Solvent and counterion cooperatively induced inversion of the stereocenter Δ/Λ and CPL in a mononuclear Eu(III) complex

Abstract

The development of lanthanide complexes with stimuli-responsive dynamic chiral inversion has significant potential for applications in chiroptical switches and chiral sensing. However, the variable coordination numbers and coordination geometries of Ln(III) ions pose substantial challenges in controlling the chiral inversion of lanthanide complexes. Herein, we present the first example of solvent and counterion cooperatively induced inversion of the Eu(III) stereocenter Δ/Λ in mononuclear complexes. In Cs[Eu(L^L)₄], where Cs⁺ serves as the counterion, the addition of chloroform to an acetonitrile solution of the complex resulted in a reversal of Eu(III) center configuration from Δ to Λ, accompanied with an inversion of the circularly polarized luminescence (CPL) signal (g_lum value shifting from +0.15 to −0.13). However, when (NMe₄)⁺ was used as the counterion, (NMe₄)[Eu(L^L)₄] did not exhibit this inversion behavior under the same conditions. Notably, the addition of Cs⁺ ions to a solution of (NMe₄)[Eu(L^L)₄] restored the inversion feature. This understanding of the impact of Cs⁺ ions and solvent on Δ/Λ inversion contributes to the development of CPL switches and sensors based on chiral lanthanide supramolecules.

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Introduction

The phenomenon of chiral inversion is widely present in biological molecules such as DNA and proteins, and the inversion is closely related to the implementation of their related functions in vital processes.^1–5 Inspired by biomacromolecules, artificial systems with chiral inversion, such as small peptides,^6,7 metal complexes,^8,9 polymers,^10,11 and supramolecules,^12,13 have been developed for their applications in fields such as asymmetric catalysis,^14,15 enantioselective recognition,^16,17 and chiral sensing.^18,19 The dynamic feature of coordination bonds and the configurational chirality (Δ or Λ) of metal centers make metal complexes an ideal platform compound for studying chiral inversion.

Recently, a few mononuclear and multinuclear complexes have achieved Δ/Λ configuration inversion under external stimuli, such as changes in solvents,^20–22 pH,^23,24 using light stimulation²⁵ and formation of host–guest complexes.^26,27 In the reported examples, the introduction of new coordinating atoms or the alteration of weak interactions between ligands are the main driving forces for inducing ligand rearrangements around the metal center.²⁸ For example, Hong and co-workers reported a chiral octahedral tris(chelate) Fe(II) complex where the modulation of hydrogen bond strength between ligands by methanol and chloroform resulted in Δ and Λ chiral inversion.²⁹ Miyake reported the inversion of stereochemistry from Λ to Δ in Co complexes induced by the coordination of NO₃⁻ ions (Scheme 1a).³⁰ However, these studies primarily focused on the modulation of stronger intramolecular supramolecular interactions, leaving the investigation of weaker intermolecular interactions for chirality regulation largely unexplored.

Scheme 1 Design concept of the dynamic chiral inversion of the metal center under external stimulation. (a) Schematic of NO₃⁻-triggered Δ/Λ inversion of Co(II) complexes. (b) The Δ/Λ configuration underwent inversion upon the binding of an Na⁺ ion to the crown-ether-like site. (c) Solvent and counterion cooperatively induced inversion of Δ/Λ in Cs[Eu(L^L)₄] and (NMe₄)[Eu(L^L)₄].

To facilitate simultaneous inversion of multiple coordination units in dynamic regulation, employing linear multidentate ligands,³¹ polypodal ligands,³² or forming multinuclear assemblies has emerged as an effective strategy.³³ The covalent linkage between coordination units enhances the mechanical coupling between ligands, thereby promoting a positive synergistic effect during ligand rearrangement. For instance, Nabeshima reported a macrocyclic copper(I) complex based on a bipodal 2,2′-bipyridine ligand, where the Δ/Λ configuration underwent inversion upon the binding of an Na⁺ ion to the crown-ether-like site of the ligand (Scheme 1b).³⁴

Compared to transition metal complexes, the high coordination number and variable coordination geometry of lanthanide ions necessitate the participation of a high number of ligands in the inversion process and overcome the unclear rearrangement orientation due to the lack of coordination guidance for lanthanides, which undoubtedly increases the difficulty of regulation.³⁵ While several lanthanide complexes have demonstrated helicity inversion through changes in solvent polarity,^36,37 coordination to Ln(III) ions,^38,39 and changing counterions,^40–44 examples of CPL inversion remain limited.^45–48 For instance, Kawai reported a solvent-dependent CPL sign reversal, where a (+)-camphorate derivative β-diketonate Eu(III) complex exhibited a significant g_lum value variation from +0.66 in acetonitrile to −0.17 with increasing acetone content.⁴⁹

Herein, we successfully report an example in which the Δ/Λ-configuration of tetrakis β-diketonate Eu(III) complexes was inverted by introducing cesium (Cs⁺) as a counter cation (Scheme 1c). As the chloroform content in the acetonitrile solution of the complex increases, Cs[Eu(L^L)₄] not only exhibits Δ → Λ configuration conversion but also causes the inversion of circularly polarized luminescence activity of Eu³⁺ ion luminescence, with the g_lum value flipping from +0.15 to −0.13. In contrast, the homologous (NMe₄)[Eu(L^L)₄] with tetramethylammonium (NMe₄)⁺ does not exhibit this inversion property. The addition of Cs⁺ ions into the solution of (NMe₄)[Eu(L^L)₄] enables realizing the Δ → Λ inversion.

Results and discussion

The design of ligands and crystal structures of Cs[Eu(L^L)₄] and (NMe₄)[Eu(L^L)₄]

The variable coordination numbers and geometries of Ln(III) ions make the control of Δ/Λ configuration chirality in lanthanide complexes more challenging compared to that in transition metal complexes. To effectively control the chirality of Ln(III) complexes, significant steric hindrance between the ligands is required to reduce the large conformational freedom resulting from the large ionic radii and the absence of coordination directionality.⁵⁰ However, from the perspective of dynamically controlling the configurational inversion of metal complexes, such large steric hindrance is clearly detrimental to Δ/Λ reversal. Therefore, resolving the contradiction between spatial constraints and dynamic regulation for chiral configuration control necessitates the careful design of ligands. Herein, we introduced two menthol moieties at the phenyl ring of benzoyltrifluoacetone (BTFA), forming a symmetric “tripodal” structure (Scheme 1c). The menthol group possesses molecular chirality, moderate rigidity–flexibility, and a large volume. When forming the Eu(L^L)₄ structure, the convergence of the eight menthol units around the Eu³⁺ ion within the limited space is expected to control the unidirectional arrangement of the ligands through weak interactions between the menthol groups and/or benzene rings. On the other hand, the moderate flexibility of menthol provides a structural basis for conformational adjustment to accommodate Δ/Λ configuration inversion under external stimuli. Additionally, the negative charge of [Eu(L^L)₄]⁻ enables the possibility of utilizing a cation to regulate structural transformations. To enhance the ion-pair interaction, we employed multi-fluoroalkyl groups as the terminal substituents of BTFA, which can engage in strong multiple hydrogen bonding or ion-dipole supramolecular interactions with (NMe₄)⁺ and Cs⁺ ions.

The synthesis and characterization of enantiomerically pure L^L and L^D, as well as their corresponding complexes Cs[Eu(L^L/D)₄] and (NMe₄)[Eu(L^L/D)₄], are listed in Scheme S1 and Fig. S1–S24.† Cs[Eu(L^L/D)₄] and (NMe₄)[Eu(L^L/D)₄] were prepared by reacting ligand L^L/D and Eu³⁺ in a 4 : 1 ratio in acetonitrile, using cesium hydroxide (CsOH, 50% w/w in water) and tetramethylammonium hydroxide (25% w/w in water) as bases, respectively. The formation of the complexes was confirmed by single-crystal X-ray diffraction analysis. Colorless crystals of Cs[Eu(L^L)₄] and (NMe₄)[Eu(L^L)₄], suitable for single-crystal X-ray diffraction, were obtained by slow evaporation of their acetonitrile/acetone/methanol-containing solutions. Crystallographic analysis revealed that Cs[Eu(L^L)₄] crystallizes in the Sohncke space group C2, while (NMe₄)[Eu(L^L)₄] crystallizes in the Sohncke space group P2₁2₁2. Notably, Cs[Eu(L^L)₄] exhibits a pseudo-racemate with Δ- and Λ-isomers in a 1 : 1 ratio within the crystal structure (Fig. 1a). In each Δ- or Λ-isomer, four ligands coordinate to the Eu³⁺ ion in a head-to-tail alternating arrangement (C₃F₇ as heads and menthol tails), with four menthol groups on two ligands adopting interdigitated arrangement (Scheme 1c, menthol group denoted with a cartoon gear). This arrangement leads to multiple hydrophobic interactions between the adjacent menthol groups, with H⋯H distances ranging from 1.8 to 3.9 Å. Notably, the methyl and isopropyl groups on one menthol moiety exhibit C–H⋯π interactions with the benzene ring of a neighboring ligand, with distances ranging from 2.5 to 3.8 Å. The Cs⁺ counterion is located within the cavity formed by the perfluoroalkyl chains and menthol groups, engaging in ion-dipole interactions with fluorine atoms (Cs⋯F distance, 3.1 to 3.4 Å).

Fig. 1 X-ray crystallographic structures and coordination geometries of (a) Cs[Eu(L^L)₄] and (b) (NMe₄)[Eu(L^L)₄] (color codes: Eu, blue-purple sphere; O, red; F, green; Cs, purple sphere). Some atoms are omitted for clarity.

In contrast to Cs[Eu(L^L)₄], (NMe₄)[Eu(L^L)₄] adopts only a Λ configuration, lacking the pseudo-racemate Δ- and the Λ-isomer pair. Furthermore, the ligand arrangement differs between the two complexes. In Cs[Eu(L^L)₄], eight oxygen atoms surrounding the Eu³⁺ ion form a triangular dodecahedron coordination geometry, with each ligand located at the edge of the polyhedron. In (NMe₄)[Eu(L^L)₄], the eight oxygen atoms form a square antiprism (SAP) coordination geometry. Notably, two bidentate diketonate units (O5, O7 and O6, O8) are located on the square planes of the SAP. The (NMe₄)⁺ counterion resides within the cavity formed by the polyfluoroalkyl chains and menthol groups. The H⋯H and C–H⋯F distances between the (NMe₄)⁺ and the ligands are in the range of 2.3 to 3.7 Å and 2.1 to 3.6 Å, respectively.

Solvent-induced inversion of Eu(III) stereocenter Δ/Λ and CPL

The presence of diastereoisomeric Δ-Eu(L^L)₄ and Λ-Eu(L^L)₄ in the crystalline state suggests that the menthol groups in the ligand do not provide sufficient steric hindrance to favor the formation of a single diastereoisomer. This observation implies that the complexes possess the potential for dynamic interconversion between Λ and Δ configurations in the solution upon exposure to external stimuli. To investigate this possibility, the solution structures of the two complexes were initially characterized in acetonitrile. High-resolution mass spectrometry (HR-MS) confirmed the formation of tetrakis β-diketonate complexes Cs[Eu(L^L/D)₄] and (NMe₄)[Eu(L^L/D)₄], where the isotopic distribution of the molecular ion peaks corresponding to the anionic species [Eu(L^L)₄]⁻ closely matches the simulated distribution (Fig. 2a). Notably, the ¹H NMR spectra of the complex Cs[Eu(L^L)₄] exhibited a single set of signals (Fig. 2b), rather than the anticipated two sets of peaks associated with the Δ and Λ diastereoisomers observed in the crystalline state. This suggests that the complex Cs[Eu(L^L)₄] either exists as a single Λ- or Δ-configurational isomer in the solution or undergo rapid interconversion between the two isomers. In the case of (NMe₄)[Eu(L^L)₄], the protons of the counterion (NMe₄)⁺ appeared at 3.09 ppm, which aligns with the chemical shift of the free (NMe₄)⁺ in acetonitrile at 3.07 ppm (Fig. S25†), indicating complete dissociation of (NMe₄)⁺ from [Eu(L^L)₄]⁻. Based on this observation, we speculate that the absence of ion-pair interaction likely facilitates the Λ ↔ Δ configurational conversion.

Fig. 2 (a) ESI-MS spectrum of (NMe₄)[Eu(L^L)₄] in CH₃CN. (b) ¹H NMR spectra of free ligand L^L, (NMe₄)[Eu(L^L)₄] and Cs[Eu(L^L)₄] in CD₃CN (400 MHz, 298 K). (c) CD spectra of (NMe₄)[Eu(L^L)₄] (red dashed line) and Cs[Eu(L^L)₄] in CH₃CN (c = 1.0 × 10⁻⁴ M). (d) CPL spectra of (NMe₄)[Eu(L^L)₄] (red dashed line) and Cs[Eu(L^L)₄] in CH₃CN (c = 1.0 × 10⁻⁵ M).

Circular dichroism (CD) spectroscopy is one of the most useful techniques for analyzing the configuration of metal complexes. As depicted in Fig. 2c, the CD spectrum of (NMe₄)[Eu(L^L)₄] exhibits a distinct Cotton effect within the ligand absorption band, ranging from 250 to 380 nm. The negative exciton couplet observed in this spectrum suggests an Δ configuration at the Eu³⁺ ion center.⁵¹ Consistent with this finding, the circularly polarized luminescence (CPL) spectra of (NMe₄)[Eu(L^L)₄] exhibit a positive sign at the magnetic dipole ⁵D₀ → ⁷F₁ transition (595 nm) and a negative sign at the electron dipole ⁵D₀ → ⁷F₂ transition (612 nm), further indicating a Δ configuration at the Eu³⁺ ion center.^52–54 Similarly, the CD and CPL spectra of Cs[Eu(L^L)₄] in CH₃CN display spectral patterns identical to those observed in (NMe₄)[Eu(L^L)₄], confirming the Δ configuration of the metal center (Fig. 2d). These findings indicate a Λ → Δ configurational conversion upon dissolution of the crystalline complexes in CH₃CN.

Based on the hypothesis that the ion pair dissociation in acetonitrile drives the configuration conversion, we propose that reducing ion pair dissociation by introducing a weakly polar solvent will hinder this conversion, potentially even reversing the configuration from Δ to Λ. To investigate this possibility, we selected chloroform (CHCl₃) as a weakly polar solvent to study the potential inverse process. In the ¹H NMR spectrum, the significant downfield shift of (NMe₄)⁺ from 3.09 ppm to 16.53 ppm in CDCl₃ strongly suggests the formation of an ion pair (Fig. 3b). This substantial shift is attributed to the paramagnetic effect of the Eu³⁺ ion. Additionally, the identical diffusion coefficients of (NMe₄)⁺ and [Eu(L^L)₄]⁻ in ¹H DOSY further support the formation of an ion pair (Fig. S14†). However, the CD spectrum (Fig. 3c) of (NMe₄)[Eu(L^L)₄] in CHCl₃ exhibits the same exciton coupling pattern as that observed in CH₃CN, indicating that the ion pair formation does not lead to a reversal of the configuration. A slight variation is observed in the luminescence dissymmetry factor (g_lum, ⁵D₀ → ⁷F₁, 595 nm), which increases from +0.142 to +0.197.

Fig. 3 Solvent-induced inversion of Eu(III) stereocenter Δ/Λ and CPL. (a) Solvent induced Δ ⇌ Λ inversion of Cs[Eu(L^L)₄]. (b) ¹H NMR spectra of (NMe₄)[Eu(L^L)₄] in CDCl₃ and CD₃CN and ¹H NMR spectra of Cs[Eu(LL)₄] in CDCl₃ (400 MHz, 298 K). (c) CD spectra of (NMe₄)[Eu(L^L)₄] in CH₃CN and CHCl₃ (c = 1.0 × 10⁻⁴ M). (d) CPL spectra of (NMe₄)[Eu(L^L)₄] in CH₃CN and CHCl₃ (c = 1.0 × 10⁻⁵ M). (e) CD spectral changes in Cs[Eu(L^L)₄] with increasing CHCl₃ content in CH₃CN (c = 2.5 × 10⁻⁵ M). (f) CPL spectral changes of Cs[Eu(L^L)₄] with increasing CHCl₃ content in CH₃CN (c = 1.0 × 10⁻⁵ M). (g) The g_lum value changes of Cs[Eu(LL)₄] with increasing CHCl₃ content in CH₃CN.

In contrast, Cs[Eu(L^L/D)₄] exhibits a configurational inversion in CHCl₃, as evidenced by the reversed CD and CPL spectra. As shown in Fig. 3e, the CD spectrum displays a positive exciton couplet, while the CPL spectrum (Fig. 3f) exhibits a negative signal at 595 nm and a positive signal at 612 nm, both suggesting a Λ configuration of the complex in CHCl₃. This finding indicates that the configuration conversion can be induced in the solution by varying the solvent composition. As expected, both CD and CPL spectra reveal an inversion process upon increasing the chloroform content in CH₃CN. The CD spectrum demonstrates that the molar extinction coefficient Δε = −20 M⁻¹ cm⁻¹ undergoes a signal reversal at a CH₃CN/CHCl₃ volume ratio of 3/7, eventually reaching a maximum positive value in chloroform (Δε = 23 M⁻¹ cm⁻¹). Similarly, the CPL spectrum also exhibits a signal inversion, accompanied by a g_lum value inversion from +0.15 to −0.13 (Fig. 3g). This Δg_lum value of 0.28 is considerably larger and more practically useful compared to the conventional Δg < 10⁻² CPL reversal magnitude change observed in fluorescent dyes.⁵⁵ Additionally, subtle differences in the fine structure of the emission spectra (Fig. S44†) indicate variations in the Eu³⁺ ion coordination environment in acetonitrile and chloroform. Different solvents also result in distinct luminescence quantum yields (QYs) for Cs[Eu(L^L)₄], with values of 36% (CHCl₃) and 45% (CH₃CN). The high QYs benefit from the good energy level match between the triplet state (T₁, 20 400 cm⁻¹) of the ligand (estimated from the 0 → 0 transition of Cs[Gd(L^L)₄]) (Fig. S47†) and the ⁵D₀ energy of the Eu³⁺ ion.

This solvent-dependent inversion process was also monitored by ¹H NMR spectroscopy (Fig. S54†). As the chloroform (CDCl₃) content increased, the peaks of all the protons exhibited broadening, particularly those from the methyl group (H_a) and the menthol moiety protons (H_d,d′). However, no two sets of signals associated with the Δ and Λ diastereoisomers were observed. To avoid the influence of the peak broadening on spectral resolution caused by the paramagnetism of the Eu³⁺ ion, the isomorphic Cs[Lu(L^L)₄] was used as a substitute for the ¹H NMR experiment (Fig. S55†). However, the ¹H NMR spectra still displayed a single set of signals, indicating that a rapid Δ ↔ Λ interconversion between the two isomers occurred within the timescale of the ¹H NMR experiment. Although the content ratio of Δ- and Λ-Cs[Lu(L^L)₄] diastereoisomers cannot be accurately determined, the chiroptical variations unequivocally verify the existence of configurational inversion at the Eu³⁺ ion center and demonstrate the role of the Cs⁺ ion in dynamic chirality reversal regulation.

Mechanism of the Eu³⁺ stereocenter Δ/Λ inversion

The aforementioned ¹H NMR, ESI-TOF-MS, and chiroptical experiments have established a correlation between configuration reversal and enhanced ion pair interactions. In polar CH₃CN, the strong solvation effect on the anion and cation leads to complete dissociation of Cs⁺ and [Eu(L^L)₄]⁻. This indicates that the complex exhibits a stereochemical preference for the Δ configuration in the absence of a counterion. To validate this hypothesis, DFT calculations were performed to optimize the structures of Δ-[Eu(L^L)₄]⁻ and Λ-[Eu(L^L)₄]⁻ in the absence of counterions, mimicking the situation of the complexes in CH₃CN. The initial structural coordinates were obtained directly from the crystal structure. Precise energy calculations revealed that the free Δ-[Eu(L^L)₄]⁻ exhibits lower system energy compared to the Λ-[Eu(L^L)₄]⁻ isomer (ΔG = +10.73 kJ mol⁻¹). This suggests that Δ-[Eu(L^L)₄]⁻ is the thermodynamically stable product in the absence of counterions, consistent with the predictions of a Δ configuration or an excess of the Δ configuration inferred from CD and CPL spectra in CH₃CN. The optimized structures and calculated system energies are depicted in Fig. 4a and S56.†

Fig. 4 DFT calculation (energies are given in kJ mol⁻¹), IGM and Hirshfeld surface analysis of Δ/Λ-Cs[Eu(LL)₄]. (a) Energy diagrams of the Δ/Λ-Cs[Eu(LL)₄] in the absence and presence of the counterion Cs⁺. (b) IGM and Hirshfeld surface analysis of the weak interactions of Δ/Λ-Cs[Eu(LL)₄] (δg_inter = 0.008). The percentage represents the contribution of C–F⋯Cs and C–O⋯Cs contacts to the total Hirshfeld surface area between Cs⁺ and the complexes.

To verify the inversion arising from the interaction between Cs⁺ and [Eu(L^L)₄]⁻, geometry optimizations and energy calculations were performed on the ion pairs, Δ-Cs[Eu(L^L)₄] and Λ-Cs[Eu(L^L)₄] (Fig. 4a and S57†). As expected, with Cs⁺ as the counterion, Λ-Cs[Eu(L^L)₄] exhibits a lower system energy than Δ-Cs[Eu(L^L)₄] (ΔG = −7.18 kJ mol⁻¹). This result aligns with the deduction of a Δ → Λ conversion due to the formation of ion pairs in CHCl₃. While the calculated ΔG was consistent with the experimental inversion results, DFT calculations have limitations, such as a computational accuracy of only 8.5–13.0 kJ mol⁻¹.⁵⁶ Therefore, DFT calculations can only provide a reference for this inversion process from the thermodynamic point of view. However, from the observed trends of energy, uphill and downhill in the complex, before and after Cs⁺ ion binding, we believe that the DFT results still support the assertion that ion pairing can alter the thermodynamic equilibrium between diastereomers.

To further elucidate the influence of counterions (Cs⁺) on the thermodynamic stability of Δ- and Λ-isomers, non-covalent interactions between Cs⁺ and [Eu(L^L)₄]⁻ were investigated using independent gradient model (IGM) analysis and Hirshfeld surface analysis.^57–59 The IGM analysis revealed green regions between Cs⁺ and [Eu(L^L)₄]⁻, indicating the presence of various weak intermolecular interactions (Fig. 4b). To quantify these supramolecular interactions, Hirshfeld surface analysis was conducted. This analysis highlighted strong contacts between Cs⁺ and [Eu(L^L)₄]⁻, predominantly driven by C–F⋯Cs and C–O⋯Cs interactions. The results showed that compared to Δ-Cs[Eu(L^L)₄], the contacts between Cs⁺ and [Eu(L^L)₄]⁻ are closer in Λ-Cs[Eu(L^L)₄], with a larger contribution from C–F⋯Cs interactions (35.05% for Δ-Cs[Eu(L^L)₄] and 36.17% for Λ-Cs[Eu(L^L)₄]). Additionally, various weak interactions were observed between the menthol groups in the neighboring ligands and the menthol/benzene ring. IGM analysis indicated that the contact areas between ligands are greater in Λ-Cs[Eu(L^L)₄] compared to those in Δ-Cs[Eu(L^L)₄] (C–H⋯π) (Fig. S59†). These results demonstrate that the energy difference between the two stereoisomeric pairs, estimated from DFT calculations, likely arises from variations in these weak interaction strengths. Overall, the DFT calculations and weak interaction analyses provide strong evidence for the solvent- and counterion-dependent inversion process.

Counterion exchange induced inversion of Δ → Λ and CPL

The above CD and CPL titration experiments show that the chiral inversion of the complexes is not only dependent on the solvent but also on the selection of counterions, Cs⁺ and (NMe₄)⁺. Therefore, we speculate that the addition of Cs⁺ ions to the solution of (NMe₄)[Eu(L^L)₄] probably results in the replacement of (NMe₄)⁺, and induces the configurational inversion from Δ to Λ. The ¹H NMR titration experiment demonstrates the occurrence of ion exchange. As depicted in Fig. 5b, upon adding various equivalents of the Cs⁺ ion to the CD₃CN/CDCl₃ (1 : 9, v/v) solution of (NMe₄)[Eu(L^L)₄], all proton resonances underwent shifts, with the methyl group of (NMe₄)⁺ exhibiting the most significant change. When 2.0 equivalents of Cs⁺ were introduced, the methyl signal of (NMe₄)⁺ shifted from the initial 12.58 ppm to 3.23 ppm, very close to the chemical shift of the free (NMe₄)⁺ at 3.20 ppm (Fig. S65†), indicating the complete replacement of (NMe₄)⁺ by Cs⁺. The fitting results obtained using BindFit software demonstrated that the binding of Cs⁺ with [Eu(L^L)₄]⁻ follows a 1 : 1 binding model, with a calculated relative binding constant K = K_Cs/K_(NMe4) = 303 M⁻¹ (Fig. S63†).

Fig. 5 Counterion exchange induced inversion of Δ → Λ and CPL. (a) Configuration inversion of Δ-(NMe₄)[Eu(L^L)₄] → Λ-Cs[Eu(L^L)₄] triggered by counterion exchanging. (b) Changes in ¹H NMR spectral upon adding various equivalents of Cs⁺ to the solution of (NMe₄)[Eu(L^L)₄] (400 MHz, 298 K, CD₃CN/CDCl₃ = 1 : 9). (c) Changes in the g_lum value upon adding various equivalents of CsTFPB to the solution of (NMe₄)[Eu(L^L)₄] in CH₃CN/CHCl₃ (1 : 9, v/v). (d) Changes in CD spectral upon adding various equivalents of Cs⁺ (c = 7.0 × 10⁻³ M) to the solution of (NMe₄)[Eu(L^L)₄] (c = 1.0 × 10⁻⁴ M) in CH₃CN/CHCl₃ (1 : 9, v/v). (e) Changes in CPL spectral upon adding various equivalents of Cs⁺ (c = 1.5 × 10⁻³ M) to the solution of (NMe₄)[Eu(L^L)₄] (c = 1.0 × 10⁻⁵ M) in CH₃CN/CHCl₃ (1 : 9, v/v).

CD and CPL spectra provided further evidence for the role of counterion exchange in regulating the Δ → Λ configuration inversion. As depicted in Fig. 5d, upon the addition of varying amounts of the Cs⁺ salt, the CD spectrum gradually transitions from negative exciton coupling to a positive form, reaching a maximum value of 75 M⁻¹ cm⁻¹ at approximately 2.0 eq. of Cs⁺ (Fig. S67†). A similar trend was observed in the CPL spectrum (Fig. 5e). Initially, (NMe₄)[Eu(L^L)₄] exhibits a positive CPL signal at the ⁵D₀ → ⁷F₁ transition, with a g_lum value of +0.20. However, with the introduction of Cs⁺ salt, the CPL signal intensity gradually diminishes, and at around 0.4 eq. of Cs⁺, the CPL sign begins to invert, eventually reaching a maximum negative value of −0.25 at approximately 2.0 eq. of Cs⁺ (Fig. S68 and S69†). The g_lum value varies with the changing Cs⁺ equivalents, as shown in Fig. 5c. Additionally, the replacement of (NMe₄)⁺ by Cs⁺ is also evident in the alterations of the emission spectral pattern for the ⁵D₀ → ⁷F₂ transition (Fig. S70†).

Conclusions

In summary, we present a case demonstrating that the chiral inversion of a mononuclear Eu(III) complex is dependent on the cooperative effect of the solvent and the counterion. The enhanced ion pair interaction between the Cs⁺ ion and the anionic [Eu(L^L)₄]⁻, driven by a decrease in solvent polarity, was shown to be the driving force for this configurational conversion. However, the absence of inversion for (NMe₄)[Eu(L^L)₄] indicates that the inversion is not solely determined by ion-pair interaction but also by the stereochemical preference of the final complex. These findings offer a novel perspective on studying dynamic chirality reversal in lanthanide complexes, highlighting their unique potential as CPL switches due to substantial variations in the CPL signal (Δg_lum = 0.28 in this work).

Author contributions

H. F. L. conceived and supervised the project. W. H. L. performed the experiments and analyzed the data. S. Y. and Z. Y. S. collected diffraction data and solved and refined the X-ray crystal structures. H. F. L., T. G., Y. Y. Z. and P. F. Y. reviewed and edited the paper. All authors contributed to the final draft of the paper.

Data availability

The data that supports the findings of this study are available in the ESI† of this article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 52273263, 52203219 and 52073080), Heilongjiang Provincial Natural Science Foundation Joint Guidance Projects (no. LH2023B022), Scientific Research Project of Basic Scientific Research Operating Expenses of Colleges and Universities in Heilongjiang Province (2021-KYYWF-0029 and 2021-KYYWF-0041). Heilongjiang Province key research and development plan (2022ZX07D04).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2386501 and 2386518. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02730b

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