14742
J. Phys. Chem. B 2008, 112, 14742–14750
Assembly, Characterization, and Photoluminescence of Hybrids Containing Europium(III) Complexes Covalently Bonded to Inorganic Si-O Networks/Organic Polymers by Modified β-Diketone Xiaofei Qiao† and Bing Yan*,†,‡ Department of Chemistry, Tongji UniVersity, Siping Road 1239, Shanghai 200092, China, and State Key Lab of Rare Earth Materials Chemistry and Applications, Peking UniVersity, Beijing 100871, China ReceiVed: March 17, 2008; ReVised Manuscript ReceiVed: September 6, 2008
1-(2-Naphthoyl)-3,3,3-trifluoroacetonate (NTA) was grafted to the coupling agent 3-(triethoxysilyl)-propyl isocyanate (TEPIC) and used as the first kind of precursor, and other kinds of precursors (PVPD, PMAA, and PVPDMAA) were synthesized through the addition polymerization reactions of the monomer 4-vinylpyridine and methacrylic acid. Then, these precursors coordinated to rare earth ions, and the three kinds of hybrid polymeric materials were obtained after hydrolysis and copolycondensation with the tetraethoxysilane (TEOS) via a sol-gel process. FTIR, ultraviolet, ultraviolet-visible diffuse reflection and photoluminescent spectra, electronic microscopy diagraphs, room-temperature X-ray diffraction patterns, and TG plots were characterized, and the results reveal that the hybrid materials imbedded into the single polymer (PVPD and PMAA) showed more uniformity in the microstructure, more efficient intramolecular energy transfer between europium ions and the modified ligand NTA-Si and more excellent characteristic emission of europium ions under UV irradiation with higher 5D0 luminescence quantum efficiency and longer lifetime than the hybrid materials imbedded into the multipolymer (PVPDMAA). 1. Introduction In the few past decades, the trivalent lanthanide ions have been known widely for their unique optical properties such as linelike emission spectra and high luminescence quantum efficiency, so much research has been almost exclusively devoted to the investigations about europium and terbium luminescence.1,2 Since Weissman discovered that the excitation may be accomplished in the lanthanide complex under suitable conditions because of the other components in the rare earth compound such as the organic ligands, which have absorbed the light energy and transferred it to the rare earth ions, numerous applications of the lanthanide complexes have played an important role in many areas.3-6 Certain organic ligands such as β-diketones, aromatic carboxylic acids, and heterocyclic derivatives are usually used to coordinate with the rare earth ions to form the lanthanide complexes so as to protect metal ions from vibrational coupling and increase the light absorption cross section by the “antenna effect”.7-9 This intermolecular transferring process is much more effective than direct excitation, since the absorption coefficients of organic ligands are many times as large as the molar absorption coefficients of trivalent lanthanide ions.10 In many lanthanide complexes, Eu3+ complexes especially containing β-diketones have been intensively studied owing to their inherent sharp emission peaks and high quantum efficiency.11,12 Some of them demonstrate potential applications in efficient light-conversion molecular devices and organic light-emitting devices.13-15 However, this kind of lanthanide complex has been prevented from practical applications as tunable solid-state laser or phosphor devices due * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. Fax: +81-21-65982287. Phone: +81-21-65984663. † Tongji University. ‡ Peking University.
to its poor stability under high temperature or moisture conditions and low mechanical strength. Thus, the lanthanide complexes should be incorporated into inorganic or organic/ inorganic matrixes to overcome the disadvantages by the low temperature soft-chemistry process, especially the sol-gel method and hydrothermal synthesis process. According to the interaction between the organic and inorganic components or phases in hybrid systems, these hybrid materials can be divided into two major classes.16 One class is physically synthesized through weak interactions (hydrogen bonding, van der Waals force, or weak static effects) between the organic and inorganic phases; the other is with powerful covalent bonds linking the organic and inorganic parts.17-21 The former class hybrid materials cannot solve the problems of the quenching effect of luminescent centers, inhomogeneous dispersion of two phases, or leaching of the photoactive molecules due to the high energy vibration aroused by the surrounding hydroxyl groups and weak interactions, so the concentration of the complex is usually greatly reduced.16 Since the latter class of hybrid materials belongs to the molecular-based composite materials with excellent chemical stability and monophasic appearance even at a high concentration of lanthanide complexes,22-35 we have realized the possibility of tailoring novel multifunctional advanced materials through the combination with chemical bonds between the different components in the single phase material.36,37 However, the above rare earth hybrid complexes merely contain either an organic ligand component or an inorganic silicon-oxygen network component, and the polymerization reactions occur just between silicanes and water molecules (so-called inorganic polymerization) to form a Si-O-Si network in the sol-gel procedure. Recently, more professional investigations have been altered to focus on the rare
10.1021/jp806341f CCC: $40.75 2008 American Chemical Society Published on Web 10/24/2008
Hybrid Europium Materials earth hybrid materials involving the inorganic and organic polymerization reactions or imbedding certain polymers containing long chains.38-42 It is proved that the hybrid materials with the inorganic networks and polymers with high molecular weight have excellent effective properties, since each dispersed molecule is a luminescent unit so that the transparency, the dimension of the composite, the interfacial interaction between the rare earth organic complex and the polymer matrix, and the amount of dispersed rare earth complex molecules are primary factors to determine the final luminescent behavior of the composite. Furthermore, the dispersion of smaller-sized particles of the complex leads to a higher transparency of the composite and a larger interfacial region between the complex particles and the matrix polymer, both of which would improve the efficiency of excitation. Compared to the rare earth hybrid material synthesized through a simple inorganic polymerization procedure, the larger interfacial region and the stronger interaction between the rare earth complex and polymer matrix in the final obtained rare earth complex/polymer composite, synthesized through organic and inorganic polymeric procedures by sol-gel technology, might accelerate energy transfer between them, facilitate the emission of the fluorescence, and enhance the luminescent intensities of the composites. Our research team has dedicated ourselves to the design of the rare earth hybrid materials with inorganic networks or organic polymeric long chains for several years, and in this paper, we put forward a novel path to assembly of hybrid material containing inorganic and organic networks. We have synthesized the polymer (PVPD, PMAA, and PVPDMAA) to construct the polymeric chains (C-C), using the monomer 4-vinylpyridine or methacrylic acid as the raw materials. Subsequently, we have modified NTA with the electrophilic reagent 3-(triethoxysilyl)-propyl isocyanate (TEPIC) so as to synthesize the functionalized covalent-bond-linking precursor (NTA-Si), and observed the red emissions of europium-β-diketone complexes and their outstanding luminescent yields. Then, we obtained the final hybrid polymers containing europium-β-diketone complexes and silicon-oxygen networks, after the coordination reaction between rare earth ions, precursor, and the polymers and the hydrolysis crosslinking reaction. Moreover, the luminescence properties, microstructure, and thermal stabilities were analyzed in detail in our paper. 2. Experimental Section 2.1. Chemicals. Europium nitrate was obtained by dissolving Eu2O3 into concentrated nitric acid. Tetraethoxysilane (TEOS, Aldrich) was distilled and stored under a nitrogen atmosphere. 3-(Triethoxysiyl)-propyl isocyanate (TEPIC) and the solvent tetrahydrofuran (THF) were used after desiccation with anhydrous calcium chloride. All of the other reagents are analytically pure. 2.2. Synthetic Procedures. 2.2.1. Synthesis of the CrossLinking Precursor Containing Si-O Chemical Bonds (NTASi). 1-(2-Naphthoyl)-3,3,3-trifluoroacetonate (NTA) (1 mmol, 0.2662 g) was first dissolved in 20 mL of dehydrate tetrahydrofuran (THF), and NaH (2 mmol, 0.048 g) was added into the solution stirring at a temperature of 65 °C. Two hours later, 2.0 mmol (0.495 g) of 3-(triethoxysilyl)-propyl-isocyanate (TEPIC) was added into the refluxing solution. The mixture was heated at 65 °C in a covered flask for approximately 12 h in the nitrogen atmosphere. After isolation and purification, the brown black viscous liquid was obtained, NTA-Si (C34H51O10F3N2Si2).
J. Phys. Chem. B, Vol. 112, No. 47, 2008 14743 2.2.2. Synthesis of Polymer Precursors (PVPD, PMAA, and PVPDMAA). 4-Vinylpyridine (2 mmol) for PVPD, methacrylic acid (2 mmol) for PMAA, and 4-vinylpyridine (2 mmol) and methacrylic acid (2 mmol) for PVPDMAA were dissolved in a small quantity of the solution tetrahydrofuran (THF) (6 mL) with the initiator (BPO, benzoyl peroxide) (0.01 g), and then, the polymers were obtained through the addition polymerization under argon atmosphere purging, respectively. The reaction temperature was maintained at 50 °C and the reaction time is about 4 h for PMAA to obtain the white turbid liquid and 6 h for PVPD and PVPDMAA to obtain the brown turbid liquid. The obtained materials were concentrated under room temperature to remove the solvent THF using a rotary vacuum evaporator, and the viscous liquid was obtained (PVPD [C7H7N]n, PMAA [C4H6O2]n, and PVPDMAA [C11H13O2N]n). The three kinds of precursors were dissolved by the solvent DMF for the coordinate reaction with rare earth ions later. 2.2.3. Synthesis of Three Kinds of the Hybrid Materials Imbedded into Si-O Networks and Organic Carbon Chains (NTA-S-Eu-PVPD/PMAA/PVPDMAA). The precursors NTA-Si and PVPD (PMAA and PVPDMAA) were dissolved in N,N-dimethyl formamide (DMF) solvent, and a stoichiometric amount of Eu(NO3)3 · 6H2O (0.3 mmol, 0.136 g) was added into the solution while stirring. After six hours, a stoichiometric amount of TEOS and H2O was added into the mixed solution after the coordination reaction was completed between precursors and europium ions, which accompanied the addition of one drop of dilute hydrochloric acid to promote hydrolysis. The molar ratio of RE(NO3)3 · 6H2O:NTA-Si:PVPD (PMAA and PVPDMAA):TEOS:H2O was 1:3:1:6:24 (the polymer obtained in the above process, TEOS 0.45 mL, 0.833 g and H2O 0.288 g). After the hydrolysis, an appropriate amount of hexamethylene-tetramine was added to adjust the pH to 6-7. The mixture was agitated magnetically to achieve a single phase after three days, and then, it was aged at 70 °C until the onset of gelation in about 5 days. The gels were collected as monolithic bulks and were ground into powdered materials for the photophysical studies (seen from Figure 1). 2.3. Physical Measurement. FTIR spectra were measured within the 4000-400 cm-1 region on an infrared spectrophotometer with the KBr pellet technique. The ultraviolet absorption spectra (DMF solution) and the ultraviolet-visible diffuse reflection spectra of the powder samples were recorded by an Agilent 8453 spectrophotometer and a BWS003 spectrophotometer, respectively. The X-ray diffraction (XRD) measurements of the powdered sample were carried out by a Bruker D8 diffractometer (40 mA-40 kV) using monochromated Cu Ka1 radiation (k ) 1.54 Å) over the 2θ range of 10-70 °. Scanning electronic microscopy (SEM) images were obtained on a Philip XL30 instrument. The luminescence excitation and emission spectra were obtained on a Perkin-Elmer LS-55 spectrophotometer. Luminescence lifetime measurements were carried out on an Edinburgh FLS920 phosphorimeter using a 450 W xenon lamp as the excitation source. The outer luminescent quantum efficiency was determined using an integrating sphere (150 mm diameter, BaSO4 coating) from an Edinburgh FLS920 phosphorimeter. The spectra were corrected for variations in the output of the excitation source and for variations in the detector response. The quantum yield can be defined as the integrated intensity of the luminescence signal divided by the integrated intensity of the absorption signal. The absorption intensity was calculated by subtracting the integrated intensity of the light source with the sample in the integrating sphere from the integrated intensity of the light source with a blank
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Figure 1. Scheme of the synthesis processes of the hybrid polymeric materials NTA-Si-Eu-PVPD, NTA-Si-Eu-PMAA, and NTASi-Eu-PVPDMAA.
sample in the integrating sphere. All of the above measurements were completed at room temperature. Thermogravimetry (TG) was obtained on a Netzsch, model STA 409C, instrument under
the following conditions: atmosphere of oxygen air, heating/ cooling rate of 10 °C/min, 18.78 mg of powder, and crucibles of Al2O3.
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Figure 2. Fourier transform infrared spectra of the free ligand NTA (A), the precusor NTA-Si (B), and the hybrid material NTA-Si-Eu-PVPD (C).
3. Results and Discussion 3.1. FTIR Spectra. The IR spectra of the free β-diketone ligand, the precursor (NTA-Si), and the hybrid polymeric material are shown in Figure 2 (A, NTA; B, NTA-Si; C, NTA-Si-Eu-PVPD). In view of the free ligand NTA (A), the vibration of -CH2- was observed at 3108 cm-1 and it was replaced by a strong broadband located at 2971 cm-1 in B, which originated from the three methylene groups of 3-(triethoxysilyl)-propyl isocyanate (TEPIC). In addition, the spectra of NTA-Si are dominated by ν (C-Si, 1167 cm-1) and ν (Si-O, 1071 cm-1) absorption bands, characteristic of a trialkoxylsilyl function group, and the band centered at 3377 cm-1 corresponds to the stretching vibration of -NH- groups and the hydroxyl groups of adsorbed water together in B. Ulteriorly, the bending vibration (δNH, 1523 cm-1) further proves the formation of amide groups. New peaks at 1702 and 1635 cm-1 in B were attributed to the absorptions of the -CONHgroup deriving from the cross-linking reagent TEPIC, proving that 3-(triethoxysilyl)-propyl isocyanate was successfully grafted onto the ligand NTA.43 Furthermore, from the spectra of C, the peaks at 1656 and 1598 cm-1 originating from the -CONHgroup of NTA-Si can also be observed, which is consistent with the fact that the NTA group in the framework remains invariably after the hydrolysis-condensation reaction, since the two carbonyl groups of β-diketones have coordinated to the rare earth ions, not the carbonyl groups of the TEPIC.35 The spectra of the hybrid polymeric material (C) indicated the formation of the Si-O-Si framework, which is evidenced by the broad bands located at about 1080-1168 cm-1 (νas, Si-O), 823 cm-1 (νs, Si-O), and 461 cm-1(δ, Si-O-Si), which is attributed to the success of hydrolysis and copolycondensation.44 (ν represents stretching, δ represents plane bending, s represents symmetric, and as represents asymmetric vibrations) However, the strong sharp peak at about 1602 cm-1 in A, assigned to the existence of the carbonyl groups of the β-diketones, has disappeared in C, also indicating that the carbonyl groups of the ligand β-diketones have successfully coordinated to the rare earth ions, which bring on the same conclusion as the above one. 3.2. Ultraviolet and Ultraviolet-Visible Diffuse Reflection Absorption Spectra. Figure 3 exhibits ultraviolet absorption spectra of NTA (A), NTA-Si (B), and NTA-Si-Eu-PVPD (C). From the spectra, it can be observed that an obvious red
Figure 3. Ultraviolet absorption spectra of the free ligand NTA (A), the precusor NTA-Si (B), and the hybrid material NTA-SiEu-PVPD (C).
shift (about 18 nm) of the major π-π* electronic transitions A f B (from 248 to 268 nm, from 330 to 345 nm) indicates the electron distribution of the modified NTA-Si has changed compared to free ligand NTA due to the introduction of carbonyl groups, which estimates that the form of the NTA-Si decreases the energy difference among electron transitions. In addition, it is observed that the peak shape has not changed basically, just shifting a small distance on the horizon. Therefore, we can deduce that the 3-(triethoxysilyl)-propyl isocyanate has been grafted to the ligand β-diketones (NTA) successfully. After the coordinate reactions, the absorption spectra of the hybrid material C (NTA-Si-Eu-PVPD) are a little different from the precursor B (NTA-Si). Both of the peak intensities have changed, and the peak at about 345 nm in B has shifted to 335 nm in C, suggesting the blue shift of the major π-π* electronic transitions occurred, the energy difference among electron transitions increased, and the electron distribution of the modified NTA-Si-Eu-PVPD has also changed compared to the precursor (NTA-Si) due to completion of the coordination process with europium ions and the addition of the polymer PVPDMAA. All of these changes mean that the complexation
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Figure 4. Ultraviolet-visible diffuse reflection absorption spectra of the hybrid polymeric materials and the ligand: (A) NTA-Si-Eu-PVPD; (B) NTA-Si-Eu-PMAA; (C) NTA-Si-Eu-PVPDMAA; (D) NTA.
Figure 5. X-ray diffraction patterns for obtained hybrid materials: (A) NTA-Si-Eu-PVPD; (B) NTA-Si-Eu-PMAA; (C) NTA-SiEu-PVPDMAA.
in carbonyl groups of NTA, the carboxyl groups of the polymer PVPD, and the europium ions has been accomplished obviously. The ultraviolet-visible diffuse reflection absorption spectra of the hybrid materials are given in Figure 4. The lines denote the rare earth hybrid polymeric materials and the free ligand NTA (A, NTA-Si-Eu-PVPD; B, NTA-Si-Eu-PMAA; C, NTA-Si-Eu-PVPDMAA; D, NTA). It is observed that the wide bands of parts A, B, and C are located at about 300-375 nm, which partially overlap with the luminescence excitation spectra (wide bands at 290-350 nm in Figure 8a) and also with the absorption spectra of the free ligand (NTA) at about 300-380 nm. It can be primarily predicted that the energy difference between NTA and Eu3+ is 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 excellent luminescence properties after the intramolecular energy transfer process, which is proved in the luminescence spectra in Figure 8. It is observed that the peak shapes of the NTA-Si-Eu-PVPD and NTA-Si-Eu-PVPDMAA have some resemblance, a little different from NTA-Si-Eu-PMAA. One possibility we presumed is that the 4-vinylpyridine has the pyridine ring, which has played an important role in the dimensional conformation within the coordinate and hydrolysis
Qiao and Yan
Figure 6. Thermogravimetry trace (TG) and differential thermogravimetry trace (DTG) of the hybrid material NTA-Si-Eu-PVPD.
reactions, while it has not been changed at all in polymerization or copolymerization reactions. However, the methacrylic acid has the carbon chains merely, when the polymerization reaction has occurred, the structure on its own has been changed entirely, chains becoming longer and longer without the existence of primary conformation. Therefore, the hybrid polymeric materials containing the pyridine ring show us similar properties, and this phenomenon is also observed in the scanning electron micrographs in Figure 7. 3.3. Powder XRD. The room-temperature X-ray diffraction patterns (from 10 to 70°) of the hybrid materials unfolded in Figure 5 exhibit that the obtained hybrid materials are amorphous at a wide range (A, NTA-Si-Eu-PVPD; B, NTA-SiEu-PMAA; C, NTA-Si-Eu-PVPDMAA). The broad peaks at about 20° are attributed to the amorphous siliceous backbone of the hybrids.39 Moreover, the polymeric chains of the polymer are essentially in an ordered arrangement, while from the patterns it cannot be observed that the existence of the polymers could result in a decrease of the overall disorder of the siliceous skeleton. The absence of any crystalline regions in these samples correlates with the presence of host inorganic frameworks. 3.4. Thermogravimetric Analysis. Figure 6 shows the thermogravimetry trace (TG) (a) and differential thermogravimetry trace (DTG) (b) of the hybrid polymeric material NTA-Si-EuPVPD. Seen from the TG curve a, the hybrid material NTASi-Eu-PVPD has lost mass (about 12%) from 123 to 270 °C. It is deduced that the adsorbed water and residual solvent evaporated, without any decomposition of the chemical bonds. According to the structural molecule of the hybrid material NTA-Si-Eu-PVPD, the polymer PVPD occupies about one-fourth (23%), and the graph shows us that there is a mass loss of about 27% from 319 to 408 °C. In addition, the hybrid material has lost weight with the most rapid speed when the temperature has climbed to 362 °C, which is observed from the DTG data with the sharpest peak at 362 °C. Therefore, the PVPD began to decompose when the temperature reached 319 °C, and when the temperature reached 408 °C, the polymeric precursors PVP departed from the hybrid materials completely. Finally, the NTA-Si-EuPVPD retained the mass (about 35%) at last. 3.5. Scanning Electron Micrographs. The scanning electron micrographs of the hybrid polymeric materials demonstrate that the homogeneous, molecular-based materials were obtained with strong covalent bonds between the organic β-diketone ligand and the inorganic matrixes, and the coordinate bonds between
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Figure 7. SEM images of the hybrid materials: (A) NTA-Si-Eu-PVPD; (B) NTA-Si-Eu-PMAA; (C) NTA-Si-Eu-PVPDMAA.
organic ligand β-diketone or polymer ligand and rare earth ions, which belong to a complicated huge molecular system in nature (see Figure 7) (A, NTA-Si-Eu-PVPD; B, NTA-Si-EuPMAA; C, NTA-Si-Eu-PVPDMAA). Compared with the hybrid materials with doped lanthanide complexes generally experiencing the phase separation phenomena, in this paper, the inorganic and organic phases can exhibit their distinct properties together in the hybrid materials we obtained containing covalent bonds.45,46 The rare earth complex A (NTA-Si-Eu-PVPD) is composed by many regular and uniform hiberarchies and dendritic
stripes (seen from Figure 7A1), and there are many granules with the same size of about 200 nm around the hiberarchies disposed in order (seen from Figure 7A2). It is also observed from the scanning electron micrograph of NTA-Si-EuPMAA and NTA-Si-Eu-PVPDMAA in Figure 7B and C. The difference between NTA-Si-Eu-PVPD (A2) and NTA-Si-Eu-PVPDMAA (C2) is that the sizes of regular granules have a little difference; that is, the latter size is a little smaller. Consequently, it is speculated that the appropriate time of the coordination reaction or the completion of the aging procedure have a large influence on the growth
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Figure 8. Fluorescent excitation (a) and emission (b) spectra of the hybrid materials: (A) NTA-Si-Eu-PVPD; (B) NTA-Si-Eu-PMAA; (C) NTA-Si-Eu-PVPDMAA.
of granules, and the growth of granules was stopped owing to the lack of time in the latter. Another possibility is that the polymer precursor was synthesized through the copolymerization reaction between the 4-vinylpyridine and methacrylic acid to form the longer chains, bringing on the more polymeric units in limited space. Therefore, the growth of these granules is hard or slow in such crowded circumstances with the unordered array. Moreover, the precursor NTA-Si is a derivative from 1-(2naphthoyl)-3,3,3-trifluoroacetonate, which has two carbonyl groups so that its corresponding rare earth complex is ready to form one-dimensional chainlike structure while not the polymeric structure, and this one-dimensional chainlike structural tendency will compete with the construction of a polymeric network structure of Si-O-Si in the hydrolysis and copolycondensation processes of silica or with the formation of the coordinate bonds between rare earth ions and polymer precursors containing the polymeric structure. Finally, both of the tendencies have respective influence on the formation of the obtained hybrid polymeric materials, even leading to the final micromorphology seen in Figure 7. Furthermore, there are some branches at the end of every trunk stripe and the stripes will continue to grow according to the directions of these branches, which forms the final structure as we see in the micrographs in Figure 7A1. Moreover, the microstructure of NTA-Si-Eu-
Qiao and Yan PMAA shows that some imbricate veins were dispersed regularly on the surface of the trunk stripes seen from the amplificatory pictures in Figure 7B2 besides the ordered trunk stripes. It is observed that the microstructures of the NTA-SiEu-PVPD and NTA-Si-Eu-PVPDMAA are different from those of NTA-Si-Eu-PMAA, which has also been obtained in the ultraviolet-visible diffuse reflection absorption spectra, and the reasons and possibilities has been stated in the discussion. Therefore, it is predicted that the different dimensional conformation could have an important influence on many properties of the hybrid polymeric materials. 3.6. Photoluminescence Properties. Figure 8 shows the excitation (a) and emission (b) spectra of hybrid polymeric materials containing Eu covalently bonded into silicon-oxygen networks and carbon chains (A, NTA-Si-Eu-PVPD; B, NTA-Si-Eu-PMAA; C, NTA-Si-Eu-PVPDMAA). The excitation spectra of these materials were all obtained by monitoring the strongest emission wavelength of the Eu3+ at 613 nm. In the excitation spectra, the broad absorption peak is located at 330-385 nm in A, 325-382 nm in B, and 323-380 nm in C, respectively, suggesting the intromolecular energy transfer system between Eu3+ and 1-(2-naphthoyl)-3,3,3-trifluoroacetonate (NTA) has formed and effective absorption of the Eu-NTA system has occurred. Therefore, in order to compare the emission spectra of these three hybrid materials with the same conditions, it should be monitored using the same and the most appropriate excitation wavelengths, 370 nm. As a result, in Figure 8b, the emission lines of the three hybrid materials were assigned to the 5D0 f 7FJ (J ) 0, 1, 2, 3, 4) transitions at about 578, 589, 613, 650, and 699 nm for Eu3+. It is noteworthy that the ligand emission background completely vanished in the emission spectra under the excitation of 370 nm, and only the characteristic emissions of Eu ions can be found. Seen from Figure 8b, among these emission peaks of the complexes of chelated Eu3+, red emission intensities (arbitrary unit, a.u.) of the electric dipole transition of 5D0 f 7F at about 613 nm (1094.12 for A, 1104.52 for B, and 479.7 2 for C) are all stronger than the orange emission intensities of magnetic dipole transition of 5D0 f 7F1 at about 589 nm (117.39 for A, 195.39 for B, and 60.81 for C, respectively). Since the 5D f 7F transition strongly varies with the local symmetry 0 2 of Eu3+ ions, while the 5D0 f 7F1 transition is independent of the host materials, the emission spectra indicate that the Eu3+ site is situated in an environment without inversion symmetry.47Therefore, it is indicated that the effective energy transfer took place between the precursors and the chelated rare earth ions. Other factors cannot still be excluded such as the relatively rigid structure of silica gel, which limits the vibration of ligand in the hybrid materials and prohibits nonradiative transitions. Accordingly, we may expect that through this efficient way leaching of the photoactive molecules and clustering of the emitting centers could be avoided, and higher concentration of metal ions will be realized. Ulteriorly, in Figure 8b, the emission spectra of A and B show some resemblance on the relative fluorescence intensities, a little stronger than C, and in view of different points, we presumed that there are some possible explanations. From the point of view of an efficient intramolecular energy transfer system, according to Dexter’s exchange energy transfer theory48 and the luminescence theory of lanthanide complexes, an efficient intramolecular energy transfer system requires the energy difference between the triple state energy level of the ligand and the resonance state energy level of the central Eu3+ to be in the range 500-2500
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cm-1. Thus, the energy difference ∆E (Tr - Eu3+) between the lowest triple state energy level of NTA (19600 cm-1) and the resonance energy level of Eu3+ (5D1, 19020 cm-1)49,50 is 580 cm-1, and it can be predicted that the triplet state energy of NTA (19600 cm-1) is very suitable for the luminescence of Eu3+. Therefore, the efficient intramolecular energy transfer in the Eu-NTA system mainly has occurred. From the other point of the dimensional effect, the polymer precursors PVPD-Si or PMAA-Si could successfully be grafted to the Eu3+, bringing the excellent luminescence properties, since both the oxygen and nitrogen atoms have prominent coordination abilities with lanthanide ions. While the precursor PVPDMAA has the huge conformation with the enormous chains and the pyridine rings together, and when coordinating with the rare earth ions, it is difficult for the carboxyl groups and pyridine rings to graft onto Eu3+ in such a small space due to the tremendous steric hindrance effect. Obviously, the different emission intensity of hybrid materials depends on many other factors, all of which could have crucial influence on the emission intensity, such as the real doping concentration, the efficiencies of initial absorption, subsequent energy transfer, final depopulation, and so on. 3.7. Luminescence Decay Times (τ) and Emission Quantum Efficiency (η). According to the emission spectra and the lifetime of the Eu3+ first excited state (τ, 5D0), the emission quantum efficiency (η) of the 5D0 excited state can be determined. Assuming that only nonradiative and radiative processes are essentially involved in the depopulation of the 5D0 state, η can be defined as follows:51
η)
Ar Ar + Anr
(1)
Here, Ar and Anr are the radiative and nonradiative transition rates, respectively. Ar can also be obtained by summing over the radiative rates A0J for each 5D0 f 7FJ (J ) 0-4) transition of Eu3+.
Ar )
∑ A0J ) A00 + A01 + A02 + A03 + A04
(2)
The branching ratio for the 5D0 f 7F5,6 transitions can be neglected, as they are not detected experimentally, whose influence can be ignored in the depopulation of the 5D0 excited state. Since 5D0 f 7F1 belongs to the isolated magnetic dipole transition, it is practically independent of the chemical environments around the Eu3+, and can be considered as an internal reference for the whole spectra. The experimental coefficients of spontaneous emission A0J can be calculated according to the equation52-55
A0J ) A01(I0J /I01)(ν01 /ν0J)
(3)
Here, A0J is the experimental coefficient of spontaneous emission. A01 is Einstein’s coefficient of spontaneous emission between the 5D0 and 7F1 energy levels. In a vacuum, A01 can be considered as a value of 14.65 s-1, when the average index of refraction n is equal to 1.506, so the value of A01 can be determined to be approximately 50 s-1 (A01 ) n3A01(vac)).56 I01 and I0J are the integrated intensities of the 5D0 f 7F1 and 5D0 f 7FJ transitions (J ) 0-4) with ν01 and ν0J (ν0J ) 1/λJ) energy centers, respectively. ν0J refers to the energy barrier and can be
determined with the emission bands of Eu3+’s 5D0 f 7FJ emission transitions. The emission intensity I, taken as the integrated intensity S of the 5D0 f 7F0-4 emission curves, can be defined as follows
Ii-j ) pωi-j Ai-j Ni ≈ Si-j
(4)
Here, i and j are the initial (5D0) and final levels (7F0-4), respectively, ωi-j is the transition energy, Ai-j is Einstein’s coefficient of spontaneous emission, and Ni is the population of the 5D0 emitting level. On the basis of refs 57-61, the value of A01 is about 50 s-1, and the lifetime (τ) and radiative (Ar) and nonradiative (Anr) transition rates are related through the following equation:
Atot ) 1/τ ) Ar + Anr
(5)
On the basis of the above discussion, the quantum efficiencies of the three kinds of europium hybrid polymeric materials can be determined, as shown in Table 1. Seen from the equation to calculate the quantum efficiency (η), the value η mainly depends on the values of two factors: one is the lifetime and the other is I02/I01. As can be clearly seen from Table 1, the results of the quantum efficiencies of the three kinds of hybrid polymeric materials confirm the conclusion that the polymer and organic networks have successfully been grafted into the europium ions with the chemical bonds and the effective intermolecular transfer system has completely been accomplished in the final products. Ulteriorly, the quantum efficiencies of NTA-Si-Eu-PVPD (η ) 27.4%) and NTA-Si-Eu-PMAA (η ) 20.3%) are higher than that of NTA-Si-Eu-PVPDMAA (14.0%), which can be ascribed to the huge copolymeric structure in such a short unit with tremendous steric hindrance effect, which could restrict the efficiency of the intermolecular transfer mechanism, or decrease the nonradiative multiphonon relaxation by coupling to -OH vibrations and the nonradiative transition rate. The results present that the fluorescence lifetime of hybrid polymer NTA-Si-Eu-PVPD is shorter than that of NTA-Si-EuPMAA owing to the possible quenching by -OH- or silanol groups in the former, while the quantum efficiencies of the NTA-Si-Eu-PVPD are better, since the ratio of I02/I01 is bigger than that of the latter (the detailed data presented in Table 1), indicating that the hybrid polymer NTA-Si-Eu-PVPD has more effective red emission and the higher color purity. Besides, we also determined the luminescent quantum efficiencies of the three hybrids from measurements (see the data in the bracket for η), which show agreement with the values from the above calculation from spectra and lifetimes. The values of luminescent quantum efficiencies from measurements are lower than those from calculation by lifetimes and spectra, suggesting that the factor influence of spectra on the quantum efficiency is complicated. Anyway, both of them show the similar rule and are suitable to compare the luminescent behavior of different materials. 4. Conclusion In summary, we have developed a representative method for assembling luminescent rare earth molecular-based polymeric hybrid materials with chemical bonds, containing the long organic carbon chains and organic network (Si-O-Si) through the sol-gel process. The polymer precursors PVPD, PMAA, and PVPDMAA were synthesized through the polymerization reactions, and the NTA-Si was constructed through the grafting reaction. Then, the hybrid polymer was imbedded into the
14750 J. Phys. Chem. B, Vol. 112, No. 47, 2008 TABLE 1: Luminescence Quantum Efficiencies and Lifetimes of Solid Hybrid Polymeric Materials
systems
hybrid ploymer NTA-Si-EuPVPD
hybrid polymer NTA-Si-EuPMAA
hybrid polymer NTA-Si-EuPVPDMAA
υ00a (cm-1) υ01a (cm-1) υ02a (cm-1) υ03a (cm-1) υ04a (cm-1) I00b I01b I02b I03b I04b A00 (s-1) A01 (s-1) A02 (s-1) A03 (s-1) A04 (s-1) τc (ms) Ar (s-1) τexp-1 (s-1) Anr (s-1) η (%)
17274 16972 16292 15373 14324 92.5 117.3 1094.1 23.2 14.1 38.7 50.0 485.4 10.9 7.1 0.463 592.2 1768.0 1617.0 27.4 (24.1)d
17289 16961 16268 15396 14312 179.1 195.3 1104.5 21.3 16.3 44.9 50.0 294.6 6.0 5.0 0.506 400.6 2741.2 2575.7 20.3 (18.2)d
17274 16932 16308 15403 14304 47.1 60.8 479.7 13.1 8.1 38.0 50.0 409.5 11.8 7.9 0.269 517.3 3282.9 3134.0 14.0 (12.4)d
a The energies of the 5D0 f 7FJ transitions (υ0J). b The integrated intensity of the 5D0 f 7FJ emission curves. c The luminescence decay times for 5D0 f 7F2 transitions. d The luminescent quantum efficiency value from the measurement (i.e., outer luminescent quantum efficiency).
silicon-oxygen networks and carbon chains through the coordination and hydrolysis and condensation processes. Furthermore, the data of the photoluminescence properties, the 5D0 lifetimes, and the 5D0luminescence quantum efficiency and the SEM diagraphs illuminate that the hybrid materials with the single class polymer (PVPD and PMAA) present the more effective intramolecular energy transfer mechanism, more uniform microstructures than that with the two-class copolymer due to the tremendous steric hindrance effect within such a narrow space in the coordination reactions. Therefore, these kinds of homogeneous molecular-based hybrid polymeric materials can be expected to have potential and significant application in optical and electronic devices in the future. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20671072). Supporting Information Available: Figures showing schemes of the synthesis processes of the precursors for NTA-Si, PVPD-Si, PMAA-Si, and PVPDMAA-Si. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sabbatini, N.; Guardigli, M.; Lehn, J.-M. Coord. Chem. ReV. 1993, 123, 201. (2) Elbanowski, M.; Makowska, B. J. Photochem. Photobiol. 1996, 99, 85. (3) Weissman, S. I. J. Chem. Phys. 1942, 10, 214. (4) Melby, L. R.; Rose, N. J.; Abramson, E.; Caris, J. C. J. Am. Chem. Soc. 1964, 86, 5117. (5) Binnemans, K.; Lenaerts, P.; Driesen, K.; Go¨rller-Walrand, C. J. Mater. Chem. 2004, 14, 191. (6) Carlos, L. D.; Sa’Ferreira, R. A.; Rainho, J. P.; de Zea Bermudez, V. AdV. Funct. Mater. 2002, 12, 819. (7) Bekiari, V.; Lianos, P. AdV. Mater. 1998, 10, 1455. (8) Driesen, K.; Deun, R. V.; Go¨rller-Walrand, C.; Binnemans, K. Chem. Mater. 2004, 16, 1531. (9) Sabbatini, N.; Guardigli, M.; Lehn, J. M. Coord. Chem. ReV. 1993, 123, 201.
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