Received 14th September 2009, Accepted 14th December 2009First published as an Advance Article on the web 25th January 2010DOI: 10.1039/b919008b
A series of novel ternary organic–inorganic mesoporous polymeric hybrids TTFA-S16-Eu-PMMA,TTFA-S16-Eu-PMAA, and TTFA-S16-Eu-PVP (TTFA = 2-Thenoyltrifluoroacetone; PMMA =polymethyl methacrylate; PMAA = polymethacrylic acid; PVP = polyvinylpyrrolidone) have beenassembled by the Eu3+ complex covalently attaching to the TTFA directly functionalized orderedmesoporous SBA-16 and organic polymer. FTIR, UV, XRD, TEM, N2 adsorption measurements,photoluminescent spectra, and TG plots were characterized, and the results reveal that they all havehigh surface area, uniformity in the mesostructure, and good crystallinity. In addition, the ternary rareearth mesoporous polymeric hybrids show an overall increase in luminescent lifetime and quantumefficiency compared to binary rare earth mesoporous hybrid TTFA-S16-Eu, especially the mesoporoushybrid with PVP exhibits the highest luminescence quantum efficiency and longest lifetime.
1. Introduction
The photophysical properties of rare earth complexes with organicligands have been the hot subject of much interest because thesefunctional complex systems can be used as active centers ofluminescent materials or the structural and functional probesfor chemical or biological macromolecule systems.1 In manyrare earth complexes, Eu3+ complexes especially containing b-diketones have been intensively studied owing to their inherentsharp emission peaks and high quantum efficiency.2 Some of themdemonstrate potential applications in efficient light-conversionmolecular devices and organic light-emitting devices.3 However,the practical application of these complexes as luminescentdevices or tunable solid-state lasers has not been realized becauseof their poor stabilities under high temperature or moistureconditions and low mechanical strength. In order to overcomethese shortcomings, the concept of “hybrid organic–inorganic”exploded in the eighties with the expansion of soft inorganicchemistry processes. Indeed, the mild synthetic conditions offeredby the sol–gel process which is based on hydrolysis and polycon-densation reactions, is the most commonly employed method forthe preparation of inorganic–organic hybrid materials. Becauseof its low processing temperature, active species such as rare-earth/organic complexes can be incorporated into the host matrix(inorganic matrix or organic polymer), and the obtained hybridmaterials exhibit improved processability, chemical stability, andmechanical strength.4 Moreover, by modifying the sol–gel process-ing conditions, the microstructure, the external shape, or the degreeof combination between the organic and the inorganic phasescan be further controlled.5 However, the conventional doping
Department of Chemistry, Tongji University, Siping Road 1239, Shanghai,200092, China. E-mail: [email protected]; Fax: +86-21-65982287;Tel: +86-21-65984663† Electronic supplementary information (ESI) available: Predicted struc-tures, pore size distributions, FTIR spectra, TGA, PL spectra and SEMimage. See DOI: 10.1039/b919008b
method in a sol–gel procedure seems unable to solve the problemof the quenching effect of luminescent centers because onlyweak interactions exist between organic and inorganic moieties.Moreover, inhomogeneous dispersion of two phases and leachingof the photoactive molecules frequently occur in this sort of hybridmaterial for which the concentration of complexes is also greatlyreduced. Therefore, another appealing method of synthesizinghybrid materials containing covalent bonds has emerged, andthe derived molecular-based materials show improved chemicalstability and have a monophasic appearance similar in natureto that of the complicated molecular polymeric Si–O network.6,7
Our research team recently has been dedicated to the design ofrare-earth hybrid materials with inorganic networks, and we havesuccessfully realized six paths to construct functional silylatedprecursors.8,9 Compared to chemically bonded rare-earth hybridswith Si–O polymeric networks, fewer reports on molecular hybridmaterials fabricated with rare-earth/organic polymers have beenpublished, which is possibly due to the fact that suitable reactivepolymers or monomers are hard to select. Recently, more pro-fessional investigations have been adapted to focus on rare earthhybrid materials concerning inorganic and organic polymerizationreactions or embedding certain polymers containing long carbonchains by covalent bonds.10,11 It was proved that hybrid materialswith inorganic networks and polymers with high molecular weightexhibited excellent photophysical properties and high thermalstability.
In the past few years, the mesoporous molecular sieves (MMSs)used as a support for rare earth complexes have started toattract much attention since they offer many novel and uniqueproperties, such as rigidity, photostability, and well-defined hy-drophilic/hydrophobic phase separation allowing for a more so-phisticated tuning of the rare earth complex microenvironment.12
As a host material for rare earth complexes, SBA-16 is expectedto be superior to hexagonal structures with its unique 3D channelnetwork. This is believed to provide a highly open porous host witheasy and direct access for guest species, which facilitates inclusion
or diffusion throughout the pore channels without pore blockage.Furthermore, there are a large number of hydroxyls in SBA-16,which provide necessary qualification for the modification of innerface and self assembly of guest macromolecules. These propertiestogether its their thermal and mechanical stability make it anideal host for incorporation of active molecules. However, tothe best of our knowledge, previous work most concentrated onthe lanthanide complexes covalently bonded mesoporous SBA-15hybrid materials,3c,4b,11a,11d while, the syntheses and luminescenceproperties of rare earth complexes supported on functionalizedSBA-16 has not been reported before.
Under the above considerations, in this paper, a novel path wasproposed to assemble mesoporous hybrids containing inorganicSi–O networks and organic polymeric chains. In this paper,2-thenoyltrifluoroacetone (TTA)-functionalized SBA-16 meso-porous hybrid (TTFA-S16) was prepared, in which TTFA wascovalently bonded to the framework SBA-16 by co-condensationof modified TTFA (TTFASi) and tetraethoxysilane (TEOS) in thepresence of Pluronic F127 surfactant as a template. Subsequently,the addition polymerization reaction was performed to constructthe polymeric chains (C–C). Then, the final mesoporous polymerichybrids TTFA-S16-Eu-PMMA/PMAA/PVP were obtained afterthe coordination reaction between rare earth ions, TTFA-S16,and the polymers. In addition, for comparison, SBA-16 covalentlybonded to the binary Eu3+ complexes with TTFA ligand was alsosynthesized, denoted as TTFA-S16-Eu. Full characterization anddetailed studies of the luminescence properties of all synthesizedmaterials were investigated and compared.
2. Experimental
Reagents
Eu(NO3)3 ethanolic solution (EtOH) was obtained by dissolvingEu2O3 in concentrated nitric acid. Tetraethoxysilane (TEOS,Aldrich) was distilled and stored under a nitrogen atmosphere.3-(Triethoxysilyl)-propyl isocyanate (TEPIC), Pluronic F127(EO106PO70EO106) and 2-thenoyltrifluoroacetone (TTFA) werepurchased from the Lancaster company, Poly(vinylpyrrolidone)(PVP with the molecular weight of 8000), methyl methacrylate(MMA) and methacrylic acid (MAA) were purchased from theShanghai chemical plant, and the solvent tetrahydrofuran (THF)was used after desiccation with anhydrous calcium chloride. Allthe other reagents were analytically pure.
Synthetic procedures
Synthesis of polymer precursor (PMMA and PMAA). Methylmethacrylate (2 mmol) for PMMA and methacrylic acid (2 mmol)for PMAA were dissolved in a small quantity of tetrahydrofuran(THF) solution (6 mL) with the initiator (BPO, benzoyl peroxide)to initiate the addition polymerization under argon atmospherepurging. The reaction temperature was maintained at 70 ◦C andthe reaction times are about 6 h for PMMA and 50 ◦C for about4 h to obtain the colorless viscid liquid PMAA. The obtainedmaterials were concentrated under room temperature to removeTHF solvent using a rotary vacuum evaporator, and a viscous liq-uid was obtained (PMMA [C5H8O2]n and PMAA [C4H6O2]n) (seeFig. 1). The average molecular weight and molecular distribution
Fig. 1 Synthesis of polymer precursors (PMMA and PMAA) by additionpolymerization.
are 9.7 ¥ 103 and 1. 38 for PMMA, and 1.41 ¥ 104 and 1.48 forPMAA, respectively.
Synthesis of TTFA-functionalized SBA-16 mesoporous material(TTFA-S16). The modified precursor TTFASi was synthesizedmainly as described in the literature.11a,11d,13 Typically, 1 mmol 2-thenoyltrifluoroacetone (TTFA) was first dissolved in 20 mL ofdehydrate tetrahydrofuran (THF), and NaH (2 mmol, 0.048 g) wasadded into the solution with stirring. Two hours later, 2.0 mmol(0.495 g) of 3-(triethoxysilyl)-propyl-isocyanate (TEPIC) wasadded dropwise into the refluxing solution. The whole mixturewas refluxed at 65 ◦C under nitrogen for approximately 12 h.After isolation and purification, a yellow oil TTFASi product wasobtained. The mesoporous material TTFA-S16 was synthesizedfrom acidic mixture with the following molar composition: 0.0040F127: 0.96 TEOS: 0.04 TTFASi: 4 HCl: 130 H2O. F127 (0.508 g)was firstly dissolved in deionized water (3.4 g) and 2 M HClsolution (20 g) at 35 ◦C under vigorous stirring. A mixture of TEOSand TTFASi was added to the above solution, which was furtherstirred at 35 ◦C for 24 h and transferred into a Teflon bottle sealedin an autoclave, which was then heated at 100 ◦C for 24 h. Thesolid product was recovered by filtration, washed thoroughly withdeionized water, and air-dried at room temperature. Removal ofcopolymer surfactant F127 was conducted by Soxhlet extractionwith ethanol for 48 h. The material was dried in a vacuum andshowed a light-yellow color (See Fig. 2).
Fig. 2 Synthesis of TTFA-functionalized SBA-16 mesoporous material(TTFA-S16).
Synthesis of ternary mesoporous hybrid materials with or-ganic polymer (TTFA-S16-Eu-PMMA/PMAA/PVP). The pre-cursors TTFA-S16 and PMMA (PMAA and PVP) were soakedin N,N-dimethyl formamide (DMF) solvent, and an appropriateamount of Eu(NO3)3 ethanol solution was added into the solutionwhile stirring (the molar ratio was 1 : 3 : 2 for Eu3+ : TTFA-S16 : PMMA/PMAA, and 1 : 3 : 1 for Eu3+ : TTFA-S16 : PVP).The mixture was stirred at room temperature for 12 h, followed byfiltration and extensive washing with EtOH. The resulting mate-rials were dried overnight at 60 ◦C under vacuum. The predictedstructure of TTFA-S16-Eu-PVP was obtained as outlined in Fig. 3,and the structures of the TTFA-S16-Eu-PMMA/PMAA in detailare represented in Fig. S1 and Fig. S2, respectively.†
Fig. 3 The predicted structure of a selected mesoporous polymeric hybridmaterial TTFA-S16-Eu-PVP.
Synthesis of binary mesoporous hybrid material (TTFA-S16-Eu).The synthesis procedure for TTFA-S16-Eu was similar to that ofTTFA-S16-Eu-PMMA except that the precursors TTFA-S16 andPMMA were replaced by TTFA-S16.
Physical measurement
Fourier transform infrared spectra were obtained from KBr pelletsand were recorded on a Nexus 912 AO446 FT-IR spectropho-tometer in the range of 4000–400 cm-1. The molecular weightand weight distribution were determined through gel-permeationchromatography (GPC) by WATERS 1515 with polystyrene asthe standard sample. The ultraviolet-visible absorption spectrawere taken with an Agilent 8453 spectrophotometer. X-raypowder diffraction patterns were recorded on a Rigaku D/max-rBdiffractometer equipped with a Cu anode in a 2q range from 0.6◦
to 6◦. Nitrogen adsorption/desorption isotherms were measuredat the liquid nitrogen temperature, using a Nova 1000 analyzer.Surface areas were calculated by the Brunauer–Emmett–Teller(BET) method and pore size distributions were evaluated from thedesorption branches of the nitrogen isotherms using the Barrett–Joyner–Halenda (BJH) model. Transmission electron microscope(TEM) experiments were conducted on a JEOL2011 microscopeoperated at 200 kV or on a JEM-4000EX microscope operated
at 400 kV. The fluorescence excitation and emission spectra wereobtained on a Perkin-Elmer LS-55 spectrophotometer. Lumines-cence lifetime measurements were carried out on an EdinburghFLS920 phosphorimeter using a 450 W xenon lamp as excitationsource. Thermogravimetric analysis (TGA) was performed on aNetzsch STA 409 at a heating rate of 15 ◦C min-1 under a nitrogenatmosphere.
3. Results and discussion
TTFA-functionalized mesoporous silica SBA-16
The presence of the organic ligand covalently bonded to themesoporous SBA-16 could be proved by characterization by FTIRand UV-Vis absorption spectra. The FTIR spectra of the freeb-diketone ligand (TTFA), the precursor (TTFASi) and TTFA-functionalized mesoporous hybrid material (TTFA-S16) are com-pared and their main assignments are shown in Fig. 4. From Ato B, it can be observed that the stretching of -CH2- at 3007 cm-1
(A) was replaced by a strong broad band located at 2978 cm-1 (B),which may have originated from CH2, CH3 and ethoxy groupsof 3-(triethoxysilyl)-propyl isocyanate(TEPIC). In addition, thespectrum of TTFASi is dominated by n(C–Si, 1172 cm-1) and n(Si–O, 1091 cm-1) absorption bands. Ulteriorly, the bending vibration(dNH, 1525 cm-1) further proves the formation of amide groups. Anew band at 1698 cm-1 is probably due to the stretching vibrationof the -CONH- group in TTFASi, proving that 3-(triethoxysilyl)-propyl isocyanate was successfully covalently bonded onto theb-diketone ligand. The bands at 3406 cm-1 and 1631 cm-1 in panelB could be attributed to stretching and bending vibrations ofphysically adsorbed water, respectively. The spectra of mesoporoushybrid material (C) indicated the formation of the Si–O–Si frame-work, which is evidenced by the broad bands located at 1083 cm-1
(nas, Si–O), 795 cm-1 (ns, Si–O), and 463 cm-1 (d , Si–O–Si), whichis attributed to the success of hydrolysis and copolycondensation.Furthermore, the bands at 1654 and 1563 cm-1 originating fromthe -CONH- group of TTFASi, can also be observed in panel Cof Fig. 4, which is consistent with the fact that carbonyl groups ofthe precursor remain intact after both the hydrolysis–condensationreaction and the surfactant extraction procedure.3c
Fig. 4 Fourier transform infrared spectra of the free ligand TTFA (A),precursor TTFASi (B), and TTFA-functionalized mesoporous materialsTTFA-S16 (C).
Fig. 5 shows the ultraviolet-visible absorption spectra of TTFA(A), TTFASi (B), and TTFA-S16 (C). Comparing the absorptionspectrum of TTFASi (B) with that of TTFA (A), an obviousblue shift of the major p–p* electronic transitions can be seenfrom 266 to 255 nm. Furthermore, the appearance of the peakcentered at 322 nm, indicates that modification of TTFA, whichwas covalently bonded by 3-(triethoxysilyl)-propyl isocyanate,influences its corresponding absorption. In terms of B and C,the corresponding red shift (255→266, 322→329 nm) is observed,suggesting that the electron distribution of the conjugated systemchanged when the modified TTFA (TTFASi) was covalentlybonded to mesoporous silica SBA-15.
Fig. 5 Ultraviolet-visible absorption spectra of the free ligand TTFA (A),precursor TTFASi (B), and TTFA-functionalized mesoporous materialsTTFA-S16 (C).
The ultraviolet absorption spectra
Europium (Eu3+) complexes covalently bonded to TTFA-functionalized mesoporous SBA-16 and polymeric chains (TTFA-S16-Eu-PMMA/PMAA/PVP). The powder X-ray diffrac-tion analyses performed on TTFA-S16, TTFA-S16-Eu-PMMA,TTFA-S16-Eu-PMAA and TTFA-S16-Eu-PVP are compared inFig. 6. It can be observed that all ternary polymeric hybridmaterials still retain the typical XRD pattern of the SBA-16-type,including strong (110) reflections at a low angle and two small
Fig. 6 XRD patterns of TTFA-S16, TTFA-S16-Eu-PMMA,TTFA-S16-Eu-PMAA and TTFA-S16-Eu-PVP.
diffractions (200) and (211) at higher angles, indicating that theordered cubic mesoporous structure of SBA-16 remains intactafter introduction of Eu3+. However, it is worth noting that theseternary rare earth mesoporous polymeric hybrids appear decreas-ing in diffraction intensity as compared with the precursor TTFA-S16, which is probably due to the presence of guest moieties onthe mesoporous framework of SBA-16, resulting in the decrease ofcrystallinity but not collapse of the pore structure of mesoporousmaterials.14 It was reported that the introduction of a guest intothe pores leads to an increased phase cancellation between theguest moiety from the wall and the pore regions and accordinglyto reduced scattering intensities for the Bragg reflections.15
XRD patterns
N2 adsorption–desorption isotherms are usually used as a macro-scopic average measurement for exploring surface area, porediameter, and pore volume of the material. Fig. 7 shows thecurves of the N2 adsorption–desorption isotherms of TTFA-S16 (a), TTFA-S16-Eu-PMMA (b), TTFA-S16-Eu-PMAA (c),and TTFA-S16-Eu-PVP (d). These all display the adsorptionisotherms of the SBA-16 silica materials, which are of the typeIV adsorption isotherm according to the IUPAC classificationwith H2-type hysteresis loops typical for materials with ink-bottlepores with interconnectivity in a 3D pore system as a result ofcavitation.16 By using BET and BJH methods, the specific areaand the pore size have been calculated, which as well as thepore volume are shown in Table 1. The pore size distributions ofall mesoporous hybrid materials were shown in Fig. S3.† Withthe linking of rare earth complexes onto the parent material
Fig. 8 HRTEM images of mesoporous polymeric hybrid TTFA-S16-Eu-PVP recorded along (a) [100], (b) [110] and (c) [111] zone axes.
TTFA–S16, the pore sizes are decreased from 3.89 to 3.67, 3.62,and 2.80 nm for TTFA-S16-Eu-PMMA, TTFA-S16-Eu-PMAA,and TTFA-S16-Eu-PVP, respectively, and the BET surface areaand pore volume correspondingly decrease as well. This isconsistent with the presence of anchored Eu-PMMA, Eu-PMAA,and Eu-PVP moieties in the parent mesoporous cages. Thisphenomenon is due to the dispersion of the Eu3+ complexes onthe surface of the parent materials, resulting in the cages occupiedby these complexes.12b,15
N2 adsorption–desorption isotherms
The 3D mesostructures of TTFA-S16-Eu-PMAA are furtherconfirmed by TEM micrographs (see Fig. 8). As shown in the
figure, the europium mesoporous polymeric hybrid TTFA-S16-Eu-PMAA presents a well-ordered spherical cage structure char-acteristic of mesoporous SBA-16 material, which indicates thatthe mesostructure of TTFA-S16-Eu-PMAA can be substantiallyconserved after the complexation process. The distances betweenthe centers of the mesopore is estimated to be around 11 nm,which is in good agreement with the value determined from thecorresponding XRD data (see Table 1).
HRTEM images
The FTIR spectra of ternary mesoporous polymeric hy-brids TTFA-S16-Eu-PMMA (a), TTFA-S16-Eu-PMAA (b), andTTFA-S16-Eu-PVP (c) were measured and are shown in the
ESI (Fig. S4†). The band at about 1651 cm-1 corresponds tothe asymmetric stretch of the carbonyl group (-CONH-) forall samples, indicating that the functionalized organic groupsTTFASi remain intact after the hydrolysis/condensation reactionand complex-grafting process.
Photoluminescent properties
Luminescence spectra. b-Diketone is already well-known to bea good chelating group to sensitize the luminescence of the rare-earth ion. The mechanism is usually described as the antennaeffect: the ligands reinforce the energy absorption ability andtransfer it to the metal ion with high efficiency. Then the emissionfrom the rare-earth ions’ excited state will be observed.17
Fluorescent excitation (a) and emission (b) spectra of themesoporous hybrid material
Fig. 9 shows the excitation (a) and emission (b) spectra ofmesoporous polymeric hybrid materials containing Eu covalentlybonded into orderly silicon–oxygen networks and organic carbonchains (A, TTFA-S16-Eu; B, TTFA-S16-Eu-PMMA; C, TTFA-S16-Eu-PMAA; D, TTFA-S16-Eu-PVP). The excitation spectraof these materials were all obtained by monitoring the strongestemission wavelength of the Eu3+ at 613 nm, which exhibitsbroad bands centered at about 341–358 nm near the ultravioletregion, corresponding to the ligand-to-metal charge-transfer (CT)transition caused by interaction between the organic groups andthe rare-earth ions.18 No apparent f–f transitions could be observedin the spectra. The wide CT excitation bands will aid the energytransfer and luminescence to europium ions.
From the emission spectra of these materials in Fig. 9 (b),characteristic Eu3+ ion emissions are observed. The lines aredistributed mainly in the 450–700 nm range, which were assignedto the 5D0→7FJ (J = 0–4) transitions at about 577, 589, 613,648, and 697 nm, respectively. A prominent feature that maybe noted in these spectra is the very high intensity of 5D0→7F2
transition. As well-known, the 5D0→7F1 transition is a parity-allowed magnetic dipole transition and is nonsensitive to thelocal structure environment, while the 5D0→7F2 transition is atypical electric dipole transition and is sensitive to the coordination
environment of the Eu3+ ion. When the interactions of the rareearth complex with its local chemical environment are stronger,the complex becomes more nonsymmetrical, and the intensityof the electric-dipolar transitions becomes more intense. As aresult, the intensity (the integration of the luminescent band)ratio of the 5D0→7F2 transition to the 5D0→7F1 transition hasbeen widely used as an indicator of Eu3+ site symmetry. In theseeuropium hybrid materials, from A to D, the intensity ratios(5D0→7F2/5D0→7F1) are 2.33, 4.56, 5.37, and 7.65, respectively.This ratio is only possible when the europium ion does not occupya site with inversion symmetry.9b It is clear that strong coordinationinteractions take place between the organic groups and rare-earth ions. Another important factor is that the relatively rigidstructure of silica gel limits the vibration of the ligand in the hybridmaterials and prohibits nonradiative transitions. Accordingly,we may expect that through this efficient way, leaching of thephotoactive molecules and clustering of the emitting centers couldbe avoided, and a higher concentration of metal ions realized.Ulteriorly, compared to TTFA-S16-Eu (A), the relative emissionintensities of the mesoporous polymeric hybrids (B, C and D)exhibit obvious enhancement, and TTFA-S16-Eu-PVP (D) arealmost three or four times as large as TTFA-S16-Eu (A) inintensity. It is speculated that if the oxygen atoms in the carboxylor carbonyl groups located in the polymer chains of TTFA-S16-Eu-PMMA/PMAA/PVP (B–D) coordinated to Eu3+ ions, couldreplace the coordinated water molecules existing in complexes ofaromatic carboxylic acid ligands, the energy loss and clustering ofthe emitting centers caused by the vibration of the hydroxyl groupsof coordinated water molecules could be avoided. Moreover, therelative emission intensities of the three rare earth mesoporouspolymeric hybrids increase gradually according to the sequenceB, C and D. The relative intensities of D are the highest andthere is a very small distinction between B and C. In terms of theabove phenomena, the following conclusion might be deduced:the polymer chains of TTFA-S16-Eu-PMMA (B) and TTFA-S16-Eu-PMAA (C) share some commonality in structure, such ascoordinated atoms, coordinated numbers of the central Eu3+ ionand the organic networks composed of chemical covalent bonds(C–C), so that the relative emission intensities are analogous.However, the relative emission intensity of TTFA-S16-Eu-PMMA(B) is slightly lower than that of TTFA-S16-Eu-PMAA (C), which
Fig. 9 Fluorescent excitation (a) and emission (b) spectra of the mesoporous hybrid material: (a) TTFA-S16-Eu, (b) TTFA-S16-Eu-PMMA,(c) TTFA-S16-Eu-PMAA, and (d) TTFA-S16-Eu-PVP.
is mainly ascribed to the difference of the coordinated organicgroups. Compared to ester groups in hybrid B, it is easier forthe carboxyl groups in hybrid C to graft onto Eu3+ due to thesmall steric hindrance effect. Furthermore, the relative emissionintensity of TTFA-S16-Eu-PVP (D) is the strongest, which maybe the result of the small steric hindrance resulting from fewercoordination number of the rare-earth ion (7) with PVP and TTA-Si ligands in the polymer TTFA-S16-Eu-PVP (D) than that (8) inmesoporous polymeric hybrids TTFA-S16-Eu-PMMA (PMAA)(B and C) with TTA-Si and PMMA (PMAA) ligands. The aboveanalysis indicates that the relative emission intensities are generallyaffected by conjugated systems configuration, the steric exclusioneffect and the coordination environment. Moreover, the differentemission intensity of hybrid materials also depends on many otherfactors, such as the real doping concentration, the efficiencies ofthe initial absorption, the subsequent energy transfer and finaldepopulation.
Luminescence decay times (s) and emission quantum efficiency(g). The typical decay curves of the Eu3+ mesoporous hybridmaterials were measured, and they can be described as a singleexponential in the form ln[S(t)/S0] = -k1t = -t/t , demonstratingthat all the Eu3+ ions locate in the same average local environmentin the obtained hybrid materials. The resulting lifetime data ofeuropium hybrids are given in Table 2. It was found that the life-times of the ternary mesoporous polymeric hybrids with organiccarbon chains present longer lifetimes than the correspondingbinary mesoporous hybrid with pure Si–O networks, suggestingthat the introduction of an organic polymeric chain can enhancethe luminescence stability of the overall hybrid system.
According to the emission spectra and the lifetime of the Eu3+
first excited level (t , 5D0), the emission quantum efficiency (h) ofthe 5D0 excited state can be determined. The quantum efficiency ofthe luminescence step, h expresses how well the radiative processes
(characterized by the rate constant Ar) compete with non-radiativeprocesses (overall rate constant Anr).19
h = Ar/(Ar + Anr) (1)
Assuming that only nonradiative and radiative processes areessentially involved in the depopulation of the 5D0 state, radiativeand nonradiative processes influence the experimental lumines-cence lifetime by the equation:
t exp = (Ar + Anr)-1 (2)
Here Ar can also be obtained by summing over the radiativerates A0J for each 5D0→7FJ (J = 0–4) transitions of Eu3+.
Ar = R A0J = A00 + A01 + A02 + A03 + A04 (3)
Since the branching ratio for the 5D0→7F5,6 transitions can beneglected as they are not detected experimentally, their influencecan be ignored in the depopulation of the 5D0 excited state. Since5D0→7F1 belongs to the isolated magnetic dipole transition, it ispractically independent of the chemical environment around Eu3+,and thus can be considered as an internal reference for the wholespectrum, the experimental coefficients of spontaneous emissionA0J can be calculated according to the equation as follows:20
A0J = A01(I 0J/I 01)(n01/n0J) (4)
Here, A0J is the experimental coefficient of spontaneous emis-sion. A01 is the Einstein’s coefficient of spontaneous emissionbetween the 5D0 and 7F1 energy levels. In a vacuum A01 has avalue of 14.65 s-1, while in an air atmosphere the value of A01 canbe determined to be 50 s-1 approximately (A01 = n3A01 (vac)),19b whenan average index of refraction n equal to 1.506 was considered.I 01 and I 0J are the integrated intensities of the 5D0 → 7F1 and 5D0
→ 7FJ transitions (J = 0–4) with n01 and n0J (n0J =1/lJ) energycenters respectively. n0J refers to the energy barrier and can be
Table 2 Photoluminescent data of all mesoporous materials
Systems TTFA-S16-Eu TTFA-S16-Eu-PMMA TTFA-S16-Eu-PMAA TTFA-S16-Eu-PVP Eu(TTFA)3PVP Eu-PVP
determined with the emission peaks of Eu3+’s 5D0 → 7FJ emissiontransitions.
On the basis of the above discussion, the quantum efficiencies ofthe four kinds of europium mesoporous hybrid materials can be de-termined, as shown in Table 2. From the equation to calculate thequantum efficiency (h), it can be seen that the value of h mainly de-pends on two factors: one is the lifetime and the other is I 02/I 01. Ifthe lifetimes and red/orange ratio are large, the quantum efficiencymust be high. As can be clearly seen from Table 2, the three rare-earth/inorganic/organic ternary mesoporous polymeric hybridsexhibit higher luminescence quantum efficiencies than the rare-earth/inorganic binary mesoporous hybrid, and in particular, theTTFA-S16-Eu-PVP hybrid with the poly(vinylpyrrolidone) chainexhibits the highest luminescence quantum efficiency. This findingis in agreement with the results from luminescent intensities.With increasing organic polymer units, the luminescent quantumefficiency is enhanced, indicating that the organic chain maybe ofsome benefit for the luminescence of Eu3+ species. For comparison,the luminescence properties of materials Eu(TTFA)3PVP and Eu-PVP have also been studied (Fig. S5†). The quantum efficiencyof the hybrid TTFA-S16-Eu-PVP is almost three times as largeas Eu-PVP, which is mainly due to TTFA-S16 replacing thecoordinated water molecules existing in the complex, and theenergy loss and clustering of the emitting centers caused by thevibration of the hydroxyl groups of coordinated water moleculesbeing avoid. In addition, compared with Eu(TTFA)3PVP, whenthe complex is covalently bonded to mesoporous silica SBA-15, the luminescence lifetimes and quantum efficiencies of themesoporous polymeric hybrid TTFA-S16-Eu-PVP exhibit anobvious enhancement, which indicates that in the Eu(TTFA)3PVPcomplex, a quenching of the europium luminescence can beinduced by the concentration effect and/or the electron–phononcouplings with the third vibrational overtone of the closing-lyingOH oscillator. This exhibited behavior means that SBA-16 isan excellent host for the luminescent complex Eu(TTFA)3PVP,because the luminescence quenching of the Eu3+ ion can beeffectively decreased in this host.
Experimental intensity parameters (X). To investigate thepossible structural changes around the center ion among thesehybrid materials, the experimental intensity parameters X shouldbe calculated from the emission spectra as described previously.22
The spontaneous emission probability A of the transition is relatedto its dipole strength according to the equation19b,21
A = (64p4n3)/[3h(2J + 1)]{[(n2+2)2/9n]S(ED) + n2S(MD)} (5)
n is the average transition energy in cm-1, h is the Planck constant,2J + 1 is the degeneracy of the initial state (1 for 5D0). S(ED) andS(MD) are the electric and magnetic dipole strengths, respectively.The factors containing the medium’s refractive index n result fromlocal field corrections that convert the external electromagneticfield into an effective field at the location of the active centerin the dielectric medium. Among all the transitions, from 5D0
to 7F0, 3, 5 (J = 0, 3, 5) are forbidden both in the magnetic andthe induced electric dipole schemes (S(ED) and S(MD) are zero).The transition from 5D0 to 7F1 (J = 1) is the isolated magneticdipole transition and has no electric dipole contribution, whichis practically independent of the ion’s chemical environment andcan be used as a reference as mentioned above. In addition, the
5D0→7F6 transition could not be experimentally detected and itis not necessary to determine its J–O parameter. So we only needto estimate the two parameters (X2, X4) related to the two purelyinduced electric dipole transitions 5D0→7F2, 4 on the basis of onlythree parameters Xl using eqn (6)19b,22
A = (64e2p4n3)/[3h(2J + 1)]{[(n2
+ 2)2/9n]R Xl|<J‖U (l)‖|J¢>|2 (6)
e is the electronic charge. With the refraction index n = 1.50619d and<J‖U (l)‖|J¢>|2 values are the square reduced matrix elementswhose values are 0.0032 and 0.0023 for J = 2 and 4,23 respectively.The X2, X4 intensity parameters for all the samples are shown inTable 2. It is observed that the three mesoporous polymeric hybridspossess relatively high values of the X2 intensity parameter, whichmight be interpreted as being the consequence of the hypersensitivebehavior of the 5D0→7F2 transition, indicating that the Eu3+ ion islocated in a polarizable chemical environment for luminescence.
Thermogravimetric analysis (TGA)
To investigate the thermal stability of all the obtained mesoporouspolymeric hybrids, thermogravimetric analyses (TGA) were car-ried out at a heating rate of 15 ◦C min-1 under a nitrogenatmosphere. Fig. 10 presents the thermogravimetric weight losscurve (TG) and differential thermogravimetry trace (DTG) ofthe mesoporous polymeric hybrid material TTFA-S16-Eu-PMAAas an example. As seen from the TG curve, the mesoporoushybrid material TTFA-S16-Eu-PMAA has a mass loss (about2.8%) from 30 ◦C until 121 ◦C, which is attributed to the loss ofphysically absorbed water. On further heating, the compound losesweight continuously, the weight losses in the temperature range121–497 ◦C amounted to a weight loss of 17.0%, correspondingto the decomposition of incompletely removed surfactant. Thethird weight loss (7.2%) peak at 526 ◦C can be ascribed to thedecomposition of the organic rare earth complex. In addition,compared with the weight loss peak (approximately 42% atabout 317 ◦C) of pure complex TTFA-Eu-PMAA (see ESI Fig.S 6a†), the decomposition point of TTFA-S16-Eu-PMAA washigher than that of pure TTFA-Eu-PMAA, suggesting that thethermal stability of the rare earth complex was enhanced as it wascovalently bonded to the mesoporous matrix. For comparison, thethermal decomposition behavior of materials TTFA-S16, TTFA-S16-Eu, and TTFA-S16/PMAA have also been studied (Fig. S6b–d†). From the figures, it can be seen that the decomposition
Fig. 10 Thermogravimetry trace (TG) and differential thermogravime-try trace (DTG) of the mesoporous polymeric hybrid materialTTFA-S16-Eu-PMAA.
temperatures of the organic groups in TTFA-S16, TTFA-S16-Eu,and TTFA-S16/PMAA were 479, 504 and 506 ◦C, respectively.The decomposition temperature of the organic ligands in TTFA-S16-Eu-PMAA (526 ◦C) was higher than that in the binarymesoporous hybrid TTFA-S16-Eu (503 ◦C), which indicates thatthe introduction of polymers could improve the thermal stabilityof mesoporous hybrid materials. Compared with mesoporous-functionalized material TTFA-S16, when the polymer PMAAwas doped into TTFA-S16, the decomposition temperature of theorganic ligand in the TTFA-S16/PMMA material was enhancedby 26 ◦C, which is consistent with the above conclusion.
4. Conclusion
We have designed and assembled some novel mesoporous rareearth molecule-based polymeric hybrid materials with cova-lent bonds (TTFA-S16-Eu-PMMA, TTFA-S16-Eu-PMAA, andTTFA-S16-Eu-PVP) that involve long organic carbon chains andordered organic networks (Si–O–Si) through the sol–gel process.Meanwhile, we also prepared the material TTFA-S16-Eu forcomparison. All mesoporous hybrid materials were characterizedin detail, ranging from structures to properties. The resultsdemonstrate that they all preserve their mesoscopically orderedstructures and show highly uniform pore size distributions.Further investigation on the luminescence properties show that theternary mesoporous polymeric hybrid materials exhibit strongerluminescent intensities, longer lifetimes, and higher quantum effi-ciencies than that of the binary material, which is attributed to theintroduction of organic polymeric chains. And especially TTFA-S16-Eu-PVP hybrid systems exhibit the most excellent luminescentbehaviors. Therefore, these kinds of rare earth molecular-basedpolymeric hybrid materials with excellent luminescent propertiesand highly ordered cubic structures can be expected to havepotential and significant applications in optical and electronicareas in the future.
Acknowledgements
This work was supported by the National Natural ScienceFoundation of China (20971100) and Program for New CenturyExcellent Talents in University (NCET-08-0398).
References
1 (a) B. Yan, H. J. Zhang, S. B. Wang and J. Z. Ni, Mater. Chem. Phys.,1997, 51, 92–96; (b) C. F. Meares and T. G. Wensel, Acc. Chem. Res.,1984, 17, 202–209; (c) T. Jin, S. Tsutsumi, Y. Deguchi, K. I. Machidaand G. Y. Adachi, J. Alloys Compd., 1997, 252, 59–66; (d) B. Yan, H. J.Zhang, S. B. Wang and J. Z. Ni, Mater. Res. Bull., 1998, 33, 1517–1525.
2 (a) A. P. Bassett, S. W. Magennis, P. B. Glover, D. J. Lewis, N. Spencer, S.Parsons, R. M. Williams, L. De Cola and Z. Pikramenou, J. Am. Chem.Soc., 2004, 126, 9413–9424; (b) J. A. Fernandes, R. A. S. Ferreira, M.Pillinger, L. D. Carlos, J. Jepsen, A. Hazell, P. Ribeiro-Claro and I. S.Goncalves, J. Lumin., 2005, 113, 50–63; (c) A. M. Konkowski, S. Lis,M. Pietraszkiewicz, Z. Hnatejko, K. Czarnobaj and M. Elbanowski,Chem. Mater., 2003, 15, 656–663.
3 (a) Y. X. Zheng, L. S. Fu, Y. H. Zhou, J. B. Yu, Y. N. Yu, S. B. Wangand H. J. Zhang, J. Mater. Chem., 2002, 12, 919–923; (b) J. M. Lehn,Angew. Chem., Int. Ed. Engl., 1990, 29, 1304–1319; (c) Y. Li, B. Yanand H. Yang, J. Phys. Chem. C, 2008, 112, 3959–3968.
4 (a) M. R. Robinson, M. B. O’Regan and J. C. Bazan, Chem. Commun.,2000, (17), 1645–1646; (b) L. N. Sun, H. J. Zhang, Q. G. Meng, F. Y.
Liu, L. S. Fu, C. Y. Peng, J. B. Yu, G. L. Zheng and S. B. Wang, J. Phys.Chem. B, 2005, 109, 6174–6182.
5 (a) L. R. Matthews and E. T. Knobbe, Chem. Mater., 1993, 5, 1697–1700; (b) P. Innocenzi, H. Kozuka and T. J. Yoko, J. Phys. Chem. B,1997, 101, 2285–2291; (c) J. L. Liu and B. Yan, J. Phys. Chem. B, 2008,112, 10898–10907.
6 (a) L. D. Carlos, R. A. Ferreira, R. N. Pereira, M. Assuncao and V. D.Bermudez, J. Phys. Chem. B, 2004, 108, 14924–14932; (b) L. D. Carlos,R. A. S. Ferreira, J. P. Rainho and V. D. Bermudez, Adv. Funct. Mater.,2002, 12, 819–823; (c) H. R. Li, J. Lin, H. J. Zhang and L. S. Fu, Chem.Mater., 2002, 14, 3651–3655.
7 (a) S. S. Nobre, P. P. Lima, L. Mafra, R. A. S. Ferreira, R. O. Freire,L. S. Fu, U. Pischel, V. D. Bermudez, O. L. Malta and L. D. Carlos,J. Phys. Chem. C, 2007, 111, 3275–3284; (b) A. C. Franville, D. Zambonand R. Mahiou, Chem. Mater., 2000, 12, 428–435; (c) Q. M. Wang andB. Yan, J. Photochem. Photobiol., A, 2006, 178, 70–75; (d) Q. M. Wangand B. Yan, Inorg. Chem. Commun., 2004, 7, 1124–1127.
8 (a) Q. M. Wang and B. Yan, J. Mater. Chem., 2004, 14, 2450–2455;(b) Q. M. Wang and B. Yan, Cryst. Growth Des., 2005, 5, 497–503;(c) Q. M. Wang and B. Yan, J. Organomet. Chem., 2006, 691, 540–545.
9 (a) H. F. Lu and B. Yan, J. Non-Cryst. Solids, 2006, 352, 5331–5336;(b) B. Yan and H. F. Lu, Inorg. Chem., 2008, 47, 5601–5611; (c) J. L.Liu and B. Yan, J. Phys. Chem. C, 2008, 112, 14168–14178; (d) B. Yanand Q. M. Wang, Cryst. Growth Des., 2008, 8, 1484–1489.
10 (a) V. Bekiari, G. Pistolis and P. Lianos, Chem. Mater., 1999, 11, 3189–3195; (b) L. H. Wang, W. Wang, W. G. Zhang, E. T. Kang and W.Huang, Chem. Mater., 2000, 12, 2212–2218; (c) Q. M. Wang and B.Yan, J. Photochem. Photobiol., A, 2006, 177, 1–5.
11 (a) Y. J. Li and B. Yan, Inorg. Chem., 2009, 48, 8276–8285; (b) B. Yanand X. F. Qiao, J. Phys. Chem. B, 2007, 111, 12362–12374; (c) X. F.Qiao and B. Yan, J. Phys. Chem. B, 2008, 112, 14742–14750; (d) B. Yanand Y. Li, Dalton Trans., 2010, DOI: 10.1039/b914586a.
12 (a) Y. H. Luo, Q. Yan, S. Wu, W. X. Wu and Q. J. Zhang, J. Photochem.Photobiol., A, 2007, 191, 91–96; (b) X. M. Guo, H. D. Guo, L. S. Fu,R. P. Deng, W. Chen, J. Feng, S. Dang and H. J. Zhang, J. Phys. Chem.C, 2009, 113, 2603–2610.
13 X. F. Qiao and B. Yan, Inorg. Chem., 2009, 48, 4714–4723.14 L. N. Sun, J. B. Yu, H. J. Zhang, Q. G. Meng, E. Ma, C. Y. Peng
and K. Y. Yang, Microporous Mesoporous Mater., 2007, 98, 156–165.
15 L. S. Li, Y. Zhang, J. B. Yu, C. Y. Peng and H. J. Zhang, J. Photochem.Photobiol., A, 2008, 199, 57–63.
16 (a) D. Y. Zhao, Q. S. Huo, J. L. Feng, B. F. Chmelka and G. D. Stucky,J. Am. Chem. Soc., 1998, 120, 6024–6036; (b) G. Chandrasekar, K. S.You, J. W. Ahn and W. S. Ahn, Microporous Mesoporous Mater., 2008,111, 455–462.
17 (a) G. B. Deacon and R. J. Philips, Coord. Chem. Rev., 1980, 33, 227–250; (b) H. F. Lu, B. Yan and J. L. Liu, Inorg. Chem., 2009, 48, 3966–3975.
18 (a) K. Binnemans, P. Lenaerts, K. Driesen and C. Gorller-Walrand,J. Mater. Chem., 2004, 14, 191–201; (b) K. Binnemans, Chem. Rev.,2009, 109, 4283–4374; (c) O. L. Malta, H. F. Brito, J. F. S. Menezes,F. R. G. E. Silva, S. Alves, F. S. Farias, A and V. M. deAndrade,J. Lumin., 1997, 75, 255–268.
19 (a) O. L. Malta, M. A. C. DosSantos, L. C. Thompson and N. K. Ito,J. Lumin., 1996, 69, 77–84; (b) M. H. V. Werts, R. T. F. Jukes and J. W.Verhoeven, Phys. Chem. Chem. Phys., 2002, 4, 1542–1548; (c) E. E. S.Teotonio, J. G. P. Espinola, H. F. Brito, O. L. Malta, S. F. Oliveria,D. L. A. de Faria and C. M. S. Izumi, Polyhedron, 2002, 21, 1837–1844; (d) J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini and M.Bettinelli, J. Phys. Chem. B, 2004, 108, 20137–20143.
20 (a) L. D. Carlos, Y. Messaddeq, H. F. Brito, R. A. S. Ferreira, V. D.Bermudez and S. J. L. Ribeiro, Adv. Mater., 2000, 12, 594–598; (b) M. F.Hazenkamp and G. Blasse, Chem. Mater., 1990, 2, 105–110.
21 (a) X. F. Qiao and B. Yan, J. Phys. Chem. B, 2009, 113, 11865–11875;(b) G. F. de Sa, O. L. Malta, C. de Mello Donega, A. M. Simas, R. L.Longo, P. A. Santa-Cruz and E. F. da Silva, Coord. Chem. Rev., 2000,196, 165–195.
22 (a) R. A. S. Ferreira, L. D. Carlos, R. R. Goncalves, S. J. L. Ribeiro andV. D. Bermudez, Chem. Mater., 2001, 13, 2991–2998; (b) P. C. R. Soares-Santos, H. I. S. Nogueira, V. Felix, M. G. B. Drew, R. A. S. Ferreira,L. D. Carlos and T. Trindade, Chem. Mater., 2003, 15, 100–108.
23 C. A. Kodaira, A. Claudia, H. F. Brito and M. C. F. C. Felinto, J. SolidState Chem., 2003, 171, 401–407.
