Novel luminescent europium(III) complexes covalently bonded to bis(phosphino)amine oxide functionalized MCM-41

Inorganic Chemistry Communications 12 (2009) 48–51

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Novel luminescent europium(III) complexes covalently bonded to bis(phosphino)amine oxide functionalized MCM-41 Qian-Yong Cao *, Yan-Hui Chen, Jiang-Hua Liu, Xi-Cun Gao * Department of Chemistry, Nanchang University, Nanchang 330031, China

a r t i c l e

i n f o

Article history: Received 21 July 2008 Accepted 25 October 2008 Available online 5 November 2008 Keywords: MCM-41 Luminescence hybrid materials Covalently bonded Europium complex

a b s t r a c t A novel organo-functionalized mesoporous MCM-41 type of hybrid materials MCM–Si–DPBB was synthesized by co-condensation of bidentate Si(OR)3 substituted N,N-2-diphenyloxyphosphine-4-bromomethyl-benzenamine (Si–DPBB) and tetraethoxysilane (TEOS) in the presence of the cetyltrimethylammonium bromide (CTAB) surfactant as template. Its ternary europium complex covalently bonded to the silica-based network MCM–Si–DPBB–Eu was also prepared by introduction the Eu(DBM)3(H2O)2 into the hybrid materials. The hybrid material MCM–Si–DPBB–Eu has strong luminescence, and when excited by the ligands absorption wavelength (386 nm), it displays the emission of the Eu3+ 5D0-7FJ (J = 0, 1, 2, 3 and 4) transition lines due to the efficient energy transfer from the ligands to Eu3+. Ó 2008 Elsevier B.V. All rights reserved.

Lanthanide complexes have attracted much attention for their sharp, intense emission lines upon ultraviolet light irradiation [1]. However, the practical application of lanthanide complexes in optical devices is limited to a large extent by their poor thermal stability and low mechanical strength. To overcome these shortcomings, the complexes should be immobilized on a stable hybrid host such as ionic liquid [2], polymer [3,4] or inorganic material [5–10]. Recently, lanthanide organic–inorganic hybrid materials have a good option because the hybrid materials can not only enhance the photophysical properties of lanthanide complexes by interaction with the host structure, but also have relatively high thermal stability [5]. Based on the interaction among the different components or phases in hybrid systems, these hybrid systems can be divided into two major classes [11]. Class I is so-called physically mixed with weak interactions (hydrogen bonding, Van der Waals force or weak static effects) between the organic and inorganic phases; this method can lead to clustering of lanthanide complexes and hence a decrease of the luminescence intensity. Class II is named as chemically bonded with powerful covalent bonds linking the organic and inorganic parts, in which the hybrid materials are monophase even at a high concentration of lanthanide complexes, and the thermal stability and the photophysical properties of the complexes were improved by the matrices. The critical step to prepare Class II hybrid materials is to synthesize a novel bifunctional ligand as a covalent bridge which can not only coordinate lanthanide ions but also act as precursors of inorganic network. Some reported bifunctional ligands include modification * Corresponding authors. Tel.: +86 791 3969252; fax: +86 791 3969386. E-mail addresses: [email protected] (Q.-Y. Cao), [email protected] (X.-C. Gao). 1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2008.10.024

of b-diketones [8], aromatic carboxylic acids [9] and heterocyclic ligands [10]. It is well-known that phosphine oxides have good binding ability toward the lanthanide ions [12], and a group of lanthanide(III) complexes with phosphine oxides derivatives as the neutral ligands have been studied extensively [13,14]. Embert et al. have reported Si(OR)3 substituted monodentate phosphine oxides as precursors to prepare monophasic luminescence europium(III)containing hybrid materials [15]. Here, we present a simple synthesis of Eu(DBM)3(DPBB) (DBM is 1,3-diphenylpropane-1,3-dione and DPBB is N,N-2-diphenyloxyphosphine-4-bromomethyl-benzenamine) functionalized MCM-41 mesoporous hybrid material (MCM–Si–DPBB–Eu), in which DPBB is a novel bidentate bis(phosphino)amine oxide and was covalently bonded to the framework of MCM-41 by co-condensation of the Si(OR)3 substituted DPBB (denoted as Si–DPBB) and tetraethoxysilane (TEOS). Full characterization and detailed studies of the luminescence properties of all synthesized materials were investigated. The synthetic route for Si–DPBB and the hybrid materials MCM– Si–DPBB–Eu is shown in Scheme 1, and detailed synthesis process and structure characterization see Supplementary material. Firstly, novel bidentate phosphine oxides compound DPBB (3) was synthesized from commercially available p-toluidine and Ph2PCl by aminolysis then oxidation reaction. Compound 3 was brominated by N-Bromosuccinimide (NBS) and then reacted with 3-(triethoxysilyl) propan-1-amine, the bifunctional precursor Si–DPBB was obtained. The structures of DPBB and Si–DPBB have been characterized by 1H NMR, elemental analyses and IR spectrum. The presence of the (EtO)3Si–CH2CH2NH– group for Si–DPBB was confirmed by (i) the 1H NMR spectrum, which contains a broad res-

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Q.-Y. Cao et al. / Inorganic Chemistry Communications 12 (2009) 48–51

P

PPH2Cl

NH2

P

H2O2

N

N P

P

2

1

P

NBS Br

O

P

P

Eu(DBM)3

O

N

O O

N Br

P

O

O

Eu

O

O 3

3 (DPBB)

P N NH

P

6 (DPBB-Eu)

O

P

Eu(DBM)3

N

O

NH

O

P

O O

Eu

O O

O

Si O O

Si O O

3

4 (Si-DPBB) 7 (Si-DPBB-Eu)

MCM-41

O O Si O

N H

N

PO P

Eu(DBM)3 MCM-41

O

O O Si O

N H

P O Eu N O P

O O 3

8 (MCM-Si-DPBB-Eu)

5 (MCM-Si-DPBB) Scheme 1.

100

Intensity(a.u.)

onance at d = 2.00 for the N–H protons, and d = 1.24 and 3.20 ppm for –OCH2CH3; (ii) the IR spectrum, which shows new bands at 3310 cm1 for m(N–H), and the m(P@O) at 1209 cm1 (1210 cm1 for DPBB) still exists. The Bidentate phosphine oxides modified MCM-41 functional materials MCM–Si–DPBB was synthesized by reaction Si–DPBB with TEOS, CTAB in NH3  H2O solution by typical co-condensation protocol reported by Zhang and co-workers [10], with a molar ratio Si–DPBB:TEOS:CTAB:NH3  H2O:H2O = 0.04:1.0:0.139:3.76:66.57. Reaction the MCM–Si–DPBB with excess Eu(DBM)3(H2O)2 in ethanol solution then washing with acetone in order to eliminate the excess Eu(DBM)3(H2O)2 and dried at 60 °C under vacuum overnight obtain novel luminescence hybrid materials MCM–Si– DPBB–Eu. The Eu3+ content was found to be 1.4 wt.% by ICP-AES. For comparison, the mode compound Si–DPBB–Eu and DPBB–Eu were also prepared, and their structures were characterized by elemental analyses and IR spectrum. X-ray diffraction patterns of parent MCM-41 and the newly synthesized hybrid materials are shown in Fig. 1. All the materials exhibit three well-resolved diffraction peaks that can be indexed as (1 0 0), (1 1 0), and (2 0 0) reflections associated with 2D hexagonal symmetry, confirming a well-ordered mesoporous structure in these samples. Compared with the XRD pattern of pure MCM-41, the d100 spacing values of MCM–Si–DPBB and MCM–Si–DPBB–Eu are nearly unchanged, indicating that the ordered hexagonal mes-

A

110

200

B C

2

4

6

2θ (degree) Fig. 1. XRD profiles of MCM-41 (A), MCM–Si–DPBB (B) and MCM–Si–DPBB–Eu (C).

oporous structure of MCM-41 remains intact after introduction of organic ligand and Eu3+. The N2 adsorption and desorption isotherms of MCM-41, MCM– Si–DPBB and MCM–Si–DPBB–Eu measured at 77 K (not shown), and texture data are given in Table 1. They all display type IV iso-

Q.-Y. Cao et al. / Inorganic Chemistry Communications 12 (2009) 48–51

Table 1 Texture parameters of MCM-41 samples from N2 Isotherms at 77 Ka.

MCM-41 MCM–Si–DPBB MCM–Si–DPBB–Eu a

799 572 546

V (cm3  g1) 0.71 0.33 0.30

1.5

DBJH (nm)

1.2

3.5 2.9 2.7

SBET – the BET surface area, V – the pore volume, and DBJH – the pore diameter.

Absorption

SBET (m2  g1)

Sample

2.0

C

1.4

Absorption

50

1.0

C'

1.0 A'

0.5

A 0.8

B'

0.0 300

400

500

Wavelength (nm)

0.6 D

0.4 1050

0.2

C

Transmittance

B

Wavelength (nm) B

Fig. 3. Diffuse reflectance UV–Vis spectra of DPBB–Eu (A), MCM–Si–DPBB (B) and MCM–Si–DPBB–Eu (C).

1182 A 1209 3000

2500

2000

1500

1000

500

-1

Wavenumber( cm ) Fig. 2. IR spectra of Si–DPBB (A), Si–DPBB–Eu (B), MCM–Si–DPBB (C) and MCM–Si– DPBB–Eu (D).

therm curves with an H1 hysteresis loop at low relative pressure according to the IUPAC classification [16], characteristic of mesoporous materials with highly uniform size distribution. Compared with MCM-41, MCM–Si–DPBB and MCM–Si–DPBB–Eu have exhibit a smaller specific area and a slightly smaller pore size and pore volume, which might be due to the presence of organic ligand and the inclusion of the Eu3+ complex.

The incorporation of organic ligand DPBB and its Eu3+ complexes in the mesoporous MCM-41 framework can be confirmed by IR and UV–Vis absorption spectra (Figs. 2 and 3). The IR spectrum of Si–DPBB (Fig. 2 A) is dominated by m(Si–O) (1045 cm1) absorption bands characteristic of trialkoxysilyl functions, and the band located at 1209 cm1 can be ascribed to the vibration of m(P@O). While in the spectrum of its ternary Eu(III) complex Si–DPBB–Eu (Fig. 2B), the band attributed to m(P@O–Eu) correspondingly moves to 1182 cm1, which value are almost equal to m(P@O–Eu) (1179 cm1) in DPBB–Eu, suggesting the formation of coordinated bonds between the P@O group and Eu3+. A new peak appears at 1538 cm1, originating from Eu3+ coordinated with DBM, can also be observed in Si–DPBB–Eu. When the organic ligand and its Eu3+ complex are anchored in MCM-41 (Fig. 2C and 2D), a broad band at about 1050 cm1, which characterized the formation of the Si–O–Si framework [17], appears. In addition, the vibration mode centered at 1590, 1547, 1538 and 1388 cm1, originating from Si–DPBB groups and DBM groups, can also be observed both in Si–DPBB–Eu and MCM–Si–DPBB–Eu.

Fig. 4. The excitation (left) and emission (right) spectra of DPBB–Eu (A), Si–DPBB–Eu (B) and MCM–Si–DPBB–Eu (C).

Q.-Y. Cao et al. / Inorganic Chemistry Communications 12 (2009) 48–51

The UV–Vis spectra of DPBB–Eu (A), MCM–Si–DPBB (B) and MCM–Si–DPBB–Eu (C) in solid state and their model compound Si–DPBB (B0 ) and Si–DPBB–Eu (C0 ) in dry EtOH solution are shown in Fig. 3. From Fig. 3, we can see that Si–DPBB (B0 ) and hybrid materials MCM–Si–DPBB (B) have only weak absorption spectra in the UV–Vis region, while their ternary europium complexes show characteristic absorption of ligand DBM p–p* electronic transition with bands at 268 nm and 386 nm for MCM–Si–DPBB–Eu and 245 nm and 355 nm for Si–DPBB–Eu, respectively. Compare to the pure complex DPBB–Eu, the major bands of MCM–Si–DPBB– Eu are shifted toward shorter wavelengths (from 304 to 268 nm, and from 408 to 386 nm, respectively), while bands of its model compound Si–DPBB–Eu are shifted toward longer wavelengths in solution (the major bands from 355 nm to 358 nm). The blue shift confirms that the rare-earth complex is incorporated into the MCM-41 channels, which influences its corresponding absorption. A similar phenomenon was also observed for the other modified MCM-41 materials [18]. In general, UV–Vis and IR spectra indicate that the Eu3+ is present in the coordinated form covalently bond to the MCM-41 network. Fig. 4 gives excitation and emission spectra of DPBB–Eu (A), Si– DPBB–Eu (B) and MCM–Si–DPBB–Eu (C) in solid state. The excitation spectra of these materials were all obtained by monitoring the strongest emission wavelength of the Eu3+ ions at 612 nm. As shown in Fig. 4A, the excitation spectrum of the pure DPBB–Eu complex exhibits a broad excitation band between 300 and 410 nm (kex = 374 nm), which can be assigned to the p–p* transition of the ligands, and its excitation spectrum in dry EtOH solution is at 381 nm with 7 nm blue shift (see Fig. 4 insert). Compared with the pure DPBB–Eu complex, the excitation bands of Si–DPBB–Eu (Fig. 4B) and MCM–Si–DPBB–Eu (Fig. 4C) become narrower, and the maximum excitation wavelength shifts from 374 to 356 and 354 nm, respectively. The blue shift of the excitation bands upon introduction of Eu3+ complex into the mesoporous material MCM-41 is due to a hypsochromic effect resulting from the change in the polarity of the environment surrounding the europium complex in the mesoporous material [18,19]. All the three materials show typical five 5D0-7FJ (J = 0, 1, 2, 3 and 4) emission peaks at 577, 589, 612, 650, and 705 nm, respectively, where the 5D0-7F2 is a typical electric dipole transition and strongly varies with the local symmetry of Eu3+ ions, while the 5D0-7F1 transition corresponds to a parity-allowed magnetic dipole transition, which is practically independent of the host material. Among these transitions, the 5D0-7F2 show the strongest emission, and the presence of local-ligand field splitting of the 5D0-7F1, 2 transition (3 and 3 Stark components for MCM–Si–DPBB–Eu as an example, see magnified figure in Fig. 4 marked with arrows) suggesting the chemical environment around the Eu3+ ions is in low symmetry without an inversion centre. The emission quantum yield of all the three compounds is measured by literature method [20,21] with values 0.56, 0.37 and 0.17 for MCM–Si–DPBB–Eu, DPBB–Eu and Si–DPBB–Eu, respectively. The highest emission intensity of MCM–Si–DPBB–Eu in these samples suggests that the MCM-41 is a good host for decreasing clustering of DPBB–Eu, because the luminescence quenching of Eu3+ ion can be effectively decreased in this host. The lowest intensity for Si–DPBB–Eu (Fig. 4B) is probably due to the introduction of

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three flexibility –OCH2CH3 groups [22], which may increase the structure relaxation in the excited state. In conclusion, by employing MCM-41 as the surfactant together with TEOS and Si–DPBB as silica source, we have prepared a novel mesoporous silica MCM-41 hybrid materials covalently bonded with europium complex. The luminescence spectra results show that Si-DPBB-Eu functionalized MCM-41 exhibits efficient narrow bandwidth emission of red light with high spectral purity. Acknowledgement This work was supported by the Program for Innovative Research Team of Nanchang University.

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