Organic Polymeric Hybrid Materials: Molecular

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J. Phys. Chem. B 2007, 111, 12362-12374

Rare-Earth/Inorganic/Organic Polymeric Hybrid Materials: Molecular Assembly, Regular Microstructure and Photoluminescence Bing Yan* and Xiao-Fei Qiao Department of Chemistry, Tongji UniVersity, Siping Road 1239, Shanghai 200092, China ReceiVed: May 9, 2007; In Final Form: August 14, 2007

2-Hydroxynicotinic acid (HNA) was grafted by 3-(triethoxysilyl)propyl isocyanate (TEPIC) to achieve the molecular precursor HNA-Si through the hydrogen-transfer nucleophilic addition reaction between the hydroxyl group of HNA and the isocyanate group of TEPIC. Then, a chemically bonded rare-earth/inorganic polymeric hybrid material (A) was constructed using HNA-Si as a bridge molecule that can both coordinate to rareearth ions (HNA-Si-RE) and form an inorganic Si-O network with tetraethoxysilane (TEOS) after cohydrolysis and copolycondensation processes. Further, three types of novel rare-earth/inorganic/organic polymeric hybrids (B-D) were assembled by the introduction of three different organic polymeric chains into the above system. First, methacrylic acid (MAA) [or methacrylic acid and acrylamide (ALM) in the molar ratio of 1:1] was mixed to polymerize (or copolymerize) with benzoyl peroxide (BPO) as the initiator to form poly(methacrylic acid) (PMAA) [or poly(methacrylic and acrylamide) (PMAALM)], and then PMAA or PMAALM was added to the precursor HNA-Si before the assembly of HNA-Si-RE, resulting in the hybrid materials HNA-Si-REPMAA (B) and HNA-Si-RE-PMAALM (C). Second, poly(vinylpyrrolidone) (PVP) was added to coordinate to the rare-earth ions by the carbonyl group in the complex HNA-Si-RE, to achieve the hybrid HNA-Si-REPVP (D). All of these hybrid materials exhibit homogeneous, regular, and ordered 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 polymeric hybrids present stronger luminescent intensities, longer lifetimes, and higher luminescent quantum efficiencies than the binary rare-earth/inorganic polymeric hybrids, indicating that the introduction of the organic polymer chain is a benefit for the luminescence of the overall hybrid system.

Introduction Recently, lanthanide complexes of organic ligands have attracted increasing attention for use as the active species in luminescent materials or devices. Lanthanide ions have sharp and intense emission bands based on their f-f electronic transitions and a wide range of luminescent lifetimes, suitable for various applications, but they have low absorption coefficients because f-f electronic transitions are forbidden. Therefore, lanthanide inorganic-organic hybrid materials in which lanthanide coordination compounds with organic ligands, such as β-diketones, aromatic carboxylic acids, and heterocyclic derivatives fabricated by a sol-gel procedure, exhibit a bright and narrow emission characteristic of metal ions and offer several advantages for the design of efficient light-conversion molecular devices (LCMDs).1,2 The absorption coefficients are increased and the corresponding emissions are often enhanced in these materials, because the ligands absorb light energy and transfer the absorbed energy to the emitting metal ions effectively (so-called antenna effect). To date, sol-gel technology, which is based on hydrolysis and polycondensation reactions, is one of the most versatile methods for the preparation of inorganic-organic hybrid materials. Because of its low processing temperature, active species such as rare-earth/organic complexes can be incorporated into host matrix (inorganic or organic polymer matrix), and the obtained hybrid materials exhibit improved processability, chemical stability, and me* To whom the correspondence should be addressed. Tel.: +86-2165984663. Fax: +86-21-65982287. E-mail: [email protected].

chanical strength because of the nature of polymer.3-5 Moreover, the prominent advantage of sol-gel technology is that the microstructure, external shape, and degree of combination between the two phases can be controlled by altering the solgel processing conditions. However, the conventional doping method in a sol-gel procedure seems unable to solve the problems of clustering of emitting centers, inhomogeneous dispersion of the two phases, and leaching of the photoactive molecules, because only weak interactions (such as hydrogen bonding, van der Waals forces, and weak static effects) exist between the organic and inorganic components.6 Therefore, another appealing method of synthesizing hybrid materials containing covalent bonds has emerged, and the as-derived molecular-based materials show improved chemical stability and have a monophasic appearance similar in nature to that of the complicated molecular polymeric Si-O network.7-25 Our research team is dedicated to the design of the rare-earth hybrid materials with inorganic networks.11-21 Compared to chemically bonded rare-earth hybrids with Si-O polymeric networks, fewer reports on molecular hybrid materials fabricated with rare-earth/ organic polymers have been published,26-30 possibly because suitable polymers or monomers are hard to select. Ternary rareearth/inorganic/organic polymeric hybrid materials with covalent bonds are even less likely to be found. Many investigations have focused on rare-earth hybrid materials containing inorganic and organic polymerization reactions, and it has been shown that these kinds of material can be expected to exhibit more effective properties, because each dispersed complex molecule is a

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Rare-Earth/Inorganic/Organic Polymeric Hybrid Materials luminescent unit so that the transparency, dimensions of the hybrids, interfacial interactions between the rare-earth luminescent species and the polymer matrixes, and molecular degrees of dispersion are primary factors determining the final luminescent behavior of the hybrids. Compared to rare-earth hybrid materials synthesized through simple inorganic polymeric procedures, the larger interfacial region and the stronger interaction between the rare-earth complex and the polymer matrix in the obtained rare-earth/inorganic/organic polymeric hybrids might accelerate energy transfer between them and enhance the luminescent efficiency of the hybrids. In this article, we describe a novel path to the assembly of hybrid materials containing the inorganic polymeric Si-O network and organic polymeric chains. The hydroxyl group of the organic ligand 2-hydroxynicotinic acid was modified with 3-(triethoxysilyl)propyl isocyanate (TEPIC) to form the precursor (HNA-Si), and hydrolysis and polycondensation reactions promoted the formation of the inorganic network (Si-O-Si). Then, an addition polymerization was performed to construct the polymeric chains (C-C) belonging to the organic component, and the cross-linking precursors (HNA-Si) with the Si-O network belonged to the inorganic component. The organic and inorganic parts were joined together and coordinated to the rareearth ions by their own carboxyl groups simultaneously in each component. Finally, we discuss and analyze the photophysical properties of the resulting hybrid materials in detail and demonstrate that the characteristics of the hybrid materials with inorganic/organic networks have been improved in some aspects. Experimental Section Chemicals and Procedures. Reagents. Terbium and europium nitrates were obtained by dissolving their oxides (Tb4O7 and Eu2O3) in concentrated nitric acid. 2-Hydroxynicotinic acid (HNA) and 3-(triethoxysilyl)propyl isocyanate (TEPIC) were supplied by Lancaster Synthesis Ltd. Poly(vinylpyrrolidone) (PVP), methacrylic acid (MAA), and acrylamide (ALM) were purchased from a Shanghai chemical plant. All other reagents were analytically pure. Synthesis of Polymer Precursors (PMAA and PMAALM). Methacrylic acid (1 mmol, 0.087 g) and acrylamide (1 mmol, 0.071 g) were dissolved in N,N-dimethyl formamide (DMF) solution with the initiator (BPO) to initiate the addition polymerization under argon purging. The reaction temperature was maintained at 70 °C for about 6 h. The coating liquid was concentrated at room temperature to remove the solvent DMF using a rotary vacuum evaporator, and a slimy yellow liquid was obtained, identified as [C4H6O2]n ([C7H11NO3]n) (see Figure 1A). Synthesis of Molecular Precursor HNA-Si with Si-O CoValent Bonds. A typical procedure for the modification of HNA to HNA-Si involved the following steps: 2-Hydroxynicotinic acid (HNA) (2 mmol, 0.278 g) was dissolved in 15 mL of acetone solvent with stirring. Then, 2.0 mmol (0.495 g) of 3-(triethoxysilyl)propyl isocyanate (TEPIC) was added dropwise into the solution with stirring. The mixture was heated to reflux at 50 °C in a covered flask for approximately 16 h under argon atmosphere. The coating liquid was concentrated to remove the solvent acetone using a rotary vacuum evaporator, and a yellow slimy liquid sample was obtained. Next, the yellow liquid sample was dissolved in absolute ethanol, and 20 mL of hexane was added to the solution to cause precipitation. Subsequently, the solution was filtered, and a white powder was obtained. The above procedures of dissolution and filtration were repeated three times. Finally, a pure white powder was obtained and dried

J. Phys. Chem. B, Vol. 111, No. 43, 2007 12363 in a vacuum. HNA-Si (see Figure 1B) [C16H26N2O7Si]: 1H NMR (deuterated DMSO), δ 0.42 (2H, t), 1.62 (2H, m), 2.52 (9H, t), 3.21 (2H, t), 3.43 (6H, m), 5.98 (1H, t), 6.71 (1H, d), 7.95 (1H, d), 8.42 (1H, d), 12.41 (1H, s). Synthesis of Molecular-Based Hybrid Material (HNA-Si-RE, Hybrid A). The sol-gel-derived hybrid material HNA-Si-RE was prepared as follows: The precursor HNA-Si was dissolved in N,N-dimethyl formamide (DMF) solvent, and a stoichiometric amount of Tb(NO3)3‚6H2O (0.32 g) or Eu(NO3)3‚6H2O (0.31 g) was added to the solution. After 3 h, TEOS and H2O were added to the mixture with stirring, and then one drop of dilute hydrochloric acid was added to promote hydrolysis. The RE(NO3)3‚6H2O/HNA-Si/TEOS/H2O molar ratio was 1:3:6:24 (TEOS, 0.45 mL, 0.833 g; H2O, 0.288 g). After the treatment of hydrolysis and polycondensation, an appropriate amount of hexamethylenetetramine was added to adjust the pH to 6-7. The mixture was agitated magnetically in a covered Teflon beaker to achieve a single phase, and then it was aged at 70 °C to reach the onset of gelation in about 5 days. The gels were collected as monolithic bulks and were ground into powdered material for the photophysical studies (see Figure 1B). Synthesis of Ternary Rare Earth/Inorganic/Organic Polymeric Hybrids HNA-Si-RE-PMAA (or -PMAALM or -PVP) (Hybrids B-D). The synthesis procedure for the rare-earth complex/ polymer hybrids HNA-Si-RE-PVP was as follows: The precursor HNA-Si was dissolved in N,N-dimethyl formamide (DMF) solvent, and a stoichiometric amount of Tb(NO3)3‚6H2O (0.32 g) or Eu(NO3)3‚6H2O (0.31 g) was added dropwise to the solution with stirring. After 3 h, a stoichiometric amount of poly(vinylpyrrolidone) (PVP) was added to the mixture to coordinate with the rare-earth ions, and then, after the coordination reaction between HNA-Si-RE and PVP had completed, TEOS and H2O were added to the mixed solution, along with one drop of dilute hydrochloric acid to promote hydrolysis. The RE(NO3)3‚6H2O/ HNA-Si/PVP/TEOS/H2O molar ratio was 1:3:1:6:24 (PVP, 0.077 g; TEOS, 0.45 mL, 0.833 g; H2O, 0.288 g). After the treatment of hydrolysis, an appropriate amount of hexamethylenetetramine was added to adjust the pH to 6-7. The mixture was agitated magnetically in a covered Teflon beaker to achieve a single phase, 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 material for the photophysical studies (see Figure 2A). The synthesis procedure for the rare-earth complex/polymer hybrids HNA-Si-RE-PMAA and -PMAALM was as follows: The precursor HNA-Si was dissolved in N,N-dimethyl formamide (DMF) solvent, and a stoichiometric amount of Tb(NO3)3‚ 6H2O (0.32 g) or Eu(NO3)3‚6H2O (0.31 g) and the polymer precursor PMAA or PMAALM were added dropwise to the solution with stirring. After 3 h, TEOS and H2O were added into the mixture with stirring, and then one drop of diluted hydrochloric acid was added to promote hydrolysis. The RE(NO3)3‚6H2O/HNA-Si/(PMAA or PMAALM)/TEOS/H2O molar ratio was 1:2:1:6:24 (TEOS, 0.45 mL, 0.833 g; H2O, 0.288 g). After the treatment of hydrolysis, an appropriate amount of hexamethylenetetramine was added to adjust the pH to 6-7. The mixture was agitated magnetically in a covered Teflon beaker to achieve a single phase, and then it was aged at 70 °C to reach the onset of gelation in about 5 days. The gels were collected as monolithic bulks and were ground into powdered material for the photophysical studies (see Figure 2B). The hybrid materials with simple inorganic network (SiO-Si) are denoted as HNA-Si-RE, and the rare-earth complex/ polymer hybrids containing the inorganic network and long

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Figure 1. Schemes of syntheses of (A) polymer precursors (PMAALM and PMAA) by addition polymerization and (B) precursor HNA-Si and rare-earth molecular-based hybrid material in simple inorganic Si-O-Si network.

organic chains are denoted as HNA-Si-RE-PVP, HNA-Si-REPMAA, and HNA-Si-RE-PMAALM, respectively. Physical Measurements. Fourier transform infrared spectra were obtained from KBr pellets and were recorded on a Nexus 912 AO446 FT-IR spectrophotometer in the range of 4000400 cm-1. 1H NMR spectra were recorded in deuterated DMSO on a Bruker AVANCE-500 spectrometer with tetramethylsilane (TMS) as the internal reference. Ultraviolet absorption spectra of powder samples [5 × 10-4 mol·L-1 tetrahydrofuran (THF) solution] were recorded on an Agilent 8453 spectrophotometer. Solid-state ultraviolet-visible spectra of the hybrid materials were measured with an Instant SpecTM-BWS003 spectrophotometer. Fluorescence excitation and emission spectra were obtained on a Perkin-Elmer LS-55 spectrophotometer (excitation slit width, 3 nm; emission slit width, 5 nm). Luminescent lifetimes for the hybrid materials were obtained with an Edinburgh Instruments FLS 920 phosphorimeter using a 450-W xenon lamp as the excitation source (pulse width, 3 µs). The microstructures were checked by scanning electronic microscope

(SEM, Philips XL-30). X-ray diffraction (XRD) measurements were carried out on powdered samples with a Bruker D8 diffractometer (40 mA, 40 kV) using monochromated Cu KR1 radiation (k ) 1.54 Å) over the 2θ range of 10-70°. All of the above measurements were performed at room temperature. Thermogravimetry (TG) and differential scanning calorimetry (DSC) data were obtained on a Netzsch model STA 409C instrument under the following conditions: nitrogen air atmosphere, 10 °C/min heating/cooling rate, 13 mg of powder, and Al2O3 crucibles. Results and Discussion Synthesis and General Characterization. The synthesis schemes for the polymer precursors (PMAA and PMAALM) by addition polymerization and for the hybrid materials HNASi-RE, HNA-Si-RE-PVP, and HNA-Si-RE-PMAA are shown in Figures 1 and 2, respectively. The hybridization of TEOS and HNA-Si proceeds through a polycondensation reaction between the terminal silanol groups of HNA-Si and the OH

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Figure 2. Schemes of the syntheses of (A) the rare-earth molecular-based hybrid material HNA-Si-RE-PVP containing organic polymeric chains (C-C) and inorganic network (Si-O-Si) and (B) the rare-earth molecular-based hybrid material HNA-Si-RE-PMAA containing organic polymeric chains (C-C) and inorganic network (Si-O-Si).


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Figure 3. Fourier transform infrared spectra of (A) HNA, (B) HNA-Si-Eu, and (C) HNA-Si-Eu-PMAA.

Figure 4. Ultraviolet absorption spectra of (A) HNA, (B) HNA-Si, (C) HNA-Si-Eu, and (D) HNA-Si-Eu-PMAA.

groups of hydrolyzed TEOS. At the beginning of the reaction, the individual hydrolyses of HNA-Si and TEOS are predominant, and then the polycondensation reactions between the hydroxyl groups of HNA-Si and TEOS become predominant, which introduces rare-earth ions into the Si-O-Si network through covalent bonds. Therefore, we denote the cooperation of both HNA-Si and TEOS during the in situ sol-gel process as cohydrolysis and copolycondensation. The Fourier transform infrared spectra of (A) HNA, (B) HNA-Si-Eu, and (C) HNA-Si-Eu-PMAA are shown in Figure 3. In Figure 3A, the peak at 1728 cm-1 corresponds to the asymmetric stretch of the carbonyl group in 2-hydroxynicotinic acid, which is shifted in B, indicating that a new conjugated system formed during the coordination between the rare-earth ions and HNA.26 The peak located at 1643 cm-1 in B due to the absorption of amide groups (CONH) suggests that 3-(triethoxysilyl)propyl isocyanate (TEPIC) was successfully grafted onto 2-hydroxynicotinic acid; it is shifted to 1660 cm-1 in C upon the addition of PMAA as a result of small changes in the conjugated system.10 The peaks located at 1209 and 1210 cm-1

in (B) HNA-Si-Eu and (C) HNA-Si-Eu-PMAA are attributed to the Si-C stretching vibration, which also indicates the success of the grafting reaction of 3-(triethoxysilyl)propyl isocyanate (TEPIC) in harmony with the above conclusion. Thus, it was found that no Si-C bond splitting occurred during the course of the hydrolysis/polycondensation reactions. Moreover, a broad peak at 1039-1147 cm-1 emerged in B and shifted to 10521169 cm-1 in C that is related to the stretching vibration of Si-O-Si; together with the bending vibration at 458 cm-1, this indicates the appearance of siloxane bonds.31 The wide-band absorption at about 3400 cm-1 is the overlap of the asymmetric and symmetric H-O-H stretches in the crystal water (36003000 cm-1) and the double multiple absorption of CdO (35003400 cm-1) and N-H (3500-3200 cm-1) in B and C. Figure 4 shows ultraviolet absorption spectra of (A) HNA, (B) HNA-Si, (C) HNA-Si-Eu, and (D) HNA-Si-Eu-PMAA. From the spectra, one can see that a red shift (A f B) of the major π-π* electronic transitions (from 243 to 253 nm) occurred and the small broad peak at 282 nm disappeared, which indicates that the diverse ligands promoted the conjugating effect of double bonds and reduced the energy differences among the electron transitions. Moreover, a large number of changes in the peak shapes between A and B are apparent, which can be seen from the facts that the peak value of the sharp peak (243 nm) is lower than that of the broad peak (325 nm) in curve A whereas the peak value of the broad peak (253 nm) is much higher than that of the broad peak (325 nm) in curve B. In summary, it is estimated that TEPIC was grafted onto HNA and that covalent chemical bonds were formed successfully. In terms of B and C, the peak shapes also changed greatly, which is observed from the disappearance of the broad peak at 253 nm in B and a blue shift of the peak from 325 nm (B) to 322 nm (C) as a result of the change in the conjugated structure of HNA after coordinating to Eu3+. Furthermore, compared to C, an obvious blue shift of 7 nm is observed in D upon the addition of polymer PMAA to hybrid material C (from 322 to 315 nm), which indicates the formation of a conjugated system between HNA-Si, Eu3+, and PMAA, different from the conjugated system between HNA-Si and Eu3+ in C. Because PMAA and HNA-Si coordinate with rare-earth ions Eu3+ simultaneously,

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Figure 5. (a,c) Excitation and (b,d) emission spectra of (a,b) terbium and (c,d) europium complexes: (A) HNA-Si-RE, (B) HNA-Si-RE-PMAA, (C) HNA-Si-RE-PMAALM, and (D) HNA-Si-RE-PVP.

a steadier conjugated system between them forms in the rareearth complex/polymer hybrids with the organic/inorganic networks (Si-O-Si and C-C) synthesized through cohydrolysis, copolycondensation, and addition polymerization, which is different from the conjugated system with the simple inorganic network (Si-O-Si) in C. Luminescent Properties. The luminescent spectra of the obtained polymeric hybrid materials are shown in Figure 5. Plots a (c) and b (d) show the excitation and emission spectra of the terbium (europium) complexes obtained through inorganic hydrolysis and copolycondensation and organic addition polymerization, and curves A-D represent the spectra of hybrid materials HNA-Si-RE, HNA-Si-RE-PMAA, HNA-Si-REPMAALM, and HNA-Si-RE-PVP, respectively. The excitation spectra (Figure 5a and c) were obtained by monitoring the emission of Tb3+ or Eu3+ at 543 or 613 nm, respectively, dominated by a series of broad bands centered at about 334346 or 339-355 nm near the ultraviolet region; in addition they show the characteristic absorption of the rare-earth complexes arising from the efficient transition based on the conjugated Cd O double covalent bonds of 2-hydroxynicotinic acid. As a result, the emission lines of the hybrid polymeric material were

obtained from the 5D4 f 7FJ (J ) 3 - 6) transitions at 488, 543, 583, and 620 nm for terbium ions or from the 5D0 f 7FJ (J ) 1 - 4) transitions at 576, 589, 613, 648, and 694 nm for europium ions under excitation at wavelengths of 334, 334, 339, and 346 nm or 352, 339, 340, and 337 nm, respectively (Figure 5b and d). This indicates that effective energy transfer took place and that conjugated systems formed between the ligands and the chelated rare-earth ions in hybrids A-D. Because of the formation of the powerful chemical bonds of the -Si-O-Sibackbone and the complexation of Tb3+ (Tb-O bond) or Eu3+ (Eu-O bond) in rare-earth hybrid materials A-D, the hybrid materials exhibit relatively strong emissions under such unique stable chemical environments of rigid molecular network structures. As shown in Figure 5b, the green emission intensities are the strongest in all emissions of the Tb3+ complexes, which can be explained by the conclusion that the leaching of the photoactive molecules was avoided and a higher concentration of metal ions was obtained. As seen from Figure 6d, among the emission peaks of the complexes of chelated Eu3+, red emission intensities (arbitrary units) of the 5D0 f 7F2 electric dipole transition at about 613 nm (295 for A, 502 for B, 520 for C, and 790 for D) are all stronger than the orange emission


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Figure 6. Part 1 of 3

intensities of the 5D0 f 7F1 magnetic dipole transition at about 589 nm (118 for A, 148 for B, 170 for C, and 281 for D), which indicates an Eu3+ site in an environment without inversion symmetry.32,33 Moreover, we compared the relative luminescent intensities of the three rare-earth/inorganic/organic polymeric hybrids HNA-Si-RE-PMAA, -PMAALM, and -PVP (B-D) containing the inorganic and organic networks with rare-earth complex HNA-Si-RE (A) containing inorganic networks, and the detailed data are listed in Tables 1and 2. Compared to rare-earth complex HNA-Si-RE (A), the relative emission intensities of hybrid polymer materials B-D exhibit obvious enhancements, and the relative emission intensities of hybrid material HNA-Si-REPVP (D) are even 3 or 4 times those of HNA-Si-RE (A). It is

speculated that, because the nitrogen atoms or carboxyl groups located in the polymer chains of HNA-Si-RE-PMAA, -PMAALM, and -PVP (B-D) coordinate to Tb3+ or Eu3+ ions, which replace the coordinated water molecules existing in complexes of aromatic carboxylic acid ligands, the energy loss and clustering of the emitting centers caused by the vibration of the hydroxyl groups of coordinated water molecules could be avoided. Furthermore, the relative emission intensity of the three rare-earth complex/polymer hybrids increases gradually according to the sequence B < C < D, and the emission intensities of D are much stronger than those of B or C, because there exists a very small distinction between the relative emission intensities of B and C and an obvious distinction between B or C and D. In terms of the above phenomena, the



Figure 6. Part 2 of 3

following conclusion might be deduced: The polymer chains of HNA-Si-RE-PMAA (B) and HNA-Si-RE-PMAALM (C) share some commonalities in the structures, such as the carboxyl groups and the organic networks composed of chemical covalent bonds (C-C), so that the relative emission intensities are analogous. However, the relative emission intensity of HNASi-RE-PMAA (B) is slightly lower than that of HNA-Si-REPMAALM (C), because the polymer chain of the latter is longer than that of the former, thus weakening the steric exclusion effects when the polymer ligands coordinate to the rare-earth ions. Furthermore, the relative emission intensity of HNA-SiRE-PVP (D) is the strongest, because the coordination number of rare-earth ion is 7 in the polymer material (Figure 2A), which is larger than that in polymer materials HNA-Si-RE-PMAA and -PMAALM (B and C; Figure 2B). The above analysis indicates

that the relative emission intensities are generally affected by the conjugated systems of the rare-earth complexes (B-D) and the coordination numbers of the rare-earth ions. The typical decay curves of the Tb and Eu hybrid materials were measured, and they can be described as a single exponential in the form ln[S(t)/S0] ) -k1t ) -t/τ, indicating that all Tb3+ ions occupy the same average coordination environment. The resulting lifetimes of the Tb and Eu hybrids are included in Tables 1 and 2, respectively. It was found that the rare-earth/inorganic/organic ternary polymeric hybrids present longer luminescent lifetimes than the corresponding rare-earth/ inorganic binary hybrids without organic polymers, suggesting that the introduction of organic polymeric chain can enhance the luminescence stability of the overall hybrid system. In addition, the luminescent lifetimes of rare-earth/inorganic/


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Figure 6. SEM images of molecular-based hybrid materials with organic and inorganic networks: (a) HNA-Si-Tb, (b) HNA-Si-Eu, (c) HNA-SiTb-PMAA, (d) HNA-Si-Eu-PMAA, (e) HNA-Si-Tb-PMAAALM, (f) HNA-Si-Eu-PMAALM, (g) HNA-Si-Tb-PVP, (h) HNA-Si-Eu-PVP.

TABLE 1: Luminescent Property Data of Terbium Molecular Hybrids HNA-Si-Tb and HNA-Si-Tb-PMAA, -PMAALM, and -PVP molecular hybrid

emission bands (nm)

relative intensitiesa

lifetime (ms)b

HNA-Si-Tb (A) HNA-Si-Tb-PMAA (B) HNA-Si-Tb-PMAALM (C) HNA-Si-Tb-PVP (D)

488.0, 543.1, 583.1, 620.1 488.5, 543.2, 583.3, 619.8 488.8, 544.0, 584.4, 619.2 489.1, 544.8, 584.1, 620.7

48.9, 140.7, 20.1, 13.1 140.5, 307.2, 34.5, 17.5 169.5, 403.7, 34.6, 18.1 259.1, 717.5, 63.8, 32.3

0.65 0.94 1.12 1.23

a Relative intensities (arbitrary units) were obtained by the calculation of the integral area of the same emission bands. b For the 5D4 f 7F5 transition of Tb3+.

organic ternary polymeric hybrids with different organic polymer chain species differ slightly, which indicates that the organic polymeric chain has a small influence on the stability of the

hybrids. Furthermore, we selectively determined the emission quantum efficiencies of the 5D0 europium ion excited state for Eu3+ hybrids on the basis of the emission spectra and lifetimes



TABLE 2: Luminescent Property Data of Europium Molecular Hybrids HNA-Si-Eu and HNA-Si-Eu-PMAA, -PMAALM, and -PVP molecular hybrid

emission bands (nm)

relative intensitiesa

Lifetime (ms)b

HNA-Si-Eu (A) HNA-Si-Eu-PMAA (B) HNA-Si-Eu-PMAALM (C) HNA-Si-Eu-PVP (D)

576.9, 588.2, 610.8, 648.5, 694.1 576.4, 588.6, 613.7, 648.5, 694.1 575.9, 588.6, 613.2, 648.5, 694.1 576.3, 588.0, 612.2, 648.0, 694.1

66.0, 118.1, 294.9, 17.1, 15.1 53.7, 148.1, 502.5, 17.5, 14.9 68.5, 169.5, 519.5, 17.9, 15.3 135.9, 280.5, 789.5, 29.7, 27.5

0.41 0.56 0.64 0.83

a Relative intensities (arbitrary units) were obtained by the calculation of the integral area of the same emission bands. b For the 5D0 f 7F2 transition of Eu3+

TABLE 3: Luminescence Quantum Efficiencies and Relevant Property Data of Europium Molecular Hybrids HNA-Si-Eu-PMAA, -PMAALM, and -PVP radiative decay luminescence quantum efficency (%) rate (s-1)

molecular hybrid HNA-Si-Eu (A) HNA-Si-Eu-PMAA (B) HNA-Si-Eu-PMAALM (C) HNA-Si-Eu-PVP (D)

210.71 240.86 226.37 218.84

8.72 13.61 14.49 18.16

of the 5D0 emitting level using the four main equations according to refs 34-41. The detailed principles and method were adopted from ref 42, and the data are reported in Table 3.

A0J ) A01(I0J/I01)(υ01/υ0J) Arad )

∑A0J ) A00 + A01 + A02 + A03 + A04

(1) (2)

τ ) Arad-1 + Anrad-1

(3)

η ) Arad/(Arad + Anrad)

(4)

Here, A0J represents the experimental coefficients of spontaneous emission, where A01 is Einstein’s coefficient of spontaneous emission between the 5D0 and 7F1 energy levels, which can be determined39-41 to be approximately 50 s-1 and was taken as a reference to calculate the other A0J values. I is the emission intensity and can be taken as the integrated intensity of the 5D0 f 7FJ emission bands.34,35 υ0J refers to the energy barrier and can be determined from the emission bands of the Eu3+ 5D0 f 7F emission transitions. A J rad and Anrad represent the radiative and nonradiative transition rates, respectively, where Arad can be determined from the summation of A0J (eq 2). Thus, the luminescence quantum efficiency can be calculated from the luminescent lifetimes and the radiative and nonradiative transition rates. The three rare-earth/inorganic/organic ternary polymeric hybrids exhibit higher luminescence quantum efficiencies than the rare-earth/inorganic binary hybrids, and in particular, the HNA-Si-Eu-PVP hybrid with the poly(vinylpyrrolidone) chain shows the highest luminescence quantum efficiency. This finding agrees with the results from the luminescent intensities and lifetimes. The fabrication of organic polymer chains in the hybrid systems benefits the luminescent properties by increasing the ratio of radiative transitions. However, it can be seen that the enhancement of the luminescence quantum efficiency data is not as great as for the luminescent intensities, which is related to the absorption properties of these hybrids. Regular Microstructure. The scanning electron micrographs of eight kinds of the rare-earth/inorganic/organic polymeric hybrids are shown in Figure 6. Images a, c, e, and g are the micrographs of HNA-Si-Tb, HNA-Si-Tb-PMAA, HNA-Si-TbPMAALM, and HNA-Si-Tb-PVP, respectively, and images b, d, f, and h show the micrographs for the corresponding hybrids containing Eu. The quite uniform frameworks on the faces demonstrate that homogeneous, molecular-based materials were

obtained containing a functional bridge ligand with strong double chemical bonds between the inorganic and organic phases, which belongs to a complicated huge molecular system. Compared to hybrid materials with doped lanthanide complexes, which generally experiencing phase separation phenomena, the two phases in the hybrid materials with chemical covalent bonds (Si-O) can exhibit their distinct properties together.43,44 The regular ordered microstructures for all of these hybrid materials suggest that a self-assembly process might occur during the polymerization reaction of the inorganic or organic polymers. We can achieve control of different regular ordered microstructures of rare-earth/inorganic/organic polymeric hybrids through strong chemical bonds. Both rare-earth ions and organic polymers are important factors. As can be seen in Figure 6a, a large number of regular columns and cuboids with sizes of about 200-500 nm are dispersed homogenously on the hierarchies, which is also observed from the scanning electron micrograph of HNA-SiEu (Figure 6b). The precursor HNA-Si is a derivative of the organic aromatic ligand 2-hydroxylnicotinic acid with one hydroxyl and one carboxyl group, so its corresponding rareearth complex is prone to form polymeric structures easily,45-48 and this tendency will compete with the tendency for the construction of the three-dimensional polymeric network structure of Si-O-Si in the hydrolysis and copolycondensation reactions of silica. The final result is that the hybrid material HNA-Si-Tb (-Eu) retains the tendency of growing into polymeric network structure and reserves the coordinated positions in the corresponding bulk materials observed from the microstructure view in Figure 6a,b. In addition, the particle size of HNA-Si-Eu is smaller than that of HNA-Si-Tb, which might be due to the slightly different ionic radii of Eu and Tb: the larger radius of Eu3+ makes it possible to form higherdimensional polymeric structures so that the resulting hybrid network structure has a smaller size. In contrast, Tb3+ might form only lower-dimensional polymeric structures and produce the larger hybrid systems. Thus, the rare-earth ions can also affect the assembly of hybrid materials, which is a very complicated problem for the composition of hybrid materials. The hybrid material HNA-Si-Tb-PMAA exhibits a bulk dendritic stripe microstructure, which is dispersed homogenously and regularly seen from the scanning electron micrograph in Figure 6c. This indicates that the tendency to form a polymeric Si-O-Si network in the hydrolysis and copolycondensation reactions of silica becomes the primary tendency when it competes with the tendency of the rare-earth complexes to form one-dimensional chainlike structures. Because PMAA has longer polymeric chains and they substitute the precursor HNA-Si, when PMAA coordinates to the rare-earth ions, there is less steric exclusion in the formation of a uniform tubular microstructure with a diameter size of 2 µm. In contrast, they are very complicated for the HNA-Si-Eu-PMAA hybrid material. The scanning electron micrographs of hybrid material HNASi-Eu-PMAA (d1-3) exhibit a homogeneous sandwich structure

12372 J. Phys. Chem. B, Vol. 111, No. 43, 2007 composed of plentiful planar circular disks on a face with a diameter size of about 5 µm, and a large number of pinholes with a uniform diameter of about 500 nm are in the circles as observed from the enlarged structure in Figure d4. The pinholes might be due to the large difference in thermal expansion coefficients between the cross-linked polymer PMAA and the silica or to the evaporation of solvent during the heating and aging procedure.49 Therefore, polymer PMAA plays an important role in the preparation of rare-earth complex/polymer hybrids and induces the pinholes to be encased in the planar circles. Furthermore, another unexpected phenomenon was found in Figure 6 (d5,6) that there exist plenty of threedimensional globes with a uniform diameter size of about 500 nm spread regularly and homogenously on the profile of the hybrid material HNA-Si-Eu-PMAA that have diameter dimensions similar to those of the planar circles in Figure 6d4. The above phenomenon might be explained by the conclusion that the planar circles (d3,4) grow into the three-dimensional globes (d5,6) on some profiles that have structural and environmental advantages to accelerate the growth of these globes, but that on other different profiles, the growth process of the threedimensional globes might be prevented by the nonhomogeneous thermal treatment procedure. Alternatively, another explanation is that plenty of three-dimensional globes exist on every profile, but they are destroyed on some profiles in the grinding procedure to prepare sample powder. The differences between HNA-SiTb-PMAA and HNA-Si-Eu-PMAA further verify that rare-earth ions play an important role in the molecular assembly of the polymeric hybrids. Compared to the above hybrids of HNA-Si-RE, the trunk microstructure arranged quite regularly on the slippery and clean surfaces and formed homogeneous bulk stripe or tubular microstructure seen from the micrographs in Figure 6e,f of the hybrid materials HNA-Si-RE-PMAALM. Because the rare-earth ions coordinated with 2-hydroxylnicotinic acid (HNA) and PMAALM simultaneously, and not just with 2-hydroxynicotinic acid (HNA), the dominant growth tendency was along the direction of Si-O-Si polymeric network in the hydrolysis and copolycondensation procedures, unlike the dominant growth tendency in HNA-Si-RE. Moreover, it is assumed that there is less steric hindrance in the conjugated system in the rare-earth complexes (HNA-Si-RE-PMAALM) than in HNA-Si-RE or HNA-Si-RE-PMAA, when the rare-earth ions coordinate with 2-hydroxynicotinic acid and PMAALM as a result of the formation of the larger polymeric chains of PMAALM through the addition reaction. As can be seen from the scanning electron micrographs of hybrid material HNA-Si-Tb-PVP (Figure 6g), this material is composed of many regular layers that are full of foursquare blocks on the faces. The tendency to form one-dimensional chainlike structures in the corresponding 2-hydroxynicotinic acid/rare-earth complex competes with the tendency to construct a polymeric network structure of Si-O-Si units through the hydrolysis and copolycondensation reactions of silica. As a result, neither of the growth tendencies becomes dominant, and competition between the two results in the final structure and morphology of the hybrid material HNA-Si-Tb-PVP. The scanning electron micrographs of HNA-Si-Eu-PVP (Figure 6h) reveal that many acicular clusters that are composed of plentiful needlelike columned configurations with widths of 100 nm and lengths of 1 µm are distributed homogenously. The structures of these materials are slightly different from that of HNA-SiRE, in that many homogeneous needlelike columns exist in the former whereas many regular columns and cuboids exist in the

Yan and Qiao

Figure 7. Thermogravimetry (TG) and differential scanning calorimetric (DSC) data for the obtained hybrid materials: (A) HNA-Si-Eu and (B) HNA-Si-Eu-PMAA.

latter. It is speculated that the addition of the polymer PVP changes the conjugated system slightly so that the coordination number of rare-earth ions becomes 7 for HNA-Si-RE. However, a large distinction appears between the microstructures of HNASi-RE-PVP and HNA-Si-RE-PMAA (-PMAALM), because the HNA-Si/RE/PVP molar ratio is 3:1:1, whereas the HNA-Si/ RE/PMAA (PMAALM) molar ratio is 2:1:1, and the coordination number of the rare-earth ion increases to 7 in the former, whereas in the latter, it remains 6, which is the same as the coordination number of HNA-Si-RE. Furthermore, the rare-earth ions coordinate with three carboxyl groups provided by 2-hydroxynicotinic acid and the nitrogen atom provided by PVP in the HNA-Si-RE-PVP, whereas in HNA-Si-RE-PMAA (-PMAALM), they coordinate with the three carboxyl groups provided by 2-hydroxynicotinic acid and the polymer PMAA (PMAALM) without the nitrogen atom, which produces a significant distinction in the formation of their conjugated systems. It is deduced that different coordination numbers and conjugated systems of rare-earth complexes largely affect the microconfiguration. Thermal Behavior and XRD. Figure 7 shows the selected thermogravimetry (TG) traces and differential scanning calo-


J. Phys. Chem. B, Vol. 111, No. 43, 2007 12373 numbers of the rare-earth ions, conforming to the conclusions obtained from the fluorescence spectra and SEM micrographs. However, so far, we are puzzled by the problem that the molecular weights of the polymers could not been determined in the investigations of the obtained hybrid materials, because tetrahydrofuran (THF), chloroform, and water solvents cannot dissolve the final materials. Therefore, we will continue exploring the polymer character to find a suitable solvent in the future. Conclusion

Figure 8. X-ray diffraction patterns for the obtained hybrid materials HNA-Si-Tb and HNA-Si-Tb-PVP.

rimetric (DSC) plots of the obtained hybrid materials, HNASi-Eu with the inorganic network for A and HNA-Si-Eu-PMAA with the inorganic/organic networks for B. The TG curve of HNA-Si-Eu shows a large mass decrease (19.8%) in heat flow around 211 °C, which is attributed to the loss of the ligand 2-hydroxynicotinic acid (HNA) seen from Figure 7A, and the TG curve of HNA-Si-Eu-PMAA shows two decreases, namely, mass changes of 29.8% and 19.8% around 179.6 and 406.5 °C, which are attributable to the loss of the solvent N,N-dimethyl formamidec (DMF) and the ligand 2-hydroxynicotinic acid (HNA), respectively (as shown in Figure 7B). The energy changes indicated by the DSC curves also conform to the results of the TG curves, which shows that the thermodynamic character of hybrid material HNA-Si-Eu-PMAA containing the inorganic network and long organic polymeric chains has been improved. According to calculations, the mass of HNA contributes about 22.9% of the total mass of HNA-Si-Eu, and when the temperature increases to 211 °C, it is lost, so that there is a mass change of 19.8% in the TG curve of HNA-Si-Eu. In HNA-Si-EuPMAA, according to the mass calculations, HNA comprises 19.49% of the total mass, and there is a mass change of 19.8% for the loss of HNA in the TG curve at around 406.5 °C. It is deduced that, because the addition of PMAA substitutes the same molar ligand 2-hydroxynicotinic acid (HNA) in the coordination procedure and causes the conjugated system to change largely between the rare-earth ions and the carboxyl groups in hybrid materials, the loss temperature of 2-hydroxynicotinic acid (HNA) increases from 211 to 406.5 °C, indicating the improved thermal stability of HNA-Si-Eu-PMAA. Selected room-temperature X-ray diffraction patterns of the hybrid materials HNA-Si-Tb and HNA-Si-Tb shown in Figure 8 verify that the obtained rare-earth complexes are amorphous from 10 to 70 °C. The broad peak is centered at 23.728°, and the structural unit distance is 3.745 Å in the XRD pattern of HNA-Si-Tb because of the amorphous siliceous backbone of the hybrids,50 which shifts to 20.366° as the structural unit distance changes to 4.357 Å in the XRD pattern of HNA-SiTb-PVP. The absence of any crystalline regions in these samples correlates with the presence of organic chains in the host inorganic framework. Moreover, it is apparent that the polymeric chains of PVP are essentially in a disordered state and the addition of PVP to hybrid material HNA-Si-Tb results in an increase of the overall disorder of the siliceous skeleton, which brings the changes in the conjugated systems and coordination

We have developed some novel binary and ternary rare-earth/ inorganic/organic polymeric hybrid materials with covalent bonds (HNA-Si-RE, HNA-Si-RE-PMAA, HNA-Si-REPMAALM, and HNA-Si-RE-PVP; RE ) Eu and Tb) that involve long organic polymeric chains through addition polymerization and an organic network (Si-O-Si) through cohydrolysis and copolycondensation reactions in a sol-gel process. The physical properties, especially the microstructure and photoluminescence properties, were studied in detail. It is worth pointing out that all of these hybrids present novel regular ordered microstructures and the different compositions of the hybrids and different organic polymeric chains influence the microstructures of the resulting hybrids, which suggests that a self-assembly process might occur during the formation of polymeric hybrids constructed with strong chemical bonds. Both the inorganic polymeric network and the organic polymer chain can control the micromorphology though coordination to the rare-earth ions. Correspondingly, the ternary rare-earth/inorganic/ organic polymeric hybrid materials exhibit stronger luminescent intensities, longer lifetimes, and higher quantum efficiencies, which is attributed to the introduction of organic polymeric chain. In particular, the HNA-Si-RE-PVP hybrid systems exhibit the most excellent luminescent behaviors. The relation between the microstructure and photoluminescence in the different hybrids, however, needs to be fundamentally investigated further. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20671072). References and Notes (1) Richardson, F. S. Chem. ReV. 1982, 82, 541. (2) Sabbatini, N.; Guardigli, M.; Lehn, J. M. Coord. Chem. ReV. 1993, 123, 201. (3) Robinson, M. R.; O’Regan, M. B.; Bazan, J. C. Chem. Commun. 2000, 17, 1645. (4) Sun, L. N.; Zhang, H. J.; Meng, Q. G.; Liu, F. Y.; Fu, L. S.; Peng, C. Y.; Yu, J. B.; Zheng, G. L.; Wang, S. B. J. Phys. Chem. B 2005, 109, 6174. (5) Hiroyuki, O.; Hideyuki, Y. Solid State Ionics 1995, 80, 251. (6) Sanchez, C.; Ribot, F. New J. Chem. 1994, 18, 1007. (7) Li, H. R.; Lin, J.; Zhang, H. J.; Fu, L. S. Chem. Mater. 2002, 14, 3651. (8) Li, H. R.; Lin, J.; Zhang, H. J.; Fu, L. S. Chem. Commun. 2001, 1212. (9) Dong, D. W.; Jiang, S. C.; Men, Y. F.; Ji, X. L.; Jiang, B. Z. AdV. Mater. 2000, 12, 646. (10) Franville, A. C.; Zambon, D.; Mahiou, R. Chem. Mater. 2000, 12, 428. (11) Wang, Q. M.; Yan, B. J. Mater. Chem. 2004, 14, 2450. (12) Wang, Q. M.; Yan, B. Cryst. Growth Des. 2005, 5, 497. (13) Wang, Q. M.; Yan, B. J. Mater. Res. 2005, 20, 592. (14) Wang, Q. M.; Yan, B. J. Photochem. Photobiol. A: Chem. 2006, 175, 159. (15) Wang, Q. M.; Yan, B. J. Photochem. Photobiol. A: Chem. 2006, 178, 70. (16) Wang, Q. M.; Yan, B. J. Organomet. Chem. 2006, 691, 540. (17) Wang, Q. M.; Yan, B. J. Organomet. Chem. 2006, 691, 3567. (18) Yan, B.; Ma, D. J. J. Solid State Chem. 2006, 179, 2059.

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Organic Polymeric Hybrid Materials: Molecular

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