Effects of Support and Ligand on the Photoluminescence Properties of

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J. Phys. Chem. C 2009, 113, 5784–5791

Effects of Support and Ligand on the Photoluminescence Properties of Siliceous Grafted Europium Complexes C. Tiseanu,*,† V. I. Parvulescu,‡ M. U. Kumke,§ S. Dobroiu,‡ A. Gessner,§ and S. Simon| National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-36, RO 76900, Bucharest-Magurele, Romania, UniVersity of Bucharest, Department of Chemical Technology and Catalysis 4-12 Regina Elisabeta BouleVard, Bucharest 030016, Romania, Institute of Chemistry, Physical Chemistry, UniVersity of Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany, and Babes-Bolyai UniVersity, Faculty of Physics, Institute for Interdisciplinary Experimental Research, 400084 Cluj-Napoca, Romania ReceiVed: September 22, 2008; ReVised Manuscript ReceiVed: January 9, 2009

Europium ions were introduced in SiO2 and MCM-41 via two different pathways: (1) grafting the europium complexes with two alkoxide structures, 3-(2-imidazolin-1-yl)-propyl-triethoxysilane (IPTES) and aminopropyltrimethoxysilane (APTMS), and (2) functionalization of the SiO2 support with silicon 4-carboxylbutyltriethoxide followed by subsequent addition of the europium ions. The new materials were characterized using nitrogen adsorption isotherms at -196 °C, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, Fourier transform infrared, NMR, DR-UV-vis, steady-state emission and excitation, and time-resolved photoluminescence spectroscopy. Spectral changes found in the time-resolved photoluminscence spectra strongly point to the distribution of europium ions on a range of environments in both SiO2 and MCM-41 supports. The average europium photoluminescence lifetimes decrease within the order: Eu3+-IPTES/SiO2 (550 µs) > Eu3+-APTMS/ SiO2 (425 µs) > Eu3+-APTMS/MCM-41 (370 µs) > Eu3+-IPTES/MCM-41 (320 µs) > Eu3+-CABES/SiO2 (240 µs). The photoluminescence quantum efficiency has the largest value, of 22%, for Eu3+-IPTES/SiO2, while the most reduced value, of 9%, was measured for Eu3+-CABES/SiO2. I. Introduction The chemical modification of silica by organosilicon compounds is already a common technique to tune its surface characteristics which opens a range of new and interesting applications in chromatography, in catalysis, wastewater purification, optical sensing, photonics, and drug delivery.1-10 At the same time, there is a growing interest in the preparation of lanthanides (Ln3+) containing hybrid materials due to potentially innovative luminescence applications. A fine-tuning of the photoluminescence (PL) properties (spectral range, lifetime, and quantum efficiency) can thus be envisaged through combining the organic and inorganic constituents with the lanthanide ions. Thus, PL properties of lanthanide based hybrid materials are covering a wide range of PL lifetimes (from nano- to milliseconds) and emission wavelengths, from VIS (Eu3+, Tb3+) to NIR (Nd3+, Er3+, Yb3+ Ho3+, Pr3+, Sm3+, or Tm3+).11-22 Silicon alkoxides and derived organically modified supports are generally the main precursors used for lanthanide-containing materials.23-27 Doping of the mesoporous silica with lanthanide complexes, in which only weak physical interactions (hydrogen bonding, van der Waals force, or weak static effect) exist between the support and the immobilized lanthanide complexes, causes several drawbacks, such as clustering of the emitting centers, inhomogeneous dispersion of both components, and leaching of the lanthanide’s complexes that are not anchored to the silica matrix (class I materials).26a A more promising * To whom correspondence should be addressed. E-mail: tiseanuc@ yahoo.com. † National Institute for Laser, Plasma and Radiation Physics. ‡ University of Bucharest. § University of Potsdam. | Babes-Bolyai University.

approach is to use a homogeneous dispersion of the lanthanide cations within the hybrid materials as a result of the hydrolysis and polycondensation of the lanthanides complexes with ligands bearing hydrolyzable Si(OR)3 groups. This method allows the synthesis of monophase organic-inorganic nanostructured hybrid materials with strong-type interactions (covalent or ionocovalent bonds) acting between the inorganic and organic counterparts. The class II materials25a,26a can be prepared either by postgrafting of as-synthesized materials via reaction with a selected organosilane or in one step by co-condensation of a tetra-alkoxysilane and one (or more) organoalkoxysilane(s). Here two approaches are used for the incorporation of the europium complexes/ions within the SiO2 and MCM-41 supports: (1) grafting reaction of the organosilylated europium complexes onto SiO2 and MCM-41 supports and (2) complexation reaction of europium ions with a previously functionalized silica. For the first approach, the ligands used were 3-(2imidazolin-1-yl)-propyl-triethoxysilane (IPTES) and aminopropyl-trimethoxysilane (APTMS), while for the second approach the SiO2 was functionalized with 4-carboxylbutyltriethoxide (CABES) prior to Eu3+ addition. To our knowledge, there are only a few reports on the grafting reaction of organosilylated lanthanides complexes on preformed silica.17 The new materials were characterized by nitrogen adsorption isotherms at -196 °C, thermogravimetric analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), DR-UV-vis, as well as steady-state and time-resolved photoluminescence spectroscopy. The aim of the present study was to investigate the distribution of the europium ions in the five types of materials. We found that, in combination with europium as luminescence probe, time-resolved PL spectra (TRES) are able to evidence

10.1021/jp808411e CCC: $40.75  2009 American Chemical Society Published on Web 03/09/2009

PL Poperties of Siliceous Grafted Europium Complexes

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5785

SCHEME 1: Structure of the IPTES Ligand

SCHEME 2: Structure of the APTMS Ligand

SCHEME 3: Structure of the CABES Ligand

even weak effects of the ligand and support material on the PL properties as well as differentiate the distribution nature (homogeneous versus heterogeneous) of the europium complexes/ ions within the SiO2 and MCM-41 supports. A comparative investigation of the quantum efficiency of emission as well as structure of the first coordination shell of europium was also pursued with respect to the ligand, support or synthesis approach. II. Experimental Section 1. Synthesis. The introduction of Eu3+ ions within the two siliceous supports, SiO2 and MCM-41, was done via two different pathways: (i) grafting of the europium complexes of two alkoxide structures, 3-(2-imidazolin-1-yl)-propyl-triethoxysilan (IPTES, Scheme 1) and aminopropyl-trimethoxysilan (APTMS, Scheme 2) on the above supports and (ii) functionalization of the SiO2 support with silicon 4-carboxylbutyltriethoxide (CABES, Scheme 3) followed by complexation with Eu3+. Silica was prepared via the sol-gel method using tetraethylorthosilicate as a precursor. The gel was prepared using a molar Si(OEt)4:H2O:HCl molar ratio of 1:4:0.01. After a strong stirring for 4 h a suspension of C14H29 (CH3)3 NBr in water (10%) was added, and the mixture has been stirred for another 2 h. The jellification was carried out for 3 days at room temperature, and the resulted gel was dried at 383 K for 12 h. The siliceous MCM-41 support was prepared according to the methodology reported in ref 27. Both supports were calcined at 773 K for 5 h. For the SiO2 support, grafting of the europium -IPTES complexes was carried out using the following procedures: first a quantity of 10 g of IPTES (approximately 0.03 moles) was diluted in benzene (adding five times more benzene) and then mixed with 0.11 g of Eu(NO3)3 · 5H2O (2.5 × 10-4 moles Eu(NO3)3 · 5H2O) and vigorously stirred for 1 h. Next, the solution was diluted with benzene resulting in a volume of 40 mL and then mixed with 0.3 g of silica. The mixture was introduced in a Teflon-lined autoclave, washed with argon, and heated under inert atmosphere and vigorous stirring (700 rpm) at 323 K for 72 h. After cooling to room temperature, in order to eliminate the ungrafted complex, the separated solid was washed twice with 40 mL of benzene and then dried under vacuum at 333K for 6 h, rewashed twice with 40 mL isopropanol, and finally dried under vacuum at 333 K for 12 h. The solid is denominated as Eu3+-IPTES/SiO2. A similar procedure was used for the grafting using APTMS: a quantity of 0.54 g of APTMS was diluted in 5 mL of EtOH and mixed with 0.11 g of Eu(NO3)3 · 5H2O (2.5 × 10-4 moles Eu(NO3)3 · 5H2O) by vigorous stirring for 1 h. Then the solution

Figure 1. XRD pattern of MCM-41 and of Eu3+-APTMS/MCM-41.

was diluted with ethanol to a final volume of 40 mL and then mixed with 0.3 g of silica. The mixture was introduced in a Teflon-lined autoclave, washed with argon, and heated under inert atmosphere and vigorous stirring (700 rpm) at 308 K for 72 h. After cooling at room temperature, to eliminate the ungrafted complex, the separated solid was washed twice with 40 mL of ethanol and then dried under vacuum at 333 K for 6 h, rewashed twice with 40 mL of isopropanol, and finely dried under vacuum at 333 K for 12 h. In case of MCM-41, grafting of the europium complexes of IPTES or APTMS was done by using the same protocol as used for the silica support. The solids are denominated as Eu3+-IPTES (APTMS)/SiO2 (MCM-41). For the functionalized SiO2 support, prior to the addition of Eu3+, the support was pretreated with silicon 4-carboxylbutyltriethoxide (CABES). Thus, 1 g of SiO2 was treated with 20 mL of solution of silicon 4-carboxylbutyltriethoxide in ethanol in a Teflon-lined autoclave under a vigorous stirring (700 rpm) at 343 K for 48 h using a CABES/support ratio of 1.2 mmol/g. The subsequent treatment followed the protocols already described. The content in grafted CABES was determined to about 1 mmol/g support from thermogravimetric analyses by heating these samples to 873 K (at 10 K min-1). The functionalized SiO2 (0.3 g) was mixed under vigorous stirring at 308 K for 72 h with 40 mL of a solution of Eu(NO3)3 · 5H2O (0.11 g) in ethanol. After cooling the mixture down to room temperature, the solid was separated, washed three times with 40 mL of ethanol, and dried under vacuum at 333 K for 12 h. The solid is denominated as Eu3+-CABES/SiO2. 2. Materials Characterization. 2.1. Structural and Textural Properties. Structural characterization of the grafted materials was performed using nitrogen adsorption isotherms at -196 °C. The specific surface areas of the samples and the volumes were determined after degassing the samples in situ at 120 °C for 5 h in order to remove any adsorbed gases. The data analysis was performed using the commercial micropore program (Micromeritics) for nitrogen adsorption isotherms collected at -196 °C with a Micromeritics ASAP2020 instrument. Powder X-ray diffraction patterns (PXRD) were collected on a Siemens D-5000, using Cu KR radiation (λ ) 1.54050 Å) and quartz as an external standard. FTIR as well as diffuse reflectance infrared Fourier transform (DRIFT) data were collected on a Nicolet 4700 FTIR spectrometer. FTIR measurements were performed using the KBr pellet technique, and DRIFT were carried out under heating the samples in helium with a rate of 1 °C/min with a plateau of 1 h at each measuring temperature. Thermogravimetric (TG) analysis was perforned on a SETARAM 92 16.18 apparatus in air using a heating rate of 10 °C/min. TEM images of the samples were collected on a Tecnai F20 200kV microscope from Philips. The instrument is equipped with a field emission gun and provides accelerating

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Tiseanu et al.

TABLE 1: Chemical Composition and Textural Properties of the Investigated Materials Eu3+ content, wt %

material SiO2 MCM-41 Eu3+-IPTES-SiO2 Eu3+-APTMS-SiO2 Eu3+-IPTES/MCM-41 Eu3+-APTMS/MCM-41 Eu3+-CABES-SiO2

ligand content, mmol g-1

surface area, m2 g-1

pore size, nm

0.10 2.81 0.12 2.80 1.2

96 942 54 34 748 584 46

10 3.2 8.6 7.1 1.8 1.2 5.8

1.25 12.3 1.19 12.1 12.8

voltages of up to 200 kV. The point resolution and the line resolution were 0.24 and 0.12 nm, respectively, at a focal length of 1.7 mm. The solid samples were dispersed in pure ethanol by sonication, and approximately 0.05 mL of this suspension was dropped on a copper mesh coated with an amorphous holey carbon film. The ethanol was evaporated at room temperature prior to the TEM analysis performed at 200 kV accelerating voltage. SEM measurements were carried out on a Hitachi S3000N scanning electron microscope, equipped with a Horiba electron microprobe. 13C and 29Si magic-angle spinning (MAS) NMR spectra were collected using a full digital BRUKER Avance 400 UltraShieldTM spectrometer, with superconductor magnet of 9.4 T operated at spinning speeds of 6 and 15 kHz, respectively. 3. Photoluminescence (PL) Measurements and Data Analysis. TRES data were recorded using a wavelength tunable Nd: YAG-laser/OPO system (Spectra Physics/GWU) operating at 20 Hz as excitation light source and an intensified CCD camera (Andor Technology) coupled to a spectrograph (MS257 Model 77700A, Oriel Instruments) as detector. The TRES data were collected using the box car technique. In a typical experiment, the sample was nonselectively excited at λex ) 394 nm (corresponding to the 7F0-5L6 transition of Eu3+), the initial gate delay (delay after laser pulse, δt) was set to 2 µs, and the gate width was adjusted to 50 µs. The PL was detected in the spectral range of 500 < λem < 750 nm with a maximum spectral resolution of 0.08 nm. All the PL measurements were performed at room temperature. The PL decays were analyzed by fitting of the integrated photoluminescence decays within wavelength intervals corresponding to the different europium luminescence transitions with a multiexponential function, f(t) n

f(t) )

∑ Ai exp(-t/τi) + B

(1)

i)1

where Ai is the decay-related amplitude, B a constant (the baseline offset) and τi the lifetime. The average PL lifetimes were calculated using the following formula

〈τ〉 )

∫0t ∫0t

max

tI(t)

max

I(t)

(2)

where I(t) represents the PL intensity at time t corrected for the background and the integrals are evaluated within range of 0 < t < tmax, where tmax was typically 5 ms. Excitation spectra were recorded on a Fluoromax3P (Jobin Yvon) spectrometer operated in the phosphorescence mode. III. Results and Discussion 1. Structural and Textural Characterization. Table 1 compiles the chemical composition and the textural properties of the investigated materials. The amount of the APTMS or IPTES ligand was similar irrespective of the support, being more

than an order of magnitude greater for the APTMS (2.8 compared to 0.1 mmol g-1). Following grafting, both surface area and pore size of SiO2 and MCM-41 were strongly reduced relative to the values for the parent supports. The BrunauerEmmett-Teller (BET) surface is reduced by almost 40 and 60% following grafting of the Eu3+-APTMS complex on MCM-41 and SiO2, respectively. For Eu-APTMS, the pore diameter is reduced by 30% for SiO2 while an even more pronounced reduction of 60% was measured for the MCM-41 support. The greatest Eu3+ content was measured for the CABES ligand (12.9 wt %), while for IPTES, the Eu3+ content was only 1.2 wt % in both supports. The decrease of these parameters parallels the loading and size of complexes and provides evidence that the lanthanides complexes were confined within the pores of MCM41 or SiO2. However, a mixed grafting mode is expected, inside the pores and on the external surface as well28 being also confirmed by our PL measurements. The XRD patterns indicated that the parent SiO2 support was completely amorphous, which is in a good agreement with previous reports concerning the synthesis of such materials following a sol-gel procedure.29 Grafting of the described complexes was not accompanied by any change of the XRD patterns. Following grafting of Eu3+-APTMS (Figure 1) or Eu3+IPTES, the regular pore structure of MCM-41 was preserved; however, a decrease in the intensity was observed. The reduction in diffraction intensity can be explained by the presence of an organic component covalently grafted inside the pore channels of the MCM-41 samples, which results in the decrease of the mesoscopic order of the organic moieties inside the pore channels. The preservation of this structure is a good evidence for the stability of this texture against the hydrolysis of the silicon alkoxide, regardless the ligand type. Parts a-c of Figure 2 represent the TEM and SEM pictures of Eu3+-IPTES-SiO2 and Eu3+-IPTES/MCM-41. In both cases a uniform distribution of the submicronic particles has been achieved. For silica, the diameter of the particles was determined to 0.8 µm (Figure 2b), while for the MCM-41 the value was almost three times smaller (around 0.3 µm, Figure 2c). Grafting the europium complexes did not induce any change in the morphology of the resulted materials. For the MCM-41, grafting of the complexes led to a partial damage of the surface, in perfect agreement with the XRD patterns (Figure 1). The 29>Si and 13C CP/MAS NMR spectra (not shown) exhibit a large peak at -111 ppm with a shoulder at about 101 ppm in all investigated materials. These were assigned to Q3 and Q4 silicon species of the siliceous support.30 Additional lines at 66-69 ppm and 58-60 ppm were also detected with all samples corresponding to the different environments for the siloxane groups. They were assigned to T2 (terminal groups that are only bound to one neighboring siloxane) and T3 (cross-linked groups that are bound to two neighboring siloxanes) from the APTMS, IPTES, and CABES ligands. The T1 (related to the isolated siloxane group) was present only as a small shoulder feature on T2. The predominance of the T3 peak sustains for the closed-



Figure 3. FTIR spectra of the investigated materials.

Figure 4. DR-UV-vis spectra of Eu3+-APTMS/MCM-41, Eu3+APTMS/SiO2, and Eu3+-IPTES/MCM-41. Inset: PL excitation spectra of Eu3+-IPTES /MCM-41 and Eu3+-APTMS/MCM-41 (λem ) 614 nm).

Figure 2. (a) TEM picture of Eu3+-IPTES/MCM-41. (b) SEM picture of Eu3+-IPTES/SiO2. (c) SEM picture of Eu3+-IPTES/MCM-41.

packed conformation of the grafted groups, regardless the support. By comparison with estimates reported in literature, the surface coverage with the grafted ligands can be tentatively approximated to be greater than 50%.30b FTIR spectra also supported the successful grafting of europium complexes. The presence of the CH2 groups in the range 1100-1600 cm-1 and those at 2900-3000 cm-1 due to the existence of the nitrogen species represent another good confirmation of the presence of the grafted Eu-IPTES or EuAPTMS complexes (Figure 3). For Eu-CABES/SiO2, the bands at 1203, 1303, 1440, 1725, 2855, 2926, and 2961 cm-1 are assigned to C-H skeletal vibration, C-O stretching vibration, C-O-C stretching vibration, CH2 and CH deformation vibration, and C-H stretching vibration. The band associated to the surface silanols (3400-3600 cm-1) in SiO2 and MCM-41 was almost completely consumed following grafting suggesting the increase of the surface hydrophobicity of the two supports.

Figure 4 shows the DR-UV-vis spectra of selected materials. The band at λ ≈ 310 nm related to aminosilane absorption31 for Eu3+-APTMS/SiO2 is red-shifted to λ ≈ 325 nm in SiO2 confirming the different interactions of Eu3+-APTMS complex within the two supports. A number of absorption peaks in the 350 m < λ < 550 nm region, i.e., λ ) 394 and 467 nm, were attributed to the f-f transitions of Eu3+. They were more clearly identified in the PL excitation spectra (inset of Figure 4). From the DR-UV/vis spectra it can be seen that the APTMS or IPTES ligands show no significant absorption at λ > 300 nm. Only the materials containing IPTES show a strong absorption around λ ) 350 nm. However, from the comparison of DR-UV/vis and excitation spectra it was inferred that the europium PL sensitization via intramolecular energy transfer from the IPTES ligand can be neglected as well. 2. PL Spectra and Decays. Figures 5 and 6 illustrate the PL spectra and decays of the investigated materials. The inset of Figure 5 shows a representative PL spectrum of the investigated samples following the nonselective excitation at λex ) 394 nm at a delay of δt ) 2 µs after the excitation. The emission contains the 5D0-7FJ (J ) 0, 1, 2, 3, 4) transitions at about 579, 582, 614, 653, and 698 nm. Weak 5D1 emission at ca. 524, 535, and 555 nm was also observed. The most important probe transitions for europium are 5D0-7F0,1,2 which are governed by selection rules which depend on the covalence effects and local symmetry of the crystal field around the ion.32-37 Albeit weak, the electric and the magnetic forbidden 5 D0-7F0 transition was detected as a weak shoulder on the 5 D--7F1 transition in all materials. It is well-known that this transition is strictly forbidden in a symmetric crystalline field. Hence, the presence of the band λem ) 579 nm suggests that Eu3+ occupies sites with low symmetry without an inversion center. For Eu3+-APTMS/SiO2, Eu3+-APTMS/MCM-41, Eu3+-


Tiseanu et al. Eu3+-IPTES grafted on SiO2 was found to be greatest among all investigated materials (around 22%), while Eu3+-CABES/ SiO2 exhibited the smallest quantum efficiency (9%), (Table 2). The quantum efficiency of the lanthanides emission in sol-gel-derived materials is low as a result of the nonradiative quenching induced by the -OH groups, mainly from water molecules. The number of water molecules bound within the first coordination shell of Eu3+ in solution or solid state can be estimated by using the updated empirical formula of Horrocks40a

nw ) 1.11(1/τ - Arad - 0.31) -1

Figure 5. Normalized D0- F0 emission spectra corresponding to Eu APTMS/MCM-41, Eu3+-IPTES/SiO2, and Eu3+-CABES/SiO2. Inset: The representative PL spectrum of the investigated materials (λex ) 394 nm). 5

7

3+

Figure 6. PL decays of the investigated materials (λex ) 394 nm and λem ) 614 nm).

IPTES/MCM-41, and Eu3+-IPTES/SiO2 the spectral shape was asymmetric toward the shorter wavelengths, while for Eu3+CABES/SiO2 an almost symmetrical Gaussian shape was evidenced. The width (fwhm) of the 5D0-7F0 emission is smallest for Eu3+-CABES/SiO2 (Figure 5). The 5D0-7F1 transition is a magnetic dipole allowed transition, and its intensity scarcely varies with the bonding environment of the europium ions. Generally, the shape of the 5D0-7F1 emission contains three spectral features (λem ) 589, 593, and 596 nm) at δt ) 2 µs. In contrast to the 5D0-7F1 transition, the intensity of the forced electric dipole 5D0-7F2 transition is hypersensitive to the variation of the bonding environment of the Eu3+ ion. By consideration of the intensity of the 5D0-7F1 transition as an internal reference, the asymmetry ratio defined as the intensity ratio, R ) I(5D0-7F2)/I(5D0-7F1) increases with the covalence of the Eu3+-oxygen bonds or/and the local asymmetry. The R values estimated from the PL spectra measured at δt ) 2 µs after the laser pulse, were 3.7 (Eu3+-APTMS/MCM-41, Eu3+APTMS/SiO2, and Eu3+-IPTES/SiO2), 3.4 (Eu3+-IPTES/MCM41), and 4.7 (Eu3+-CABES/SiO2), respectively. Except for Eu3+-CABES/SiO2 the PL decays of the investigated samples measured around λem ) 614 nm following excitation at λex ) 394 nm were nonexponential (Figure 6). Up to three exponentials were used in the PL decays fitting (eq 1). The average europium PL lifetimes calculated according to eq 2 decrease within the order: IPTES/SiO2 (550 µs) > APTMS/ SiO2 (425 µs) > APTMS/MCM-41 (370 µs) > IPTES/MCM41 (320 µs) > CABES/SiO2 (240 µs). Europium is the only lanthanide ion that allows the calculation of the radiative lifetime of Eu3+ from its corrected emission spectrum.38 Following the protocol described in previous works,38,39 the estimated value for the quantum efficiency of

(3)

Here, both τ and Arad are expressed in ms with τ representing the average PL lifetime calculated using eq 2. Although the noninteger values included the effect of the unbound closely diffusing H2O molecules, we assign them mostly to the uncertainty of this empirical formula. Therefore, in Table 2 the nw values were also rounded to the nearest integer. The formula was derived from the former equation of Horrocks and Sudnick,40 which was supposed to be valid for lanthanides in both solution and solid state. For low number of nw however, the reliability of the equation was reported to fail and claimed that it cannot be applied to nonmolecular solids, e.g., the coupling strength of water molecules was found to depend on the surroundings of the Eu3+ ion.41 This formula proved however useful in assessing the nonradiative relaxation rate induced by -OH groups in various europium-doped silica materials,18c-e,22c,d and we also use it here to compare the investigated materials. Thus, it was found that a relatively large number of water molecules (about 4) is directly coordinated to Eu3+ in CABES/ SiO2, which explains its low quantum efficiency (Table 2). On the other side, about one water molecule is estimated to be in the first coordination shell of the Eu3+ in the case of Eu3+IPTES/SiO2, which represents the minimum value among for the investigated materials. It is interesting to note that the parameter values of the quantum efficiency (22%) or average lifetime (550 µs) promote Eu3+-IPTES/ SiO2 as a relatively highly luminescent material comparable with the europium complexes covalently bound to a previously modified silica support. For these materials, the quantum efficiency and (average) lifetime were of of 29% and 480 µs (for a ternary europium complex Eu(NTA)3bpy covalently bonded to SBA15, where NTA is 1-(2-naphthoyl)-3,3,3-trifluoroacetonate and bpy is bipyridine),16b 37% and 490 µs (for a ternary complex Eu(TTA)3phen covalently bonded to SBA-15),22e or 28% and 360 µs (for a ternary complex Eu(TTA)3phen covalently periodic mesoporous organosilica where TTA is 2-thenoyltrifluoroacetone and phen is 1,10-phenanthroline),22d respectively. 2.1. Effects of the Support. In contrast to the PL spectra, the PL decays showed a dependency on the support with a faster PL dynamics evidenced for the europium complexes grafted onto MCM-41 (Figure 6). This is tentatively assigned to the increased contribution of vibrational coupling of europium excited states to the OH-groups in MCM-41. On the other side, the asymmetry ratios, R, determined at δt ) 2 µs are close to 3.7 for Eu3+-APTMS and Eu3+ -IPTES grafted onto SiO2 and MCM-4,1 which may indicate similar distorted environments around europium ions within the two supports. However, with increasing δt, the asymmetry ratio R evolved differently with respect to the ligand and support. For Eu3+-APTMS, R increased from 3.7 up to 4.8 (SiO2) or 5 (MCM-41) at δt ) 1.5 ms. In the case of Eu3+-IPTES, R increases from 3.7 up to 4.2 (SiO2) and from 3.4 to 5.4 (MCM-41) within the same time range. Further differences induced by the two supports are exhibited by the time evolution of the 5D0-7F1 emission (see Figure 7). For Eu3+-APTMS/SiO2, the Stark splitting structure of the



TABLE 2: PL Properties of the Investigated Materials material

average PL lifetime 〈τ〉 ( 5% (µs)

Arad(ms-1)

η (%)

nW ( 0.1

Eu -APTMS/MCM-41 Eu3+-APTMS/SiO2 Eu3+-IPTES/MCM-41 Eu3+-IPTES/SiO2 Eu3+-CABES/SiO2

370 425 350 550 240

345 335 320 360 400

11 12 10 22 9

2.9 (3) 2.6 (3) 3.2 (3) 1.2 (1) 3.9 (4)

3+

D0-7F1 emission is changed from a three spectral features shape (λem ) 589, 593, and 596 nm) at δt ) 2 µs to only two (λem ) 592 and 596 nm) at δt ) 1.5 ms after the laser pulse. For Eu3+IPTES/SiO2 the Stark splitting structure of the 5D0-7F1 emission remained almost unchanged with δt. As the splitting of the 5 D0-7F1 is generally governed by variations of the crystal field strength and symmetry,33 it can obviously be attributed to the existence of slightly different crystal-field symmetries at the europium sites in Eu3+-APTMS/SiO2, while for Eu3+-IPTES (APTMS)/MCM-41, Eu3+-IPTES/SiO2, and Eu3+-CABES/SiO2 the opposite is observed. The above results corroborated with the differences measured for the PL decays and TRES at δt ) 1.5 ms (Figures 6 and 8) and DR-UV/vis spectra (Figure 4) sustain close but not identical environments for Eu3+-APTMS and Eu3+-IPTES complexes grafted on MCM-41 and SiO2. In principle, there may be several sources responsible for the nonexponential behavior of the europium PL decay in the investigated materials. In some reports, the nonexponential character of 5D0 emission decay measured for various europium complexes immobilized on silica is erroneously assigned to the concentration induced quenching or cross-relaxation via ion-ion interactions. The 5D0 state of europium is not involved in crossrelaxation processes due to the absence of energy levels matching in contrast with the next excited state 5D1 state.37 Nonexponential decay related to the 5D0 emission may be related to the energy migration between closely located europium ions with subsequent nonradiative relaxation to traps. Migration effects on the PL decay are therefore expected to be more pronounced with more concentrated europium samples, such as Eu3+-APTMS, which is obviously not the case if we compare the PL decay of Eu3+-APTMS/MCM-41 with that of the more diluted Eu3+-IPTES/MCM-41 sample (Table 1 and Figure 6). Therefore we assign the nonexponential nature of the europium PL decay in the investigated materials to the existence of multiple europium species. An alternate indication for the heterogeneous distribution of europium in these materials is given by the time evolution of the asymmetry ratio, R, illustrated in Figure 8. For the Eu3+ ions located in one chemical environment (a single Eu3+ species), the R value is expected to remain constant with time delay after the laser pulse (δt), while for a system with a discrete or continuous distribution of the luminescent species, R becomes time dependent15c (Figure 8). Spectral changes displayed with TRES corresponding to the 5 D0-7F1 emission (Figure 7) and time evolution of the asymmetry ratio, R, (Figure 8) argue for a distribution of Eu3+APTMS and Eu3+-IPTES complexes in a range of environments rather then the existence of a number of species distributed on well-defined sites in both SiO2 and MCM-41 supports. This range of sites includes the pore channels and the external surface28 where several coordination modes of the lanthanides complexes following grafting can occur, including binding to the surface silanolate (SiO) groups.19 Albeit the amount of grafted complexes is the same in SiO2 and MCM-41 (ca. 2.8 mmol/g for APTMS and 0.1 mmol/g for IPTES, Table 1), the PL decays are shortened in MCM-41 compared to SiO2 (Figure 6). On the basis of the amorphous 5

nature of wall structures of MCM-41, a similar surface chemistry of MCM-41 and silica was expected. Further, the distribution of the grafted complexes within the mesoporous ordered MCM41 (pore size of 3.2 nm) and amorphous SiO2 (greater pore size of 10 nm) can be regarded as similar with the condition that the pore blocking is avoided during the grafting process.42 Pore blocking can be neglected considering the initial pore diameter of 3.2 nm of MCM-41 and the low APTMS and IPTES content (Table 1).43 A tentative explanation of the different PL dynamics of europium in MCM-41 compared to SiO2 considers the perpendicular orientation of the europium complexes to the surface of SiO2, which reduces the probability of interaction of europium with the different Si-OH groups (i.e., single, hydrogenbonded, or geminal). 2.2. Effects of the Ligand. For the SiO2 support, the asymmetry ratio, R, values increased with δt for both APTMS and IPTES ligands, but the R values measured for the IPTES are clearly less dispersed (Figure 8). Such a result supports a

Figure 7. TRES (area normalized) corresponding to the 5D0-7F1 transition measured at δt ) 2 (bold line) and δt ) 1.5 ms (thin line).


Figure 8. Evolution with time of the asymmetry ratio, R, for the investigated materials.

Tiseanu et al. exibits a relative strong 5D1 emission with an emission yield of about 10% of the total 5D0, 1-7F0, 1, 2, 3 emission (inset of Figure 9). The environment at the europium site is the most distorted compared to the rest of the investigated materials quantified by the largest asymmetry ratio, R, of 4.7 (Table 2). From the maximum of the 5D0-7F0 emission relative to the rest of the investigated materials (Figure 5), it was inferred that the most covalent bonding environment for europium among the investigated materials is provided by the CABES ligand. The fwhm value corresponding to the 5D0-7F0 transition is only 0.8 nm (24 cm-1) in CABES/SiO2 compared to ca. 1.3 nm (40 cm-1) for the rest of materials. Besides carboxylate groups that bind predominantly as bidentate ligands to the lanthanide’s ions a number of four molecules was estimated to be present in the first coordination shell of europium in CABES/SiO2 (Table 2). The narrow emission bands along with the single exponential nature of the PL decay provide clear evidence for a single europium species in CABES/SiO2. However, a “good” fit of the PL decay curve to single exponential does not necessarily imply that the PL arises from a single, discrete component. Narrow lifetime distributions can be fitted to a single exponential within the statistical error provided by the used instrumentation as well. Indeed, the slight spectral changes evidenced with δt (Figures 8 and 9) point to the distribution of europium ions on a range of similar but not identical environments in CABES/ SiO2. IV. Conclusions

Figure 9. Comparison between the high-resolution TRES of Eu3+CABES/SiO2 measured at δt ) 2 µs (solid line) and 1.5 ms (dotted line) after the laser pulse (λex ) 394 nm) in the spectral range of 570-640 nm. Inset: PL spectrum measured at δt ) 2 µs after the laser pulse. With numbers 0, 1, 2, and 3 are demoted the 5D1-7F0, 5D1-7F1, 5 D1-7F2, and 5D1-7F3 transitions, respectively.

more homogeneous distribution of Eu3+-IPTES compared to Eu3+-APTMS complexes following grafting on the SiO2 support. Further evidence is given by the time evolution of the 5D0-7F1 emission (Figure 7) that was previously discussed and the shape of the PL decay (Figure 6). For the MCM-41 support, data are less conclusive, and a similar tendency could not be established. Though the large fwhm values preclude an accurate determination of the energy values of 5D0-7F0 emission in all investigated materials, a clear trend of its emission peak with the ligand type was obtained, (Figure 5). Thus, the 5D0-7F0 transition is red-shifted within the order: Eu3+-IPTES/SiO2 (579.82 nm (17247 cm-1) > Eu3+-APTMS/SiO2 ≈ Eu3+APTMS/MCM-41 (579.7 nm (17250 cm-1) > Eu3+-CABES (579.5 nm (17256 cm-1). The red-shift of the 5D0-7F0 emission is related to the nephelauxetic effect, which is generally ascribed to the covalency contributions of the ligands in the first coordination shell via the phenomenological equation of Frey and Horrocks.44 Accordingly, it is realistic to consider that Eu3+ coordination to different types of oxygen and nitrogen based environments as well as changes on the total coordination number may cause different degrees of covalency and local polarization in the first coordination shell of Eu3+-IPTES (APTMS)/MCM-41 (SiO2) and Eu3+-CABES/SiO2. Thus the least covalent europium bonds are ensured by the IPTES ligand, while the most covalent environment is provided by the CABES ligand (both in SiO2). 2.3. Eu3+-CABES/SiO2. Figure 9 illustrates the TRES of Eu3+-CABES/SiO2. While for the europium complexes in silica only a weak 5D1 emission is measured,22c Eu-CABES/SiO2

The different influence of the support material (SiO2 or MCM41) on the PL properties of the grafted Eu3+-APTES and Eu3+IPTES complexes is tentatively assigned to differences in the vibronic coupling to the matrix (especially -OH groups and remaining water molecules) and distribution effects. The coupling seems to be stronger for the MCM-41 support and can be best seen by the differences in the average PL lifetimes. For Eu3+-IPTES/SiO2 about one water molecule is found in the first coordination shell compared to three for Eu3+-APTMS/ SiO2.. As a result, for Eu3+-IPTES/SiO2 the average PL lifetime and the PL efficiency are the largest among the investigated materials (550 µs and 22%, respectively). Spectral changes found in the TRES such as the time evolution of the asymmetry ratio as well as the slight changes of the 5D0-7F1 emission strongly point to the distribution of Eu3+-APTMS and Eu3+IPTES complexes on a range of environments rather then the existence of a number of species distributed on well-defined sites in both SiO2 and MCM-41 supports. Among the investigated materials Eu3+-CABES/SiO2 showed the most distinct PL properties related to a narrow distribution of the europium environments and a fastest PL decay with a lifetime of 240 µs. The low quantum efficiency (about 9%) is explained by a relatively large number of water molecules (about 4) directly bound to europium in CABES/SiO2. References and Notes (1) Blitz, J. P.; Little, C. B. Fundamental and Applied Aspects of Chemically Modified Surfaces, Royal Society of Chemistry: Cambridge, U.K., 1999. (2) Taguchi, A.; Schuth, F. Microporous Mesoporous Mater. 2005, 77, 1. (3) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. ReV. 2002, 102, 3615. (4) Wark, M.; Rohlfing, Y.; Altindag, Y.; Wellmann, H. Phys. Chem. Chem. Phys. 2003, 5, 5188. (5) Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001, 13, 3140. (6) Yoshitake, H.; Yokoi, T.; Tatsumi, T. Chem. Mater. 2002, 14, 4603.

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JP808411E

Effects of Support and Ligand on the Photoluminescence Properties of

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