Synthesis and photophysical properties of rare earth-containing luminescent silicone resin from cooperative molecular design and assembly

Journal of Non-Crystalline Solids 356 (2010) 1581–1586

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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

Synthesis and photophysical properties of rare earth-containing luminescent silicone resin from cooperative molecular design and assembly Xiaochen Wang, Haifeng Lu, Hua Wang, Shengyu Feng ⁎ Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, PR China School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China

a r t i c l e

i n f o

Article history: Received 23 November 2009 Received in revised form 27 April 2010 Available online 22 June 2010 Keywords: Photophysical property; Luminescence; Silicone resin; Sol–gel process

a b s t r a c t Two series of rare earth-containing luminescent silicone resins have been achieved by sol–gel method. The precursors which were modified via acylamidation reactions behave as structural molecular bridges. The silanol groups were used to form the host of silicone resin and the chromophore groups were used to exhibit the luminescence by the cooperative assembly courses with rare earth ions (Eu3+, Tb3+, and Dy3+). The modifications were determined by Fourier transform infrared (FTIR), 1H-NMR spectra and diffuse reflectance ultraviolet–visible spectra (DRUVS). Scanning electronic microscopy (SEM, micrometric scale) and X-ray diffraction studies (XRD, nanometric scale) were employed to evaluate the frameworks of silicone resin materials. Narrow-width red emission was observed in Eu (III) resin materials and green emissions were obtained in Tb (III) resin materials and Dy (III) resin materials, indicating that an efficient intramolecular energy transfer took place in these resins. Further investigations into the luminescent properties of these materials show that the luminescence in these materials is quite operative by means of the design of molecular structure. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction Silicone resin materials combining diverse unique properties of building parts have been applied in the domains of important advanced materials [1–4]. These resin materials present improved mechanical and thermal stabilities provided by inorganic segments and show multifunctional characters that came from organic parts [5]. On the basis of previous research, there are two typical classes of luminescent resin materials in terms of interaction among organic and inorganic components [1]. The first class [6–9] is mechanical mixed materials which could cluster the emitting centers through weak interactions (van der Waals force, hydrogen bonds, etc). However, fluorescence quenching easily occurs in these materials and hence limits their utilities. Furthermore, there exist an inhomogeneous dispersion phenomenon and a lower energy transition process among central and host moieties [10–15]. It is satisfied that the second class of resin materials can meet the requirements mentioned above. These resins link different constituents with covalent bonds and tailor them to be a monophasic system [1]. This modification can overcome the defects of the first class with the synergy of combination and the unique role of inner interfaces. In view of their long-live excited-state characteristic and efficient strong

⁎ Corresponding author. Tel.: + 86 531 88364866; fax: + 86 531 88564464. E-mail address: [email protected] (S. Feng).

narrow-width emission band in the visible region [16–19], rare earth ions (Eu3+, Tb3+, and Dy3+, for example) are prior to be used to construct luminescent resin materials. Previously the sol–gel-derived matrixes attracted many interests, which were composed of aromatic carboxylic acids, β-diketones and heterocyclic ligands tethering rare earth ions [20–22]. With the explosive development of the rare earthcontaining resin materials, it is especially important to investigate the relations between the molecular structure and the luminescent properties, namely to find the guideline for the different possible synthetic strategies. Sol–gel route has been regarded as the typical approach for the preparation of resin materials [23–25], due to shaping easily and processing at a low temperature. This method can obtain the anticipant silica-based or siloxane-based resin materials which blend with rare earth complexes by hydrolysis and polymerization. Moreover, non-hydrolyzable Si–C covalent bonds which graft rare earth complexes to precursor backbone improved the properties of organic–inorganic resin materials [26–34]. Here we propose cooperative molecular design and assembly of rare earth complexes by way of chelate effect of organic ligands. Benzoyl chloride (PhCOCl, abbreviated as A) and acetyl chloride (CH3COCl, abbreviated as B) are selected as original agent separately. They are modified via acylamidation reaction with cross-linking molecular N-β-aminoethyl-γ-aminopropyl-methyldimethoxysilane, (H2N(CH2)2HN(CH2)3SiCH3(OCH3)2, abbreviated as AEP) to derive organosilane precursors which could be chelated to rare earth ions. Then the precursors were complexed with rare earth ions in sol–gel

0022-3093/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.059

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process and hydrolyzed to obtain resin materials. The photophysical properties, texture and assembly structure of resin materials were investigated in detail. Moreover the relations between photophysical properties and coordinated acylamido ligands containing diverse chromophoric conjugate groups were discussed to find a guideline for the design of rare earth-containing luminescent resin materials. 2. Experimental section 2.1. Chemicals and procedures All the starting materials and solvents were purchased from China National Medicines Group and were distilled before utilization according to published procedures [35,36]. The rare earth oxides were purchased from China National Medicines Group. Europium (III) nitrates, terbium (III) nitrates and dysprosium (III) nitrates were obtained from the corresponding oxides in dilute nitric acid. 2.2. Preparation of precursors 2.2.1. Synthesis of A-AEP The precursors were prepared as follows (Scheme 1): AEP (1.236 g, 6 mmol) was first dissolved in 15 mL of pyridine under stirring and then A (0.843 g, 6 mmol) was added to the solution by drops. The mixture was refluxing at 70 °C for 4 h. After the precipitation was filtered, the solution was condensed to evaporate the solvent and then the residue was dried on a vacuum line. Yellow oil was afforded. Yield: 92%. A-AEP: C15H26N2O3Si: 1H-NMR (CDCl3): δ7.49 (d, 2H, j1-phenylH–), 7.41 (d, 1H, k1-phenyl-H–), 7.38 (d, 2H, i1-phenyl-H–), 7.86 (s, 1H,

h1-phenyl-CONH–), 3.76 (s, 6H, t1-OCH3), 3.28 (t, 2H, g1-CH2–), 3.17 (t, 2H, f1-CH2–), 2.94 (t, 2H, d1-CH2–), 1.95 (s, IH, e1-N–H–), 1.63 (m, 2H, c1-CH2–), 0.64 (t, 2H, b1-CH2–), 0.15 (t, 3H, a1-Si–CH3). 2.2.2. Synthesis of B-AEP AEP (1.236 g, 6 mmol) was dissolved in 15 mL pyridine under stirring and then B (0.471 g, 6 mmol) was added to the solution by drops. The mixture was refluxing at 70 °C for 4 h. After the precipitation was filtered, the solution was condensed to evaporate the solvent and then the residue was dried on a vacuum line. Yellow oil was afforded. Yield: 94%. B-AEP: C15H26N2O3Si: 1H-NMR (CDCl3): δ6.75 (s, 1H, h2-CH3– CONH–), 3.51 (s, 6H, t2-OCH3), 3.02 (t, 2H, g2-CH2–), 2.90 (t, 2H, f2CH2–), 2.77 (t, 2H, d2-CH2–), 2.09 (s, 3H, i2-CH3), 1.86 (s, 1H, e2-N–H–), 1.61 (m, 2H, c2-CH2–), 0.63 (t, 2H, b2-CH2–), 0.15 (t, 3H, a2-Si–CH3). 2.3. Preparation of acylamido-functionalized resin materials 0.6 mmol of precursor (A-AEP or B-AEP) and 1.2 mmol of tetraethoxysilane (TEOS) were dissolved in 5 mL of ethanol with stirring. The mixture was agitated magnetically to achieve a single phase in a covered Teflon beaker for 4 h, and then 10 mL of water was added under gentle magnetic stirring for an hour. After that, it was dried on a vacuum line at 60 °C immediately. After aged until the onset of gel occurred, the gels were collected for the photophysical property studies. The white powder resin material from benzoyl chloride (A for short) was named resin I-A with yield of 94%. The white powder resin I-B with yield of 96% was from acetyl chloride (B for short). 2.4. Preparation of luminescent resin materials According to the above preparation of acylamido-functionalized silicone resin material, by using 0.6 mmol of precursor A-AEP (or 0.6 mmol of precursor B-AEP), 0.2 mmol of rare earth nitrate (Eu (NO3)3·6H2O, Tb(NO3)3·6H2O or Dy(NO3)3·6H2O) and 1.2 mmol of tetraethoxysilane (TEOS) as starting agent. The mixture was agitated magnetically in a covered Teflon beaker for 4 h, and then 10 mL of water was added under gentle magnetic stirring for an hour. After that, it was dried on a vacuum line at 60 °C immediately. After aged until the onset of gel occurred, the gels were rare earth-containing resin materials. They were white powders and named resin II-A-RE (III) (yields of 90%) and resin II-B-RE (III) (yields of 90%), respectively. 2.5. Measurements Fourier transform infrared (FTIR) spectra were recorded on a Bruker TENSOR27 infrared spectrophotometer with the KBr pellet technique within the 4000–400 cm− 1 region. Proton Nuclear Magnetic Resonance (1H NMR) spectra were determined in CDCl3 on a BRUKER AVANCE-400 spectrometer without internal reference. Diffuse reflectance ultraviolet–visible spectra (DRUVS) were performed with a Shimadzu UV-2500. Luminescence (excitation and emission) spectra were measured with a Perkin-Elmer LS-55 spectrophotometer and the excitation and emission slits were 5 and 2.5 nm, respectively. Scanning electronic microscopy (SEM, micrometric scale) images were obtained by JEOL JSM-7600F. The X-ray diffraction (nanometric scale) measurements were carried out on powdered samples via a “BRUKER D8” diffractometer (40 mA_40 kV) using monochromated Cu Kα1 radiation (λ = 1.54 Å) over the 2θ range of 10° to 70°. 3. Results

Scheme 1. The preparation procedures for precursors of A-AEP and B-AEP.

The IR spectra for AEP (a), modified precursors A-AEP (b) and B-AEP (c), resin II-A-Dy (III) (d) and resin II-B-Dy (III) (e) are shown in Fig. 1.

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Curve (a) shows the stretching vibration peaks of N–H at 3372 cm− 1 and 3302 cm− 1, and a bending vibration peak of N–H at 1593 cm− 1 [37]. Different from the IR spectrum of AEP, the double N–H absorption peaks turn to single peak. 3269 cm− 1 for N–H stretching vibration and

1577 cm− 1 for N–H bending vibration appear in (b) A-AEP. There are 3264 cm− 1 for N–H stretching vibration and 1549 cm− 1 for N–H bending vibration in B-AEP (c). The C O stretching vibration peaks appear at 1635 cm− 1 in A-AEP (b) and 1637 cm− 1 in B-AEP (c). It is verified that the –CONH– groups formed via acylamidation reactions. Meanwhile, the absorption peaks of C–H in phenyl group appear at 3057 cm− 1 in A-AEP (b). The stretching vibration (ν Si–C) appears around 1262 cm− 1 in above curves. Compared with precursors A-AEP (b), the ν (C O) vibration peak is observed at 1620 cm− 1 in resin II-A-Dy (III) (d). The coordination between the oxygen atoms in C O group and the metallic ion can be proved by the shift (from 1635 cm− 1 to 1620 cm− 1, Δ = 15 cm− 1). Meanwhile, the C O stretching vibration peak appears at 1624 cm− 1 in resin II-B-Dy (III) (e), while it appeared at 1637 cm− 1 in B-AEP (c). It proves the coordination between oxygen atoms in C O group and the metallic ion in resin II-B system. This coordination provides an energy transfer between the ligands and the rare earth ions [6]. Furthermore, the stretching vibration (ν Si–C, 1262 cm− 1) still exists in the IR spectra of resin materials, corresponding to the fact that no (Si–C) bond split happens during the hydrolysis and condensation reactions [33,34]. The broad absorption band at 1120–1000 cm− 1 (ν Si–O–Si) indicates the formation of siloxane bonds. Fig. 2 shows the diffuse reflectance ultraviolet–visible spectra (DRUVS) of resin I-A (a), resin II-A-Tb (III) (b), resin I-B (c) and resin II-B-Tb (III) (d). In curve (a), the absorption peak from 220 nm to 260 nm corresponded to the π→π* electronic transition of the benzoylamido group [37]. In curve (b), the absorption band was enlarged slightly which almost completely overlapped from 200 nm to 300 nm, which could also be assigned to the transition of the benzoylamido group [38]. The differences may come from the rare earth ions, indicating that there exists the interaction between the rare earth ions and the benzoylamido group. The similar phenomenon can also be observed between curve (c) and curve (d) in Fig. 2. And it can be noticed that the absorption of acylamido group in system A is higher compared to system B in UV range. The excitation spectrum of resin II-A-Eu (III) (b) exhibits a broad excitation bands centered at 260 nm in the UV range, while the others show poor excitation characters (λem = 617 nm, Fig. 3). The corresponding overlap between DRUVS spectra and excitation spectra exhibits the effective sensitization of rare earth ions by benzoylamido groups. The strong absorption of benzoylamido groups and the energy transfer from benzoylamido groups to rare earth ions result in the intense emission of rare earth ions [38].

Fig. 2. Diffuse reflectance ultraviolet–visible spectra of resin I-A (a), resin II-A-Tb (III) (b), resin I-B (c) and resin II-B-Tb (III) (d).

Fig. 3. The excitation spectra of resin I-A (a), resin II-A-Eu (III) (b), resin I-B (c) and resin II-B-Eu (III) (d) (λem = 617 nm).

Fig. 1. Infrared spectra of AEP (a), modified precursors of A-AEP (b) and B-AEP (c), resin II-A-Dy (III) (d) and resin II-B-Dy (III) (e).

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Fig. 6. Emission spectra of dysprosium (III) resin materials: resin II-A-Dy (III) (a) and resin II-B-Dy (III) (b) (λex = 300 nm). Fig. 4. Emission spectra of terbium (III) resin materials: resin II-A-Tb (III) (a) and resin II-B-Tb (III) (b) (λex = 300 nm).

The luminescence behaviors of all the materials have been investigated at 298 K by direct excitation of the ligands. Representative emission spectra are given in Figs. 4–6. It is a narrow-width red emission of europium (III) resin materials. The narrow-width green emissions are observed in terbium (III) resin materials and dysprosium (III) resin materials. Fig. 4 illustrates typical photoluminescence spectra of terbium (III) resin material (λex = 300 nm). Narrow-width emission bands with maxima at 487 nm and 545 nm are recorded. These bands are related to the f–f transition and attributed to the 5D4→7F6 and 5D4→7F5 transitions of Tb3+ ions. Fig. 5 illustrates photoluminescence spectra of europium (III) resin materials (λex = 270 nm). The maxima of these bands are at 580, 590 and 616 nm which is associated with 5D0→7F0, 5 D0→7F1 and 5D0→7F2 transitions of Eu (III). The emission bands of dysprosium (III) resin materials (λex = 300 nm, Fig. 6) were assigned to the 4F9/2→6H15/2 (484 nm) and 4F9/2→6H13/2 (578 nm) transitions of Dy3+. 4. Discussion

Fig. 5. Emission spectra of europium (III) resin materials: resin II-A-Eu (III) (a) and resin II-B-Eu (III) (b) (λex = 270 nm).

Carbonyl compounds have been proved to be good chelating groups to sensitize the luminescence of rare earth ions [6]. Generally, the mechanism can be described as antenna effect: the ligands reinforce the energy absorbability and transfer energy to the metal ion with high efficiency. Then the emission from the rare earth ions' excited state will be observed [12]. A very strong emission before 400 nm (Fig. 4(b)) was detected, which could be associated to fluorescence from excited single state of the acylamido group. However, this emission band is not observed in the resin II-A-Tb (III) compounds (Fig. 4(a)). It is indicated that energy transfer process from the lowest triplet state of the benzoylamido group in resin II-A to the rare earth ions is very efficient. The similar situations are also observed in europium (III) ions containing resin materials and dysprosium (III) ions containing resin materials, which further confirm that the benzoylamido groups in resin II-A materials

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are better functional ligand to sensitize the luminescence of rare earth ions than the acetylamido groups in resin II-B materials. This indicates that the luminescence of the rare earth-containing resin materials could be operated by variating the acylamido groups. Indeed, it is well-known that the f–f transitions of the rare earth ions are very weak. In order to obtain a good luminescent material, one has to take advantage of the strong absorbing capacity of acylamido ligands with π-electrons, and the possibility of transferring the excitation energy from the triplet states of the ligands to the higher energy levels of the 4fn configuration of the rare earth ion [39]. The resin II-A-RE (III) has a superior luminescent performance in comparison with resin II-B-RE (III). A major improvement is to select more suitable acylamido group. The scanning electron micrographs (SEM) of europium (III) resin materials (resin II-A-Eu (III) (a) and resin II-B-Eu (III) (b) in Fig. 7) demonstrate that homogeneous materials were obtained. No phase separation was observed because of strong covalent bonds bridging between the inorganic and organic phases. In addition, it is interesting to note that many large pores are dispersed in these materials, which mainly formed in the sol–gel process. When aerogel is formed by dried over a vacuum line, the pore narrowing by capillary attraction is excluded, and so large pores volume is formed without damaging the solid part [40]. The X-ray diffraction figures of resin I-A (a), resin II-A-Eu (III) (b) and resin I-A doped with europium (III) nitrates (c) are shown in Fig. 8. The diffractogram of the resin materials reveals that all of the resin materials with 10° ≤ n ≤ 70° are totally amorphous. For resin I-A (a), there is only a broad peak centered 22° while there are some narrow peaks protrude from the baseline in the curve of resin II-A-Eu (III) (b). To exclude the impact of rare earth nitrates, resin I-A doped

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Fig. 8. The X-ray diffraction spectra of resin I-A (a), resin II-A-Eu (III) (b), resin I-A doped with europium (III) nitrates (c), resin I-B (d), resin II-B-Eu (III) (e) and resin I-B doped with europium (III) nitrates (f).

with europium (III) nitrates according to the ratio of resin II-A-Eu (III) is prepared and its XRD spectrum (c) is measured. Between (a) and (c), the major differences take place around 10° which mean that the peaks for europium (III) nitrates are located about 10° and simply mix could not lead to a total amorphous. Comparing (b) with (c), peaks of europium (III) nitrates and the narrow peaks that appeared in (c) are all different. From the spectra, it is clear that neither free europium (III) nitrates salt nor crystalline rare earth complexes occurs throughout the range of those materials. The absence of any crystalline regions in these resin samples correlates well with the conclusion which came from the scanning electron micrographs. The similar situation was also observed among resin I-B (d), the resin II-B-Eu (III) (e) and resin I-B doped with europium (III) nitrates (f) in Fig. 8. 5. Conclusions In summary, two series of rare earth ion centered luminescent resin materials were achieved via sol–gel process. It is anticipated that this homogeneous and porous resin material with strong covalent bonds will attract interest for its utilization in optical or electronic applications. Further investigations into the luminescence properties of these materials show that the benzoylamido group is quite suitable for sensitization of rare earth ions. And the luminescent performance in these materials is quite operative. Acknowledgements This work was supported by the China Postdoctoral Science Foundation Funded Project (20080441135) and National Natural Science Foundation of China (20574043 and 20874057). References

Fig. 7. Scanning electronic microscopy images of cooperative molecular resins: resin II-A-Eu (III) (a) and resin II-B-Eu (III) (b).

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