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Journal of Luminescence 130 (2010) 598–602

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

n Corr

of Educ

130024

E-m

journal homepage: www.elsevier.com/locate/jlumin

Preparation, characterization, and luminescence properties of novel hybridmaterial containing europium(III) complex covalently bonded to MCM-41

Dan Wang a, Bin Li a,b,n, Liming Zhang b, Jun Ying a, Xiudong Wu a

a Polyoxometalate Science Key Laboratory of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR Chinab Key Laboratory of Excited State Processes, Changchun Institute of Optics Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, PR China

a r t i c l e i n f o

Article history:

Received 11 January 2009

Received in revised form

30 October 2009

Accepted 5 November 2009Available online 11 November 2009

Keywords:

Europium complex

MCM-41

Photostability

Covalently bond

13/$ - see front matter & 2009 Published by

016/j.jlumin.2009.11.002

esponding author at: Polyoxometalate Scienc

ation, Faculty of Chemistry, Northeast No

, PR China. Tel: +86 431 86176935.

ail address: lib020@yahoo.cn (B. Li).

a b s t r a c t

In this paper, a novel luminescent organic–inorganic hybrid material containing covalently bonded

ternary europium complex in mesoporous silica MCM-41 has been successfully prepared by co-

condensation of tetrethoxysilane (TEOS) and the modified ligand 2-phenyl-1H-imidazo[4,5-f]

[1,10]phen-3-(triethoxysilyl)propylcarbamate (PIP-Si) in the presence of cetyltrimethylammonium

bromide (CTAB) surfactant as template. PIP-Si containing 1,10-phenanthroline covalently grafted to

3-(triethoxysilyl)propyl isocyanate is used not only as a precursor but also as the second ligand for

Eu(TTA)3 �2H2O (TTA: 2-thenoyltrifluoroacetate) complex to prepare a novel functionalized mesopor-

ous material. The resulted mesoporous composite materials, which demonstrate strong characteristic

emission lines of Eu3 + 5D0-7FJ (J=0, 1, 2, 3, 4), were characterized by Fourier transform infrared (FT-IR),

small-angle X-ray diffraction, excited-state decay analysis. Emission intensity of the Eu(III) complex

covalently linked to MCM-41 (Eu-MCM-41) increases with the increasing irradiation time, demonstrat-

ing better photostability compared with both pure Eu(III) complex and physically incorporated sample.

& 2009 Published by Elsevier B.V.

1. Introduction

Historically, rare-earth complexes have been well known togive sharp, intense emission lines upon ultraviolet light irradia-tion because the effective intramolecular energy transfer fromcoordinated ligands to luminescent central lanthanide ion, whichin turn undergoes corresponding radiative emitting process (theso-called ‘‘antenna effect’’) [1–3]. However, the poor photostabil-ity of rare-earth complexes limits their practical applications.Recently, it is reported that their photophysical properties couldbe modified by interaction of a host structure. Consequently,luminescence properties of rare-earth complexes supported onsolid matrixes are studied extensively. For instance, rare-earthcomplexes were covalently linked to or physically incorporatedon host materials, including sol–gel matrix [4] and silica matrix[5]. Among them, mesoporous molecular sieve used as a supportfor rare-earth complexes has attracted particular attention [6–9].Xu et al. [10,11] and Zhang and co-workers [12–15] encapsulateddifferent rare-earth complexes into mesoporous channels ofMCM-41 and SBA-15 via wet impregnation. In the past few years,many studies focus on covalently grafting of ligands of rare-earthcomplexes to backbone of networks via Si–C bonds [16,17].

Elsevier B.V.

e Key Laboratory of Ministry

rmal University, Changchun

However, there is no report of study on photostability of Eu(III)complex covalently grafted to mesoporous materials.

In this paper, we report a novel Eu(III) complex, which iscovalently bonded to the framework of MCM-41 by co-condensationof modified phenanthroline (PIP-Si) and tetraethoxysilane (TEOS)using cetyltrimethylammonium bromide (CTAB) surfactant astemplate. Highly luminescent complex Eu(TTA)3PIP functiona-lized MCM-41 (denoted as Eu-MCM-41) is obtained by introdu-cing Eu(TTA)3 �2H2O into the PIP-MCM-41 hybrid materials with aligand exchange reaction. Thus, the ternary europium complexEu(TTA)3PIP is successfully linked to the framework of MCM-41via a covalently bonded phen group. Its photophysical propertiesare systematically studied. The photostability of Eu-MCM-41 issuperior to those of pure europium complex and physicallyincorporated ones in MCM-41.

2. Experimental

2.1. Materials and reagents

Analytical grade solvents and compounds were used forpreparations. TEOS (Tianjin Chemicals Co.), 3-(triethoxysilyl)-propyl isocyanate (ICPTES, Aldrich), CTAB (Aldrich). All reagentswere of analytical grade. Water used in our work was deionized.Europium chloride was obtained by dissolving Eu2O3 (99.99%,Shanghai yuelong) in hydrochloric acid.

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Scheme 1. Synthesis procedure and the predicted structure of the novel hybrid material Eu-MCM-41.

D. Wang et al. / Journal of Luminescence 130 (2010) 598–602 599

2.2. Synthesis of hydrolyzable 1,10-phenanthroline functionalized

PIP-Si ligand

The PIP-Si was prepared using 2-phenyl-1H-imidazo [4,5-f][1,10]phenanthroline (PIP-OH) and ICPTES as staring materials.The synthesis of PIP-OH was performed by oxidation of 1,10-phenanthroline in a mixture of concentrated sulfuric acid andnitric acid, followed by condensation of 1,-10-phenanthroline-5,6-dione and 4-hydroxy-benzaldehyde, according to a slightlymodified imidazole ring preparation method [18–20]. Thedetailed synthetic procedure of PIP-Si has been described else-where [21] and can be summarized as follows: PIP-OH (0.25 g)was mixed with an excess of ICPTES (1 mL) in a round-bottomflask, then the mixture was kept under nitrogen in an ultrasonicbath for about 15 min. After having stirred under nitrogen at 80 1Cfor 72 h, the residual was added dropwise into about 30 mL ofcold hexane to precipitate a white-yellow solid from the mixture.The final filtered-off precipitate was washed with several portionsof cold hexane and then dissolved in absolute ethanol. Thesolution was filtered, and ethanol was eliminated by evaporationto obtain a solid compound. The compound can be dissolved in asmall portion of dichloromethane, suggesting that the reactionbetween PIP-OH and ICPTES occurred since the starting PIP-OHwas not soluble in dichloromethane. The solution was added

dropwise into about 30 mL of cold hexane to reprecipitate thecompound and dried in vacuo. The product obtained by themethod described above is pure enough for use as a startingproduct for the preparation of hybrid materials.

2.3. Synthesis of phen-functionalized MCM-41 mesoporous material

(PIP-MCM-41)

The synthetic procedure of intermediate mesoporous silicaPIP-MCM-41 was similar to the previous publication with someminor modifications [15]. The synthetic procedure was as follows:concentrated NH3 �H2O (4.5 mL) was mixed with deionized water(10 mL) and CTAB (0.41 g) at 35 1C. To this homogeneous solutionTEOS (1.9 mL) and PIP-Si (0.11 g) were added under vigorousstirring. The molar ratio of the synthetic mixture was PIP-Si/TEOS/CTAB/NH3H2O/H2O=0.02:1.0:0.139:3.76:66.57. The mixture wasstirred for 10 h at room temperature and then transferred into aTeflon bottle sealed in an autoclave, which was then heated at100 1C for 48 h. The solid product was filtered, washed with H2O,and dried for 12 h at 60 1C. The surfactant was removed by acid/solvent extraction, using a solution of 1 M HCl in ethanol. Thismixture was refluxed for 72 h, then filtered and washed withEtOH to remove the residual HCl. The product was dried at 60 1Cfor 12 h in vacuo to obtain the synthesized PIP-MCM-41.

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Fig. 1. Representative FT-IR spectra of PIP-Si (A), surfactant-extracted PIP-MCM-41

(B), and Eu-MCM-41 (C).

Fig. 2. SAXRD patterns of surfactant-extracted PIP-MCM-41 (A) and Eu-MCM-41 (B).

D. Wang et al. / Journal of Luminescence 130 (2010) 598–602600

2.4. Synthesis of MCM-41 mesoporous material covalently bonded

with Eu(TTA)3PIP ternary complex (Eu-MCM-41)

The PIP-MCM-41 was added to an Eu(TTA)3 �2H2O ethanolsolution under stirring. Eu(TTA)3 �2H2O was prepared by themethod described in Ref. [2]. The mixture was heated under refluxfor 12 h and was recovered by filtration. Then the sample wassoaked in an appropriate amount of acetone and heated underreflux for half an hour, followed by filtration and extensivewashing with acetone to remove the excess of Eu(TTA)3 �2H2O. Inthe obtained material, the PIP-Si acts as a bidentate ligand andexpels two water molecules from the first coordination sphere ofEu(TTA)3 �2H2O complex. In this way, Eu(TTA)3PIP was formedand the Eu(III) complex was attached to the MCM-41 mesoporousmaterial. The resulting sample was dried at 60 1C under vacuumfor 12 h and the hybrid mesoporous product Eu-MCM-41 wasobtained as outlined in Scheme 1.

2.5. Synthesis of MCM-41 mesoporous silica with the physically

incorporated Eu(TTA)3Phen complex (MCM-41/Eu)

The Eu(TTA)3Phen complex was synthesized following theliterature procedure [22]. The MCM-41/Eu composite system wasprepared similar to that of the Eu-MCM-41 except that PIP-MCM-41and Eu(TTA)3 �2H2O were replaced by pure MCM-41 and theEu(TTA)3Phen complex, respectively.

2.6. Instrumentation and measurements

The FT-IR spectra were recorded within a 400–4000 cm-1 regionby an FT-IR spectrophotometer (Model Bruker Vertex 70 FTIR) witha resolution of 74 cm�1 using the KBr pellet technique. Small-angle X-ray diffraction (SAXRD) patterns were recorded with aRigaku-Dmax 2500 diffractometer using Cu Ka1 (l=0.15405 nm)radiation at a 0.02o (2y) canning step. Excitation and emissionspectra were recorded at room temperature using a Hitachi F-4500spectrophotometer equipped with a continuous 150 W Xe-arclamp. In the experiments of spectral change induced by UV lightirradiation, monochromic light separated from the same Xe-arclamp was used as the irradiation source, with a slit of 10 nm. Thefluorescence lifetime of Eu(III) in Eu-MCM-41 was obtained with a355 nm light generated from the Fourth-Harmonic-Generatorpump, using pulsed Nd:YAG laser as the excitation source. Allmeasurements were performed at room temperature.

3. Results and discussion

3.1. FT-IR spectra results

Fig. 1 shows the FT-IR spectra of PIP-Si (A), surfactant-extracted PIP-MCM-41 (B), and Eu-MCM-41 (C), which provesthe successful covalent grafting between Eu(TTA)3PIP andfunctionalized MCM-41 type mesoporous material. In Fig. 1A,the spectrum of PIP-Si is dominated by nas(C–Si, 1193 cm�1) andnas(Si–O, 1078 cm�1) absorption bands, characteristic oftrialkoxylsilyl functions. While, in Fig. 1B and C, the occurrenceof the grafting reaction is evidenced by the bands located at 1653and 1645 cm�1, originating from the –CONH– amide group of PIP-Si. The formation of the Si–O–Si framework is evidenced by thebands located at 1080 cm�1 (nas, Si–O), 799 cm�1 (ns, Si–O), and458 cm�1 (d, Si–O–Si) (n represents stretching, d in-plane bending,s symmetric, and as asymmetric vibrations). The appearance ofabsorption peaks of these groups in the final Eu-MCM-41 sample isconsistent with the fact that the organic groups have been

successfully grafted to the Si–O–Si networks of the inorganicmatrix through the hydrolysis–condensation reaction. And fromFig. 1B, the surfactant-extracted PIP-MCM-41 exhibits very weakn(C–H) vibrations in the region of 2700–3000 cm�1, which confirmsthat most of the surfactant has been removed.

3.2. SAXRD analysis

The powder X-ray diffraction analyses performed on surfac-tant-extracted PIP-MCM-41 (A) and Eu-MCM-41 (B) are comparedin Fig. 2. These patterns feature distinct Bragg peaks in the 2yrange of 0.8–2.51, which can be indexed as (1 0 0), (1 1 0), and(2 1 0) reflections of a two-dimensional hexagonal structure ofMCM-41 material [23]. The presence of these three diffractionpeaks indicates that all of the samples have a MCM-41 typearchitecture with long range order. Compared with the d100

spacing value of PIP-MCM-41, that of Eu-MCM-41 is nearlyunchanged, indicating that the framework hexagonal ordering

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Fig. 4. Fluorescence decay curve of Eu-MCM-41.

Fig. 5. Dependence of emission intensity at 614 nm on irradiation time of Eu-

MCM-41 (A), MCM-41/Eu (B), and Eu(TTA)3Phen (C) (lex=365 nm).

Fig. 3. Excitation and emission spectra of Eu-MCM-41 (A), MCM-41/Eu (B), and

Eu(TTA)3 phen (C) (lex=365 nm, lem=614 nm).

D. Wang et al. / Journal of Luminescence 130 (2010) 598–602 601

has been retained very well upon the introduction of Eu(TTA)3 �

2H2O.

3.3. Excitation and emission spectra

Fig. 3 shows the excitation and emission spectra for Eu-MCM-41(A), MCM-41/Eu (B), and Eu(TTA)3Phen (C) in solid state at roomtemperature. The excitation spectra of all materials were obtainedby monitoring the emission wavelength of Eu3+ ions at 614 nm.The excitation spectrum of Eu-MCM-41 becomes narrowerthan those of pure Eu(TTA)3Phen complex and MCM-41/Eu, whichsuggests that the ligand environment has been changed whenEu(TTA)3 �2H2O covalently links to MCM-41. No f–f transition ofEu3+ ions is observed in the excitation spectrum shown Fig. 3A,indicating that the energy transfer from ligands to Eu3+ ion is moreefficient in Eu-MCM-41 than that in pure Eu(TTA)3Phen complexand MCM-41/Eu. The emission lines of Eu-MCM-41 located at 590,614, 650, and 700 nm are assigned to 5D0-

7FJ transitions, for J=1, 2,3, and 4, respectively. The 5D0-

7F2 emission around 614 nm is themost predominant transition, so-called hypersensitive transitions,which is sensitive to the symmetry of coordination sphere.

3.4. Excited state lifetime

Fig. 4 shows the typical excited-state intensity decay profilefor Eu-MCM-41. The excited-state decay curve of Eu-MCM-41sample can also be well fitted by single exponential equation,confirming that Eu(III) complex covalently grafted to the networkof functionalized mesoporous MCM-41 are located on the samemicroenvironment sites, and the distribution is uniform andhomogeneous.

3.5. Photoluminescence stability

Stability of rare-earth complexes under UV irradiation is veryimportant for practical applications. To investigate the photolumi-nescence stability, ultraviolet light irradiation-induced spectralchanges in different samples are also studied. Fig. 5 shows thedependence of Eu-MCM-41 (A), Eu/MCM-41 (B) and Eu(TTA)3Phen(C) normalized emission intensity at 614 nm on irradiation time. Asshown in Fig. 5, the emission intensity of 5D0-

7F2 in pure complexdecreases with increasing exposure time, whereas, that in sample Bis nearly constant. It is more interesting to observe that theintensity in sample A even increases with increasing exposure time.Xu’s group also observed the improvement of photostability of Eucomplexes encapsulated in MCM-41 pores [10,11]. However, theimprovement is not as large as our results, which can be explainedas follows. The Eu(III) complex in the present sample is betterdistributed into the mesoporous molecule sieves and covalentlygrafted is so effective in stabilizing intensity of the Eu(III) complex.Actually, both the functionalized MCM-41 and pure MCM-41provide rigid environments to reduce energy consumption onvibration of ligands and collision of intermolecular of complexes.The emission intensity enhancement with exposure time in sampleA can be attributed to optical modification of surface defects. Duringsample preparation, a large number of surface defects are involvedin the inner surface of pores, acting as nonradiative relaxationpaths. Under UV light exposure, the defects are modified gradually,causing photoluminescence increase [24,25].

4. Conclusion

In this article, we report a covalent grafting strategy using one-pot co-condensation methods about preparation of the materials

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D. Wang et al. / Journal of Luminescence 130 (2010) 598–602602

based on Eu(III) complex covalently assembled within functiona-lized MCM-41 mesoporous materials. Their excitation andemission spectra, lifetimes and photoluminescence stabilitywere systemically studied. Eu(III) complex covalently grafted inMCM-41 emitted bright characteristic emission of Eu(III) ions andits photoluminescence stability was improved considerably incomparison to the pure complex and corresponding one thatphysically incorporated in MCM-41, which was attributed to themodification of surface defects under the exposure of ultravioletlight. Improved photostability of Eu(III) organic complex makespossible for their applications in areas of optical and electricaldevices.

Acknowledgements

The authors gratefully thank the financial supports of OneHundred Talents Project from Chinese Academy of Sciences andthe National Natural Science Foundations of China (Grant no.50872130).

References

[1] S. Sato, M. Wada, Bull. Chem. Soc. Jpn. 1970 (43) (1970) 1955.[2] M. Guan, L.H. Gao, S.S. Wang, C.H. Huang, K.Z. Wang, J. Lumin. 127 (2007) 489.

[3] A. Jain, M. Mohapatra, S.V. Godbole, B.S. Tomar, Spectrochim. Acta A 71(2008) 1007.

[4] L.R. Matthews, E.T. Knobbe, Chem. Mater. 5 (1993) 1697.[5] O.A. Serra, E.J. Nassar, G. Zapparoll, I.L.V. Rosa, J. Alloys Compd. 454 (1994)

207.[6] J.L. Liu, B.J. Yan, Phys. Chem. B 112 (2008) 10898.[7] Y. Li, B. Yan, H. Yang, J. Phys. Chem. C 112 (2008) 3956.[8] J.L. Liu., B. Yan, J. Phys. Chem. C 112 (2008) 14168.[9] B. Yan, H.F. Lu, Inorg. Chem. 47 (2008) 5601.

[10] Q.H. Xu, L.S. Li, B. Li, R.R. Xu, Microporous Mesoporous Mater. 38 (2000) 351.[11] Q.H. Xu, L.S. Li, X.S. Liu, R.R. Xu, Chem. Mater. 14 (2002) 549.[12] C.Y. Peng, H.J. Zhang, J. Phys. Chem. B 109 (2005) 15278.[13] H.R. Li, J. Lin, H.J. Zhang, H.C. Li, L.S. Fu, Q.G. Meng, Chem. Commun. 13 (2001)

1212.[14] H.R. Li, H.J. Zhang, Chem. Mater. 14 (2002) 3651.[15] L.N. Sun, H.J. Zhang, C.Y. Peng, J.B. Yu, Q.G. Meng, L.S. Fu, X.M. Guo, J. Phys.

Chem. B 110 (2006) 7249.[16] B.J. Scott, G. Wirnsberger, G.D. Stucky, Chem. Mater. 13 (2001) 3140.[17] A.C. Franville, D. Zambon, R. Mahiou, Chem. Mater. 12 (2000) 428.[18] E.A. Steck, A.R. Day, J. Am. Chem. Soc. 65 (1943) 452.[19] B. Jing, T. Wu, C. Tian, M. Zhang, T. Shen, Bull. Chem. Soc. Jpn. 73 (2000) 1749.[20] Z.Q. Bian, K.Z. Wang, L.P. Jin, Polyhedron 21 (2002) 313.[21] P. Lenaerts, A. Storms, J. Mullens, J. D’Haen, C. Gorller-Walrand, K. Binnemans,

K. Driesen, Chem. Mater. 17 (2005) 5194.[22] L.R. Melby, N.J. Rose, E. Abramson, J.C. Caris, J. Am. Chem. Soc. 86 (1964) 5117.[23] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt,

C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L.Schlenker, J. Am. Chem. Soc. 114 (1992) 10834.

[24] S.W. Li, H.W. Song, W.L. Li, X.G. Ren, S.Z. Lu, G.H. Pan, H.Q. Fan, H. Zhang,R.F. Qin, Q.L. Dai, T. Wang, J. Phys. Chem. B 110 (2006) 23164.

[25] H. Zhang, H.W. Song, H.Q. Yu, X. Bai, S.W. Li, G.H. Pan, Q.L. Dai, T. Wang, W.L.Li, S.Z. Lu, X.G. Ren, H.F. Zhao, J. Phys. Chem. C 111 (2007) 6524.