Complexes Covalently Bonded - American Chemical Society

Feb 19, 2008 - 3-(triethoxysilyl)propyl isocyanate (TEPIC) was used as the precursor for the preparation of mesoporous materials. SBA-15 consisting of...
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J. Phys. Chem. C 2008, 112, 3959-3968

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Construction, Characterization, and Photoluminescence of Mesoporous Hybrids Containing Europium(III) Complexes Covalently Bonded to SBA-15 Directly Functionalized by Modified β-Diketone Ying Li,† Bing Yan,*,†,‡ and Hong Yang§ Department of Chemistry, Tongji UniVersity, Shanghai 200092, China, Lab of AdVanced Materials, Fudan UniVersity, Shanghai 200433, China, and State Key Lab of Rare Earth Materials Chemistry and Applications, Beijing 100871, China ReceiVed: October 15, 2007; In Final Form: December 25, 2007

Novel organic-inorganic mesoporous luminescent hybrid materials were prepared by linking the binary and ternary Eu3+ complexes to the functionalized ordered mesoporous SBA-15 with the modified 1-(2-naphthoyl)3,3,3-trifluoroacetonate (NTA) by co-condensation of tetraethoxysilane (TEOS) in the presence of Pluronic P123 surfactant as a template. 1-(2-Naphthoyl)-3,3,3-trifluoroacetonate (NTA) grafted to the coupling agent 3-(triethoxysilyl)propyl isocyanate (TEPIC) was used as the precursor for the preparation of mesoporous materials. SBA-15 consisting of the highly luminescent ternary complex Eu(NTA)3bpy covalently bonded to the silica-based network, which was designated as Eu(NTA-SBA-15)3bpy, was obtained by introducing EuCl3 and 2,2′-bipyridine (bpy) complex into the hybrid material via a ligand-exchange reaction. In addition, for comparison, SBA-15 doped with Eu(NTA)3‚2H2O and Eu(NTA)3bpy complexes and SBA-15 covalently bonded with the binary Eu3+ complex with NTA ligand were also synthesized, denoted as Eu(NTA)3/SBA-15, Eu(NTA)3bpy/SBA-15, and Eu(NTA-SBA-15)3, respectively. The luminescence properties of these resulting materials were characterized in detail, and the results reveal that they all have high surface area, uniformity in the mesostructures, and crystallinity. The efficient intramolecular energy transfer in mesoporous material Eu(NTA-SBA-15)3bpy mainly occurs between the modified ligand NTA-Si and the central Eu3+ ion. Furthermore, Eu(NTA-SBA-15)3bpy exhibited the characteristic emission of Eu3+ ion under UV irradiation with higher 5D0 luminescence quantum efficiency than the pure Eu(NTA)3bpy complex and the other materials. Thermogravimetric analysis on the mesoporous material Eu(NTA-SBA-15)3bpy indicated that the thermal stability of the complex was improved as it was covalently bonded to the host SBA-15 matrix.

1. Introduction Rare-earth (RE) 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 ion.1-3 In the past decades, Eu3+ complexes have been intensively studied due to their inherent extremely sharp emission peaks and high quantum efficiency.4-12 Some of the RE complexes demonstrate potential applications in efficient light-conversion molecular devices and organic light-emitting devices.13-15 However, they have so far been excluded from practical applications as tunable solid-state laser or phosphor devices, essentially due to their poor stabilities under high temperature or moisture conditions and low mechanical strength. In order to circumvent these shortcomings, the lanthanide complexes should be incorporated into inorganic or organic/ inorganic matrices by low-temperature soft-chemistry processes, including the sol-gel method and hydrothermal synthesis process. Recently, lanthanide organic-inorganic hybrid materials incorporation of rare-earth complexes in the inorganic matrices have attracted considerable interest, and the lumines* To whom the correspondence should be addressed. Phone: +81-2165984663. Fax: +81-21-65982287. E-mail: [email protected]. † Tongji University. ‡ State Key Lab of Rare Earth Materials Chemistry and Applications. § Fudan University.

cence properties of lanthanide complexes supported on a solid matrix have been studied extensively because their photophysical properties could be modified by interaction with the host structure. The hybrid materials enable both inorganic and organic dopants to be incorporated with relatively high thermal stability.16-18 According to the chemical nature or different synergy between components, hybrids can be categorized into two main classes. The first class concerns all systems where no covalent bond is present between organic and inorganic parts but only weak interactions (such as hydrogen bonding, van der Waals forces, or electrostatic forces) occur;19,20 the corresponding conventional doping methods seem hard to prohibit the problem of the quenching effect on luminescent centers due to the high vibration energy of the surrounding hydroxyl groups. In addition, inhomogeneous dispersion of two phases and leaching of the photoactive molecules frequently occur in this sort of hybrid materials for which the concentration of complex is also largely restricted. Therefore, another attractive possibility in regard to complexation of rare-earth ions using ligands that are covalently bonded to the inorganic networks has emerged. Our research group is concentrating on covalently grafting the ligands to the inorganic networks in which lanthanide complexes luminescent centers are bonded with a siloxane matrix through Si-O linkage using different modified routes, including modification of active amino group, hydroxyl groups, and carboxyl groups with coupling agent, etc.21-27 These studies indicate that

10.1021/jp710023q CCC: $40.75 © 2008 American Chemical Society Published on Web 02/19/2008

3960 J. Phys. Chem. C, Vol. 112, No. 10, 2008 the thermal stabilities and photophysical properties of the lanthanide complexes were improved by the matrices. So far, incorporation of luminescent lanthanide complexes in solid matrices is of widespread interest in materials science as it allows construction of functional materials with various optical properties. Mesoporous materials are a special type of nonmaterials with ordered arrays of uniform nanochannels, and the mesoporous molecular sieves used as a support for RE complexes have attracted particular attention.28-31 Among them, SBA-15 is by far the largest pore-size mesochannel with thick walls, adjustable pore size from 3 to 30 nm, and high hydrothermal and thermal stability.32-35 These properties together with the thermal and mechanical stabilities make it an ideal host for incorporation of active molecules, and some work has already been devoted to this field.36-38 In particular, a few studies on the coordination of lanthanide ions or transition-metal ions with organic ligands covalently bonded to SBA-15 mesoporous silica have been widely investigated because surface modification permits tailoringofthesurfacepropertiesfornumerouspotentialapplications.39-41 Besides the advantages of very high surface area, large pore volume, and outstanding thermal stabilities, there are a large number of hydroxyls in SBA-15, which provides necessary qualification for modification of the inner face and self-assembly of huge guest molecules, namely, providing outstanding hosts for self-aggregation chemistry. Many research efforts, which have focused on preparing the organic/inorganic hybrids through functionalization of the exterior and/or interior surfaces, prompted the utilization of mesoporous SBA-15 in many areas. It is shown that the promising visible-luminescent properties can be obtained by linking the ternary europium complexes to the mesoporous materials. However, the synthesis and luminescence properties of SBA-15 mesoporous materials covalently bonded with lanthanide complexes by the modified β-diketonates have not been explored to date. Here, we report on the synthesis and characterization of 1-(2naphthoyl)-3,3,3-trifluoroacetonate (NTA) functionalized SBA15 mesoporous hybrid material (NTA-SBA-15) in which NTA was covalently bonded to the framework of SBA-15 by cocondensation of the modified NTA (denoted as NTA-Si) and the tetraethoxysilane (TEOS) using the Pluronic P123 surfactant as template. The highly luminescent ternary complex Eu(NTA)3bpy-functionalized SBA-15, denoted as Eu(NTA-SBA15)3bpy, was obtained by introducing EuCl3 and 2,2′-bipyridine (bpy) complex into the hybrid material via a ligand-exchange reaction. Thus, the ternary europium complex Eu(NTA)3bpy was successfully linked to the framework of SBA-15 by a covalently bonded NTA group. In addition, for comparison, SBA-15 doped with Eu(NTA)3‚2H2O and Eu(NTA)3bpy complexes and SBA15 covalently bonded with the binary Eu3+ complex with NTA ligand were also synthesized, denoted as Eu(NTA)3/SBA-15, Eu(NTA)3bpy/SBA-15, and Eu(NTA-SBA-15)3, respectively. Full characterization and detailed studies of the luminescence properties of all synthesized materials were investigated in relation to guest-host interactions between the organic complex and the silica matrix. 2. Experimental Section 2.1. Chemicals. Pluronic P123 (EO20PO70EO20, Aldrich), tetraethoxysilane (TEOS, Aldrich), and 3-(triethoxysilyl)propyl isocyanate (TEPIC, Lancaster) were used as received. The solvent tetrahydrofuran (THF) was used after desiccation with anhydrous calcium chloride. Europium chloride (EuCl3) ethanol solution (EtOH) was prepared by dissolving Eu2O3 in concentrated hydrochloric acid (HCl).

Li et al. 2.2. Synthetic Procedures. Synthesis of NTA-Functionalized SBA-15 Mesoporous Material (NTA-SBA-15). 1-(2-Naphthoyl)3,3,3-trifluoroacetonate (NTA) was prepared using a Claisen condensation between 2-acetonaphthone and ethyl trifluoroacetate in the presence of sodium ethoxide. After refluxing in anhydrous ethanol at 70 °C for 24 h, the solid was purified by recrystallization from ethanol. Anal. Calcd for C14H9O2F3: C, 63.1; H, 3.38. Found: C, 62.6; H, 3.29. Mp: 75 °C. 1H NMR (CDCl3): δ 6.71 (1H, s), 7.25 (1H, m), 7.58 (1H, m), 7.90 (1H, d), 7.98 (1H, d), 8.51 (1H, s), 15.25 (1H, b). A typical procedure for the preparation of the modified precursor NTA-Si was as follows: 1-(2-naphthoyl)-3,3,3trifluoroacetonate (NTA) (1 mmol, 0.2662 g) was first dissolved in 20 mL of dehydrated tetrahydrofuran (THF), and NaH (2 mmol, 0.048 g) was added to the solution with stirring. Two hours later, 2.0 mmol (0.495 g) of 3-(triethoxysilyl)propyl isocyanate (TEPIC) was added dropwise into the refluxing solution. The mixture was heated at 65 °C in a covered flask for approximately 12 h in a nitrogen atmosphere. After isolation and purification, a yellow oil sample NTA-Si was obtained. NTA-Si (C34H51O10F3N2Si2): IR -CONH- 1661 cm-1, -(CH2)3- 2926 cm-1, Si-O 1084 cm-1. Then the mesoporous material NTA-SBA-15 was synthesized from an acidic mixture with the following molar composition: 0.0172:0.96:0.04:6: 208.33 P123:TEOS:NTA-Si:HCl:H2O. P123 (1.0 g) was dissolved in deionized water (7.5 g) and 2 M HCl solution (30 g) at room temperature. A mixture of NTA-Si and TEOS was added into the above solution with stirring for 24 h and transferred into a Teflon bottle sealed in an autoclave, which was heated at 100 °C for 48 h. The solid product was filtrated, washed thoroughly with deionized water, and dried at 60 °C. Removal of copolymer surfactant P123 was conducted by Soxhlet extraction with ethanol under reflux for 2 days. The material was dried in a vacuum and showed a light-yellow color. Synthesis of SBA-15 Mesoporous Material CoValently Bonded with the Ternary Eu3+ Complex (Denoted as Eu (NTA-SBA15)3bpy). The NTA-SBA-15-derived hybrid mesoporous material containing Eu3+ was prepared as follows: While being stirred, NTA-SBA-15 was soaked in an appropriate amount of EuCl3 and 2,2′-bipyridine (bpy) ethanol solution with the molar ratio of Eu3+:NTA-Si:bpy ) 1:3:1. The mixture was stirred at room temperature for 12 h followed by filtration and extensive washing with EtOH. The resulting material Eu (NTASBA-15)3bpy was dried at 60 °C under vacuum overnight. The predicted structure of Eu (NTA-SBA-15)3 bpy was obtained as outlined in Figure 1. The modified ligand was covalently bonded to the SBA-15 host like an arm through Si-O-Si by the hydrolysis-condensation of organic functionality NTASi. Synthesis of SBA-15 Mesoporous Material CoValently Bonded with the Binary Eu3+ Complex (Denoted as Eu (NTA-SBA15)3). The synthesis procedure for Eu (NTA-SBA-15)3 was similar to that of Eu (NTA-SBA-15)3bpy except that the mixed ethanol solution of EuCl3 and bpy was replaced by EuCl3. Synthesis of SBA-15 Doped with the Binary and Ternary Eu3+ Complexes. SBA-15 host structure was synthesized according to the reported procedure using Pluronic P123 as a structuredirecting agent and tetraethyl orthosilicate (TEOS) as a silicon source under acidic conditions.42,43 Typically, 1.0 g of P123 was dissolved in 7.5 g of H2O and 30 g of dilute HCl solution (2.0 M) with stirring at 35 °C. Then 2.08 g of TEOS was added dropwise to the solution with stirring for 24 h and transferred into a Teflon bottle sealed in an autoclave, which was heated at 100 °C for 24 h. The solid product was filtered, washed

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Figure 1. Scheme of the synthesis process of NTA-Si, and predicted structure of hybrid mesoporous material Eu(NTA-SBA-15)3bpy.

thoroughly with deionized water, and dried at 60 °C. The assynthesized material was calcined from room temperature to 550 °C at a heating rate of 2-6 °C/min for 5 h to remove the templates and obtained fine mesoporous SBA-15. The syntheses of pure complexes Eu(NTA)3‚2H2O and Eu(NTA)3bpy were similar to a published procedure.44 The encapsulation method of the mesoporous silicon SBA15 doped with Eu3+ complexes was by mixing Eu(NTA)3‚2H2O or Eu(NTA)3bpy in ethanol solution together with SBA-15. The mixture was stirred at room temperature for 12 h, and the resulting samples were filtered, washed with ethanol, and dried at 60 °C under vacuum overnight. The mesoporous materials were denoted as Eu(NTA)3/SBA-15 and Eu(NTA)3bpy/SBA15, respectively. 2.3. Characterization. IR spectra were measured within the 4000-400 cm-1 region on an infrared spectrophotometer with the KBr pellet technique. 1H NMR spectra were recorded in CDCl3 on a Bruker AVANCE-500 spectrometer with tetramethylsilane (TMS) as an internal reference. The ultraviolet absorption spectra were taken with an Agilent 8453 spectro-

photometer. X-ray powder diffraction patterns were recorder on a Rigaku D/max-rB diffractometer equipped with a Cu anode in the 2θ range from 0.6° to 6°. Nitrogen adsorption/desorption isotherms were measured at liquid nitrogen temperature using a Nova 1000 analyzer. Surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, and pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the Barrett-Joyner-Halenda (BJH) model. The fluorescence excitation and emission spectra were obtained on a Perkin-Elmer LS-55 spectrophotometer. All spectra are normalized to a constant intensity at the maximum. Luminescence lifetime measurements were carried out on an Edinburgh FLS920 phosphorimeter using a 450 W xenon lamp as the excitation source. Thermogravimetric analysis (TGA) was performed on a Netzsch STA 409 at a heating rate of 10 °C/ min under nitrogen atmosphere. Scanning electronic microscopy (SEM) was measured on a Philip XL30. Transmission electron microscopy (TEM) experiments were conducted on a JEOL2011 microscope operated at 200 kV or on a JEM-4000EX microscope operated at 400 kV.

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Figure 2. IR spectra for NTA (A), NTA-Si (B), and NTA-functionalized hybrid mesoporous material NTA-SBA-15 (C).

Figure 3. UV absorption spectra for (A) NTA, (B) NTA-Si, and (C) NTA-SBA-15.

3. Results and Discussion 3.1. NTA-Functionalized Mesoporous Silica SBA-15. The presence of the organic ligand NTA covalently bonded to the mesoporous SBA-15 was characterized by IR and UV absorption spectra. The IR spectra for NTA (A), NTA-Si (B), and NTAfunctionalized hybrid mesoporous material NTA-SBA-15 (C) are shown in Figure 2. From A to B, it can be observed that the vibration of -CH2- at 3113 cm-1 (A) was replaced by a strong broad band located at 2974 cm-1 (B), which originated from the three methylene groups of 3-(triethoxysilyl)propyl

isocyanate (TEPIC). Furthermore, in Figure 2B the spectrum of NTA-Si is dominated by ν(C-Si, 1188 cm-1) and ν(SiO, 1090 cm-1) absorption bands, characteristic of trialkoxylsilyl functions. The band centered at 3383 cm-1 corresponds to the stretching vibration of grafted -NH- groups. In addition, the bending vibration (δNH, 1563 cm-1) further proves formation of amide groups. New peaks at 1765 and 1664 cm-1 were due to the CdO absorptions of TEPIC, proving that 3-(triethoxysilyl)propyl isocyanate was successfully grafted onto the -CH2groups of the coupling agent. In Figure 2C formation of the Si-O-Si framework is evidenced by the bands located at 1084 (νas, Si-O), 805 (νs, Si-O), and 461 cm-1 (δ, Si-O-Si) (ν represents stretching, δ in-plane bending, s symmetric, and as asymmetric vibrations). Furthermore, the peaks at 1634 and 1565 cm-1 originating from the -CONH- group of NTA-Si can also be observed in hybrid mesoporous material NTA-SBA15 (C), which is consistent with the fact that the NTA group in the framework remains intact after both hydrolysis-condensation reaction and the surfactant extraction procedure.42 Figure 3 shows the UV absorption spectra of (A) NTA, (B) NTA-Si, and (C) NTA-SBA-15 in DMF. Comparing the absorption spectrum of NTA-Si (B) with that of NTA (A), we can see a blue shift of the major π-π* electronic transitions (from 346 to 331 nm) and the disappearance of the peak centered at 212 nm, indicating that modification of NTA, which was grafted by 3-(triethoxysilyl)propyl isocyanate, influences its corresponding absorption. In Figure 3C, the corresponding red shift (257 f 268, 284 f 297, 331 f 350 nm) is observed, substantiating that a more extensive π-π* conjugating system

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TABLE 1: Textural Data of SBA-15, NTA-SBA-15, Eu(NTA-SBA-15)3, and Eu(NTA-SBA-15)3bpya sample

d100(nm)

SBET(m2/g)

V(cm3/g)

DBJH(nm)

a0

t

SBA-15 NTA-SBA-15 Eu(NTA-SBA-15)3 Eu(NTA-SBA-15)3bpy

10.68 10.11 10.08 10.15

746 626 609 549

0.97 0.85 0.82 0.67

5.64 5.41 5.36 4.92

12.33 11.67 11.64 11.72

6.69 6.26 6.28 6.80

a d100 is the d(100) spacing, a0 is the cell parameter (a0 ) 2d100/x3), SBET is the BET surface area, V is the pore volume, D is the pore diameter, and t is the wall thickness, calculated by a0 - D.

was formed due to the grafting reaction and the NTA groups were located on the surface of the mesoporous material SBA15. 3.2. Europium (Eu3+) Complexes Covalently Bonded to NTA-Functionalized Mesoporous SBA-15. The small-angle X-ray diffraction (SAXRD) patterns and nitrogen adsorption/ desorption isotherms are popular and efficient methods to characterize highly ordered mesoporous material with hexagonal symmetry of the space group P6mm. The SAXRD patterns of a pure SBA-15 mesoporous silicon (a), NTA-SBA-15 (b), Eu(NTA-SBA-15)3 (c), and Eu(NTA-SBA-15)3bpy (d) are presented in Figure 4. All the materials exhibit three wellresolved diffraction peaks that can be indexed as (100), (110), and (200) reflections associated with 2D hexagonal symmetry (P6mm), confirming a well-ordered mesoporous structure in these samples. Compared with the SAXRD pattern of pure SBA15, the d100 spacing values of Eu(NTA-SBA-15)3 and Eu(NTA-SBA-15)3bpy are nearly unchanged (see Table 1), indicating that the ordered hexagonal mesoporous structure of SBA-15 remains intact after introduction of Eu3+. However, it is worth noting that the intensity of these characteristic diffraction peaks decreases slightly in the Eu3+ complexes functionalized mesoporous materials as compared with SBA15, which is probably due to the presence of guest moieties on the mesoporous framework of SBA-15, resulting in the decrease of crystallinity but not the collapse in the pore structure of mesoporous materials.45 The N2 adsorption-desorption isotherms of pure SBA-15 (a), Eu(NTA-SBA-15)3 (b), and Eu(NTA-SBA-15)3bpy (c) samples are shown in Figure 5. They all display Type IV isotherms with H1-type hysteresis loops at high relative pressure according to the IUPAC classification,46-48 characteristic of mesoporous materials with highly uniform size distributions. From the two branches of adsorption-desorption isotherms, the presence of a sharp adsorption step in the P/P0 region from 0.6 to 0.8 and a hysteresis loop at the relative pressure P/P0 > 0.7 shows that the materials process a well-defined array of regular mesopores. The specific area and pore size have been calculated using Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The structure data of all these mesoporous materials (BET surface area, total pore volume, and pore size, etc.) are summarized in Table 1. It is known from Table 1 that calcined SBA-15 has a high BET surface area (746 m2/g) and a large pore volume (0.97 cm3/g) and pore size (5.64 nm), indicative of its potential application as a host in luminescence materials. After functionalized with NTA, the NTA-SBA-15 exhibits a smaller specific area and a slightly smaller pore size and pore volume in comparison with those of pure SBA-15, which might be due to the presence of organic ligand NTA on the pore surface and the co-surfactant effect of NTA-Si, which interacts with surfactant and reduces the diameter of the micelles.49 Furthermore, upon introduction of Eu3+ into the NTA-SBA-15, the specific area, pore size, and pore volume of the materials containing Eu3+ are less than those of NTA-SBA-15. This further confirmed incorporation of the Eu3+ complexes in the channels of SBA-15.

From the HRTEM images (as shown in Figure 6) of Eu(NTA-SBA-15)3bpy, we found that the ordered pore structure was still substantially conserved after complexation. It confirms that the suggested P6mm symmetry and a well-ordered hexagonal structure, which is also in agreement with the SAXRD and N2 adsorption/desorption isotherms. The distance between the centers of the mesopore is estimated to be 10.12 nm, in good agreement with the value determined from the corresponding XRD data (see Table 1). The IR spectra of SBA-15 (a), NTA-SBA-15 (b), and Eu(NTA-SBA-15)3bpy (c) are shown in Figure 7. In the SBA15 host material (a), the evident peak appearing at 1082 cm-1 is due to asymmetric Si-O stretching vibration modes (νas, SiO) and the peak at 796 cm-1 can be attributed to the symmetric Si-O stretching vibration (νs, Si-O). The Si-O-Si bending vibration (δ, Si-O-Si) can also be observed at 457 cm-1, and the band at 968 cm-1 is associated with silanol (Si-OH) stretching vibrations of surface groups.50 In addition, the presence of hydroxyl can be clearly evidenced by the peak at 3449 cm-1. Compared with SBA-15, the hybrid mesoporous materials not only exhibit the similar infrared absorption bands as the silica framework but also the peaks at 1380-1555 cm-1 in NTA-SBA-15 and Eu(NTA-SBA-15)3bpy, which originated from the -CONH- group of NTA-Si, indicating that NTASi has been grafted onto the wall of SBA-15. 3.3. Photoluminescence Properties. Luminescence measurements have been carried on the ternary Eu3+ complexes functionalized hybrid mesoporous materials Eu(NTA-SBA15)3bpy at room temperature. The efficient ligand to the central ion energy transfer in Eu(NTA-SBA-15)3bpy was investigated by the energy difference between the triplet states of organic ligands and the resonance energy level of the central Eu3+ ion. According to the energy-transfer and intramolecular energy mechanism,51-53 the most important factor influencing the luminescence properties of rare-earth complexes is the intramolecular energy-transfer efficiency, which mainly depends on the two energy-transfer processes.54 One is from the lowest triplet level of ligands to the emissive energy level of Ln3+ ion (rareearth ion) by Dexter’s resonant exchange interaction theory;55,56 the other is the reverse energy transition by the thermal deactivation mechanism. The energy-transfer rate constants (kT) are dependent on the energy difference (∆E (Tr - Ln3+)) between the lowest triplet level energy of ligands and the resonant emissive energy of the central Ln3+. On the basis of the above two facts, the conclusion can be drawn that ∆E (Tr - Ln3+) can have contrast influence on the two energy-transfer process mentioned and there should exist an optimal energy difference between the triplet position of ligands and the emissive energy level Eu3+ in the resulting material Eu(NTASBA-15)3bpy. If the energy difference is too big, the energytransfer rate constant will decrease due to the diminution in the overlap between the energy donor (NTA and bpy) and acceptor (Eu3+ ion). On the contrary, if the energy difference is too small, the energy can be back-transferred from the Eu3+ ion to the triplet state of the ligands.57 According to the luminescence theory of lanthanide complexes, the requirement for an efficient

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Figure 4. SAXRD patterns of SBA-15 (a), NTA-SBA-15 (b), Eu(NTA-SBA-15)3 (c), and Eu(NTA-SBA-15)3bpy (d).

Figure 5. N2 adsorption/desorption isotherms of pure SBA-15 (a), Eu(NTA-SBA-15)3 (b), and Eu(NTA-SBA-15)3bpy (c).

intramolecular energy transfer is that the energy difference between the triple state level of the ligand and the resonance energy level of the central Eu3+ ion be in the range 500-2500 cm-1. Thus, the energy difference ∆E (Tr - Eu3+) between the lowest triple state energy levels of NTA (19 600 cm-1) and bpy (22 913 cm-1) ligands and the resonance energy levels of Eu3+ (5D1, 19 020 cm-1)51,52 are 580 and 3893 cm-1, respectively, and it can be predicted that the triplet state energy of NTA (19600 cm-1) is more suitable for the luminescence of Eu3+ ion than bpy. Thus, it can be concluded that the efficient intramolecular energy transfer in mesoporous material Eu(NTA-SBA-15)3bpy mainly occurs between the NTA-Si ligand and the Eu3+ ion. Figure 8 shows the normalized excitation and emission spectra for the pure Eu(NTA)3bpy complex (A) and SBA-15 mesoporous materials covalently bonded with the binary and ternary Eu3+ complexes Eu(NTA-SBA-15)3 (B) and Eu(NTA-SBA15)3bpy (C). The excitation spectra of these materials were all obtained by monitoring the strongest emission wavelength of the Eu3+ ions at 613 nm. As shown in Figure 8A, the excitation spectrum of the pure Eu(NTA)3bpy complex exhibits a broad excitation band between 220 and 450 nm (λex ) 387 nm), which can be assigned to the π f π* transition of the ligands.48,58,59 Compared with the pure Eu(NTA)3bpy complex, the excitation band becomes narrower and the maximum excitation wavelength

Figure 6. HRTEM images of Eu(NTA-SBA-15)3bpy recorded along the [100] (A) and [110] (B) zone axes.

shifts from 387 to 356 and 337 nm for Eu(NTA-SBA-15)3 (see Figure 8B) and Eu(NTA-SBA-15)3bpy (see Figure 8C), respectively. The blue shift of the excitation bands upon introduction of Eu3+ complex into the mesoporous material SBA-15 is due to a hypsochromic effect resulting from the change in the polarity of the environment surrounding the europium complex in the mesoporous material.60 In the case of Eu(NTA)3/SBA-15 (as shown in Supporting Information Figure S1a) and Eu(NTA)3bpy/SBA-15 (Figure S1b), the maximum absorptions of their excitation spectra shift to 356 and 347 nm, respectively, which are similar the observed blue shift in Eu(NTA-SBA-15)3 and Eu(NTA-SBA-15)3bpy. From the emission spectra of Eu(NTA-SBA-15)3bpy in Figure 8C, characteristic Eu3+ ion emissions are observed. Bands in the 450-700 nm range can be clearly seen, which are

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Figure 7. IR spectra of SBA-15 (a), NTA-SBA-15 (b), and Eu(NTASBA-15)3bpy (c).

assigned to the 5D0 f 7FJ (J ) 0-4) transitions at 578, 590, 611, 650, and 700 nm, The 5D0 f 7F2 transition is a typical electric dipole transition and strongly varies with the local symmetry of Eu3+ ions, while the 5D0 f 7F1 transition corresponds to a parity-allowed magnetic dipole transition, which is practically independent of the host material. Among these transitions, the 5D0 f 7F2 transition shows the strongest emission, suggesting the chemical environment around Eu3+ ions is in low symmetry.61,62 In the pure Eu(NTA)3bpy complex (Figure 8A) the Stark splitting reveals an ordered “crystalline” rare-earth-ion environment rather than an amorphous one. However, for Eu(NTA-SBA-15)3 (B) and Eu(NTA-SBA15)3bpy (C) the broadened emission lines and decreased number of Stark components suggest less ordered crystalline environments of the Eu3+ ion in these materials.63-65 In addition, the emission spectrum of Eu(NTA-SBA-15)3 contains not only the emission of Eu3+ ion but also a broad emission band in the blue spectral region peaking around 460 nm (Figure 8B). Compared with the typical emissions of the central Eu3+, the broad emission band which mainly originated from emission of the host SBA-15 and the π-π* relaxation of the free NTA ligand moiety is very weak, only suggesting that the organic ligand does not completely transfer the absorbed energy to the central Eu3+ ion. Furthermore, upon illumination of Eu(NTASBA-15)3 and Eu(NTA-SBA-15)3bpy with a UV lamp, the resulting Eu(NTA-SBA-15)3bpy shows a pure red light and much stronger luminescence, which is in agreement with their emission spectra. Thus, the presence of both β-diketone ligand NTA and the second ligand bpy in the mesoporous matrix improved the luminescence properties of the mesoporous material Eu(NTA-SBA-15)3bpy. As a result, the strong red luminescence was observed in the emission spectra, which indicated that the effective energy transfer took place between the modified NTA and the chelated Eu3+ ions. The ternary hybrid mesoporous material Eu(NTASBA-15)3bpy shows relatively strong emission due to the chemically covalently bonded molecular Si-O network structure between the complex and the mesoporous silica. Furthermore, the luminescence intensities of the 5D0 f 7F2 transition for pure Eu(NTA)3bpy complex and the mesoporous hybrids were all

Figure 8. Excitation and emission spectra for the pure Eu(NTA)3bpy complex (A), Eu(NTA-SBA-15)3 (B), and Eu(NTA-SBA-15)3bpy (C).

compared. The relative intensity of Eu(NTA-SBA-15)3bpy mesoporous material is stronger than that of pure Eu(NTA)3bpy complex and Eu(NTA-SBA-15)3, which indicates that the ligand NTA-Si has a strong absorption in the UV region and can transfer energy to the central Eu3+ efficiently. Since the coordination ability of oxygen with lanthanide ions is stronger than that of nitrogen, the bpy cannot successfully compete with the water molecules for a place in the first coordination sphere

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Li et al.

TABLE 2: Photoluminescent Data of All Mesoporous Materials ν00 (cm-1) ν01 (cm-1) ν02 (cm-1) ν03 (cm-1) ν04 (cm-1) I01 I02 I02/I01 τ (ms) 1/τ(ms-1) Ar Anr η (%)

Eu(NTA)3bpy

Eu(NTA-SBA-15)3bpy

Eu(NTA-SBA-15)

17 301 16 977 16 366 15 313 14 285 36.04 276.01 7.67 0.515 1.942 486 1456 25.03

17 301 16 949 16 366 15 384 14 265 97.05 971.45 10.01 0.48 2.083 601 1482 28.85

17 331 16 977 16 366 15 408 14 285 50.16 482.63 9.62 0.306 3.267 631 2636 19.31

around the lanthanide ions in the absence of β-diketonate66,67 and introduction of bpy in the mesoporous hybrids to better sensitize Eu3+ luminescence by replacing H2O groups that can quench the luminescence of Eu3+ ions. In addition, the 5D0 f 7F transition can be used as a reference to compare luminescent 1 intensities of different Eu3+-based materials due to its magnetic dipole nature. The relative luminescent intensities of the 5D0 f 7F1 transition (I01) and the 5D0 f 7F2/5D0 f 7F1 intensity ratios (red/orange ratio) for all materials are listed in Table 2. 3.4. Luminescence Decay Times (τ) and Emission Quantum Efficiency (η). The typical decay curves of the Eu3+ mesoporous hybrid materials were measured at room temperature using a selective excitation wavelength of 340 nm, which can be assigned to the π f π* transition of the ligands, and all decay curves can be described as a single exponential (ln(S(t)/ S0) ) -k1t ) -t/τ), indicating that all Eu3+ ions occupy the same average coordination environment (see Supporting Information Figure S2). From the resulting lifetime data (shown in Table 2) it appears that the lifetimes of pure Eu(NTA)3bpy complex are longer than those of hybrid materials, indicating the possible quenching by -OH- or silanol groups in hybrid materials. Furthermore, the fluorescence lifetime of Eu3+ ion in Eu(NTA-SBA-15)3bpy (0.45 ms) is much longer than that in Eu(NTA)3/SBA-15, which can be attributed to the quenching effect by the -OH- group from the coordinated H2O in Eu(NTA)3/SBA-15.68,69 According to the emission spectrum and lifetime of the Eu3+ first excited level (τ, 5D0), the emission quantum efficiency (η) of the 5D0 Eu3+ 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 follows70

Ar η) Ar + Anr

(1)

where Ar and Anr are 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 ratios 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+ ion and thus can be considered as an

Eu(NTA)3/SBA-15

Eu(NTA)3bpy/SBA-15

17 301 16 977 16 366 15 360 39.13 119.41 5.10 0.205 4.878 257 4621 5.27

17 331 16 949 16 366 15 408 87.26 228.42 2.62 0.26 3.846 388 3458 10.09

internal reference for the whole spectrum; the experimental coefficients of spontaneous emission, A0J, can be calculated as follows71-74

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 vacuum, A01 has a value of 14.65 s-1; when an average index of refraction n equal to 1.506 was considered, the value of A01 can be determined to be approximately 50 s-1 (A01 ) n3A01(vac)).75 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. υ0J refers to the energy barrier and can be determined from the emission bands of Eu3+’s 5D0 f 7FJ emission transitions. The emission intensity, I, taken as integrated intensity S of the 5D0f7F0-4 emission curves can be defined as

Ii-j ) pωi-j Ai-jNi ≈ Si-j

(4)

where 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 65-69 the value of A01 ≈ 50 s-1 and the lifetime (τ), radiative transition rate (Ar), and nonradiative (Anr) transition rate are related through the following equation

Atot ) 1/τ ) Ar + Anr

(5)

On the basis of the above discussion, the quantum efficiencies of the four kinds of europium mesoporous hybrid materials can be determined as shown in Table 2. From the equation of η it can be seen that the value η mainly depends on the values of two quantum: one is lifetimes and the other is I02/I01. As can be clearly seen from Table 2, the quantum efficiency of Eu(NTA-SBA-15)3bpy (η ) 28.85%) is higher than that of pure Eu(NTA)3bpy and the other materials, which can be ascribed to substitution of the silanol with covalently bonded NTA groups in the pore channel of mesoporous SBA-15, resulting in the decrease in the level of nonradiative multiphonon relaxation by coupling to -OH vibrations and nonradiative transition rate. This clearly demonstrates the modifications in the Eu3+ ion local environment as Eu(NTA)3bpy is covalently bonded to the mesoporous SBA-15. The results described above further confirm that Eu(NTA)3bpy is successfully covalently bonded to the SBA-15 network. Furthermore, Eu(NTA-SBA-15)3 (η ) 19.31%) exhibits much lower emission quantum efficiency

Mesoporous Hybrids Containing Europium(III) Complexes than Eu(NTA-SBA-15)3bpy (η ) 28.85%), which indicates that introduction of the bpy ligand in can efficiently activate the luminescence of Eu3+ ion. Further, we selectively determined the energy-transfer efficiency for the two covalently bonded mesoporous hybrid materials For the covalently bonded mesoporous hybrids, ternary Eu(NTA-SBA-15)3bpy and binary Eu(NTA-SBA-15) systems, NTA-SBA-15 play two roles: both the host and the energy donor for Eu3+ (energy acceptor). On the basis of refs 76 and 77 the energy-transfer efficiency from NTA-SBA-15 to Eu3+ of Eu(NTA-SBA-15)3bpy (63%) is lower than that of Eu(NTA-SBA-15)3bpy (75%). This is due to the fact that NTA-SBA-15 is the energy donor in ternary systems Eu(NTA-SBA-15)3bpy and bpy also behaves as an energy donor for the luminescence of Eu3+, so the contribution of energy-transfer efficiency of NTA-SBA-15 decreases in the total luminescence quantum efficiency of Eu3+. 3.5. Thermogravimetric Analysis (TGA). The thermogravimetric weight loss curve of Eu (NTA-SBA-15)3bpy is given in Supporting Information Figure S3. From the TGA curve we can see that the mesoporous material Eu(NTA-SBA-15)3bpy has little weight loss. The first weight loss peak at around 100 °C is due to physically adsorbed water, and this is followed by a weight loss peak at 300 °C which may be ascribed to the thermal decomposition of incompletely removed surfactant. The weight loss (approximately 15%) peak at around 560 °C can be attributed to decomposition of the organic complex. In addition, compared with the weight loss peak (approximately 53% at about 350 °C) of pure complex Eu(NTA)3bpy (see Supporting Information Figure S4), the decomposition point of Eu(NTA-SBA-15)3 bpy was higher than that of pure Eu(NTA)3bpy, indicating that the thermal stability of the pure complex was enhanced as it was covalently bonded to the mesoporous matrix. 4. Conclusion In summary, the Eu3+ complexes have been covalently immobilized in the ordered SBA-15 mesoporous host through modification of 1-(2-naphthoyl)-3,3,3-trifluoroacetonate (NTA) with 3-(triethoxysilyl)propyl isocyanate (TEPIC) using a cocondensation method. Synthesis of NTA-SBA-15 provides a convenient approach for tailoring the surface properties of mesoporous silicates by organic functionalization, and the resulting materials all retain the ordered mesoporous structures. Further investigation on the luminescence properties of Eu(NTA-SBA-15)3bpy mesoporous materials shows that the characteristic luminescence of the corresponding Eu3+ through the intramolecular energy transfers from the modified ligand (NTA-Si) to the central Eu3+ ions. Meanwhile, the differences in luminescence intensity of the 5D0 f 7F2 transition, the 5D0 lifetimes, and the 5D0 luminescence quantum efficiency among all the synthesized materials confirm that the ternary complex Eu(NTA)3bpy is covalently bonded to the SBA-15 silicon network and Eu(NTA-SBA-15)3bpy exhibits higher 5D0 luminescence quantum efficiency and longer lifetime than the pure Eu(NTA)3bpy complex and the other materials. Thus, it can be concluded that the method of covalently bonding organometallic complexes to the silica backbone is more effective than the conventional method of doping mesoporous silica with organometallic complexes. Moreover, compared with SBA-15 covalently bonded with the binary europium complex Eu(NTASBA-15)3, introduction of bpy ligand into the mesoporous matrix better sensitizes Eu3+ luminescence by replacing H2O groups that can quench the luminescence of Eu3+ ions.

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