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ELSEVIER Journal of Non-Crystalline Solids 223 (1998) 123-132 IOURNAI~ OF ltll Luminescence properties of lanthanide complexes incorporated into sol-gel derived inorganic-organic composite materials Tetsuro Jin, Satoshi Inoue, Shuji Tsutsumi, Ken-ichi Machida *, Gin-ya Adachi Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565, Japan Received30 December1996;revised 19 August 1997 Abstract Lanthanide complexes such as [Tb(bpy)2]C13 and [Eu(phen)2]C13 incorporated into the organically modified silicates (ormosil) were prepared and their luminescence properties measured. The ormosil composite materials incorporated with [Tb(bpy)2]C13 and [Eu(phen)2]C13 had green and red emissions and, by optimization of the complex concentration amounts and heat treatment conditions, they provided emission intensities with 100 and 80%, respectively, of those for conventional phosphors used as lamp illumination. The resulting composite materials also possessed excellent mechanical strength, hydrolytic stability, and, consequently good durability for the emissions from the lanthanide compositions after being exposed to laboratory atmosphere for 100 days. © 1998 Elsevier Science B.V. 1. Introduction Light emitting e.g. photoluminescence (PL), elec- troluminescence (EL) and thermoluminescence (TL) devices activated by the lanthanide ions such as Eu 3+ and Tb 3÷ provide emissions with good effi- ciency [1-3] by the intramolecular 4f electron transi- tions. These have been used as illumination lamps and cathode ray tube (CRT) displays, because of their colorimetric purity. However, the materials made for the purpose of the practical uses, e.g., phosphors [4-7] or laser devices [8], are almost limited to inorganic solids and the metal complexes or organometallic compounds consisting of lan- * Correspondingauthor. Tel: +81-6 879 7353; fax: +81-6 876 4754; e-mail: [email protected]. thanide ions have been excluded from such applica- tions although they also have good luminescence properties for use as phosphors. The lanthanide complexes and organometallic compounds emit when in aqueous or organic solu- tions and these emissions have been investigated [9-11]. The lanthanide ions, e.g. Eu 3+ and Tb 3÷ form stable crystalline complexes with heterocyclic ligands such as 1,10-phenanthroline (phen) and 2,2- bipyridine (bpy) which have efficient energy trans- fers from the coordinated ligands to the chelated lanthanide ions [12-14]. The potential applications of these compounds are based on the fact that they can absorb ultraviolet light in the organic portion of molecules and produce emissions in the red and green regions of the spectrum with good efficiency via the subsequent intramolecular energy transfers [15-17]. However, the thermal stability and the me- 0022-3093/98/$19.00 © 1998 ElsevierScienceB.V. All rights reserved. Pll S0022-3093(97)00425-0

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Page 1: Luminescence properties of lanthanide complexes incorporated into sol-gel derived inorganic-organic composite materials

ELSEVIER Journal of Non-Crystalline Solids 223 (1998) 123-132

IOURNAI~ OF

ltll

Luminescence properties of lanthanide complexes incorporated into sol-gel derived inorganic-organic composite materials

Tetsuro Jin, Satoshi Inoue, Shuji Tsutsumi, Ken-ichi Machida *, Gin-ya Adachi Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565, Japan

Received 30 December 1996; revised 19 August 1997

Abstract

Lanthanide complexes such as [Tb(bpy)2]C13 and [Eu(phen)2]C13 incorporated into the organically modified silicates (ormosil) were prepared and their luminescence properties measured. The ormosil composite materials incorporated with [Tb(bpy)2]C13 and [Eu(phen)2]C13 had green and red emissions and, by optimization of the complex concentration amounts and heat treatment conditions, they provided emission intensities with 100 and 80%, respectively, of those for conventional phosphors used as lamp illumination. The resulting composite materials also possessed excellent mechanical strength, hydrolytic stability, and, consequently good durability for the emissions from the lanthanide compositions after being exposed to laboratory atmosphere for 100 days. © 1998 Elsevier Science B.V.

1. Introduction

Light emitting e.g. photoluminescence (PL), elec- troluminescence (EL) and thermoluminescence (TL) devices activated by the lanthanide ions such as Eu 3+ and Tb 3÷ provide emissions with good effi- ciency [1-3] by the intramolecular 4f electron transi- tions. These have been used as illumination lamps and cathode ray tube (CRT) displays, because of their colorimetric purity. However, the materials made for the purpose of the practical uses, e.g., phosphors [4-7] or laser devices [8], are almost limited to inorganic solids and the metal complexes or organometallic compounds consisting of lan-

* Corresponding author. Tel: +81-6 879 7353; fax: +81-6 876 4754; e-mail: [email protected].

thanide ions have been excluded from such applica- tions although they also have good luminescence properties for use as phosphors.

The lanthanide complexes and organometallic compounds emit when in aqueous or organic solu- tions and these emissions have been investigated [9-11]. The lanthanide ions, e.g. Eu 3+ and Tb 3÷ form stable crystalline complexes with heterocyclic ligands such as 1,10-phenanthroline (phen) and 2,2- bipyridine (bpy) which have efficient energy trans- fers from the coordinated ligands to the chelated lanthanide ions [12-14]. The potential applications of these compounds are based on the fact that they can absorb ultraviolet light in the organic portion of molecules and produce emissions in the red and green regions of the spectrum with good efficiency via the subsequent intramolecular energy transfers [15-17]. However, the thermal stability and the me-

0022-3093/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. Pll S0022-3093(97)00425-0

Page 2: Luminescence properties of lanthanide complexes incorporated into sol-gel derived inorganic-organic composite materials

124 T. Jin et al . / Journal of Non-Crystalline Solids 223 (1998) 123-132

chanical strength of these lanthanide heterocyclic complexes are less than those of inorganic com- pounds used as phosphors and laser devices, and furthermore, the emission intensity is less owing to a multiphonon relaxation of excited energy via hy- drated water molecules on the lanthanide complexes.

In our previous works [18,19], luminescence properties of the europium and terbium complexes before and after incorporation into silica gel matrices by the sol-gel method were studied. Although the compos i t e mate r ia l s i nco rpo ra t ed wi th [Eu(phen)2]C13 • 2H20 and [Tb(bpy)z]C13 • 2H20 provide red and green emissions with colorimetric purity and furthermore the thermal stability and me- chanical strength of the complexes are improved by silica gel matrix, their relative emission intensities are small compared to those of the original com- plexes themselves because of the dilution of the rare earth concentration in the silica gel matrix. There exists a concentration limitation for incorporating the complexes ( < several weight percents) and if the gel solutions containing large amounts of the complexes completely take place, the resulting composites be- come brittle and the transparency is decreased be- cause of formation of cracks and segregation of the complexes.

On the other hand, silica-based hybrid matrices with three dimensional skeleton structures of silica partially combining with oligomeric organosilane monomer unit, that is, organically modified sili- cates (ormosil), have been synthesized and their properties measured, particularly optical properties [20-25]. These materials are expected to possess the following advantages for processing and for proper- ties: (1) the preparation temperature is relatively low (< 150°C); (2) the density of resulting materials is large compared to plastics; (3) the obtained mono- liths are transparent in the range from 400 to 800 nm; (4) properties such as refractive index can be controlled by the chemical composition of starting gels; and (5) the viscous gels as precursors are easily molded or spun into useful materials in various shapes such as thin films and fibers [20-27].

Our preliminary work has demonstrated that the ormosil matrices can retain large amount of rare earth complexes and provide the composite type phosphors with good luminescence properties com- pared to the conventional silica one [28].

In this work, the ormosil matrices containing propylmethacrylate, methyl or phenyl groups as an organic component are incorporated with the lan- thanide complexes, [Eu(phen)2]C13 • 2H20 and [Tb(bpy)2]C13 • 2HzO by the sol-gel method and their luminescence properties are determined on the basis of the measurements for luminescent spectra, thermal and hydration stability.

2. Experimental

Organic lanthanide complexes, [Tb(bpy)2]C13 • 2H20 and [Eu(phen)2]C13 • 2H20, were prepared ac- cording to previous reports [18,19]. Appropriate amounts of respective lanthanide complexes dis- solved in N,N-dimethylformamide (DMF, 99.9%, Wako Pure Chemical) were added in a mixed solu- tion of water, ethanol (99.9%, Wako Pure Chemical), tetraethoxysilane (TEOS, 99.9%, Wako Pure Chemi- cal) and an organosilane such as diethoxydimethylsi- lane (DEDMS, 99.9%, Shin-Etsu Chemical), di- ethoxydiphenylsilane (DEDPS, 99.7%, Shin-Etsu Chemical) or 3-(trimethoxysilyl)propyl methacrylate (TMSPM, 97%, Aldrich Chemical) with molar ratio of 11:7:3:1 by reflux thoroughly for 1 h. Dilute hydrochrolic acid (0.005 M) was added to the sol solutions as a catalyst. The gel solutions obtained were cured at 50°C over 10 days. The solidified ormosil composite materials were then heated at 100 to 350°C for 5 h in air. The excitation and emission spectra, IR spectra and relative emission intensities for these ormosil composite materials were measured by a method described previously [28].

3. Results

3.1. General results

The organosilanes, TMSPM, DEDMS and DEDPS, were used as the oligomers because the organosilicates derived from them have similar back- bone structures, - S i - O- , to that of the silica matrix obtained from TEOS by the sol-gel method. The plasticity of these gels were improved compared with that of the conventional silica gels [20-27]. Model structures for the hybrid networks of inor-

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T. Jin et al. / Journal of Non-Crystalline Solids 223 (1998) 123-132 125

ganic and organic silicate units studied in this work are shown in Fig. 1. The organic network formers, DEDMS and DEDPS (see Fig. la and b), have parts of Si-methyl and Si-phenyl which cannot polymerize with other groups. Only the -Si(OH) 2 unit derived from -Si(OC2Hs) 2 in DEDMS or DEDPS can react with TEOS [28]. On the other hand, the organic network former, TMSPM is usually connected to the silicon by a Si-C bond, and the -Si(OH) 3 derived from -Si(OCH3) 3 mixes with TEOS (see Fig. lc).

I I o o

I I - - O - - Si-- O - - S i - - O - -

I I o o

I I

(a) SiO 2

I O CH 3 I I

- - O - - S i - - O - - S i - - O - -

I I O CH 3 I

(b) SiO 2 / Si(CH3)20

I o c6rt 5 I I

- - 0 - - Si-- O - - S i - - 0 - -

0 C6H 5 I

(c) SiO 2 / Si(C6H5)20

I o o o

I II I - - O - - Si-- (CH2) 3 - O - C - C - C H 3 - - O - - Si--

I I o CH 2 O

I I H3C -- C -- C -- O- - (CH2) 3 - - Si--

I II O O

O - -

O - -

(d) SiO 2 / SPM

Fig. 1. Network former and modifier units in ormosil matrices.

Particularly, the propylmethacrylate groups in TM- SPM polymefize one with another. In this paper, we define the ormosil matrix derived from DEDMS and TEOS as SiO2/Si(CH3)20, from DEDPS and TEOS as SiO2/Si(C6H5)20, and from TMSPM and TEOS as S iOJSPM.

The physical properties of the resulting ormosil matrices were as follows: density, ~ 1.4 g cm-3; index of refraction, 1.38. However, the pore size of samples was too small to measure on an apparatus using N 2 as a probe gas. The softening temperature was not observed because the thermal decomposition of lanthanide complexes (at ~ 300°C) took place before such softening. Also, the hardness could not be evaluated quantitatively. However, conventional glass abrasive tests showed that the composites ob- tained in this work were much harder than conven- tional organic resin glasses.

Fig. 2 shows a typical photograph of SiO2/SPM composite materials doped with 10 mol% of the lanthanide complexes (SiO2/SPM:[Tb(bpy) 2 ]3 + (10 mol%) and SiO2/SPM:[Eu(phen)2] 3+ (10 mol%)) irradiated by ultraviolet (UV) light along a direction perpendicular to the sheet. Both the samples pro- duced were transparent and free from cracks, and the red and green emissions were observed from within the samples, respectively. Therefore, it is possible that the lanthanide complexes are incorporated into the derivatives of ormosil matrix.

Typical infrared(IR) spectra of lanthanide com- plex, [Tb(bpy)2]C13, and its composite materials, SiO2/SPM:[Tb(bpy)2] 3+ (1-20 mol%), heated at 150°C for 5 h in air are shown in Fig. 3. The composite materials had typical absorption bands in the regions of A (C=C and C=N stretch) and B (C-H out of plane bend) [29]. The intensities of these bonds were increased with increasing concen- tration of [Tb(bpy)2]C13 . 2H20, apart from a strong absorption band based on the Si-O stretch in wavenumber region C, although they overlapped on the bands of ormosil. In addition, for the ormosil:[Tb(bpy)2 ]3÷ composite materials containing excess amount of the complex (> 10 mol%), the original absorption band of the [Tb(bpy)2]C13 com- plex as observed in the wave number region of C increased and dominated the original absorption band based on the Si-O stretching band of the ormosil matrix. This increase indicates that the [Tb(bpy) 2 ]C13

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126 T. Jin et al. / Journal of Non-Crystalline Solids 223 (1998) 123-132

Fig. 2. Photograph of the SiO2/SPM:[Tb(bpy)2] 3+ and SiO2/SPM:[Eu(phen)2] 3÷ composite materials incorporated with 10 mol% of (a) [Tb(bpy)2]C13 and (b) [Eu(phen)2]C13 after aging and drying at 50°C for more than two weeks.

• 2H20 complex is incorporated into the ormosil matrix without decomposition or modification. The absorption band at 1670 cm - I , especially shown in Fig. 4, was assigned to the C = C stretching mode for propylmethacrylate groups of TMSPM [29], indicat- ing that the acrylate groups still remained in the ormosil matrix without polymerization. However, this band almost disappeared after heating at a tempera- ture > 150°C. Infrared data for the ormosil matrices incorporated with [Eu(phen)2]C13 were the same as described above.

3.2. Europium composite materials

E x c i t a t i o n and e m i s s i o n s pec t r a o f SiO2/SPM:[Eu(phen)z] 3+ (10 tool%) as heated at 50, 150 and 300°C for 5 h are shown in Fig. 5. The measured emission profile was similar to the emis- sion spectrum pattern of the original [Eu(phen) z If13 • 2HzO complex [16]: The red emissions were ob- served even on the composite materials with large amounts of [Eu(phen)2]C13.2H20 and were as-

signed to the original [Eu(phen)2] 3+ complex cation (see Fig. 5a and b). On the other hand, two absorp- tion bands existed in the excitation spectrum, one at ~ 280 nm and another at ~ 300 nm which were caused by the SiO2/SPM matrix and the lanthanide complex, respectively [ 13,14,30]. The excitation band ascribed to ormosil matrix decreased with increasing heat treatment temperature. Therefore, the heat-resis- tance temperature of the ormosil matrix was less than that of the SiO 2 gel matrix prepared by the sol-gel method. The fact that the absorption band at around 325 nm due to the ~t-Tr * band of phen ligands [13,14] adds to the excitation spectrum indi- cates that the [Eu(phen) 2 ]3+ complex cations exist in the SiO2/SPM matrix and maintain their original molecular composition and structure. Therefore, the excitation spectrum is amplified by the phen ligands under UV irradiation of ~ 325 nm as follows:

I= f g(v)dv,

where g ( v ) is the function describing the shape of

Page 5: Luminescence properties of lanthanide complexes incorporated into sol-gel derived inorganic-organic composite materials

T. Jin et al. / Journal of Non-Crystalline Solids 223 (1998) 123-132 127

the absorption amplitude (= g(t')max). However, the excitation band intensity decreased after heat treat- ment at 300°C. Consequently, the emission spectra of the ormosil composite materials were less than that of SiO2:[Eu(phen)2] 3+ heated at 450°C [19] and only emission lines ascribed to the free Eu 3+ ion derived via decomposition of the complex were ob- served (see Fig. 5c).

Heat treatment temperature dependencies of the relative emission intensity for SiO2/SPM:[Eu (phen) 2 ]3+ composite materials with various concen- trations of the europium complex are shown in Fig. 6. The relative emission intensity of the SiO2/SPM:[Eu(phen)2] 3+ composite materials was

1670

t,,-

E t ' -

I--

i I r I I I

t9 t- in

E 09 t -

F-

2000 1500 1000 500

W a v e n u m b e r / cm -1

Fig. 3. Typical IR spectra of (a) lanthanide complex ([Tb(bpy) 2 ](713) itself and (b-e) lanthanide complex ormosil com- posite materials, SiO 2/SPM:[Tb(bpy)2] 3+ (x mol%), with vari- ous concentrations of [Tb(bpy)2]3+: (b) x = 1; (c) x = 3 ; (d) x = 10; (e) x = 20. The composite materials were treated at 150°C for 5 h in air.

2000 1500 1000 500

Wavenumber / cm "1

Fig. 4. IR spectra of SiO 2/SPM:[Tb(bpy)2] 3÷ (10 mol%): (a) after heat treatment at 150°C for 5 h; (b) without any heat treatment.

greater than that of the SiO::[Eu(phen)2] 3+ with incorporation of the same amounts of complexes. In the low temperature region, < 150°C, the emission intensity of ormosil composite materials increased with increasing heat treatment temperatures. This increase would be due to the removal of solvated water from both the complexes and silica matrix and indicates that the condensation among the remaining silanol groups takes place by heat treatment [18,19]. As shown in Fig. 7, the emission intensity for two composite materials at 150°C was a maximum when 10 mol% of [Eu(phen)2]C13 was incorporated, pro- viding a relative intensity of about 80%. It is noted that the specific intensity per complex formula unit for the SiO2/SPM:[Eu(phen)2] 3+ is several times greater than that of the original [Eu(phen)2]C13 • 2H20 complex and SiO2:[Eu(phen)2] 3+. Particu- larly, since the lanthanide complex incorporated into the ormosil matrix was protected from moisture by the surrounding organosilane units shown in Fig. lb and c, their luminescence property was stable after

Page 6: Luminescence properties of lanthanide complexes incorporated into sol-gel derived inorganic-organic composite materials

128 T. Jin et al. / Journal of Non-Crystalline Solids 223 (1998) 123-132

exposure to air. We will present this evidence later. In this case, the relative intensity was a maximum after heat treatment at the relative high temperature o f 1 5 0 ° C . T h e r e f o r e , the r e s u l t i n g SiO2/SPM:[Eu(phen)2] 3+ composite materials with ~ 10 mol% of [Eu(phen)2]Cl3.2H20 had a good

thermal stability as well as the original [Eu(phen)2]C13-2H20 complex compared to [Eu(bpy)2]Cl3.2H20, but the heat-resistance tem- perature was less than that of SiO2:[Eu(phen)2] 3- ( ~ 10 tool%).

] O0 620(5D0"7F2) (a) ~0 ~ x . Em. 60 5D0"7F4

2040 r ~k " 15D°'7F1j

.-,A 0 I O0 / 1 (b)

,~ 80 O9 c 60

o c 40

.~ 20 E m 0

1 O0 (c)

80

60

40

0 200 300 400 500 600 700 800

Wavelength I n m

Fig. 5. Exci ta t ion and emiss ion spectra of the SiO 2/SPM:[Eu(phen)2] 3+ (10 tool%) composite materials heated at various temperatures for 5 h in air: (a) 50°C; (b) 150°C; (c) 300°C.

o-e 8o

gt~ 60 _E.~.

"~ 40

cc 0

50 I00 150 200 250 300 350

Temperature /

Fig. 6. Heat-temperature dependencies of the relative emission intensity for SiO 2/SPM:[Eu(phen)2] 3+ (x tool%) with concen- tration values of [Eu(phen)2]3+: ( 0 ) x = l ; (11) x = 3 ; (&) x = 10 and ( 0 ) x = 20. Heat treatments: in air, 5 h. Each mean value plotted contained the variation less than 10%. The lines are drawn as guides for the eyes.

3.3. Terbium composite materials

Excitation and emission spectra of the SiO2/SPM:[Tb(bpy)2] 3+ (10 mol%) composite ma- terial heated at 50, 150 and 300°C are shown in Fig. 8. The former spectra were recorded at the peak positions of the emission, 545 nm, and the latter were obtained at the maximum peak positions of excitation, 310 to 340 rim, respectively. Bands at 310-340 nm as observed in the excitation spectra of

1 O0

8O

-~ "~ 6O ~o

._o_" 40

~ zo

a~ 0

:" 7 " 7 " 4 ~ / . . . . . / j

- I ~ . J Y "

I I I I

0 5 10 15 20

[Eu(phen)2]CI3 / tool% Fig. 7. Complex-concentration dependencies of the relative emis- sion intensity for the ormosil:[Eu(phen)2] 3+ composite materials obtained from various matrices: ( 0 ) ormosil = SiO 2/SPM; (A) SiO 2/Si(C6H5)20 and (11) SiO 2/Si(CH3)20. Heat treatments: at 150°C in air, 5 h. Each mean value plotted contained the variation less than 10%. The lines are drawn as guides for the eyes.

Page 7: Luminescence properties of lanthanide complexes incorporated into sol-gel derived inorganic-organic composite materials

T. Jin et al. / Journal of Non-Crystalline Solids 223 (1998) 123-132 129

the SiO2//Si(C6Hs)20:[Tb(bpy)2] 3+ composite ma- terials heated to 50 and 150°C (see Fig. 8a and b), were assigned to the "rr-~ * transition of bpy ligands [13,14], and the band at 270 nm was due to the absorption by the ormosil matrix. Therefore, we conclude that the [Tb(bpy)2]C13 complex still exists in the ormosil matrix prepared via the sol-gel pro- cess and the same molecular composition and struc- ture as the original [Tb(bpy) 2 ]C13 • 2H20 complex is maintained even after heat treatment to ~ 150°C in a manner similar to SiO2/SPM:[Eu(phen)2] 3+. How- ever, the excitation peak position of the composite

° ~

(o c-

c-

t-

O ° ~

co o~

o ~

E w

lOO

80

60

40

20

o

lO0

80

60

40

2o

o

lO0 o/f 60

40

2O

0 i 200 300

E x .

5D4-7F 6

/

- I v /

Fig. 8. Exc i t a t ion

545(SD4"7F5) (a) Em.

5D4-7F 4

( b )

(c

400 soo 600 700 800

Wavelength / nm

and e m i s s i o n spec t ra o f the SiO 2/SPM:[Tb(bpy)z] 3+ (1 mol%) composite materials heated for 5 h in air at various temperatures: (a) 50°C; (b) 150°C; (c) 300°C.

l O0 / ~

8o

c o 60 ._o ~

• 2 4o E ~ l ~ l - J

-"4 . . . . . - - - . . . . . . . . . 2O

-~ , , , . - - - - . .~ .~;~ m 0

0 50 lO0 150 200 250 300

Temperature / °C

Fig. 9. Temperature dependencies of the relative emission intensi- ties for SiO2//SPM:[Tb(bpy)2] 3+ (x mol%) with various concen- trations of [Tb(bpy)2]3+: (O) x = 1; (11) x = 3; ( A ) x = 10 and ( , ) x = 20. Heat treatments: in air, 5 h. Each mean value plotted contained the variation less than 10%. The lines are drawn as guides for the eye.

material heated to 300°C shifted to about 310 nm and the corresponding emission spectrum was weak- ened compared with the data of samples heated at 50 and 150°C. This difference means that the [Tb(bpy)2] 3+ complex is gradually decomposed at temperatures > 150°C. The emission spectrum of SiOE/Si(CH 3)20:[Th(bpy)2 ]3 + composite materials consisted of four main lines at 489 nm (SD4-7F6), 545 nm (SD4-TFs), 586 nm (SD4-7F 4) and 622 nm (SD4-TF 3) [31,32], and among them the emission lines at 545 nm had the largest amplitude. The emission intensity of SiO2/SPM:[Tb(bpy)2] 3+ com- posite materials was greater than those of SiO2:[Tb(bpy)2] 3+ [18,19]. However, the emission intensity for the composite material heated to 300°C decreased and there was no contribution to the en- ergy transfer between the bpy ligand and the Zb 3+

ion [31,32]. The relationships between the treatment tempera-

tures and emission intensities for SiO2/SPM:[Tb ( b p y ) 2 ] 3+ composite materials are shown in Fig. 9. In the temperature region < 150°C, the emission intensity of terbium complexes increased by incorpo- rating them into the SiO2/SPM matrix with increas- ing treatment temperature. This increase is due to the dehydration of both the complexes and ormosil ma- trix, which decrease the probability of excitation energy loss via the multiphonon non-radiative pro-

Page 8: Luminescence properties of lanthanide complexes incorporated into sol-gel derived inorganic-organic composite materials

130 T. Jin et al. / Journal of Non-Crystalline Solids 223 (1998) 123-132

cess by water molecules [33]. This effect could be explained by the fact that the multiphonon relaxation is due to the hydrated water molecules [3]. The emission intensity of SiO2/SPM:[Tb(bpy)2] 3÷ (1 reel%) was constant, within errors of measurement, up to 150°C because the ormosil matrix had been completely polymerized without the heat treatment and the [Tb(bpy)2]C13 • 2H20 complex was incorpo- rated as fully dispersed in the ormosil matrix [28]. The ormosih[Tb(bpy)2 ]3 + composite materials (ormosil = SiO2/Si(C6H5)20, SiO2/Si(CH3)20, SiO2/SPM) with an amount of the complex around 10 mol% provided a maximum emission intensity at the heat temperature of 150°C as shown in Fig. 10, indicating that the [Tb(bpy)2]C13 • 2H20 was effec- tively dispersed and incorporated into cage of or- rnosil matr ix in a m a n n e r s imilar to ormosih[Eu(phen)2] 3+ (ormosil = S iO2/SPM, SiO2/Si(C6H5)20) [3]. Particularly, we note that the relative emission intensity (103%) of the SiO2/SPM:[Tb(bpy)2] 3+ (10 mol%) composite ma- terial is comparable to that of the conventional phos- phor, LaPO4:Ce, Tb used for lamp illumination [3]. The increased emission intensity by heat treatment at the appropriate temperature is the same as those of the original [Tb(bpy)2]C13 • 2H20 complex and the SiO2 :[Tb(bpy)2 ]3 + composite materials of a simple matrix [18,19] and is due to the dehydration of complexes. However, the thermal stability of those matrices was less than that of the silica matrix

o~ t O0

~p__ 80 c

=o 60 .0 '~i"

.~O 40 E ~ W - - J

~ 2o ¢0 CD rr 0

0

I I [ (

5 10 15 20

[Tb(bpy)2]CI3 / mot%

Fig. 10. Complex-concentration dependencies of the relative emission intensity for the ormosih[Tb(bpy) 2 ]3 + composite materi- als derived from various matrices: (O) ormosil = SiO 2/SPM; (A) SiO 2/Si(C6H5)20 and (m) SiO 2/Si(CH3)20. Heat treat- ments: at 150°C in air, 5 h. Each mean value plotted contained the variation less than 10%. The lines are drawn as guides for the eye.

100 ~

~: 80~ o "6 g 6o

E U.I

ID cc

40

20

• ~ =

0 I I I I I

0 20 40 60 80 100 Time / day

Fig. 11. Relative emission intensities vs. standing time for: ( 0 ) S iO 2 / S P M : [ T b ( b p y ) 2 ] 3+ (10 m o l % ) ; ( 1 1 ) SiO 2/SPM:[Eu(phen)2] 3+ (10 mol%); (O) [Tb(bpy)2]C13 itself; and ( [ ] ) [Eu(phen) 2 ]C13 itself. The samples were exposed to air. The lines are drawn as guides for the eye.

derived from TEOS only, and the emission intensity gradually decreased by heat treatment above 150°C because of the decomposition of terbium complex.

3.4. Stability against moisture

Relationship between the relative emission inten- sities of the ormosil composite materials or lan- thanide complexes themselves and the standing time in air are shown in Fig. 11. From this result, the lanthanide complexes were found to be effectively surrounded by ormosil matrices so that they were shielded from the moisture in air, although the emis- sion intensities of free lanthanide complexes de- creased in several days. Therefore, we conclude that the ormosil stability is sufficient for use as high-per- formance transparent light-emitting materials.

4. Discussion

In the preparation process, although, phase sepa- ration took place in ormosil materials prepared with- out refluxing in a sol solution containing 0.005 M HC1 as a catalyst. However, no such separation nor cracking was observed on the resulting matrices after refluxing for 1 h. This absence would be due to the complete condensation between the silanol groups in TEOS and organosilane precursors which proceeds by refluxing [20-27].

The ormosil composite materials incorporated with lanthanide compounds have been prepared in a ho- mogeneous form as shown in the photograph pre-

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T. Jin et aL / Journal of Non-Crystalline Solids 223 (1998) 123-132 131

sented in Fig. 2. In the cases of aqueous solutions of the lanthanide complexes, homogeneous material was not obtained because of the precipitation of the complex. This precipitation is caused by the fact that the vaporizing rate of the solvent for lanthanide complex is almost equal to the condensation rate of silanol in the ormosil matrices since the boiling point of DMF is higher than those of EtOH and water. Furthermore, DMF is an aprotic solvent and has an affinity for organic compounds due to its large polar- ity [34]. Therefore, we conclude that the lanthanide complexes are effectively dispersed and embedded in the ormosil matrices before precipitating the com- plexes.

The lanthanide complexes which are properly in- corporated into the sol-gel derived ormosil host materials are surrounded by the three-dimensional network in these matrices. Even at the lower temper- atures for heat treatment (< 150°C), the flexible three-dimensional network composed by copolymer- ization among silica and propylmethacrylate units are expected [20-27], which are cross-linked and in which the complexes can be embedded. Therefore, the previous result that the larger amount of lan- thanide complexes cannot be well incorporated in the silica matrix is ascribed to the rigid framework of silica which prevents their incorporation into the simple matrix of SiO 2 [18,19].

On the other hand, although the condensation reactions among the silanol groups have been re- ported to be accelerated by addition of a base cata- lyst [24], the complexes are not incorporated into the ormosil matrices. This failure means that the conden- sation rate is too fast for the incorporation of the complexes into the ormosil matrices. This result indicates that the initial stage for network construc- tion takes place around the lanthanide complexes as templates at the sol formation step of the sol-gel process. The interactions between complexes and ormosil matrix probably exist even in the 'sol-phase' (solution) before the prolonged condensation pro- ceeds, as shown by the failure of incorporation under the basic conditions. The final stage of the network construction is during the drying process of the matrix, resulting in complete encapsulation of the lanthanide complexes.

Heat-resistance temperatures at which the emis- sion intensity of the lanthanide complexes incorpo-

rated into the ormosil matrices started to decrease were evaluated to be about 150°C for SiO2/SPM:[Tb(bpy)2] 3+ and 150 to 200°C for SiOe/SPM:[Eu(phen)2] 3÷. The ormosil matrices formed via the sol-gel route were flexible compared with the silica gel prepared by the same way, and furthermore pore radii were estimated to be smaller than in a sol-gel silica matrix [35]. Therefore, since the ormosil matrices containing lanthanide complex can effectively disperse the lanthanide complexes in the ormosil matrices, the composite materials are consequently transparent. As stated above, the lan- thanide complexes, [Tb(bpy)2]Cl3.2H20 and [Eu(phen)e]C13 • 2HzO, are perhaps completely in- troduced in the network formed by the organic group such as CH 3, C6H 5, and propylmethacrylate in or- mosil, and effectively stabilized from the moisture in air [36].

The increase in the intensities of that the red and green emissions by heat treatments are due to the elimination of the solvated water which plays a role as an inhibitor for the energy transfer from the excited ligands to the lanthanide ions in the matrix [3], and also such elimination induces further poly- merization among the organosilane and TEOS residues by which the complexes are fully incorpo- rated and homogeneously dispersed in the matrix. These results are supported by the temperature de- pendence of emission intensity shown in Fig. 9 and our previous results [18,19,28], which provide opti- mum heat treatment conditions for the maximum emission intensity.

5. Conclusions

The following conclusions were drawn. (1) A large amount of [Eu(phen)2]C13 and

[Tb(bpy)2]C13 complexes is incorporated into the ormosil matrices, compared to the simple silica ma- trix, in which they are effectively isolated from ambient moisture. They provide red and green emis- sions as strong as conventional lamp phosphors, Y(P, V)O4:Eu and LaPO4:Ce, Tb, respectively.

(2) The ormosil matrices are of good moisture resistivity compared to the rare earth complexes themselves, and therefore the ormosil composite ma- terials, e.g. S i O : / S P M : [ E u ( b p y ) 2 ] 3+ and

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132 T. Jin et al. / Journal of Non-Crystalline Solids 223 (1998) 123-132

SiO2 /SPM: [Tb(bpy )2 ]3+, are noted as phosphors for

pract ical uses.

Acknowledgements

The authors wish to thank Nich ia Kagaku for

supplying the standard phosphors, LaPOa:Ce, Tb and

Y(P, W)O4:Eu. This work was supported by Grant-

in -Aid for Scient i f ic Research on Priori ty Areas ' N e w

D e v e l o p m e n t o f Rare Earth C o m p l e x ' Nos. 06241106

and 06241107 f rom the Minis t ry o f Educat ion, Sci-

ence, Sports, and Culture of Japan.

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