Lanthanide-organic complexes based on polyoxometalates: Solvent effect on the luminescence properties

Journal of Solid State Chemistry 190 (2012) 85–91

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Lanthanide-organic complexes based on polyoxometalates: Solvent effect on the luminescence properties Qun Tang, Shu-Xia Liu n, Da-Dong Liang, Feng-Ji Ma, Guo-Jian Ren, Feng Wei, Yuan Yang, Cong-Cong Li Key Laboratory of Polyoxometalate Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun, Jilin 130024, China

a r t i c l e i n f o

abstract

Article history: Received 29 June 2011 Received in revised form 29 January 2012 Accepted 5 February 2012 Available online 13 February 2012

A series of lanthanide-organic complexes based on polyoxometalates (POMs) [Ln2(DNBA)4(DMF)8] [W6O19] (Ln ¼ La(1), Ce(2), Sm(3), Eu(4), Gd(5); DNBA ¼ 3,5-dinitrobenzoate; DMF ¼ N,N-dimethylformamide) has been synthesized. These complexes consist of [W6O19]2  and dimeric [Ln2(DNBA)4 (DMF)8]2 þ cations. The luminescence properties of 4 are measured in solid state and different solutions, respectively. Notably, the emission intensity increases gradually with the increase of solvent permittivity, and this solvent effect can be directly observed by electrospray mass spectrometry (ESIMS). The analyses of ESI-MS show that the eight coordinated solvent DMF units of dimeric cation are active. They can move away from dimeric cations and exchange with solvent molecules. Although the POM anions escape from 3D supramolecular network, the dimeric state structure of [Ln2(DNBA)4]2 þ remains unchanged in solution. The conservation of red luminescence is attributed to the maintenance of the aggregated state structures of dimeric cations. & 2012 Elsevier Inc. All rights reserved.

Keywords: Polyoxometalate Lanthanide Luminescence property Solvent effect

1. Introduction The polyoxometalate (POM)-based lanthanide-organic complexes, which are composed of POMs and lanthanide-organic coordination polymers, have attracted much attention of synthetic chemists and material chemists in the past decade. This is not only because POM anions have remarkable physical and chemical properties of metal oxide surfaces and diverse geometric patterns [1–3], but also because lanthanide-organic coordination polymers reveal intriguing structural features and unique functionalities [4–7]. The combination of POMs and lanthanideorganic coordination polymers could produce new species, which possess unique side in many areas, such as optics, magnetism, catalysis, and medicine [8]. Luminescence is an attractive property of lanthanide-organic complexes based on polyoxometalates [9–11]. The luminescence properties of lanthanide complexes have a close connection with their structures [12]. These POMs-incorporated lanthanide complexes employed organic ligands as chromophores, which acted as antennae to sensitize the otherwise weakly luminescent metal centers [13]. This fact not only suggests a fundamental strategy for regulating the emission intensity of complexes, but provides a chance for understanding the relationship between the structures and luminescence properties. Recently, Boskovic et al. reported

n

Corresponding author. Fax: þ86 43185099328. E-mail address: [email protected] (S.-X. Liu).

0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2012.02.006

two POM-based terbium-organic complexes, which represented fundamentally different luminescence behaviors due to the difference of the Tb coordination environments, and illuminated the correlation between structures with luminescence properties [14]. Gunnlaugsson reported a cationic dinuclear complex with two metal-bound water molecules [15]. This dinuclear complex could recognize or sense N,N-dimethylaminocarboxylic acid and bis(carboxylate) terephthalic acid in solution. It is mainly because the metal-bound water molecules were active, could move away through the metal center, then the bidentate anions coordinated lanthanide ions by a short semirigid bridge. Although this process was monitored by the increase and quenching of luminescence intensity, investigating its definite mechanisms by other characterization methods could be essential to understand the structures of dinuclear complexes in solution and explain the correlation between the structure and luminescence property. The transition 5D0-7F2 in Eu3 þ complexes is hypersensitive to the coordination environment of Eu3 þ center due to its strong electric dipole character, which allows the use of the relative intensity of this transition to probe the nature of the linker environment. At present, revealing the correlation between the luminescence properties and the structures of POM-based lanthanide-organic hybrid materials is still a challenging issue. Specifically, when the complexes have been dissolved in solvents, the structure of complexes may change, which should result in the change in luminescence properties. This phenomenon can be attributed to a solvate effect [16]. However, the origin of the solvate effect is not as yet quite clearly understood. Eu3 þ -based

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complexes are very promising for efficient light conversion molecular devices on account of their potential applications, such as luminescent labels for luminescent chemical sensors, luminescent stains in clinical immunoassays, and luminescent probes [17]. We have successfully synthesized a series of 3D compounds [Ln2(DNBA)4(DMF)8][W6O19] (Ln¼La(1), Ce(2), Sm(3), Eu(4), Gd(5); DNBA¼ 3,5-dinitrobenzoate; DMF¼N,N-dimethylformamide). They consist of [W6O19]2  and dimeric [Ln2(DNBA)4 (DMF)8]2 þ cations. The luminescence activity of 4 was determined in detail, and its emission properties in solution showed the solvate effect. The origin of the solvate effect was probed by electrospray mass spectrometry (ESI-MS), which revealed the correlation between the structures and luminescence properties.

2. Experimental

0.40 g (64.41% based upon W). Elemental anal. Calcd for C52H68Sm2W6N16O51: C, 19.91; H, 2.18; N, 7.14; W, 35.16; Sm, 9.59. Found: C, 19.92; H, 2.18; N, 7.15; W, 35.15; Sm, 10.74. FT-IR data (cm  1): 3100 (w), 2937 (w), 1649 (s), 1591 (w), 1541 (s), 1499 (w), 1458 (w), 1437 (w), 1403 (m), 1348 (s), 1252 (w), 1111 (w), 979 (s), 813 (s), 723 (s), 676 (m). 2.2.4. Synthesis of [Eu2(DNBA)4(DMF)8][W6O19] (4) The preparation of 4 was similar to that of 1, except that EuCl3  6H2O (0.37 g) was used in place of LaCl3  7H2O. Yield: 0.28 g (44.80% based upon W). Elemental anal. Calcd for C52H68Eu2W6N16O51: C, 19.89; H, 2.18; N, 7.14; W, 35.13; Eu, 9.68. Found: C, 19.82; H, 2.20; N, 7.11; W, 35.15; Eu, 9.70. FT-IR data (cm  1): 3102 (w), 2937 (w), 1649 (s), 1585 (w), 1539 (s), 1499 (w), 1457 (w), 1437 (w), 1304 (m), 1349 (s), 1252 (w), 1111 (w), 978 (s), 813 (s), 722 (s), 677 (m).

2.1. Materials and general methods All chemicals were analytical reagents, commercially purchased, and used without further purification. [(n-C4H9)4N]2 W6O19 was synthesized by the methods of the literature [18], and characterized by IR spectra and TG analyses. Elemental analyses (C, H, N) were performed on a Perkin–Elmer 2400 CHN elemental analyzer. IR spectra were recorded in the range 400– 4000 cm  1 on an Alpha Centaurt FT-IR spectrophotometer using KBr pellets, and the IR spectra of 1–5 are very similar, see Fig. S1. Thermal stability analysis was performed on a Perkin–Elmer TGA7 instrument in flowing N2 atmosphere with a heating rate of 10 1C min  1. Photoluminescence spectra were measured using a FLSP 920 Edinburgh instrument (Eng) with 450 W Xenon lamp monochromatized by double grating. Mass spectra were measured on a Bruker micrOTOF mass spectrometer in electrospray ionization (ESI) mode. 2.2. Synthesis 2.2.1. Synthesis of [La2(DNBA)4(DMF)8][W6O19] (1) 5 mL glacial acetic acid (HOAc) solution of 3,5-dinitrobenzoic acid (1 mmol, 0.21 g) were added dropwise to 12 mL DMF and 3 mL water solution of LaCl3  7H2O (1 mmol, 0.37 g). The mixture solution was stirring 0.5 h at 70 1C, then [(n-C4H9)4N]2W6O19 (0.2 mmol, 0.38 g) was added. The light yellow solution was stirred for 2 h. The resulting solution was filtered and then allowed to evaporate in air at room temperature. Slow evaporation afforded light yellow crystal of 1. Yield: 0.36 g (57.68% based upon W). Elemental anal. Calcd for C52H68La2W6N16O51: C, 20.06; H, 2.20; N, 7.20; W, 35.42; La, 8.92. Found: C, 20.10; H, 2.19; N, 7.22; W, 35.40; La, 8.94. FT-IR data (cm  1): 3104 (w), 2935 (w), 1651 (s), 1588 (w), 1542 (s), 1497 (w), 1458 (w), 1437 (w), 1379 (m), 1348 (s), 1250 (w), 1109 (w), 981 (s), 812 (s), 723 (s), 672 (m). 2.2.2. Synthesis of [Ce2(DNBA)4(DMF)8][W6O19] (2) The preparation of 2 was similar to that of 1, except that CeCl3  6H2O (0.35 g) was used in place of LaCl3  7H2O. Yield: 0.37 g (58.73% based upon W). Elemental anal. Calcd for C52H68Ce2W6N16O51: C, 20.04; H, 2.20; N, 7.19; W, 35.39; Ce, 8.99. Found: C, 20.11; H, 2.20; N, 7.19; W, 35.40; Ce, 8.98. FT-IR data (cm  1): 3104 (w), 2936 (w), 1651 (s), 1588 (w), 1542 (s), 1497 (w), 1458 (w), 1437 (w), 1377 (m), 1348 (s), 1250 (w), 1109 (w), 981 (s), 813 (s), 722 (s), 672 (m). 2.2.3. Synthesis of [Sm2(DNBA)4(DMF)8][W6O19] (3) The preparation of 3 was similar to that of 1, except that SmCl3  6H2O (0.36 g) was used in place of LaCl3  7H2O. Yield:

2.2.5. Synthesis of [Gd2(DNBA)4(DMF)8][W6O19] (5) The preparation of 5 was similar to that of 1, except that GdCl3  6H2O (0.37 g) was used in place of LaCl3  7H2O. Yield: 0.29 g (46.39% based upon W). Elemental anal. Calcd for C52H68Gd2W6N16O51: C, 19.82; H, 2.18; N, 7.11; W, 35.01; Gd, 9.98. Found: C, 19.81; H, 2.18; N, 7.10; W, 35.07; Gd, 9.93. FT-IR data (cm  1): 3101 (w), 2937 (w), 1650 (s), 1586 (w), 1539 (s), 1499 (w), 1457 (w), 1404 (w), 1374 (m), 1349 (s), 1252 (w), 1112 (w), 978 (s), 813 (s), 722 (s), 678 (m). 2.3. X-ray crystallographic study Diffraction intensities for compounds 1–5 were collected on an Oxford Diffraction Gemini R Ultra diffractometer (a Bruker Smart Apex CCD diffractometer for 5) with Mo Ka monochromatic ˚ at 293 K (296 K for 5). The linear radiation (l ¼0.710 73 A) absorption coefficients, scattering factors for the atoms, and anomalous dispersion corrections were taken from International Tables for X-ray Crystallography [19]. The structures of 1–5 were solved by direct methods and refined by the full-matrix leastsquares method on F2 using the SHELXTL crystallographic software package [20]. All non-hydrogen atoms in 1–5 were refined anisotropically. The crystal data and structure refinement results of compounds 1–5 are summarized in Table 1. The bond valence sums (BVS) of all Ln cations (Ln¼La, Ce, Sm, Eu, and Gd) are presented in Table S1. Selected bond lengths for compounds 1–5 are provided in Tables S2–S6 in the supporting material, and the selected Ln–O bond lengths, which are also significant for the lanthanide coordination.

3. Results and discussion The highest yield of compounds 1-5 is obtained when the anions [W6O19]2  are added to DNBA–HOAc–LnCl3–DMF–H2O solution at 70 1C. The ratio of DMF–H2O mixed solution is 4:1, which must be controlled strictly. Excess H2O leads to precipitation of [(n-C4H9)4N]2W6O19, or crystals do not form during the lack of H2O. The salient point is that the DNBA should be added to DMF–H2O solution of LnCl3 before the [(n-C4H9)4N]2W6O19, otherwise only colorless crystals of [(n-C4H9)4N]2W6O19 are obtained. The HOAc is indispensable, it not only protects Ln3 þ from being hydrolyzed, but keeps coordinative active of Ln3 þ at an opportune pH¼5. Moreover, the effect of temperature on the reaction is not negligible. When it is higher than 80 1C, only low yields are obtained, and lower than 60 1C, the unsuitable colorless crystals are produced.

Q. Tang et al. / Journal of Solid State Chemistry 190 (2012) 85–91

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Table 1 Crystal data and structure refinements for 1–5. Complex

1

2

3

4

5

Formula Formula weight T (K) ˚ Wavelength (A)

C52H68La2W6N16O51 3114.08 293(2) 0.71073

C52H68Ce2W6N16O51 3116.5 293(2) 0.71073

C52H68Sm2W6N16O51 3136.99 293(2) 0.71069

C52H68Eu2W6N16O51 3140.20 293(2) 0.71073

C52H68Gd2W6N16O51 3150.76 296(2) 0.71073

Cryst. Syst. Space group ˚ a (A)

Triclinic P-1 13.3315(14)

Triclinic P-1 13.2697(13)

Monoclinic Cc 29.260(5)

Triclinic P-1 12.865(2)

Triclinic P-1 12.927(8)

˚ b (A) ˚ c (A)

13.8926(12)

13.8499(19)

14.698(5)

13.2016(18)

13.338(8)

14.2678(12)

14.224(2)

24.363(5)

15.054(3)

15.204(9)

a (deg.)

102.322(7) 100.982(8) 116.338(10) 2187.5(5)

102.624(12) 100.984(10) 115.959(12) 2168.2(5)

90.000(5) 123.822(5) 90.000(5) 8705(4)

109.352(12) 110.565(14) 98.394(11) 2156.6(7)

110.011(8) 110.226(7) 100.687(8) 2170(2)

1 2.364 8.915 1.012 0.0322 0.0398

1 2.387 9.059 0.901 0.0397 0.0557

4 2.394 9.329 0.758 0.0339 0.0434

1 2.418 9.506 1.034 0.0702 0.1557

1 2.411 9.530 1.097 0.1165 0.3059

b (deg.)

g (deg.) V (A˚ 3) Z Dc (g cm  3) m (mm  1) GOF on F2 R1 [I4 2s(I)]a wR2 (all data)b a b

P P R1 ¼ 9 9Fo9–9Fc99/ 9Fo9. P P wR2 ¼ { [w(F2o  F2c )2]/ [w(F2o )2]}1/2.

Fig. 1. (a) The coordination fashion of the dimer cation of 1. (b) The coordination fashion of the dimer cation of 2–5, the 2 is used to illustrate the coordination fashion.

3.1. Description of crystal structures The structural analyses of 1 and 2 indicate that the compounds are isostructural with [Ln2(DNBA)4(DMF)8][Mo6O19] and [Ln2(DNBA)4(DMF)8][Mo6O19] [9a]. The polyanion [W6O19]2 in each compound is a typical iso-polyoxotungstate with Oh symmetry, which agrees with the structures of previously reported isolated cluster anions in solid state [1b, 21]. The Ln3 þ centers of dimeric cations have different coordinated environments. The La3 þ of compound 1 is nine coordinate, with two La3 þ bonded to four DNBA and eight DMF molecules (Fig. S2). Two DNBA ligands display the same bidentate coordination mode, and are different from the other two DNBA tridentate ligands (Fig. 1(a)). The eight membered ring La–O11–C1–O15–LaA–O11A–O1A–O15A is planar, and the corresponding two DNBA ligands are also in the same plane. The Ce3 þ ion of compound 2 is eight coordinate (Fig. S3). All of DNBA ligands display identical bidentate coordination mode, accordingly two ligand molecules are parallel to each other (Fig. 1(b)). The dimeric cations of compounds 3–5 are isostructural to 2, with the main differences being the angles of the adjacent DNBA ligands plane, anticipated because of the distortion of the DNBA ligands. The angles for the adjacent DNBA ligands plane of 1–5 are shown in Figs. S4–S6. The ranges for Ln–O bonds and the ionic radii of Ln3 þ are shown in Table S1, all within the ranges reported for similar compounds.

The title compounds consist of POM anions and dimeric [Ln2(DNBA)4(DMF)8]2 þ cations having a center of symmetry. A 2D grid-like network is formed by p–p stacking interactions between the aromatic groups, then further extend into ‘‘host’’ 3D supramolecular networks by hydrogen bonding interactions of the oxygen atoms from the nitro group of DNBA linking to carbon atoms from the methyl group of DMF molecules in another 2D layer (Figs. S7 and S8). In the 2D grid-like network, the distance of adjacent Ce–Ce is 14.22 and 13.85 A˚ between different dimeric [Ln2(DNBA)4(DMF)8]2 þ cation groups, respectively. The size of 1D box-like channels is 8.23  8.70 A˚ 2, favorable for the ‘‘guests’’ Lindquist [W6O19]2 anion (5.66  5.70 A˚ 2) (Fig. 2). When other anions replace the [W6O19]2 Lindquist anion under the same conditions, we do not obtain the 3D supramolecular network. So the size and charge of Lindquist anion are suited for the 1D boxlike channels of 3D supramolecular network, for instance the [Mo6O19]2  compounds reported by Wang [9a]. Our results show that the POM anions play an important role in the yields of title compounds. It also indicates that the POMs-templating is a critical factor for self-assembly of 3D supramolecular networks. 3D supramolecular networks of compounds 3–5 are different from 1 to 2. The compound 3 crystallizes in the monoclinic space group Cc (No. 15), the other compounds crystallizes in the triclinic space group P-1 (No. 2). The differences in the angles of the adjacent DNBA ligands plane make the acentric and unique compound 3 in

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Fig. 4. View of the polyhedral-and-stick representation of compounds 4 and 5. The DMF molecules and hydrogen atoms have been omitted for clarity. Fig. 2. View of the polyhedral-and-stick representation of compounds 1 and 2. The DMF molecules and hydrogen atoms have been omitted for clarity.

organic ligands. The total weight loss of 45.31% agreed with the calculated value of 44.35%. The TGA curves of compounds 2–5 exhibit similar weight loss stages to those of compound 1 (See Supporting Information Figs. S10–S13). All the five compounds have no further weight loss until 600 1C. 3.3. Luminescence properties and ESI-MS

Fig. 3. View of the polyhedral-and-stick representation of compound 3. The DMF molecules and hydrogen atoms have been omitted for clarity.

these compounds. Thus the 2D grid-like network is distorted (Fig. 3). The angles of the adjacent DNBA ligands plane for the isostructural compounds 4 and 5 are different from 1 to 2. So the 1D channels formed by 2D grid-like network stacking are rhombus (Fig. 4), and not the squareness of compounds 1 and 2. 3.2. Thermogravimetric analyses The thermogravimetric analyses (TGA) were performed with N2 atmosphere for all compounds in the range 20–600 1C. The TGA curve of compound 1 shows continuous weight loss from 180 to 400 1C (Fig. S9), which corresponds to the decomposition of all

The photoluminescence spectrum of powder samples of compound 4 at room temperature exhibit the characteristic transitions of the Eu3 þ ion 5D0-7FJ (J¼0, 1, 2, 3, 4), under the excitation wavelength 394 nm. The excitation and emission spectra of solid-state 4 are shown in Fig. 5. The emission bands at 579, 592, 618, 650, and 699 nm correspond to the 5D0-7F0, 5 D0-7F1, 5D0-7F2, 5D0-7F3, and 5D0-7F4 transitions, respectively. Study on luminescence properties of compounds containing Eu3 þ , the 5D0-7F0 transition is strictly forbidden in a field of symmetry. Thus, the above results reveal that Eu3 þ in 4 occupies sites with low symmetry and without an inversion center [22], which is consistent with the crystal structures. The 5D0-7F1 and 5 D0-7F2 transitions are ascribed to the magnetic and electric dipole transitions, respectively, and the I(5D0-7F2)/I(5D0-7F1) ratio is widely used as a measure of the coordination state and site symmetry of the lanthanide [23]. For compound 4, the ratio is about 4.64, indicating the low site symmetry of the Eu3 þ . It is exciting that the red luminescence from the colorless and transparent crystals under UV irradiation (365 nm) can be observed macroscopically (Fig. S14). The photoluminescence spectra of 4 were further investigated in different solvents (Concentration: 2  10  3 M): DMSO (dimethyl sulphoxide), DMF, EtOH (ethanol), THF (tetrahydrofuran), and HOAc. The excitation wavelength of emission spectra is 378 nm at room temperature. Interestingly, the emission intensity of 4 increases gradually with the increase of solvent permittivity (Fig. 6); this shows the solvate effect. The emission intensity and solvent permittivity are shown in Table 2. The characteristic emission bands of solution 4 are almost identical to those of solid-state 4 (Fig. S15), which indicates that the coordination state and site symmetry of the lanthanide ion hardly change at all. It is noteworthy that the red luminescence of solvents under UV irradiation (365 nm) is concordant with solid

Q. Tang et al. / Journal of Solid State Chemistry 190 (2012) 85–91

Fig. 5. Excitation spectrum (lem ¼ 617 nm) of solid 4 (dashed line), emission spectrum (lex ¼394 nm) of solid 4 (solid line) showing characteristic EuIII emission.

in DMF solution, as shown in Fig. 7. The peaks of the corresponding symmetric products at m/z ¼783.87 and 1408.53 can be clearly assigned to the [W6O19]2  and [HW6O19]  anion. The signals at m/z ¼355.87, 471.82, and 783.87 are assigned to [W3O10]2  , [W4O13]2  and [HW10O32]3  , respectively, which suggest that the isopolytungstate anion clusters partially disintegrate in solution. The partial [W6O19]2  anions have been shown to fragment at high temperatures in electrospray ionizations sources [24]. The {[Ln2(DNBA)4(DMF)8](C2H6NCO)4}2  can be foud at m/z ¼1010.66, which shows the interaction between solvate DMF molecules and dimeric [Eu2(DNBA)4(DMF)8]2 þ . Exhaustive analyses of the ESI-MS data show that 4 DMF solution complexes can be identified in positive ion mode (Fig. 8). The signals 647.97, 721.02, and 794.07 unambiguously correspond to [Eu2(DNBA)4(DMF)2]2 þ , [Eu2(DNBA)4(DMF)4]2 þ and [Eu2(DNBA)4(DMF)6]2 þ , respectively. The results infer that the eight coordination solvent DMF units of every dimeric cation are active, and can be detached from [Eu2(DNBA)4(DMF)8]2 þ in DMF solution. Clearly, these results provide an important evidence for the solvate effect between [Eu2(DNBA)4(DMF)8]2 þ and DMF solution. For the ESI-MS data of 4 EtOH solution (Fig. S17), the signals at m/z¼579.56, 666.20, 712.20, 721.19, 737.06, and 794.25 are assigned to [Eu2(DNBA)4(EtOH)]2 þ , [Eu2(DNBA)4 (EtOH)4]2 þ , [Eu2(DNBA)4(EtOH)6]2 þ , [Eu2(DNBA)4(DMF)4]2 þ , [Eu2 (DNBA)4(EtOH)7]2 þ , and [Eu2(DNBA)4(DMF)(EtOH)7]2 þ , respectively. The assignments of these signals illustrate that the solvate EtOH molecules could coordinate with dimeric cations [Eu2(DNBA)4]2 þ and exchange with coordinated solvent DMF units. The number of solvent molecules coordinating with dimeric [Eu2(DNBA)4]2þ has changed. By the solution luminescence and ESI-MS analyses, it can be known that the structure of dimer [Eu2(DNBA)4]2þ cation remains unchanged in solution (Fig. S18). The leaving of DMF molecules and the coordination of the solvent molecules with Ln3 þ cause the slight changes of the coordination environment, and therefore they also lead to the change of luminescent intensity. The results reveal that the solvate effect influence luminescent properties. Similarly, the solvent effect on the luminescence property of the reported compound [Eu2(DNBA)4(DMF)8][Mo6O19] was studied. The similar luminescence properties not were observed. It is due to this compound not dissolve in many solvents.

Fig. 6. Emission spectra of 4 in difference solvents (2  10  3 M) at room temperature (lex ¼378 nm).

Table 2 The 5D0-7F2 emission intensity and solvent permittivity of 4 in different solvents (lex ¼378 nm). Sovlents

5

D0-7F2 emission intensity

Permittivity

DMSO DMF EtOH THF HOAc

99.89 70.06 31.05 9.80 8.92

47.2 36.7 25.7 7.58 6.15

state 4; the intensity of red luminescence agrees with emission intensity (Fig. S16). To have a better understanding of the luminescent variation of the solvate effect, ESI-MS studies of 4 were carried out in DMF and EtOH solutions. The ESI-MS studies of 4 DMF solutions in negative ion mode reveal that the ‘‘guests’’ [W6O19]2  anion escape from 3D ‘‘host’’ network, and exist as individual objects

89

Fig. 7. The negative ion mass spectrum of 4 in DMF solution.

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University (NCET-07-0169), Fundamental Research Funds for the Central Universities ((Grant Nos. 09ZDQD0015) and Program for Changjiang Scholars and Innovative Research Team in University.

Appendix A. Supporting information CCDC 802447–802451 contain the supplementary crystallographic data for compounds 1–5. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif. Supplementary material associated with this article can be found in the online version at doi:10.1016/j.jssc.2012.02.006.

References [1] (a) (b) (c) (d) Fig. 8. The positive ion mass spectrum of 4 in DMF solution.

Although the structures of POMs-based lanthanide organic complexes were well characterized by single-crystal X-ray diffraction, the structures have never been reported in solvents. The unambiguous assignments of the signals from ESI-MS allow us to understand the emission intensity study of 4 in solution, because of the presence of solvate effect. At the same time, this result also provides extremely useful information to confirm the coordination states and site symmetry of the central Eu3þ . These observations demonstrate that the aggregated states structures of POMs-based lanthanide-organic complexes in solution are different from crystallographic structures. The 3D supramolecular network based on POMs is not so stable, but the dimeric [Eu2(DNBA)4]2þ has higher stability. The interaction of [Eu2(DNBA)4]2 þ with different solvents is embodied in the changes of emission intensity.

(e) [2] (a) (b) (c) (d) (e) [3] (a) (b) (c) (d) [4] (a) (b) (c) [5] (a) (b) (c)

4. Conclusions In summary, five compounds 1–5 constructed from Lindquist POM anions and lanthanide dimeric cations have been isolated under conventional conditions. Our studies on the luminescence properties of complex 4 (observing that the emission intensity of 4 increases gradually with the increase of solvent permittivity) revealed a unique solvent effect. The origin of such solvent effect can be understood and explained on the basis of the existence of coordinated active sites by ESI-MS studies. The existence of coordinated active sites and the maintenance of dimeric cations both play key roles in the emission properties. This work presents the relationship between the structures and luminescence properties. The structures of POMs-based lanthanide-organic complexes in solution are more readily characterized by solution luminescence and ESI-MS, which puts forward a new and effective method for investigating the interaction of complexes and solvent in solution. Further efforts will be focused on the construction of lanthanide complexes by selecting other organic ligands and POMs, leading to highly luminescent POMs-based lanthanide-organic hybrid complexes. Those complexes might be exploited as the advanced materials, like efficient red light.

(d) (e) [6] (a) (b) (c) (d) [7] (a) (b) (c) (d) [8] (a) (b) (c) (d) (e) [9] (a) (b)

Acknowledgments

(c) (d)

This work was supported by NSFC (Grant Nos. 20871027 and 20973035), Program for New Century Excellent Talents in

(e)

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