps, and the other two compounds have only one weightloss step, respectively. For 1, the first to second weight losssteps are from about 215 to 730 °C, and the total weight loss

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Fig. 4 (a) Coordination environment of the Zn centre. (b) The 2D layerconnected by bpy ligands. (c) Schematic representation of the4-connected topology.

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of the two steps is 88.03% (calculated 88.39%), whichcorresponds to the decomposition of the organiccomponents. The remaining weight of 11.98% corresponds tothe percentage of Cu and O components (calculated 11.61%),indicating that the final product is CuO. For 2, the weightloss of 5.35% (calculated 5.19%) before 220 °C correspondsto the loss of coordinated water molecules. The overallframework begins to collapse at about 240 °C, and the Co2O3

residue of 24.47% (calculated 23.95%) is observed at about650 °C. For 3, the overall weight loss corresponds to theremoval of coordinated water molecules before 145 °C(observed 4.65%, calculated 5.03%), there is no weight lossuntil the decomposition of the framework occurs at about210 °C. A Mn and O component residue of 12.46%(calculated 12.21%) is obtained at about 630 °C, indicatingthat the final product is MnO. The curve of 4 shows a one

This journal is © The Royal Society of Chemistry 2014

step weight loss process from about 240 °C to 740 °C, aresidue of 21.81% (calculated 21.66%) is obtained and theultimate residue should be Ag. For 5, a one step weight lossprocess before about 140 °C corresponds to the loss ofcoordinated water (observed 3.84%, calculated 3.46%), noobvious weight loss is observed for 5 until the decompositionof the framework occurs at about 250 °C. At about 750 °C, aCdO residue of 24.21% (calculated 24.68%) is obtained. For6, the first step weight loss before about 130 °C is theremoval of free water molecules (observed 2.47%, calculated1.94%); the second process from about 240 to 730 °Ccorresponds to the decomposition of the framework. A whiteresidue of ZnO (observed 18.19%, calculated 17.57%) isobtained at the end.

Photophysical properties. Coordination polymers with TMatoms are promising candidates for photoactive materialswith potential applications as luminescent materials.10 Thus,the solid-state photoluminescence properties of compounds1–6 and the corresponding free H3L/bpy ligands have beeninvestigated at room temperature under the same experimen-tal conditions. All the excitation spectra display an intenseand broad band with a maximum at around 350 nm, whichis attributed to the π–π* electron transition of the H3L andbpy ligands. As shown in Fig. 5b, the free H3L ligand displaysphotoluminescence with an emission maximum at 472 nmupon 350 nm excitation, which can be attributed to the π*–πtransition of the p electrons of the aromatic rings. The freebpy ligand shows a weak emission band at λmax = 421 nm,which is also attributable to a ligand-centered (LC) π*–πtransition. With the same excitation at 350 nm (Fig. 5a),compounds 2–6 exhibit photoluminescence with emissionmaxima at 565, 460, 535, 395 and 402 nm, respectively(Fig. 5b). However, compound 1 displays very weak lumines-cence and almost no emission, which is probably attributedto the fluorescence quenching of Cu2+ by the H3L ligand.11

The emission wavelength (421 nm) of the free bpy ligand isfar shorter than those of compounds 2–4 and therefore it hasessentially little contribution to the luminescence of 2–4.12

The emissions of compounds 5 and 6 exhibit blue-shifts withrespect to the free bpy ligand. It should be pointed out thatsince Zn2+ and Cd2+ ions are difficult to oxidize or reducedue to the features of their electronic configurations, theemission bands of compounds 5 and 6 are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal chargetransfer (LMCT) in nature.13 Thus they are most probablyattributed to intraligand and/or ligand-to-ligand charge trans-fer (LLCT).14 The emission peak of compound 3 is slightlyblue-shifted relative to the free H3L, which can probably beexplained as arising from the strong interaction between theligand and metal. On the contrary, compounds 2 and 4 showred-shifts relative to the free H3L ligand. Compound 4 issupported by strong Ag–Ag and π–π interactions. The π–π

interactions between adjacent conjugated linkers or betweena linker and a guest molecule can produce an excited com-pound that typically exhibits broad, featureless lumines-cence.11 Thus the red-shift for the energy band at 535 nm for

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Fig. 5 (a) Solid-state excitation spectra of free H3L, bpy ligand andcompounds 1–6 with normalized intensities. (b) Solid-state emissionspectra of free H3L, bpy ligand and compounds 1–6 with normalizedintensities. (c) Bright field and photoluminescent images of compounds1–6 under UV radiation (254 nm). (d) CIE coordinates forphotoluminescence of compounds 2–6.

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compound 4 is most probably due to the formation of the Ag+

supramolecular structure and may originate from the chargetransition between the ligands and metal ions.15 Additionally,except for 4, higher red-shift is also observed in compound 2.Strong hydrogen interactions are obtained in 2 rather thanthe π–π interactions. Obviously, the red shifts of the fluores-cence are considered to mainly arise from the coordination ofmetal atom to ligand and the hydrogen interactions.11–15 It isworth mentioning that 2 shows much weaker PL intensity anda narrow band compared with 3–6, which is most probablyattributed to the effect of the metal centre when Co2+ coordi-nated to the H3L ligand (Fig. S7†).

At room temperature, compounds 1–6 show blue(compound 1), pink (compound 2), yellow (compounds 3, 5and 6), and colorless (compound 4) colours. Upon irradiationwith a standard UV lamp (λex = 254 nm) at room temperature,weak Co(II)-based yellow, intense Mn(II)-based blue, Ag(I)-basedgreen, Cd(II)- and Zn(II)-based violet luminescence are observedfor solid samples of 2–6, respectively, with no visible lumines-cence apparent for 1, as shown in Fig. 5c. The luminescence of2–6 is also reflected in the Commission International del'Eclairage (CIE) chromaticity coordinates (Fig. 5d), which arein the yellow region with a value of (0.410, 0.585) for 2, blueregion with a value of (0.188, 0.43) for 3, light green region witha value of (0.326, 0.593) for 4, and in the blue violet region witha value of (0.178, 0.167) for 5 and (0.187, 0.186) for 6.

QY (Φ) measurements on these systems reveal the extentof the quantum efficiency related to the excimer emission,which is the number of molecules that have been photo-bleached divided by the total number of photons absorbedover a given time interval. The absolute luminescence QYs ofcompounds 2–6 as well as the H3L and bpy ligands weredetermined by means of an integrating sphere at roomtemperature under the excitation wavelengths.

The solid-state measurements gave QYs of 6.16% for com-pound 3, 6.27% for compound 4, 11.51% for compound 5and 10.95% for compound 6, respectively, which are similarto those of traditional phosphors.16 But compound 2 featuresa weak QY (2.05%), which is similar to the QYs of H3L(2.75%) and bpy (2.34%).

Time-correlated single-photon-counting measurementswere carried out for the luminescence decay (τ) of compounds2–6 and the H3L/bpy ligands. The luminescence decay curvesby monitoring the most intense emission (λem) of the respec-tive samples are shown in Fig. 6 and S8.† A multiexponentialdecay behavior was again observed, which could be fit satis-factorily to a monoexponential function of compound 2, andthe H3L and bpy ligands, with lifetime values of τ = 0.31, 0.15and 0.21 ns, respectively. Further, the emission decay curvescan be well fitted with triexponential functions for 3–6, withluminescent lifetimes as follows: τ1 = 0.48 ns, τ2 = 5.10 ns, andτ3 = 76.20 ns for 3; τ1 = 0.44 ns, τ2 = 4.85 ns, and τ3 = 85.72 nsfor 4; τ1 = 0.50 ns, τ2 = 5.34 ns, and τ3 = 83.28 ns for 5; and τ1 =0.61 ns, τ2 = 3.91 μs, and τ3 = 86.51 ns for 6. Among thesecompounds, 6 has the longest lifetime on account of the 3Dcondensed structure, which suggests that the Zn2+ ions in 6

This journal is © The Royal Society of Chemistry 2014

Fig. 7 Effect of UV photo-irradiation time on the fluorescence inten-sity of 5 (green squares) and 6 (purple circles).

Fig. 6 Fluorescence decay curve of compound 6.

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are well shielded from nonradiative deactivations.17 In con-clusion, the short fluorescence lifetimes of 2–6 are indicativeof the radiative recombination of the excitons giving rise tofluorescence.18 The luminescence data of compounds 2–6, aswell as the H3L ligand, were studied and the data are listed inTable 2.

Further studies on photostability revealed that the Cd(II)-and Zn(II)-based violet luminescence compounds 5 and 6show good photobleaching stability on excitation with UVlight. The fluorescence of 5 and 6 show no signs of anyfluorescence quenching and measurable attenuation ofshort-wavelength under the same illumination conditions, asshown in Fig. 7, as green squares and purple circles, respec-tively. Simultaneously, the photostability of 5 and 6 are alsocharacterized by the photobleaching QY under the sameexcitation conditions, and the QYs stayed almost constant atdifferent UV photo-irradiation times. Consequently, thesefluorescent properties and excellent stability provide strongevidence about the application possibilities of 5 and 6 inlighting, display, sensing, and optical devices.11,19

Moreover, according to the diverse extendable ligands andTMs, it could be expected that such a luminescence tuningstrategy with TMs would expand the functional design ofsuperior luminescent CPs.

Conclusions

In summary, six new compounds of TMs have been synthe-sized under hydrothermal conditions. These title compoundspossess various structures, including 1D linear chains for1–3, a 1D double chain for 4, a 2D square-grid sheet for5 and a 3D window-shaped net for 6. Notably, different

This journal is © The Royal Society of Chemistry 2014

Table 2 Luminescence data for compounds 2–6 and the H3L and bpy ligan

Compounds λmax (nm) Φ

H3L 472 2bpy 421 22 565 23 460 64 535 65 395 116 402 10

metals induced different luminescent emissions. Additionof Co2+/Mn2+/Ag+/Cd2+/Zn2+ metal salts realized color tun-ability of weak Co(II)-based yellow, intense Mn(II)-basedblue, Ag(I)-based green, Cd(II)-based violet and Zn(II)-basedviolet luminescence upon irradiation with a standard UVlamp (λex = 254 nm) at room temperature. However theCu(II)-based compound shows fluorescence quenching. Therelatively high QYs and short fluorescence lifetimes of 2–6are indicative of the radiative recombination of the excitonsgiving rise to fluorescence. It should be further pointed outthat compounds 5 and 6 also show good photobleachingstability on excitation with UV light for as long as twelvehours. The strong luminescent properties reinforce thesecompounds as good candidates for potential photoactivematerials that could be possibly used in chemical sensors,photochemistry, electroluminescent display, lighting andoptical devices. On the basis of this work, further synthesisand luminescent property studies of TMs with H3L andother pyridine ligands are under way in our laboratory.

Acknowledgements

This work is supported by the National Basic ResearchProgram of China (973 Program) (No. 2012CB825800,2013CB932702), National Natural Science Foundation ofChina (NSFC) (No. 51132006), a project funded by the PriorityAcademic Program Development of Jiangsu Higher EducationInstitutions (PAPD), a Suzhou Planning Project of Scienceand Technology (ZXG2012028).

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ds

(%) τ (ns)

.75 0.31

.34 0.21

.05 0.15

.16 0.48(88.92%), 5.10(6.64%), 76.20(4.44%),

.27 0.44(88.67%), 4.85(6.21%), 85.72(5.12%)

.51 0.50(95.17%), 5.34(3.03%), 83.28(1.81%)

.95 0.61(82.68%), 3.91(10.73%), 86.51(6.59%)

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