Lanthanide (Eu3+, Tb3+) Centered Hybrid Materials using Modified

Aug 12, 2008 - Lanthanide (Eu3+, Tb3+) Centered Hybrid Materials using Modified Functional Bridge Chemical ... a molecular precursor BFPP−Si through...
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10898

J. Phys. Chem. B 2008, 112, 10898–10907

Lanthanide (Eu3+, Tb3+) Centered Hybrid Materials using Modified Functional Bridge Chemical Bonded with Silica: Molecular Design, Physical Characterization, and Photophysical Properties J. L. Liu and B. Yan* Department of Chemistry, Tongji UniVersity, Siping Road 1239 Shanghai 200092, PR China ReceiVed: May 3, 2008; ReVised Manuscript ReceiVed: July 11, 2008

1,3-Bis(2-formylphenoxy)-2-propanol (BFPP) was first synthesized and then grafted to 3-(triethoxysilyl)propyl isocyanate (TESPIC) to achieve a molecular precursor BFPP-Si through the hydrogen-transfer nucleophilic addition reaction between the hydroxyl group of BFPP and the isocyanate group of TESPIC. Then, a chemically bonded lanthanide/inorganic/organic hybrid material (BFPP-Si-Ln) was constructed using BFPP-Si as a bridge molecule that can both coordinate to lanthanide ions (Eu3+ or Tb3+) and form an inorganic Si-O network with tetraethoxysilane (TEOS) after cohydrolysis and copolycondensation processes. Furthermore, two types of ternary rare-earth/inorganic/organic hybrids (BFPP-Si-Dipy-Ln and BFPP-Si-Phen-Ln) were assembled by the introduction of the second ligands (4,4′-bipyridyl and 1,10-phenanthroline) into the above system. All of these hybrid materials exhibit homogeneous microstructures and morphologies, suggesting the occurrence of self-assembly of the inorganic network and organic chain. Measurements of the photoluminescent properties of these materials show that the ternary rare-earth/inorganic/organic hybrids present stronger luminescent intensities, longer lifetimes, and higher luminescent quantum efficiencies than the binary hybrids, indicating that the introduction of the second ligands can sensitize the luminescence emission of the lanthanide ions in the ternary hybrid systems. 1. Introduction The search for new materials for new applications is a challenge for materials science research. In this context, the possibility comes up to combine organic and inorganic components, at a molecular or a nanometric level, generating new materials which are known as organic-inorganic hybrid materials.1-4 The organic-inorganic hybrid materials combine some advantages of organic compounds (easy processing with conventional techniques, elasticity, and organic functionalities) with properties of inorganic oxides (hardness, thermal and chemical stability, transparency) and thus have attracted considerable attention.5,6 The sol-gel route is the most commonly employed method for the preparation of organic-inorganic hybrids at macro-/microscale, even at a molecular level under mild conditions, Moreover, by modifying the sol-gel processing conditions, the microstructure, the external shape, or the degree of combination between the organic and the inorganic phases can be further controlled.7-9 It is well known that lanthanide complexes have characteristic luminescence properties and give sharp, intense emission lines upon ultraviolet light irradiation, because of the effective intramolecular energy transfer from the coordinated ligands to the luminescent central lanthanide ions.10-12 In the past few decades, inorganic matrices doped with metal complexes, especially lanthanide organic complexes introduced into a silica matrix, have already been found to show superior emission intensities, and organic components are considered to be efficient sensitizers for the luminescence of lanthanide ions.13-15 However, this conventional doping method seems unable to solve the problem of the quenching effect of luminescent centers because only weak interactions (such as * To whom correspondence should be addressed. E-mail: byan@ tongji.edu.cn. Fax: +86-21-65982287. Telephone: +86-21-65984663.

hydrogen bonding, van der Waals forces, or weak static effects) exist between organic and inorganic moieties. Moreover, inhomogeneous dispersion of two phases and leaching of the photoactive molecules frequently occur in this sort of hybrid material for which the concentration of complexes is also greatly reduced. As a consequence, another appealing method has emerged which concerns the covalently bonded hybrids (class-hybrid materials16), and the as-derived molecular-based materials exhibit monophasic appearance even at a high concentration of rare-earth complexes17-22 for they belong to complex molecular network systems. In addition, the thermal and mechanical resistances and the chemical stability of this class of materials have been greatly reinforced. The incorporation of organic compounds into silica gels and glasses is of the utmost interest for a variety of technological applications including optical devices, for the silica network provides good mechanical resistance, extraordinary thermal stability and entirely amorphous systems. Bridged polysilsesquioxanes are a class of hybrid organic-inorganic materials that contain a variable organic bridging group and trifunctional silyl groups.23-25 In these materials the organic group is covalently anchored to the trifunctional silicon groups through Si-C bonds and thus makes the organic group an integral part of the materials. The key procedure to construct such molecular-based materials is to design a functional bridge molecule (precursor) by the grafting reaction, which can not only chelate to lanthanide ions but also allow sol-gel process to constitute covalent Si-O network. Some preceding reports have concentrated on the modification of pyridine-dicarboxylic acid, and the feasibility of the dicarboxylic acid system has been firmly proved.22 Zhang and his co-workers focus on the modification of the heterocyclic ligands like 1,10-phenanthroline and 2,2′-bipyridyl,26,27 Carlos and co-workers have prepared hybrids that contain OCH2CH2

10.1021/jp803915g CCC: $40.75  2008 American Chemical Society Published on Web 08/12/2008

Lanthanide Centered Hybrid Materials (polyethylene glycol, PEG) repeat units grafted on to a siliceous backbonebyurea(-NHC(dO)NH-),orurethane(-NHC(dO)O-) bridges.28,29 Our research team recently did extensive work on the topic, and we have successfully realized five paths to construct functional silylated precursors. The first path is to modify the amino groups of aminobenzoic acids using 3-(triethoxysilyl)propyl isocyanate (abbreviated as TESPIC) through a hydrogen transfer addition reaction between the amino group of acids and internal ester group (isocyanate) of TESPIC.30,31 The second path is to modify the carboxylate group of aromatic carboxylic acids through the amidation reaction between the carbonyl groups of acids and the amino groups of amino-silane cross-linking reagents.32,33 The third path is to modify the hydroxyl groups of hydroxyl compounds also using the internal ester groups of TESPIC.34 The fourth path is to modify the sulfonic groups of an acylamine derivation to graft with 3-aminopropyl trimethoxysilane.35 The fifth path uses methylene modification by grafting the dibenzoylmethane (DBM) to the coupling reagent TESPIC.36 After the modification, we assemble the above modified bridge ligands with lanthanide ions and tetraethoxysilane (TEOS) to compose hybrid systems with covalent bonds. On the basis of the former work, in this paper, we first synthesize a kind of organic compound, 1,3-bis(2-formylphenoxy)-2-propanol (denoted as BFPP), containing two formylphenoxy groups. It is then grafted to 3-(triethoxysilyl)propyl isocyanate (TESPIC) to achieve a molecular precursor BFPP-Si through a hydrogen-transfer nucleophilic addition reaction between the hydroxyl group of BFPP and the isocyanate group of TESPIC. And then, we constructed a chemically bonded rareearth/inorganic/organic hybrid material (BFPP-Si-Ln) using BFPP-Si as a bridge molecule that can both coordinate to lanthanide ions (Eu3+ or Tb3+) and form an inorganic Si-O network with tetraethoxysilane (TEOS) after cohydrolysis and copolycondensation processes. Furthermore, in order to enhance luminescent intensities of the above binary hybrids, we also prepared two types of ternary rare-earth/inorganic/organic hybrids (BFPP-Si-Dipy-Ln and BFPP-Si-Phen-Ln) by the introduction of the second ligands (4,4′-bipyridyl and 1,10phenanthroline) into the above system. Measurements of the photoluminescent properties of these materials show that the ternary rare-earth/inorganic/organic hybrids present stronger luminescent intensities, longer lifetimes, and higher luminescent quantum efficiencies than the binary hybrids, indicating that the introduction of the second ligands can sensitize the luminescence emission of the lanthanide ions in the ternary hybrid systems. 2. Experimental Section 2.1. Physical Measurements. All measurements were completed under room temperature. 1H NMR spectra were recorded on a Bruker AVANCE-500 spectrometer with tetramethylsilane (TMS) as internal reference. FT-IR spectra were measured within the 4000-400 cm-1 region on a Nicolet model 5SXC spectrophotometer with the KBr pellet technique. Melting points were measured on a XT4-100XA apparatus and were uncorrected. Ultraviolet absorption spectra of these samples (5×10-4 moll-1 chloroform solution) were recorded with an Agilent 8453 spectrophotometer. Scanning electronic microscope (SEM) images were obtained with a Philips XL-30. The X-ray diffraction (XRD) measurements were carried out on powdered samples via a BRUKER D8 diffractometer (40 mA /40 kV) using monochromated Cu KR1 radiation (λ ) 1.54 Å) over the 2θ range 10-70°. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on a

J. Phys. Chem. B, Vol. 112, No. 35, 2008 10899 NETZSCH STA 449C with a heating rate of 10 K/min under a nitrogen atmosphere (flow rate: 40 mL/min). Reflectivity spectra were recorded on a Bws003 spectrometer equipped with a diffuse reflectance accessory. Fluorescence excitation and emission spectra were obtained on a Perkin-Elmer LS-55 spectrophotometer with 3 nm excitation and 5 nm emission slits. Luminescent lifetimes were recorded on an Edinburgh FLS 920 phosphorimeter using a 450W xenon lamp as excitation source (pulse width, 3 µs). 2.2. Materials. Starting materials were purchased from China National Medicines Group. All the reagents are analytically pure. Tetraethoxysilane (TEOS) was distilled and stored under a N2 atmosphere. The solvents were purified according to literature procedures.37 Other starting reagents were used as received. Europium and terbium nitrates were obtained by dissolving Eu2O3 and Tb4O7 in concentrated nitric acid, respectively. 2.3. Synthesis. Synthesis of 1,3-Bis(2-formylphenoxy)-2propanol (Denoted as BFPP). 38 A 4.4 g (0.11 mol) sample of NaOH and 13.6 g (0.11 mol) of salicyladehyde were added to 100 mL of water, the mixture was warmed to 60 °C and then 4.36 g (0.05 mol) of epichlorohydrin was added over 2 h under argon. The mixture was stirred at 60°C for additional 4 h. After cooling, the precipitate was filtered off, washed with water and dried. The crude product was purified by recrystallization from methanol-water (8 : 1, v/v). BFPP was finally obtained as white needles, yield 4.8 g (42%), m.p. 109.5-110.5 °C. 1H NMR (CDC13, 500 MHz): δ 3.20 (br, lH, OH), 4.30-4.54 (m, 5H, CH2CHCH2), 6.98-7.87 (m, 8H, ArH), 10.40 (s, 2H, CHO) ppm. Synthesis of Silylated Precursors (BFPP-Si). To a warm solution of BFPP (1.0 mmol) in 10 mL of pyridine, was added 3-(triethoxysilyl)propyl isocyanate (denoted as TESPIC) (2.0 mmol) dissolved in 10 mL of pyridine, dropwise with stirring, and the mixture was warmed at 80 °C overnight under argon in a covered flask. After cooling, the solvent was removed in vacuo, and the residue was washed with 20 mL of hexane three times, and then a clear yellow oil was obtained in a 96.9 % yield. 1H NMR (CDC13, 500 MHz): δ 0.62 (t, 2H, CH2Si); 1.12 (m, 2H, NHCH2CH2CH2Si); 1.22 (t, 9H, CH3CH2); 2.68(m, 2H, NHCH2); 3.82(q, 6H, SiOCH2); 4.33-4.52 (m, 5H, CH2CHCH2); 7.04-7.81 (m, 8H, ArH); 7.29 (t, 1H,NH); 10.43 (s, 2H, CHO) ppm. Synthesis of the Molecular Hybrid Materials Containing Lanthanide (BFPP-Si-Ln, BFPP-Si-Dipy-Ln, BFPP-SiPhen-Ln, Ln ) Eu or Tb) through a Sol-Gel Procedure. A typical procedure for the preparation of the hybrid materials was as follows. The precursor BFPP-Si and the second ligands (free for BFPP-Si-Ln, 4,4′-bipyridyl for BFPP-Si-Dipy-Ln and 1,10-phenanthroline for BFPP-Si-Phen-Ln materials) were dissolved in N,N-dimethyl formamide (DMF) solution, a stoichiometric amount of Ln(NO3) · 6H2O was added. After 3 h, TEOS and H2O were added while stirring, and then one drop of diluted hydrochloric acid was added to promote hydrolysis. The mole ratio of Ln(NO3) · 6H2O/BFPP-Si/bipyridyl (or phenanthroline)/TEOS/H2O was 1:3:1:6:24. After the treatment of hydrolysis for 6 h, an appropriate amount of hexamethylenetetramine was added to adjust the pH value to 6-7. The resulting mixture was agitated magnetically to achieve a single phase, and thermal treatment was performed at 70 °C in a covered Teflon beaker for about 6 days until the sample solidified. The obtained gels were washed with ethanol and dried at 90 °C for another 2 days. The final molecular hybrid materials were collected as monolithic bulks and were ground into powdered materials for the photophysical studies.

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Figure 1. Scheme of the synthesis process of the organic component BFPP, the silylated precursor BFPP-Si (A) and the predicted structure of the hybrid BFPP-Si-Phen-Eu (B).

3. Results and Discussion 3.1. Organic Compound BFPP and the Silylated Precursor BFPP-Si. As detailed in the Experimental Section, 1H NMR spectra relative to the organic compound BFPP and the silylated precursor BFPP-Si are in full agreement with the proposed structures. The 1H NMR chemical shift relative to OH bond is observed at 3.20 ppm in BFPP and has disappeared in the corresponding silylated precursor BFPP-Si, which indicate the accomplishment of the hydrogen transfer reaction between OH and the TESPIC. The signal observed for amino group attributed to -CONH- group can further prove the grafting reaction. Furthermore, integration of the 1H NMR signal corresponding to ethoxy group shows that no hydrolysis of the precursor occurred during the grafting reaction. The synthesis of organic compound BFPP and the grafting reaction of BFPP with TESPIS can also be confirmed by the FT-IR. Figure 2 (I) shows the IR spectra of BFPP (A), TESPIC (B) and BFPP-Si (C) in the 4000-400 cm-1 range, and the assignments of the main infrared absorption bands are shown

in Table 1. The emergence of the strong vibration bond of OH at 3466 cm-1, together with the characteristic absorption peaks of Ar-O-CH2 group located at around 1240, 1035 cm-1, proves the formation of the organic compound BFPP. The presence of a series of strong bands at 2975, 2928, 2886 cm-1 due to the vibrations of methylene -(CH2)3- and the disappearance of the stretch vibration of the absorption peaks at 2250-2275 cm-1 for NdCdO of TESPIC indicate that BFPP has been successfully grafted on to TESPIC. Besides, the stretching vibration of Si-C, located at about 1200 cm-1, and the stretching vibration of Si-O at 1103 and 1079 cm-1, together with the bending vibration at 460 cm-1, indicate the absorption of the siloxane bonds. 3.2. HybridMaterials(BFPP-Si-Ln,BFPP-Si-Dipy-Ln, BFPP-Si-Phen-Ln, Ln ) Eu or Tb). 3.2.1. FI-IR Spectra. All of the obtained hybrid materials were also characterized by infrared spectroscopy. The IR spectra of BFPP-Si-Eu (A), BFPP-Si-Tb(B),BFPP-Si-Dipy-Eu(C),BFPP-Si-Dipy-Tb (D), BFPP-Si-Phen-Eu (E), BFPP-Si-Phen-Tb (F) and the

Lanthanide Centered Hybrid Materials

J. Phys. Chem. B, Vol. 112, No. 35, 2008 10901

Figure 2. Infrared spectra of (I) BFPP (A), TESPIC (B) and BFPP-Si (C) and (II) BFPP-Si-Eu (A), BFPP-Si-Tb (B), BFPP-Si-Dipy-Eu (C), BFPP-Si-Dipy-Tb (D), BFPP-Si-Phen-Eu (E) and BFPP-Si-Phen-Tb (F) in the 4000-400 cm-1 range.

TABLE 1: Assignments of the Main Infrared Absorption Bands for BFPP, TESPIC, BFPP-Si, and the Corresponding Hybrids BFPP-Si-Eu, BFPP-Si-Tb, BFPP-Si-Dipy-Eu, BFPP-Si-Dipy-Tb, BFPP-Si-Phen-Eu, and BFPP-Si-Phen-Tb compounds

ν(OH)

BFPP BFPP-Si BFPP-Si-Eu BFPP-Si-Tb BFPP-Si-Dipy-Eu BFPP-Si-Dipy-Tb BFPP-Si-Phen-Eu BFPP-Si-Phen-Tb

3466 3415 3420 3395 3422 3422 3395

ν(CH2) 2955, 2975, 2980, 2978, 2981, 2985, 2983, 2980,

2881, 2928, 2931, 2939, 2938, 2936, 2936, 2938,

ν(CHO) 2846 2886 2884 2882 2885 2885 2882 2884

2760, 2764, 2765, 2764, 2759, 2763, 2760, 2764,

1690 1690 1675 1670 1674 1677 1679 1680

assignments of main infrared absorption bands are shown in Figure 2 (II) and Table 1, respectively. The broad absorptions of the ν(Si-C) vibration, which are located in 1200-1192 cm-1 wavelength range, and the (ν(Si-O-Si)) vibration, which are located in 1109-1060 cm-1 wavelength range can be seen in all of the spectra. The presence of the ν(Si-C) absorption is consistent with the fact that no (Si-C) bond cleavage occurs22, while the absorption of ν(Si-O-Si) indicates the formation of

Ar 1601, 1600, 1601, 1600, 1601, 1601, 1601, 1601,

1485 1485 1485 1487 1487 1488 1484 1487

Ar-O-CH2 1247, 1240, 1245, 1245, 1244, 1244, 1245, 1245,

1033 1038 1040 1034 1036 1035 1038 1035

1,2-Ar

ν(Si-C)

759 760 763 762 761 760 761 761

1192 1200 1193 1197 1197 1198 1200 1196

ν(Si-O) 1104, 1107, 1107, 1109, 1108, 1106, 1109, 1108,

1080 1077 1079 1069 1075 1081 1079 1074

siloxane bonds during the hydrolysis/condensation reactions. Furthermore, the complexation of Ln3+ with the organic compound in all of the hybrids can be clearly shown by infrared spectroscopy. Compared with the silylated precursor BFPP-Si, the ν(CdO) vibrations of the CHO groups are shifted to lower frequency (from 1690 cm-1 in BFPP-Si to 1670-1680 cm-1 in hybrids, ∆ν ) 10-20 cm-1). This is ascribe to the complexation of the Ln3+ ion with the oxygen atom of the CdO

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Figure 3. Ultraviolet absorption spectra of BFPP-Si, BFPP-Si-Eu, and BFPP-Si-Tb.

group in hybrids22. The Ar-O-CH2 vibrations in all of the hybrids are nearly not shifted, which means that the oxygen atom of the Ar-O-CH2 group don’t take part in the complexation with Ln3+ ion. However, in the spectra, the ν(O-H) vibration at around 3400 cm-1 can also be observed, which means the existence of residual silanol groups and the presence of H2O molecule. And the ν(Si-OH) stretching vibration at 956 cm-1 is a further evidence of the incompleteness of condensation reactions. In consideration of the infrared spectroscopy and other references previously reported,22,39 the resulting predicted structure of the hybrid BFPP-Si-Phen-Eu is shown in Figure 1B as an example. However, the presence of residual Si-OH groups in the complexes cannot be excluded. 3.2.2. UltraWiolet Spectra. The formation of a complex between Ln3+ and BFPP-Si can also be proved by the ultraviolet spectra. The ultraviolet absorption spectra of BFPP-Si, BFPP-Si-Eu and BFPP-Si-Tb are shown in Figure 3. From the spectra, we find a blue shift of the major electronic transitions (from 272 to 266 nm of Tb3+ to BFPP-Si and from 272 to 262 nm of Eu3+ to BFPP-Si, ∆ν ) 7 and 10 nm, respectively). This may because the coordination interaction between rate earth ions and BFPP-Si enlarges the energy levels of the major π f π* transition and thus exhibits an obvious blue shift. 3.2.3. Diffuse ReflectiWity Spectra. Diffuse reflectance experiments were performed on powdered materials for all the materials. The corresponding absorption spectra of BFPP-Si-Eu, BFPP-Si-Dipy-Eu, BFPP-Si-Phen-Eu (A) and BFPP-SiTb, BFPP-Si-Dipy-Tb, BFPP-Si-Phen-Tb (B) are shown in Figure 4. All of the spectra exhibit a broad absorption band in the UV-VIS range (200-400nm). This absorption band corresponds to transition from the ground state of the organic ligand to the first excited state (S0 f S1). It is more specifically attribute to π f π* transition of the aromatic groups. Following Dexter’s exchange energy transfer theory,40 the luminescence intensity of hybrid material depends on the matching degree between the ligand’s triplet state energy and lanthanide iron’s emission energy. In terms of the above phenomena, it can be primarily predicted that the energy levels matching degree between BFPP-Si and RE ions is suited and appropriate so that the organic ligand can absorb abundant energy in ultraviolet-visible extent to transfer the energy to the corresponding hybrid materials. Then the final hybrid materials can be expected to have strong luminescence after intramolecular energy transfer has accomplished. For BFPP-Si-Ln, BFPP-SiPhen-Ln and BFPP-Si-Phen-Ln, a blue shift of the absorp-

Figure 4. Ultraviolet-visible diffuse reflection absorption spectra of (A) BFPP-Si-Eu, BFPP-Si-Dipy-Eu, BFPP-Si-Phen-Eu and (B) BFPP-Si-Tb, BFPP-Si-Dipy-Tb, BFPP-Si-Phen-Tb.

tion band is observed in spectra A and B; this may be due to the complexation effect of RE ions with the silylated precursor BFPP-Si and the second ligands 4,4′-bipyridyl or 1,10phenanthroline. It is presumed that different structures and coordination number in the lanthanide hybrid materials will induce to the different energy absorption in ultraviolet-visible extent, different intramolecular energy transfer coefficient and different fluorescence emission intensities. After the addition of the second ligands, a more stable eightfold-coordinated Ln complexation was formed without the participant of H2O molecule, because the ability of 4,4′-bipyridyl or 1,10-phenanthroline to coordinate with RE ions is much stronger than H2O molecule. It can also be proved by the emission spectra of these materials. The emission peaks of 614 nm in Figure 5A and 545 nm in Figure 5B were assigned to the transitions from the 5D0 f 7F2 transitions for Eu3+ ions and the 5D4 f 7F5 transitions for Tb3+ ions, respectively. The fluorescence emission intensities of these kinds of materials are determined in the order: BFPP-SiPhen-Ln > BFPP-Si-Dipy-Ln > BFPP-Si-Ln. In BFPPSi-Phen-Eu hybrids, even the transitions from the 5D0 f 7F1 transitions at 590 nm for Eu3+ can be clearly seen from the spectra, and the 5D4 f 7Fj (j ) 6, 4, 3) transitions at 490, 583 and 622 nm for Tb3+ can also be obviously observed in both BFPP-Si-Dipy-Tb and BFPP-Si-Phen-Tb hybrids. It is indicate that the addition of the second ligands can largerly sensitize the emission of RE ions, which was proved by the fluorescence spectra in Figure 8.

Lanthanide Centered Hybrid Materials

Figure 5. X-ray diffraction (XRD) graphs of the hybrids BFPP-Si-Eu, BFPP-Si-Dipy-Eu, BFPP-Si-Phen-Eu, BFPP-Si-Tb, BFPP-SiDipy-Tb, and BFPP-Si-Phen-Tb.

Figure 6. TGA traces of the hybrids BFPP-Si-Eu, BFPP-SiDipy-Eu, BFPP-Si-Dipy-Tb, BFPP-Si-Phen-Eu, and BFPPSi-Phen-Tb.

3.2.4. Powder XRD. The X-ray diffraction patterns of the hybrid materials BFPP-Si-Eu, BFPP-Si-Dipy-Eu, BFPPSi-Phen-Eu, BFPP-Si-Tb, BFPP-Si-Dipy-Tb, and BFPPSi-xPhen-Tb reproduced in Figure 5 reveal that all the materials are totally amorphous. All the diffraction curves show similar broad peaks, with angel 2θ centered around 23°, which is characteristic of amorphous silica materials.41 The structural unit distance, calculated using the Bragg law, is approximately 3.94 Å. This may be ascribed to the coherent diffraction of the siliceous backbone of the hybrids.42,43 The absence of any crystalline regions in these samples is due to the presence of organic chains in the host inorganic framework.44 By comparison with all patterns, it seems that the introducing of the second ligand (4,4′-bipyridyl or 1,10-phenanthroline) has no influence on the disorder structure of the siliceous skeleton. In addition, none of the hybrid materials contains measurable amounts of phases corresponding to the pure organic compound or free RE nitrate, which is an initial indication for the formation of the true covalent-bonded molecular hybrid materials. 3.2.5. Differential Scanning Calorimetry (DSC) and ThermograWimetric Analysis (TGA). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on all of the amorphous materials. Figure 6 shows the TGA traces of the hybrids BFPP-Si-Eu, BFPP-Si-Dipy-Eu, BFPP-Si-Dipy-Tb, BFPP-Si-Phen-Eu. and BFPP-SiPhen-Tb. From the TGA curves we can see that, all the samples show the similar change trends in weight loss, and three main

J. Phys. Chem. B, Vol. 112, No. 35, 2008 10903 degradation steps can be obviously observed. The first step of weight loss (about 6.0% for BFPP-Si-Dipy-Ln and BFPP-SiPhen-Ln, 8.0% for BFPP-Si-Eu) from 150 °C to 200 °C in heat flow around 177.0 °C could be attributed to the desorption of physically absorbed or bonded water and the residuary solvent DMF, which is an endothermic process observed from the DSC curves (not given). The second weight loss (about 16.1% for BFPP-Si-Dipy-Ln and BFPP-Si-Phen-Ln, 14.2% for BFPP-Si-Eu) between 230°C and 350 °C in heat flow around 270.5 °C was assigned to the decomposition of organic ingredients. Whereas the slight weight loss beyond 350 °C was ascribed to the release of water formed from the further condensation of silanols in the silica framework. The slight difference of the weight loss for BFPP-Si-Eu and BFPP-SiDipy(Phen)-Ln may be due to the amount of the coordinate H2O molecular. The addition of the second ligand (bipyridyl or phenanthroline) takes the place of the coordinated H2O molecule and thus influences the weight loss in 150-200 °C ranges. Compared with the organic compound BFPP whose melting point is only 110 °C, the thermal stabilities of this kind of hybrids were largerly improved. This improvement of the thermal stability is interpreted by the silica component inducing a protective barrier against thermal degradation for organic species.45 3.2.6. Scanning Electron Micrograph (SEM). The scanning electron micrographs for the hybrid materials demonstrate that a homogeneous system was obtained. Because the organic compounds were covalent linked into silica matrixes by the strong Si-C bonds and a complicated huge molecule was formed, they were composed quite uniformly via a self-assembly process during the hydrolysis/polycondensation process so that the inorganic and the organic phases can exhibit their distinct properties together and no phase separation happened.46-49 Figure 7 shows the selected micrographs for (A) BFPP-Si-Eu, (B) BFPP-Si-Dipy-Eu, (C) BFPP-Si-Phen-Eu, (D) BFPPSi-Tb, (E) BFPP-Si-Dipy-Tb, (F) BFPP-Si-Phen-Tb. There are many speckle-like microstructures with the same size of about 0.1 µm on the smooth surface of the BFPP-Si-Eu seen from Figure 7A. However, the BFPP-Si-Dipy-Eu and the BFPP-Si-Phen-Eu complex are composed by many regular and uniform granules microstructures with a larger size of about 0.2 µm observed from Figure 7B and Figure 7C. The differences of the microstructures may be due to the addition of the second ligands. After the addition of the 4,4′-bipyridyl or 1,10-phenanthroline, a more stable eightfold-coordinated structure with Eu3+ ions was formed, which may readily provide orientation and induction ability. The tendency to form the threedimensional Si-O-Si network has been strengthened compared with BFPP-Si-Eu. For Tb hybrids, it is very interesting to find that there are many leaflike microstructures in BFPP-Si-Tb hybrids, which is also regular and uniform with the size of about 2 µm in length and 0.8 µm in width. This may be due to the enlarged extent of polycondensation reaction during the copolycondensation processes of silica and thus lead to the large size of the microstructure. Meanwhile, the different tendency of the polycondensation reaction of Si-O-Si in the threedimension caused the different sizes in length and in width. The BFPP-Si-Dipy-Tb hybrids wear the bulk visible dendritic stripe microstructure and micromorphology. It is indicated that the tendency of Tb complex to form one-dimensional chainlike structure after the addition of the second ligand of 4,4′bipyridyl has become the important tendency competed with the tendency to form the polymeric Si-O-Si network, which formsthefinaldendriticstripemicrostructure.TheBFPP-Si-Phen-Tb

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Figure 7. SEM images for molecular-based hybrid materials with organic and inorganic networks: (A) BFPP-Si-Eu, (B) BFPP-Si-Dipy-Eu, (C) BFPP-Si-Phen-Eu, (D) BFPP-Si-Tb, (E) BFPP-Si-Dipy-Tb, and (F) BFPP-Si-Phen-Tb.

hybrids wear the similar granule-like microstructures with the BFPP-Si-Phen-Eu. The size of BFPP-Si-Phen-Tb is little smaller than that of BFPP-Si-Phen-Eu. For 1,10-phenanthroline is not a interlink ligand and can only coordinated with one RE ions, the BFPP-Si-Phen-Ln materials can not provide the one-dimensional chain-like structure suck as BFPP-SiDipy-Ln, so the microstructures of BFPP-Si-Phen-Ln do not show the dendritic stripe. In conclusion, the self-assembly process can be influenced not only by the different organic compounds and the RE ions but also by the different coordination modality of Ln ions in the hybrid materials. The additions

of the second ligands can largely influence the microstructures of the hybrid materials. 3.2.7. Photoluminescence Properties. On the basis of the enhancement of luminescent intensities of active lanthanide ions (such as Eu3+ and Tb3+) by the addition of the second ligands in solution or solid complexes, we selected the 4,4′-bipyridyl and 1,10-phenanthroline as the second ligands to coordinate with RE ions in the co-hybrid molecular materials. Figure 8 shows the excitation and emission spectra of the europium hybrid materials (A) BFPP-Si-Eu, BFPP-Si-Dipy-Eu, BFPP-SiPhen-Eu and the terbium hybrid materials (B) BFPP-Si-Tb,

Lanthanide Centered Hybrid Materials

J. Phys. Chem. B, Vol. 112, No. 35, 2008 10905 excitation spectra and as a result, the emission lines were assigned to the 5D4 f 7Fj transitions located at 490, 544, 587 and 622 nm, for j ) 6, 5, 4, and 3, respectively. The most striking green fluorescence (5D4 f 7F5) was observed due to the fact that this emission is the most intense one. Corresponding to the emission spectra of Eu3+ hybrids, the fluorescent intensities of Tb3+ hybrids change with the same sequence, that is BFPP-Si-Phen-Tb > BFPP-Si-Dipy-Tb > BFPP-Si-Tb. Although the BFPP can sensitize Eu3+ and Tb3+ ions and the corresponding molecular hybrids can exhibit good luminescence characteristics of Eu3+ and Tb3+, its intensities are weak. However, the fluorescence intensities of molecular hybrids are much enhanced when the second ligands are added to the hybrids, especially for the addition of 1,10-phenanthroline. This is a good method to prepare materials with strong luminescence intensities. 3.2.8. Luminescence Decay Times (τ) and Emission Quantum Efficiency (η). To further investigate the luminescence efficiency of these covalent hybrids, we selectively determined the emission quantum efficiencies of the 5D0 excited state of europium ion for Eu3+ hybrids on the basis of the emission spectra and lifetimes of the 5D0 emitting level. Assuming that only nonradiative and radiative processes are essentially involved in the depopulation of the 5D0 state, radiative and nonradiative processes influence the experimental luminescence lifetime by the equation:50-58

τexp ) (Ar+Anr)-1

(1)

where Ar and Anr are radiative and nonradiative transition rates, respectively. The quantum efficiency of the luminescence step, η, can be defined as how well the radiative processes compete with nonradiative processes.50-58 Figure 8. Excitation and emission spectra of the europium hybrid materials (A) BFPP-Si-Eu, BFPP-Si-Dipy-Eu, BFPP-Si-PhenEu and the terbium hybrid materials (B) BFPP-Si-Tb, BFPP-SiDipy-Tb, BFPP-Si-Phen-Tb.

BFPP-Si-Dipy-Tb, BFPP-Si-Phen-Tb. The excitation spectra were obtained by monitoring the emission of Eu3+ or Tb3+ at 614 or 545 nm. For Eu3+ hybrids, all the systems have similar excitation spectra that were dominated by a broad band from 296 to 350 nm with the maximum peak at about 347 nm. As a result, the emission lines of the hybrid materials were assigned to the characteristic 5D0 f 7F1 and 5D0 f 7F2 transitions at 590 and 614 nm, respectively, while the emission lines of 5D0 f 7F3 and 5D0 f 7F4 are too weak to be observed. The 5D0 f 7F2 emission around 614 nm is the most predominant transition, which agrees with the amorphous characters of the hybrid materials. From the excitation spectra, we can see that the fluorescence emission intensities of these kinds of materials are determined in the order: BFPP-Si-Phen-Eu > BFPP-SiDipy-Eu > BFPP-Si-Eu, which indicate that the second ligands 4,4′-bipyridyl and 1,10-phenanthroline can efficiently sensitize the luminescence of Eu3+ ions. For Tb3+ hybrids, a broad band centered at around 350 nm was observed in the

η )

Ar Ar + Anr

(2)

So, quantum efficiency can be calculated from the radiative transition rate constant and experimental luminescence lifetime from the following equation:50-58

η ) Arτexp

(3)

Here Ar can be obtained by summing over the radiative rates A0j for each 5D0 f 7Fj transitions of Eu3+.50-58

∑ A0j ) A00+A01+A02+A03+A04

Ar )

(4)

Since 5D0 f 7F1 belongs to the isolated magnetic dipole transition, it is practically independent of the chemical environments around the Eu3+ ion, and thus can be considered as an internal reference for the whole spectrum, the experimental coefficients of spontaneous emission, A0j can be calculated according to the equation.50-58

A0j ) A01(I0j ⁄ I01)(υ01 ⁄ υ0j)

(5)

Here A0j is the experimental coefficients of spontaneous emissions (j ) 0, 1, 2, 3, 4), for the branching ratio for the 5D0 f

TABLE 2: Luminescence Efficiencies and Lifetimes for the Europium Hybrid Materials BFPP-Si-Eu, BFPP-Si-Dipy-Eu, and BFPP-Si-Phen-Eu hybrids

υ01 (cm-1)a

υ02 (cm-1)a

A0j (s-1)

Aexp (s-1)

Arad (s-1)

Anrad (s-1)

τ (µs)b

η (%)

BFPP-Si-Eu BFPP-Si-Dipy-Eu BFPP-Si-Phen-Eu

16978 16978 16978

16313 16313 16313

50, 181 50, 301 50, 261

1647 975 915

231 351 311

1416 624 604

607 1026 1093

14.0 36.0 34.0

a

The energies of the 5D0 f 7Fj transitions (υ0j). b For the 5D0 f 7F2 transition of Eu3+.

10906 J. Phys. Chem. B, Vol. 112, No. 35, 2008

Liu and Yan

7F 5,6

transitions can be neglected as they both are not detected experimentally, whose influence can be ignored in the depopulation of the 5D0 excited state.50-58 Among A01 is the Einstein’s coefficient of spontaneous emission between the 5D0 and 7F1 energy levels. In vacuum, the value of A01 can be determined to be 50 s-1 approximately (A01) n3A01 (vacuum)).51,56 I is the emission intensity and can be taken as the integrated intensity of the 5D0 f 7Fj emission bands.57,58 υ0j refers to the energy barycenter and can be determined from the emission bands of Eu3+’s 5D0 f 7Fj emission transitions. Here the emission intensity, I, taken as integrated intensity S of the 5D0 f 7F0-4 emission curves, can be defined as below:

Ii-j ) pωi-jAi-jNi ≈ Si-j

(6)

where i and j are the initial (5D0) and final levels (7F0-4), respectively, ωi-j is the transition energy, Ai-j is the Einstein’s coefficient of spontaneous emission, and Ni is the population of the 5D0 emitting level. On the basis of the above discussion, it can be seen the value ηmainly depends on the values of two quanta: one is lifetime and the other is I02/I01 (red/orange ratio). If the lifetimes and red/orange ratio are large, the quantum efficiency must be high. As shown in Table 2, the quantum efficiencies of the three kinds of europium hybrid materials can be determined in the order: BFPP-Si-Dipy-Eu > BFPP-Si-Phen-Eu > BFPP-Si-Eu. So we can see that the addition of the second ligands into the hybrids not only enhances the materials’ luminescent intensities, but also extends the materials’ luminescent lifetimes and thus improve the materials’ quantum efficiencies. 4. Conclusions In summary, three types of lanthanide/inorganic/organic hybrid materials (BFPP-Si-Ln, BFPP-Si-Dipy-Ln and BFPP-Si-Phen-Ln) were first prepared using an BFPP-Si compound as a bridge molecule that can both coordinate to lanthanide ions (Eu3+ and Tb3+) and form an inorganic Si-O network with tetraethoxysilane (TEOS) after cohydrolysis and copolycondensation processes though a sol-gel process. These materials display the connection of inorganic and organic parts on a molecular level. SEM shows that all of these hybrid materials exhibit homogeneous microstructures and morphologies, suggesting the occurrence of self-assembly of the inorganic network and organic chain. Measurements of the photoluminescent properties of these materials show that the ternary rare-earth/inorganic/organic hybrids present stronger luminescent intensities, longer lifetimes, and higher luminescent quantum efficiencies than the binary hybrids, indicating that the introduction of the second lignads can sensitize the luminescence emission of the overall hybrid system. As the synthesis process can be easily applied to other organic ligands and to different alkoxysilanes, we may expect to obtain stable and efficient hybrid materials in optical or electronic areas for the desired properties can be tailored by an appropriate choice of the precursors and the addition of the second ligands. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20671072). References and Notes (1) Ramos, G.; Belenguer, T.; Levy, D. J. Phys. Chem. 2006, 110, 24780.

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