Preparation and Luminescence Properties of Organically Modified

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Chem. Mater. 1999, 11, 3171-3176

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Preparation and Luminescence Properties of Organically Modified Silicate Composite Phosphors Doped with an Europium(III) β-Diketonate Complex Huihui Li, Satoshi Inoue, Ken-ichi Machida,* and Gin-ya Adachi* Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1Yamadaoka, Suita, Osaka 565-0871, Japan Received April 29, 1999. Revised Manuscript Received July 30, 1999

The phenanthroline adduct of the tris(2-thenoyltrifluoroaceto-O,O′)europium(III) complex, Eu(TTA)3phen, was doped into organically modified silicate (ORMOSIL) matrixes via the sol-gel process, and the luminescence properties of the resultant ORMOSIL composite phosphors (ORMOSIL:Eu(TTA)3phen) were characterized. The emission intensity of the composite phosphors maximized at ∼50% vs the commercially available lamp phosphor Y(P,V)O4:Eu, and transparent ORMOSIL:Eu(TTA)3phen composite phosphor disks (45 mm in diameter by 1.5 mm) were obtained under appropriate complex concentration and matrix composition. Moreover, the emission intensity of the composite phosphors was found to be maintained at the same level even after standing for up to 180 days in air, but lowered after heat treatments (100-300 °C), possibly due to the transformation of the β-diketonate ligand from the photoactive π electron-conjugated enolate to the corresponding nonphotoactive ketone form. In particular, the ORMOSIL:Eu(TTA)3phen composite phosphor powders treated with (CH3)3SiNHSi(CH3)3 (hexamethyldisilazane, HMDS) showed a remarkable increase in emission intensity, owing to the improved water repellency resulting from the implantation of the -OSi(CH3)3 (trimethylsilyl substituent: TMS) in the ORMOSIL composites and the favorable reconversion of the ligand from the nonconjugated β-diketone to the photoactive conjugated enolate form as induced by the NH3 evolved during the TMS modification process. A composite phosphor with high emission intensity (∼70% vs Y(P,V)O4:Eu) was obtained after the modification.

Introduction The sol-gel process has been of great interest and has been noted as an excellent candidate for the preparation of inorganic-organic composite materials in recent years.1-5 In particular, sol-gel-derived organically modified silicate (ORMOSIL) host materials were found to have the advantage of easy realization of versatility in optical and mechanical properties by employing various kinds of the alkoxides as starting materials and/or optimizing their compositions; moreover, the obtained ORMOSIL materials possess matrix properties which combine both inorganic and organic characters.6 Thus, many organic dyes, which are susceptible to high-temperature degradation, have been incorporated into sol-gel-derived host materials under mild conditions and various inorganic-organic hybrid materials that have promising optical properties have * To whom all correspondence should be addressed. Phone: +816-6789-7353. Fax: + 81-6-6789-7354. E-mail: adachi@ ap.chem.eng.osaka-u.ac.jp. (1) Kobayashi, Y.; Imai, Y.; Kurokawa, Y. J. Mater. Sci. Lett. 1988, 7, 1148. (2) Knobbe, E. T.; Dunn, B.; Fuqua, P. D.; Nishida, F. Appl. Opt. 1990, 29, 2729. (3) Gvishi R.; Reisfeld, R. J. Non-Cryst. Solids 1991, 128, 69. (4) Jin, T.; Tsutsumi, S.; Deguchi, Y.; Machida, K.; Adachi, G. J. Electrochem. Soc. 1995, 142, 195. (5) Yanagi, H.; Hishiki, T.; Tobitani, T.; Otomo, A.; Mashiko, S. Chem. Phys. Lett. 1998, 292, 332. (6) Lintner, B.; Arfsten, N.; Dislich, H.; Schmidt, H.; Philipp, G.; Seiferling, B. J. Non-Cryst. Solids 1988, 100, 378.

been prepared, aimed at the development of solid tunable laser devices,7-10 nonlinear optics,11 and holeburning materials.12 Apart from organic dyes, recently lanthanide complexes, such as [Eu(phen)2]Cl3 and [Tb(bpy)2]Cl3, have also been incorporated into the ORMOSIL matrixes according to the sol-gel method.13,14 The obtained ORMOSIL composites have been found to show strong red or green emission and have been applied to amorphous15,16 and crystalline16 silicon solar cells to enhance their photovoltaic output. However, the emission intensity of powdered ORMOSIL:[Eu(phen)2]3+ and ORMOSIL:[Tb(bpy)2]3+ composite materials was not maintained at the initially high level and gradually (7) Altman, J. C.; Stone, R. E.; Nishida, F.; Dunn, B. Sol-Gel Optics II. SPIE 1992, 1758, 507. (8) Rahn, M. D.; King, T. A.; Capozzi, C. A.; Seddon, A. B. Sol-Gel Optics III. SPIE 1994, 2288, 364. (9) Reisfeld R.; Gvishi, R.; Burshtein, Z. J. Sol-Gel Sci. Technol. 1995, 4, 49. (10) Law, H.; Tou, T.; Ng, S. Appl. Opt. 1998, 37, 5694. (11) Kim, J.; Plzawsky, J. L.; Van Wagenen, E.; Korenowski, G. M. Chem. Mater. 1993, 5, 1118. (12) Kulikov, S. G.; Veret-Lemarinier, A. V.; Galaup, J. P.; Chaput, F.; Boilot, J. P. Chem. Phys. 1997, 216, 147. (13) Jin, T.; Tsusumi, S.; Deguchi, Y.; Machida, K.; Adachi, G. J. Electrochem. Soc. 1996, 143, 3333. (14) Jin, T.; Inoue, S.; Tsutsumi, S.; Machida, K.; Adachi, G. J. NonCryst. Solids 1998, 223, 123. (15) Jin, T.; Inoue, S.; Tsutsumi, S.; Machida, K.; Adachi, G. Chem. Lett. 1997, 171. (16) Jin, T.; Inoue, S.; Machida, K.; Adachi, G. J. Electrochem. Soc. 1997, 144, 4054.

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decreased with standing in air, owing to the moisture absorbed from the surrounding atmosphere and consequentially increased probability of nonradiative multiphonon decay caused by the stretching vibration of the -OH groups. In addition, much attention has been directed recently to water-repellent materials, such as hydrophobic aerogels17 and thin films18-20 which have important applications in heat insulators and in coatings for glasses and for windshields of automobiles. Particularly, the TMS modification, where -OH groups are substituted by -SiO(CH3)3 and hence increased hydrophobicity of the modified materials is achieved, has been proved to be an excellent chemical approach to increase the waterrepellency of sol-gel-derived materials.17 It has been reported that the moisture resistance was drastically increased while the transparency and density of the trimethylsilyl modified silica aerogel (TMSA) was maintained after such modification. This fact naturally leads to the following idea: the decrease in the emission intensity of the ORMOSIL composites, as caused by the moisture absorbed from the air and the consequential aggravated nonradiative multiphonon relaxation through -OH groups, might be effectively depressed by the TMS modification. Lanthanide β-diketonate chelates have long been known to give intense emission lines upon UV light irradiation, because of the effective intramolecular energy transfer from the coordinating ligands to the central lanthanide ions, which in turn undergo the corresponding radiative emitting process (“the antenna effect”).21 In particular, some complexes in this category have been noted to show laser action in solutions;22-24 therefore, they are expected to be promising luminescent dopants for the preparation of hybrid phosphors and other optical sources. However, to date, most of the research concerning lanthanide β-diketonate chelates have been mainly concentrated on the solid complexes themselves or their solutions,25-31 and although recently a few research groups have carried out some studies on the synthesis and fluorescence properties of β-diketonate complexes incorporated into solid matrixes,32,33 many (17) Yokogawa, H.; Yokoyama, M. J. Non-Cryst. Solids 1995, 186, 23. (18) Langroudi, A. E.; Mai, C.; Vigier, G.; Vassoille, R. J. Appl. Polym. Sci. 1997, 65, 2387. (19) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 1040. (20) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 3213. (21) Sato, S.; Wada, M. Bull. Chem. Soc. Jpn. 1970, 43, 1955. (22) Samelson, H.; Lempicki, A.; Brophy, V. A.; Brecher, C. J. Chem. Phys. 1964, 40, 2547. (23) Bjorklund, S.; Kellermeyer, G.; Hurt, C. R.; McAoy, N.; Filipescu, N. Appl. Phys. Lett. 1967, 10, 160. (24) Whittakker, B. Nature, 1970, 228, 157. (25) Samelson, H.; Lempicki, A.; Brecher, C.; Brophy, V. A. Appl. Phys. Lett. 1964, 5, 173. (26) (a) Sager, W. F.; Filipescu, N.; Serafin, F. A. J. Phys. Chem. 1965, 69, 1092. (b) Filipescu, N.; Sager, W. F.; Serafin, F. A. J. Phys. Chem. 1964, 68, 3324. (27) Samelson, H.; Brecher, C.; Lempicki, A. J. Mol. Spectrosc. 1996, 19, 349. (28) Dutt, N. K.; Sur, S.; Rahut, S. J. Inorg. Nucl. Chem. 1971, 33, 1717. (29) Bu¨nzli, J. G.; Moret, E.; Foiret, V.; Schenk, K. J.; Wang, M.; Jin, L. J. Alloys Compd. 1994, 207/208, 107. (30) Uekawa, M.; Miyamoto, Y.; Ikeda, H.; Kaifu, K.; Nakaya, T. Synth. Met. 1997, 91, 259. (31) Tsaryuk, V. I.; Zolin, V. F.; Kudryashova, V. A. Synth. Met. 1997, 91, 357.

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aspects of the preparation of inorganic-organic hybrid materials incorporating lanthanide β-diketonate complexes, as well as the luminescence properties and practical applications of such composite materials have yet to be studied. In the present work, the ORMOSIL composite phosphors doped with the phenanthroline adduct of the tris(2-thenoyltrifluoroaceto-O,O′)europium(III) complex, Eu(TTA)3phen, were prepared via the conventional solgel process, and the luminescence and thermal properties of the ORMOSIL:Eu(TTA)3phen composite phosphors were investigated. In addition, the TMS modification was performed on the ORMOSIL:Eu(TTA)3phen composite phosphors. The variation in the emission intensity was interpreted mainly from the viewpoint of the nonradiative decay process through the -OH groups and the structural transformation of the β-diketonate ligand. Experimental Section Appropriate amounts of organic ligands, β-diketone, and 1,10-phenanthroline (phen) of a molar ratio of 3:1, were dissolved in ethanol, and the pH value of the resultant solution was adjusted to 6-7 with NH(CH2CH2OH)2. A stoichiometric amount of EuCl3 to the ligands was then dissolved in ethanol and added dropwise to the above solution. After the solution was stirred at ambient temperature overnight, the resultant precipitate was filtered from the mixture and washed with ethanol. Finally, the europium(III) complex was obtained after drying in a vacuum for 5 h without further purification. The composition of the complex was checked by elemental analysis. The ORMOSIL composite phosphors doped with the β-diketonate complex Eu(TTA)3phen were prepared as follows: After mixtures of tetraethyl orthosilicate (TEOS), triethoxylphenylsilane (TEPS), tetrahydrofuran (THF), EtOH, and H2O in a molar ratio of x:(1 - x):4:2:4 (x ) 0.2-0.8) were refluxed for 1 h to give homogeneous sol solutions, appropriate amounts of a DMF solution of the complex were added until the desired concentrations were obtained (1-5 mol % vs silane). The complex-containing sol solutions were subsequently cast into covered translucent plastic cups and cured at 50 °C for 5-14 days to accomplish their gelation. Solidified ORMOSIL composite phosphors were obtained after further aging for 7-12 days. Additional heat treatments were carried out at various temperatures (100-300 °C) for 5 h in air. In addition, the TMS modifications were performed according to the following steps: The bulk composites samples were ground into powders (particle size: ∼50 µm in diameter) and added into a (CH3)3SiNHSi(CH3)3 ethanol solution (10-20 wt %). The mixtures were then stirred for 24 h at room temperature. The modified powder samples were filtered off and dried in air. The fluorescence properties of the complex and its ORMOSIL composites were measured on a Hitachi F-4500 fluorescence spectrophotometer, equipped with a xeon lamp as a light source. Emission and excitation spectra were measured in the “front face” orientation, and all spectra were corrected. The emission intensity values were normalized to that of the commercially available lamp phosphor Y(P,V)O4:Eu (Nichia Chemical Industries, Japan) by comparing the integrated area of the emission bands (570-720 nm) recorded under the optimum excitation wavelength for each sample with that of the above standard phosphor. FT-IR spectra were recorded on a JAS FT/IR-430 spectrophotometer by the KBr method, and transmittance spectra were recorded on a Shimadzu UV 2200 double-beam spectrophotometer. (32) Matthews, L. R.; Knobbe, E. T. Chem. Mater. 1993, 5, 1697. (33) Matthews, L. R.; Wang, X.; Knobbe, E. T. J. Sol-Gel Sci. Technol. 1994, 2, 627.

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Table 1. Luminescence Properties of EuL3phen Complexes EuL3phen

Ex. (nm)a

Em. (nm)b

emission intensity (%) vs Y(P, V)O4:Eu

Eu(TTA)3phenc Eu(DBM)3phend Eu(BTFA)3phene Eu(FTA)3phenf Eu(BAC)3pheng

374 373 367 382 368

613 613 613 613 613

89.6 49.5 66.7 41.5 33.1

a Wavelength corresponding to the maximum excitation peak. Wavelength corresponding to the maximum emission peak. c TTA: Thenoyltrifluoroacetone. d DBM: Dibenzoylmethane. e BTFA: Benzoyltrifluoroacetone. f FTA: Furoyltrifluoroacetone. g BAC: Benzoylacetone. b

Figure 2. Emission intensity for the ORMOSIL:Eu(TTA)3phen (TEOS/TEPS ) 6/4) phosphors incorporated with (a) 1 mol % and (b) 2 mol % of Eu(TTA)3phen as a function of the matrix composition.

Figure 1. Photograph of the ORMOSIL:Eu(TTA)3phen composite phosphor (TEOS/TEPS ) 6/4, 1 mol % of Eu(TTA)3phen) taken under UV irradiation.

Results and Discussion A series of Eu(III) adduct complexes with β-diketonate ligands and 1,10-phenanthroline (EuL3phen) were synthesized, and their luminescence properties were measured. The obtained results are summarized in Table 1. It can be seen that the Eu(TTA)3phen complex possesses the highest emission intensity as well as the best thermal stability among them as confirmed by thermal gravimetry (TG), and hence this β-diketonate complex was selected as the candidate to be incorporated into the ORMOSIL matrixes for the preparation of Eu(III) β-diketonate complex-containing ORMOSIL composite phosphors. The Eu(TTA)3phen complex was incorporated into the ORMOSIL matrixes with a wide range of matrix compositions via the sol-gel process. In general, the complexdoped ORMOSIL composite phosphors as obtained were pale yellow or yellow, and this was dependent on the concentration of the Eu(TTA)3phen complex. Moreover, it was found that bulk composite phosphor disks with high transparency were obtained when the TEOS/TEPS ratio was within the intermediate range of 0.5-0.6, while cracked composite samples were produced if the ORMOSIL matrixes were derived from mixtures with a high content of either TEOS or TEPS as starting materials to form the ORMOSIL networks. A photograph of the ORMOSIL:Eu(TTA)3phen composite material (TEOS/TEPS ) 6/4, Eu(TTA)3phen ) 1 mol %) taken under UV light irradiation is shown in Figure 1. It is obvious that the phosphor disk is transparent, and

although not shown, it gives out an intense red emission upon UV light irradiation. In addition, the emission intensity of the ORMOSIL composite phosphors with various matrix compositions doped with 1 or 2 mol % of the Eu(TTA)3phen complex were measured, and the relative emission intensities vs the composition of the ORMOSIL:Eu(TTA)3phen materials are plotted in Figure 2. The emission intensity tended to increase with the content of the organic component forming the ORMOSIL matrixes. This is due to the fact that a more homogeneous dispersion of the complex molecules may take place in the ORMOSIL matrix when the TEPS component, which forms a flexible ORMOSIL network, increases. Since the Eu(TTA)3phen complex is extremely hydrophobic, it is supposed that the compatibility between the organically modified matrix network and the hydrophobic Eu(TTA)3phen complex dopant will become better when the amount of the TEPS component for the preparation of the ORMOSIL phosphors increases, and thus the Eu(TTA)3phen molecules doped in the ORMOSIL matrix will be more effectively protected from aggregation. As a result, the nonradiative decay caused by the energy transfer between the neighboring Eu(TTA)3phen molecules should be decreased. ORMOSIL:Eu(TTA)3phen phosphor bulk samples were ground into powders and their emission intensity was monitored as a function of time. The emission intensities of the parent Eu(TTA)3phen complex and the ORMOSIL:Eu(TTA)3phen composite powders were found to maintain the same value even after exposure in air for up to 180 days, as shown in Figure 3. This observation is in distinct contrast with [Eu(phen)2]Cl3 and its ORMOSIL composite powder samples, which usually demonstrate a remarkable decrease in emission intensity on standing in air.14 The main reason for this discrepancy is ascribed to a kind of “hydrophobic insulating wall”, consisting of the organic ligands coordinating with the central Eu3+ ion, which effectively prevent the Eu(TTA)3phen molecules against water solvation. As a result, the probability of the nonradiative multiphonon relaxation of the Eu(TTA)3phen complex and its ORMOSIL:Eu(TTA)3phen composite phosphors through H2O was far less than that of the ORMOSIL phosphors doped with [Eu(phen)2]Cl3, which tends to absorb moisture from the surrounding environment and

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Figure 3. Time dependencies for the relative emission intensity of (a) Eu(TTA)3phen and (b) ORMOSIL:Eu(TTA)3phen composite phosphor (TEOS/TEPS ) 6/4, 3 mol % of Eu(TTA)3phen).

Figure 4. Temperature dependencies (solid line) for the emission intensity and TG curve (dashed line) of Eu(TTA)3phen.

consequently suffers enhanced nonradiative decay through -OH groups. The typical temperature dependency for the relative emission intensity of the Eu(TTA)3phen complex is shown in Figure 4, together with the TG curve. In addition, those of the ORMOSIL: Eu(TTA)3phen (TEOS/ TEPS ) 6/4) composite materials are presented in Figure 5. As can be seen in Figure 4, the emission intensity of the Eu(TTA)3phen complex holds at the original value in the temperature region up to 200 °C, while no emission is observed after heat treatment for 5 h at 250 °C, owing to the thermal decomposition of the complex as supported by the TG analysis (dashed line, in Figure 4). On the other hand, the emission intensity of ORMOSIL:Eu(TTA)3phen composite phosphors gradually decreases with an elevation in the heattreatment temperature through the whole temperature range, and it is considered that the interaction between the complex molecules and their surrounding local chemical environment may play a significant role in determining the emission intensity of the ORMOSIL: Eu(TTA)3phen composite materials. Even though the pure Eu(TTA)3phen complex tends to maintain its molecular structure and hence the emission intensity is maintained after heat treatment until thermal decomposition takes place, the situation should be much more complicated when the complex is incorporated into host matrixes, owing to the interaction between the

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Figure 5. Temperature dependencies for the emission intensity of the ORMOSIL:Eu(TTA)3phen (TEOS/TEPS ) 6/4) phosphors incorporated with (a) 1 mol % and (b) 2 mol % of Eu(TTA)3phen.

complexes and the surrounding environment both during the evolution of the sol-gel solutions and the subsequent further solidification process to form the three-dimensional network of the ORMOSIL matrix. It was also found that the emission intensity of the ORMOSIL:Eu(TTA)3phen composite powders considerably increased after immersing in a dilute ammonia solution. For instance, a relative emission intensity of 57% vs Y(P, V)O4:Eu was observed for the treated ORMOSIL:Eu(TTA)3phen (3 mol %, TEOS/TEPS ) 6/4) composite powder while the untreated sample gave an intenisity of only 44%. As is well-known, β-diketone ligands can generally exist in either the keto or enol form, and in most cases, the keto form is far more favorable than the enol one.34 Moreover, lanthanide ions are expected to coordinate with only β-diketone ligands existing in the π electron-conjugated and photoactive enolate form, which is formed in the presence of a base.32 Thus, it is possible that during the heat treatment and preparation process of the ORMOSIL composite phosphors as well, some of the Eu(TTA)3phen complexes may decompose owing to the transformation of the β-diketonate ligands from the enolate to the neutral keto form, since the solvents and the acid silanol groups might provide protons to some of the enolate ligands existing in their vicinities. Conversely, the treatment of the composite phosphor powders by ammonia solution leads to the reversion of the above transformation process and results in the increase of the emission intensity. It should be noted that the red emission is still observed on the composite samples under UV light irradiation even after heat treatment for 5 h at 300 °C. The luminescence spectra of the ORMOSIL composite phosphor (TEOS/TEPS ) 6/4) doped with 1 mol % of Eu(TTA)3phen after a series of heat treatments at various temperatures are shown in Figure 6. The broad band ascribed to the π-π* electron transition29 of the organic ligands is observable in the excitation spectrum of the sample even after the heat treatment at 300 °C, while no appreciable absorption peak (394 nm) assigned to the Eu3+ ions is observed in the spectrum profile. In addition, similar emission spectra consisting of four intense emission lines, peaking at 591 nm (5D0-7F1), (34) Reid, J. C.; Calvin, M. J. Am. Chem. Soc. 1950, 72, 2948.

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Figure 7. Emission intensity of the ORMOSIL:Eu(TTA)3phen (TEOS/TEPS ) 6/4) phosphors incorporated with various amount of Eu(TTA)3phen (1-5 mol %) before (solid line) and after (dashed line) the TMS modification.

Figure 6. Luminescence spectra of the ORMOSIL:Eu(TTA)3phen (TEOS/TEPS ) 6/4, 1 mol % of Eu(TTA)3phen) phosphor after various heat treatments at (a)100 °C, (b) 200 °C, and (c) 300 °C, for 5 h in air.

613 nm (5D0-7F2), 652 nm (5D0-7F3), and 703 nm (5D0-7F4), which are characteristic of the Eu3+ ions can be obviously observed in all of the emission spectra of the samples after various heat treatments. Therefore, the luminescence characteristics are concluded to originate from Eu(TTA)3phen, and the thermal resistance of the complex is improved after its incorporation into the ORMOSIL matrixes. The TMS modification was carried out for the ORMOSIL:Eu(TTA)3phen composite phosphor powders by using (CH3)3SiNHSi(CH3)3, and the luminescence properties of the obtained modified samples were characterized. The emission intensity profiles of the ORMOSIL: Eu(TTA)3phen phosphor powders before and after the TMS modification as a function of the Eu(TTA)3phen concentration are shown in Figure 7. It can be seen that the emission intensity obviously increased after the modification for all of the samples, especially for those with the higher Eu(TTA)3phen concentrations. Increases by 53% and 40% were obtained for the ORMOSIL composite powders doped with 3 and 5 mol % of Eu(TTA)3phen, as referred to the initial emission intensity values of the unmodified samples, respectively. In comparison with a maximum emission intensity of ∼50% vs Y(P,V)O4:Eu obtained for the untreated ORMOSIL:Eu(TTA)3phen (5 mol %) phosphor, the emission intensity of the same composite phosphor powder after the TMS modification maximized at ∼70%. The increase in the emission intensity is mainly attributed to the substitution of the -OH groups by -Si(CH3)3 in the ORMOSIL phosphor powders, which results in a substantial decrease of the nonradiative multiphonon decay as caused by the -OH groups. This is well supported by the FT-IR measurement, as shown in

Figure 8. FT-IR spectra of the ORMOSIL:Eu(TTA)3phen composite phosphors (TEOS/TEPS ) 6/4, 3 mol % of Eu(TTA)3phen): (a) before and (b) after the TMS modification.

Figure 8. The absorption peaks ascribed to the -OH stretching vibration A (∼3400 cm-1) and in-plane bending mode B (∼1650 cm-1) as well as the Si-OH symmetrical stretching vibration D (∼960 cm-1) are greatly weakened, while peak C (∼1080 cm-1) assigned to the Si-O-Si symmetrical stretching mode is enhanced after the TMS modification, strongly suggesting the effective substitution of the -OH groups by -Si(CH3)3 in the composite powders. Besides, the structural transformation of those β-diketone ligands from the keto form to the enolate one is possibly another significant factor which promotes the increase in the emission intensity, since such β-diketone molecules might perform the reconversion process via reaction with NH3, which is released during the reaction of HMDS with the -OH groups of the ORMOSIL composites,17 in a similar manner as being treated with dilute ammonia solution as stated above. Conclusions A series of ORMOSIL composite materials doped with Eu(TTA)3phen, showing strong red emission characteristic of Eu3+ ions under UV light irradiation, were produced, and they could be obtained as transparent

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ORMOSIL:Eu(TTA)3phen composite phosphor disks by optimizing the preparation conditions. The emission intensity of the ORMOSIL:Eu(TTA)3phen phosphors is independent of the standing time in air even for the powdered samples, since hydrophobic organic ligands such as TTA are tightly surrounding the central Eu3+ ions. However, the emission intensity gradually decreased with an elevation in the heat-treatment temperature, ascribed to the probable structural transition from the π-electron-conjugated photoactive enolate to the keto form of the β-diketonate ligands for the Eu(TTA)3phen complexe dispersed in the ORMOSIL matrixes. In contrast to this, a remarkable increase in the emission intensity was observed on the ORMOSIL: Eu(TTA)3phen composite samples after treatment with HMDS and intense emission intensity as high as ∼70% vs Y(P,V)O4:Eu was achieved. This intensity increase can be mainly attributed to the substitution of the -OH

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groups by the -OSi(CH3)3 ones and the consequentially decreased probability of nonradiative multiphonon relaxation through the -OH groups, together with the possible reconversion of the ligands from keto to the photoactive enolate form via reaction with NH3. The TMS modification with HMDS is therefore considered to be an effective approach for property improvement of inorganic-organic composite materials such as ORMOSIL:Eu(TTA)3phen. Acknowledgment. This work was mainly supported by “Research for the Future” Program, “Photoscience” from the Japan Society for the Promotion of Science and Grant-in-Aid for Scientific Research nos. 09229235, 10123217, and 10874093 from the Ministry of Education, Science, Sports, and Culture of Japan. CM990251A