Synthesis, characterisation and luminescence investigations of Eu activated CaAl2O4 phosphor

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Microporous and Mesoporous Materials 113 (2008) 453–462

Synthesis, characterisation and luminescence properties ofMCM-41 impregnated with an Eu3+ b-diketonate complex

Sofia M. Bruno a, Rute A. Sa Ferreira b, Luıs D. Carlos b,*, Martyn Pillinger a,Paulo Ribeiro-Claro a, Isabel S. Gonc�alves a,*

a Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugalb Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

Received 24 September 2007; received in revised form 29 November 2007; accepted 5 December 2007Available online 15 December 2007

Abstract

A luminescent material has been prepared by formation of the Eu3+ b-diketonate complex Eu(NTA)3L00 {NTA = 1-(2-naphthoyl)-3,3,3-trifluoroacetonate; L00 = ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate} within the channels of MCM-41-type ordered mesoporous silica.Using simple wet impregnation methods, the ligand L00 was first encapsulated followed by treatment with the complex Eu(NTA)3(H2O)2.An analogous Gd3+ supported material was also prepared, as well as the model complexes Ln(NTA)3L0 {Ln = Eu, Gd; L0 = 2-[3(5)-pyr-azolyl]pyridine}. The materials were characterized by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), FTIR andFT Raman spectroscopies, 13C and 29Si MAS NMR, and photoluminescence spectroscopy. The spectroscopic studies showed thatexchange of the H2O ligands for the bidentate ligand L00 occurred to give the desired 8-coordinate species Ln(NTA)3L00, with a localLn3+ coordination environment similar to that for the model complexes. The low (�259 �C) and room temperature emission spectraof the Eu-modified mesoporous material displayed the typical Eu3+ intra-4f6 lines ascribed to transitions between the 5D0,1 excited statesand the ground multiplet (7F0–4). Negligible emission from the organic part of the encapsulated species was observed, indicating thatenergy transfer from the ligands to the Eu3+ ion was quite efficient. The 5D0 quantum efficiency was estimated to be 21.1%, comparedwith 46.1% for the model complex Eu(NTA)3L0. This difference is attributed to the presence of an excited state of the organic ligands inthe supported material that lies approximately resonant with the 5D1 intra-4f6 level, opening up an additional non-radiative channel thatis not present in the corresponding model complexes.� 2007 Elsevier Inc. All rights reserved.

Keywords: Mesoporous materials; Europium; Gadolinium; b-Diketone; Luminescence

1. Introduction

Europium ions and their complexes have characteristicluminescent properties and are of great interest for a widerange of applications such as tuneable solid-state lasers,optical amplifiers, displays, phosphor devices and lumines-cent labels [1,2]. Lewis-base adducts of europium(III)

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.12.004

* Corresponding authors. Tel.: +351 234 378190; fax: +351 234 370084(I.S. Gonc�alves).

E-mail addresses: [email protected] (L.D. Carlos), [email protected](I.S. Gonc�alves).

tris(b-diketonate) complexes, with the general formulaEu(diket)3Ln, have received particular attention as theyexhibit very efficient emission under ultraviolet excitation[2]. Through the so-called ‘‘antenna effect”, the metal-cen-tred luminescence can be optimised by choosing ligandswith organic chromophores that have high absorptioncross-sections and promote an efficient intramolecularenergy transfer to the lanthanide ion.

Despite the advances made with these types of com-plexes, they have been excluded from practical applicationsdue, essentially, to their low thermal and photochemicalstability and mechanical properties. In order to overcomethese drawbacks, the lanthanide complexes have been

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454 S.M. Bruno et al. / Microporous and Mesoporous Materials 113 (2008) 453–462

incorporated into inorganic, organic or hybrid inorganic–organic hosts by low-temperature soft-chemistry processes[3]. Examples of suitable hosts include cyclodextrins [4],liquid crystals [5], sol–gel-derived hybrids such as di-urea-sils [6], zeolites [7] and mesoporous materials [8]. Orderedmesoporous materials, prepared using surfactant tem-plates, are particularly interesting as hosts for optical func-tionalities [8]. Europium tris(b-diketonates) Eu(diket)3,with or without additional Lewis-base ligands suchas H2O, triphenylphosphine oxide, 2,20-bipyridine and1,10-phenanthroline (phen), have been immobilised inhexagonal and cubic mesoporous silicas by simple wetimpregnation methods and chemical vapour infiltration[9]. The mesoporous supports were used either in the purelysiliceous form or modified with methyl, propylamine orethylenediamine groups. Various types of host–guest inter-actions were described for the resultant materials, rangingfrom hydrogen-bonding interactions between the rare-earth complex and the silylating agent, to iono-covalentSi–O–Eu bonds. Another approach concerns the complex-ation of lanthanides by ligands that are covalentlyanchored to the support [10]. Specifically, Ln(diket)3 com-plexes (Ln = Eu, Gd, Er, Nd, Yb) were immobilised inmesostructured materials by a ligand exchange reactionbetween Ln(diket)3(H2O)2 precursors and tethered pyrazol-ylpyridine [10a] or phen [10b,10c,10d] groups. In the for-mer case (Fig. 1), the spectroscopic studies confirmed thesuccessful immobilisation of coordinatively saturated com-plexes and pointed to the existence of an unusual two-stepintermolecular energy transfer between ‘‘free” and com-plexed pyrazolylpyridine ligands, which culminated in theobservation of enhanced Eu3+ luminescence.

In the present work, we have used a simple impregna-tion method to encapsulate europium and gadoliniumtris(b-diketonate) complexes bearing a bidentate pyrazolyl-pyridine ligand in an MCM-41-type mesoporous silica.The final hybrid mesostructured materials have beencharacterised by a combination of techniques such aspowder XRD, TGA, solid-state NMR, vibrational and

Fig. 1. Representation of the tethered europium(III) tris(b-diketonate)complex described in Ref. [10a].

photoluminescence spectroscopies. Our interest in theimmobilisation of these complexes in MCM-41 stems fromthe fact that the photostability under UV radiation isenhanced and the ligand-to-metal energy transfer couldbe substantially improved, increasing, therefore, the emis-sion quantum yield. Furthermore, the nature of the interac-tion between the surface of the host and the lanthanide-coordinated ligands (covalently anchored or weaklybonded) plays an important role in that energy transferprocess.

2. Experimental

2.1. Characterisation methods

Microanalyses for CHN and ICP-AES determination ofEu and Gd were performed at the University of Aveiro.TGA was carried out under air using a Shimadzu TGA-50 system at a heating rate of 5 �C min�1. Powder XRDdata were collected on a Philips X’pert diffractometer usingCu Ka radiation filtered by Ni (k = 1.5418 A). Sampleswere step-scanned in 0.02� 2h steps with a counting timeof 2 s per step. FTIR spectra were obtained with a FTIRMattson-7000 infrared spectrophotometer, using KBr pel-lets. Raman spectra were recorded on a Bruker RFS100/S FT instrument (Nd:YAG laser, 1064 nm excitation, InG-aAs detector). Solid-state magic-angle-spinning (MAS)NMR spectra were recorded at 79.49 MHz for 29Si and125.76 MHz for 13C on Bruker Avance 400/500 spectrom-eters. 29Si MAS NMR spectra were recorded with 40�pulses, spinning rates of 5.0–5.5 kHz and 60 s recycledelays. 29Si CP MAS NMR spectra were recorded with5.5 ls 1H 90� pulses, a contact time of 8 ms, a spinning rateof 5 kHz and 4 s recycle delays. 13C CP MAS NMR spectrawere recorded with 3.5 ls 1H 90� pulses, a contact time of2 ms, a spinning rate of 7 kHz and 4 s recycle delays. Thephotoluminescence spectra were recorded between�259 �C and room temperature on a Fluorolog-3 ModelFL3 – 2T spectrometer with double excitation and aTRIAX 320 single emission spectrometer coupled to aR928P photomultiplier, using a front face acquisitionmode. The excitation source was a 450 W Xenon lamp.Emission was corrected for the spectral response of themonochromators and the detector using the typical correc-tion spectrum provided by the manufacturer and the exci-tation spectra were corrected for the spectral distributionof the lamp intensity using a photodiode reference detector.The emission decay curves were acquired between �259 �Cand room temperature with the setup described aboveusing a pulsed Xe–Hg lamp (6 ls pulse at half width and20–30 ls tail). The absolute emission quantum yields weremeasured at room temperature using a Quantum YieldMeasurement System C9920-02 from Hamamatsu with a150 W Xenon lamp coupled to a monochromator forwavelength discrimination, an integrating sphere as sam-ple chamber and a multi-channel analyser for signaldetection.

S.M. Bruno et al. / Microporous and Mesoporous Materials 113 (2008) 453–462 455

2.2. Starting materials and intermediates

Purely siliceous MCM-41 was synthesised as describedpreviously using [CH3(CH2)13N(CH3)3]Br as the templat-ing agent [11]. The surfactant template was removed fromthe as-synthesised material by calcination at 540 �C underair for 6 h. Powder XRD (2h/�, hk l in parentheses):2.51 (100), 4.31 (110), 4.97 (20 0), 6.59 (210); a =2d100

p3 = 40.6 A. Europium(III) chloride hexahydrate,

gadolinium(III) chloride hexahydrate, 1-(2-naphthoyl)-3,3,3-trifluoroacetone, CHCl3, CDCl3, CH2Cl2, tolueneand n-hexane were purchased from Aldrich and usedas received. Literature procedures were used to prepare2-[3(5)-pyrazolyl]pyridine (L0) [12a], ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate (L00) [12b] and Ln(NTA)3(H2O)2

(Ln = Eu, Gd) [13]. Before use, physisorbed water wasremoved from the ligands L and calcined MCM-41 byheating at 75 and 180 �C, respectively, under reduced pres-sure for 2 h. All preparations were carried out using stan-dard Schlenk techniques under argon.

2.3. Ln(NTA)3{2-[3(5)-pyrazolyl]pyridine} [Ln = Eu (1),

Gd (2)]

A solution of 2-[3(5)-pyrazolyl]pyridine (L0) (0.15 g,1 mmol) in CHCl3 (10 mL) was added to a solution ofLn(NTA)3(H2O)2 (1 mmol) in CHCl3 (5 mL) at room tem-perature. After stirring the resultant solution for 4 h, thesolvent was removed under reduced pressure and the solidwashed with n-hexane, and finally dried under reducedpressure.

2.3.1. Eu(NTA)3{2-[3(5)-pyrazolyl]pyridine} (1)

Yield: 0.81 g (74%). Anal. Calcd for C50H31EuF9N3O6

(1092.7): C, 54.96; H, 2.86; N, 3.85. Found: C, 54.80; H,3.04; N, 3.44%. IR (KBr, cm�1): 1677m, 1612s, 1570w,1530w, 1510w, 1460w, 1430w, 1384w, 1354m, 1295 s,1251w, 1227w, 1217w, 1187m, 1133s, 1073vw, 1053vw,957m, 936w, 863m, 791s, 765m, 749m, 683s, 567s,518m, 470s, 458sh. Raman (cm�1): 3126w, 3057m,3019m, 2924w, 1677w, 1628s, 1596s, 1572w, 1532w,1511w, 1468s, 1445w, 1432m, 1386vs, 1354w, 1297m,1228w, 1219w, 1198w, 1146w, 1127w, 1070vw, 1052vw,1019w, 1003vw, 963w, 936w, 870w, 771m, 722vw,711vw, 684vw, 517m, 468w, 393w, 373vw, 308vw,289vw, 229w, 194w.

2.3.2. Gd(NTA)3{2-[3(5)-pyrazolyl]pyridine} (2)

Yield: 1.04 g (95%). Anal. Calcd for C50H31GdF9N3O6

(1098.0): C, 54.69; H, 2.85; N, 3.83. Found: C, 55.15; H,3.30; N, 3.75%. IR (KBr, cm�1): 1677m, 1612vs, 1570s,1531s, 1509s, 1459m, 1430m, 1385w, 1353m, 1299vs,1250m, 1227w, 1217w, 1197s, 1135s, 1074w, 1053w,958m, 936w, 864m, 823w, 792s, 765m, 749m, 684m,568m, 520w, 472w. Raman (cm�1): 3125w, 3057m,3019w, 2925w, 1678vw, 1629s, 1596m, 1572w, 1533w,1511w, 1468s, 1445w, 1432m, 1386vs, 1354w, 1297m,

1228w, 1219w, 1199w, 1147w, 1127w, 1071vw, 1052vw,1019w, 1003vw, 962w, 937w, 871vw, 771m, 721vw,711vw, 685vw, 517m, 393w, 375vw, 308vw, 289vw,229vw, 192vw.

2.4. Encapsulation of ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate (L00) in MCM-41

A solution of L00 (0.2 g, 0.86 mmol) in CH2Cl2 (4 mL)was added to a suspension of MCM-41 in toluene(20 mL). The mixture was refluxed under argon overnight.The solution was filtered off and the solid washed withCH2Cl2, and dried under reduced pressure at 140 �C for1 h. The resultant material will be denoted as L00-MCM-41. Elemental analysis found: C, 8.75; H, 1.27; N, 2.86%.IR (KBr, cm�1): 1741m, 1633m, 1572vw, 1498w, 1465w,1401vw, 1363vw, 1214sh, 1083vs, 957s, 796s, 575m, 460s.Raman (cm�1): 3150w, 3071m, 2948m, 2882w, 1636w,1604s, 1573m, 1530vs, 1499m, 1447m, 1407m, 1365w,1284w, 1242w, 1155w, 1060w, 1006m, 1011m, 958m,870w, 798w, 694w, 629w. 13C CP MAS NMR: d = 167.6(C@O), 150.0 (C3, C7, C11), 133.4 (C5, C9), 123.0 (C8,C10), 105.0 (C4), 61.9 (C13), 52.9 (C12), 11.9 (C14) ppm.29Si MAS NMR: d = �101.5 (Q3), �109.0 (Q4) ppm. 29SiCP MAS NMR: d = �90.5 (Q2), �100.6 (Q3), �107.9(Q4) ppm.

2.5. Immobilisation of Ln(NTA)3

A solution of Ln(NTA)3(H2O)2 (0.14 mmol) in CHCl3(15 mL) was added to a suspension of L00-MCM-41(0.20 g, containing 0.14 mmol encapsulated ligand L00) inCHCl3 (15 mL) and the mixture stirred at room tempera-ture for 24 h. The solution was then filtered off, and thesolid washed several times with CHCl3 and dried underreduced pressure. The resultant materials will be denotedas Eu/L00-MCM-41 and Gd/L00-MCM-41.

2.5.1. Eu/L00-MCM-41

Elemental analysis found: C, 4.90; H, 1.60; N, 0.37; Eu,1.20%. IR (KBr, cm�1): 1628m, 1533w, 1383w, 1294m,1230sh, 1090vs, 963m, 794m, 667w, 556sh, 466s. Raman(cm�1): 3148w, 3060m, 2937m, 2883w, 1629vs, 1597m,1573m, 1531m, 1469s, 1432m, 1386vs, 1354w, 1301m,1228m, 1198m, 1148w, 1129w, 1054w, 1013m, 962m,938w, 873w, 770 m, 686w, 517m, 392w, 305w, 229w,187w.

2.5.2. Gd/L00-MCM-41

Elemental analysis found: C, 4.66; H, 1.51; N, 0.40; Gd,1.17%. IR (KBr, cm�1): 1635m, 1533w, 1456w, 1399w,1299sh, 1228sh, 1087vs, 963m, 794m, 684vw, 558sh, 462s.Raman (cm�1): 3147w, 3063m, 2985w, 2938m, 2903w,1630vs, 1598s, 1572m, 1531m, 1469s, 1433m, 1385vs,1354w, 1302m, 1229m, 1218m, 1198m, 1147w, 1129w,1054w, 1019m, 963m, 938w, 872w, 770m, 687w, 517m,392w, 306w, 228w, 187w.

Fig. 2. Powder XRD patterns of (a) MCM-41, (b) L00-MCM-41 and (c)Eu/L00-MCM-41.


3. Results and discussion

3.1. Synthesis of model compounds and supported materials

The model complexes Ln(NTA)3L0 [Ln = Eu (1), Gd(2)] bearing the ligand 2-[3(5)-pyrazolyl]pyridine (L0) wereobtained as light yellow solids by the reaction ofLn(NTA)3(H2O)2 with 1 equiv. of L0 in CHCl3 at roomtemperature (Chart 1). Both complexes were characterisedby thermogravimetric analysis, vibrational and photolumi-nescence spectroscopies. The europium(III) complex 1

decomposes in three main steps between 125 and 525 �C,leaving a residual mass of 18% (not shown). The masslosses for each of these steps are 16% up to 210 �C, 53%between 210 and 350 �C, and 10% between 450 and525 �C. The last two steps match those exhibited by thestarting material Eu(NTA)3(H2O)2, and can therefore beattributed to decomposition of the NTA anions. TGA ofEu(NTA)3(H2O)2 also shows a mass loss up to 100 �C cor-responding to the removal of coordinated water. This stepis not observed for complex 1, indicating that the two H2Omolecules in each complex were successfully exchanged byone ligand L0. Indeed, the first step in the TGA curve of 1

can be assigned to the decomposition of the pyrazolylpyri-dine ligand since TGA of the pure ligand L0 shows that thecompound sublimes and decomposes in one step in therange 125–225 �C. The FTIR and Raman spectra of com-plexes 1 and 2 are dominated by the bands due to theNTA ligands, which undergo only minor shifts or changesin relative intensity compared with the correspondingbands for Ln(NTA)3(H2O)2. In the Raman spectra, thevery weak bands at about 1572, 1445, 1052, 1003, 711and 289 cm�1 are assigned to the ligand L0. A band atabout 374 cm�1, which is not exhibited by either the freeligand L0 or Ln(NTA)3(H2O)2, is tentatively assigned toan Ln–N stretching mode. A similar assignment was madepreviously for the 366 cm�1 band exhibited by the com-plexes Ln(NTA)3phen (Ln = Eu, Gd) [14].

Treatment of purely siliceous MCM-41 with the ligandethyl[3-(2-pyridyl)-1-pyrazolyl]acetate (L00) in refluxing tol-uene/CH2Cl2 gave a supported material (hereafter denotedas L00-MCM-41) with a ligand loading of 0.61 mmol g�1,based on the carbon content (and the assumption thatthe ligand was not chemically modified during the immobil-isation process; this was later confirmed by the 13C MAS

Chart 1.

NMR studies described below). Powder XRD showed thatthe structural integrity of the mesoporous host was pre-served during the immobilisation process (Fig. 2). TGAof L00-MCM-41 revealed a single decomposition step of11.1% between 200 and 400 �C (Fig. 3). For comparison,the pure ligand L00 decomposes in one step between 150and 260 �C with nearly 100% mass loss. The mass loss of11.1% for L00-MCM-41 is lower than the calculated valueof 14.1% based on the ligand loading of 0.61 mmol g�1.

Fig. 3. TGA curves of ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate (L00) (—),L00-MCM-41 (- - -), Eu(NTA)3L00 (� � �) and Eu/L00-MCM-41 (�–�–).

Scheme 1. Schematic drawing of the supported materials, showing thenature of the likely host–guest interactions. For clarity, only one of thethree NTA ligands in Ln/L00-MCM-41 is fully represented.


However, the TGA of L00-MCM-41 shows that the sampleadsorbed a significant amount of water during transfer ofthe sample to the TGA instrument under air, whichresulted in a dehydration step of about 20% from roomtemperature to 125 �C. It follows that the TG-derivedligand loading in L00-MCM-41 is 13.9% on an anhydrousbasis, which is in good agreement with the value deter-mined from microanalysis for carbon. The higher thermalstability of the organic species in L00-MCM-41 is consistentwith the successful encapsulation of the ligand within thechannels of the silica support and points to the presenceof a significant host–guest interaction.

FTIR spectra were measured for L00, MCM-41 and L00-MCM-41 in the range 400–4000 cm�1. MCM-41 exhibitsa very broad band centred at about 3500 cm�1 assignedto O–H stretching of adsorbed water molecules and/orhydrogen-bonded surface Si–OH groups. Under the exper-imental conditions used to acquire the spectra, it was notpossible to observe the absorption band for free OH groupson the silica surface, which usually appears as a sharp bandat about 3740 cm�1. The Si–O–Si vibrations appear as avery strong band at 1083 cm�1 and a shoulder at about1220 cm�1 (asymmetric stretching), a medium intensityband at 796 cm�1 (symmetric stretching), and a strongband at 460 cm�1 (in-plane bending). The medium inten-sity band at 963 cm�1 and the weak shoulder at about555 cm�1 are attributed to Si–O vibrations in silanols[15]. After inclusion of the ligand L00 in MCM-41, the Si–O–Si absorption bands were unaffected. On the other hand,the 963 and 555 cm�1 bands shifted to 957 and 575 cm�1,which may be an indication for hydrogen-bonding interac-tions between L00 and MCM-41 [16], most likely involvingthe 2-substituted pyridyl rings and surface silanol groups(Scheme 1) [17]. The existence of such an interaction is rein-forced by the solid-state crystal structure of ethyl[3-(2-pyr-idyl)-1-pyrazolyl]acetate monohydrate [18], in which eachN-atom of a pyridyl ring is involved in a strong and highlydirectional O–H� � �N hydrogen-bonding interaction withone water molecule of crystallisation. The FTIR spectrumof L00-MCM-41 in the region 1300–1800 cm�1 containedseveral weak bands that could be assigned to the guest mol-ecule: 1741 (mC@O), 1572, 1498, 1465 and 1401 cm�1. Thesebands are shifted by 3–6 cm�1 to higher frequency com-pared with the corresponding bands for L00 in the solid-state. Similarly, the Raman spectrum of L00-MCM-41 inthe regions 900–1650 cm�1 and 2850–3150 cm�1 showedseveral bands for the ligand modes that were either unshif-ted compared with the corresponding bands for the freeligand, or shifted to higher frequency by up to 10 cm�1.

Fig. 4 shows the 13C and 29Si (CP) MAS NMR spectraof L00-MCM-41. The two broad overlapping peaks at�101.5 and �109.0 ppm in the 29Si MAS NMR spectrumare assigned to Q3 and Q4 species of the silica framework,respectively [Qn = Si(OSi)n(OH)4�n]. Application of crosspolarisation allowed the Q2 peak to be observed as a weakpeak at about �91 ppm, and there is a marked increase inthe intensity of the Q3 signal relative to that for the Q4

peak. These changes are as expected given that the Q2

and Q3 silicons are attached to hydroxyl groups. Very sim-ilar spectra were obtained for the MCM-41 precursor (notshown), suggesting that the surface of the support was notchemically modified to a measurable extent during theinclusion process. This conclusion was supported by the13C CP MAS NMR spectrum, which could be assigned inaccordance with the 13C NMR spectrum of the free ligandL00 in solution. The resonances for the CO, pyrazolyl andpyridyl ring carbon atoms appear in the range d = 100–170 ppm, the methylene group presents a signal atd = 52.9 ppm, and the resonances for the ethyl groupappear at d = 11.9 and 61.9 ppm. All of these signals areessentially unshifted compared with those for the freeligand L00, suggesting that the ligand was not chemicallymodified during the inclusion process. The applied reactionconditions are therefore not conducive to the covalentgrafting of the ligand through a transesterification reaction,i.e. reaction of the surface silanol groups with the estergroup (COOEt) of L00 to give surface attached species ofthe type Si–O(CO)R [19].

Fig. 4. (a) 29Si MAS and CP MAS NMR spectra of L00-MCM-41. (b) 13CCP MAS NMR spectrum of L00-MCM-41 compared with the resonancesof L00 in solution (dashed lines give relative intensities).

Fig. 5. Raman spectra of Eu/L00-MCM-41 (—), L00-MCM-41 (- - -) andEu(NTA)3(H2O)2 (� � �) in the range 1325–1650 cm�1.


Treatment of the material L00-MCM-41 with chloroformsolutions of Ln(NTA)3(H2O)2 (Ln = Eu, Gd; 1 equiv. withrespect to L00) gave two supported materials with lantha-nide loadings of about 1.2 wt.%, hereafter denoted as Ln/L00-MCM-41. The C/Ln molar ratio for both materialswas about 52, which is close to the expected value of 54for complexes of the type Ln(NTA)3L00. Comparison ofthe calculated ligand (L00) contents for L00-MCM-41 andLn/L00-MCM-41 indicates that about 90% of the pyrazoly-pyridine ligand was extracted into solution during thetreatment of L00-MCM-41 with Ln(NTA)3(H2O)2. Thedecrease in the L00 loading is presumably related with thesteric bulk of the Ln(NTA)3 fragment. By comparison withprevious reports concerning the immobilisation of the tet-rakis(b-diketonate) complex [Eu(TTA)4]� (TTA = 2-thenoyltrifluoroacetone) in MCM-41 [20], we assume thatthe host–guest interaction in the materials Ln/L00-MCM-41 involves hydrogen-bonding between the F atoms ofthe complexes (and possibly also the O atoms of carbonylgroups) and the H atoms of silanol groups (Scheme 1).

Powder XRD of the materials Ln/L00-MCM-41 showedthat the long-range hexagonal symmetry of the mesopor-ous host was not significantly disrupted after incorporationof Ln(NTA)3 into L00-MCM-41 (Fig. 2). Fig. 3 shows theTGA curves of Eu/L00-MCM-41 and the complexEu(NTA)3L00. A two-step decomposition is observed forEu(NTA)3L00 in the temperature range of 225–575 �C, witha mass loss of 80.5%. Although the material Eu/L00-MCM-41 exhibits a similar onset point, the subsequent decompo-sition takes place smoothly over the temperature range of225–500 �C, with several overlapping steps. The mass lossof 8.0% is in good agreement with the theoretical valueof 7.6%, calculated assuming that the material containsonly encapsulated Eu(NTA)3L00 complexes at a loading of0.08 mmol g�1 and that heating to 500 �C will lead to81% mass loss. The FTIR spectra of Ln/L00-MCM-41showed some very weak absorption bands for the NTAligands at 682, 1295, 1512, 1533 and 1549 cm�1. Due tothe lower ligand (L00) content in Ln/L00-MCM-41 comparedwith L00-MCM-41, bands for the L00 modes could not bediscerned, and the 957 and 575 cm�1 bands for L00-MCM-41 reverted back to 963 and 556 cm�1, that is, thetypical frequencies observed for the MCM-41 precursor.The Raman spectra were more useful for characterisingthe incorporated lanthanide complexes, showing most ofthe strong and medium bands due to Ln(NTA)3 and L00.These were essentially unshifted compared with the freeligand L00 and the complexes Ln(NTA)3(H2O)2. However,there were some changes that could be attributed to theformation of encapsulated complexes of the typeLn(NTA)3L00 (Fig. 5). As observed in a previous study con-cerning the immobilisation of Eu(NTA)3 in MCM-41 func-tionalised with a pyrazolylpyridine ligand [10a], the bandsat 1448 and 1499 cm�1 for L00 included in MCM-41(assigned to C–N stretching of the pyrazole ring andinter-ring C–C stretching, respectively) are clearly affected


by the inclusion process, and this is consistent with coordi-nation of the bidentate ligand to the lanthanide ion.

Fig. 7. Room-temperature emission spectra of Eu(NTA)3L00 (1) at 315 nm(� � �) and 534 nm (—). The insets show in detail the 5D0 ?

7F0–2 transitions.

3.2. Photoluminescence studies

Fig. 6 shows the low-temperature emission spectraacquired under excitation via the ligand’s excited states(370 nm) and through direct intra-4f6 excitation (5D2,465 nm) for Eu/L00-MCM-41. The spectra display the typi-cal Eu3+ intra-4f6 lines ascribed to transitions between the5D0,1 excited states and the ground multiplet (7F0–4). Aband with negligible intensity is also observed between410 and 560 nm (magnified in Fig. 6), which may beascribed to the emission arising from the triplet states ofthe ligands, as described below. The negligible intensityof this band points to an efficient energy transfer betweenthe ligands and the Eu3+ ions. The energy and number ofemission lines do not depend on the excitation wavelength.Furthermore, the presence of a single line for the non-degenerate 5D0 ?

7F0 transition together with the Starksplitting of the 5D0 ?

7F1,2 transitions into 3 and 5 compo-nents are all consistent with Eu3+ ions occupying the sameaverage local environment. Nevertheless, the high full-width at half maximum (fwhm) of the 5D0 ?

7F0 transition(�48 cm�1, Fig. 6b) and the changes in the relative inten-sity of the Eu3+ transitions as the excitation wavelengthis varied point to a high distribution of similar Eu3+ sites,probably related with different relative positions or confor-mations of the NTA ligands. At room temperature, theluminescence features resemble those reported at �259 �C.

The emission spectra of the model complex Eu(NTA)3L0

(1) are displayed in Fig. 7. The energy, number of emissioncomponents and the dependence of the relative intensity ofthe emission lines on the excitation wavelength are approx-imately the same for both complex 1 and Eu/L00-MCM-41,indicating that the Eu3+ local coordination environment issimilar in the two compounds. Nevertheless, the model

Fig. 6. (a) Emission spectra (�259 �C) of Eu/L00-MCM-41 under excita-tion wavelengths of 370 nm (� � �) and 465 nm (—). The 5D0 ?

7F0–2

transitions are shown in detail in parts (b), (c) and (d).

complex displays narrower Eu3+ transitions; for examplethe 5D0 ?

7F0 transition presents an fwhm of �29 cm�1.This indicates a higher distribution of Eu3+ local environ-ments for the complex immobilised in the MCM-41 host.

Fig. 8 shows the excitation spectra of Eu/L00-MCM-41and complex 1 monitored within the Eu3+ 5D0 ?

7F2 emis-sion lines. The spectrum for the supported material dis-plays a large broad band ascribed to the excited states ofthe ligands and a low-intensity intra-4f6 line ascribed tothe 7F0 ?

5D2 transition. The higher intensity of the broadband excitation component with respect to that of theintra-4f6 lines indicates that the Eu3+ sensitisation processis more efficient than direct intra-4f6 excitation. The corre-sponding broad band excitation component for 1 is largerand deviated to the red, and the relative intensities of theintra-4f6 lines are higher. These differences presumably

Fig. 8. Room-temperature excitation spectra of Eu/L00-MCM-41 (—) andEu(NTA)3L00 (1) (� � �) monitored at 612 nm.

Table 1Room-temperature 5D0 lifetime (s), radiative (kr) and non-radiative (knr) transition probabilities, quantum efficiency values (q) and absolute emissionquantum yields (/) for complex 1 and the supported material Eu/L00-MCM-41 under excitation via the excited states of the ligands

kex (nm) s (ms) kr (ms�1) knr (ms�1) q (%) / (%)

Eu(NTA)3L00 (1) 315 0.508 ± 0.002 0.908 1.061 46.1 35Eu/L00-MCM-41 320 0.229 ± 0.002 0.920 3.447 21.1 15

Fig. 9. Room-temperature emission spectra of Gd/L00-MCM-41 (—) andGd(NTA)3L00 (2) (� � �) excited at 380 and 370 nm, respectively.


arise due to the subtle differences in the structures of the Eucomplexes in the two systems, and the apparent beneficialeffect that the mesoporous host has on the sensitisationprocess.

The 5D0 lifetimes of Eu/L00-MCM-41 and complex 1

were estimated based on the emission decay curves moni-tored around the 5D0 ?

7F2 emission lines under an excita-tion wavelength that favours the Eu3+ emission, namely viathe ligand states at 320 nm for Eu/L00-MCM-41 and315 nm for 1. The curves display a single exponentialbehaviour (not shown), yielding lifetimes of 0.23 ms forthe supported material and 0.51 ms for 1 (Table 1). Toget further insight into the depopulation rates of Eu/L00-MCM-41 and complex 1, the radiative (kr) and non-radia-tive (knr) transition probabilities, and the 5D0 quantumefficiency (q), were estimated based on the data obtainedunder excitation via the ligand states at 315–320 nm (Table1). The methodology used has been extensively described inthe literature (see, for example, Ref. [14]). An index ofrefraction of 1.5 was considered for all the materials, andit was assumed that only non-radiative and radiative pro-cesses were essentially involved in the depopulation of the5D0 excited state. The q value for complex 1 (46.1%) issomewhat lower than that reported previously for the com-plex Eu(NTA)3{ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}(52.9%) [10a], even though the latter complex gave aslightly lower kr value. The difference in the q valuesfor these two complexes is explained by the different knr

values: 1.061 ms�1 for 1 compared with 0.739 ms�1 forEu(NTA)3L00. The even higher knr value of 3.447 ms�1 forEu/L00-MCM-41 accounts for the lower q value of 21.1%.The presence of a non-radiative channel in the supportedmaterial that is not present in the pure complexes is rein-forced by the lower absolute emission quantum yield mea-sured for Eu/L00-MCM-41 (15%) compared with thatestimated for 1 (35%). A q value of 39.9% was reportedfor MCM-41 derivatized with the tethered complex repre-sented in Fig. 1 [10a]. The difference between this valueand that determined for the model complex Eu(NTA)3L00

is again explained by a higher knr value of 1.270 ms�1 forthe supported material, showing the appearance of non-radiative channels.

To further interpret the role of the excited states of theligands in the supported material Eu/L00-MCM-41, theemission features of MCM-41, Gd(NTA)3{2-[3(5)-pyrazol-yl]pyridine} (2) and Gd/L00-MCM-41 were studied. While itis has been reported that MCM-41 is a visible broad bandemitter, with two emissions peaking in the red and in theblue-green spectral regions [21], the material prepared in

the present work is optically inactive and therefore its con-tribution to the overall emission features of the supportedmaterials can be neglected. As shown in Fig. 9, the emis-sion spectrum of complex 2 displays an asymmetricalbroad band between 400 and 600 nm, peaking around480 nm. This band resembles that exhibited by the com-plexes Gd(NTA)3L00 [10a] and Gd(NTA)3phen [14], andcan be ascribed to the convolution of emissions from sin-glet (S1, 480 nm) and triplet (T1, higher wavelength region)states of the NTA ligands [14]. The emission spectrum ofGd/L00-MCM-41 occurs in the same spectral region as thatfor complex 2, with an increase in the intensity of the T1

component (530 nm) relative to that of the S1 state(480 nm). This means that internal conversion and intersys-tem crossing between S1 and T1 seems to be improved inthe former material since the triplet emission is raised rela-tive to that of complex 2.

The room-temperature emission decay curves were mon-itored at 480 and 530 nm for Gd/L00-MCM-41, at 480 nmfor complex 2, and at 380 nm for L00-MCM-41 (not shown).The time scale behind the emissions peaking at 480 nm forGd/L00-MCM-41 and complex 2, and 380 nm for L00-MCM-41, lied beyond the detection limits of our equip-ment (10�5 s), which is consistent with the singlet stateassignment. The emission decay curve monitored at530 nm for Gd/L00-MCM-41 was well fitted using a singleexponential function and yielded a lifetime of 1.282 ±0.030 ms. This value is similar to that reported forGd(NTA)3L00 (1.85 ± 0.03 ms at �259 �C) [10a], adding


weight to the assignment of the peak at 530 nm in the emis-sion spectrum of Gd/L00-MCM-41 to a triplet state of theNTA ligands. The position of the triplet state for Gd/L00-MCM-41 (18868 cm�1) is red-shifted relative to that forGd(NTA)3L00 (553 nm, 18083 cm�1) [10a]. Interestingly,the magnitude (785 cm�1) of the energetic shift of the T1

states is similar to that observed when comparingGd(NTA)3L00 with the tethered complex represented inFig. 1 [10a]. Thus, we may also suggest that while a higherligand-to-Eu3+ energy transfer rate is expected as theT1 ?

5D1 energy transfer pathway is the higher energytransfer rate channel (due to dominance of the exchangemechanism), a higher energy back-transfer rate might alsooccur, contributing to an increase in the knr parameter[10a].

4. Conclusions

As mentioned in Section 1, there is considerable interestin finding materials that can support luminescent lantha-nide complexes through a non-covalent host–guest interac-tion. Cyclodextrins are a good example of organic hoststhat have been successfully applied in this respect. Here,we have shown that the Eu3+b-diketonate complexEu(NTA)3L00 can be securely encapsulated within the cylin-drical pores of MCM-41 without the need to functionalisethe silica surface of the host. The host–guest interactionprobably involves hydrogen-bonding between surface sila-nol groups and the carbonyl group in the ligand L00 and/orthe fluorine atoms in the ligand NTA. Although theseinteractions are weak, they appear to affect the emissionproperties of the rare earth complex by distorting the coor-dination geometry. By preparing the analogous gadolin-ium-containing material, we have been able to show thatthe emission quantum yield for the europium-containingmaterial is lower than that for model complexes containingpyrazolylpyridine ligands due to the presence of an excitedstate of the organic ligands that lies approximately reso-nant with the 5D1 intra-4f6 level, opening up an additionalnon-radiative channel that may be responsible for thehigher non-radiative transition probability.

Acknowledgments

We are grateful to the FCT, POCI 2010, OE and FED-ER for funding (Project POCI/CTM/58507/2004). S.M.B.thanks the FCT for a doctoral grant.

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Synthesis, characterisation and luminescence investigations of Eu activated CaAl2O4 phosphor

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