Sandwich lanthano-silicotungstates: Structure, electrochemistry and photoluminescence properties

Polyhedron 52 (2013) 308–314

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Sandwich lanthano-silicotungstates: Structure, electrochemistry and photoluminescence properties Diana Julião a, Diana M. Fernandes a, Luís Cunha-Silva a, Duarte Ananias b, Salete S. Balula a,⇑, Cristina Freire a,⇑ a b

REQUIMTE & Department of Chemistry and Biochemistry, Faculty of Sciences, Universidade do Porto, 4169-007 Porto, Portugal CICECO, Department of Chemistry, Universidade of Aveiro, 3810-193 Porto, Portugal

a r t i c l e

i n f o

Article history: Available online 23 September 2012 Dedicated to Alfred Werner on the 100th Anniversary of his Nobel Prize in Chemistry in 1913. Keywords: Lanthanide Polyoxometalate Silicotungstates Electrochemistry Luminescence

a b s t r a c t Sandwich-type of luminescent silicotungstates were prepared using the inorganic ligand [SiW11O39]8 (SiW11) coordinating different lanthanide ions (Eu3+, Tb3+, Dy3+): Eu(SiW11)2, Tb(SiW11)2 and Dy(SiW11)2. The potassium salts of these compounds were characterised by several techniques and the crystal structure of Eu(SiW11)2 was studied. The lanthanide ion in the sandwich structure reveled to have some influence in the electrochemical behaviour of these compounds in aqueous solutions, mainly in the first of the two reduction processes observed for the tungsten atoms. The influence of scan rate on the voltammetric characteristics of the first tungsten reduction process led to the conclusion that the process was diffusion-controlled. The pH of the electrolyte solution showed significant effect on the electrochemical behaviour of these compounds, suggesting that the tungsten reductions processes are accompanied by addition of two to three protons. The photoluminescence properties of Eu(SiW11)2 and Tb(SiW11)2 were investigated in some detail, namely as a function of temperature and pressure (with exposure to a high vacuum of ca. 5  106 mbar). Both compounds are iso-structural with a single Ln3+ site and with absence of water molecules coordinated to the lanthanide ions. Contrary to Eu(SiW11)2, the Tb(SiW11)2 compound is not optically active at room temperature and shows a very sensitive photoluminescence with the temperature on the range of 11–150 K which was interpreted by the effect of a back energy transfer de-excitation process. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Polyoxometalates (POMs) are a versatile class of inorganic compounds with remarkable chemical, structural and electronic versatility, having high potential for practical applications in several areas such as catalysis, conductivity, photo- and electrochromic devices and molecular electronics [1,2]. These compounds possess well-defined structures and shapes obtained by coordinating metal–oxygen building units. They are characterised by metallic centres M (WVI, MoVI, VV) called the addenda atoms, surrounded by bridging or terminal oxygen atoms, forming MO6 octahedra linked together through oxygen bridges. Among the most studied POMs are the Keggin anions with the general formula [XM12O40]n, having the central XO4 tetrahedron surrounded by the twelve MO6 octahedra. The physical performance of POMs can be modified by the substitution of one or more of their addenda atoms by other metal elements, by changing primary heteroatom X and modifying counter ions [3]. In fact, the reactivity of the Keggin anion can be

⇑ Corresponding authors. Tel.: +351 220402576; fax: +351 220402659. E-mail addresses: [email protected] (S.S. Balula), [email protected] (C. Freire). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.09.019

dramatically increased by preparing vacant derivatives which are formed by the loss of one or more M atoms with their terminal oxygen atoms. The resulting vacant anion (for example the monovacant [XM11O39]m) are more nucleophilic and hence more reactive and are considered as inorganic multidentate ligands capable to coordinate several elements. Rare earth metals are suitable to link vacant POMs giving rise to a new class of materials with potentially useful luminescence properties. Because the lanthanide ions are larger than the transition metal ions, these cannot be fully incorporated into the vacant site and their high coordination number (P6) leads to a tendency to link two or more vacant POM units [4,5]. Lanthanide containing POMs have attracted a large attention owing to their structure between cations Ln3+ and POM ligands and their characteristic luminescence properties, such as extremely sharp emission bands, long lifetime and good fluorescence monochromaticity [6]. These properties allow their application in different fields such as fluorescence probes in biochemistry, laser and electroluminescent optical devices [4,6]. In fact, some examples can be found in the literature reporting different structures formed by the coordination of monovacant Keggin and Wells–Dawson-type POMs and the lanthanide ions. One of the first compounds formed by the coordination of

D. Julião et al. / Polyhedron 52 (2013) 308–314

POMs and Ln3+ were reported by Peacock and Weakley [7]. In this study the authors showed that the interaction of the monovacant a-[SiW11O39]8 (L) and the Ln3+ could form LnL or LnL2 (sandwich-type) complexes. However, no structural information was provided. In 1991, it was published the synthesis of several compounds of type [Ln(b2-SiW11O39)2]13 (Ln = La, Ce, Pr, Nd Sm, Gd, Er). Some years later, Pope and co-workers reported the synthetic strategies to obtain compounds with different ratios POM:Ln, [SiW11O39]8 (L) unit as coordinating ligand and several Ln3+ [5]. Recently, it was published the single crystal structure of LnL2 complexes with L = b2-[SiW11O39] and Ln = La, Ce, Sm, Eu, Gd, Tb, Yb, Lu [8]. The crystal structure of the sandwich-type complexes with the isomer a-[SiW11O39]8 is only known for [Ln(a-SiW11O39)2]13 (Ln = La and Ce) [9–11]. Lanthano-POMs containing Eu3+, Tb3+ and Dy3+ have shown to be the most interesting compounds because they can produce strong fluorescence [6]. The lanthanide elements such as terbium and europium have attracted more interest for application as high-potential visible luminescence sources. In this paper, we report the preparation and characterisation of three different sandwich-type complexes KxH13X[Ln(a-SiW11O39)2] (Ln(SiW11)2), Ln3+ = Eu, Tb, Dy). The single crystal structure was determined for Eu(SiW11)2 and the electrochemical characterisation was performed for the sandwich-type lanthanide–POMs. In fact, POMs can exhibit fast and reversible multi-electron redox transformations, mainly associated to WVI M WV processes, without any significant structural alteration [12–14]. The influence of the lanthanide nature in the sandwich POM structure is here analysed. Furthermore, the photoluminescent properties of these compounds are also presented.

2. Experimental 2.1. Materials and methods Sodium tungstate (Sigma), sodium metasilicate (Sigma), europium chloride (Sigma), terbium chloride (Sigma), dysprosium chloride (Sigma), chloridric acid (Panreac), potassium chloride (Riedel-de-Haen) and sulfuric acid (Merck) were used as received. Elemental analysis for K, W, Si and lanthanide (Eu, Tb, Dy) was performed by ICP spectrometry (University of Santiago). Hydration water contents were determined by thermogravimetric analysis performed in air between 30 and 700 °C, with heating speed of 5 °C/min, using a TGA-50 Shimadzu thermobalance. Infrared absorption spectra were obtained on a Jasco FT/IR-460 Plus spectrophotometer in the range 400–4000 cm1, using a resolution of 4 cm1 and 32 scans. The spectra were obtained in KBr pellets (Merck, spectroscopic grade). FT-Raman spectra were recorded on a RFS-100 Bruker FT-spectrometer, equipped with a Nd:YAG laser with excitation wavelength of 1064 nm, with laser power set to 200 mW. Electronic absorption spectra were recorded on a Varian cary 50 Bio spectrophotometer in the range 190–1100 nm, at room temperature, using quartz cells with 1 cm optical path. X-ray power diffraction patterns were measured on a RigakuD/Max III instrument using Cu Ka radiation in the range 2b = 3–50°. Cyclic voltammetry measurements were carried out using an Autolab PGSTAT 30 potentiostat/galvanostat (EcoChimie B.V.) controlled by GPES software. Emission and excitation spectra were recorded using a Fluorolog-2Ò Horiba Scientific (Model FL3-2T) spectroscope, with a modular double grating excitation spectrometer (fitted with a 1200 grooves/mm grating blazed at 330 nm) and a TRIAX 320 single emission monochromator (fitted with a 1200 grooves/mm grating blazed at 500 nm, reciprocal linear density of 2.6 nm mm1), coupled to a R928 Hamamatsu photomultiplier, using the front face acquisition mode.

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2.2. Synthesis of sandwich lanthanide–POMs Sandwich lanthanide–POMs KxHy[Ln(SiW11O39)2]nH2O (Ln (SiW11)2) with Ln3+ = Eu, Tb and Dy were prepared following literature procedures [5]. Anal. Calc. for K13[Eu(SiW11O39)2]18H2O (Eu(SiW11)2) (wt%): K, 8.2; W, 60; Eu, 1.4; Si, 1.1; FT-IR (cm1) 1011 (s), 958 (vs), 902 (vs, sh), 835 (vs, sh), 774 (m), 683 (m), 541 (w). Anal. Calc. for K13[Tb(SiW11O39)2]11H2O (Tb(SiW11)2) (wt%): K, 7.9; W, 58; Tb, 1.9; Si, 0.85; FT-IR (cm1) 1005 (s), 956 (vs), 904 (vs, sh), 835 (vs, sh), 765 (m), 682 (m), 548 (w). Anal. Calc. for K9H4[Dy(SiW11O39)2]14H2O (Dy(SiW11)2) (wt%): K, 5.5; W, 66; Dy, 1.9; Si, 1.2; FT-IR (cm1) 1009 (s), 959 (vs), 907 (vs, sh), 838 (vs, sh), 770 (m), 686 (m), 552 (w). These compounds were characterised by elemental and thermal analysis, Fourier transformed infrared (FT-IR) and Raman (FT-Raman) absorption spectroscopy, electronic absorption spectroscopy and powder X-ray diffraction. The results were in agreement with those previously published [5]. 2.3. X-ray crystallography A single crystal of Eu(SiW11)2 was harvested and mounted in a Hampton CryoLoop using viscous FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13) [15]. Diffraction data were collected at 150 K on a Bruker X8 Kappa APEX II charge-coupled device (CCD) area-detector diffractometer (Mo Ka graphite-monochromated radiation, k = 0.71073 Å; crystal positioned at 45 mm from the detector and using 10 s of exposure time) controlled by the APEX2 software [16], and equipped with an Oxford Cryosystems Series 700 cryostream monitored remotely by the CRYOPAD interface [17]. Images were processed using the software package SAINT+ [18], and the absorption correction was performed by the multi-scan semi-empirical method implemented in SADABS [19]. The structure was solved using the direct methods implemented in SHELXS-97 [20,21], allowing the immediate location the Eu-atom and most of the W-atoms. The remaining non-H atoms were located from difference Fourier maps calculated from successive full-matrix least squares refinement cycles on F2 using SHELXL-97 [20,22]. All atoms belonging to the POM and the charge balancing K+ cations were successfully refined using anisotropic displacement parameters, while the water molecules were only refined with isotropic parameters. The H-atoms of the water molecules of crystallization could not be located from difference Fourier maps or even placed in calculated positions, however, all these H atoms were added to the empirical formula of the compound. Crystal data for Eu(SiW11)2: EuH50K13O103Si2W22, M = 6459.54, triclinic, a = 13.6679(11) Å, b = 19.657(3) Å, c = 20.4813(17) Å, a = 110.195(5)°, b = 105.364(4)°, c = 98.710(5)°, V = 4797.1(9) Å3, T =  Z = 2, l = 27.611 mm1, 145 998 reflec150(2) K, space group P1, tions measured, 21 839 independent reflections (Rint = 0.0532). The final R1 values were 0.0382 [I > 2r(I)] and 0.0395 (all data), and the final wR(F2) values were 0.0717 [I > 2r(I)] and 0.0793 (all data). 2.4. Electrochemical studies A conventional three-electrode compartment cell was used with a glassy carbon electrode, GCE, (3 mm diameter, BAS, MF2012) as the working electrode. The auxiliary and reference electrodes were platinum wire (7.5 cm, BAS, MW-1032) and Ag/AgCl (sat. KCl) (BAS, MF-2052), respectively. The cell was enclosed in a grounded Faraday cage and kept under flowing argon during experiments. Prior to use, the GCE was conditioned by a polishing/cleaning procedure. The GCE was polished with aluminium oxide of particle size 0.3 lm (Buehler-Masterprep) on a microcloth polishing pad (BAS Bioanalytical Systems Inc.), and then the electrode was rinsed with ultra-pure water and finally sonicated for

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5 min in an ultrasonic bath (FUNGILAB). Electrolyte solutions for voltammetry were prepared using ultra-pure water (resistivity 18.2 MX cm at 25 °C, Millipore). H2SO4/Na2SO4 buffer solutions within the pH range 3.0–4.5 were prepared by mixing appropriate amounts of a 0.2 mol dm3 H2SO4 solution with a 0.5 mol dm3 Na2SO4 solution. Voltammetric measurements were performed at room temperature. A combined glass electrode (Crison) connected to a pH meter Basic 20+ (Crison) was used for the pH measurements. 2.5. Photoluminescence studies Emission and excitation spectra were recorded at 11 and 298 K. The excitation source was a 450 W Xe arc lamp. Emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. Time-resolved measurements have been carried out using a 1934D3 phosphorimeter coupled to the FluorologÒ-3, and a Xe–Hg flash lamp (6 ls/pulse half width and 20–30 ls tail) was used as the excitation source. The variable temperature measurements were performed using a helium-closed cycle cryostat with vacuum system measuring 5  106 mbar and a Lakeshore 330 auto-tuning temperature controller with a resistance heater. The temperature can be adjusted from 11 to 450 K. 3. Results and discussion 3.1. Characterisation of Ln(SiW11)2 Potassium salts of sandwich lanthanide-silicotungstates [Ln(SiW11O39)2]13 (Ln(SiW11)2) with Ln = Eu3+, Tb3+ and Dy3+ were prepared following publishing procedures [5]. FT-IR spectra of Ln(SiW11)2 compounds are presented in Fig. S1 in Electronic Supporting Information (ESI) and the intense bands observed in the range of 500–1010 cm1 are typical for the Keggin derivative structure [23]. The FT-IR spectra of the three Ln(SiW11)2 compounds are very similar, showing the typical bands attributed to asymmetric stretching vibrations of the silicotungstate: terminal bonds W–Od at 960 cm1, the tetrahedral Si–O bond at 940 cm1 and two types of bridging W–O–W bonds at 830 (W–Ob–W) and 770 cm1 (W–Oc–W), where Ob is the bridge oxygen of two octahedral sharing a corner and Oc is the bridged oxygen of two octahedral sharing an edge. Similar patterns are also found for the FT-Raman spectra of Ln(SiW11)2 (Fig. S1 in ESI). The symmetric stretching vibration of the W–Od terminal oxygen can be identified near 960 cm1 [23]. The similarity of the FT-IR and FT-Raman spectra of the different potassium salts of Ln(SiW11)2 suggests that the nature of the lanthanide cation does not change significantly the framework of the ligand SiW11. The thermogravimetric (TG) curves of Ln(SiW11)2 compounds are shown in Fig. S2 in ESI. These exhibits one step of weight loss in the temperature range of 50–250 °C attributed to release of crystal water: 5.07% (calc. 5.10%) for Eu(SiW11)2, 3.22% (calc. 3.18%) for Tb(SiW11)2 and 4.14% (calc. 4.12%) for Dy(SiW11)2. From these weight losses it was possible to determinate the water content present in the molecular formula of each compound (presented in Section 2.2). The TG curves of the three compounds are similar, which suggest their structure analogy. The absorption spectra of Ln(SiW11)2 solutions (3  105 mol dm3) exhibit a well observed band at 248 nm assigned to the bridge oxygen Ob(c)–W charge transfer transition (Fig. S3 in ESI). At lower wavelength (near 200 nm) is observed other charge transition band that corresponds to the terminal oxygen Od–W. This latter band is

well observed for Eu(SiW11)2 and less pronounced for Tb(SiW11)2 and Dy(SiW11)2, where only a small shoulder near 200 nm is observed. Both charge transfer transitions are characteristic bands of Keggin type compounds and these are in agreement with literature results [24–27]. Suitable single crystals of the Eu3+ based POM for X-ray diffraction analysis could also be isolated and the crystalline structure obtained confirms unequivocally the formation of polyoxoanion, Eu(SiW11)2. To obtain the single crystals of these compounds, it was not necessary to perform hydrothermal synthesis, as described before in the literature for similar lanthanide compounds [9]. The  space group, crystal structure was determined in the triclinic P1 and reveals two monovacant Keggin units [SiW11O39]8 sandwiching one Eu3+ cation; Fig. 1a). The lanthanide ion connects four Oatoms in the lacuna of each Keggin fragment, leading to an eight coordinated centre [EuO8] in a slightly distorted square-antiprismatic geometry (pseudo-D4d symmetry; Fig. 1b), as consequence of the relative rotation of about 40° in the two undecatungstosilicate moieties. The interplanar distance (dpp, defined as the distance between the two square planes containing the four O-atoms) and the average in-plane OO distance (din is average OO distance within the oxygen-based square planes) are 2.610 Å and 2.876 Å, respectively, while the angle between the normal vectors of the oxygen-based square planes (u) is 2.8°. These values and the main structural features are comparable to that already reported for related compounds [8,28,29]. Typically, the sandwich type europium silicotungstate anions Eu(SiW11)2 are surrounded by potassium cations for charge balance and large number of hydration water molecules. The powder XRD patterns of the Eu(SiW11)2, Tb(SiW11)2 and Dy(SiW11)2 compounds are depicted in Fig. S1 in ESI. All the diffractograms are identical, particularly at low angles (2h between 5° and 12°), indicating that the three compounds were isolated as pure phases in the same crystalline system. The minor differences observed in the diffraction patterns at high angles (2h larger than 20°) are consequence of the high positional disorder of the K+ cations and crystallization water molecules typical in most of the POMs.

Fig. 1. (a) Mixed ball-and-stick and polyhedral representation of the sandwich type lanthano-silicotungstate anion, [Eu(SiW11O39)2]13. (b) The [EuO8] coordination centre showing a square-antiprismatic geometry.

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3.2. Electrochemical behaviour of Ln(SiW11)2 in aqueous solution

30

Silicotungstates are unstable in neutral and basic aqueous solutions; consequently the study of their electrochemical behaviour is usually carried out in acid aqueous solutions. Voltammetric studies in aqueous solution (pH range of 2.0–4.5) with the lacunary SiW11 were performed previously [30] and the main features are indicated here for a better understanding of the results for the lanthanide silicotungstates [Ln(SiW11)2]n. In aqueous acid solution, the lacunary anion presented two two-electron electrochemical reductions at Epc1 = 0.501 and Epc2 = 0.705 V, corresponding to the reduction of WVI atoms. The reduction involved the uptake of protons to prevent build-up of negative charge: for this polyanion a 2e/2H+ global process was obtained (slope of Epc versus pH = 0.067 V) [30]. Considering this, the electrochemical studies of potassium salts of Ln(SiW11)2, with Ln3+ = Eu, Tb and Dy, were carried out in a systematic way in aqueous buffer solutions (H2SO4/Na2SO4) in the pH range of 3.0–4.5 in order to evaluate the influence of different Ln3+ cations on the electrochemical responses of Ln(SiW11)2 salts. Fig. 2 shows the cyclic voltammograms of Eu(SiW11)2, Tb(SiW11)2 and Dy (SiW11)2 (5  104 mol dm3) in H2SO4/Na2SO4 buffer solution (pH 3.5) at different scan rates. In the potential range from +0.2 to 1.1 V, Eu(SiW11)2 presents two reductions processes at Epc1 = 0.630 V and Epc2 = 0.886 V (Table S1 in ESI) assigned to the reduction of W atoms [30]. Tb(SiW11)2 also exhibits two reduction processes at Epc1 = 0.750 V and Epc2 = 0.877 V (Table S1 in ESI), but the former process is only observed for scan rates lower than 0.04 V s1. The cyclic voltammograms of Dy(SiW11)2 revealed only reduction processes at more negative potentials, Epc2 = 0.880 V, independently of scan rate (Table S1 in ESI). This suggests that for Tb(SiW11)2 and Dy(SiW11)2, the first tungsten reduction process involves a slow charge transfer, slower for the latter compound since this reduction process is never observed regardless of the scan rate used. However, for the second reduction process, the peak potentials for the three POMs do not differ much from each other (Table S1 in ESI) which indicates that the presence of different Ln3+ atoms have little effect on the second W reduction process. Peak-to-peak separation (DEp) were calculated for the first tungsten reduction process for Eu(SiW11)2 at different scan rates and the values are between 0.031 and 0.037 V, which indicates a two-electron process. Values of |Epc  Ep/2| between 0.028 and 0.042 V support this hypothesis. In the experimental timescale employed (scan rates in the range 0.02–0.5 V s1) the values of peak potentials for the second electrochemical process changed slightly with scan rate (0.006 V for Eu(SiW11)2, 0.019 V for Tb(SiW11)2 and 0.027 V for Dy(SiW11)2). The plots of log ip versus log m show slopes of 0.5, with 0.998 P r P 0.991 (insets in Fig. 2), indicating that this electrochemical process is diffusion controlled. These results are in good agreement with previously reported data [30,31]. The pH of the electrolyte solution has a marked effect on the voltammetric behaviour of polyoxometalates [30,32,33]. With increasing pH, from 3.0 to 4.5, the peak potentials shift to more negative values. Fig. 3 shows plots of peak potential versus pH for Eu(SiW11)2 for Epc1 and Epc2 and the slopes obtained are 0.082 (r = 0.999) and 0.079 V/pH unit (r = 0.991), respectively, for Epc2. For Tb(SiW11)2 and Dy(SiW11)2 the slopes obtained were 0.083 V/pH unit (r = 0.995) and 0.077 V/pH unit (r = 0.991), respectively (see Figs. S5 and S6 in ESI for the plots). The slopes show that the redox mechanism at the tungsten atoms confirms the involvement of protons. So, in the experimental conditions, assuming Nernstian behaviour, and considering previously published works were a slope of 0.067 V/pH unit corresponds to a 2e/2H+ process and 0.122 V/pH unit to a 2e/4H+ process [30],

20 10 0 -5.2 -5.3

-20

log (ipc / A)

i / μA

-5.1

-10

-30

-5.4 -5.5 -5.6

-40

-5.7

-50

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

log (v / V s-1)

-1.2

-0.9

-0.6

-0.3

0.0

0.3

-3

E / V vs. Ag/AgCl (3 mol dm KCl) 40

20

0 -4.5

-20

-4.6 log (ipc / A)

i / μA

-4.4

-40

-4.7 -4.8 -4.9 -5.0 -1.8

-60 -1.2

-0.9

-0.6

-1.6

-1.4

-1.2 -1.0 -0.8 log (v / V s-1)

-0.3

-0.6

0.0

-0.4

0.3

-3

E / V vs. Ag/AgCl (3 mol dm KCl) 40

20

-4.4 -4.5

-20

-4.6 log (ipc / A)

i / μA

0

-40

-4.7 -4.8 -4.9 -5.0 -1.8

-60

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

log (v / V s-1)

-1.2

-0.9

-0.6

-0.3

0.0

0.3

-3

E / V vs. Ag/AgCl (3 mol dm KCl) Fig. 2. Cyclic voltammograms of K+ salt of Eu(SiW11)2 (top), Tb(SiW11)2 (middle) and Dy(SiW11)2 (bottom) with 5  104 mol dm3 and pH 3.5 H2SO4/Na2SO4 buffer solution at scan rates of 0.02, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 and 0.5 V s1. Inset shows log ipc vs. log m for second reduction peak for Eu(SiW11)2 and for the first reduction peak for Tb(SiW11)2 and Dy(SiW11)2.

the tungsten reductions of the Ln(SiW11)2 in this study are accompanied by addition of 2–3 protons.

3.3. Photoluminescence studies of Ln(SiW11)2 The excitation spectra of Eu(SiW11)2 recorded at room-temperature (298 K) (Fig. 4) display only a series of sharp lines, from 355 to 580 nm, characteristics of the Eu3+ intra-4f6 transitions namely,

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-0.56

Epc1 Epc2

-0.64

Epc / V

-0.72

-0.80

-0.88

-0.96

3.0

3.3

3.6

3.9

4

4.5

pH Fig. 3. Plots of peak potential vs. pH for Eu(SiW11)2.

7

F0,1 ? 5D4-0, 5L6 and 5G2-6. However at low temperature (11 K) the corresponding spectrum also presents a structured broad UV band ranging from 240 to 355 nm which may be attributed to a Ligandto-Metal Charge Transfer band (LMCT). In other hand Tb(SiW11)2 just shows photoluminescence at low temperature. The corresponding excitation spectrum at 11 K (Fig. 4) displays the UV broad band similar to the Eu(SiW11)2 compound and the Tb3+ intra-4f8 7 F6 ? 5D4 transition. Considering the similarity between the excitation UV broad bands of Eu(SiW11)2 and Tb(SiW11)2, and their corresponding absorption spectra at room-temperature, all the components of the structured UV band (peaking at ca. 273, 310 and 333 nm) must be attributed to the O ? W LMCT processes. Also, the absence of photoluminescence of Tb3+ at room temperature and the lack of the most energetic excited levels of Eu3+ (namely, 5H3-7 and 5F1-5) and Tb3+ (for instance 5D3) should be due to a back energy transfer process from the Ln3+ ions to the surrounding species (the [SiW11O39]8 Keggin ligands) followed by non-radiative relaxation. The emitting level of Tb3+ (5D4, 485 nm/ 20 620 cm1) is located at higher energy than the Eu3+ (5D0, 578 nm/17 300 cm1) one, given a smaller energy difference from non-radiative surrounding state. Because that, the back energy

7

transfer de-excitation, namely by phonon assisted vibronic relaxation, is more effective for Tb(SiW11)2. In accord with this the Tb3+ emission has a strong dependence with the temperature. As shown in Fig. S7 in ESI (in excitation mode) the photoluminescence decrease linearly from 11 to 125 K and is almost absent at 150 K. The emission spectra of Eu(SiW11)2 recorded at 298 and 11 K (excited at 393 nm) are provided in Fig. 5. The sharp lines are assigned to transitions between the first excited non-degenerate 5 D0 state and the 7F0-4 levels of the fundamental Eu3+ septet. Except for 5D0 ? 7F1, which has a predominant magnetic-dipole character independent of the Eu3+ crystal site, the observed transitions are mainly of electric-dipole nature. The local-field splitting of the 7 F0,1 levels into one and three Stark components, respectively, and the predominance of the 5D0 ? 7F2 transition relatively to the 5D0 ? 7F1, indicates the presence of a single Eu3+ environment as expected from their crystal structure determination. Eu3+ emission is highly sensitive to small modifications on the first coordination sphere of the metal, such as the variation of the number and type of coordinated moieties. For instance, the ratio between the integrated intensities of the 5D0 ? 7F2 and 5D0 ? 7F1 transitions, Ið5 D0 !7 F2 Þ =Ið5 D0 !7 F1 Þ , also known as the asymmetric ratio (R), gives values of 1.45 and 1.64 for the ambient pressure and high vacuum, respectively. These values, typical of relatively high symmetrical local environments, points to a slightly more distorted environment of the Eu3+ coordination polyhedron with the vacuum exposure (please note: a low R-value indicates a high local symmetry for Eu3+). Thus, the observed changes in the room-temperature emission spectrum with the application of a high vacuum of ca. 5  106 mbar (Fig. 5) clearly indicates a structural change on the Eu(SiW11)2 compound probably due to some release of adsorbed water molecules. To further investigate this, the ambient temperature (298 K) 5D0 lifetime of Eu3+ in the Eu(SiW11)2 sample was determined by monitoring the emission decay curves within the maximum of the 5D0 ? 7F2 transition, using an excitation at 393 nm (see Fig. S8 in ESI). The decay curves were well fitted by single exponential functions, yielding lifetimes of 2.34 ± 0.01 and 2.27 ± 0.01 ms, for ambient pressure and high vacuum, respectively. The unique lifetime determination confirms the presence of a unique Eu3+ crystallographic site on the compound. Because the values are similar, with a slight decrease with the vacuum exposure, indicates that the differences observed in the emission spectra cannot be attributed to the release of water molecules coordinated to Eu3+. Even, the calculated number of coordinated

5

Intensity (a. u.)

5

5

3+

F6→ D4 (Tb )

5

250

300

350

400

5

F0,1→ D2

450

7

5

F0,1→ D1

500

550

7

5

F0→ D0 600

5

7

7

D0→ F0

5

F0,1→ D3

5

7

D0→ F3

7

D0→ F2 5

7

D0→ F4

5

7

7

D0→ F1

7

5

F0,1→ GJ

7

7

LMCT

F0,1→ D4

Intensity (a. u.)

F0→ L6

Wavelength (nm)

580 Fig. 4. Excitation spectra of Eu(SiW11)2 at 298 K (black line) and 11 K (red line) while monitoring the emission at 613.8 nm and the excitation spectrum of Tb(SiW11)2 at 11 K (green line) while monitoring the emission at 544.6 nm. The room-temperature absorption spectrum of Eu(SiW11)2 (dash line on top) is also provided for comparison. The intensity is only comparable for the Eu(SiW11)2 excitation spectra. (Colour online.)

600

620

640 660 Wavelength (nm)

680

700

Fig. 5. Emission spectra of Eu(SiW11)2 as a function of temperature and vacuum (kexc = 393 nm): black line – ambient temperature (298 K) and pressure (1 bar); green line – temperature of 298 K and pressure of ca. 5  106 mbar (high vacuum); red line – temperature of 11 K and ca. 5  106 mbar of pressure. (Colour online.)

D. Julião et al. / Polyhedron 52 (2013) 308–314

5

7

Intensity (a. u.)

D4→ F5

5 5

7

D4→ F4

7

D4→ F6

5

7

500

550

600 Wavelength (nm)

7 5

D4→ F1

7

5

V

V

D4→ F2

D4→ F3

650

Fig. 6. Emission spectra of Tb(SiW11)2 at 11 (black line) and 125 K (red line) with the excitation fixed at 311 nm. (Colour online.)

313

redox process no significant influence of lanthanide was detected. The tungsten reductions processes were diffusion controlled and were accompanied by addition of two to three protons. Photoluminescent studies were performed for Eu(SiW11)2 and Tb(SiW11)2. The emission spectra and the 5D0 decay curves of Eu3+ clearly demonstrate the presence of a unique Eu3+ environment without coordinated water molecules on the Eu(SiW11)2 compound. The Eu3+ photoluminescence features also indicates a slight more distorted environment for the sample exposed to a high vacuum (ca. 5  106 mbar), probably due to the release of the adsorbed water molecules. The Tb(SiW11)2 compound showed a strong dependence of the Tb3+ photoluminescence with the temperature, totally absent at room temperature, which was interpreted with an effective back energy transfer process from the Tb3+ to the SiW11 ligands. Consequently, the Tb3+ presents a more complex emitting mechanism, responsible for the bi-exponential nature of their 5D4 decay curve, with lifetimes much shorter than the one expected from the normal photoluminescence features of the Tb3+ relatively to the Eu3+. Acknowledgments

water molecules using the empirical formula of Kimura and Kato [34] (nw ¼ A  ð1=sExp Þ  B, were A = 1.1 and B = 0.71 for Eu3+; intrinsic error of ±0.25) for the room-temperature lifetimes without and with vacuum gives values of 0.24 and 0.23 which indicates the absence of coordinated water molecules to the Eu3+. Although compound Tb(SiW11)2 presents a significant photoemission at 11 K (Fig. 6), with the characteristics sharp lines of the Tb3+ intra-4f8 5D4 ? 7F6-1 transitions, their photoluminescence strongly decreases with the increase of temperature (shown in Fig. 6 for 125 K) and virtually collapse at 150 K as state above in the excitation section. The small bands labelled with ‘‘V’’ in Fig. 6 are attributed to cooperative vibronic transitions with a energy difference of ca. 750 cm1, from the corresponding neighbouring electronic transition located at higher energy, which can be hardly identified in the Tb(SiW11)2 Raman spectra. The Tb3+ 5D4 decay curve at 11 K (Fig. S9 in ESI) is only conveniently fitted by a biexponential function given lifetimes of 0.52 ± 0.02 and 1.74 ± 0.01 ms, contrary to the corresponding Eu3+ 5D0 singleexponential decay curve with a fitted lifetime of 2.58 ± 0.01 ms. These results are totally unexpected, as Tb(SiW11)2 and Eu(SiW11)2 are iso-structural with a single Ln3+ site and because the luminescence lifetimes of Tb3+ 5D4 level are commonly, by far, longer than the ones of Eu3+ 5D0 level. For instance, the hydrated salts in the solid state shows a lifetime of four times longer for Tb3+ 5D4 level relatively to the Eu3+ 5D0 one [35]. The discrepancy of these results can just be rationalised by the previous mentioned back energy transfer process, responsible by the strong reduction on the expected Tb3+ 5D4 lifetime and by a more complex emission mechanism which originates the bi-exponential emission decay.

4. Conclusions Three different sandwich-type lanthano-silicotungstates (Eu(SiW11)2, Tb(SiW11)2, Dy(SiW11)2) were successfully prepared and the influence of the lanthanide nature in the sandwich structure was analysed. A pronounced similarity between the three potassium salts compounds were found by the different techniques used for their characterisation, mainly FT-IR, FT-IR, thermogravimetry, UV–Vis spectroscopy and XRD powder diffraction. Furthermore, the influence of the lanthanide ions in the sandwich silicotungstate structure was investigated by studying their electrochemical behaviour in aqueous solutions (pH range of 3.0–4.5). The main differences were found for the first reduction process associated to the tungsten atoms; however, for the second

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