Spectroscopic properties of neodymium(III)-containing polyoxometalates in aqueous solution

Spectrochimica Acta Part A 62 (2005) 478–482

Spectroscopic properties of neodymium(III)-containing polyoxometalates in aqueous solution Slawomir But a , Stefan Lis a , Rik Van Deun b , Tatjana N. Parac-Vogt b , Christiane G¨orller-Walrand b , Koen Binnemans b, ∗ b

a Faculty of Chemistry, Adam Mickiewicz University, 60-780 Pozna´ n, Poland Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200F, B-3001 Leuven, Belgium

Received 7 January 2005; accepted 27 January 2005

Abstract The spectroscopic properties of the neodymium(III)-containing polyoxometalates (POMs) [Nd(PW11 O39 )2 ]11− , [Nd(PMo2 W9 O39 )2 ]11− , [Nd(PMo4 W7 O39 )2 ]11− , [Nd(PMo6 W5 O39 )2 ]11− , [Nd(SiMo2 W9 O39 )2 ]13− , [Nd(P2 W17 O61 )2 ]17− , [NdW10 O36 ]9− , [NdP5 W30 O110 ]12− and [NdAs4 W40 O140 ]25− are described. Absorption spectra of aqueous solutions of the complexes have been recorded and the transition intensities are parameterised in terms of the Judd–Ofelt intensity parameters Ωλ (λ = 2, 4, 6). Marked differences were found between the luminescence lifetimes of the complexes of the type Nd(POM) and those of the type Nd(POM)2 , due to a better shielding of the neodymium(III) ions from the bulk water molecules in the latter type of complexes. © 2005 Elsevier B.V. All rights reserved. Keywords: Heteropolyanions; Judd–Ofelt theory; Lanthanides; Neodymium; Polyoxometalates; POMs; Rare earths

1. Introduction The chemistry of polyoxometalate (POM) anions is a modern and important subfield of inorganic chemistry. These compounds have received much attention because of their interesting chemical and physicochemical properties. The polyoxometalates (POMs) are important as reagents in analytical chemistry, and they find widespread applications in catalysis, molecular biology, materials sciences and medicine [1–6]. In our previous investigations on lanthanide(III)containing polyoxometalates, we used the hypersensitive transitions of neodymium(III)- and erbium(III)-containing species to study the complex formation between the trivalent lanthanide ions and polyoxometalates in aqueous solution and in non-aqueous solvents [7–12]. The absorption spectra recorded in the region of the hypersensitive transitions of solutions of the neodymium(III) ∗

Corresponding author. Tel.: +32 16 32 74 46; fax: +32 16 32 79 92. E-mail address: [email protected] (K. Binnemans). 1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.01.019

salts and polyoxometalate anions show characteristic maxima (λmax ) corresponding to the solvated Nd3+ ion and to the Nd(POM) and Nd(POM)2 species. The analysis of absorption spectra revealed appearance of Nd(POM), Nd(POM)2 and Nd2 (POM) type of complexes, depending on the molar ratio between the neodymium(III) ions and the polyoxometalate anions [9]. Computer-assisted target factor analysis has been applied to evaluate the absorption spectra of neodymium(III) in DMSO solutions containing varying amounts of POMs. The conditional formation constants with log β12 = 8.6 ± 0.5, log β12 = 8.2 ± 0.4 and log β12 = 9.6 ± 0.7 were obtained for the complexes with (Ph3 PEt)5 H3 SiMo2 W9 O39 , (Ph3 PC16 H33 )7 HSiMo2 W9 O39 and (Ph3 PCPh3 )5 H3 SiMo2 W9 O39 , respectively [12]. The Judd–Ofelt theory has often been applied to absorption spectra of trivalent lanthanide ions for describing the intensities of the induced electric dipole transitions [13–16]. Studies on different types of matrices revealed that for most trivalent lanthanide(III) ions, a good agreement between the experimental and calculated dipole strengths (or oscillator strengths) is obtained. A well-known exception is the

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praseodymium(III) ion, and modifications to the Judd–Ofelt theory have been proposed to obtain a better agreement between experiment and theory [15]. To the best of our knowledge, only Peacock has applied the Judd–Ofelt theory to polyoxometalates. He obtained intensity parameters for the complexes K7 [LnW10 O35 ], where Ln = Pr, Nd, Eu, Dy, Ho and Er [17]. In the present paper, we have investigated the absorption spectra of the neodymium(III)-containing polyoxometalates (K11 [Nd(PMox W11−x O39 )2 ], x = 0–7; K13 [Nd(SiMo2 W9 O39 )2 ], K10 [Nd(P2 W17 O61 )2 ], Na9 [NdW10 O36 ], K12 [NdP5 W30 O110 ], K25 [NdAs4 W40 O140 ] and K16 [NdSb9 W21 O86 ]) in aqueous solution. The intensities of the f–f transitions are parameterised in terms of the Judd–Ofelt theory. The nearinfrared luminescence spectra of the different polyoxometalates have been measured and the differences between the Nd(POM) and the Nd(POM)2 complexes will be discussed.

479

was for all measurements 0.002 mol dm−3 . The steady state luminescence spectra and the lifetime measurements in the infrared region were measured on an Edinburgh Instruments FS920P near-infrared spectrometer, with a 450 W xenon lamp as the steady state excitation source, a double excitation monochromator (1800 lines mm−1 ), an emission monochromator (600 lines mm−1 ) and a liquid nitrogen cooled Hamamatsu R5509-72 near-infrared photomultiplier tube. For the lifetime measurements, the setup included a Nd:YAG laser, which allows laser excitation of the sample at 355 nm or 532 nm. The repetition rate was 10 Hz and the pulse width was 3–5 ns. The luminescence lifetimes have been determined by measurement of the luminescence decay curves. The concentration of the aqueous solutions for luminescence studies was 0.005 M.

3. Theoretical background 2. Experimental procedures All reagents used in these studies were of at least analytical grade. Nd(NO3 )3 ·6H2 O was prepared from Nd2 O3 (99.9%), and the potassium salts of the POMs studied were synthesised and characterised according to literature methods [5–10,18]. Polyoxometalate complexes with lacunary structures (for example Keggin’s type of polyoxoanions [PW11 O39 ]7− ) were obtained by partial degradation of the corresponding plenary structures ([PW12 O40 ]3− ) [19]. The neodymium(III)containing sandwich complexes Nd(POM)2 were obtained according to a method described by Peacock and Weakley [20], and modified by us [8]. In the same way, the Dawson’s type of POM anion [P2 W17 O61 ]10− and the corresponding neodymium(III)-containing sandwich complex [Nd(P2 W17 O61 )2 ]17− were prepared. Elemental analysis was made on an Elemental Analyser model VARIO ELIII, and the data were used to calculate the number of moles of crystallization water in the complexes. Molybdenum and tungsten were determined using our spectrophotometric method [21]. Thermogravimetric analysis was performed on a SETARAM SETSYS TG-DSC 15 system. The measuring conditions were as follows: temperature interval 20–550 ◦ C, heating rate: 2 ◦ C/min, specimen weight: 8–12 mg, air flux: 1.9 L/h. CuSO4 ·5H2 O was used as the reference material. The IR spectra were obtained by means of a FTIR Bruker IFS 113v spectrometer, and the samples (∼2 mg) were incorporated in KBr pellets. Identification of the synthesised compounds was done by comparison of their IR spectra with those of previously in the literature reported spectra, and with the use of elemental and thermogravimetric analysis data. The absorption spectra were recorded at ambient temperature on a Varian Cary 5000 UV–vis–NIR spectrophotometer, with the use of a 5 cm quartz cell. The metal complexes were dissolved in water in volumetric flasks with a content of 5 mL. The concentration of the neodymium(III) solutions

The transitions observed in the absorption spectra of the trivalent lanthanide ions are intraconfigurational f–f transitions. The majority of these transitions are induced electric dipole (ED) transitions, although a few magnetic dipole (MD) transitions are known [15]. The intensities of the transitions can be characterised by the dipole strength D: D=

1 108.9Cd




A (¯ν) d¯ν ν¯

(1)

where C is the concentration of the lanthanide ion (mol L−1 ), d the optical path length (cm), A the absorbance (A = −log(I/I0 )) and ν¯ the wavenumber (cm−1 ). We are conscious of the fact that we do not use S.I., but the C.G.S. system. In spectroscopy, the S.I.-units are not easy to handle and their use often obscures simple relations between different quantities. The dipole strength is expressed in D2 (Debye2 ). According to the Judd–Ofelt theory [13–16], the intensity of induced electric dipole transitions can be described in terms of three phenomenological intensity parameters Ωλ (λ = 2, 4 and 6):

D=

2 1036 (n2 + 2) 2  Ωλ |JU (λ) J  |2 e 2J + 1 9n

(2)

λ=2,4,6

The factor 1036 is used for the conversion between D2 units and esu2 cm2 (1 D = 10−18 esu cm). The elementary charge e is 4.803 × 10−10 esu. The degeneracy of the ground state is 2

+2) equal to 2J + 1. The factor (n 9n takes into account that the lanthanide ion is not in a vacuum, but in a dielectric medium (n is the refractive index of the solution). Finally, the |JU(λ) J |2 are the squared reduced matrix elements. The Ωλ parameters can be determined by a standard-least squares fitting method. 2

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4. Results and discussion Nine different types of neodymium(III)-containing polyoxometalates (POMs) have been prepared in order to have a broad variety of structures: [Nd(PW11 O39 )2 ]11− , [Nd (PMo2 W9 O39 )2 ]11− , [Nd(PMo4 W7 O39 )2 ]11− , [Nd(PMo6 W5 O39 )2 ]11− , [Nd(SiMo2 W9 O39 )2 ]13− , [Nd(P2 W17 17− O61 )2 ] , [NdW10 O36 ]9− , [NdP5 W30 O110 ]12− and [NdAs4 W40 O140 ]25− . The compounds were obtained as potassium salts, except [NdW10 O36 ]9− , which was prepared in the form of a sodium salt. The compounds contain different quantities of crystal water, and the number of moles of crystal water per mole of polyoxometalate was determined by thermogravimetric analysis. The polyoxometalate complexes are highly hydrated species. The number of water molecules per molecule of polyoxometalate complex varied between 16 for K11 [Nd(PW11 O39 )2 ]·16H2 O and 54 for K25 [NdP5 W30 O110 ]·54H2 O. An observed trend is that the hydration number increases with increasing charge of the polyoxometalate anion. The polyoxometalate complexes we studied can be divided in two classes, the Nd(POM) and the Nd(POM)2 complexes. The polyoxometalates [NdP5 W30 O110 ]12− , [NdAs4 W40 O140 ]25− and K16 [NdSb9 W21 O86 ]16− are of the type Nd(POM) and can be considered as inorganic cryptands and have 3–4 water molecules in the inner coordination sphere of the neodymium(III) ion in solids and in aqueous solutions [9]. The other neodymium(III)-containing polyoxometalates are sandwich complexes of the type Nd(POM)2 . These Dawson’s type of polyoxometalates have no water molecules in the first coordination sphere of the neodymium ion. The europium(III) analogues show a high luminescence intensity and long luminescence lifetimes, both in the solid state and water solutions [8–11]. For instance the polyoxometalate [EuW10 O36 ]9− is one of the best luminescent POM compounds [9,22]. The absorption spectra of the neodymium(III)-containing polyoxometalate complexes were recorded in aqueous solution at room temperature. Room temperature spectra are necessary for application of the Judd–Ofelt theory, because this theoretical model assumes that all crystal-field levels of the ground state are equally populated [13–15]. This condition is fairly good fulfilled at room temperature, if the crystalfield splitting is not too large (a few hundred cm−1 ). All the transition observed in the absorption spectrum start from the 4I 2S+1 L levels 9/2 ground state. Transitions to the following J 4 2 4 4 4 4 were observed: F3/2 , H9/2 , F5/2 , F7/2 , S3/2 , F9/2 , 2 H11/2 , 4G , 2G , 4G , 2K 4 2 4 2 7/2 7/2 13/2 , G9/2 , K15/2 , G11/2 , D3/2 and 5/2 2 G . Although in some complexes the 2 P 4 9/2 1/2 ← I9/2 transition could be observed (at about 23 200 cm−1 ), it is obscured in other complexes by the tail of the charge transfer transition O → M (M = Mo, W). Due to the charge transfer transitions, no f–f transitions could be observed in the ultraviolet spectral region. The absorption spectrum of [Nd(PW11 O39 )2 ]11− is shown in Fig. 1. The absorption spectra of the complexes of the other neodymium(III)-containing polyoxometalates of

Fig. 1. Absorption spectrum of the neodymium(III)-containing polyoxometalate [Nd(PW11 O39 )2 ]11− in aqueous solution (concentration 0.002 M, ambient temperature). All the transitions start from the 4 I9/2 ground state of Nd3+ . The most intense transitions have been labeled.

the type Nd(POM)2 are very similar, both what concerns the fine structure of the absorption bands and the intensities. Even the hypersensitive transition 4 G5/2 ,2 G7/2 ← 4 I9/2 at about 17 200 cm−1 reveals no differences in spectroscopic behaviour of the various complexes of type Nd(POM)2 . The transitions in the absorption spectrum were assigned by comparing with the values reported by Carnall et al. for LaF3 :Nd3+ [23]. From the spectra, the experimental dipole strengths were derived and these were used to determine the Judd–Ofelt intensity parameters Ωλ (λ = 2, 4 and 6). The matrix elements in the fitting procedure were these given by Carnall et al. for Nd3+ in aqueous solution [24]. In the case of overlapping transitions, the matrix elements of the corresponding transitions were summed. All the transitions observed in the spectrum are induced electric dipole transitions. No magnetic dipole contributions were taken into account. The spectral assignments, the experimental and calculated electric dipole strengths for Nd3+ in the polyoxometalate complex [Nd(PW11 O39 )2 ]11− are given in Table 1. The intensity results for the POM complexes are analogous, so that we restrict ourselves to report only the Ωλ parameters (Table 2). The similarity between the absorption spectra of the neodymium(III)-containing polyoxometalates is also reflected by the Judd–Ofelt intensity parameters. Within the limits of the experimental errors, all the sets of Ωλ parameters of the Nd(POM)2 complexes are identical. The Judd–Ofelt parameters of the Nd(POM) differ from those of the Nd(POM)2 complexes. The Ωλ parameters of the [NdW10 O36 ]9− and the [NdP5 W30 O110 ]25− complexes are smaller than those of the Nd(POM)2 complexes, whereas significantly larger values are observed for the [NdAs4 W40 O140 ]25− complexes. For all the complexes except for [NdAs4 W40 O140 ]25− , the following trend is observed for the numerical values of the Judd–Ofelt param-

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Table 1 Measured and calculated dipole strengths for the transitions in the absorption spectrum of [Nd(PW11 O39 )2 ]11−a Transition ← 4 I9/2

Wavenumber (cm−1 )

Dexp (×10−6 D2 )

Dcalc (×10−6 D2 )

Dexp − Dcalc (×10−6 D2 )

Dexp /Dcalc

4F

11450 12410 13540 14780 15920 17130 19060 20890 23200

313 1295 1616 104 21 1442 766 205 18

358 1426 1553 105 27 1455 559 129 42

−46 −131 +63 −1 −5 −13 +207 +76 −24

0.87 0.91 1.04 0.99 0.78 0.99 1.37 1.59 0.43

3/2 2H , 4F 9/2 3/2 4F , 4S 7/2 3/2 4F 9/2 2H 11/2 4G , 2G 5/2 7/2 4G , 2K 4 7/2 13/2 , G9/2 2K 4 15/2 , G11/2 2D , 2G 3/2 9/2

a The Judd–Ofelt parameters used for the intensity calculation are: Ω = (2.61 ± 0.88) × 10−20 cm2 , Ω = (3.70 ± 1.29) × 10−20 cm2 and 2 4 Ω6 = (8.33 ± 0.58) × 10−20 cm2 . The R.M.S. value is 110 × 10−6 Debye2 .

Table 2 Judd–Ofelt intensity parameters Ωλ (λ = 2, 4, 6) and observed luminescence lifetimes τ for neodymium(III)-containing polyoxometalates Complex

Ω2 (×10−20 cm2 )

Ω4 (×10−20 cm2 )

Ω6 (×10−20 cm2 )

K11 [Nd(PW11 O39 )2 ]·16H2 O K11 [Nd(PMo2 W9 O39 )2 ]·28H2 O K11 [Nd(PMo4 W7 O39 )2 ]·28H2 O K11 [Nd(PMo6 W5 O39 )2 ]·28H2 O K13 [Nd(SiMo2 W9 O39 )2 ]·30H2 O K17 [Nd(P2 W17 O61 )2 ]·32H2 O Na9 [NdW10 O36 ]·19H2 O K25 [NdP5 W30 O110 ]·54H2 O K25 [NdAs4 W40 O140 ]·39H2 O

2.61 ± 0.88 3.25 ± 1.15 3.54 ± 1.1 3.59 ± 1.04 3.81 ± 1.08 3.69 ± 1.35 1.95 ± 0.47 1.54 ± 0.33 6.13 ± 1.07

3.70 ± 1.29 5.27 ± 1.68 5.32 ± 1.61 4.59 ± 1.52 4.8 ± 1.58 5.7 ± 1.98 3.3 ± 0.69 2.24 ± 0.49 6.06 ± 1.56

8.33 9.29 9.12 8.42 8.81 10.15 5.22 2.79 8.92

eters: Ω2 < Ω4 < Ω6 . For [NdAs4 W40 O140 ]25− , we have: Ω2 ∼ = Ω4 < Ω6 . The intensity ratio Ω6 /Ω4 of the Nd(POM)2 complexes varies between 1.71 for [Nd(PMo4 W7 O39 )2 ]11− and 2.25 for [Nd(PW11 O39 )2 ]11− . Lower values of the Ω6 /Ω4 ratio are observed for the Nd(POM) complexes, with the lowest value being 1.25 for [NdP5 W30 O110 ]25− . These observations make clear that the Ω6 /Ω4 ratios for the neodymium(III)-containing polyoxometalates are significantly different from the values observed for Nd3+ in inorganic glasses, where we find empirically that Ω6 ∼ = Ω4 [25]. Although the Nd3+ ion is well-known to be an nearinfrared emitter, no near-infrared luminescence is in general observed for neodymium(III) compounds in aqueous solution, because of efficient radiationless deactivation of the excited states of the neodymium(III) ion (multiphonon relaxation). We made measurements of the luminescence of different neodymium(III)-containing polyoxometalates in aqueous solution. Although the solutions were only weakly luminescent, marked differences could be observed for the luminescence lifetimes of the complexes: the lifetimes of the complexes of the type Nd(POM)2 are much longer than the lifetimes of the complexes of the type Nd(POM). For instance, the luminescence lifetime of the complex [Nd(PW11 O39 )2 ]11− is 411 ± 6 ns, whereas a luminescence lifetime of only 67 ± 2 ns is observed for the [NdAs4 W40 O140 ]25− . The observed lifetimes are also included in Table 2. These findings can be rationalised by the fact that in the Nd(POM)2 the Nd3+ ion is much better shielded from its environment than in the Nd(POM) complexes. As mentioned above, in the Nd(POM)2 complexes no

± ± ± ± ± ± ± ± ±

0.58 0.76 0.72 0.68 0.71 0.84 0.31 0.22 0.71

τ (ns) 411 394 381 376 394 432 151 64 67

± ± ± ± ± ± ± ± ±

6 5 4 5 5 4 5 6 2

water molecules are present in the first coordination sphere of the neodymium(III) ion, whereas in the Nd(POM) complexes three or four water molecules are situated in the first coordination sphere of the neodymium(III) ion. Radiationless deactivation of the excited states is much less efficient in the Nd(POM)2 complexes than in the case of the Nd(POM) complexes. The near-infrared luminescence spectrum of [Nd(P2 W17 O61 )2 ]17− is shown in Fig. 2. The observed sharp peaks are transitions between the 4 F3/2 level and the different J levels of the ground state 4 I term (4 IJ , J = 9/2–13/2). The most intense transition is the 4 F3/2 → 4 I11/2 transition at 9450 cm−1 (about 1060 nm).

Fig. 2. Near-infrared luminescence spectrum of [Nd(P2 W17 O61 )2 ]17− in aqueous (concentration 0.005 M, ambient temperature).

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5. Conclusions An intensity study of the neodymium(III)-containing polyoxometalates [Nd(PW11 O39 )2 ]11− , [Nd(PMo2 W9 O39 )2 ]11− , [Nd(PMo4 W7 O39 )2 ]11− , [Nd(PMo6 W5 O39 )2 ]11− , [Nd(SiMo2 W9 O39 )2 ]13− , [Nd(P2 W17 O61 )2 ]17− , [NdW10 O36 ]9− , [NdP5 W30 O110 ]12− and [NdAs4 W40 O140 ]25− reveals that the intensity of the f–f complexes is mainly determined by the type of complex, being Nd(POM) or Nd(POM)2 , rather than by the chemical constitution of the complex. The Judd–Ofelt analysis indicates that the values of the Ωλ intensity parameters are of medium size, with the parameters following the trend: Ω2 < Ω4 < Ω6 . For [NdAs4 W40 O140 ]25− , the order is Ω2 ∼ = Ω4 < Ω6 . The aqueous solutions of neodymium(III)-containing POMs are only weakly luminescent in the near-infrared region.

Acknowledgments KB, RVD and TNPV thank the FWO-Flanders (Belgium) for a Postdoctoral Fellowship. Financial support from the FWO-Flanders (G.0117.03) and from the K.U.Leuven (GOA 03/03) is gratefully acknowledged. This work has been funded by the Flemish and Polish government (bilateral project BIL03/17 between Flanders and Poland).

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