Ancillary ligand modulated stereoselective self-assembly of triple-stranded Eu(III) helicate featuring circularly polarized luminescence

Abstract

Creating optically pure metal assemblies is a hot research topic in the realms of chiral supramolecules. Here, three new triple-stranded europium(III) helicates Eu₂L₃(L′)₂ [L = 4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)diphenyl sulphide; L′ = 1,10-phenanthroline (Phen) or R/S-2,2′-bis(diphenylphosphinyl)-1,1′-binaphthyl (R/S-BINAPO)] were synthesized in order to investigate the effects of ancillary ligands on controlling the stereoselective self-assembly of lanthanide helicates. X-ray single crystal structure analysis showed that Eu₂L₃(Phen)₂ crystalized in an achiral space group P1̄ with the equivalent amount of P and M helicates in one single cell. The isolated Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ were verified to be enantiopure by ¹H, ¹⁹F, ³¹P NMR and DOSY NMR analyses. Additionally, the mirror-image CD spectra also demonstrated the successful syntheses of the enantiomers and the presence of an effective chirality transformation from BINAPO to achiral L. Furthermore, the perfect mirror-image circularly polarized luminescence (CPL) spectra of Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ indicated the existence of the excited state chirality of the Eu³⁺ center associated with |g_lum| values reaching 0.112. In addition, the photophysical properties of three helicates were also discussed.

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Introduction

In recent years, circularly polarized luminescence (CPL) materials¹ have attracted great interest because of their potential application in 3D displays,² optical storage,³ communication of spin information⁴ and CPL probes.⁵ Lanthanide helicates are regarded as the favorable candidates of CPL materials in view of their supramolecular helical chirality and the characteristic electron transition from Ln³⁺ ions.⁶ The partial magnetic dipole character endowed the Ln³⁺ ions with a higher luminescence dissymmetry factor (g_lum),⁷ typically in the range of 10⁻²–0.5,⁸ which is obviously larger than those of pure organic luminophores (g_lum = 10⁻⁵–10⁻³).⁹ To date, the highest g_lum value for CPL materials in solution was recorded to be 1.38 from a Eu³⁺ complex, Cs[Eu((+)-hfbc)₄].¹⁰

On the other hand, the chirality amplification effect arising from the supramolecular chirality provides a structural foundation for the helicates to produce the large g_lum values. In 2009, Gunnlaugsson et al. prepared a chiral lanthanide helicate and reported its CPL property for the first time.¹¹ In recent years, Gunnlaugsson,¹² Sun,¹³ and Law¹⁴ have successfully prepared a few optical pure dinuclear lanthanide helicates based on chiral 2,6-dipicolinic amides derivatives, and some of which have been used for CPL study. Although the g_lum values were satisfactory, the examples for the preparation of chiral lanthanide helicates were still rare. The introduction of chiral ancillary ligand was proved to be an effective way to control the stereoselective of lanthanide assemblies.¹⁵

Our group has been concerned with the research of luminescent lanthanide helicates composed of β-diketonate ligands.¹⁶ The highly effective sensitizing ability of β-diketones ensured the high luminescence quantum yields (QYs) of helicates. More recently, we reported several examples of chiral helicates featured with CPL properties, which showed the moderate |g_lum| values.¹⁷ Generally, the large degree of twisting of helix is favorable for chirality amplification, and probably brings the increase of luminescence dissymmetry.

Herein, three optical active and inactive helicates were constructed by employing an achiral Phen and two chiral R- and S-BINAPO as the second ligands. Single crystal X-ray crystallographic analysis showed Eu₂L₃(Phen)₂ was a racemic mixture with a pair of ΔΔ- and ΛΛ-helicate existing in one single cell. While the diastereoselective self-assembly during the preparation of Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ can be verified by NMR spectra and semiempirical geometry optimization. The photophysical measurements showed that the helicates all present the satisfactory luminescence quantum yields, 31% and 34% for Eu₂L₃(Phen)₂ and Eu₂L₃(R/S-BINAPO)₂, respectively. In addition, the chiral helicates Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ present a modest circularly polarized luminescence dissymmetry (|g_lum| = 0.112 at 595 nm).

Results and discussion

Characterization of L and complexes

The syntheses routes of L and its corresponding complexes Eu₂L₃(Phen)₂ and Eu₂L₃(R/S-BINAPO)₂ are outlined in Scheme 1. The C₂-symmetric achiral bis-β-diketone 4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)phenyl sulphide (L) was prepared via two steps, the first was the Friedel–Crafts acylation of phenyl sulfide, then followed with a Claisen condensation of 4,4′-diacetyl phenyl sulfide and ethyl trifluoroacetate. The corresponding intermediate and ligand were characterized by ESI-TOF-MS and ¹H NMR (Fig. S1–S3†).

Scheme 1 Synthetic routes of L and the corresponding complexes Eu₂L₃(Phen)₂ and Eu₂L₃(BINAPO)₂.

The triple-stranded helicates Ln₂L₃(L′)₂ were isolated by stirring a 3 : 2 : 2 mixture of L, L′ and LnCl₃·6H₂O (Ln = Gd, Eu) at room temperature in CH₃OH. High-resolution ESI-TOF-MS analyses affirmed the successful preparation of triple-stranded helicates. For instance, Fig. 1a presents three clusters of peaks, corresponding to the positive charged [M + H]⁺, [M + K]⁺ and [M + Na]⁺ species of Eu₂L₃(S-BINAPO)₂, at m/z 2995.1158, 3017.1653 and 3033.1340. The mass spectra of the other complexes also showed the expected molecule ions peaks (Fig. S4–S7†). The assignments were further affirmed by comparing the isotopic distributions of the experimental with simulated results.

Fig. 1 (a) ESI-TOF-MS of Eu₂L₃(S-BINAPO)₂ with inset representing the simulated (Sim.) and observed (Obs.) isotopic distributions. (b) ¹H NMR (400 MHz) spectra of L, R-BINAPO and Eu₂L₃(R-BINAPO)₂ and Eu₂L₃(S-BINAPO)₂ (THF-d₈). (c) ¹H DOSY spectrum of Eu₂L₃(S-BINAPO)₂ in THF-d₈.

¹H, ³¹P, ¹⁹F NMR, and DOSY NMR results also supported the successful preparation of Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ and their stabilities in THF (Fig. 1b, S8 and S9†). As shown in Fig. 1b, ¹H NMR of Eu₂L₃(S-BINAPO)₂ shows a single set of clear and broadening signals from L and R/S-BINAPO. Considering the absence of C₃ symmetry of the helicates, three sets of H signals from three different ligands should be observed. However, with the influence of paramagnetism of Eu³⁺ ion, for instance, only a single peak attributed to methine H_C resonance of β-diketone was observed at 5.34 ppm, not the expected three peaks. A more facile way to infer the formation of a single species was the employment of ³¹P and ¹⁹F NMR, in which only the singlet peaks were observed (Fig. S8 and S9†). Additionally, one single diffusion rate of Eu₂L₃(S-BINAPO)₂ (D = 5.88 × 10⁻⁶ cm² s⁻¹) in ¹H NMR diffusion ordered spectroscopy (DOSY) also supported the presence a single species in THF (Fig. 1c).

X-ray single crystal structure of Eu₂L₃(Phen)₂

The single crystals of Eu₂L₃(Phen)₂ were grown by diffusion of diethyl ether to a THF/toluene solution of the complex. The structural analysis revealed that Eu₂L₃(Phen)₂ was the expected triple-stranded helicate with each Eu³⁺ center ligated by six oxygen (O) atoms from three β-diketones and two nitrogen (N) atoms from Phen (Fig. 2a), and adopting a square antiprism as coordination geometry (Fig. S10†). The Eu–N and Eu–O distances are in ranges of 2.331–2.383 Å and 2.574–2.617 Å, respectively. The Eu–Eu distance in one helicate reached to 13.229 Å. The corresponding crystallographic parameters were list in Table S1.† It was found that the Eu₂L₃(Phen)₂ crystalized in an achiral triclinic space group P1̄, where a pair of racemic mixtures of ΔΔ-Eu₂L₃(Phen)₂ and ΛΛ-Eu₂L₃(Phen)₂ were observed in one single cell. This result indicated that the absence of chiral element in self-assembly process will result in the formation of the racemic helicate.

Fig. 2 (a) X-ray crystallographic structure of Eu₂L₃(Phen)₂ and (b) Sparkle/RM1 optimized geometries of Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂.

As reported by us and others,¹⁸ the chiral ancillary ligands could induce the metal supramolecule assemblies to form a single diastereoisomer. Herein, we employed the chiral BINAPO as ancillary ligands to construct the diastereopure helicates. Unfortunately, the single crystals of Eu₂L₃(R/S-BINAPO)₂ were not obtained, although several methods had been tried. To better understand the chiral induction, we resorted to a molecular mechanical modeling built from LUMPAC 3.0 software with a Sparkle/RM1 model.¹⁹ Taking the ΔΔ-Eu₂L₃ helicate as an example, its assembly with R-BINAPO and S-BINAPO were both considered. As can be seen in Fig. S11,† the ΔΔ-Eu₂L₃ with R-BINAPO as ancillary ligand has a relative lower system energy than that with S-BINAPO. Thus, it was proposed that the R-BINAPO could induce the Eu₂L₃ to form P helical conformation with the metal center the ΔΔ configuration.

Photoluminescent properties of the complexes

The UV-vis spectra of Eu₂L₃(Phen)₂, Eu₂L₃(R/S-BINAPO)₂ and free L in THF are shown in Fig. 3. In free ligand, two clear bands with λ_max = 241 nm (1.8 × 10⁴ dm³ mol⁻¹ cm⁻¹) and λ_max = 366 nm (4.1 × 10⁴ dm³ mol⁻¹ cm⁻¹) were observed, which could be attributed to the π–π* transitions localizing at benzene ring and the ILCT transition from benzene ring to β-diketone units, respectively. In comparison with free ligand, three complexes all present the obvious hypsochromic shifts with the absorption maxima from 370 nm to 350 nm. This significant shift arose from the large twisting of L in complexes, which disrupted the conjugation of β-diketone moieties. While the high energy absorbances at 277 and 237 nm were attributed to the absorptions of Phen and R/S-BINAPO, respectively.

Fig. 3 UV-vis absorption spectra of L (3.3 × 10⁻⁶ M) and helicates Eu₂L₃(Phen)₂ and Eu₂L₃(R/S-BINAPO)₂ (1.0 × 10⁻⁵ M) in THF.

The excitation and emission spectra of three Eu³⁺ complexes are shown in Fig. 4 and S12.† Upon excitation at 370 nm, three Eu³⁺ complexes displayed five sharp emission bands at 580, 594, 613, 650 and 702 nm, respectively, relating to the ⁵D₀ → ⁷F_J (J = 0–4) transitions. In excitation spectra, the spectral bands could be well matched with the absorption bands of complexes in 280–400 nm regions, while the possible energy transfer from ancillary ligands could be excluded due to the absence of corresponding absorbance at high energy bands of 240–260 nm.

Fig. 4 Emission spectra of complexes Eu₂L₃(Phen)₂ and Eu₂L₃(R-BINAPO)₂ in THF (λ_ex = 370 nm, c = 1.0 × 10⁻⁵ M).

In emission spectra of Eu³⁺ complexes, the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions are of special importance. The former is the electric dipole hypersensitive transition, which is usually suggested to be sensitive to the coordination geometry of Eu³⁺ ion. Whereas, the ⁵D₀ → ⁷F₁ transition is insensitive to the coordination sphere. Thus, the integrated intensity ratio of I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁) was often employed as a probe to infer the coordination geometry symmetry of Eu³⁺ ion, the larger of this value generally means the larger deviation of the Eu³⁺ from the highly symmetric coordination geometry.²⁰ Here, the intensity ratios reached to 25.4 and 28.3 for Eu₂L₃(Phen)₂ and Eu₂L₃(R/S-BINAPO)₂, respectively. It indicated the presence of a lower symmetry around Eu³⁺ ion.

Another manner to infer the symmetry of Eu³⁺ site is to count the numbers of crystal-field splits.²¹ On the basis of ⁵D₀ → ⁷F₂ transitions with the five peaks at 612 nm (detailed splits can be seen from the deconvolution analyses in Fig. S13†), the local symmetry around Eu³⁺ ions in Eu₂L₃(Phen)₂ and Eu₂L₃(BINAPO)₂ were most probably to be C₁. However, the assignment of the point group symmetry was not in line with that observed D_4d coordination geometry calculated from the single crystal structure of Eu₂L₃(Phen)₂ (Fig. S10†). It is reasonable to consider the possibility of the presence of different coordination geometries around Eu³⁺ ion in solution and in solid state.²²

The large ratio of I(⁵D₀ → ⁷F_J)/I(⁵D₀ → ⁷F₁) transition generally implies that the complex has a relatively large radiative transition probability as judged from eqn (1).

The radiative rate constant (k_r) is proportional to the intensity ratio of I_tot/I_MD, I_tot and I_MD represent the total integrated emission of ⁵D₀ → ⁷F_J and ⁵D₀ → ⁷F₁ transitions, respectively. A_MD,0 is a constant (14.65 s⁻¹), representing the spontaneous emission probability of ⁵D₀ → ⁷F₁ transition. n is the refractive index of solution.

The large k_r value does not indicate the high luminescence efficiency of Eu³⁺ center, because the non-radiative transition (k_nr) is inevitable. The both deactivation pathways finally determine the intrinsic quantum yields (Φ_Eu) as shown in eqn (2).

τ_obs is the experimental lifetimes. From the emission decay curves of Eu³⁺ ion monitored at the ⁵D₀ → ⁷F₂ transition (Fig. S14 and S15†), the lifetimes of Eu₂L₃(Phen)₂, Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ were fitted to be 613, 635 and 635 μs, respectively. On the basis of the calculated radiative lifetimes (τ_rad) and measured lifetimes, Φ_Eu were calculated to reach up to 74% and 84% for Eu₂L₃(Phen)₂ and Eu₂L₃(R/S-BINAPO)₂. With eqn (2), the radiative (k_r) and nonradiative rate constants (k_nr) were estimated and listed in Table 1. Additionally, the complexes also showed the large Eu³⁺ center luminescence quantum yields (Φ_overall) of 31% and 34% for Eu₂L₃(Phen)₂ and Eu₂L₃(R/S-BINAPO)₂ (Fig. S16–S18†).

Φ_overall = η_senΦ_Ln (3)

Table 1 Radiative (k_r) and nonradiative (k_nr) decay rate constants, measured emission lifetime of Eu³⁺ (τ_obs), sensitization efficiency (η_sens), intrinsic quantum yield (Φ_Eu), and overall quantum yield (Φ_overall), g_lum values for ⁵D₀ → ⁷F_J of Eu³⁺ ion. Error in τ_obs: ±0.05 ms; 10% relative error in the other values; λ_ex = 370 nm

Complexes	kr (s^−1)	knr (s^−1)	τobs (μs)	ΦEu (%)	ηsens (%)	Φoverall (%)	glum ^5D0 → ^7FJ (J = 0, 1, 2, 3, 4)
							J = 0	J = 1	J = 2	J = 3	J = 4
Eu2L3(R-BINAPO)2	1328	245	635	84	40	33.7	0.006	0.112	−0.009	0.003	−0.001
Eu2L3(S-BINAPO)2	1327	247	635	84	41	34.3	−0.006	−0.112	0.009	−0.029	0.001
Eu2L3(Phen)2	1208	422	613	74	42	30.8	—	—	—	—	—

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From the eqn (3), the sensitizing efficiency (η_sen) was also calculated. In Eu³⁺ complexes, the energy level difference ΔE (T₁ − ⁵D₀) between the triplet states (T₁) of ligands and ⁵D₀ energy level of Eu³⁺ ion usually determined the sensitizing efficiency. An empirical energy gap to realize a highly effective energy transfer was proposed to be in range of 2500–5000 cm⁻¹ for Eu³⁺ complexes.²³ By virtue of phosphorescence spectra of the Gd³⁺ complexes, the ΔE were estimated to be 3246 and 2540 cm⁻¹ for Eu₂L₃(Phen)₂ and Eu₂L₃(R/S-BINAPO)₂ (Fig. S19†), which exactly located at the proposed energy gap range. In addition, the possible energy transfer from ancillary ligands to Eu³⁺ ion was excluded because of the absence of the excitation bands attributing to Phen and BINAPO.

Chiroptical properties of Eu₂L₃(R/S-BINAPO)₂

In view of the introduction of chiral R-BINAPO and S-BINAPO and above spectral analyses, it was inferred that chiral ancillary could regulate the diastereoselective self-assembly of triple-stranded helicate. Therefore, the chirality of the helicate was firstly characterized by CD spectroscopy. From the Fig. 5, the two complexes Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ showed mirror-image Cotton effect in ranges of 250–400 nm, relating to the UV-vis absorption region of helicates. Moreover, two clear exciton couplings were observed in range of 250–315 and 315–400 nm, which were ascribed to the transitions from chiral BINAPO and achiral L ligand, respectively. In Eu₂L₃(R-BINAPO)₂, a negative exciton coupling (a negative and positive signals at 300 and 260 nm) attributed to R-BINAPO was observed; conversely, a positive exciton coupling was obtained in case of S-BINAPO. Notably, the achiral L also present the chiroptical signal with a positive exciton coupling for Eu₂L₃(S-BINAPO)₂ and a negative case for Eu₂L₃(R-BINAPO)₂. It implied that the chirality was transferred from chiral phosphine oxide ancillary ligands to achiral L through the generation of supramolecular chirality.

Fig. 5 CD spectra of Eu₂L₃(S-BINAPO)₂ (black line) and Eu₂L₃(R-BINAPO)₂ (red line) in THF (1.0 × 10⁻⁵ M).

The exciton coupling in metal complexes originated from the interactions of chromophores, in which the electron transition was no longer localized at one ligand, but became delocalization over the entire assemblies. Here, three bis-β-diketone were ligated at two metal centers, which forced them to close together. From the suggested empirical rule,²⁴ the relationships between the absolute configurations of the metal centers and the couplet signs have been built. Generally, the negative exciton couplet related to a Δ configuration of metal center; conversely, the positive exciton couplet correlated a Λ configuration. From this rule, the configurations estimated from the optimized molecular structures were in line with the observed CD patterns.

The excited state chiroptical property can be confirmed from the CPL spectra of helicates. As shown in Fig. 6, a pair of mirror-image CPL signals appeared at the ⁵D₀ → ⁷F_J (J = 0–4) transition region. In these bands, the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions present the larger CPL intensities than other ⁵D₀ → ⁷F_0,3,4 transition. The strong ⁵D₀ → ⁷F₂ emission was due to the largest emitting intensity at 612 nm. In contrast, the ⁵D₀ → ⁷F₁ transition at 595 nm also displayed a relatively large intensity, although its total emission was low. This mainly benefitted from its magnetic dipole transition characteristic. The luminescence dissymmetry factor, g_lum value (−2 ≤ g_lum ≤ 2) is the standard to estimate the excess of left vs. right circularly polarized luminescence, where g_lum = 2(I_L − I_R)/(I_L + I_R), I_R and I_L refer to the left and right circularly polarized emission intensities, respectively. The |g_lum| values of the ⁵D₀ → ⁷F_J transitions were summarized in Table 1. The largest |g_lum| values were measured to be about 0.112 for ⁵D₀ → ⁷F₁ transition at 595 nm, while the ⁵D₀ → ⁷F₂ transitions were about 0.009. These results were comparable to the previously reported CPL Eu³⁺ complexes.²⁵

Fig. 6 CPL spectra of Eu₂L₃(S-BINAPO)₂ (black line) and Eu₂L₃(R-BINAPO)₂ (red line) (λ_ex = 370 nm, 1.0 × 10⁻⁵ M in THF).

Conclusions

In summary, we successfully synthesized an optical inactive helicate Eu₂L₃(Phen)₂ and a pair of optical active enantiomers Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ by employing achiral Phen and chiral R/S-BINAPO as second ligands. X-ray crystallographic analysis revealed that achiral ancillary could not control the stereoselective in the self-assembly process of the helicate, only a racemate could be obtained. Whereas, in the case of chiral BINAPO as ancillary ligands, the diastereoselectivity was realized. The mirror-image CD and CPL signs combining the NMR and semiempirical mechanical methodology proved that the chiral BINAPO could regulate the diastereoselective self-assembly of lanthanide helicate. In addition, the introduction of ancillary ligands not only offer the possibility for obtaining optical pure lanthanide helicate, but also increase the luminescence quantum yields from Eu³⁺ center and achieve a modest circularly polarized luminescence (CPL) with |g_lum| values reaching to 0.112.

Experimental

Materials and instruments

Diphenyl sulfide, ethyl trifluoroacetate, 1,10-phenanthroline, (R/S)-BINAPO and LnCl₃·6H₂O salts (Ln = Eu and Gd) were purchased from Chembee Chemical Reagent Co., Ltd. (China); other chemicals and solvents are of analytical reagent grade.

The NMR experiments were measured on a Bruker Avance III 400 MHz spectrometer. EI-MS and ESI-TOF-MS were recorded on Agilent 5975N and Bruker maXis mass spectrometers, respectively.

Elemental analyses were recorded on an Elementar Vario EL cube analyzer. Single crystal X-ray crystal data of Ln₂L₃(Phen)₂ were recorded on a Xcalibur, Eos, Gemini diffractometer with Mo Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods and refined on F² by full-matrix least-squares using the SHELXTL-2018 program.²⁶ UV-vis spectra were measured by a PerkinElmer Lambda 25 spectrometer. Photoluminescence spectra containing the emission, excitation and luminescence lifetime measurements were recorded on an Edinburgh FLS 1000 spectrophotometer, and the luminescence quantum yield (Φ) were estimated by an integrating sphere equipped on this instrument. The Φ values were calculated using the following equation:

where E_reference is the emission spectrum of the excitation light as only the reference in the sphere, E_sample is the emission spectrum of the excitation light as only the sample in the sphere, L_emission is the emission spectrum of the sample. The method is accurate within 10%. CD and CPL experiments were performed on an Olis DM245 spectrofluorimeter.

Synthetic procedures

4,4′-Diacetyldiphenyl sulphide. Anhydrous AlCl₃ (2.15 g, 16.1 mmol) was added to 30 mL dichloromethane in a 100 mL round bottom flask and stirred for 5 min. Then acetyl chloride (1.27 g, 16.1 mmol) in 15 mL of dichloromethane was slowly added dropwise. After cooling to 0 °C with ice-bath, diphenyl sulfide (1.00 g, 5.37 mmol) in 15 mL of dichloromethane was added. After stirring overnight, the reaction solution was poured into 150 mL ice-water, then hydrolyzed with 0.5 M HCl. The crude product was extracted with CH₂Cl₂ for two times, the organic layer was combined and dried with anhydrous Na₂SO₄, then evaporated to remove the solutions in vacuo. Recrystallization the crude product from ethanol led to the white flake crystals (1.24 g, 86%). ¹H NMR (400 MHz, CDCl₃) δ 7.94–7.86 (m, 4H), 7.44–7.37 (m, 4H), 2.58 (s, 6H). EI-MS m/z 271.35 (M + H)⁺.

4,4′-Bis(4,4,4-trifluoro-1,3-dioxobutyl)diphenyl sulphide (L). L was prepared by the Claisen condensation of 4,4′-diacetyldiphenyl sulphide and ethyl trifluoroacetate in 1,2-dimethoxyethane (DME). To add freshly prepared sodium methoxide (0.80 g, 14.8 mmol) and ethyl trifluoroacetate (2.11 g, 14.8 mmol) into a Schlenk bottle containing 30 mL DME, and stirred the solution to become clear and transparent. Then 4,4′-diacetyl phenyl sulfide (1.00 g, 3.7 mmol) was added, and then solution was stirred for 24 hours at room temperature. The final reaction solution was quenched with ice-water, then acidified to pH 2–3 using 2.0 M HCl solution, the resulting yellow precipitate was filtered and recrystallized from ethyl acetate solvents to give yellow flake crystal (1.26 g, 87%). ¹H NMR (400 MHz, THF-d₈): 8.09–8.07 (m, 4H), 7.55–753 (m, 4H), 6.83 (s, 2H). ESI-MS m/z 549.07 [L + (CH₃OH)₂ + Na]⁺.

Synthesis of Ln₂L₃(Phen)₂

L (1.00 g, 2.10 mmol), NEt₃ (0.70 g, 7.2 mmol) and 1,10-phenanthroline (0.50 g, 2.8 mmol) were dissolved in 30 mL methanol and stirred for 5 min, then LnCl₃·6H₂O (1.4 mmol) in 5 mL methanol was added, which was accompanied by the presence of a large amount of precipitation. The mixture was then refluxed for 12 h. After cooling to room temperature, the precipitation was filtered and washed by 10 mL CH₃OH and 10 mL H₂O, respectively, then dried under vacuum.

Eu₂(L)₃(Phen)₂. Yield: 72%. Anal. Calc. for C₈₄H₄₆F₁₈N₄O₁₂S₃Eu₂ (2045.4026): C, 49.33; H, 2.27; N, 2.74; S, 4.70. Found: C, 49.41; H, 2.49; N, 2.71; S, 4.82. ESI-MS: m/z = 2046.0363.

Gd₂(L)₃(Phen)₂. Yield: 82%. Anal. Calc. for C₈₄H₄₆F₁₈N₄O₁₂S₃Gd₂ (2055.9777): C, 49.33; H, 2.27; N, 2.74; S, 4.70. Found: C, 49.41; H, 2.29; N, 2.71; S, 4.82. ESI-MS: m/z = 2056.0778.

Synthesis of Ln₂(L)₃ ((R/S)-BINAPO)₂

L (1.00 g, 2.10 mmol), NEt₃ (0.70 g, 7.2 mmol) and (R/S)-BINAPO (1.9 g, 2.8 mmol) were dissolved in 30 mL methanol and stirred for 5 min, then LnCl₃·6H₂O (1.4 mmol) in 5 mL methanol was added, which was accompanied by the presence of a large amount of precipitation. The mixture was then refluxed for 12 h. After cooling to room temperature, the precipitation was filtered and washed by 10 mL CH₃OH and 10 mL H₂O, respectively, then dried under vacuum.

Eu₂L₃(R-BINAPO)₂. Yield: 81%. Anal. Calc. for C₁₄₈H₉₄F₁₈O₁₆S₃P₄Eu₂ (2994.34): C, 59.37; H, 3.16; S, 3.21. Found: C, 59.14; H, 3.21; S, 3.47. ¹H NMR (400 MHz, THF-d₈) δ 9.49 (s, 8H), 9.43–9.41 (m, 4H), 8.31–8.27 (m, 12H), 8.02–7.85 (m, 24H), 7.74–7.65 (m, 8H), 7.58–7.52 (m, 4H), 7.38–7.29 (m, 12H), 7.03–6.96 (m, 4H), 6.61–6.59 (m, 12H), 5.34 (s, 6H). ESI-MS: m/z = 2995.1156 [M + H]⁺; 3017.1649 [M + Na]⁺; 3033.1338 [M + K]⁺.

Eu₂L₃(S-BINAPO)₂. Yield: 82%. Anal. Calc. for C₁₄₈H₉₄F₁₈O₁₆S₃P₄Eu₂ (2994.34): C, 59.37; H, 3.16; S, 3.21. Found: C, 59.26; H, 3.33; S, 3.09. ¹H NMR (400 MHz, THF-d₈) δ 9.49 (s, 8H), 9.43–9.41 (m, 4H), 8.31–8.27 (m, 12H), 8.02–7.85 (m, 24H), 7.74–7.65 (m, 8H), 7.58–7.52 (m, 4H), 7.38–7.29 (m, 12H), 7.03–6.96 (m, 4H), 6.61–6.59 (m, 12H), 5.34 (s, 6H). ESI-MS: m/z = 2995.1158 [M + H]⁺; 3017.1653 [M + Na]⁺; 3033.1340 [M + K]⁺.

Gd₂L₃(R-BINAPO)₂. Yield: 84%. Anal. Calc. for C₁₄₈H₉₄F₁₈O₁₆S₃P₄Gd₂ (3004.28): C, 59.16; H, 3.15; S, 3.20. Found: C, 59.14; H, 3.19; S, 3.22. ESI-MS: m/z = 3005.9389 [M + H]⁺.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 520730805 & 51872077 & 51773054) for supporting this work.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 994501. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1ra01583d

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