Study of lanthanide complexes in silica gels by photoacoustic spectroscopy

Materials Science and Engineering B 116 (2005) 82–86

Study of lanthanide complexes in silica gels by photoacoustic spectroscopy Yang Yuetao∗ , Zhang Shuyi State Key Laboratory of Modern Acoustics, Institute of Acoustics, Nanjing University, Nanjing 210093, PR China Received 11 June 2004; received in revised form 13 September 2004; accepted 20 September 2004

Abstract Via a sol–gel process, lanthanide complexes Ln(bipy)2 Cl3 ·2H2 O (Ln3+ :La3+ , Nd3+ , Tb3+ ; bipy: 2,2 -bipyridyl) are incorporated into silica gels by the hydrolysis and condensation of tetraethoxysilane (TEOS). Upon heat treatment at 150 ◦ C, PA intensity of the ligand increases for Tb3+ , La3+ and Nd3+ complexes in silica gels, respectively, while this difference cannot be observed for samples without heat treatment. Different PA intensities of lanthanide complexes in silica gels are interpreted by comparison with their luminescence spectra. Spectral results indicate that bipy does not coordinate with lanthanide ions in gels without a suitable heat treatment. The formation of lanthanide complexes in silica gels is discussed from two aspects: radiative and non-radiative processes firstly. The nephelauxetic parameters and PA branching vectors of f–f transitions of Nd3+ complex in silica gels, which exhibit highly sensitivity to the coordination environment, are also discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Lanthanide complex; Sol–gel technique; Photoacoustic spectroscopy; Relaxation process

1. Introduction In general, excellent luminescence properties of lanthanide complexes are attributed to the intramolecular energy transfer between the ligands and chelated metal, that is, the absorption of excitation energy mainly takes place in the organic part of chelate ligands (‘antenna effect’) instead of the bound central lanthanide ion [1]. Phosphors and laser devices made for practical use, up to date, are limited mostly to inorganic solids [2,3]. The complexes or organic compounds containing lanthanide ions, however, have been excluded from such application because of their poor thermal resistivity, photochemical stability, moisture stability and mechanical strength, although they also have good luminescence properties for use as phosphors, laser devices and biomimetic materials [4,5]. Recently, sol–gel technique has been proven to be a promising approach to prepare unique inorganic luminescent solid materials hybridized with organic lanthanide complexes ∗

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[6]. Inorganic solid matrices, due to their good optical, thermal and chemistry properties, are suitable candidates as the hosts for lanthanide complexes. Various kinds of lanthanide complexes have been incorporated into silica matrix, organically modified silicate matrix and inorganic–organic matrix [7–9]. However, lanthanide complexes would be unstable in the most common sol–gel precursor solution. For examples, lanthanide heterocyclic complexes tend to decompose when they contact with water, and lanthanide complexes with aromatic acids are decomposed by a few drops of HCl [10,11]. Therefore, it is important to study the formation of lanthanide complexes in silica gels. Luminescence spectroscopy has been widely used in studies of lanthanide complexes doped silica gels, and electronic absorption spectroscopy has been used to determine the absorption properties and extract Judd–Ofelt intensity parameters for these composite materials [12,13]. Photoacoustic (PA) spectroscopy is a relatively new technique for studies of the properties of opaque or scattering substances [14,15]. PA spectroscopy is a direct monitor of the non-radiative relaxation channel after excitation and therefore, it is the complement of luminescence spectroscopy. PA technique has been

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found to be suitable for investigating solid lanthanide compounds according to our recent work. It is interesting to study lanthanide complexes doped into sol–gel glasses by PA spectroscopy. In this paper, lanthanide complexes with bipy are incorporated into silica gels by a sol–gel method. The formation of lanthanide complexes in silica gels is interpreted by their PA and luminescence spectra from two aspects: non-radiative and radiative processes. 2. Experimental 2.1. Sample preparation Lanthanide complex Ln(bipy)2 Cl3 ·2H2 O (Ln:La3+ , Nd3+ or Tb3+ ; bipy: 2,2 -bipyridyl) was conventionally prepared by addition of 2,2 -bipyridyl to an ethanol solution containing lanthanide chloride LnCl3 ·6H2 O. After stirring overnight, the resulting white precipitate was collected. A hydrated complex Ln(bipy)2 Cl3 ·2H2 O was obtained from the solid by washing with small portions of cold ethanol several times and dried over vacuum at room temperature. The complexes were characterized by IR spectroscopy and by CHN elemental analysis. For the acid-catalyzed tetraethoxysilane (TEOS) sol preparation, exactly measured volumes of TEOS and ethanol were mixed. To this mixture was slowly added an aqueous HCl solution, resulting in a solution with pH = 2. The mole ratio of TEOS, ethanol and water was 1:4:4. The mixture was allowed to stand for several hours under vigorous stirring. Lanthanide complex was introduced to the above precursor solution, so that the Ln/Si ratio was 0.08. The resulting sols were cast into cylindrical-shaped plastic boxes and sealed with wax film, then set aside at room temperature. One week after gelation, holes were poked in the wax film to allow solvent escape. Thermal densification of the gels was performed in a box furnace. The temperature increased to 150 ◦ C stepby-step within 24 h. The samples were open to air during the heating process and held for 24 h at 150 ◦ C. The term, wet gel is used in this paper to refer to the samples that had undergone aging and drying at room temperature only. 2.2. Spectroscopic measurements PA spectra were measured on a single-beam spectrometer constructed in our laboratory [16]. Excitation source was a 500 W Xenon lamp. The optical system was a monochromator and a variable speed mechanical chopper at a frequency of 33 Hz, which was used to modulate the light source intensity. The acoustic signal was monitored with the sample placed in an indigenous photoacoustic cell fitted with an electret microphone. The output signal from the microphone was amplified by a preamplifier, and then fed to a lock-in-amplifier with a reference signal imputed from the chopper. The final signal was normalized for the changes in lamp intensity using carbon black as a reference. PA spectra of all the samples were recorded at room temperature in the region of 300–800 nm.

Fig. 1. PA spectra of Nd3+ (1), Tb3+ (2) and La3+ (3) complexes in wet gels.

Luminescence spectra of the samples were taken with Hitachi 850 fluorescence spectrophotometer. Measurement of infrared spectra for the composite materials was carried out using conventional KBr pellet technique.

3. Results and discussion 3.1. Photoacoustic spectra Photoacoustic signal is obtained by detecting the heat generated through non-radiative transitions by the sample after absorbing a periodically varying incident light. The PA signal (P) can be written as [17]: P = kAabs γ

(1)

where Aabs is the absorbency of the sample, γ the probability of non-radiative transitions after excitation, and k a coefficient which is determined by the thermal property of the sample and by the spectrometer. PA spectra of lanthanide complexes doped silica gels are shown in Figs. 1 and 2. The broad absorption band around 310 nm is assigned to the ␲–␲* transition of bipy [18]. For lanthanide ions, f–f transitions are only observed from 350 to 800 nm. Since the ␲–␲* transition of bipy is far more stronger than the parity forbidden f–f transitions, f–f transitions of lanthanide ions in the region 300–350 nm are lost in the strong ␲–␲* transition. In the UV–vis region, La3+ has no absorption. The absorptions of different energy levels of Nd3+ are clearly shown in the PA spectra. The numerous closely packed energy levels

Fig. 2. PA spectra of Nd3+ (1), La3+ (2) and Tb3+ (3) complexes in gels after heat treatment.

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Fig. 3. Excitation (a) and emission (b) spectra of Tb3+ complex in wet gel.

of Nd3+ are often intermixed, and Nd3+ has a high probability to relax by non-radiative transitions. For Tb3+ , which has strong luminescence properties, however, relaxation of 5 D4 cannot be monitored by PA spectroscopy, and PA signal of 5 D at 375 nm are also quite weak. It can be interpreted by the 3 relaxation model according to our previous paper similarly [19]. In the region of ligand absorption, PA intensity is nearly the same for lanthanide complexes in wet gels. Since PA signal is proportional to the probability of non-radiative transitions γ, there is almost no difference for γ of bipy in the samples. After heat treatment, PA intensity of the ligand increases for Tb3+ , La3+ and Nd3+ complexes in silica gels, respectively. It indicates that γ of bipy is the largest for Nd3+ complex doped gel and is the least for Tb3+ complex doped gel. Heat treatment changes the relaxation processes of lanthanide complexes in silica gels. These changes can be reflected in their luminescence spectra. 3.2. Luminescence and infrared spectra Luminescence spectra of lanthanide complexes in silica gels are shown in Figs. 3 and 4. Excitation spectrum of Tb3+ luminescence in wet gel is similar to that of the aqueous Tb3+ ion. The narrow bands in the excitation spectrum are due to f–f absorptions of Tb3+ , which suggests that bipy does not coordinate with Tb3+ . It should be mentioned that for the complexes in wet gels, a similar broad emission band of bipy is observed upon direct excitation of the ligand. For

Fig. 5. IR spectra of Ln(bipy)2 Cl3 ·2H2 O pure complex (. . .), Ln3+ complex in silica gel after heat treatment (—).

Tb3+ complex in silica gel after heat-treatment, its excitation spectrum turns out to be a broad band, and its characteristic emissions increase significantly compared with its wet gel sample. For the broad excitation band is in the region of bipy absorption, the characteristic emissions of Tb3+ demonstrate an energy transfer from bipy to Tb3+ . It indicates that bipy coordinates with Tb3+ upon heat treatment. For La3+ complex in silica gel after heat treatment, the broad emission band in the region of 350–500 nm is attributed to the emission of bipy. La3+ with 4f0 configuration has no low-lying excited states. The energy absorbed by bipy cannot transfer to La3+ , but relax through its own lower energy levels, which results the emission of bipy. The energy levels of Nd3+ are often intermixed and provide paths for efficient quenching of the excited state of bipy. The emission of bipy is very weak for Nd3+ complex in silica gel after heat treatment. The result of luminescence spectra is complementary with that of PA spectra. For lanthanide complexes, PA intensity of the ligand is the sum of non-radiative transition of the ligand, the energy transfer and the following non-radiative relaxation of the central Ln3+ [20,21]. Upon excitation of the ligand, the total emission intensity increases for Nd3+ , La3+ and Tb3+ complexes in gels after heat treatment, respectively. As the probability of radiative transition increases, PA intensity exhibits a corresponding decrease. IR spectrum patterns observed on Ln(bipy)2 Cl3 ·2H2 O pure complex and in silica gel are shown in Fig. 5. For the sample with heat treatment, absorption peaks assigned to Table 1 Calculated values of various covalency parameters of (I) Nd3+ complex in wet gel; (II) Nd3+ in wet gel; (III) Nd3+ complex in gel after heat treatment

Fig. 4. Excitation (a) and emission (b) spectra of Tb3+ (1), La3+ (2) and Nd3+ (3) complexes in gels after heat treatment.

β b1/2 δ

I

II

III

99.50% 0.050 0.50

99.52% 0.049 0.48

98.62% 0.083 1.40

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Table 2 Band assignments and branching vectors of PA bands of (I) Nd3+ complex in wet gel; (II) Nd3+ in wet gel; (III) Nd3+ complex in gel after heat treatment. (Ground state: 4 I9/2 ) Energy level

4F

5/2

4F

7/2

4F

9/2

+ 4 S3/2

2H

11/2

4G

5/2

4G

7/2

4G

9/2

2D

3/2

4G

11/2

2P

1/2 ␲–␲*

+ 4 G7/2

+ 2 G9/2

Assignment (cm−1 ) I

II

795.0 741.5 679.3 632.2 580.0 523.9 516.2 478.7 466.1 432.2 <340

795.0 740.6 679.5 632.1 579.6 523.1 516.4 479.1 466.2 432.8

Range (cm−1 )

Branching vector B

III 749.1 684.6 635.3 585.2 529.0 519.9 483.5 470.2 436.0 <340

lanthanide bipy complex, C N, C C stretching (1598 and 1455 cm−1 ) and C H out-of-plane bend (766 cm−1 ), are superimposed on the original patterns of SiO2 matrix. This further supports the above discussion. 3.3. Effect of environment on f–f transitions of Nd3+ complex in silica gel Lanthanide ions are frequently used as structural probe for biological systems and inorganic matrices [22]. It is interesting to analysis f–f transitions of Nd3+ in different silica gels to evaluate the formation of complex. Despite the shielding effect of 5s and 5p atomic shells, 4f electrons may partly participate in the formation of metal–ligand bands in ligand fields. The “degree of covalency” can be estimated from nephelauxetic ratio β, bonding parameter b1/2 and Sinha parameter δ [23]. Calculated values of covalency parameters are listed in Table 1, based on PA assignments in Table 2. The parameters of only Nd3+ in wet gel, which is prepared under the same condition, are also presented for comparison. The nephelauxetic effect (1 − β) of Nd3+ complex in wet gel is consistent with that of only Nd3+ in wet gel, and is comparable with that of the aqueous Nd3+ ion. As room temperature drying only removes the solvent molecules physisorbed on the walls of the open pores, there are still sufficient trapped water and ethanol in wet gels. The excess water is more prone to coordinate with Nd3+ than bipy [10]. The remaining solvent may form a solvation shell around Nd3+ , resulting in the quite low nephelauxetic effect of Nd3+ complex in wet gel. Obvious increase of the nephelauxetic effect is observed for the sample after heat treatment. During heat treatment, water, ethanol and HCl vaporize further. At this stage, polymerization is not completed. The flexibility of siloxane threedimensional network still permits easy diffusion of bipy molecules and lanthanide ions through the walls, which results in formation of the complex. Nd3+ may also participate in the continuous polymerisation–condensation reactions, forming (Si O)n Nd bonds and releasing HCl [24]. Coordination of Nd3+ with bipy and Si O groups eventually

I

II

III

775–721 721–615

0.33 0.10

0.33 0.11

0.27 0.09

615–550

0.29

0.28

0.43

550–498

0.20

0.21

0.17

498–451

0.08

0.07

0.04

increases the degree of covalency for the sample, though the exact coordination sphere of Ln3+ complex in silica gel still needs further study. The absence of precise determination of molar absorption coefficient in PA measurement makes it difficult for us to discuss the relationship between PA band intensity and the influence of ligand fields. However, the branching vector defined by Strek et al. [25] can be used to characterize any changes qualitatively in the intensities of PA bands [26]. The branching vector (BPA ) was defined as the ratio of the integrated PA band intensity of one of the bands (IPA ) in spectrum to the total integrated intensity of all bands, as: [B1PA , B2PA , . . . , BkPA , . . . , BnPA ]

(2)

where BkPA = BkPA /




IiPA ,

(i = 1, . . . , n)

(3)

PA branching vectors of Nd3+ complex in silica gels are listed in Table 2. For wet gel samples doped with Nd3+ complex and with only Nd3+ ion, their PA branching vectors are similar and the branching vectors of hypersensitive energy levels 4G 2 5/2 + G7/2 are coincident with each other. This indicates that perturbation of ligand fields is similar, and the environment effect is nearly the same for Nd3+ ions in two wet gel samples. For Nd3+ complex in silica gel after heat treatment, branching vector of energy levels 4 G5/2 + 2 G7/2 increases remarkably, while the others decrease or change little compared with those of wet gel samples. This is because that nephelauxetic effect increases for Nd3+ complex in silica gel upon heat treatment. As the degree of covalence increases, the oscillator strength of hypersensitive transition exhibits a corresponding increase [27]. Here, PA spectra confirm that the transitions 4I 4 2 9/2 to G5/2 and G7/2 , which satisfies the selection rule |J| ≤ 2, are hypersensitive ones. Those transitions manifest some extraordinary sensitivity to the ligand environment.

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Acknowledgment The authors thank the National Natural Science Foundation of PR China for supporting this program.

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