Adjustment of coordination environment of Ln3+ ions to modulate near-infrared luminescent properties of Ln3+ complexes

Inorganic Chemistry Communications 14 (2011) 200–204

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Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e

Adjustment of coordination environment of Ln3+ ions to modulate near-infrared luminescent properties of Ln3+ complexes Yani Hui a, Weixu Feng a, Tao Wei a, Xingqiang Lü a,b,⁎, Jirong Song a, Shunsheng Zhao c, Wai-Kwok Wong c, Richard A. Jones d a

Shaanxi Key Laboratory of Degradable Medical Material, Shaanxi Key Laboratory of Physico–inorganic Chemistry, Northwest University, Xi'an 710069, Shaanxi, China Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, Fujian, China Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, China d Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, TX 78712-0615, United States b c

a r t i c l e

i n f o

Article history: Received 23 September 2010 Accepted 21 October 2010 Available online 28 October 2010 Keywords: Adjustment of coordination environment Zn2Ln arrayed Schiff-base complexes Sensitization and energy transfer Modulation of NIR luminescence

a b s t r a c t With the Zn-Schiff-base [ZnL(Py)] from the pure Salen-type Schiff-base ligand H2L (H2L = N,N′-bis (salicylidene)ethylene-1,2-diamine) as the precursor, a series of hetero-trinuclear Zn2Ln complexes [Zn2Ln (L)2(NO3)2(SCN)(Py)2] (Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3 and Ln = Gd, 4) are obtained by the further reaction with Ln(NO3)3·6H2O and KSCN on the condition of stoichiometry control, respectively. The influence of molecular structures on the photophysical properties shows their NIR luminescent properties are hypersensitive to the composition and symmetry of the coordination environments of Ln3+ ions bonded with the mixed anions, which provides a wide variety of means to modulate the NIR luminescence of Ln3+ complexes. © 2010 Elsevier B.V. All rights reserved.

Much recent interest has been devoted to near-infrared (NIR) luminescent Ln3+ (Nd3+, Yb3+ or Er3+) complexes with long-lived lifetime (μs or ms), large Stokes' shift and characteristic narrow-line emission [1], which have potential applications in laser systems [2], optical amplifiers for fiber-optic networks [3], functional devices for Organic light-emitting diodes (OLED) [4] and biological imaging [5]. Though many organic (cyclic [6] or acyclic [7]) ligands and d [8] or fblock [9] metal complexes have been used as antennae or chromophores for the effective sensitization of NIR luminescence of these Ln3+ ions with the desired excited states (1LC, 3LC, 3MLCT or 3LMCT), the sensitization of NIR luminescence for high quantum yields remains a real challenge. In this respect, the avoiding or decreasing the luminescent quenching effect arising from OH―, CH― or NH-oscillators of the solvates around the Ln3+ ions [10] is necessary, and the effective energy transfer, indirectly, should be obtained [11], which are relative to the coordination environment of the Ln3+ ions, such as coordination number, composition of the coordination sphere, site symmetry, strength or distance of the ligand-to-metal bonds. Our past studies [12] have focused on the choice of Zn2+ complexes of Salen-type Schiff-base ligands with the outer O2O2 moieties, as antenna or sensitizers for NIR luminescence of Ln3+ ions. To effectively prevent the quenching effect arising from OH―, CH― or NH-oscillators of the solvates around the Ln3+ ions, the fixation of ⁎ Corresponding author. Shaanxi Key Laboratory of Degradable Medical Material, Shaanxi Key Laboratory of Physico–inorganic Chemistry, Northwest University, Xi'an 710069, Shaanxi, China. Tel./fax: +86 29 88302312. E-mail addresses: [email protected] (X. Lü), [email protected] (W.-K. Wong). 1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2010.10.022

pyridyl [13] or bipyridyl [14] ligands on the axial position of Zn2+ ion and the linkage of bicarboxylate [15] or isonicotinate [16] ligands between the Zn2+ and Ln3+ ions are used to obtain the heterometallic polynuclear or polymeric complexes with the enhanced NIR luminescence. However, the two Zn2+ complexes from the pure Salen-type Schiff-base ligands without the outer O2O2 moieties can further coordinate the same 4f ion from their phenolic O atoms, giving the formation of hetero-metallic Zn2Ln complexes [17]. In the view of the coordination environment of the Ln3+ ions, monodentate and/or bidentate modes of NO− 3 anions from Ln(NO3)3 exist, which should remind the chemical environment of 4f ions adjustable. Through the point, the specialty of the coordination environment of the Ln3+ ions could be studied in detail, and the modulation of their NIR luminescence should be relatively effective. Herein, with the ZnSchiff-base [ZnL(Py)] complex from the pure Salen-type Schiff-base ligand H2L (H2L = N,N′-bis(salicylidene)ethylene-1,2-diamine) as the precursor, a series of hetero-trinuclear Zn2Ln complexes [Zn2Ln(L)2 (NO3)2(SCN)(Py)2] (Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3; Ln = Gd, 4) are − obtained by the involvement of mixed anions (NO− 3 and SCN ) from Ln(NO3)3·6H2O and KSCN. The photophysical properties of the mixed-anions-induced hetero-metallic complexes are reported, and the sensitization and the energy transfer for the NIR luminescence of the Ln3+ ions are also discussed. As shown in Scheme 1, reaction of equimolar amount H2L, Zn (OAc)2·2H2O and absolute pyridine (Py) in absolute MeOH, afforded the precursor [ZnL(Py)] in good yield of ca. 80%. Further reaction of the precursor with Ln(NO3)3·6H2O (Ln = Nd, Yb, Er or Gd) and KSCN on the control of stoichiometry, the series of hetero-trinuclear Zn2Ln

Y. Hui et al. / Inorganic Chemistry Communications 14 (2011) 200–204

KSCN

Zn O

N

N

N

O

N O Zn N O

O

O

N

N

O O O O

Ln N

201

Ln N O N Zn O N

Yb(2) Er(3)

Ln(NO3)3

N

Nd(1)

Gd(4)

S KSCN

Py

Zn(OAc)2

N

N

OH

HO

N

N O Zn N O

O

O

N

N

O O O O

O

Ln O

N O N Zn O N

N O Scheme 1. Controlled design of the series of trinuclear Zn2Ln complexes 1–4.

complexes [Zn2Ln(L)2(NO3)2(SCN)(Py)2] (Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3; Ln = Gd, 4) are obtained, respectively (in Supporting information). The four complexes 1–4 are insoluble in water while soluble in common organic solvents. These complexes were characterized by EA, FT-IR, 1H NMR, ESI-MS and X-ray quality crystals were obtained, with the tables of selected crystal properties of complex 1 given in Tables 1 and 2 s. For complex 1, as shown in Fig. 1, the four phenoxo oxygen atoms of the two [ZnL(Py)] components coordinate to one Nd3+ ion, resulting in the formation of a hetero-trinuclear Zn2Nd complex. Each

Zn2+ (Zn1 or Zn2) ion has the similar five-coordinate environment and adopts a distorted square pyramidal geometry, composed of the inner N2O2 core from the respective Schiff-base (L)2− ligand as the base plane, one N atom from the coordinated Py group at the corresponding apical position. The Nd3+ ion is nine-coordinate and surrounded by eight oxygen atoms and one nitrogen atom: four phenoxo oxygen atoms from the two [ZnL(Py)] components, four oxygen atoms from two bidentate NO− 3 anions and one nitrogen atom from the coordinated SCN− anion. The Zn⋯Nd separations from two Zn2+ (Zn1 and Zn2) with Nd3+ ions bridged by four phenoxo O atoms

Fig. 1. Perspective drawing of complex 1, and H atoms are omitted for clarity.

Y. Hui et al. / Inorganic Chemistry Communications 14 (2011) 200–204

of two Salen-type Schiff-base ligands, are 3.465(2) or 3.497(2) Å, respectively. The Nd-O bond lengths, depending on the nature of the oxygen atoms, vary from 2.438(6) to 2.555(8) Å, and the bond lengths from oxygen atoms of NO− 3 anions are distinctively longer than those from phenoxo oxygen atoms. The Nd―N bond length lies between those of the two sets of the Nd―O bond lengths, showing the feasibility of the partial or complete replacement of NO− 3 anions with SCN− anions. It should be noted that the effect of Py groups at the axial position of 3d Zn2+ ions on the formation of a trinuclear Zn2Nd array in complex 1, is incomparable to that of trinuclear Zn2Ln complexes from the Salen-type Schiff-base ligands with the outer MeO groups [18], in which, the coordination preference for the nitrogen atom from the coordinated SCN− anion at the axial position of 3d Zn2+ ions occurs in the absence of Py groups. It is of special interest to compare the stepwise formation of the complexes 1–4 with our reported [Zn2Ln(L)2(NO3)3(Py)2] (Ln = Nd, Yb, Er or Gd) from the same precursor [ZnL(Py)]. For complexes [Zn2Ln(L)2(NO3)3(Py)2], the presence of Py groups induced the three 3+ NO− ions in mixed 3 anions binding to the hard acidic Ln (monodentate and bidentate) modes. While on the condition of stoichiometry control (3:1 molar ratio) of Ln(NO3)3·6H2O (Ln = Nd, Yb, Er or Gd) and KSCN, the SCN− anion could selectively replace the 3+ monodentate NO− ions with the 3 anion and coordinate to the Ln nitrogen atom, the trinuclear Zn2Ln array in complexes 1–4 could − 3+ obtained with mixed anions (NO− ions. 3 and SCN ) around the Ln As a matter of fact, the reaction of the precursor with the Ln (NO3)3·6H2O (Ln = Nd, Yb, Er or Gd) and KSCN at a 3:1 molar ratio in one pot, or the reaction of the reported [Zn2Ln(L)2(NO3)3(Py)2] with the KSCN on the condition of stoichiometry control (3:1 molar ratio), complexes 1–4 could also obtained in spite of the lower yields. (In Supporting information). The synthesis study shows that the existence of Py groups in the assembly, leads to the border-line lewis acidity of the hard acidic Ln3+ ions, which completes its coordination environment with the N atom not the S atom from the coordinated SCN− anion. The IR spectra of complexes 1–4 show the strong characteristic absorption band (2070 cm− 1 for 1, 2050 cm− 1 for 2, 2053 cm− 1 for 3 or 2060 cm− 1 for 4) and two strong absorption bands at 1450–1471 and 1280–1290 cm− 1, attributed to v(SCN−) and v(NO− 3 , bidentate) [19], respectively, which indicates the presence of mixed anions in the four solid complexes. The powder X-ray diffraction measurement of the selected bulk as-prepared products 1 and 4, gives a reasonable XRD pattern (as shown in Fig. 1 s), which closely matches the simulated one from the single-crystal data of 1, indicating that the isostructural complexes 1–4 can be quantitatively constructed in pure phase. For complex 1, the room temperature 1H NMR spectrum in CD3CN exhibits large shifts (δ from 10.08 to −6.37 ppm) of the photon resonances of the L2− ligands due to the Nd3+-induced shift, significantly spread in relative to those of the free H2L ligand (δ from 13.34 to 3.90 ppm) and the precursor [ZnL(Py)] (δ from 8.60 to 3.72 ppm). The ESI-MS spectra of the four complexes (1–4) exhibit one peak at m/z 1148.89 (1), 1177.69 (2), 1171.91 (3) and 1161.90 (4), respectively, corresponding to the major species ([Zn2Ln(L)2(NO3)2(SCN)(Py)2-H]+ (Ln = Nd, 1; Ln= Yb, 2; Ln=Er, 3 or Ln=Gd, 4), further indicating the existence of the discrete neutral Zn2Ln molecule in the respective dilute MeCN solution. The photophysical properties of H2L, [ZnL(Py)] and complexes 1–4 have been examined in dilute MeCN solution at room temperature, and summarized in Table 3 s and Figs. 2–4. As shown in Fig. 2, the similar ligand-centered solution absorption spectra (231–234, 256– 260 and 325–331 nm) of complex 1–4 in the UV–visible region are observed, red-shifted upon coordination to metal ions compared with that (213, 255, 317 nm) of the free Salen-type Schiff-base ligand H2L, while similar to that (223, 260 and 352 nm) of the [ZnL(Py)] precursor with the lowest energy absorption peak being blue-shifted by 21–25 nm. The molar absorption coefficients of complexes 1–4 in all the three bands are more than two orders of magnitude larger than

Fig. 2. UV–Visible absorption spectra of H2L, [ZnL(Py)] and complexes 1–4 in MeCN solution (2 × 10− 5 M) at room temperature.

those of the free H2L ligand or [ZnL(Py)] precursor due to the involvement of more energy donors. For each of complexes 1–3, the residual ligand-centered fluorescence in the visible region is almost quenched in dilute absolute MeCN solution at room temperature, however, as shown in Fig. 3, photo excitation of the antennae at the range of 200–470 nm (λex =349 nm for 1, 345 nm for 2 or 346 nm for 3), gives rise to the characteristic emissions of Nd3+ ion (4F3/2 → 4IJ/2, J =9, 11, 13), the Yb3+ ion (2F5/2 → 2F7/2) and the Er3+ ion (4I13/2 → 4I15/2) in the NIR region, respectively: the emissions at 904, 1082 and 1357 nm for 1 can be assigned to 4F3/2 → 4I9/2, 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2 transitions of the Nd3+ ion, respectively, and the emissions at 1012 and 1559 nm can be assigned to the 2F5/2 → 2F7/2 transition of Yb3+ ion in 2 and the 4I13/2 → 4I15/2 transition of Er3+ ion in 3, respectively. For the isostructural complex 4, the chromophore-based visible luminescence (λem =455 nm) is just partially quenched, exhibiting the weakened emission in contrast to that (λem =450 nm) of the precursor [ZnL(Py)], as shown in Fig. 4. It is worth noting that for complexes 1–4, the similar excitation spectrum monitored at the respective NIR emission peak (1082 nm for 1, 1012 nm for 2 or 1559 for 3) or the residual visible emission peak (λem =455 nm for 4,), clearly showing both the visible and NIR emissions are originated from the same π–π* transitions of the H2L Schiff-base ligand, suggests that the energy transfer from the antenna to the Ln3+ ions takes place efficiently in complexes 1–3 [20]. Complex 4 does not exhibit the NIR luminescence under the same condition, due to

10 Ex for 1 Em for 1 Ex for 2 Em for 2 Ex for 3 Em for 3

8

Intensity

202

6

4

2

0 200

300

400

800 1000 1200 1400 1600 1800

Wavelength (nm) Fig. 3. NIR emission and excitation spectra of complexes 1–3 in MeCN solution (2×10− 5 M) at room temperature.

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to quenching by the distant C―H oscillators of the ligands [25] besides the less opportunities of non-radiative migration from the single emitting transition (2F5/2 → 2F7/2) of Yb3+ ion in 2. In conclusion, with the Zn-Schiff-base [ZnL(Py)] from the pure Salen-type Schiff-base ligand H2L without the outer O2O2 portion as the precursor, a series of hetero-trinuclear Zn2Ln complexes [Zn2Ln (L)2(NO3)2(SCN)(Py)2] are obtained by the inducement of mixed anions. Moreover, their NIR luminescent properties are hypersensitive to the composition and symmetry of the coordination environments of Ln3+ ions bonded with the mixed anions, which provides a wide variety of means to modulate the NIR luminescence of Ln3+ complexes. The specific design of anion-controlled hetero-metallic polynuclear complexes from the pure Salen-type Schiff-base ligands in facilitating the NIR sensitization is now under way. Acknowledgements Fig. 4. Visible emission and excitation spectra of [ZnL(Py)] and complex 4 in MeCN solution (2 × 10− 5 M) at room temperature.

the absence of energy transfer, because the Gd3+ ion has no energy levels below 32,000 cm− 1 and therefore cannot accept any energy from the antenna excited state (π*) [21]. While in dilute MeCN solution at 77 K, the longer luminescent lifetimes (λ em = 451 nm, τ = 6.78 ns and λem = 501 nm, τ=8.23 ms) than that of 4 at room temperature, shows that the sensitization of the NIR luminescence in the isostructural complexes 1–3 should arise from both the 1LC and the 3LC excited state of the ligand at low temperature [17]. Furthermore, for complexes 1–2, the luminescent decay curves obtained from time-resolved luminescent experiments can be fitted mono-exponentially with time constant of microseconds (2.35 μs for 1 and 23.16 μs for 2), and the intrinsic quantum yield (0.94% for 1 or 1.16% for 2) of Ln3+ emission may be estimated by ФLn = τobs/τ0, where τobs is the observed emission lifetime and τ0 is the “neutral lifetime”, viz 0.25 ms and 2.0 ms for Nd3+ and Yb3+ ions [22], respectively. This indicates the presence of single emitting center for both 1 and 2 in MeCN solution [23]. Due to the limitation of our instrument, we were unable to determine the τobs for the Er3+ ion and thus could not estimate the intrinsic quantum yield of Er3+ emission in complex 3, though the weak NIR emission of Er3+ ion is observed for complex 3. We were interested in the influence of anion-induced structural differences between the two series of hetero-trinuclear 1–3 and the reported [Zn2Ln(L)2(NO3)3(Py)2] (Ln= Nd, Yb or Er) on their photophysical properties in the NIR region. Although upon excitation in the UV region, all of the two series of complexes display the characteristic emission spectra in the NIR region, for the complexes 1–3, no typical ligand-field splittings are observed, in contrast to those of the reported [Zn2Ln(L)2(NO3)3(Py)2]. In fact, the linearity of the SCN− anion through the replacement of monodentate NO− 3 anion, despite no distinctive effect on the residual visible emissions, endows the well site symmetry of the chemical environments of the Ln3+ ions in complexes 1–3 besides the change of compositions of the inner-coordination spheres. On the other hand, the mixed-anions binding in complex 4, also leads to the change of the energy levels of the ligand-centered 0–0 transition for the singlet (1LC, 22,676 cm− 1) and triplet (3LC, 19,960 cm− 1) states at 77 K, in comparison with that of the reported [Zn2Gd(L)2(NO3)3(Py)2], which results in the large differences in the NIR luminescence of its isostructural complexes 1–3: the energy loss during the energy transfer was much greater for 3 than for complexes 1 and 2, which should be due to the larger energy gap between the same energy-donating level (3LC) and the emitting level (4I13/2, 6610 cm− 1) of the Er3+ ion [24]; Moreover, though the energy gap is smaller in Nd3+ (4F3/2, 11,257 cm− 1) for 1 compared to Yb3+ (2F5/2, 10,200 cm− 1) for 2, the higher efficiency of 2 is shown than that of 1, possibly because the excited state of Nd3+ ion is more sensitive

This work is funded by the National Natural Science Foundation (20871098), the State Key Laboratory of Structural Chemistry (20100014), the Provincial Key Item of Shaanxi and Graduate Crossdiscipline Funds (09YJC23) of Northwest University, Hong Kong. Research Grants Council (HKBU 202407 and FRG/06-07/II-16) in P. R. of China, the Robert A. Welch Foundation (Grant F-816), the Texas Higher Education Coordinating Board (ARP 003658-0010-2006) and the Petroleum Research Fund, administered by the American Chemical Society (47014-AC5). Appendix A. Supplementary material The syntheses and characterization of H2L, [ZnL(Py)] and complexes 1–4, are founded in the supporting information. The comparison between the Powder X-ray patterns of complexes 1 and 4 and the simulation based on the X-ray single-crystal analysis of complex 1 is shown in Fig. 1 s. The photophysical properties of the H2L, [ZnL(Py)], and complexes 1–4 in MeCN solution at 2× 10− 5 M at room temperature are founded in Table 3 S. The crystallographic data for 1 are founded in Tables 1–2 s, and have been deposited at the Cambridge Crystallographic Data Center, CCDC-733875 (for 1). The data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html. or from the Cambridge CB21EZ, UK. Supplementary data to this article can be found online at doi:10.1016/ j.inoche.2010.10.022. References [1] S. Comby, J.-C. G. Bünzli, in Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner Jr, J.-C. G. Bünzli, V. K. Pecharsky, Elsevier Science B. V., Amsterdam, 2007 vol. 37, ch. 235. [2] (a) Z. Mierczyk, Z. Frukacz, Opto-Electron. Rev. 8 (2000) 67; (b) P.A. Martin, Chem. Soc. Rev. 31 (2002) 201. [3] (a) W.J. Miniscalco, L.J. Andrews, Mater. Sci. Forum 32–33 (1988) 501; (b) B.T. Wu, J. Ruan, J.J. Ren, D.P. Chen, C.S. Zhu, S.F. Zhou, J.R. Qiu, Appl. Phys. Lett. 92 (2008) 041110; (c) A. Monguzzi, R. Tubino, F. Meinardi, A. Orbelli Biroli, M. Pizzotti, F. Demartin, F. Quochi, F. Cordella, M.A. Loi, Chem. Mater. 21 (2009) 128. [4] (a) S.W. Magennis, A.J. Ferguson, T. Bryden, T.S. Jones, A. Beeby, I.D.W. Samuel, Syn. Metals 138 (2003) 463; (b) A. O'Riordan, R. Van Deun, E. Mairaux, S. Moynihan, P. Fias, P. Nockemann, K. Binnemans, G. Redmond, Thin Solid Films 516 (2008) 5098; (c) Z.Q. Chen, F. Ding, Z.Q. Bian, C.H. Huang, Org. Electron. 11 (2010) 369. [5] (a) R. Weissleder, N. Ntziachristos, Nat. Ned. 9 (2003) 123; (b) S. Kim, Y.T. Lim, E.G. Soltesz, A.M. De Grand, J. Lee, A. Nakayama, J.A. Parker, T. Mihaljevic, R.G. Laurence, D.M. Dor, L.H. Cohn, M.G. Bawendi, J.V. Frangioni, Nat. Biotechnol. 22 (2004) 93; (c) P. Escribano, B. Julian-Lopez, J. Planelles-Arago, E. Cordoncillo, B. Viana, C. Sanchez, J. Mater. Chem. 18 (2008) 23. [6] (a) J.-C.G. Bünzli, C. Piguet, Chem. Soc. Rev. 34 (2005) 1048; (b) S.V. Eliseeva, J.-C.G. Bünzli, Chem. Soc. Rev. 39 (2010) 189. [7] (a) J.-C.G. Bünzli, C. Piguet, Chem. Rev. 102 (2002) 1897; (b) C.M.G. dos Santos, A.J. Harte, S.J. Quinn, T. Gunnlaugsson, Coord. Chem. Rev. 252 (2008) 2512. [8] (a) M.D. Ward, Coord. Chem. Rev. 251 (2007) 1663;

204

Y. Hui et al. / Inorganic Chemistry Communications 14 (2011) 200–204

(b) F.F. Chen, Z.Q. Chen, Z.Q. Bian, C.H. Huang, Coord. Chem. Rev. 254 (2010) 991. [9] (a) S. Faulkner, S.J.A. Pope, J. Am. Chem. Soc. 125 (2003) 10526; (b) Q. Zhong, H.S. Wang, G.D. Qian, Z.Y. Wang, J.H. Zhang, J.Q. Qiu, M.Q. Wang, Inorg. Chem. 45 (2006) 4537. [10] L. Winkless, R.H.C. Tan, Y. Zheng, M. Motevalli, P.B. Wyatt, W.P. Gillin, Appl. Phys. Lett. 89 (2006) 111115. [11] J.-C.G. Bünzli, S. Comby, A.-S. Chauvin, C.D.B. Vandevyver, J. Rare Earths 25 (2007) 257. [12] (a) W.K. Wong, H.Z. Liang, W.Y. Wong, Z.W. Cai, K.F. Li, K.W. Cheah, New J. Chem. 26 (2002) 275; (b) W.K. Lo, W.K. Wong, J.P. Guo, W.Y. Wong, K.F. Li, K.W. Cheah, Inorg. Chim. Acta 337 (2004) 4510; (c) W.K. Lo, W.K. Wong, W.Y. Wong, J.P. Guo, K.T. Yeung, Y.K. Cheng, X.P. Yang, R. A. Jones, Inorg. Chem. 45 (2006) 9315; (d) X.P. Yang, R.A. Jones, Q.Y. Wu, M.M. Oye, W.K. Lo, W.K. Wong, A.L. Holmes, Polyhedron 25 (2006) 271; (e) W.Y. Bi, X.Q. Lü, W.L. Chai, J.R. Song, W.Y. Wong, W.K. Wong, R.A. Jones, J. Mole. Struct. 891 (2008) 450. [13] (a) W.Y. Bi, X.Q. Lü, W.L. Chai, J.R. Song, W.K. Wong, X.P. Yang, R.A. Jones, Z. Anorg. Allg. Chem. 634 (2008) 1795; (b) X.Q. Lü, W.X. Feng, Y.N. Hui, T. Wei, J.R. Song, S.S. Zhao, W.Y. Wong, W.K. Wong, R.A. Jones, Eur. J. Inorg. Chem. (2010) 2714. [14] X.Q. Lü, W.Y. Bi, W.L. Chai, J.R. Song, J.X. Meng, W.Y. Wong, W.K. Wong, R.A. Jones, New J. Chem. 32 (2008) 127.

[15] (a) X.P. Yang, R.A. Jones, W.K. Wong, M.M. Oye, A.L. Holmes, Chem. Commun. (2006) 1836; (b) X.Q. Lü, W.Y. Bi, W.L. Chai, J.R. Song, J.X. Meng, W.Y. Wong, W.K. Wong, X.P. Yang, R.A. Jones, Polyhedron 28 (2009) 27. [16] W.Y. Bi, X.Q. Lü, W.L. Chai, T. Wei, J.R. Song, S.S. Zhao, W.K. Wong, Inorg. Chem. Commun. 12 (2009) 267. [17] W.Y. Bi, T. Wei, X.Q. Lü, Y.N. Hui, J.R. Song, W.K. Wong, R.A. Jones, New J. Chem. 33 (2009) 2326. [18] S.S. Zhao, X.Q. Lü, A.X. Hou, W.Y. Wong, W.K. Wong, X.P. Yang, R.A. Jones, Dalton Trans. (2009) 9595. [19] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th edn.Wiley, New York, 19868 Sect. II. 8. [20] M. Albrecht, O. Osetska, J. Klankermayer, R. Fröhlich, F. Gumy, J.-C.G. Bünzli, Chem. Commun. (2007) 1834. [21] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4443. [22] M.J. Weber, Phys. Rev. 171 (1968) 283. [23] C. Görller-Walrand, K. Binnemans, in Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner, Jr, L. Eyring, Elsevier Science B. V., Amsterdam, 1996 vol. 37, ch. 155. [24] J.-C.G. Bünzli, in Lanthanide Probe in life, Chemical and Earth Sciences, Theory and Practice, ed. J.-C. G. Bünzli, G. R. Choppin, Elsievier Science Pub1. B. V., Amsterdam, 1989, ch. 7, pp. 219–293. [25] A. Beeby, I.M. Clarkson, R.S. Dockins, S. Faulkner, D. Parker, L. Royle, A.S. de Souda, J.A.G. Williams, M. Woods, J. Chem. Soc. Perkin Trans. 2 (1999) 493.