Microporous and Mesoporous Materials 128 (2010) 62–70

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials

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

Lanthanide (Tb3+, Eu3+) functionalized MCM-41 through modifiedmeta-aminobenzoic acid linkage: Covalently bonding assembly,physical characterization and photoluminescence

Ying Li, Bing Yan *

Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 April 2009Received in revised form 21 July 2009Accepted 7 August 2009Available online 12 August 2009

Keywords:Mesoporous materialChemically bondedLanthanide complexPhotoluminescence

1387-1811/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.micromeso.2009.08.005

* Corresponding author. Tel.: +86 21 65984663; faxE-mail address: byan@tongji.edu.cn (B. Yan).

Novel luminescent organic–inorganic mesoporous materials were synthesized by linking lanthanide(Tb3+, Eu3+) complexes to the functionalized ordered mesoporous MCM-41 with modified meta-amino-benzoic acid (MABA-Si) by co-condensation of tetraethoxysilane (TEOS) in the presence of the cetyltri-methylammonium bromide (CTAB) surfactant as template. meta-Aminobenzoic acid (MABA) grafted tothe coupling agent 3-(triethoxysilyl)-propyl isocyanate (TEPIC) was used as the precursor for thepreparation of mesoporous materials. The luminescence properties of these resulting materials (denotedas Ln-MABA-MCM-41, Ln = Tb, Eu) were characterized in detail, and the results reveal that luminescentmesoporous materials have high surface area, uniformity in the mesopore structure and good crystallin-ity. Moreover, the mesoporous material covalently bonded Tb3+ complex (Tb-MABA-MCM-41) exhibitsthe stronger characteristic emission of Tb3+ and longer lifetime than Eu-MABA-MCM-41 due to the tripletstate energy of organic ligand MABA-Si matches with the emissive energy level of Tb3+ very well. Com-pared with pure Tb(MABA)3 complex, the increase of luminescence intensity in Tb-MABA-MCM-41 showsthat the mesoporous MCM-41 is an excellent host for the luminescence Tb(MABA)3 complex. In addition,the luminescence lifetime and emission quantum efficiency of 5D0 Eu3+ excited state also indicates that amore efficient intramolecular energy transfer process in Tb-MABA-MCM-41 than in Eu-MABA-MCM-41.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

Lanthanide (Tb3+, Eu3+) complexes have characteristic lumines-cence properties and give sharp, intense emission lines upon ultra-violet light irradiation, because of the effective intramolecularenergy transfer from the coordinated ligands to the luminescentcentral lanthanide ion [1–3]. Therefore, they are expected to bepromising luminescent dopants for the preparation of hybrid phos-phors and other optical sources. In recent years, lanthanide organ-ic–inorganic hybrid materials, incorporation of rare earthcomplexes in the inorganic matrices have attracted considerableinterest, and the luminescence properties of lanthanide complexessupported on a solid matrix have been studied extensively becausetheir photophysical properties could be modified by interactionwith the host structure [4]. Furthermore, the incorporation of lan-thanide complexes with aromatic carboxylic acids [5], b-diketones[6], and heterocyclic ligands [7] into sol–gel-derived host struc-tures has been extensively investigated. Our research group is con-centrated on covalently grafting the ligands to the inorganicnetworks in which lanthanide complexes luminescent centers are

ll rights reserved.

: +86 21 65982287.

bonded with a siloxane matrix through Si–O linkage using differentmodified routes, including the modification of active amino group,hydroxyl groups and carboxyl groups with coupling agent, etc. [8–14]. These studies indicate that the thermal stabilities and photo-physical properties of the lanthanide complexes were improvedby the matrixes. So far, the incorporation of luminescent lantha-nide complexes in solid matrices is of wide spread interest in mate-rial science as it allows construction of functional materials withvarious optical properties [15].

Mesoporous materials M41S, have attracted much attentiondue to their unique surface area and uniform pore structure sincethey were synthesized in 1992. MCM-41, one member of theM41S family, possesses regularly hexagonal arrays of mesopores,changeable pore diameter between 1.5 and 30 nm and tailorableinterior surfaces [16,17]. These properties together with the ther-mal and mechanical stabilities make it as an ideal host for incorpo-ration of active molecules and some work has already beendevoted on this field [18–20]. For example, Zink and co-workerstook advantage of the different chemical and physical propertiesof the regions in the sol–gel films and developed new strategiesto place the active molecules, e.g. lanthanide complex luminescentmolecules, in desired inorganic silicate framework [21]. Mitchell-Koch and Borovith [22] immobilized europium complexes within

Y. Li, B. Yan / Microporous and Mesoporous Materials 128 (2010) 62–70 63

porous organic polymer hosts, and obtained luminescent materialspotentially suitable for chemical sensing. As a good host material,studies of the encapsulation and assembly of guest molecules inthe mesoporous channels have been very extensive [23–25]. Re-cently, Carlos and co-workers [26] have reported the syntheses ofMCM-41 mesoporous materials covalently bonded with ternaryeuropium complexes. It is shown that the promising visible-lumi-nescent properties can be obtained by linking the ternary euro-pium complexes to the mesoporous materials. However, thesynthesis and luminescence properties of MCM-41 mesoporousmaterials covalently bonded with lanthanide complexes by themodified aromatic carboxylic acids have not been explored to date.

Herein, we report a direct synthesis of meta-aminobenzoic acid(MABA)-functionalized MCM-41 mesoporous hybrid mate-rial(MABA-MCM-41), in which MABA was covalently bonded tothe framework of MCM-41 by co-condensation of the modifiedMABA (denoted as MABA-Si) and the tetraethoxysilane (TEOS) byusing the cetyltrimethylammonium bromide (CTAB) surfactant astemplate. The luminescent lanthanide (Tb3+, Eu3+) complexes onfunctionalized MCM-41 with modified meta-aminobenzoic acid(denoted as Ln-MABA-MCM-41 Ln = Tb, Eu) were obtained byintroducing Ln3+ into the MABA-MCM-41 hybrid material. Thus,the lanthanide complex Ln(MABA)3 was successfully linked tothe framework of MCM-41 via a covalently bonded MABA group.Full characterization and detail studies of luminescence propertiesof all these synthesized materials were investigated in relation toguest–host interactions between the organic complex and the sil-ica matrix.

2. Experimental

2.1. Materials

Cetyltrimethylammonium bromide (CTAB, Aldrich), tetraeth-oxysilane (TEOS, Aldrich), 3-(triethoxysiyl)-propyl isocyanate (TE-PIC, Lancaster), meta-aminobenzoic acid (MABA), and ethanolwere used as received. The solvent chloroform (CHCl3) was usedafter desiccation with anhydrous calcium chloride.

LnCl3 (Ln = Tb, Eu) ethanol solution (EtOH) was prepared as fol-lows: the rare earth oxide (Tb4O7, Eu2O3,) was dissolved in concen-trated hydrochloric acid (HCl), and the surplus HCl was removedby evaporation. The residue was dissolved with anhydrous ethanol.The concentration of the rare earth ion was measured by titrationwith a standard ethylenediamine tetraacetic acid (EDTA) aqueoussolution.

2.2. Synthetic procedures

2.2.1. Preparation of MABA-functionalized MCM-41 mesoporousmaterial (MABA-MCM-41)

The modified meta-aminobenzoic acid (MABA-Si) was preparedas follows: 1 mmol meta-aminobenzoic acid was first dissolved in20 mL CHCl3 with stirring. Then, 2.0 mmol (0.495 g) of 3-(trieth-oxysilyl)-propyl-isocyanate (TEPIC) was dropwise added into therefluxing solution. The mixture was heated at 75 �C in a coveredflask for approximately 12 h at the nitrogen atmosphere. After iso-lation and purification, a yellow oil sample MABA-Si was obtained.Anal: calcd. for C27H49N3O10Si2: C, 51.35; H, 7.99; N, 6.85%. Found:C, 51.62; H, 7.86; N, 6.49%. 1H NMR (CDCl3): d 12.45 (1H, s), 8.75(1H, s), 7.94 (1H, s), 7.57 (1H, t), 7.24 (1H, t), 3.77 (2H, m), 3.45(3H, m), 3.06 (2H, s), 2.66 (10H, d), 1.89 (1H, s), 1.50 (3H, t), 1.24(1H, s), 1.15 (3H, m), 1.06 (18H, m), 0.86 (1H, s). 13C NMR (CDCl3):d 169.8 (C8), 152.8 (C7), 148.5 (C1), 140.3 (C5), 128.5–128.1 (C2–C4),121.7 (C6), 117.3 (C3), 56.0 [CH2(OEt)], 45.4 (C9), 22.9 (C10), 18.2[CH3(OEt)], 10.3 (C11).

The mesoporous material MABA-MCM-41 was synthesized asfollows: CTAB (1.1 g) was dissolved in concentrated NH3�H2O(12 mL), to which deionized water (26 mL), TEOS (5.5 mL), andMABA-Si were added with the following molar composition: 0.12CTAB:0.5 NH3�H2O:0.96 TEOS:0.04 MABA-Si:58.24 H2O. The mix-ture was stirred at room temperature for 24 h and transferred intoa Teflon bottle sealed in an autoclave, which was then heated at100 �C for 48 h. Then the solid product was filtrated, washed thor-oughly with deionized water, and air-dried for 12 h at room tem-perature. Removal of the surfactant CTAB was conducted bySoxhlet extraction with ethanol for 2 days to give the sample de-noted as MABA-MCM-41.

2.2.2. Preparation of MCM-41 mesoporous material covalently bondedwith the lanthanide (Ln3+) complexes (denoted as Ln-MABA-MCM-41,Ln = Tb, Eu)

The hybrid mesoporous material was prepared by hydrother-mal process as follows: While being stirred, MABA-MCM-41 wassoaked in an appropriate amount of LnCl3 ethanol solution withthe molar ratio of Ln3+:MABA-Si being 1:3. The mixture was heatedunder reflux for 12 h, followed by filtration and extensive washingwith EtOH. The resulting Ln-MABA-MCM-41 was dried under vac-uum overnight. And the hybrid mesoporous product Ln-MABA-MCM-41 was obtained as outlined in Fig. 1.

2.3. Characterization

FT-IR spectra were measured within the 4000–400 cm�1 regionon an infrared spectrophotometer with the KBr pellet technique.1H NMR spectra were recorded in CDCl3 on a Bruker AVANCE-500 spectrometer with tetramethylsilane (TMS) as internalreference. Elemental analyses (C, H, N) were determined with anElementar Cario EL elemental analyzer. The Ultraviolet absorptionspectra of the samples in chloroform (CHCl3) solution were takenwith an Agilent 8453 spectrophotometer. X-ray powder diffractionpatterns were recorder on a Rigaku D/max-rB diffractometerequipped with a Cu anode in a 2h range from 0.6� to 6�. Nitrogenadsorption/desorption isotherms were measured at the liquidnitrogen temperature, using a Nova 1000 analyzer. Surface areaswere calculated by the Brunauer–Emmett–Teller (BET) methodand pore size distributions were evaluated from the desorptionbranches of the nitrogen isotherms using the Barrett–Joyner–Halenda (BJH) model. The fluorescence excitation and emissionspectra were obtained on a RF-5301 spectrophotometer. Lumines-cence lifetime measurements were carried out on an EdinburghFLS920 phosphorimeter using a 450 W xenon lamp as excitationsource. Scanning electronic microscope (SEM) was measured onPhilips XL30 operated. Transmission electron microscope (TEM)experiments were conducted on a JEOL2011 microscope operatedat 200 kV or on a JEM-4000EX microscope operated at 400 kV.

3. Results and discussion

3.1. MABA-Functionalized mesoporous silica MCM-41

The presence of the organic ligand MABA covalently bonded tothe mesoporous MCM-41was characterized by FT-IR and UVabsorption spectra. The IR spectra for meta-aminobenzoic acid(A), MABA-Si (B) and MABA-functionalized hybrid mesoporousmaterial MABA-MCM-41 (C) are shown in Fig. 2. In Fig. 2B, thespectrum of MABA-Si is dominated by m(C–Si, 1199 cm�1) and m(Si–O, 1089 cm�1) absorption bands, characteristic of trialkoxylsi-lyl functions. The occurrence of the grafting reaction was sup-ported by the bands located at 1668 cm�1, which originated fromthe absorption of amide groups (–CONH–). In addition, the bending

Fig. 1. Synthesis procedure and predicted structure of Ln-MABA-MCM-41 (Ln = Tb, Eu).

Fig. 2. IR spectra for meta-aminobenzoic acid (A), MABA-Si (B) and covalently bonded hybrid mesoporous material MABA-MCM-41 (C).

64 Y. Li, B. Yan / Microporous and Mesoporous Materials 128 (2010) 62–70

vibration (dNH, 1559 cm�1) further proves the formation of amidegroups. Then, a series of strong bands at around 2977, 2932,2887 cm�1 are due to the vibrations of methylene –(–CH2)3– in

the TEPIC, proving that 3-(triethoxysilyl)-propyl isocyanate hasbeen successfully grafted onto meta-aminobenzoic acid. In panelC of Fig. 1, the formation of the Si–O–Si framework is evidenced

Fig. 3. UV absorption spectra for (A) meta-aminobenzoic acid, (B) MABA-Si and (C) MABA-MCM-41 in CHCl3 solution.

Fig. 4. XRD patterns of MCM-41, MABA-MCM-41, and Ln-MABA-MCM-41 (Ln = Tb, Eu).

Y. Li, B. Yan / Microporous and Mesoporous Materials 128 (2010) 62–70 65

by the bands located at 1085 cm�1 (mas, Si–O), 802 cm�1 (ms, Si–O),and 460 cm�1 (d, Si–O–Si) (m represents stretching, d in plane bend-ing, s symmetric, and as asymmetric vibrations). Furthermore, thepeaks at 1638 and 1555 cm�1 originating from –CONH– group ofMABA-Si, can also be observed in hybrid mesoporous materialMABA-MCM-41 (C), which is consistent with the fact that theMABA group in the framework remains intact after both hydroly-sis–condensation reaction and the surfactant extraction procedure[27].

Fig. 3 shows the UV absorption spectra of (A) meta-aminoben-zoic acid, (B) MABA-Si and (C) MABA-MCM-41. Comparing theabsorption spectrum of MABA-Si (B) with that of MABA (A), wecan see a blue-shift of the major p–p* electronic transitions (from251 to 242 nm) and the disappearance of the peak centered at315 nm, indicating that modification of meta-aminobenzoic acid,

which was grafted by 3-(triethoxysilyl)-propyl isocyanate, influ-ences its corresponding absorption [28]. Moreover, a red-shift(from 243 nm in Fig. 3B to 250 nm in Fig. 3C) is observed for asol–gel film of amorphous silica containing hydrolyzed MABA-Si,suggesting that the meta-aminobenzoic acid groups were locatedon the surface of the materials [29].

3.2. Lanthanide (Ln3+) complexes covalently bonded to MABA-Functionalized mesoporous MCM-41

3.2.1. Powder XRDThe power XRD, especially small-angle X-ray diffraction (SAX-

RD) patterns and nitrogen adsorption/desorption isotherms arepopular and efficient methods to characterize highly ordered mes-oporous material with hexagonal symmetry of the space group

Table 1Textural data of MCM-41, MABA-MCM-41, and Ln-MABA-MCM-41a (Ln = Tb, Eu).

Sample d100

(nm)SBET

(m2/g)V(cm3/g)

DBJH

(nm)a0 t

MCM-41 4.15 1028 0.81 3.13 4.79 1.66MABA-MCM-41 4.07 927 0.69 2.97 4.70 1.73Tb-MABA-MCM-41 3.88 880 0.57 2.55 4.48 1.93Eu-MABA-MCM-41 3.80 872 0.54 2.50 4.39 1.89

a d100 is the d(100) spacing, a0 – the cell parameter(a0 = 2d100/p

3), SBET – the BETsurface area, V – the pore volume, D – the pore diameter, and t the wall thickness,calculated by a0–D.

66 Y. Li, B. Yan / Microporous and Mesoporous Materials 128 (2010) 62–70

p6mm. The SAXRD patterns of a pure MCM-41 mesoporous silica(which prepared according to reference [30,31]), MABA-MCM-41and a series of lanthanide-complexes functionalized MCM-41materials are presented in Fig. 4. For all materials, the patternsclearly show the order of the hexagonal array of the MCM-41structure and exhibit distinct Bragg peaks in the 2h range of 0.6–6�, which can be indexed as (1 0 0), (1 1 0), and (2 0 0) reflections.Compared with the SAXRD pattern of MCM-41, the d100 spacingvalues of Ln-MABA-MCM-41 (Ln = Tb, Eu) are nearly unchanged(see Table 1), indicating that the framework hexagonal orderinghas been retained very well upon the introduction of Ln3+

[32,33]. In addition, it is worth noting that lanthanide-complexfunctionalized materials Ln-MABA-MCM-41 appear decreasing indiffraction intensity as compared with MABA-MCM-41, which isprobably due to the presence of guest moieties inside the porechannels of host MCM-41 material, resulting in the decrease ofthe mesoporous order, but not the collapse in the pore structureof mesoporous materials [34].

3.2.2. Nitrogen adsorption–desorption isothermsN2 adsorption–desorption isotherms are used as a macroscopic

average measurement for exploring surface area, pore diameter,

Fig. 5. N2 adsorption/desorption isotherms of MCM-41 (a), MABA-MCM-41 (b), Tb-MABABJH pore distributions.

and pore volume of the material. The N2 adsorption–desorptionisotherm and pore size distribution for MABA-MCM-41, Ln-MABA-MCM-41 (Ln = Tb, Eu), and pure MCM-41 samples areshown in Fig. 5. They all display Type IV isotherms with H1-typehysteresis loops at low relative pressure according to the IUPACclassification [35,36,19,37], characteristic of mesoporous materialswith highly uniform size distributions. From the two branches ofadsorption–desorption isotherms, the presence of a sharp adsorp-tion step in the P/P0 region from 0.3 to 0.5 and a hysteresis loopat the relative pressure P/P0 > 0.35 shows that all materials processa well defined array of regular mesopores. The specific area and thepore size have been calculated by using Brunauer–Emmett–Teller(BET) and Barrett–Joyner–Halenda (BJH) methods, respectively.And the structure data of all these mesoporous materials (BET sur-face area, total pore volume, and pore size) were summarized inTable 1. It can be clearly seen that MABA-MCM-41 exhibits a smal-ler specific area and a slightly smaller pore size and pore volume incomparison with those of pure MCM-41, which might be due to thepresence of organic ligand MABA on the pore surface and the co-surfactant effect of MABA-Si, which interacts with surfactant andreduces the diameter of the micelles [38,39]. Furthermore, uponintroduction of Ln3+ (Tb3+, Eu3+) into the MABA-MCM-41, the spe-cific area, pore size, and pore volume of Ln-MABA-MCM-41 are lessthan those of MABA-MCM-41. This further confirmed the incorpo-ration of the Ln(MABA-Si)3 complexes in the channels of MCM-41.

3.2.3. High-resolution transmission electron microscopy (HRTEM)and Scanning electron micrograph (SEM)

From the TEM images (as shown in Fig. 6) of Tb-MABA-MCM-41, it can be found that the ordered pore structure was stillsubstantially conserved after the complexation. It confirms thesuggested p6mm symmetry and a well-ordered hexagonalstructure, which is also in agreement with the SAXRD and N2

adsorption/desorption isotherms. The distance between the

-MCM-41 (c), Eu-MABA-MCM-41 (d) materials. The insets show the corresponding

Fig. 6. HRTEM images of Tb-MABA-MCM-41 recorded along the [1 0 0] (A) and [1 1 0] (B) zone axes.

Y. Li, B. Yan / Microporous and Mesoporous Materials 128 (2010) 62–70 67

centers of the mesopore is estimated to be 3.91 nm, in good agree-ment with the value determined from the corresponding XRD data(see Table 1). Fig. 7 gives the scanning electron micrograph of themesoporous material containing Tb3+. It demonstrates thathomogeneous, molecular-based material Tb-MABA-MCM-41 wasobtained where no phase separation was observed because ofstrong covalent bonds bridging between the inorganic and organicphase, and that they composed quite uniformly so that the twophases can exhibit their distinct properties together.

3.2.4. Photoluminescence propertiesLuminescence measurements have been carried on these lan-

thanide-complexes functionalized hybrid mesoporous materialsat room temperature. The excitation spectra of Tb-MABA-MCM-41 (top) and the absorption spectra of MABA-Si (bottom) areshown in Fig. 8. The absorption band of MABA-Si is shorter thanthe excitation band of Tb-MABA-MCM-41, which makes it possible

Fig. 7. Scanning electron micrograph (SEM) of Tb-MABA-MCM-41.

for the energy transfer between the energy donor (MABA-Si) andenergy acceptor (Tb3+). The corresponding overlap between thesetwo spectra seems not large, which maybe due to the differencebetween Tb- MABA-MCM-41 (ordered mesoporous host Si–Oframework) and the MABA-Si (normal Si–O network without or-dered mesoporous microstructure). In addition, the efficient ligand

Fig. 8. Excitation spectrum of Tb-MABA-MCM-41 (top) and the absorption spec-trum of MABA-Si (below).

68 Y. Li, B. Yan / Microporous and Mesoporous Materials 128 (2010) 62–70

to metal ion energy transfer in Tb-MABA-MCM-41 is also investi-gated by energy difference between the triplet states of ligandand the resonance energy level of the central lanthanide ion.According to the energy transfer and intramolecular energy mech-anism [40–42], the most important factor influencing the lumines-cence properties of rare earth complexes is the intramolecularenergy transfer efficiency, which mainly depends on the two en-ergy transfer processes [43]. One is from lowest triplet level of li-gands to the emissive energy level of Ln3+ ion by the Dexter’sresonant exchange interaction theory [44,45]; the other is the re-verse energy transition by the thermal deactivation mechanism.And the energy transfer rate constants (kT) are dependent on theenergy difference (DE (Tr–Ln3+)) between the lowest triplet levelenergy of ligands and the resonant emissive energy of Ln3+

[44,45]. Based on the above two facts, the conclusion can be drawnthat DE (Tr–Ln3+) can have contrast influence on the two energytransfer process mentioned, and there should exist an optimal en-ergy difference between the triplet position of MABA-Si and theemissive energy level Ln3+ (Tb, Eu), the larger or the smaller DE(Tr–Ln3+) value will decrease the luminescence properties of rareearth. Thus the energy difference between the lowest triple stateenergy of the modified ligand MABA-Si [28] (23,923 cm�1) andthe resonance energy levels of Tb3+ (5D4, 20430 cm�1), Eu3+ (5D0,17250 cm�1) [40] were calculated, and it can be predicted thatthe triplet state energy of MABA-Si (23,923 cm�1) is more suitablefor the luminescence of Tb3+ ion than Eu3+ in the materials. So, thehybrid mesoporous material Tb-MABA-MCM-41 exhibits the bet-ter luminescence properties.

Fig. 9. Excitation (a, kem = 545 nm) and emission spectra (b) of Tb (MABA)3

(kex = 358 nm) and Tb-MABA-MCM-41 (kex = 310 nm).

Fig. 9 shows the normalized excitation and emission spectra ofpure Tb(MABA)3 complex and Tb-MABA-MCM-41 mesoporousmaterial. The excitation spectrum (Fig. 9a), monitoring the stron-gest emission band of the Tb3+ ion at 545 nm, presents a largebroad band between 250 and 450 nm in Tb-MABA-MCM-41. Thebroad band is attributed to the light absorption by the modifiedMABA ligand. Compared with the pure Tb(MABA)3 complex, theexcitation becomes narrower and the maximum excitation wave-length shifts from 358 to 310 nm for Tb-MABA-MCM-41. The blueshift of the excitation bands as the introduction of Tb3+ complexinto the mesoporous material is due to a hypsochromic effectresulting from the change in the polarity of the environment sur-rounding the terbium complex in the mesoporous material [46].From the emission spectra (Fig. 9b), characteristic Tb3+ ion emis-sions are observed. It can be clearly seen that four bands in the450–650 nm range, which are assigned to the 5D4 ?

7FJ (J = 6–3)transitions at 487, 544, 582, 620 nm, respectively. As a result, thestrong green luminescence was observed in the emission spectrawhich indicated that the effective energy transfer took place be-tween the modified MABA and the chelated Tb3+ ions. The hybridmesoporous material Tb-MABA-MCM-41 shows relatively strongemission due to the chemically covalently bonded molecular Si–O network structure between the complex and the mesoporoussilica.

The fluorescent excitation and emission spectra of Eu-MABA-MCM-41 are given in Fig. 10. The excitation spectrum was obtainedby monitoring the emission of Eu3+ at 613 nm and dominated by adistinguished band centered at 324 nm. As shown in Fig. 10a, theexcitation spectrum of the mesoporous material Eu-MABA-MCM-

Fig. 10. Excitation (a, kem = 613 nm) and emission (b, kex = 324 nm) spectra for theEu-MABA-MCM-41 material.

Y. Li, B. Yan / Microporous and Mesoporous Materials 128 (2010) 62–70 69

41 exhibits p ? p* electron transition of organic ligand from220 nm to 350 nm, and a peak at 392 nm can be observed due tothe f ? f absorption transition (7F0 ?

5D2) of Eu3+ ion. The emis-sion lines of Eu-MABA-MCM-41 (Fig. 10b) were originated from5D0 ?

7F1,5D0 ?

7F2,5D0 ?

7F4 transitions at 589, 613, and703 nm of Eu3+. The 5D0 ?

7F2 transition is a typical electric dipoletransition and strongly varies with the local symmetry of Eu3+ ions,while the 5D0 ?

7F1 transition corresponds to a parity-allowedmagnetic dipole transition, which is practically independent ofthe host material. In addition, among these transitions, 5D0 ?

7F2

transition shows the strongest emission, suggesting the chemicalenvironment around Eu3+ ions is in low symmetry [47]. As a con-sequence, The Eu-containing hybrid mesoporous material Eu-MABA-MCM-41 can not give the same high emission intensitiesas Tb-MABA-MCM-41, showing that the modified ligand MABA-Siis the most efficient for Tb3+ ion and could sensitize its correspond-ing green emission due to the proper energy level match.

3.2.5. Luminescence decay times (s) and emission quantum efficiency(g)

The luminescence decay profiles relative to Ln-MABA-MCM-41(Ln = Tb, Eu) could be fitted with single exponentials, from whichthe luminescence lifetimes were calculated to confirm that allthe Ln3+ ions occupy the same average coordination environment.Fig. 11 shows the typical decay curve of the hybrid mesoporousmaterials Tb-MABA-MCM-41. The luminescent lifetime data alsoexhibit the similar feature to luminescent intensity and Tb-MABA-MCM-41 mesoporous material presents the longer lifetime(1.15 ms) than Eu-MABA-MCM-41 (0.51 ms).

According to the emission spectrum and the lifetime of the Eu3+

first excited level (s, 5D0), the emission quantum efficiency (g) ofthe 5D0 Eu3+ excited state can be determined. Assuming that onlynonradiative and radiative processes are essentially involved inthe depopulation of the 5D0 state, g can be defined as follows [48]:

Fig. 11. The luminescence decay curves of Tb-MABA-MCM-41 (5D4).

Table 2Photoluminescent data of Ln-MABA-MCM-41 (Ln = Tb, Eu).

kem (nm) kex (nm) I (a.u.) s (ms)

Tb-MABA-MCM-41 545 487.0 422.7544.0 873.9582.0 103.2 1.15620.0 50.7

Eu-MABA-MCM-41 613 589.0 176.7613.0 198.6 0.51703.0 120.3

g ¼ Ar

Ar þ Anrð1Þ

where Ar and Anr are radiative and nonradiative transition rates,respectively. Since the magnetic dipole transition 5D0 ?

7F1 is rela-tively insensitive to the chemical environments around the Eu3+ ion,and thus can be considered as a reference for the whole spectrum.The Einstein’s coefficient of spontaneous emission (A0J) can be cal-culated according to the relation [49].

A0J ¼ A01ðI0J=I01Þðm01=m0JÞ ð2Þ

In Eq. (2), I01 and I0J are the integrated intensities of the 5D0 ?7F1

and 5D0 ? 7FJ transitions (J = 0–4) with m01 and m0J (m0J = 1/kJ) energycenters respectively. And A01 is the Einstein’s coefficient of sponta-neous emission between the 5D0 and 7F1 levels. On the basis of ref-erence [50–52], the value of A01 � 50 s�1 and the lifetime (s),radiative (Ar), and nonradiative (Anr) transition rates are relatedthrough the following equation:

Atot ¼ 1=s ¼ Ar þ Anr ð3Þ

Ar can be obtained by summing over the radiative rates A0J for each5D0 ? 7FJ transition.

Ar ¼ A01m01

I01

X4

J¼0

I0J

m0J¼X

J

A0J ð4Þ

In addition, the number of coordinated water molecules (nw) can beestimated from the experimental decay time by the empirical for-mula [53]:

nw ¼ 1:05ðAtot � ArÞ ð5Þ

According to Eqs. (1)–(5) the parameters Ar, Anr, nw and the quan-tum efficiency values (g), for the 5D0 Eu3+ ion excited state in thesample Eu-MABA-MCM-41 can be obtained, as shown in Table 2.The low quantum efficiency (g = 7.5%) of Eu3+ in Eu-MABA-MCM-41 indicates that the high nonradiative (Anr = 1810 s�1) due to theluminescence quenching of the 5D0 emitting level by the vibrationof OH or silanol. Moreover, the triple state energy of the modifiedligand MABA-Si is not quite suitable for the luminescence of Eu3+

ion comparing with Tb3+, suggesting that there is a more efficientintramolecular energy transfer process in Tb-MABA-MCM-41 thanin Eu-MABA-MCM-41, which resulting in the lower emission quan-tum efficiency in material Eu-MABA-MCM-41.

4. Conclusions

In summary, the lanthanide (Tb3+, Eu3+) complexes have beensuccessfully covalently immobilized in the ordered MCM-41 mes-oporous material by the modification of meta-aminobenzoic acidgroup (MABA) with 3-(triethoxysilyl)-propyl isocyanate (TEPIC)using a co-condensation method. The synthesis of MABA-MCM-41 provides a convenient approach of tailoring the surface proper-ties of mesoporous silicates by organic functionalization, and thederivative materials Ln-MABA-MCM-41 (Ln = Tb, Eu) all retain

1/s (ms�1) Ar (ms�1) Anr (ms�1) g (%) nw

0.870 – – –

1.961 0.15 1.81 7.5 2

70 Y. Li, B. Yan / Microporous and Mesoporous Materials 128 (2010) 62–70

the mesoporous structures. Further investigation into the lumines-cence properties of Ln-MABA-MCM-41 mesoporous materialsshow that the characteristic luminescence of the correspondinglanthanide ions (Ln3+) through the intramolecular energy transfersfrom the modified ligand to the lanthanide ions. Furthermore, theluminescent lifetime of two mesoporous materials indicates thesimilar feature to the luminescence intensities, elucidating thatthe triple state energy level of MABA-Si is more quite suitable forthe central Tb3+ than Eu3+. In addition, it should be emphasizedthat Ln3+ ion are well shielded from its chemical environmentand the drawback of limited solubility of the lanthanide complexescould be largely increased during the complexes are covalentlylinked to the matrices. Since various lanthanide complexes andpore materials are now available, we believed that it is feasibleto obtain various luminescent mesoporous materials. And as themethod of synthesis can be easily applied to other compoundsand different modified organic ligand, the desired properties ofmesoporous MCM-41 can be tailored by an appropriate choice ofthe precursors and the metal ions. In conclusion, the good lumines-cent properties of these materials, together with the highly orderedhexagonal channel structures and uniform tunable pore sizes ofMCM-41 mesoporous materials will expand their applications inoptical or electronic areas.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (20671072) and Program for New Century Excel-lent Talents in University (NCET-08-0398).

References

[1] R. Reisfeld, Struct. Bond. 106 (2004) 209.[2] G.F. DeSá, O.L. Malta, C. De Mello Donegá, A.M. Simas, R.L. Longo, P.A. Santa-

Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165.[3] N. Sabbatini, M. Guardingli, J.M. Lehn, Coord. Chem. Rev. 123 (1993) 201.[4] Q.A. Serra, I.L.V. Rosa, C.L. Medeiros, M.E.D. Zaniquell, J. Lumin. 60–61 (1994)

112.[5] D.W. Dong, S.C. Jiang, Y.F. Men, X.L. Ji, B.Z. Jiang, Adv. Mater. 12 (2000) 646.[6] K. Binnermans, P. Lenaerts, K. Driesen, C. Görller-Walrand, J. Mater. Chem. 14

(2004) 191.[7] (a) H.R. Li, J. Lin, H.J. Zhang, L.S. Fu, Q.G. Meng, S.B. Wang, Chem. Mater. 14

(2002) 3651;(b) A.C. Franville, D. Zambon, R. Mahiou, Chem. Mater. 12 (2000) 428;(c) F. Embert, A. Mehdi, C. Reyé, R.J.P. Corriu, Chem. Mater. 13 (2001) 4542.

[8] Q.M. Wang, B. Yan, Inorg. Chem. Commun. 7 (2004) 1124.[9] Q.M. Wang, B. Yan, Cryst. Growth Des. 5 (2005) 497.

[10] B. Yan, L.M. Zhao, Mater. Lett. 59 (2005) 795.[11] Q.M. Wang, B. Yan, J. Photochem. Photobiol., A: Chem. 177 (2006) 1.[12] L.M. Zhao, B. Yan, Appl. Organometal. Chem. 19 (2005) 1060.[13] Q.M. Wang, B. Yan, J. Photochem. Photobiol., A: Chem. 175 (2005) 159.[14] B. Yan, F.F. Wang, J. Organomet. Chem. 692 (2007) 2395.[15] (a) D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) 5956;

(b) C. Sanchez, B. Lebeau, F. Chaput, J.P. Boilot, Adv. Mater. 15 (2003) 1969.

[16] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992)710.

[17] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am.Chem Soc. 114 (1992) 10834.

[18] B.J. Scott, G. Wirnsberger, G.D. Stucky, Chem. Mater. 13 (2001) 3140.[19] M.H. Lim, A. Stein, Chem. Mater. 11 (1999) 3285.[20] W.H. Zhang, J.L. Shi, L.Z. Wang, D.S. Yan, Chem. Mater. 12 (2000) 1408.[21] P.N. Minoofar, R. Hernandez, S. Chia, B. Dunn, J.I. Zink, A.C. Franville, J. Am.

Chem. Soc. 124 (2002) 14388.[22] J.T. Mitchell-Koch, A.S. Borovith, Chem. Mater. 15 (2003) 3490.[23] L.C. Maria, L.C. Frances, H. Garcia, V. Marti, J.C. Scaiano, J. Phys. Chem. 100

(1996) 18152.[24] Q.H. Xu, L.S. Li, B. Li, J.H. Yu, R.R. Xu, Micropor. Mesopor. Mater. 38 (2000) 351.[25] Y.Y. Feng, Z.M. Shen, S.J. Xin, J. Rare Earths 18 (2000) 3.[26] S. Gago, J.A. Fernandes, J.P. Rainho, R.A.S. Ferreira, M. Pillinger, A.A. Valente,

T.M. Santos, L.D. Carlos, P.J.A. Ribeiro-Claro, I.S. Goncalves, Chem. Mater. 17(2005) 5077.

[27] H.R. Li, J. Lin, L.S. Fu, J.F. Guo, Q.G. Meng, F.Y. Liu, H.J. Zhang, Micropor.Mesopor. Mater. 55 (2002) 103.

[28] Q.M. Wang, B. Yan, J. Mater. Chem. 14 (2004) 2450.[29] C.E. Fowler, B. Lebeau, S. Mann, Chem. Commun. 17 (1998) 1825.[30] L.N. Sun, H.J. Zhang, C.Y. Peng, J.B. Yu, Q.G. Meng, L.S. Fu, F.Y. Liu, X.M. Guo, J.

Phys. Chem. B. 110 (2006) 7249.[31] J.Y. Zhang, Z. Luz, D. Goldfarb, J. Phys. Chem. B 101 (1997) 7087.[32] J. Hukkamäki, S. Suvanto, M. Suvanto, T.T. Pakkanen, Langmuir 20 (2004)

10288.[33] A.B. Bourlinos, Th. Karakostas, D. Petridis, J. Phys. Chem. B 107 (2003) 920.[34] L.N. Sun, J.B. Yu, H.J. Zhang, Q.G. Meng, E. Ma, C.Y. Peng, K.Y. Yang, Micropor.

Mesopor. Mater. 98 (2007) 156.[35] D.H. Everett, Pure. Appl. Chem. 31 (1972) 577.[36] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscow, R.A. Pierotti, J. Rouquerol, T.

Siemieniewska, Pure Appl. Chem. 57 (1985) 603.[37] W.H. Zhang, X.B. Lu, J.H. Xiu, Z.L. Hua, L.X. Zhang, M. Robertson, J.L. Shi, D.S.

Yan, J.D. Holmes, Adv. Funct. Mater. 14 (2004) 544.[38] C.Y. Peng, H.J. Zhang, Q.G. Meng, H.R. Li, J.B. Yu, F.J. Guo, L.N. Sun, Inorg. Chem.

Commun. 8 (2005) 440.[39] Q.Y. Hu, J.E. Hampsey, N. Jiang, C.J. Li, Y.F. Lu, Chem. Mater. 17 (2005) 1561.[40] S. Sato, M. Wada, Bull. Chem. Soc. Jpn. 43 (1970) 1955.[41] G.A. Crosby, R.E. Whan, R.M. Alire, J. Chem. Phys. 34 (1961) 745.[42] B. Yan, H.J. Zhang, S.B. Wang, J.Z. Ni, J. Photochem. Photobiol., A: Chem. 116

(1998) 209.[43] S.L. Wu, Y.L. Wu, Y.S. Yang, J. Alloys Compds. 180 (1994) 399.[44] D.L. Dexter, J. Chem. Phys. 21 (1953) 836.[45] B. Yan, H.J. Zhang, J.Z. Ni, Monafsh. Chem. 129 (1998) 151.[46] C.Y. Peng, H.J. Zhang, J.B. Yu, Q.G. Meng, L.S. Fu, H.R. Li, L.N. Sun, X.M. Guo, J.

Phys. Chem. B 109 (2005) 15278.[47] (a) P. Miranda Jr., J. Zukerman-Schpector, P.C. Isolani, G. Vicentini, L.B. Zinner,

J. Alloys Compds. 344 (2002) 141;(b) X.M. Guo, L.S. Fu, H.J. Zhang, L.D. Carlos, C.Y. Peng, J.F. Guo, J.B. Yu, R.P.Deng, L.N. Sun, New J. Chem. 29 (2005) 1351.

[48] P.C.R. Soares-Santos, H.I.S. Nogueira, V. Félix, M.G.B. Drew, R.A.S. Ferreira, L.D.Carlos, T. Trindade, Chem. Mater. 15 (2003) 100.

[49] E.S. Teotonio, J.G.P. Espínola, H.F. Brito, O.L. Malta, S.F. Oliveria, D.L.A. de Foria,C.M.S. Izumi, Polyhedron 21 (2002) 1837.

[50] L.D. Carlos, Y. Messaddeq, H.F. Brito, R.A.S. Ferreira, V.D. Bermudez, S.J.L.Ribeiro, Adv. Mater. 12 (2000) 594.

[51] M.F. Hazenkamp, G. Blasse, Chem. Mater. 2 (1990) 105.[52] R.A.S. Ferreira, L.D. Carlos, R.R. Gonçalves, S.J.L. Ribeiro, V.D. Bermudez, Chem.

Mater. 13 (2001) 2991.[53] W. De, W. Horrocks, D.R. Sudnick, J. Am. Chem. Soc. 101 (1979) 334.