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  Luminescent rare earth (RE) complexes have attracted much attention because of the diversity in crystal structures and biological applications in fluorescent sensors, labels, catalysts and magnetism.^[1] Bipyridine groups are often used in the syntheses of metal ion complexes due to the chelation effect and the π-accepting ability of these moieties, ^[2] and these metal-bipyridine complexes were found various applications in many chemistry areas including catalysis, electrochemistry, drug and so on.^[3] In recent years, 2, 2'-bipyridine unit and its derivatives are used to the synthesis of macrocyclic ligands and their RE metal complexes, ^[4] and the bipyridine unit serves both as a chelating subunit and as an antenna chromophore in these complexes.^{[2, 4]} Unique luminescence properties have been found in these complexes: efficient optical absorption, narrow emission band, and high quantum efficiency.^[5] These properties make RE metal complexes containing bipyridine unit also have a lot of applications in lighting, display and medical immunoassays.

  As part of a research program aimed at the design and synthesis of lanthanide luminescence bioprobes, ^[6] we were interested in the synthesis of ligands based on a bipyridine-moiety containing an appropriate monofunctionalization group to be covalently attached to optical material. Adopting the simpler and cheaper raw material through multistep finishing process, a series of Na⁺ cryptates 17~22 and Eu³⁺ cryptates 23 and 24 were synthesized. The single crystal structure of 23 was determined to obtain detailed structural features. The PL excitation and emission spectra, the decay curve, and the quantum efficiency of 23 were investigated. These data provide useful information that would allow the use of this kind of cryptates in lighting and display.

  1   Results and discussion

  1.1   Syntheses

  The 4, 4'-diaryl-substituted-6, 6'-dimethyl-2, 2'-bipyridines 3 and 4 were prepared according to the literature^[7] from the corresponding 1, 6-diarylhexa-1, 5-diene-3, 4-dione derivatives 1 and 2 by refluxing them with acetonylpyridinium chloride and NH₄OAc, respectively (Scheme 1). The furyl-substituted compound 3 was oxidized by KMnO₄ in t-BuOH to dicarboxylic acid 5^[8], which was esterified to 6, 6'-dimethyl-2, 2'-bipyridine-4, 4'-dimethyl ester (6) and monomethyl ester 7 (Scheme 1). 6 was a very versatile starting material in this respect. Transesterification of 6 with LiO(t-Bu) in t-BuOH/toluene^[9] yielded di(tert-butyl)ester 8 in 90% yield. Compound 9 is an asymmetrical ester being made of compound 7 with dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) in t-BuOH/ CH₂Cl₂ in 50% yield.^[10] This procedure offers a convenient method for the esterification of carboxylic acid with alcohol.

  Figure Scheme 1.  Preparation of compounds 1~9

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  Two different routes to prepare 6, 6'-bis(bromomethyl)-2, 2'-bipyridine derivatives are presented in Scheme 2. The direct bromogenation of 6, 6'-dimethyl-2, 2'-bipyridines with N-bromosuccinimide (NBS) could give the target products, but the yields of 6, 6'-bis(bromomethyl)-2, 2'-bipyridine derivatives were low because of many other co-bromoge-nated compounds formed in this synthesis procedure. It was also difficult to separate any pure 6, 6'-bis(bromomethyl)-2, 2'-bipyridine from its isomeric, the unsymmetrical (dibromomethyl) methyl derivative, using flash chromatography. However, in Route 2 there was too many steps and the yield of the desired product remained low. Referring to the literature reported, ^[11] a better process was applied to get pure products with higher yields. Significant improvements were accomplished by directly converting 6, 6'-dimethyl-2, 2'-bipyidine derivatives to 6, 6'-bis(dibromomethyl)-2, 2'-bipyridine derivatives. Thus, bis(bromomethyl) derivatives 10~14 were prepared by bromination of the corresponding dimethyl derivatives with excessive NBS, then reduced by diethylphosphite and EtNⁱPr₂, respectively (Route 1).

  Figure Scheme 2.  Preparation of compounds 10~14

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  6, 6'-Bis(aminomethyl)-2, 2'-bipyridine trihydrobromide hydrates 15 and 16 were prepared from the corresponding 6, 6'-bis(bromomethyl)-2, 2'-bipyridine derivatives as previously described^[12] (Eq. 1). The macrobicyclic sodium cryptates 17~22 were synthesized from the corresponding 6, 6'-bis(bromomethyl) compounds 11~13 by refluxing them with 15 or 16 in dry CH₃CN as previously described, respectively^[12] (Figure 1). Sodium ion of 17 was displaced by simply refluxing 17 with an excess of EuCl₃ in dry CH₃CN solution affording europium(Ⅲ) cryptate 23.^[12] However, because tert-butyl ester could be easily hydro-lyzed, we didn't get the corresponding europium(Ⅲ) cryptate of 18. After being refluxed with the excess of EuCl₃, 18 was reacted with ethanediamine in dry CH₃OH, then acidized with CF₃COOH. Then 24 was obtained adopting reverse phase HPLC. Europium(Ⅲ) cryptate 24 is a commendable material to prepare fluorescent label.

  Figure 1.  Diagrammatic sketch of cryptates 17~24

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  1.2   Crystal structure of cryptate 23

  Because it was difficult to obtain the crystals of this kind of cryptates, we only determined the crystal structure of 23. The molecular diagram of 23 is shown in Figure 2, and the selected bond distances and angles are listed in Table 1. 23 has a triclinic system with the space group P-1. Center Eu³⁺ was coordinated by eight N atoms (six from three bpys and two from the bridgehead N) and two chlorine atoms. X-ray diffraction confirmed that 23 was expected to be cryptate type analogy to earlier work.^[13] It is worth noting that there are two Cl atoms bounding to Eu ion in this cryptate, which is different from literature reported in which one Cl atom and a O atom from H₂O molecule are bounded to center Eu ion.^[13] Two nitrogen atoms (N(3) and N(4)) are located at bridghead axial positions with a N(3)—Eu(1)— N(4) angle of 164.6(2)° (Table 1). The distances of Eu(1) with N(1), N(2), N(5) and N(6) atoms from two bpys are approximately equivalent and significantly shorter than those of Eu(1) with N(7) and N(8) from the third bpy. These might be due to steric effect in coordination environment of 23. Cl(1) inside is at 2.665(2) Å and Cl(2) is a little further at 2.807(2) Å from Eu(1), respectively.

  Figure 2.  An ORTEP representation of cryptate 23 with atom labeling

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  Table 1.  Selected bond lengths (Å) and angles (°) for cryptate 3

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  Eu(1)—Cl(2) 2.807(2) N(3)—Eu(1)—N(4) 164.6(2)

  Eu(1)—N(1)	2.755(6)	N(5)—Eu(1)—N(6)	61.6(2)
  Eu(1)—N(2)	2.758(6)	N(5)—Eu(1)—Cl(1)	137.71(15)
  Eu(1)—N(3)	2.728(7)	Cl(1)—Eu(1)—Cl(2)	146.43(6)
  Eu(1)—N(4)	2.741(6)	N(5)—Eu(1)—N(1)	104.0(2)
  Eu(1)—N(5)	2.607(6)	Cl(1)—Eu(1)—N(1)	82.45(16)
  Eu(1)—N(6)	2.620(6)	Cl(1)—Eu(1)—N(1)	82.45(16)
  Eu(1)—N(7)	2.962(6)	N(4)—Eu(1)—N(1)	119.12(18)
  Eu(1)—N(8)	2.945(6)	N(3)—Eu(1)—N(2)	118.56(19)
  Eu(1)—Cl(1)	2.665(2)	N(4)—Eu(1)—N(2)	62.39(18)

  Table 1.  Selected bond lengths (Å) and angles (°) for cryptate 3

  1.3   Luminescence of cryptate 23

  The PL spectra of cryptate 23 are shown in Figure 3. The excitation spectrum was observed in the wavelength region of 200~440 nm, which is well matched with the wavelength of light emission of UV-LED chips (360~400 nm).^[14] The absorption peak may be ascribed to intramolecular energy transfer from the π, π^* excited states of the ligand groups to excited levels of the Eu³⁺, which then emit from the ⁵D₀ level.^[5b] The efficiency of conversion is high probably due to unity of [Eu⊂bpy•bpy•bpy]³⁺. Upon the excitation with the near UV light 400 nm, the spectrum presents display typical ⁵D₀→⁷F_J (J=0, 1, 2, 3 and 4) emission lines of the Eu³⁺. The excitation of the phosphor is beneficial to white light emission diodes (W-LEDs) because it well matches with the output wavelength of near-UV or blue LED chips in phosphor-converted W-LEDs. It indicates that cryptate 23 is a good candidate for the white LED devise. It is a well-known fact that organic luminescence materials have great superiorities in encapsulating and preparing for W-LED devises than any inorganic phosphor.

  Figure 3.  The normalized excitation and the emission spectra of cryptate 23

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  It has been well confirmed that the selection rules and transition probabilities between the states of Eu³⁺ depend strongly on crystal field. The electrical dipole transition ⁵D₀→⁷F₂ is much affected by the ligand field around Eu³⁺, while magnetic dipole transition ⁵D₀→⁷F₁ is not sensitive to the local environment.^[15] The dominant red emission of 595 nm in Figure 3 is attributed to the ⁵D₀→⁷F₁, indicating that Eu³⁺ is located at the site of inversion symmetry. This is in agreement with the crystal structure of [Eu⊂bpy• bpy•bpy]•2Cl•Br. The luminescence color of 23 (x=0.5929, y=0.3528) was expressed according to the Commission International de I'Eclairage (CIE) (1931). This is a typical red color.

  The decay curve of cryptate 3 (Supplementary Data) presents the slight non-exponential profiles. The lifetime value can be given to the average lifetime defined as follows:

  The phosphor has a luminescence lifetime of 0.32 ms. The decay curve shows that this cryptate luminescence material can get very good application in LED lights because there will be no afterglow. The quantum efficiency was measured to be 70% at the excitation of near UV light. This value is higher than the well-known red-emitting phosphor such as commercial Y₂O₂S:Eu³⁺ (35%, λ_ex=317 nm).^[16] The high η indicates that the cryptate 23 could be an efficient red-emitting phosphor.

  2   Conclusions

  In summary, we have designed a facile and effective approach which was economy and easy to be operated to fabricate a series of sodium cryptates 17~22 and europium(Ⅲ) cryptates 23 and 24, and determined the single crystal X-ray structure of cryptate 23. Center metal Eu ion in this cryptate is bounded by eight N atoms and two Cl atoms with the coordination number of ten. The luminescence properties of 23 were investigated. Cryptate 23 is an efficient red-emitting Eu³⁺ phosphor, which can be effectively excited by near-UV and presents efficient red luminescence with the quantum efficiency of 70%. The cryptate 23 is a potential candidate for solid-state lighting based on near-UV LED chips.

  3   Experimental section

  3.1   General procedures

  All preparations and manipulations of air-and moisture-sensitive materials were carried out under argon by using standard Schlenk techniques. Tetrahydronfuran (THF) was freshly distilled from Na/benzophenone. Acetonitrile was freshly distilled. CH₃OH was freshly distilled from Na. CH₂Cl₂, diethylphosphite and ethanediamine were freshly distilled from CaH₂. N-Bromo-succinimide (NBS) was recrystallized prior to use. Other commercially available chemicals were used without further purification. Butyllithium was titrated before use with HCl, using phenolphthalein as indicator. Thin-layer chromatography (TLC) was performed on silica. TLC spots were visualised by irradiation with UV light (R_f values refer to relative mobilities on TLC plates). Column chromatography was carried out on silica gel with CH₃OH/CH₂Cl₂ (200~300 mm).

  Proton magnetic resonance spectra (¹H NMR) were recorded on a Bruker Avance DRX-400 MHz or 600 MHz spectrometer. Residual protic solvent was used as the internal reference, setting chloroform to δ 7.26, dimethylsulfoxide to δ 2.50 and H₂O to δ 4.79. Carbon magnetic resonance spectra (¹³C NMR) were recorded on a Bruker Avance 400 MHz spectrometer, referenced to the appropriate solvent peak, taking chloroform as δ 77.0. M.p.: WRR apparaatus, uncorrected. Electrospray (ES) high resolution mass spectra were obtained using a Bruker ESI-TOF. The Eu(Ⅲ) luminescence emission and excitation spectra were recorded on a Perkin-Elmer LS-50B luminescence spectrometer. The luminescence decay curve was measured from the fourth harmonics of a Nd-YAG pulsed laser under the excitation of the 266 nm.

  Diffraction data were collected with a Rigaku Mercury CCD area detector in ω scan mode using Mo-Kα radiation (λ=0.71075 Å). The diffracted intensities were corrected for empirical absorption corrections and Lorentz polarization effects. The structure was solved and refined using SHELEXL-97 and diamond programs.

  3.2   Syntheses

  3.2.4   General method for the synthesis of 6, 6'-bis-(monobromomethyl)-2, 2'-bipyridines (10~14) using N-bromosuccinimide (Route 1)

  Initially, a 6, 6'-dimethy-2, 2'-bipyridine derivative (5 mmol) was reacted with 4.2 equiv. NBS in CCl₄ at reflux temperature in the presence of a catalytic amount of benzoyl peroxide. The solution was heated to reflux under argon for 12 h. The succinimide was removed by filtration and the filtrate was evaporated to yellow solid. Without further purified, THF solution of the yellow solid was cooled to 0 ℃ in ice bath and stirred under argon atmosphere. Diethyl phosphite (3.7 mL, 30 mmol) was then added dropwise, followed by the addition of ethyldiisopropylamine (3.9 mL, 30 mmol). After the reaction mixture was stirred at room temperature for 20 h, all volatiles were removed under reduced pressure. Then it was poured into 30 mL of water and extracted with ethyl acetate (20 mL×4). The organic layer was dried with Na₂SO₄ and all volatiles were removed under reduced pressure. Residue was washed with methanol, and the product was recrystallized in CHCl₃.

  6, 6'-Bis(bromomethyl)-2, 2'-bipyridine (10): Yield 50%. m.p. > 280 ℃ (lit.^[9] > 260 ℃); ¹H NMR (400 MHz, CDCl₃) δ: 8.38 (d, J=7.9 Hz, 2H), 7.82 (t, J=7.8 Hz, 2H), 7.47 (d, J=7.7 Hz, 2H), 4.63 (s, 4H); ¹³C NMR (101 MHz, CDCl₃)δ: 155.75, 155.00, 137.45, 123.08, 120.05, 33.62.

  6, 6'-Bis(bromomethyl)-2, 2'-bipyridine-4, 4'-dimethyl ester (11): Yield 50%. m.p. 200~204 ℃ (lit.^[19] 198~200℃); ¹H NMR (400 MHz, CDCl₃) δ: 8.90 (d, J=1.4 Hz, 2H), 8.06 (d, J=1.4 Hz, 2H), 4.70 (s, 4H), 4.02 (s, 6H); ¹³C NMR (101 MHz, CDCl₃) δ: 164.81, 157.21, 155.35, 139.32, 122.87, 119.60, 52.44, 32.75.

  6, 6'-Bis(bromomethyl)-2, 2'-bipyridine-4, 4'-ditert-butyl ester (12): Yield 82%. m.p. 179~184 ℃ (lit.^[9] 183~184℃); ¹H NMR (400 MHz, CDCl₃) δ: 8.83 (d, J=1.3 Hz, 2H), 7.97 (d, J=1.3 Hz, 2H), 4.69 (s, 4H), 1.65 (s, 18H); ¹³C NMR (101 MHz, CDCl₃) δ: 163.35, 156.89, 155.40, 141.19, 122.61, 119.64, 82.38, 32.95, 27.61.

  6, 6'-Bis(bromomethyl)-4, 4'-diphenyl-2, 2'-bipyridine(13): Yield 66%. m.p. 215~219 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.69 (d, J=1.5 Hz, 2H), 7.84~7.78 (m, 4H), 7.74 (d, J=1.5 Hz, 2H), 7.57~7.49 (m, 6H), 4.74 (s, 4H); ¹³C NMR (101 MHz, CDCl₃)δ: 156.38, 155.18, 149.70, 137.32, 128.77, 128.64, 126.76, 121.19, 118.35, 33.75.

  6, 6'-Bis(bromomethyl)-2, 2'-bipyridine-4-methyl-4'-tert-butyl ester (14): Yield 80%. m.p. 191~194 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.94 (d, J=1.2 Hz, 1H), 8.90 (d, J=1.2 Hz, 1H), 8.10 (d, J=1.3 Hz, 1H), 8.03 (d, J=1.3 Hz, 1H), 4.75 (d, J=1.4 Hz, 4H), 4.07 (s, 3H), 1.70 (s, 9H); ¹³C NMR (101 MHz, CDCl₃)δ: 164.84, 163.30, 157.14, 156.95, 155.55, 155.21, 141.24, 139.26, 122.73, 119.65, 119.59, 82.41, 52.41, 32.85, 27.61. HRMS calcd for C₁₉H₂₀Br₂N₂O₄[M+H⁺] 498.9868, found 498.9890.

  3.2.7   Synthesis of sodium cryptates 17~22

  Under argon, 6, 6'-bis(monobromomethyl)-2, 2'-bipyri-dines (11~13, 2 mmol) and 6, 6'-bis(aminomethyl)-2, 2'-bipyridine trihydrobromide hydrates (15 or 16, 1 mmol) were dissolved in CH₃CN (250 mL, dry HPLC grade) and Na₂CO₃ (14 equiv.) was added. The suspension was heated to reflux for 48 h, cooled to ambient temperature, and filtered. The filtrate was concentrated under reduced pressure and the resulting residue dissolved in CHCl₃, filtered. The filtrate was concentrated under reduced pressure and the resulting residue was washed with ethyl acetate, and dried under reduced pressure. The crude product was purified by column chromatography over silica gel (V(CH₃OH): V(CH₂Cl₂)=1:50 to 1:20) to give the title compound.

  6, 6'', 6'''':6', 6''', 6'''''-Bis(nitrilotri(methylene)) tris(2, 2'-bip-yridine)-4, 4'4'', 4''', 4'''', 4'''''-hexamethyl ester (17): Yield 67%. m.p. > 280 ℃ (lit.^[19] > 260 ℃); ¹H NMR (400 MHz, CDCl₃) δ: 8.49 (d, J=0.8 Hz, 6H), 7.96 (d, J=0.8 Hz, 6H), 4.07 (s, 12H), 4.02 (s, 18H); ¹³C NMR (101 MHz, CDCl₃) δ: 165.01, 160.12, 155.72, 140.03, 123.97, 120.13, 59.31, 53.34; HRMS calcd for C₄₈H₄₂N₈NaO₁₂ 945.2814, found 945.2857.

  6, 6'', 6'''':6', 6''', 6'''''-Bis(nitrilotri(methylene)) tris(2, 2'-bipyridine)-4, 4'-dimethyl-4'', 4''', 4'''', 4'''''-tetra(tert-butyl)ester (18): Yield 63%. m.p. > 280℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.47 (s, 2H), 8.38 (s, 4H), 7.96 (s, 2H), 7.83 (s, 4H), 4.05 (d, J=21.6 Hz, 12H), 4.01 (s, 6H), 1.61 (s, 36H); ¹³C NMR (101 MHz, CDCl₃) δ: 164.39, 162.88, 159.52, 159.10, 155.12, 141.21, 139.37, 123.25, 122.94, 119.45, 119.36, 82.96, 58.65, 52.71, 27.52; HRMS calcd for C₆₀H₆₆N₈NaO₁₂[M-Na⁺+2H⁺]/2 546.2478, found 546.2295.

  6, 6'', 6'''':6', 6''', 6'''''-Bis(nitrilotri(methylene)) tris(2, 2'-bi-pyridine)-4, 4'-diphenyl-4'', 4''', 4'''', 4'''''-tetramethyl ester (19): Yield 70%. m.p. > 280 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.48 (s, 4H), 8.08 (d, J=17.0 Hz, 2H), 7.96 (d, J=22.1 Hz, 4H), 7.68 (d, J=7.0 Hz, 4H), 7.59 (s, 2H), 7.51~7.43 (m, 6H), 4.13~3.97 (m, 24H); ¹³C NMR (101 MHz, CDCl₃) δ: 164.40, 159.67, 158.32, 155.37, 155.21, 150.46, 139.30, 136.81, 129.16, 128.82, 126.65, 123.19, 121.75, 119.44, 118.05, 58.95, 58.86, 52.68; HRMS calcd for C₅₈H₄₈N₈NaO₁₂ 981.3331, found 981.3396.

  6, 6'', 6'''':6', 6''', 6'''''-Bis(nitrilotri(methylene))-4, 4'4'', 4''', 4'''', 4'''''-hexaphenyl-tris(2, 2'-bipyridine) (20): Yield 71%. m.p. > 280℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.14~8.11 (m, 6H), 7.73~7.68 (m, 12H), 7.60 (d, J=0.5 Hz, 6H), 7.52 (t, J=7.4 Hz, 12H), 7.47 (t, J=7.3 Hz, 6H), 4.13 (d, J=64.9 Hz, 12H); ¹³C NMR (101 MHz, CDCl₃) δ: 158.95, 155.72, 150.28, 137.09, 129.14, 128.87, 126.68, 121.69, 118.02, 59.32; HRMS calcd for C₇₂H₅₄N₈Na [M-Na⁺+2H⁺]/2 516.2314, found 516.2347.

  6, 6'', 6'''':6', 6''', 6'''''-Bis(nitrilotri(methylene))tris(2, 2'-bipyridine)-4, 4', 4'', 4'''-tetraphenyl-4'''', 4'''''-dimethyl ester (21): Yield 77%. m.p. > 280℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.50 (s, 2H), 8.12 (s, 4H), 7.94 (s, 2H), 7.73~7.67 (m, 8H), 7.60 (s, 4H), 7.52~7.46 (m, 12H), 4.14~3.93 (m, 18H); ¹³C NMR (101 MHz, CDCl₃)δ: 164.55, 159.98, 158.65, 155.56, 155.40, 150.42, 139.22, 136.98, 129.20, 128.87, 126.67, 123.17, 121.75, 119.43, 118.05, 59.18, 59.07, 52.71; HRMS calcd for C₆₄H₅₂N₈NaO₄ [M-Na⁺+2H⁺]/2 498.2056, found 498.2078.

  6, 6'', 6'''':6', 6''', 6'''''-Bis(nitrilotri(methylene)) tris(2, 2'-bipyridine)-4, 4'-diphenyl-4'', 4''', 4'''', 4'''''-tetra(tert-butyl)ester (22): Yield 53%. m.p. > 280 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.37 (s, 4H), 8.08 (s, 2H), 7.80 (s, 4H), 7.68 (d, J=7.4 Hz, 4H), 7.60 (s, 2H), 7.47 (t, J=7.5 Hz, 4H), 7.42 (t, J=7.3 Hz, 2H), 4.32~3.76 (m, 12H), 1.59 (s, 36H); ¹³C NMR (101 MHz, CDCl₃)δ: 162.95, 159.46, 158.54, 155.35, 155.24, 150.34, 141.02, 136.86, 129.03, 128.76, 126.64, 122.85, 121.78, 119.21, 117.88, 82.76, 58.85, 58.74, 27.50; HRMS calcd for C₆₈H₇₀N₈NaO₈[M-Na⁺+2H⁺]/2 564.2737, found 564.2738.

  3.2.1   Preparation of compounds 1~5 and 8

  The following compounds were prepared as described in the literature: 1-Acetonylpyridinium chloride^[17], 1, 6-diarylhexa-1, 5-diene-3, 4-dione derivatives 1 and 2^[18], 4, 4'-diaryl-substituted-6, 6'-dimethyl-2, 2'-bipyridines 3 and 4^[7], 6, 6'-dimethyl-2, 2'-bipyridine-4, 4'-dicarboxylic acid 5^[8], and 6, 6'-dimethyl-2, 2'-bipyridine-4, 4'-ditert-butyl ester 8^[9].

  3.2.3   Preparation of 6, 6'-dimethyl-2, 2'-bipyridine-4-methyl-4'-tert-butylester (9)

  A mixed solution of compound 7, t-BuOH and 4-di-methylaminopyridine (DMAP) in CH₂Cl₂ was added drop-wise to a stirred solution of dicyclohexylcarbodiimide (DCC) in toluene under argon protection. The solution was stirred at room temperature for 5 h and then filtered. The filtrate was evaporated to afford the white solid 9 (50%). m.p. 170~172 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.72 (s, 1H), 8.66 (s, 1H), 7.74 (s, 1H), 7.68 (s, 1H), 3.98 (s, 3H), 2.71 (s, 6H). 1.63 (s, 9H); ¹³C NMR (101 MHz, CDCl₃) δ: 165.62, 164.15, 158.75, 158.57, 156.01, 155.61, 140.12, 138.14, 122.13, 122.04, 117.22, 81.81, 52.13, 27.61, 24.12; HRMS calcd for C₁₉H₂₂N₂O₄[M+H⁺] 343.1658, found 343.1628.

  3.2.6   Preparation of 6, 6'-bis(aminomethyl)-2, 2'-bipy-ridine-4, 4'-diphenyl trihydrobromide hydrate (16)

  16 was prepared analogously as 15 from 13. m.p. 253~256 ℃; Yield 50%. ¹H NMR (400 MHz, D₂O) δ: 8.41 (s, 2H), 7.82~7.81 (m, 6H), 7.54~7.53 (m, 6H), 4.42 (s, 4H); ¹³C NMR (101 MHz, D₂O) δ: 154.42, 153.87, 153.36, 137.54, 132.75, 131.49, 129.40, 123.89, 121.75, 44.34. HRMS calcd for C₂₄H₂₂N₄ [M+Na⁺] 389.1742, found 389.1715.

  3.2.5   Preparation of 6, 6'-bis(aminomethyl)-2, 2'-bipy-ridine-4, 4'-dimethyl ester trihydrobromide hydrate (15)

  11 (276.3 mg, 0.6 mmol) was suspended in CHCl₃ (6 mL) and heated under reflux until everything was dissolved. A solution of hexamethylenetetraamine (186.1 mg, 1.3 mmol, 2.2 equivs.) in CHCl₃ (6 mL) was added dropwise and heating was continued for additional 3 h. The suspension was stored at ambient temperature overnight, then the colorless precipitate was collected, washed with CHCl₃, and dried under infrared lamp. The solid was suspended in H₂O/EtOH/ 47% HBr (aq.) (2.8 mL/11.7 mL/2 mL) and stirred at 85 ℃ until everything was dissolved. After storing the mixture at ambient temperature for 1 h, the yellow crystals was deposited at 0 ℃. The crystals collected were washed with EtOH, and dried under reduced pressure. 15 was obtained as a light yellow crystals (191 mg, 54%). m.p. 270~273 ℃; ¹H NMR (400 MHz, D₂O) δ: 8.91 (d, J=0.8 Hz, 2H), 8.04 (d, J=0.8 Hz, 2H), 4.56 (s, 4H), 4.04 (s, 6H); ¹³C NMR (101 MHz, D₂O) δ: 166.40, 154.69, 152.64, 139.34, 121.72, 119.68, 52.89, 42.30.

  3.2.2   Preparation of 6, 6'-dimethyl-2, 2'-bipyridine-4, 4'-dimethylester (6) and 6, 6'-dimethyl-2, 2'-bipyridine-4-carboxylic-4'-methyl ester (7)

  An anhydrous methanol solution (100 mL) of 5(2.5 g, 9.3 mmol) was heated to about 50 ℃, then a concentrated sulfuric acid was added dropwise until the solution became clear, refluxing the solution overnight. The solution was cooled to room temperature, then filtered. After being washed with methanol. 6 was precipitated as white crystals (2.1 g, 7 mmol, 72%). The filtrate was chromatographed (silicagel, V(CH₃OH):V(CH₂Cl₂)=1:50) to afford 7.

  6: m.p. 220~225 ℃ (lit.^[19] 222~224℃); ¹H NMR (400 MHz, CDCl₃) δ: 8.73 (d, J=0.5 Hz, 2H), 7.74 (d, J=1.0 Hz, 2H), 3.99 (s, 6H), 2.72 (s, 6H); ¹³C NMR (101 MHz, CDCl₃) δ: 166.09, 159.27, 156.27, 138.67, 122.66, 117.70, 52.64, 24.62.

  7: m.p. 261~263 ℃; ¹H NMR (400 MHz, DMSO-d₆) δ: 8.62 (s, 2H), 7.80~7.75 (m, 2H), 3.94 (s, 3H), 2.67 (d, J=2.3 Hz, 6H); ¹³C NMR (101 MHz, DMSO-d₆) δ: 166.17, 165.21, 159.35, 159.19, 155.11, 154.82, 139.65, 138.31, 122.87, 122.50, 116.78, 116.39, 52.76, 24.12. HRMS calcd for C₁₅H₁₄N₂O₄[M+H⁺] 287.1032, found 287.1019.

  3.2.8   Cryptates 23 and 24

  Under argon, EuCl₃ (2.5 equiv.) in dry CH₃CN was heated under reflux for 30 min, then 17 (1.0 equiv.) was added. The suspension was heated to reflux overnight, cooled to ambient temperature, and filtered. The filtrate was concentrated under reduced pressure. Cryptate 23 was obtained as a light yellow solid; HRMS calcd for C₄₈H₄₂-N₈EuO₁₂ [M+HCOO^-]/2 560.1058, found 560.1002.

  Under argon, EuCl₃ (2.5 equiv.) in dry CH₃CN was heated under reflux for 30 min, then 18 (1.0 equiv.) was added. The suspension was heated to reflux overnight, cooled to ambient temperature, and filtered. CF₃COOH was added to the residue, then the mixture was concentrated under reduced pressure. After the product was dissolved in CH₃OH under the condition of ice bath, a solution of ethanediamine in CH₃OH was added dropwise. The solution was stirred under the ice bath for 5 min, then stirred under the ambient temperature for another 5 h. The solid was obtained by centrifugate, dried under vacuum, and purified by preparative HPLC; HRMS calcd for C₄₆H₄₂N₁₂EuO₁₀ [M+CH₃COO^-]/2 567.1246, found 567.1196.

  The crystallographic data of 23 was saved in the Cambridge Crystallographic Data Centre, and the CCDC number is 1505978.

  Supporting information  Crystal data and NMR spectra of 1~24. These materials can be obtained free of charge from our journal website via http://sioc-journal.cn/.

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• 

  Scheme 1  Preparation of compounds 1~9

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  Scheme 2  Preparation of compounds 10~14

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  Figure 1  Diagrammatic sketch of cryptates 17~24

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  Figure 2  An ORTEP representation of cryptate 23 with atom labeling

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  Figure 3  The normalized excitation and the emission spectra of cryptate 23

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  Table 1.  Selected bond lengths (Å) and angles (°) for cryptate 3

  Eu(1)—Cl(2) 2.807(2) N(3)—Eu(1)—N(4) 164.6(2)

  Eu(1)—N(1)	2.755(6)	N(5)—Eu(1)—N(6)	61.6(2)
  Eu(1)—N(2)	2.758(6)	N(5)—Eu(1)—Cl(1)	137.71(15)
  Eu(1)—N(3)	2.728(7)	Cl(1)—Eu(1)—Cl(2)	146.43(6)
  Eu(1)—N(4)	2.741(6)	N(5)—Eu(1)—N(1)	104.0(2)
  Eu(1)—N(5)	2.607(6)	Cl(1)—Eu(1)—N(1)	82.45(16)
  Eu(1)—N(6)	2.620(6)	Cl(1)—Eu(1)—N(1)	82.45(16)
  Eu(1)—N(7)	2.962(6)	N(4)—Eu(1)—N(1)	119.12(18)
  Eu(1)—N(8)	2.945(6)	N(3)—Eu(1)—N(2)	118.56(19)
  Eu(1)—Cl(1)	2.665(2)	N(4)—Eu(1)—N(2)	62.39(18)

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•