Sol–gel approach to the novel organic–inorganic hybrid composite films with ternary europium complex covalently bonded with silica matrix

Materials Chemistry and Physics 95 (2006) 89–93

Sol–gel approach to the novel organic–inorganic hybrid composite films with ternary europium complex covalently bonded with silica matrix Dewen Dong a,b,∗ , Yongsheng Yang c , Bingzheng Jiang b b

a Department of Chemistry, Northeast Normal University, Changchun 130024, PR China Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China c Heilongjiang Institute of Sports Technology, Haerbin 150001, PR China

Received 31 January 2005; received in revised form 6 May 2005; accepted 5 June 2005

Abstract Novel organic–inorganic hybrid composite films with ternary lanthanide complex covalently bonded with silica matrix were prepared in situ via co-ordination of N-(3-propyltriethoxysilane)-4-carboxyphthalimide (TAT) and 1,10-phenanthroline (Phen) with europium ion (Eu3+ ) during a sol–gel approach and characterized by the means of spectrofluorimeter, phosphorimeter and infrared spectrophotometer (FTIR). The resulting transparent films showed improved photophysical properties, i.e. increased luminescence intensity and longer luminescence lifetime, compared with the corresponding binary composite films without Phen. All the results revealed that the intense luminescence of the composite film was attributed to the efficient energy transfer from ligands, especially Phen, to chelated Eu3+ and the reduced non-radiation through the rigid silica matrix and “site isolation”. © 2005 Elsevier B.V. All rights reserved. Keywords: Europium complex; Luminescence; Organic–inorganic composite; 1,10-Phenanthroline; Sol–gel approach

1. Introduction Optically active lanthanide ions (noted as Ln3+ , e.g. Eu3+ and Tb3+ ) within various insulating hosts have long been used in a broad spectrum of applications, including phosphors for fluorescence lighting and display monitors, solid-state lasers and amplifiers for fiber-optic communication [1,2]. The popularity of Ln3+ stems from their unique luminescence characteristics, i.e. high colorimetric purity and long excitedstate lifetime, based on their electronic transitions between their 4f energy levels [3,4]. However, these applications up to date are almost limited to inorganic solids. Organic lanthanide complexes are excluded from such application due to their poor values for thermal stability, resistance to moisture and mechanical performance. Recently, considerable efforts have been devoted towards the development of sol–gel-derived luminescence materials

∗

Corresponding author. Tel.: +86 431 5099759; fax: +86 431 5098966. E-mail address: [email protected] (D. Dong).

0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.06.005

containing lanthanide complexes [5–11]. Extensive work in this field has revealed that the inorganic matrix protects, at least to a certain extent, lanthanide complexes from moisture and mildly high temperature. Indeed sol–gel process involves in an inorganic polymerisation, which is synthetically based on hydrolysis/condensation reactions of metal alkoxide [12,13]. Its mild conditions, especially low temperature processing, make it possible to incorporate organic moieties, lanthanide complexes, for example, into inorganic network. Moreover, the properties of the final product can be tailored through rationally designing the chemical structure of sol–gel precursor, modifying the preparation conditions or varying the composition of components [14,15]. The sol–gel process has proven to be a convenient route in development of the organic–inorganic luminescence materials containing lanthanide complex. However, it is worth noting that lanthanide complexes in the above studies are commonly introduced into gels by physically dispersing them into the sol or pre-condensed oligomeric silica and then trapping them with gelation. The doped complexes in many cases leach from the matrix during

D. Dong et al. / Materials Chemistry and Physics 95 (2006) 89–93

90

Scheme 1.

the preparation since their molecular size is small enough, compared with the typical pore size of the sol–gel network, to readily diffuse through the matrix. Moreover, the resulting composites become brittle, translucent or opaque when the dopant concentration is increased to a critical value (several wt%) [8,9]. Most recently, some researchers reported organic– inorganic hybrid composites with lanthanide complex covalented to the inorganic matrix [16–18]. Chemical links were introduced between the organic ligand of the lanthanide complex and the silica matrix, which led to the elimination of many of the above-mentioned problems. In our previous work [16], we designed and synthesized a silylated ligand, N(3-propyltriethoxysilane)-4-carboxyphthalimide (TAT, see Scheme 1) that combined the roles of sol–gel precursor with co-ordination donor of lanthanide ions in the same molecule. Through this precursor, a series of nano-structured binary lanthanide complex films were further prepared in situ via a sol–gel approach. As part of further investigations, we wish to report the preparation of ternary europium complexes by the co-ordination of TAT and 1,10-phenanthroline (Phen) with Eu3+ during a sol–gel process with the expectation of obtaining intensely luminescent materials. Meanwhile, an organic ligand N-propyl-4-carboxyphthalimide (NP, see Scheme 1) and its ternary europium complex with Phen were synthesized for comparison.

2. Experimental

[19b,20]. A typical procedure was described as following: 3.05 mmol of NP and 1.05 mmol Phen were dissolved in 50 mL ethanol. The pH value of the solution was adjusted to 6.5 by adding aqueous sodium hydroxide slowly under stirring. Then, 10 mL EuCl3 alcoholic solution (0.1 M) was added to the above solution. After being stirred and refluxed for 3 h, the reaction system was cooled down to ambient temperature. A white solid was precipitated from the system, which was washed with water and ethanol, dried at ambient temperature. The resulting product was identified by elemental analysis, further confirmed by FTIR and 1 H NMR spectra. The spectra data and elemental analytical data are described below: 1 H NMR (400 MHz, DMSO-d , 25 ◦ C): δ[ppm] 9.19(2 H, 6 s, H-Phen), 8.65(2 H, s, 2 H, s, H-Phen), 8.12(2 H, s, H-Phen), 7.86(2 H, s, H-Phen), 7.34(1 H, s, ArH), 6.80(1 H, s, ArH), 6.66(1 H, s, ArH), 3.49(2 H, t, NCH2 , J 7.6 Hz), 1.62(2 H, q, CH2 , 7.6 Hz), 0.89(3 H, t, CH3 , J 7.6 Hz). IR (KBr): 1773(νs C O ), 1717(νa C O ), 1554(νa COO ), 1394(νs COO ) cm−1 . Elemental analyses (data in bracket were calculated for Eu(NP)3 Phen·2H2 O): C, 53.87 (54.14); H, 4.02 (3.98); N, 6.46 (6.55); Eu, 14.13 (14.27). 2.3. The preparation of Eu3+ -doped hybrid composite films A series of hybrid composite films with ternary lanthanide complex covalently bonded to the silica matrix were prepared from TAT, Phen and Eu3+ via a sol–gel process. A typical procedure is described below: an appropriate amount of Phen, Eu3+ solution in N,N -dimethylformamide (DMF) and deionized H2 O (acidified to 0.15 M HCl) were added to a TAT solution in DMF. The feed molar ratio of Eu3+ /Phen/TAT varied from 1/1/3, 1/1/10, 1/1/102 , 1/1/103 to 1/1/104 . The molar ratio of H2 O/TAT was 2:1. With stirring, the mixture formed a homogeneous solution immediately. The solution was spin-coated on a quartz slide as substrate, which was then kept at room temperature for 2 days, to form a wet gel film. The composite film was obtained after the wet gel was treated at 60 ◦ C for about 1 week. The composite film from TAT only was also prepared with the same procedure.

2.1. The preparation of organic ligands TAT and NP 2.4. Measurements The sol–gel precursor TAT and its model compound NP were synthesized according to literatures [16] and [19], respectively. Their structures were identified by elemental analysis, FTIR and 1 H NMR spectra, all of which were in good agreement with those in the literatures. 2.2. The preparation of ternary europium complex Eu(NP)3 Phen·2H2 O The ternary complex of Eu3+ with aromatic acid NP and Phen was prepared according to a published method

The elemental analysis was carried out with a Carioerba 1106 elemental analyzer. FTIR measurements were conducted with a Perkin-Elmer 1616 series FTIR spectrometer. Fluorescence spectra were measured on a SPEX FL-2T2 spectrofluorimeter (slit = 0.1 mm) and fluorescence decays were detected with a SPEX 1934D phosphorimeter (pulse with 3 ␮m). 1 H NMR spectra were recorded on a Bruker DRX spectrometer (400 MHz) using DMSO-d6 as solvent and tetramethylsilane as an internal standard.

D. Dong et al. / Materials Chemistry and Physics 95 (2006) 89–93

91

Fig. 2. The fluorescence spectra of ternary europium complex Eu(NP)3 Phen·2H2 O.

Fig. 1. The IR spectra of: (a) TAT and (b) Eu3+ /Phen/TAT (molar ratio: 1/1/3).

3. Results 3.1. IR spectra Fig. 1 shows the IR spectra of the composite film from: (a) TAT and (b) Eu3+ /TAT/Phen, respectively. For TAT composite, there are four strong absorption bands appearing at 1774, 1718, 1370 and 730 cm−1 . These characteristic bands relating to imide groups could be assigned to symmetric carbonyl stretching vibrations, asymmetric carbonyl stretching vibrations, C N stretching and ring carbonyl deformation, respectively. The strong absorption band at 1100 cm−1 is assigned to the Si O Si asymmetric stretching vibration indicating the formation of inorganic network based on the siloxane bonds [17a]. Meanwhile, a middle strong absorption band can be detectable at 965 cm−1 related to asymmetric stretching vibrations of Si OH. In the IR spectra of Eu3+ /TAT/Phen, the absorption bands in the range of 950–1200 cm−1 are still there which suggest the inorganic network also formed as in TAT film. However, the broad carbonyl absorptions of TAT at 2300–2700 cm−1 present in Fig. 1a are absent in Fig. 1b, whereas several new strong absorption bands are present at 1656 cm−1 (νa coo− ) and 1383 cm−1 (νs coo− ). These could be interpreted as supporting evidence for the conversion of carboxyl groups into carboxylate anions as a result of the formation of co-ordination bonds between aromatic acid TAT and Eu3+ [16]. 3.2. Luminescence In this work, N-propyl-4-carboxyphthalimide and its europium ternary complex with Phen was synthesized as model compound. The composition of the complex was

established based on the elemental analysis results and further confirmed by FTIR and 1 H NMR spectra. The ternary europium complex was investigated by fluorescence spectra, as shown in Fig. 2. The excitation spectra of the ternary complex were recorded by monitoring the 5 D → 7 F transition at 612 nm (Fig. 2, left). The spectra 0 2 are dominated by a broad and asymmetric excitation band at 230–430 nm, which is assigned to the absorption of the ␲–␲* transitions based on the conjugated double bond of aromatic ligands. The absorption at 395 and 468 nm are detected and attributed to 7 F0 → 5 L6 and 7 F0 → 5 D2 transitions of the Eu3+ 4f6 intra-shell, respectively. However, these absorption peaks are weak compared to those relative to the organic ligands, which suggests that the luminescence sensitization via excitation of ligands would be much more than the direct excitation of the Eu3+ own absorption. The emission spectra were recorded under UV excitation at 303 nm (Fig. 2, right). A group of emissions based on the 5 D0 → 7 Fj (j = 0, 1, 2, 3 and 4) transitions are observed, appearing at 579, 593, 612 (620), 650 (663) and 684 (700) nm, respectively. The most predominant emission is based on 5 D0 → 7 F2 transition, which splits into two strong peaks appearing at 612 and 620 nm. Such observation suggests that the Eu3+ ion lie in a site of low symmetry [8b,17a]. Apparently, the characteristic luminescence indicates that the surrounding aromatic ligands can absorb energy and transfer energy efficiently to the chelated Eu3+ . For the Eu3+ -doped composite films, the excitation spectra were recorded by monitoring the 5 D0 → 7 F2 transition at 617 nm. The spectra are dominated by a broad and asymmetric excitation band at 230–430 nm, which is assigned to the absorption of the ␲–␲* transitions based on the conjugated double bond of aromatic ligands, both TAT and Phen. The emission spectra were recorded under UV light excitation at 326 nm (Fig. 3). Consequently, a group of emissions are observed, which result from the relaxation from the first 5 D0 excited state of Eu3+ to the first five levels of 7 Fj (j = 0–4) ground state (Table 1). The most predominant emission is based on 5 D0 → 7 F2 transition occurring at 617 nm with the half-width of less than 10 nm. The characteristic “europium

D. Dong et al. / Materials Chemistry and Physics 95 (2006) 89–93

92

Table 1 Luminescence properties of Eu(NP)3 Phen·2H2 O and composites Eu3+ /Phen/TAT (1a–1e) Sample

Eu(NP)3 Phen·2H2 O Composite 1a Composite 1b Composite 1c Composite 1d Composite 1e

Composition molar ratioa

1/1/3 1/1/10 1/1/102 1/1/103 1/1/104

Emission lines (nm) 5D

0

579 580 580 580 580 580

→ 7 F0

5D 0

→ 7 F1

593 594 594 594 594 594

5D

0

→ 7 F2

612 (620) 617 617 617 617 617

5D

0

→ 7 F3

650 (663) 652 652 652 652 652

5D 0

Lifetimeb (ms)

Intensityc (%)

0.38 0.86 0.84 0.79 (0.68) 0.72 (0.56) 0.67 (0.41)

100 73.7 (50.4) 58.5 (36.3) 40.2 (21.8) 26.3 (10.9) 9.6 (2.7)

→ 7 F4

684 (700) 700 700 700 700 700

a

Molar ratio of Eu3+ /Phen/TAT. The values in bracket for the corresponding Eu3+ /TAT composites with the same Eu3+ molar concentration. c The emission intensity values are normalized to that of Eu(NP) Phen·2H O by comparing the integrated area of its emission bands (570–720 nm), the 3 2 values in bracket for the corresponding Eu3+ /TAT composites with the same Eu3+ molar concentration. b

Fig. 4. The fluorescence intensity decay curve of composite 1a. Fig. 3. Fluorescence emission spectra of hybrid organic–inorganic composites 1a–1e (from top to bottom), λex = 326 nm.

red” luminescence indicates that the surrounding aromatic ligands absorb energy and transfer energy efficiently to the chelated Eu3+ . The strong emission intensity and narrow half emission width represent the excellent luminescence characteristics of the resulting hybrid films. From the emission spectra, it has also been observed that the emission intensities decrease as the Eu3+ concentration lowers due to dilution effect. The decay curves of the resulting Eu3+ -doped composites were measured as shown in Fig. 4. All the decay curves fit a single-exponential law, which reveals that all Eu3+ ions lie in the same average environment, namely there is only one luminescent substance that exists in the sample. The luminescence lifetimes are calculated based on the decay curves as listed in Table 1.

4. Discussion For the ease of comparison, we listed some results from our previous work in Table 1. It is not difficult to note that

the composites of Eu3+ /TAT/Phen exhibit prolonged lifetime and much stronger emission intensities compared with the corresponding composites of Eu3+ /TAT (with the same Eu3+ concentration). The results implied that Phen, as a co-ligand, co-ordinate with Eu3+ in the composite system like TAT. Moreover, Phen might play a predominant role in the process of energy absorption and efficiency transfer to the chelated Eu3+ . Indeed, it is well known that some strongly absorbing organic ligands, such as Phen, are used to improve the intensity of lanthanide complex luminescence. Extensive work on this area have revealed that organic ligands has the dual effect of both protecting chelated lanthanide ions from luminescence quenching process and increasing light absorption cross section by the “antenna effect” [5–7]. From the above results, it is not difficult to find the differences between the model ternary complex and the Eu3+ doped composite film. In the case of the composites, the organic ligands absorption bands become narrow and the Eu3+ absorption has diminished, whereas the emission peaks become sharper without shoulder peaks compared with that of the model complex. Moreover, the composites exhibit much longer lifetime. The luminescence differences most

D. Dong et al. / Materials Chemistry and Physics 95 (2006) 89–93

properly stem from the environmental changes around the chelated lanthanide ion, Eu3+ . All Eu3+ ions of the model complex lie in the same average environment and its ligands are confined in the fine ordered crystal skeleton of complex. The broad absorption band and multi-peak emissions of the complex are due to the different vibration absorption of organic ligands and crystal lattice. However, Eu3+ ions of the composite are isolated through the carboxyl groups of the sol–gel precursor and further confined strictly by the rigid silica network. Due to the “cage effect” of silica network, some vibrations of ligands and crystal lattice might diminish or disappear completely, which results in the smooth excitation band and fewer emission peaks. Also, the “cage effect” makes contributions to the much longer lifetime of the composite. It is well known that the emission of lanthanide complex is based on the energy transfer from the absorbing organic ligands to the chelated lanthanide ions [21,22]. Under excited state, the vibrations of ligands and crystal lattice may redirect some energy to non-radiation transitions, which may decrease the fluorescence lifetime and quantum yield. Therefore, some other researchers attributed the long lifetime of lanthanide complex in silica to the restriction of the ligand vibrations by relatively rigid structure of silica matrix [8,9]. The unique chemical structure of TAT leads to the formation of one-component hybrid organic–inorganic materials with chemical links between organic ligands and silica matrix, which makes the restriction effect even much stronger in our studied system. 5. Conclusion In summary, a series of novel organic–inorganic hybrid composite films with ternary lanthanide complex covalently bonded with silica matrix were prepared in situ via coordination of TAT and Phen with Eu3+ during a sol–gel approach. The resulting transparent and crack-free composite films provided strong red emission even at low concentration of Eu3+ , e.g. 0.1 mol%. The intense luminescence was attributed to the efficient energy transfer from ligands, especially, Phen, to chelated Eu3+ and the reduced non-radiation through the rigid silica matrix and “site isolation”. Further investigations on morphology, luminescence mechanism and light emission devices of the hybrid films are in progress.

93

References [1] E. Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications, John Wiley & Sons, NY, 1994. [2] J.S. Wilkinson, M. Hempstead, Curr. Opin. Solid State Mater. Sci. 2 (1997) 194. [3] S. Hufner, Optical Spectra of Transparent Rare Earth Compounds, Academic Press, New York, NY, 1978. [4] N.S. Sabbatini, M. Guardigli, M.J. Lehn, Coord. Chem. Rev. 123 (1993) 201. [5] (a) L.R. Matthews, E.T. Knobbe, Chem. Mater. 5 (1993) 1697; (b) L.R. Matthews, X. Wang, E.T. Knobbe, J. Sol–Gel Sci. Technol. 2 (1994) 627; (c) C. Sanchez, B. Lebeau, F. Chaput, J.-P. Boilot, Adv. Mater. 15 (2003) 1969. [6] O.A. Serra, E.J. Nassar, M.E.D. Zaniquelli, I.L.V. Rosa, J. Alloys Compd. 207/208 (1994) 454. [7] L.-L. Lee, D.-S. Tsai, J. Mater. Sci. Lett. 13 (1994) 615. [8] (a) B. Yan, H. Zhang, S. Wang, J. Ni, Mater. Chem. Phys. 51 (1997) 92; (b) B. Yan, H. Zhang, S. Wang, J. Ni, J. Photochem. Photobiol. A Chem. 112 (1998) 231. [9] (a) T. Jin, S. Tsutsumi, Y. Deguchi, K. Machida, J. Electrochem. Soc. 42 (1995) 195; (b) T. Jin, S. Tsutsumi, Y. Deguchi, K. Machida, J. Alloys Compd. 252 (1997) 59; (c) T. Jin, S. Inoue, S. Tsutsumi, K. Machida, G. Adachi, J. NonCryst. Solids 223 (1998) 123. [10] (a) V. Bekiari, G. Pistolis, P. Lianos, Chem. Mater. 11 (1999) 3189; (b) V. Bekiari, P. Lianos, Adv. Mater. 10 (1998) 1455. [11] H. Li, S. Inoue, K. Machida, G. Adachi, Chem. Mater. 11 (1999) 3171. [12] L.L. Hench, J.K. West, Chem. Rev. 90 (1990) 33. [13] E.T. Knobbe, B. Dunn, P.D. Fuqua, F. Nishida, Appl. Opt. 29 (1990) 2729. [14] J. Wen, G.L. Wilkes, Chem. Mater. 8 (1996) 1667. [15] P. Tien, L.K. Chau, Chem. Mater. 11 (1999) 2141. [16] D. Dong, S. Jiang, Y. Men, X. Ji, B. Jiang, Adv. Mater. 12 (2000) 646. [17] (a) A.C. Franville, D. Zambon, R. Mahiou, Y. Troin, Chem. Mater. 12 (2000) 428; (b) A.C. Franville, D. Zambon, R. Mahiou, S. Chou, Y. Troin, J.C. Cousseins, J. Alloys Compd. 275 (1998) 831. [18] H. Li, H. Zhang, H. Li, L. Fu, Q. Meng, Chem. Commun. 13 (2001) 1212. [19] (a) D. Dong, Y. Men, S. Jiang, X. Ji, B. Jiang, Mater. Chem. Phys. 70 (2001) 249; (b) D. Dong, B. Jiang, Mater. Chem. Phys. 78 (2002) 501. [20] J.W. Moore, M.D. Gilick, W.A. Baker Jr., J. Am. Chem. Soc. 94 (1972) 1858. [21] G.A. Crosby, J. Chem. Phys. 349 (1961) 743. [22] S. Sato, M. Weda, Bull. Chem. Soc. Jpn. 43 (1970) 1955.