A hydrogen-bond-stabilized chiral tetrakis Eu(III) complex with strong circularly polarized luminescence

Abstract

We report the synthesis and properties of a hydrogen-bond-stabilized chiral tetrakis Eu(III) complex with enhanced circularly polarized luminescence (CPL). It is based on a Eu(III) tris[3-(heptafluoropropylhydroxymethylene)-camphorate] complex modified with tetrabutylammonium and imidazolium counterions. While the bulky tetrabutylammonium cation preserves the parent tris-diketonate framework, the smaller imidazolium cation forms a novel chiral tetrakis Eu(III) complex through stable NH⋯O=C hydrogen bonding between the imidazolium N–H donor and the diketonate carbonyl acceptor. A single-crystal X-ray diffraction study indicates a square-antiprismatic geometry. CPL spectra, recorded with a Raman optical activity (ROA) spectrometer under 532 nm excitation, reveal an extremely high dissymmetric factor of 1.26 at 595 nm. In chloroform solution, additional ROA signals were detected, attributed to CPL-induced Raman scattering from the solvent. These properties highlight the convenience of hydrogen-bond-assisted stabilization. The results document an efficient approach for tuning CPL-active lanthanide complexes and demonstrate the methodological advantage of ROA-based CPL detection.

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Introduction

Circularly polarized luminescence (CPL) is a convenient tool for studying chirality of the excited state, including coordination complexes and supramolecular systems.^1–3 CPL performance is commonly evaluated using the luminescence dissymmetry factor, g_lum = 2 (I_L − I_R)/(I_L + I_R), where I_L and I_R represent the intensities of left- and right-circularly polarized emission, respectively. However, high values of g_lum (close to 2) are extremely rare,^4–6 and achieving them remains a challenge in the design of CPL-active materials.

Furthermore, lanthanide complexes are particularly attractive CPL emitters. They typically provide narrow f–f bands and long lifetimes, and their properties can be tuned using organic ligands.^7–9 A landmark example is the heterobimetallic complex Cs⁺[Eu((+)-hfbc)₄]⁻ (hfbc = 3-heptafluorobutyrylcamphorate), which exhibits a remarkable g_lum of 1.38.⁴ More recent studies have indicated the decisive role of counterions. For example, replacing Cs⁺ with smaller alkylammonium cations further enhanced the CPL response.¹⁰ CH⋯F interactions with tailored counterions can also stabilize chiral Eu(III) architectures with improved CPL performance due to ligand-to-metal charge transfer (LMCT).

Accurate determination and control of CPL signals remains a methodological challenge.¹¹ Variations in the excitation wavelength, detection sensitivity, and the sample environment can produce discrepancies in reported g_lum values, complicating cross-study comparisons. Methodological advances are therefore needed to ensure both sensitivity and reproducibility in CPL measurements.

Previously, we have demonstrated that a Raman optical activity (ROA) spectrometer can be used for lanthanide CPL studies.^12–15 ROA instruments can normally detect minor differences in vibrational Raman scattering between right- and left-circularly polarized light. When operated under 532 nm excitation, their high laser intensity and artifact-resistant detection enable sensitive monitoring of even weak Eu(III) CPL bands. Because lanthanide complexes exhibit negligible absorption and electronic circular dichroism (ECD) in the visible range,^16,17 interference from these signals is minimal. At 532 nm, the vibrational transitions are usually weak and often dominated by lanthanide CPL. In contrast, near-IR excitation at 785 nm effectively suppresses fluorescence, allowing the determination of vibrational ROA and CPL contributions. For example, vibrational bands of several Eu(III) complexes could be assigned in this case with the aid of density functional theory (DFT) simulations.¹²

Here, we explore a counterion variation in Eu(III) camphorate complexes. It appears that tetrabutylammonium cations preserve the parent tris-Eu(III) framework, whereas imidazolium (ImH) cations form so far undescribed hydrogen-bond-stabilized tetrakis species, ImH⁺[Eu(hfbc)₄]⁻, with a square-antiprismatic geometry determined by single-crystal X-ray diffraction. In contrast, the introduction of sodium hydroxide as a pH balancing agent disrupted this ion pair and yielded a heterobimetallic Na⁺[Eu(hfbc)₄]⁻ complex. The CPL spectrum of ImH⁺[Eu((−)-hfbc)₄]⁻ recorded with an ROA spectrometer under 532 nm excitation exhibits a high dissymmetry factor (g_lum = −1.26 at 595 nm) for the imidazolium-stabilized complex. ROA signals from the solvent affected by Eu(III) CPL were observed in chloroform. This phenomenon is often distinct from ECD-Raman interference,¹⁷ which is negligible in this spectral region.

Experimental

All chemicals were purchased from Sigma Aldrich and used without any further purification.

Synthesis of europium(III) tris[3-(heptafluoropropylhydroxymethylene)-camphorate] Eu(hfc)₃

0.53 mmol (225 μl) of (+)-(hfc)₃ ((+)-3-(heptafluorobutyryl)-(+)-camphor) or (−)-(hfc)₃ ((−)-3-(heptafluorobutyryl)-(−)-camphor) was dissolved in 3 ml of ethanol, then an equivalent molar amount of Et₃N (0.53 mmol, 75 μl) was added slowly upon stirring, and the mixture was kept under stirring for 1 h. Then 0.177 mmol (65 mg) of EuCl₃·6H₂O in 2 ml of ethanol was added. After 2 h, ethanol was removed and treated with CH₂Cl₂ : H₂O = 1 : 1, the organic layer was further washed with water and concentrated in vacuo. The dried product (262 mg, yielding 12.3%) was used directly in the following steps.

Synthesis of imidazolium tetrakis(3-heptafluoro-butylryl-camphorato) europium(III) ImH⁺[Eu(hfbc)₄]⁻

To a 9 ml ethanol solution of 0.05 mmol (59 mg) of Eu-(+)-(hfc)₃ or Eu-(−)-(hfc)₃, 0.1 mmol (104.5 mg) of imidazole hydrochloride (ImH) dissolved in 1 ml of ethanol was added slowly under stirring, and the mixture was refluxed for 15 h. A yellow powder was obtained after the solution cooled down and the solvent was removed partially. The crude product was recrystallized in acetonitrile (MeCN) solution, and 9.48 mg of yellow needle crystals were collected, with a yield of 15%. ESI-MS (m/z): [Eu(+hfbc)₄]⁻ calcd for C₅₆H₅₆EuF₂₈O₈, 1541.27; found, 1541.27. Elemental analysis: calcd for C₅₉H₆₁EuF₂₈N₂O₈, C 44.01, H 3.82, N 1.74; found, C 43.79, H 3.66, N 1.88%. ImH⁺[Yb((−)-hfbc)₄]⁻ was prepared following the same procedure as for the Eu analogue, with Yb³⁺ used instead of Eu³⁺.

Synthesis of sodium tetrakis(3-heptafluoro-butylryl-camphorato) europium(III) Na⁺[Eu(hfbc)₄]⁻

0.37 mmol (73 mg) of tetraethylammonium iodide or equivalent imidazole hydrochloride and 0.19 mmol (81.4 mg) of Eu(NO₃)₃·5H₂O were dissolved in 3 ml of ethanol and added slowly to 3 ml ethanol solution of 0.57 mmol (242 μl) of (+)-(hfc)₃ or (−)-(hfc)₃. The pH value of the mixture was neutralized to 7 by adding drops of saturated ethanol solution of NaOH. The mixture was stirred for 1 h, and a white precipitate was filtered; a light yellow powder was obtained after the solvent was removed. The crude product was recrystallized in MeCN solution and 76 mg (yield 7%) of light yellow crystals were collected. ESI-MS (m/z): [Eu(+hfbc)₄]⁻ calcd for C₅₆H₅₆EuF₂₈O₈, 1541.27; found, 1541.27. Elemental analysis: calcd for C₅₈H₅₉EuF₂₈NO₈ (based on crystal structure data) C 43.40, H 3.71, N 0.87; found, C 43.22, H 3.45, N 1.02%.

Spectroscopic measurements

The CPL spectra were measured using a ZEBR Raman optical activity (ROA) spectrometer equipped with two CCD cameras, operating in an extended spectral range of 80–4580 cm⁻¹ (∼534–703 nm) under laser excitation of 532 nm.^18,19 For luminescence bands, the Raman shift/wavenumber (ν_R, in cm⁻¹) can be converted to the absolute wavelength scale (ν_A, in nm) using ν_A = 10⁷/(10⁷/532 − ν_R). The CPL dissymmetry factor g_lum value is reported as −2CID, where CID (circular intensity difference) is the ROA analogue of the dissymmetry factor and is expressed as CID = (I_R − I_L)/(I_R + I_L).^13,16 The laser power was 90 mW for ImH[Eu(hfbc)₄] and 30 mW for Na[Eu(hfbc)₄]; ROA experiments were carried out with an accumulation time of 5 minutes to capture the intense CPL bands, while longer acquisition times (up to 2 hours) were employed to acquire weaker spectral features with higher resolution.

The ¹H NMR spectrum of ImH⁺[Yb(hfbc)₄]⁻ was measured with a Bruker Avance III™ HD 400 MHz spectrometer at ambient temperature in deuterated chloroform.

The ECD spectra were recorded on a Jasco J-815 spectropolarimeter in the range of 350–600 nm. All experiments were performed at room temperature, in a 2 mm quartz cell, and in methanol solutions; the concentration was 2 × 10⁻³ mol L⁻¹ for both complexes.

The fluorescence/luminescence spectra of ImH⁺[Eu(hfbc)₄]⁻ and Na⁺[Eu(hfbc)₄]⁻ complexes in chloroform were recorded on a Fluorolog-QM spectrophotometer (Horiba) equipped with a xenon lamp as the excitation source. Steady-state emission spectra were measured using excitation wavelengths of 360, 370 and 380 nm, respectively. Fluorescence lifetimes at room temperature were measured with the same instrument, using a Spectro LED excitation source (Horiba).

Computations

Equilibrium geometries of ImH⁺[Eu((−)-hfbc)₄]⁻ and Na⁺[Eu((−)-hfbc)₄]⁻ were obtained by energy minimization using the Gaussian16 program²⁰ with the B3LYP functional. For C, H, N, O and F atoms, the 6-311++G(d,p) basis set was employed for the two Eu(III) complexes. The SDF10 and MWB28 pseudopotentials and basis sets were used for Na in Na⁺[Eu((−)-hfbc)₄]⁻ and for Eu in both complexes, respectively. The conductor-like polarizable continuum solvent model (CPCM) was applied to mimic the environment (CHCl₃).^12,21

Results and discussion

Recent studies have established that both counterion interactions with the tetrakis europium anion and ligand–metal coupling are critical determinants of CPL performance.¹⁰ In this work, we employed tetrabutylammonium and imidazolium cations to modulate the Eu(III) tris[3-(heptafluoropropylhydroxymethylene)-camphorate] complex. Whereas the bulky tetrabutylammonium cation leaves the parent tris-diketonate framework essentially unaltered, the smaller imidazolium cation promotes the formation of a novel chiral tetrakis Eu(III) species. In contrast, neutralization with sodium hydroxide prevents incorporation of the imidazolium counterion, affording instead the heterobimetallic complex Na⁺[Eu(hfbc)₄]⁻.

Single-crystal X-ray diffraction analyses of ImH⁺[Eu((−)-hfbc)₄]⁻ (Fig. 1) and Na⁺[Eu((−)-hfbc)₄]⁻ (Fig. 2) unambiguously confirm these structural motifs, with the key crystallographic parameters summarized in Tables S1 and S2.

Fig. 1 (A) The asymmetric unit diagram, (B) coordination environment of the Eu³⁺ ion of complex ImH⁺[Eu((−)-hfbc)₄]⁻, and (C) the presence of CO⋯HN hydrogen bonds between imidazolium and oxygen atoms of the diketonate ligand.

Fig. 2 (A) The asymmetric unit diagram, and (B) DD-8(D2h) configuration of the Na⁺[Eu((−)-hfbc)₄]⁻ complex.

The coordination geometry of the Eu core in ImH⁺[Eu((−)-hfbc)₄]⁻ adopts the typical square antiprism (SAP) arrangement (Fig. 1B), consistent with other reported chiral tetrakis Eu(III) complexes.^4,5,10 Notably, however, this complex crystallizes in the orthorhombic crystal system with the chiral space group P2₁2₁2₁, distinct from the previously reported tetrakis Eu(III) complexes stabilized by small alkylammonium cations.¹⁰ The asymmetric unit comprises four 3-heptafluorobutyrylcamphor [C₁₄H₁₅F₇O₂]⁻ anions, one Eu³⁺ ion, and one imidazolium [C₃H₄N₂]⁺ cation, corresponding to the formula [C₅₉H₆₁EuF₂₈N₂O₈] (Fig. 1A). In the eight-coordinated [EuO₈] core, the dihedral angle between the projected planes of adjacent oxygen atoms is −42° (Fig. 1B), similar to the typical square antiprism geometry of its analogue.⁵

The crystal structure reveals a key stabilization feature: a strong N–H⋯O=C hydrogen bond forms between the N–H group of the imidazolium cation and the carbonyl oxygen atom of the diketonate ligands. The donor–acceptor distance for this interaction is 1.973 Å. Furthermore, an identical hydrogen bond, with a slightly longer distance of 2.055 Å, connects the complex to a second, adjacent ImH⁺[Eu((−)-hfbc)₄]⁻ molecule (Fig. 1C). Thus, the well-defined hydrogen bonding stabilizing the complexes differs from the ligand-to-ligand hydrogen bonding observed previously.^22,23

In addition, several weaker C–H⋯FC interactions were observed between the fluorine substituents of the ligands and the imidazolium cation, with contact distances in the range of 2.672–2.827 Å. These cooperative interactions collectively stabilize the tetrakis Eu(III) framework.

In contrast, the crystal structure of Na⁺[Eu((−)-hfbc)₄]⁻ is closely related to previously described systems exhibiting a “pseudo-achiral DD (D2d)” configuration (Fig. 2B), which also crystallize in the orthorhombic P2₁2₁2₁ space group.^24–26 Here, the Na⁺ ion is nine-coordinate, encapsulated by the tetrakis europium anion through multiple Na⁺⋯O–Eu and Na⁺⋯FC contacts, giving rise to a compact ion pair. Additionally, one nitrogen atom from a solvated CH₃CN molecule binds directly to Na⁺ (Fig. 2A).

Paramagnetic NMR of the Yb³⁺ analogue is often used to gain structural insights into Eu³⁺ complexes in solution. Indeed, two broad bands at δ > 12 in the paramagnetic ¹H NMR spectrum of ImH⁺[Yb(hfbc)₄]⁻ confirm the presence of the imidazolium cation (Fig. S1). The DFT optimized geometries of both complexes closely reproduce the crystallographic structures (Fig. 3). For ImH⁺[Eu((−)-hfbc)₄]⁻, the calculated distance between the N–H group of the imidazolium cation and the carbonyl oxygen atom of the diketonate ligands of 1.760 Å is slightly shorter than in the crystal. This is consistent with the tendency of DFT to slightly strengthen directional hydrogen bonds in the absence of lattice packing effects. These data highlight the structural flexibility of the [Eu(hfbc)₄]⁻ framework: in the imidazolium salt, stabilization is dominated by directional N–H⋯O=C hydrogen bonding, while in the sodium salt, it is governed by electrostatic Na⁺⋯O/F interactions. Despite these differences, both complexes adopt the Λ-configuration of the camphorate ligands, consistent with previously reported tetrakis Eu(III) architectures.^24,25

Fig. 3 DFT optimized geometries of complexes ImH⁺[Eu((−)-hfbc)₄]⁻ (left) and Na⁺[Eu((−)-hfbc)₄]⁻·CH₃CN (right).

Raman/luminescence (I_R + I_L) and ROA/CPL (I_R − I_L) spectra of ImH⁺[Eu((±)-hfbc)₄]⁻ and Na⁺[Eu((±)-hfbc)₄]⁻ enantiomer complexes in chloroform and acetonitrile solutions were measured using an ROA spectrometer equipped with two CCD cameras covering the spectral ranges of 80–2630 cm⁻¹ (∼534–618 nm) and 2200–4580 cm⁻¹ (∼603–703 nm). Most luminescence bands of ImH⁺[Eu((−)-hfbc)₄]⁻ and Na⁺[Eu((−)-hfbc)₄]⁻ observed in the Raman scattering closely match those recorded using conventional fluorescence spectroscopy (Fig. S2 and S3). Emission decay curve measurements reveal that the luminescence lifetime of ImH⁺[Eu((−)-hfbc)₄]⁻ is shorter than that of Na⁺[Eu((−)-hfbc)₄]⁻ (Fig. S4), although this difference does not significantly influence the recorded total luminescence or CPL spectra. The CCD detector clearly resolved the characteristic Eu(III) transitions: ⁵D₁ → ⁷F₂ (∼550–560 nm), ⁵D₀ → ⁷F₀ (∼580 nm), ⁵D₀ → ⁷F₁ (∼583–598 nm), ⁵D₀ → ⁷F₂ (∼610–625 nm), ⁵D₀ → ⁷F₃ (∼640–660 nm), and ⁵D₀ → ⁷F₄ (∼690–700 nm).

The structure of ImH⁺[Eu((±)-hfbc)₄]⁻ appears highly stable in both solvents, as the CPL (I_R − I_L) patterns of the Eu(III) transitions are almost identical in chloroform (Fig. 4) and acetonitrile (Fig. 5). However, the luminescence dissymmetry factors differ significantly, with chloroform showing a much larger value of 1.26 at the ⁵D₀ → ⁷F₁ (595 nm) transition (Table 1). In the low wavenumber range of 100–760 cm⁻¹ (530–550 nm), where the Raman signals of chloroform overlap with the chirality sensitive Eu(III) absorption ⁵D₁ ← ⁷F₁ (∼535 nm)⁶ and emission ⁵D₁ → ⁷F₂ (∼550–560 nm), almost mirror image ROA bands of chloroform were detected at 262, 368, 670 and 753 cm⁻¹ (Fig. 4 left). Similar features had been previously observed for Cs⁺[Eu((±)-hfbc)₄]⁻ in chloroform.¹⁵ At the low concentration used here, the ECD of the Eu(III) ⁵D₁ ← ⁷F₁ (∼535 nm) transition was not observable using a conventional ECD spectrometer (Fig. S5), confirming that ECD-Raman effects can be neglected.^16,17 We therefore attribute the observed ROA signals in chloroform primarily to solvent polarized Raman scattering, coupled with contributions from CPL arising from Eu(III) transitions.

Fig. 4 Raman/luminescence (I_R + I_L) and ROA/CPL (I_R − I_L) spectra of ImH⁺[Eu((±)-hfbc)₄]⁻ enantiomer complexes (3 mM) in chloroform in the range of 80–2630 cm⁻¹ (left) and 2280–4580 cm⁻¹ (right). Minor artifacts (marked by asterisks [*]) are due to subtracting the chloroform signal.

Fig. 5 Raman/luminescence (I_R + I_L) and ROA/CPL (I_R − I_L) spectra of ImH⁺[Eu((±)-hfbc)₄]⁻ enantiomer complexes (1 mM) in acetonitrile in the range of 80–2630 cm⁻¹ (left) and 2280–4580 cm⁻¹ (right). Minor artifacts (marked by asterisks [*]) are due to subtracting the acetonitrile signal.

Table 1 Circularly polarized luminescence parameters of Eu(III) complexes in chloroform and acetonitrile

Complex	^7FJ	g lum in CHCl3		g lum in MeCN
ImH^+[Eu((−)-hfbc)4]^−	J = 1	−0.52	586 nm	0.34	585 nm
	J = 1	−1.26	595 nm	0.47	595 nm
	J = 2	0.17	612 nm	−0.07	612 nm
	J = 3	0.18	649 nm	0.11	650 nm
	J = 3	−0.38	653 nm	−0.21	652 nm
	J = 4	0.07	693 nm	0.05	692 nm
	J = 4	−0.04	696 nm	−0.01	696 nm
Na^+[Eu((−)-hfbc)4]^−	J = 1	−0.12	585 nm	0.05	584 nm
	J = 1	−0.13	591 nm	−0.23	585 nm
	J = 1	−0.23	595 nm	−0.41	595 nm
	J = 2	0.02	611 nm	0.05	612 nm
	J = 2	−0.05	626 nm	0.01	625 nm
				0.03	634 nm
	J = 3	0.05	651 nm	0.08	650 nm
	J = 3	−0.09	655 nm	−0.15	652 nm
				0.02	677 nm
	J = 4	−0.02	693 nm	0.04	692 nm
	J = 4	0.02	696 nm	−0.02	696 nm

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Although the g_lum values in acetonitrile are only about half of those in chloroform, no ROA bands assignable to acetonitrile vibrations were detected (Fig. 5). The reduced CPL dissymmetry factor is likely associated with solvent-induced perturbations of the Eu³⁺ excited states. Coupling of the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions with the C≡N stretching vibrations of acetonitrile (∼2250 cm⁻¹) may promote nonradiative relaxation and subtly alter the local electric dipole environment, thereby influencing the observed CPL response.

For Na⁺[Eu((±)-hfbc)₄]⁻, both the luminescence (I_R + I_L) and CPL (I_R − I_L) spectral patterns differ significantly between chloroform (Fig. 6) and acetonitrile (Fig. 7), most likely due to the coordination of one acetonitrile molecule to Na⁺ in the solid state. In contrast to the imidazolium system, where larger g_lum values are observed in chloroform than in acetonitrile, the sodium analogue shows slightly higher g_lum in acetonitrile than in chloroform. The observed CPL bands and g_lum values are similar to previous reports obtained with conventional CPL instrumentation.²⁷ In addition, lower-energy transitions such as ⁵D₀ → ⁷F₃ (∼640–660 nm) and ⁵D₀ → ⁷F₄ (∼690–700 nm) were also detected.

Fig. 6 Raman/luminescence (I_R + I_L) and ROA/CPL (I_R − I_L) spectra of Na⁺[Eu((±)-hfbc)₄]⁻ enantiomer complexes (3 mM) in chloroform in the range of 80–2630 cm⁻¹ (left) and 2280–4580 cm⁻¹ (right). Minor artifacts (marked by asterisks [*]) are due to subtracting the chloroform signal.

Fig. 7 Raman/luminescence (I_R + I_L) and ROA/CPL (I_R − I_L) spectra of Na⁺[Eu((±)-hfbc)₄]⁻ enantiomer complexes (1 mM) in acetonitrile in the range of 80–2630 cm⁻¹ (left) and 2280–4580 cm⁻¹ (right). Minor artifacts (marked by asterisks [*]) are due to subtracting the acetonitrile signal.

Remarkably, a weak but broad band at ∼269 cm⁻¹ was detected in the acetonitrile solution of Na⁺[Eu((±)-hfbc)₄]⁻ (Fig. 7 left) and is attributed to the ECD of the Eu(III) ⁵D₁ ← ⁷F₁ (∼540 nm) transition.⁶ A similar pattern was also discernible in the acetonitrile solution of Na⁺[Eu((±)-hfbc)₄]⁻ (Fig. 5 left), though somewhat obscured by adjacent solvent noise. Measurement of the degree of circularity^15,17,28 confirmed that these signals do not originate from Raman scattering. Such a feature is extremely difficult to detect in conventional ECD experiments (Fig. S6),^29,30 underscoring the sensitivity of the ROA technique.

Conclusions

In this work, we uncovered a distinct stabilization mechanism in which the imidazolium cation forms strong N–H⋯O=C hydrogen bonds with the diketonate carbonyl groups, yielding a stable chiral tetrakis Eu(III) complex, ImH⁺[Eu(hfbc)₄]⁻. This architecture represents a different structural motif compared to earlier ammonium-stabilized systems, where steric hindrance prevented hydrogen bonding. Single-crystal X-ray diffraction unambiguously confirms the unique framework.

The ImH⁺[Eu((−)-hfbc)₄]⁻ complex exhibited a large CPL dissymmetry factor of −1.26 at 595 nm, among the highest reported for hydrogen-bond-stabilized Eu(III) complexes and comparable to the benchmark Cs⁺ system.^31,32 The emergence of such high CPL activity in a solution-stable ion-pair underscores the crucial role of the imidazolium counterion in modulating the excited-state chiroptical response.

We also find the use of a ROA spectrometer for CPL detection important for the methodology as a toolkit for studying lanthanide optical activity. High-intensity laser excitation enabled the detection of weak Eu(III) bands and allowed separation of CPL from vibrational ROA signals in chloroform solutions. The observation of the Eu(III) ECD transition (⁵D₁ ← ⁷F₁) in Na⁺[Eu(hfbc)₄]⁻ further illustrated the sensitivity of ROA to subtle structural and electronic variations.

Overall, the results show that the hydrogen-bond-assisted stabilization is a powerful approach for tuning CPL-active lanthanide complexes and demonstrate the unique potential of ROA-based CPL spectroscopy. Future studies will focus on elucidating the interplay between structure, counterion effects, and solvent-induced Raman interference in lanthanide chiroptical systems.

Author contributions

Experiments: QK, BK, and TW; computation: TW; experimental data analysis: QK, BK, and TW; calculation analysis: PB and TW; writing of the manuscript: QK and TW; review and editing: all authors; funding acquisition: QK and TW.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: crystallographic data, ¹H NMR spectrum of Yb complex, emission spectra with the corresponding emission decay profiles and ECD spectra of the complexes. See DOI: https://doi.org/10.1039/d5dt02294k.

CCDC 2450564 and 2450565 contain the supplementary crystallographic data for this paper.^33a,b

Acknowledgements

This work was supported by the Czech Science Foundation (23-05378S) and the National Natural Science Foundation of China (22165014).

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