Luminescent organic–inorganic hybrid materials based

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O R I G I N A L P A P E R

Luminescent organicinorganic hybrid materials basedon lanthanide containing ionic liquids and sylilated b-diketone

Yige Wang Yu Feng Hongsheng Zhao

Quanying Gan Xiaoyan Yu

Received: 4 January 2011 / Accepted: 19 March 2011 / Published online: 2 April 2011

Springer Science+Business Media, LLC 2011

Abstract In this work, we report the luminescent

organicinorganic hybrid materials prepared by hydrolysisand condensation of sylilated b-diketone under acid con-

ditions in the presence of carboxyl-functionalized ionic

liquid in which Eu3? ions are coordinated to the oxygen

atoms of carboxylate groups from the ionic liquids. The

obtained materials were characterized with FT-IR, TG and

photoluminescence spectroscopy. FT-IR spectra imply that

Eu3? ions are still coordinated to the ionic liquid in the

hybrid materials. Excitation and emission spectra demon-

strate that the energy transfer occurs from the b-diketone

molecules covalently bonded with silica to Eu3? ions. The

Eu3? (5D0) quantum efficiency value of the hybrid mate-

rials has been estimated based on the emission spectrum

and the value of lifetime. A large value of ratio (16.44)

between the intensities of the 5D0?7F2 and

5D0?7F1

transition and high value of 5D0 quantum efficiency

(51.01%) are obtained.

Keywords Ionic liquids Lanthanide Solgel Photoluminescence

1 Introduction

The trivalent lanthanide ions display photoluminescence

properties that are favorable for optical applications such as

fiber amplifiers and solid-state lasers [13]. However, the

luminescence intensity of lanthanide ions is limited by its

poor light-absorbing ability due to the Laporte forbidden

character and intraconfigurational nature of the 4f transi-

tions. Organic ligands with large molar absorption coeffi-

cients are normally used to coordinate to lanthanide ions,

resulting in sensitized emission via the so called antenna

effect [4, 5]. These ligands can also protect the lanthanide

ions from molecules with high-energy vibrations such as

water molecules that can quench lanthanide ion lumines-

cence by radiationless deactivation. Some of the best

ligands for these purpose are b-diketonates having aro-

matic and fluorine substituents in terms of high harvest

emissions due to the effectiveness of the energy transfer

from the ligand to the Ln3? cations [6, 7]. However, in

spite of the interesting luminescence features, lanthanide

complexes have been excluded from practical applications

as tunable solid-state lasers or phosphor devices due to

their poor thermal stability and mechanical properties.

One acceptable solution to this problem is to immobilize

lanthanide complexes in solgel derived silica, resulting in

luminescent organicinorganic hybrid materials [814].

The interest in light-emitting lanthanide based organic

inorganic hybrid materials has grown considerably during

the last decade as they can find potential applications in

tunable lasers, amplifiers for optical communications,

emitter layers in multilayer light emitting diodes, efficient

light conversion molecular devices, UV dosimeters, and

light concentrators for photovoltaic devices [8]. Various

lanthanide complexes have been immobilized in the silica

matrices via the SiC covalent bond through the hydrolysis

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10971-011-2450-7 ) contains supplementary

material, which is available to authorized users.

Y. Wang (&) Q. Gan X. YuSchool of Chemical Engineering and Technology,

Hebei University of Technology, 300130 Tianjin, China

e-mail: [email protected]

Y. Feng

The College of Environmental Science and Engineering,

Nankai University, 30071 Tianjin, China

H. Zhao

Chaoyang Health School, Chaoyang, Liaoning 12200, China

123

J Sol-Gel Sci Technol (2011) 58:711715

DOI 10.1007/s10971-011-2450-7
http://dx.doi.org/10.1007/s10971-011-2450-7http://dx.doi.org/10.1007/s10971-011-2450-7


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and condensation of TEOS and silylated organic ligands

that play double roles of both coordinating to lanthanide

ions or lanthanide complexes and acting as an organosilane

precursor to form the silica network. Among of them,

immobilization of b-diketonate complexes in silica and

mesoporous materials is highly intensive due to the reasons

mentioned above. This is normally achieved either by

adduct formation with a heterocyclic molecular immobi-lized on the matrices or through the modification of

b-diketone [1517].

We recently reported a facile way to prepare lumines-

cent organicinorganic hybrid materials through hydrolysis

and condensation of silylated bipyridine that can sensitize

the luminescence of europium (III) ions in the presence of a

carboxyl-functionalized ionic liquid in which the Eu3? ions

are coordinated to the oxygen atoms of carboxylate groups

[18]. A longer lifetime of Eu3? 5D0 excited state level was

obtained in comparison with similar materials without the

addition of ionic liquids [11]. In the present work, lantha-

nide b-diketonate complexes were immobilized in ionicliquid containing hybrid organosilica via SiC bond by

hydrolysis and condensation of TTA-Si (scheme 1) in the

presence of 3-(5-carboxypropyl)-1-methylimidazolium

bromide (IL, Scheme 1)) in which the Eu3? ions are

coordinated to the oxygen atoms of carboxylate groups.

The thermal stabilities and the luminescence properties of

the resulting hybrid materials are analyzed in detail.

2 Results and discussion

FT-IR spectrum was firstly employed to characterize the

obtained materials, the FT-IR spectra of TTA-Si and the

hybrid material Eu@IL-TTA-SiO2 are shown in Fig. 1.

The main changes upon hydrolysis and condensation of

TTA-Si in europium (III)-containing ionic liquid with

respect to the FT-IR spectrum of TTA-Si is the disap-

pearance of characteristic bands of SiOCH2CH3 groups at

960 and 1,172 cm-1, indicating the complete hydrolysis of

the TTA-Si precursor. The presence of the broad band at

1,1201,000 cm-1 indicates the formation of siloxane

bonds. No aborption band at 1,729 cm-1 attributed to the

COOH (Fig. 1c) groups from the ionic liquid can be

observed, implying that Eu3? ions are still coordinated to

the ionic liquid in the hybrid materials. The band at

1,690 cm-1 can be ascribed to the absorption of CONHgroups from Si-TTA.

Thermogravimetric analysis was also performed to

determine the thermal stability of the hybrid materials.

Figure 2 presents the thermogravimetric weight loss curve

(TG). Three main degradation steps can be observed from

the TG curve. The first step of weight loss of ca.17.4%

below 200 C could be attributed to the desorption of

physically absorbed water and residual solvents. The sec-

ond weight loss of 45.4% in the range of 200600 C can

be ascribed to the decomposition of ionic liquid and the

organic moieties from Si-TTA. Finally, the slight weight

loss beyond 600 C is ascribed to the release of water

Scheme 1 Ionic liquid (IL, a) and silylated ligand (TTA-Si, b) used

in this study and the digital photo of luminescent hybrid materials

irradiated under uv lamp (kmax = 365 nm, c)

Fig. 1 FT-IR spectrum of TTA-Si (a), the hybrid material Eu@IL-

TTA-SiO2 (b) and the ionic liquid (c)

Fig. 2 TGA curve of Eu@IL-TTA-SiO2

712 J Sol-Gel Sci Technol (2011) 58:711715

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formed from the further condensation of silanols in the

silica framework.

The organicinorganic hybrid materials show an red

photoluminescence upon irradiation with UV radiation

(scheme 1). The excitation and emission spectrum of the

obtained material were measured at room temperature and

are shown in Fig. 3. The excitation spectrum obtained by

monitoring the 5D0?7F2 emission at 612 nm displays alarge broad band between 225 and 450 nm resulting from

the p?p* transition of TTA superimposed with a sharp

line characteristic of Eu3? energy level. The relatively low

intensity of the intra-4f6 transition compared with that of

the broad band in the excitation spectrum indicates that

Eu3? ions are essentially excited by a sensitized process

rather than by direct population of the intra-4f6 levels. The

luminescence spectrum was measured with 325 nm as the

excitation wavelength, several narrow peaks can be

observed and are attributed to the transitions between the5D0 excited state and the different J levels of the ground

term 7FJ, (J= 04). The 5D0?7F2 emission line at 612 nmdominates the spectrum and this luminescence line is

responsible for the red luminescence color. The presence of

the forbidden 5D0?7F0 transition indicates that Eu

3? ions

are located in a coordination sphere with low symmetry

[19]. The 5D0?7F1 transition corresponds to a parity-

allowed magnetic dipole transition that is independent of

the environment. The hypersensitive 5D0?7F2 transition

varies strongly with the local surrounding around the Eu3?

ions. Its intensity increases when the lattice environment is

distorted and contains certain components of noninversion

symmetry. [20] The ratio (R) between the intensities of the5D0?

7F2 and5D0?

7F1 transition therefore can be used as

a parameter to probe the asymmetry of the Eu3? sites. The

R value here is determined to be 16.44, indicating that the

local symmetry groups of the Eu3? chemical environment

is not characterized by an inversion centre. The typical

decay curve of the hybrid material was measured and can

be described as a single exponential (supporting informa-

tion, Fig. 1s), indicating that all Eu3? ions occupy the same

average coordination environment, and the luminescence

lifetime is determined to be 0.531 0.001 ms.

We also determine the emission quantum efficiency (q)

of the 5D0 excited state based on the emission spectra andthe lifetime of the Eu3? first excited level by using the

following equations according to reference [21]. q can be

defined by Eq. 1 if we assuming that only nonradiative and

radiative processes are involved in the depopulation of the5D0 state.

q kr

kr knr1

where kr and knr are the radiative and nonradiative

probabilities, respectively. The radiative contribution may

be calculated from the relative intensities of the 5D0?7FJ

(J= 04) and can be expressed by Eq. 2. The 5D07F5,6transitions are not taken into account for the calculus of the

efficiency of the 5D0 level, probably due to their low

intensity (compared with the other lines).

KrA01E01

S01

X4

J0

S0J

E0J2

where A01 is Einsteins coefficient of spontaneous emis-

sion between the 5D0 and7F1 level, usually considered to

be equal to 50 s-1 when an average index of refraction n

equal to 1.506 was considered. E0J and S0J are the energy

and the integrated intensity of the

5

D0?

7

FJ transitions,respectively. The obtained data are summarized in

Table 1.The q value for the sample is 51.01% that is much

higher than the luminescent hybrid materials reported

Fig. 3 Excitation (a) and emission (b) spectrum of the hybrid

material Eu@IL-TTA-SiO2

Table 1 Experimental 5D0 lifetime, calculated radiative and nonra-

diative 5D0 decay rate, and5D0 quantum efficiency value

Sample s (ms) kr (ms-1) knr (ms

-1) q (%)

Eu@IL-TTA-SiO2 0.53 0.96 0.92 51.01

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Luminescent organic–inorganic hybrid materials based