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Metal–organic framework (MOF): lanthanide(III)-doped approachfor luminescence modulation and luminescent sensing†

Feng Luo*a and Stuart R. Battenb

Received 9th February 2010, Accepted 31st March 2010First published as an Advance Article on the web 19th April 2010DOI: 10.1039/c002822n

A distinct method, lanthanide(III)-doped pathway is for thefirst time utilized to replace lanthanide(III) MOFs for theaccess of MOF-based luminescent sensing of metal ions. Theresearch results revealed that this strategy is highly effectiveand displays several outstanding features.

One of the most interesting properties of metal–organic frame-works (MOFs) is luminescence behavior.1-4 Since the first lu-minescent investigation of MOFs in 2002, we have witnessednearly 200 articles and a few reviews reporting MOF luminescentproperties.5-7

Particular interest is the very recently developed MOF-basedluminescent sensing of ions or detection of explosive molecules,8-13

not only because of the wide variety of luminescent MOFs andthe synthetic versatility inherent in these materials that wouldseem to make them ideal for molecular recognition, but alsobecause of their significant potential applications in biologicaland environmental systems.14-16

In light of the excellent luminescent merits of lanthanide ionsand its sensitive antenna effect, in 2004, a lanthanide MOF for thefirst time was proposed to display luminescent sensing of Ag+.8

Thereafter, four 3d-4f 9-10 and two other lanthanide MOFs11-12

were explored for the evidence of luminescent sensing of Zn2+,Mg2+, F-, or Cu2+. According to the research results reported byChen groups,11-12 the possible mechanism for such special functionis metal–ligand coordination interactions, or supramolecule-directing ability. Obviously, among these now-demonstratedconcepts are the design and preparation of lanthanide MOFscapable of providing coordination sites or supramolecule-directingpotential. However, based on the current state of the art incrystal engineering, the rational design and preparation of desiredlanthanide MOFs is still impossible. Thereby, despite the excitingand compelling recent developments, in our view the area of MOF-based luminescent sensing is still in an immature phase, and itis believed that the present tremendous knotty problem is thediscovery of an effective and reproducible strategy to target thegoal.

In the literature, the lanthanide(III)-doped approach is exten-sively utilized to modify the optical and magnetic properties ofmetal or inorganic materials or MOFs.17-20 Stimulated by this, inour opinion the lanthanide(III)-doped phase should be expectedto perform a similar function of luminescent sensing observed in

aCollege of Biology, Chemistry and Material Science, East China Instituteof Technology, Fuzhou, Jiangxi, China. E-mail: ecit.luofeng@gmail.combSchool of Chemistry, Monash University, Vic, 3800, Australia† Electronic supplementary information (ESI) available: Fig. S1–S4.CCDC reference number 721176. For ESI and crystallographic data inCIF or other electronic format see DOI: 10.1039/c002822n

lanthanide MOFs. Herein, we take polymer 1 ([NH4]2[ZnL]·6H2O,L = 1,2,4,5-benzenetetracarboxylate), an anionic microporousframework with [NH4]+ counterions, as an example for thepurpose, based on the consideration of its structure containingregular 1D channels with the occupation of [NH4]+ counterions.Through simple cation-exchange by replacing [NH4]+ ions withlanthanide ions the generation of the desired lanthanide(III)-dopedphase will be expected. In particular, this convenient operation canbring tunable luminescence properties (such as Eu(III) dopant/red,Tb(III) dopant/green) via the choice of different lanthanide(III)dopants. As a result, polymer 1 and its lanthanide(III)-doped phaseare proved to possess the potential of luminescent sensing of metalions, whilst depending on the nature of dopants, the original blueemission of 1 is easily modulated to be red (Eu3+ dopant) or green(Tb3+ dopant) emission.

Polymer 1 was obtained from the hydrothermal self-assembly of Zn(NO3)2·6H2O, pyromellitic dianhydride, and(NH4)6Mo7O24·4H2O.‡ The structure contains badly disorderedguest waters that could not be fully located in the crystalstructure, but were quantified by element analysis (EA) andthermal gravimetric analysis (TG-DTA). In addition, the phasepurity of bulk crystal samples was confirmed by powder X-raydiffraction (XRD) studies (Fig. S3†).§

The structure has been reported by Yang and co-workers.21 Itcontains one unique zinc ion and one unique L ligand. The zincatom is tetrahedral and bonds to four carboxylate oxygens fromfour different L ligands. The Zn–O bond lengths of 1.9724(17) to1.9761(17) are in the normal range.22 In turn, the L ligand bondsto four different metal atoms, with each of the four carboxylategroups of the ligand binding in a monodentate fashion. This leadsto an overall 3D network (Fig. 1) featuring PtS topology (Fig.S1†) - the metals act as the tetrahedral nodes and the L ligandsserve as the square planar nodes.

An outstanding feature of the structure is the 1D rhombicchannels of ca. 9.6 ¥ 9.6 A2 cross-section which lie along thec axis direction (Fig. 1). The guest water molecules and NH4

+

counterions reside within these channels. TG-DTA studies indicatea weight loss due to guest water molecules between 30-120 ◦C(calc. 23.4%, exp. 22.5%). Above 185 ◦C, the continuous weightloss implies the onset of chemical decomposition (Fig. S2†).

As shown in Fig. S4†, polymer 1 in the solid state at roomtemperature affords strong photoluminescence at 437 nm, ifexcited at 339 nm. H4L or pyromellitic dianhydride are almostnon-luminescence, thus the origin of this typical blue emission maybe attributed to the ligand-to-metal charge transfer (LMCT).7

Driven by the lanthanide-doped approach, polymer 1 wasimmersed in EuCl3 or Tb(ClO4)3 solutions with concentrationsin the range of 10-3–10-6 mol L-1 or 10-3–10-7 mol L-1, respec-tively, to form lanthanide-doped phases (Eu@1 or Tb@1) as

This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 4485–4488 | 4485

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Fig. 1 View of the 3D structure of 1 along the c axis direction, withobvious 1D rhombic channels. Atom colors are as follows: C/green,N/blue, O/red, Zn/purple tetrahedron. The NH4

+ counterions lie close tothe walls of the channels. Hydrogen atoms and guest water molecules areomitted for clarity.

microcrystalline solids for subsequent luminescence studies. Thephotoluminescence spectra of the Eu/Tb-infused phase Eu/Tb@1are shown in Fig. 2. As expected, the initial blue emission of 1disappeared, and these emission spectra exhibit the characteristictransitions of the Eu(III) or Tb(III) ions. The emission peakscorrespond to the 5D0-7Fn (n = 0, 1, 2) transitions in Eu@1,i.e. 580 nm (5D0-7F0), 592 nm (5D0-7F1), and 614 nm (5D0-7F2),while the transitions in Tb@1 are 490 nm (5D4-7F6), 544 nm (5D4-7F5), 584 nm (5D4-7F4) and 620 nm (5D4-7F3). The most intensetransition is 5D0-7F2 for Eu@1 and 5D4-7F5 for Tb@1, whichimplies red and green emission, respectively. The luminescenceintensity of Eu-infused Eu@1 from a 10-3mol L-1 EuCl3 solutionis almost 30 times of that from a 10-6mol L-1 EuCl3 solution,whereas the luminescence intensity of Tb-infused Tb@1 froma 10-3mol L-1 Tb(ClO4)3 solution is only 8 times that from a10-7mol L-1 Tb(ClO4)3 solution. When the concentration of EuCl3

or Tb(ClO4)3 solutions is below 10-6 or 10-7 mol L-1, respectively,the lanthanide-doped phases give strong blue emission, indicatingthat this amount of lanthanide dopants display a negligible effecton the luminescence modulation.

Subsequently, Eu/Tb@1 obtained from 10-3mol L-1 EuCl3 orTb(ClO4)3 solutions were immersed in 10-2mol L-1 MClx solutions(M = Na+, K+, Zn2+, Ni2+, Mn2+, Co2+, Cu2+) to form further metal-ion infused phases (M–Eu@1 or M–Tb@1) as microcrystallinesolids for further luminescence studies. As shown in Fig. 3, itis clear that the luminescence intensity of the metal-ion-infusedM–Eu@1 or M–Tb@1 is highly dependent on the nature ofmetal ion: Na+, K+, Zn2+ ions show a negligible effect on theluminescence intensity, whereas others show a range of quenchingeffects on the luminescence intensity. In particular, it is notablethat for M–Eu@1, Cu2+ ions afford the most significant effect,whilst for M–Tb@1, Co2+ ions show the most significant effect.As illustrated in Fig. 4, the luminescence intensity of Cu–Eu@1from a 10-6mol L-1 CuCl2 solution is about 16 times of that froma 10-2mol L-1 CuCl2 solution, while the luminescence intensityof Co–Tb@1 from a 10-6mol L-1 CoCl2 solution is about 28times of that from a 10-2mol L-1 CoCl2 solution. This indicatesthat the lanthanide-doped Eu@1 or Tb@1 are suitable as highlyselective, sensitive, and low-detection-limit luminescent sensors ofaqueous Cu2+ or Co2+ ions. In literature, this kind of investigationof highly selective, sensitive, and low-detection-limit luminescentsensors of aqueous transition-metal ions is highly attractive,due to its significant potential application in environment andbiological systems.23 To the best of our knowledge, the presentwork should be the first MOF solid material owning the po-tential for the luminescent sensors of aqueous transition-metalions.

To clarify the possible mechanism for this new luminescencephenomenon, EA and ICP studies were employed for 1, Eu@1,Tb@1, Cu–Eu@1, and Co–Tb@1. The detailed preparation ofthese compounds is included in the ESI.† EA indicated C/25.92,H/4.79, N/6.13% for 1, C/17.59, H/3.05, N/1.45% for Eu@1,C/18.53, H/2.66, N/1.00% for Cu–Eu@1, C/16.08, H/2.55,N/2.06% for Tb@1, and C/17.89, H/3.07, N/0.44% for Co–Tb@1. ICP analysis gives the Zn : Eu ratio of 1 : 1 for Eu@1,Zn : Eu : Cu ratio of 1 : 0.5 : 1 for Cu–Eu@1, Zn : Tb ratio of 1 : 0.9for Tb@1, and Zn : Tb : Co ratio of 1 : 0.4 : 0.8 for Co–Tb@1. Inlight of these studies, we can deduce the formula for Eu@1, Tb@1,Cu–Eu@1, and Co–Tb@1, see Table 1. As illustrated in Table 1,it is clear that from 1 to Eu@1/or Tb@1 the cation exchangeoccurs between Eu(III)/or Tb(III) and (NH4)+, then from Eu@1/or

Fig. 2 Photoluminescence spectra of polymer 1 activated in various concentrations of EuCl3 (left) or Tb(ClO4)3 (right) in water, respectively.

4486 | Dalton Trans., 2010, 39, 4485–4488 This journal is © The Royal Society of Chemistry 2010

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Fig. 3 5D0-7F2 or 5D4-7F5 transition intensities of Eu@1 (left) or Tb@1 (right) activated in different 10-2mol L-1 MClx (M = Na+, K+, Zn2+, Ni2+, Mn2+,Co2+, Cu2+) aqueous solutions, respectively.

Fig. 4 Photoluminescence spectra of polymer Eu@1 (left) or Tb@1 (right) activated in various concentrations of CuCl2 or CoCl2 solutions, respectively.

Table 1 The detailed EA and ICP studies

Compounds EA(%) ICP Formula

1 exp. C/25.92, H/4.79, N/6.13 [NH4]2[ZnL]·6H2OEu@1 calc. C/26.12, H/4.82, N/6.09 Zn : Eu = 1 : 1 [NH4]0.7[Eu][ZnL][Cl]1.7(H2O)8

Cu–Eu@1 exp. C/17.59, H/3.05, N/1.45 Zn : Eu : Cu = 1 : 0.5 : 1 [NH4]0.5[Eu]0.5[Cu][ZnL][Cl]2(H2O)7

Tb@1 calc. C/17.54, H/3.06, N/1.43 Zn : Tb = 1 : 0.9 [NH4]1.1[Tb]0.9[ZnL][ClO4]1.8(H2O)6

Co–Tb@1 exp. C/18.53, H/2.66, N/1.00 Zn : Tb : Co = 1 : 0.4 : 0.8 [NH4]0.2[Tb]0.4[Co]0.8[ZnL][ClO4](H2O)9

calc. C/18.17, H/2.74, N/1.05exp. C/16.08, H/2.55, N/2.06calc. C/15.69, H/2.42, N/2.01exp. C/17.89, H/3.07, N/0.44calc. C/17.37, H/3.03, N/0.41

Tb@1 to Cu–Eu@1/or Co–Tb@1 both Eu(III)/or Tb(III) and(NH4)+ cation ions are replaced by Cu(II)/or Co(II) ions. PXRDresearches suggest that the new phase of Eu@1 and Tb@1 havedifferent crystal lattice from 1, as well as the PXRD patterns ofCu–Eu@1 and Co–Tb@1 are also different from Eu@1 and Tb@1,respectively, see Fig. S3.†

In summary, we have for the first time disclosed a distinctlanthanide-doped approach to target MOF-based luminescentsensing. Our research results revealed that the present strategy

is highly effective. Notably, as discussed above, for M–Eu@1,Cu2+ ions afford the most significant effect, while for M–Tb@1, Co2+ ions show the most significant effect. This distinctsensing diversity depended on the nature of lanthanide dopantsis unprecedented. Moreover, this tunable solid luminescenceproperty reveals that it is possible to obtain a white emissionunder UV radiation by introducing mixed Eu(III)/Tb(III) dopantsin well-chosen relative ratios. We are currently pursuing thisobjective.

This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 4485–4488 | 4487

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Acknowledgements

This work was funded by the Start-up Foundation of East ChinaInstitute of Technology. We thank Prof. Long and Fisher for theirvaluable suggestions.

Notes and references

‡ Synthesis of 1: The mixture of Zn(NO3)3·6H2O, pyromellitic dianhydride,and (NH4)6Mo7O24·4H2O in a ratio of 1 : 1 : 0.5 was sealed in a Teflonreactor, and heated at 200 ◦C for six days, and then cooled to roomtemperature at 3 ◦C h-1. Subsequently, colorless block crystals wereobtained in 88% yield based on Zn. Element analysis (%) for 1: calc:C 27.14, H 2.96, N 6.33; found: C 27.23, H 2.92, N 6.36.§ Single-crystal X-ray crystallography: Data collections were performedwith Mo radiation (0.71073 A) on a Bruker P4 detector diffractometer.The structures were solved by direct methods and all non-hydrogen atomswere subjected to anisotropic refinement by full matrix least-squares onF2 using the SHELXTL program. The badly disordered guest watermolecules are treated by the PLATON Squeeze routine. Crystal data for 1:C10H22N2O14Zn, 459.70, T = 296(2) K, monoclinic, C2/c, a = 11.548(2) A,b = 15.319(3 A, c = 11.021(2) A, b = 92.36(3)◦, V = 1947.9(7) A3, F(000) =680, Z = 4, 9467 reflections collected, 2232 unique, and the number of‘‘observed’’ data was 1906, Rint = 0.0576, S = 1.052, R1 = 0.0389, wR2 =0.1105. The position and composition of lattice water molecules couldnot be accurately determined because of structural disorder, which areconfirmed by EA and TG-DTA analysis. CCDC number 721176(1).

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