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

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1 Spectrochimica Acta Part A 62 (2005) 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 Görller-Walrand b, Koen Binnemans b, a Faculty of Chemistry, Adam Mickiewicz University, Poznań, Poland b 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(PW 11 O 39 ) 2 ] 11, [Nd(PMo 2 W 9 O 39 ) 2 ] 11, [Nd(PMo 4 W 7 O 39 ) 2 ] 11, [Nd(PMo 6 W 5 O 39 ) 2 ] 11, [Nd(SiMo 2 W 9 O 39 ) 2 ] 13, [Nd(P 2 W 17 O 61 ) 2 ] 17, [NdW 10 O 36 ] 9, [NdP 5 W 30 O 110 ] 12 and [NdAs 4 W 40 O 140 ] 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 Ω λ (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 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.: ; fax: address: koen.binnemans@chem.kuleuven.ac.be (K. Binnemans). salts and polyoxometalate anions show characteristic maxima (λ max ) corresponding to the solvated Nd 3+ 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 Nd 2 (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 β 12 = 8.6 ± 0.5, log β 12 = 8.2 ± 0.4 and log β 12 = 9.6 ± 0.7 were obtained for the complexes with (Ph 3 PEt) 5 H 3 SiMo 2 W 9 O 39, (Ph 3 PC 16 H 33 ) 7 HSiMo 2 W 9 O 39 and (Ph 3 PCPh 3 ) 5 H 3 SiMo 2 W 9 O 39, 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 /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.saa

2 S. But et al. / Spectrochimica Acta Part A 62 (2005) 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 K 7 [LnW 10 O 35 ], 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 (K 11 [Nd(PMo x W 11 x O 39 ) 2 ], x = 0 7; K 13 [Nd(SiMo 2 W 9 O 39 ) 2 ], K 10 [Nd(P 2 W 17 O 61 ) 2 ], Na 9 [NdW 10 O 36 ], K 12 [NdP 5 W 30 O 110 ], K 25 [NdAs 4 W 40 O 140 ] and K 16 [NdSb 9 W 21 O 86 ]) 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. 2. Experimental procedures All reagents used in these studies were of at least analytical grade. Nd(NO 3 ) 3 6H 2 O was prepared from Nd 2 O 3 (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 [PW 11 O 39 ] 7 ) were obtained by partial degradation of the corresponding plenary structures ([PW 12 O 40 ] 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 [P 2 W 17 O 61 ] 10 and the corresponding neodymium(iii)-containing sandwich complex [Nd(P 2 W 17 O 61 ) 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 SE- TARAM SETSYS TG-DSC 15 system. The measuring conditions were as follows: temperature interval C, heating rate: 2 C/min, specimen weight: 8 12 mg, air flux: 1.9 L/h. CuSO 4 5H 2 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 was for all measurements 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 R 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 M. 3. Theoretical background 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 A ( ν) d ν (1) 108.9Cd ν where C is the concentration of the lanthanide ion (mol L 1 ), d the optical path length (cm), A the absorbance (A = log(i/i 0 )) 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 D 2 (Debye 2 ). 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 Ω λ (λ =2,4 and 6): D = J + 1 (n 2 + 2) 2 9n e 2 λ=2,4,6 Ω λ J U (λ) J 2 (2) The factor is used for the conversion between D 2 units and esu 2 cm 2 (1D=10 18 esu cm). The elementary charge e is esu. The degeneracy of the ground state is equal to 2J + 1. The factor (n2 +2) 2 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 J U (λ) J 2 are the squared reduced matrix elements. The Ω λ parameters can be determined by a standard-least squares fitting method.

3 480 S. But et al. / Spectrochimica Acta Part A 62 (2005) 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(PW 11 O 39 ) 2 ] 11, [Nd (PMo 2 W 9 O 39 ) 2 ] 11, [Nd(PMo 4 W 7 O 39 ) 2 ] 11, [Nd(PMo 6 W 5 O 39 ) 2 ] 11, [Nd(SiMo 2 W 9 O 39 ) 2 ] 13, [Nd(P 2 W 17 O 61 ) 2 ] 17, [NdW 10 O 36 ] 9, [NdP 5 W 30 O 110 ] 12 and [NdAs 4 W 40 O 140 ] 25. The compounds were obtained as potassium salts, except [NdW 10 O 36 ] 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 K 11 [Nd(PW 11 O 39 ) 2 ] 16H 2 O and 54 for K 25 [NdP 5 W 30 O 110 ] 54H 2 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 [NdP 5 W 30 O 110 ] 12, [NdAs 4 W 40 O 140 ] 25 and K 16 [NdSb 9 W 21 O 86 ] 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 [EuW 10 O 36 ] 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 4 I 9/2 ground state. Transitions to the following 2S+1 L J levels were observed: 4 F 3/2, 2 H 9/2, 4 F 5/2, 4 F 7/2, 4 S 3/2, 4 F 9/2, 2 H 11/2, 4 G 5/2, 2 G 7/2, 4 G 7/2, 2 K 13/2, 4 G 9/2, 2 K 15/2, 4 G 11/2, 2 D 3/2 and 2 G 9/2. Although in some complexes the 2 P 1/2 4 I 9/2 transition could be observed (at about 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(PW 11 O 39 ) 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(PW 11 O 39 ) 2 ] 11 in aqueous solution (concentration M, ambient temperature). All the transitions start from the 4 I 9/2 ground state of Nd 3+. 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 G 5/2, 2 G 7/2 4 I 9/2 at about 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 LaF 3 :Nd 3+ [23]. From the spectra, the experimental dipole strengths were derived and these were used to determine the Judd Ofelt intensity parameters Ω λ (λ = 2, 4 and 6). The matrix elements in the fitting procedure were these given by Carnall et al. for Nd 3+ in aqueous solution [24]. Inthe 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 Nd 3+ in the polyoxometalate complex [Nd(PW 11 O 39 ) 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 Ω λ 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 Ω λ 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 Ω λ parameters of the [NdW 10 O 36 ] 9 and the [NdP 5 W 30 O 110 ] 25 complexes are smaller than those of the Nd(POM) 2 complexes, whereas significantly larger values are observed for the [NdAs 4 W 40 O 140 ] 25 complexes. For all the complexes except for [NdAs 4 W 40 O 140 ] 25, the following trend is observed for the numerical values of the Judd Ofelt param-

4 S. But et al. / Spectrochimica Acta Part A 62 (2005) Table 1 Measured and calculated dipole strengths for the transitions in the absorption spectrum of [Nd(PW 11 O 39 ) 2 ] 11 a Transition 4 I 9/2 Wavenumber (cm 1 ) D exp ( 10 6 D 2 ) D calc ( 10 6 D 2 ) D exp D calc ( 10 6 D 2 ) D exp /D calc 4 F 3/ H 9/2, 4 F 3/ F 7/2, 4 S 3/ F 9/ H 11/ G 5/2, 2 G 7/ G 7/2, 2 K 13/2, 4 G 9/ K 15/2, 4 G 11/ D 3/2, 2 G 9/ a The Judd Ofelt parameters used for the intensity calculation are: Ω 2 = (2.61 ± 0.88) cm 2, Ω 4 = (3.70 ± 1.29) cm 2 and Ω 6 = (8.33 ± 0.58) cm 2. The R.M.S. value is Debye 2. Table 2 Judd Ofelt intensity parameters Ω λ (λ = 2, 4, 6) and observed luminescence lifetimes τ for neodymium(iii)-containing polyoxometalates Complex Ω 2 ( cm 2 ) Ω 4 ( cm 2 ) Ω 6 ( cm 2 ) τ (ns) K 11 [Nd(PW 11 O 39 ) 2 ] 16H 2 O 2.61 ± ± ± ± 6 K 11 [Nd(PMo 2 W 9 O 39 ) 2 ] 28H 2 O 3.25 ± ± ± ± 5 K 11 [Nd(PMo 4 W 7 O 39 ) 2 ] 28H 2 O 3.54 ± ± ± ± 4 K 11 [Nd(PMo 6 W 5 O 39 ) 2 ] 28H 2 O 3.59 ± ± ± ± 5 K 13 [Nd(SiMo 2 W 9 O 39 ) 2 ] 30H 2 O 3.81 ± ± ± ± 5 K 17 [Nd(P 2 W 17 O 61 ) 2 ] 32H 2 O 3.69 ± ± ± ± 4 Na 9 [NdW 10 O 36 ] 19H 2 O 1.95 ± ± ± ± 5 K 25 [NdP 5 W 30 O 110 ] 54H 2 O 1.54 ± ± ± ± 6 K 25 [NdAs 4 W 40 O 140 ] 39H 2 O 6.13 ± ± ± ± 2 eters: Ω 2 < Ω 4 < Ω 6. For [NdAs 4 W 40 O 140 ] 25, we have: Ω 2 = Ω4 < Ω 6. The intensity ratio Ω 6 /Ω 4 of the Nd(POM) 2 complexes varies between 1.71 for [Nd(PMo 4 W 7 O 39 ) 2 ] 11 and 2.25 for [Nd(PW 11 O 39 ) 2 ] 11. Lower values of the Ω 6 /Ω 4 ratio are observed for the Nd(POM) complexes, with the lowest value being 1.25 for [NdP 5 W 30 O 110 ] 25. These observations make clear that the Ω 6 /Ω 4 ratios for the neodymium(iii)-containing polyoxometalates are significantly different from the values observed for Nd 3+ in inorganic glasses, where we find empirically that Ω 6 = Ω4 [25]. Although the Nd 3+ 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(PW 11 O 39 ) 2 ] 11 is 411 ± 6 ns, whereas a luminescence lifetime of only 67 ± 2 ns is observed for the [NdAs 4 W 40 O 140 ] 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 Nd 3+ ion is much better shielded from its environment than in the Nd(POM) complexes. As mentioned above, in the Nd(POM) 2 complexes no 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(P 2 W 17 O 61 ) 2 ] 17 is shown in Fig. 2. The observed sharp peaks are transitions between the 4 F 3/2 level and the different J levels of the ground state 4 I term ( 4 I J, J = 9/2 13/2). The most intense transition is the 4 F 3/2 4 I 11/2 transition at 9450 cm 1 (about 1060 nm). Fig. 2. Near-infrared luminescence spectrum of [Nd(P 2 W 17 O 61 ) 2 ] 17 in aqueous (concentration M, ambient temperature).

5 482 S. But et al. / Spectrochimica Acta Part A 62 (2005) Conclusions An intensity study of the neodymium(iii)-containing polyoxometalates [Nd(PW 11 O 39 ) 2 ] 11, [Nd(PMo 2 W 9 O 39 ) 2 ] 11, [Nd(PMo 4 W 7 O 39 ) 2 ] 11, [Nd(PMo 6 W 5 O 39 ) 2 ] 11, [Nd(SiMo 2 W 9 O 39 ) 2 ] 13, [Nd(P 2 W 17 O 61 ) 2 ] 17, [NdW 10 O 36 ] 9, [NdP 5 W 30 O 110 ] 12 and [NdAs 4 W 40 O 140 ] 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 Ω λ intensity parameters are of medium size, with the parameters following the trend: Ω 2 < Ω 4 < Ω 6. For [NdAs 4 W 40 O 140 ] 25, the order is Ω 2 = Ω4 < Ω 6. 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 ) 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). References [1] M.T. Pope, Heteropoly and Isopolyoxometalates, Springer, New York, [2] M.T. Pope, A. Muller (Eds.), Polyoxometalates: From Platonic Solids to Anti-Retroviral Activity, Kluwer Academic Publishers, Dordrecht, [3] M.T. Pope, A. Muller, Angew. Chem. Int. Ed. Engl. 30 (1991) 34. [4] I.A. Weinstock, Chem. Rev. 98 (1998) 113. [5] J.T. Rhule, C.L. Hill, D.A. Judd, R.F. Schinazi, Chem. Rev. 98 (1998) 327. [6] D.E. Katsoulis, Chem. Rev. 98 (1998) 359. [7] S. Lis, J. Alloys Compd (2001) 88. [8] S. Lis, S. But, J. Inclusion Phenom. Mol. Recogn. Chem. 35 (1999) 225. [9] S. Lis, S. But, J. Alloys Compd (2000) 370. [10] G. Meinrath, S. Lis, S. But, M. Elbanowski, Talanta 55 (2001) 371. [11] S. Lis, S. But, A.M. Kłonkowski, B. Grobelna, Int. J. Photoen. 5 (2003) 233. [12] S. Lis, S. But, G. Meinrath, J. Alloys Compd. 374 (2004) 366. [13] B.R. Judd, Phys. Rev. 127 (1962) 750. [14] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [15] C. Görller-Walrand, K. Binnemans, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 25, North-Holland, Amsterdam, 1998, p. 101, Chapter 167. [16] R.D. Peacock, Struct. Bond. 22 (1975) 83. [17] R.D. Peacock, Mol. Phys. 4 (1973) 817. [18] S. Lis, M. Elbanowski, S. But, Acta Phys. Pol. A 90 (1996) 361. [19] Y. Jeannin, Martin-Frere, Inorg. Synth. 27 (1990) 71. [20] R.D. Peacock, T.J.R. Weakley, J. Chem. Soc. A (1971) [21] S. Lis, S. But, J. Alloys Compd (2000) 132. [22] T. Yamase, T. Kobayashi, M. Sugeta, H. Naruke, J. Phys. Chem. A 101 (1997) [23] W.T. Carnall, G.L. Goodman, K. Rajnak, R.S. Rana, A Systematic Analysis of the Spectra of the Lanthanides Doped into Single Crystal LaF 3, ANL-88-8 Report, Chemistry Division, Argonne National Laboratory, Argonne, IL, [24] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) [25] K. Binnemans, C. Görller-Walrand, J. Phys.: Condens. Matter 10 (1998) 167.