# Regioisomerism Induced Circularly Polarized Luminescence Switching in Natural (+)-Camphor-Based Bis(pyrazolyl)pyridine Europium(III) Complexes

Justin Cado, Alexandre Abhervé, Maxime Grasser, Nicolas Vanthuyne, Francesco Zinna, Lorenzo Di Bari, Boris Le Guennic, N. Avarvari

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To cite this version:

Justin Cado, Alexandre Abhervé, Maxime Grasser, Nicolas Vanthuyne, Francesco Zinna, et al. "Regioisomerism Induced Circularly Polarized Luminescence Switching in Natural (+)-Camphor-Based Bis(pyrazolyl)pyridine Europium(III) Complexes." Inorganic Chemistry, 2025, 64 (15), pp.12781-12791. 10.1021/acs.inorgchem.5c01706. hal-05156111

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Distributed under a Creative Commons Attribution 4.0 International License
Regioisomerism Induced Circularly Polarized Luminescence
Switching in Natural (+)-Camphor Based
Bis(pyrazolyl)pyridine Europium(III) Complexes

Justin Cado,ª Alexandre Abhervé,*,ª Maxime Grasser,#,b Nicolas Vanthuyne, c Francesco Zinna, d
Lorenzo Di Bari, d Boris Le Guennic,*,b and Narcis Avarvari* ,a

<footnote>ª Univ Angers, CNRS, MOLTECH-Anjou, SFR MATRIX, F-49000 Angers, France. E-mail: [alexandre.abherve@univ-angers.fr](mailto:alexandre.abherve@univ-angers.fr); [narcis.avarvari@univ-angers.fr](mailto:narcis.avarvari@univ-angers.fr)</footnote>

<footnote>b Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, F-35000 Rennes, France. E-mail: [boris.leguennic@univ-rennes.fr](mailto:boris.leguennic@univ-rennes.fr)</footnote>

<footnote>c Aix Marseille Univ, CNRS, Centrale Med, UAR 1739, FSCM, Chiropole, Marseille, France</footnote>

<footnote>d Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via G. Moruzzi 13, 56124, Pisa, Italy</footnote>

<footnote>\# Present address: Department of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.</footnote>
Table of Contents Entry

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Europium(III) complexes bearing the chiral tridentate bis.camphor)-2,2-bpp and -1,2-bpp (bpp = 2,6-bis(pyrazolyl)-pyridine) ligands show strong CPL activity, with an inversion of the polarization depending on the bpp regioisomer.
**ABSTRACT:** Chiral 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligands containing camphor units have been used in their enantiopure, racemic and meso forms to prepare luminescent europium(III) complexes. Thefone ligands have been fully characterized by single crystal X-ray diffraction and chiroptical spectroscopy. For one of the enantiopure ligands, namely bis(4S,7R)camphor-2,2-bpp, the regioisomer bis(4S,7R)camphor-1,2-bpp, containing one pyrazole connected through the N1 nitrogen atom, has been obtained and used to coordinate the europium(III) ion. Ancillary hexafluoroacetylacetonato ligands were used to complete the coordination sphere of the lanthanide. The complexes with the 2,2-bpp ligands have been structurally characterized by single crystal X-ray diffraction. They are isostructural in spite of the non-centrosymmetric monoclinic P2₁ space group for the enantiopure compounds and the centrosymmetric monoclinic P2₁/n space group for the meso and the racemic ones. The metal center is ennea-coordinated within slightly distorted Muffin type geometry. The three enantiopure complexes show strong circularly polarized luminescence activity, with anisotropy factors glum reaching up ±0.2 for the 7F1 ← 5D0 magnetic dipole sensitive transition. Most remarkably, the polarization of the emission is inverted for the two complexes bearing the regioisomeric bis(4S,7R)camphor-2,2-bpp and bis(4S,7R)camphor-1,2-bpp ligands, yet having the same absolute configuration of the camphor units, thus allowing an unprecedented possibility of CPL switching.
INTRODUCTION

The straightforward access to chiral functional materials remains a significant challenge in the field of coordination chemistry, driven by the intriguing properties associated with chirality, such as circular dichroism (CD)¹ and circularly polarized luminescence (CPL).² CPL-active materials are attracting increasing interest due to their broad applicability, including their use in CP-OLEDs,³,⁴,⁵ 3D displays,⁶ or advanced security inks for anti-counterfeiting.⁷ CPL refers to the differential spontaneous emission between left- and right-handed circularly polarized light, and is typically quantified by the dissymmetry factor, g<sub>lum</sub> = 2(I<sub>L</sub> - I<sub>R</sub>)/(I<sub>L</sub> + I<sub>R</sub>), where I<sub>L</sub> and I<sub>R</sub> represent the left and right circularly polarized emission components, respectively. To date, the most efficient molecular CPL emitters in solution, exhibiting the highest g<sub>lum</sub> values, are chiral lanthanide complexes.⁸,⁹,¹⁰,¹¹ These complexes usually display |g<sub>lum</sub>| values ranging from 0.1 to 1, peaking to 1.54 for the anionic europium(III) complex TEA⁺[Eu(+tfc)₄]⁻ (TEA = tetraethylammonium, +tfc = (+)-3-trifluoroacetylcamphorate),¹² for the ⁵D₀→⁷F₁ magnetically allowed transition. This value significantly surpasses the g<sub>lum</sub> value of -0.78 observed for the widely studied neutral counterpart [Eu(+tfc)₃], for the same transition.¹³ In comparison, organic and organometallic chiral molecules generally display much lower g<sub>lum</sub> values, typically in the range of 10<sup>-4</sup>-10<sup>-3</sup>,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸ reaching up to 10<sup>-2</sup> for some d-metal complexes.¹⁹,²⁰,²¹ Additionally, the emission of lanthanide complexes consists of narrow optical bands at energies that are largely independent of their coordination environment. Their emission is promoted by the antenna effect, in which energy is transferred from an excited ligand state to the emissive state of the lanthanide.⁹ This mechanism is critical, as direct excitation of lanthanide ions is inefficient because of the inherently weak extinction coefficients of f-f transitions. Therefore, the choice of the ligands is crucial for creating a chiral coordination environment and ensuring efficient excitation of the lanthanide. This, in turn, is essential for achieving high quantum yields of the emission and maximizing CPL brightness, which is expressed as B<sub>CPL</sub> = ε × Φ × |g<sub>lum</sub>|/2.²² Examples of chiral ligand families reported in the literature for CPL-active lanthanide complexes,²³ particularly europium(III) complexes, include bipyridine-based cryptates,²⁴ cyclens,²⁵,²⁶ cholate,²⁷ binole-biphosphate,²⁸ pyridine-bis(carboxyamides),²⁹,³⁰,³¹ camphor acetyl-acetonates,³²,³³,³⁴,³⁵ pyridine oxazolines,³⁶,³⁷,³⁸,³⁹,⁴⁰ pyridine-bis(triazoles),⁴¹ diamino-hydroxyl-isophthalamides⁴² and diamino-polypyridines.⁴³,⁴⁴ We recently introduced helicene-appended 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligands, with the helicene substituent in the *para* position of the pyridine ring,⁴⁵ along with several d-metal complexes. Notably, the enantiopure [6]helicene-bpp ligands provided chiral europium(III) complexes, [Eu(tta)₃(bpp-hel)] (tta = 2-thienoyltrifluoroacetonate), which showed CPL activity despite the helicene units being positioned remotely from the lanthanide coordination sphere. These complexes displayed a relatively low g<sub>lum</sub> value of ±3·10<sup>-3</sup>.⁴⁶ It is worth noting that helicene-bpp ligands completed the family of the very few previously reported chiralfone ligands,⁴⁷,⁴⁸ which had been almost exclusively used for catalytic applications.⁴⁹ To induce a stronger chiral distortion in the lanthanide coordination environment compared to our previous helicene-bpp europium complexes,⁴⁶ we have now focused on chiral bis.camphor)-bpp ligands, where camphor units are fused to the pyrazole rings (Chart 1). The synthesis of 2,6-bis[(4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-2-indazolyl]pyridine (bis(4S,7R)camphor-2,2-bpp) has been previously reported,⁵⁰ along with its coordination chemistry involving Ru, Cu or Rh, primarily in the context of chiral catalysts for enantioselective epoxidation.⁵¹,⁵² Additionally, a related naphthyridine-bis.camphorpyrazole has been investigated as a ligand for dinuclear and tetranuclear Cu(I) and Ag(I) complexes.⁵³ Furthermore, the (4S,7R)camphor-pyrazole unit has been employed as a ligand for complexation with CuCl₂⁵⁴ or as a building block to form C₃-symmetric tris(pyrazolyl)hydroborate and tris(pyrazolyl)phosphine ligands.⁵⁵ However, to date, no camphor-substitutedfone ligand has been investigated for light-emitting applications.
In this study, we report the synthesis, solid-state structures and chiroptical characterization of Eu(III) complexes based on bis.camphor)-2,2-bpp and the unprecedented bis.camphor)-1,2-bpp. Both ligands were derived from the same precursor due to tautomerism in the camphor-pyrazole precursor. Remarkably, these complexes exhibit strong CPL activity, with unexpected inversion of CPL sign between the two regioisomers. State-of-the-art calculations provide deep insights into the chiroptical properties of these Eu(III) complexes.

<figure><img src="image_4.png" /><figcaption>bis(4R,7S)camphor-2,2-bpp</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>bis(4S,7R)camphor-2,2-bpp</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(4S,7R,4'R,7'S)biscamphor-2,2-bpp</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>bis(4S,7R)camphor-1,2-bpp</figcaption></figure>

**Chart 1.** Ligands used in this work.

## EXPERIMENTAL SECTION

**Materials and Methods.** (1R)-(+)-Camphor (98%, Merck), (±)-camphor (>95%, Merck), tert-butoxy bis(dimethylamino)methane (Combi-Blocks, 90%), hydrazine monohydrate (Merck, ≥97%), 2,6-dibromopyridine (Alfa Aesar, 98%), sodium hydride (Merck, 60% dispersion in mineral oil), 1,1,1,5,5,5-hexafluoroacetylacetone (97%, Combi-Blocks) and Eu(OAc)₃,xH₂O (99.9%, Alfa Aesar) were purchased and used without further purification. [Eu(hfac)₃(H₂O)₂] was prepared from Eu(OAc)₃,xH₂O and 1,1,1,5,5,5-hexafluoroacetylacetone according to literature reports.⁵⁶ The synthesis of the bis.camphor)-bpp ligands was adapted from reported procedures.⁵⁸,⁵⁹,⁵⁰ ¹H NMR spectra were recorded with a Bruker AVANCE III 300 (300 MHz). Chemical shifts are given in ppm relative to tetramethylsilane TMS and coupling constants *J* in Hz. Residual non-deuterated solvent was used as an internal standard. Mass spectrometry analyses were performed on a MALDI-TOF JEOL JMS3000 with DCTB as matrix in positive mode. Elemental analyses were performed using a Flash 2000 Fisher Scientific Thermo Electron analyzer. Powder X-ray diffraction patterns were recorded at 20°C using a Rigaku SmartLab (Cu Kα, λ = 1.5406 Å) mounted in the Bragg-Brentano geometry (in θ-θ measurement).

**Synthesis of bis(4S,7R)camphor-2,2-bpp and bis(4S,7R)camphor-1,2-bpp ligands.**

**Synthesis of (1R,4S)-3-[(E)-(dimethylamino)methylidene]-1,7,7-trimethylbicyclo[2.2.1]heptan-2-one (1-R).** (1R)-(+)-Camphor (20 mmol, 3.04 g) was dissolved in dimethylformamide (DMF, 30 mL) and tert-butoxy bis(dimethylamino)methane was added (29 mmol, 6.7 mL) and the mixture was stirred under reflux overnight. After evaporation of the solvent, the residue was dissolved in CH₂Cl₂ (50 mL) and washed two times with water (2x40 mL). The organic phase was dried and evaporated to give an orange oil. Dissolving the oil in petroleum ether (30 mL) and keeping the solution at -20 °C afforded a yellow precipitate that could be filtered and dried under vacuum (2.6 g, 64% yield). ¹H NMR (CD₃CN):
δ 7.07 (s, 1H), 3.05 (s, 6H), 2.05 (ddt, J = 11.5, 10.4, 4.2 Hz, 1H), 1.65 (ddd, J = 12.8, 10.5, 3.5 Hz, 1H), 1.41 (ddd, J = 11.8, 9.1, 3.4 Hz, 1H), 1.25 (ddd, J = 13.4, 9.2, 4.5 Hz, 1H), 0.91 (d, J = 4.4 Hz, 1H), 0.84 (s, 3H).

**Synthesis of (4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-2H-4,7-methanoindazole ((4S,7R)-camphor-pyrazole).** A mixture of **1-R** (3.88 mmol, 804 mg), hydrazine monohydrate (4.25 mmol, 0.207 mL) and hydrochloric acid (4.25 mmol, 0.35 mL) was dissolved in methanol (50 mL) and stirred at room temperature for 24 h. The solvent was removed under vacuum to give **(4S,7R)-camphor-pyrazole** as a yellow powder (640 mg, yield 94%). ¹H NMR (CD₃CN): δ 7.14 (s, 1H), 2.77 (d, J = 4.0 Hz, 1H), 2.11 (ddt, J = 11.5, 10.6, 3.8 Hz, 1H), 1.87 (ddd, J = 11.2, 9.9, 3.2 Hz, 1H), 1.27 (s, 3H), 1.22 – 1.10 (m, 2H), 0.97 (s, 3H), 0.64 (s, 3H).

**Synthesis of 2,6-bis[(4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-2-indazolyl]pyridine (bis(4S,7R)camphor-2,2-bpp) and (4S,7R)-7,8,8-trimethyl-2-(6-((4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-1H-4,7-methanoindazol-1-yl)pyridin-2-yl)-4,5,6,7-tetrahydro-2H-4,7-methanoindazole (bis(4S,7R)camphor-1,2-bpp).** Under Argon atmosphere, (4S,7R)-camphor-pyrazole (3.63 mmol, 640 mg) was dissolved in diethylene glycol dimethyl ether (diglyme, 20 mL) and sodium hydride (4.00 mmol, 160 mg) was added dropwise. After 2 h, 2,6-dibromopyridine (1.21 mmol, 287 mg) was added and the mixture was stirred at 130 °C for 5 days. The resulting solution was concentrated under vacuum and water was added. The obtained crude precipitate was recovered and purified by column chromatography on silica (CH₂Cl₂). The bis(4S,7R)camphor-2,2-bpp (481 mg) and bis(4S,7R)camphor-1,2-bpp (23 mg) were isolated in the first and second fraction, respectively (overall yield 98%). ¹H NMR of bis(4S,7R)camphor-2,2-bpp (CD₃CN): δ 8.23 (s, 2H), 7.87 (dd, J = 8.3, 7.8 Hz, 1H), 7.53 (d, J = 8.0 Hz, 2H), 2.88 (d, J = 3.9 Hz, 2H), 2.19 (ddt, J = 11.6, 7.8, 3.8 Hz, 2H), 2.01 – 1.93 (m, 2H), 1.41 – 1.27 (m, 10H), 1.03 (s, 6H), 0.73 (s, 6H). ¹H NMR of bis(4S,7R)camphor-1,2-bpp (CD₃CN): 8.07 (s, 1H), 8.00 (t, J = 8.0 Hz, 1H), 7.77 (dd, J = 8.1, 0.8 Hz, 1H), 7.46 (dd, J = 7.8, 0.8 Hz, 1H), 7.40 (s, 1H), 2.88 (d, J = 4.0 Hz, 1H), 2.86 (d, J = 3.7 Hz, 1H), 2.24 – 2.14 (m, 3H), 1.97 (ddd, J = 10.0, 5.9, 3.5 Hz, 2H), 1.61 – 1.52 (m, 1H), 1.35 (s, 3H), 1.34 (s, 3H), 1.30 – 1.14 (m, 5H), 1.04 (s, 3H), 0.98 (s, 3H), 0.78 (s, 3H), 0.74 (s, 3H).

**Synthesis of racemic bis.camphor)-2,2-bpp.**

**Synthesis of (rac)-3-[(E)-(dimethylamino)methylidene]-1,7,7-trimethylbicyclo[2.2.1]heptan-2-one (1-rac).** (±)-Camphor (30 mmol, 4.567 g) was dissolved in dimethylformamide (DMF, 50 mL) and tert-butoxy bis(dimethylamino)methane was added (44 mmol, 10 mL) and the mixture was stirred under reflux overnight. After evaporation of the solvent, the residue was dissolved in CH₂Cl₂ (70 mL) and washed two times with water (2x60 mL). The organic phase was dried and evaporated to give an orange oil. Dissolving the oil in petroleum ether (40 mL) and keeping the solution at -20 °C afforded a yellow precipitate that could be filtered and dried under vacuum (3.4 g, 55% yield). ¹H NMR (CD₃CN): δ 7.07 (s, 1H), 3.05 (s, 6H), 2.05 (ddt, J = 11.5, 10.4, 4.2 Hz, 1H), 1.65 (ddd, J = 12.8, 10.5, 3.5 Hz, 1H), 1.41 (ddd, J = 11.8, 9.1, 3.4 Hz, 1H), 1.25 (ddd, J = 13.4, 9.2, 4.5 Hz, 1H), 0.91 (d, J = 4.4 Hz, 1H), 0.84 (s, 3H).

**Synthesis of (rac)-7,8,8-trimethyl-4,5,6,7-tetrahydro-2H-4,7-methanoindazole (rac-camphor-pyrazole).** A mixture of **1-rac** (14.4 mmol, 3 g), hydrazine monohydrate (14.4 mmol, 0.7 mL) and hydrochloric acid (14.4 mmol, 1.2 mL) was dissolved in methanol (80 mL) and stirred at room temperature for 24 h. The solvent was removed under vacuum to give **2-rac** as a yellow powder (2.03 g, yield 80%). ¹H NMR (CD₃CN): δ 7.14 (s, 1H), 2.78 (d, J = 4.0 Hz, 1H), 2.17 – 2.06 (m, 1H), 1.88 (ddd, J = 11.1, 9.9, 3.2 Hz, 1H), 1.27 (s, 3H), 1.17 (ddd, J = 18.8, 11.7, 3.4 Hz, 2H), 0.97 (s, 3H), 0.64 (s, 3H).

**Synthesis of 2,6-bis[(rac)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-2-indazolyl]pyridine (rac-biscamphor-2,2-bpp).** Under Argon atmosphere, rac-camphor-pyrazole (5.45 mmol, 960 mg) was
dissolved in diethylene glycol dimethyl ether (diglyme, 40 mL) and sodium hydride (5.99 mmol, 240 mg) was added dropwise. After 2 h, 2,6-dibromopyridine (2.73 mmol, 645 mg) was added and the mixture was stirred at 130 °C for 5 days. The resulting solution was concentrated under vacuum and water was added. The obtained crude precipitate was recovered and purified by column chromatography on silica (CH₂Cl₂). The *rac*-biscamphor-2,2-bpp was isolated as a white powder (995 mg, 85%). ¹H NMR (CD₃CN): δ 8.23 (s, 2H), 7.86 (dd, *J* = 8.4, 7.7 Hz, 1H), 7.52 (d, *J* = 8.0 Hz, 2H), 2.87 (d, *J* = 3.9 Hz, 2H), 2.23 – 2.12 (m, 2H), 1.97 (ddd, *J* = 11.6, 10.2, 3.4 Hz, 2H), 1.43 – 1.23 (m, 11H), 1.02 (s, 6H), 0.73 (s, 6H).

**Synthesis of Eu(III) complexes.** Under Argon atmosphere, [Eu(hfac)₃(H₂O)₂] (41 mg, 0.05 mmol) and the bis.camphor)-bpp ligand (21 mg, 0.05 mmol) were dissolved in 1,2-dichloroethane (5 mL) and stirred at 70 °C overnight. The solution was filtered and left to slowly evaporate. Colourless prismatic crystals were obtained after 2 days for the complexes Eu(4R,7S-2,2), Eu(4S,7R-2,2), Eu(meso-2,2) and Eu(rac-2,2). MALDI: 992.1 ([Eu(bis.camphor)-bpp)(hfac)₂]⁺). Exact mass calcd for C₃₇H₃₅N₅O₄F₁₂Eu ([Eu(bis.camphor)-bpp)(hfac)₂]⁺): 992.16904; found for Eu(4S,7R-1,2): 992.1695. Elemental analysis calcd for C₄₂H₃₆N₅O₆F₁₈Eu (%): C, 42.01, H, 3.02, N, 5.83; found for Eu(4R,7S-2,2): C, 42.16, H, 3.19, N, 5.69; for Eu(4S,7R-2,2): C, 42.29, H, 3.22, N, 5.96.

**X-ray Crystallography.** Single crystals of the compounds were mounted on glass fibre loops using a viscous hydrocarbon oil to coat the crystal and then transferred directly to the cold nitrogen stream. Data collection were performed at 180 or 230 K on an Agilent Supernova with Cu-Kα (λ = 1.54184 Å). The structures were solved by dual-space algorithm and refined against all *F*² values with the SHELX package⁵⁷ using the WinGX graphical user interface. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions and refined isotropically with a riding model. A summary of the crystallographic data and the structure refinement is given in Tables S2-S3. CCDC 2430296 (bis(4R,7S)camphor-2,2-bpp), CCDC 2430297 (bis(4S,7R)camphor-2,2-bpp), CCDC 2430298 ((4S,7R,4'R,7'S)-bis.camphor-2,2-bpp), CCDC 2430299 ([Eu(hfac)₃(bis(4R,7S)camphor-2,2-bpp))), CCDC 2430300 ([Eu(hfac)₃(bis(4S,7R)camphor-2,2-bpp))), and CCDC 2430301 ([Eu(hfac)₃(rac-bis.camphor-2,2-bpp)]) contain the supplementary crystallographic data for the paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

**(Chir)optical measurements.** UV-Vis absorption and CD spectra of 10⁻⁵ M acetonitrile solutions of both ligands and complexes were recorded in transmission mode on a JASCO J-1500 spectrometer at 20 °C. Excitation and emission spectra were recorded using a RF-6000 spectrometer in 10⁻⁶ M acetonitrile solutions and in the solid state (on polycrystalline powders). Absolute quantum yields were determined by using an integrating sphere on both solution and solid-state samples. CPL of 10⁻³ M dichloromethane solutions of the complexes were recorded on a JASCO CPL-300 spectrophotometer at 20 °C. Measurements were performed under excitation wavelength λ = 350 nm, using excitation and emission bandwidths of 5 and 1 nm, respectively, data integration time of 4 s and scanning speed of 20 nm/min. Each spectrum is the accumulation of ten scans.

# RESULTS AND DISCUSSION

## Synthesis, crystal structures and chiroptical properties of bis.camphor)-bpp

First, the enantiopure (4S,7R)camphor-pyrazole unit was synthesized according to literature reports, in two steps by reacting (1R)-(+)-camphor with bis(dimethylamino)-tertbutoxymethane (Bredereck's reagent)⁵⁸ followed by cyclocondensation of the resulting camphor enaminone with hydrazine monohydrate (Scheme 1).⁵⁹ Next, the synthesis of the bis.camphor)-bpp was adapted from the method reported by Steel et al.⁵⁰ As observed in several tetrahydroindazole derivatives, (4S,7R)-camphor-
pyrazole exists in two tautomeric forms, namely the **(4S,7R)-camphor-1H-pyrazole** and **(4S,7R)-camphor-2H-pyrazole**, with the latter being more stable in solution.⁵⁴ Consequently, double nucleophilic substitution on 2,6-dibromopyridine at 130 °C in diglyme over 5 days led to the formation of two isomeric compounds, e.g. the symmetrical **bis(4S,7R)camphor-2,2-bpp** as the major product and the unsymmetrical **bis(4S,7R)camphor-1,2-bpp** as the minor product. Note that the latter was not reported so far in the literature. The two isomers were successfully separated by column chromatography on silica.

<figure><img src="image_8.png" /><figcaption>**Scheme 1.** Synthesis of **bis(4S,7R)camphor-2,2-bpp** (major product) and **bis(4S,7R)camphor-1,2-bpp** (minor product).</figcaption></figure>

The racemic form of bis.camphor)- was prepared for the first time starting from (±)-camphor and following the same synthetic procedure (Scheme 2). Surprisingly, in this case, only formation of the symmetrical bis.camphor)-2,2-bpp could be observed, while the unsymmetrical bis.camphor)-1,2-bpp derivative could not be isolated. Nevertheless, the racemic mixture was subjected to semi-preparative chiral HPLC using a Chiralpak AZ-H column (250 x 10 mm), which successfully allowed us to separate the (4R,7S,4'R,7'S) and (4S,7R,4'S,7'R) enantiomers, as well as the meso form (4S,7R,4'R,7'S) of bis.camphor)-2,2-bpp, with a distribution ratio of 23:22:55, respectively (see experimental details in the ESI).

<figure><img src="image_9.png" /><figcaption>**Scheme 2.** Synthesis and semi-preparative chiral HPLC of rac-bis.camphor)-2,2-bpp.</figcaption></figure>

Optical rotation measurements for the two pure enantiomers revealed that **bis(4R,7S)camphor-2,2-bpp** was levorotatory, while **bis(4S,7R)camphor-2,2-bpp** was dextrorotatory (Table S1). Each fraction
was then recrystallized in acetonitrile, yielding crystals suitable for single-crystal X-ray diffraction. Structure analysis confirmed that all three bis.camphor-2,2-bpp derivatives crystallized in the orthorhombic *P*2<sub>1</sub>*2*<sub>1</sub>*2*<sub>1</sub> space group, further confirming their absolute configurations (Figure 1c, Table S2 and Figure S9). Circular dichroism (CD) and UV-Vis absorption spectra of the three enantiomerically pure ligands were recorded on acetonitrile solutions (Figure 1a-b). The bis.camphor-2,2-bpp ligands exhibited strong absorbance below 350 nm, with the two enantiomers displaying mirror-image CD spectra. Distinct peaks were observed at around 260, 285 and 325 nm. Notably, in the CD spectrum of **bis(4S,7R)camphor-1,2-bpp**, the peak at 325 nm was nearly absent, while the peaks at 260 and 285 nm showed inverted sign compared to those of **bis(4S,7R)camphor-2,2-bpp**. This represents a very strong modulation of the chiroptical properties of the bpp ligands only by changing the pyrazole nitrogen atom connected to the pyridine ring. Finally, the bis(4S,7R)camphor-2,2-bpp displays a blue luminescence with an emission band centered at 355 nm and a quantum yield of 66% (Figure 1d).

<figure><img src="image_10.png" /><figcaption>**Figure 1.** (a) CD and (b) UV-Vis absorption spectra of **bis(4R,7S)camphor-2,2-bpp** (red), **bis(4S,7R)camphor-2,2-bpp** (blue) and **bis(4S,7R)camphor-1,2-bpp** (green curves) recorded at 21°C in 10⁻⁵ M acetonitrile solutions. (c) Crystal structure of **bis(4S,7R)camphor-1,2-bpp**. (d) Emission and excitation spectra of **bis(4S,7R)camphor-2,2-bpp** recorded at 21 °C in 10⁻⁶ M acetonitrile solutions.</figcaption></figure>

## Synthesis, crystal structures and chiroptical properties of Eu complexes

Europium(III) complexes were prepared by refluxing a mixture of Eu(hfac)₃(H₂O)₂ (hfac = hexafluoro-acetylacetonato) and the bis.camphor)-cpp ligand overnight in 1,2-dichloroethane (Scheme S1 and details of experimental procedure in the ESI). For complexes based on bis.camphor-2,2-bpp ligands, namely Eu(4R,7S-2,2), Eu(4S,7R-2,2), Eu(meso-2,2) and Eu(rac-2,2), red-luminescent colorless
prismatic crystals could be obtained by slow evaporation of acetone solutions (Figure S10, ESI), while any attempt to crystallize [Eu(hfac)₃(4S,7R-bis.camphor)-1,2-bpp]], hereafter named **Eu(4S,7R-1,2)**, remained unsuccessful. All complexes crystallized in the monoclinic system, within non-centrosymmetric *P*2₁ space group for **Eu(4R,7S-2,2)** and **Eu(4S,7R-2,2)**, and centrosymmetric *P*2₁/*n* space group for **Eu(meso-2,2)** and **Eu(rac-2,2)** (Table S3). The europium coordination sphere consists of three N atoms from the 2,2-bpp ligand and six O atoms from the hfac anions (Figure 2). In the two pure enantiomers, the asymmetric unit consists of two independent complexes, where the metal centers show mirror-image coordination environments (Figure S11). Continuous SHAPE measurements (CShM)⁶⁰,⁶¹,⁶² indicate a highly similar coordination environment across all complexes, with small deviations from the ideal Muffin geometry (Cs local symmetry), showing dispersion values between 4.731 and 4.835 (Table S4). Therefore, the non-centrosymmetric nature of the two pure enantiomers in the solid state arises solely from the chirality of the camphor-based ligand. In the compounds based on meso and racemic ligands, only one independent complex is observed, while its mirror image is obtained through a glide reflection symmetry operation. In the racemic compound, a strong disorder at the camphor substituents can be observed due to the 50:50 mixture of both (4S,7R) and (4R,7S) configurations. Finally, all compounds present a similar crystal packing arrangement (Figure S12).

<figure><img src="image_11.png" /><figcaption>**Figure 2.** Crystal structures of **Eu(4R,7S-2,2)** (top left), **Eu(4S,7R-2,2)** (top right), **Eu(meso-2,2)** (bottom left) and **Eu(rac-2,2)** (bottom right).</figcaption></figure>

Powder X-ray diffraction analysis confirmed the purity of the polycrystalline material (Figure S13). Electronic circular dichroism (ECD) and UV-Vis absorption spectra of the three enantiomerically pure complexes were recorded in acetonitrile solution (Figure 3a-b). Similar to the free ligands, both **Eu(4R,7S-2,2)** and **Eu(4S,7R-2,2)** strongly absorb in the 240-360 nm wavelength region, with three absorbance maxima of 36000-38000 L.mol⁻¹.cm⁻¹ at around 262, 289 and 319 nm. They show mirror-image ECD spectra with peaks located at around 260, 286, 305 and 335 nm. In contrast, **Eu(4S,7R-1,2)**
displays a slightly different absorption profile, featuring a single maximum absorbance of 43000 L·mol⁻¹·cm⁻¹ at 308 nm and an absorbance edge at around 350 nm. Its ECD spectrum contains four peaks at wavelengths similar to those of (4S,7R,4′S,7′R)-bis.camphor)-2,2-bpp, though the peaks at 260, 305 and 335 nm have opposite signs. Upon excitation at 325 nm, Eu(4S,7R-2,2) shows the characteristic ⁷Fₙ ← ⁵D₀ emission bands of Eu(III) ion, though with lower relative intensity compared to the precursor complex [Eu(hfac)₃(H₂O)₂] (Figure S14). In addition, residual emission from the bis.camphor)-2,2-bpp ligand was observed. However, excitation at a lower energy (λexc = 365 nm) suppressed the ligand emission, enhancing the relative intensity of the red Eu(III) emission. These results were confirmed by quantum yield measurements on both solution and solid-state samples (Figure S15) revealing an overall quantum yield of 7% for the heteroleptic complexes, twice that of the precursor complex [Eu(hfac)₃(H₂O)₂] (φ = 3.5%). Notably, when [Eu(hfac)₃(H₂O)₂] was excited at 365 nm, no luminescence was observed, highlighting that in the chiral complexes, only the camphor-based bpp ligand functioned as a chromophore at this wavelength. These results highlight the strong ability of the bis.camphor)-2,2-bpp ligand to act as antenna in sensitizing Eu(III) emission. As expected, similar photoluminescence properties were observed for Eu(4S,7R-1,2), though with a slightly different excitation spectrum compared to Eu(4S,7R-2,2) (Figure S16), suggesting a distinct coordination environment around the Eu(III) ion.

To further investigate the chiroptical properties of this series of complexes, CPL measurements were performed on the three enantiopure complexes (Figure 3c-d and Figure S17). As anticipated, Eu(4R,7S-2,2) and Eu(4S,7R-2,2) exhibit mirror-image CPL spectra, with two multiplets corresponding to the ⁷F₁ ← ⁵D₀ (595 nm) and ⁷F₂ ← ⁵D₀ (616 nm) transitions, both dominated by a sharp peak. Additionally, weaker double peaks were observed for the ⁷F₃ ← ⁵D₀ (652-656 nm) and ⁷F₄ ← ⁵D₀ (690-703 nm) transitions. Remarkably, Eu(4S,7R-1,2) displays an inverted CPL spectrum compared to Eu(4S,7R-2,2). Specifically, the magnetic-dipole allowed ⁷F₁ ← ⁵D₀ transition exhibits a *g*<sub>lum</sub> value of -0.2 for Eu(4S,7R-2,2) and +0.2 for Eu(4S,7R-1,2), while the electric-dipole allowed ⁷F₂ ← ⁵D₀ transition shows a *g*<sub>lum</sub> of +0.02 and -0.02, respectively. According to the CPL brightness (*B*<sub>CPL</sub>) definition proposed by some of the authors (*B*<sub>CPL</sub> = *θ*ᵢ · ε<sub>λ</sub> · *φ* · |*g*<sub>lum</sub>|/2, with *θ*ᵢ being the branching ratio corresponding to the *i*-band),²² the corresponding values were calculated as *B*<sub>CPL</sub> = 13.5 L·mol⁻¹·cm⁻¹ for the magnetic dipole ⁷F₁ ← ⁵D₀ transition and *B*<sub>CPL</sub> = 22.7 L·mol⁻¹·cm⁻¹ for the electric dipole ⁷F₂ ← ⁵D₀ transition, in both cases upon irradiation - and consequently with ε<sub>λ</sub> - at 365 nm. These values place these complexes among the brightest red CPL emitters reported to date,²² although record values of ≈ 200 L·mol⁻¹·cm⁻¹ were also recently reported.¹² A summary of the chiroptical properties for both ligands and Eu(III) complexes is provided in Table 1.
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 3.** Chiroptical properties of Eu(4R,7S-2,2) (red), Eu(4S,7R-2,2) (blue) and Eu(4S,7R-1,2) (green) curves). (a) ECD and (b) UV-Vis absorption spectra. (c) CPL and (d) normalized emission spectra. Excitation wavelength: λ = 365 nm.</figcaption></figure>

<table><caption>**Table 1.** Chiroptical properties of both ligands and complexes described in this work.</caption><thead><tr><th>Compound</th><th>ε<sub>max</sub> in L·mol<sup>-1</sup>·cm<sup>-1</sup><br>(λ<sub>max</sub> in nm)</th><th>Δε in L·mol<sup>-1</sup>·cm<sup>-1</sup><br>(λ in nm)</th><th>g<sub>lum</sub> (λ in nm)</th></tr></thead><tbody><tr><td>bis(4R,7S)camphor-2,2-bpp</td><td>30845 (323)</td><td>+5.4 (285)<br>-3.7 (325)</td><td>-</td></tr><tr><td>bis(4S,7R)camphor-2,2-bpp</td><td>30510 (323)</td><td>-5.2 (285)<br>+5.0 (325)</td><td>-</td></tr><tr><td>bis(4S,7R)camphor-1,2-bpp</td><td>19810 (310)</td><td>+4.5 (285)<br>+2.9 (310)</td><td>-</td></tr><tr><td>Eu(4R,7S-2,2)</td><td>38610 (320)</td><td>+4.5 (305)<br>-5.0 (330)</td><td>+0.20 (595)<br>-0.02 (615)</td></tr><tr><td>Eu(4S,7R-2,2)</td><td>36190 (320)</td><td>-4.3 (305)<br>+2.7 (330)</td><td>-0.19 (595)<br>+0.02 (615)</td></tr><tr><td>Eu(4S,7R-1,2)</td><td>42975 (308)</td><td>+5.2 (305)<br>-0.7 (330)</td><td>+0.20 (594)<br>-0.02 (616)</td></tr></tbody></table>

In order to shed light on the nature of the transitions involved in the polarized emission, the CPL spectra of Eu(4S,7R-2,2) and Eu(4R,7S-2,2) were simulated at the SA-CAS(6,7)SCF/RASSI-SO level of theory (Figure 4), following a computational protocol previously applied successfully to various
lanthanide complexes (see experimental part for the computational details).<sup>63,64,65,66</sup> The simulations for both enantiomers exhibit comparable trends and show good agreement with the experiments. For both complexes, the <sup>7</sup>F<sub>0</sub> ← <sup>5</sup>D<sub>0</sub> transition was calculated to have very low intensity (Tables S5 and S6), consistent with its negligible contribution to the experimental spectrum. In the case of the <sup>7</sup>F<sub>1</sub> ← <sup>5</sup>D<sub>0</sub> and <sup>7</sup>F<sub>2</sub> ← <sup>5</sup>D<sub>0</sub> transitions, only the 1 ← 0' and the 3 ← 0' transitions, respectively, were computed with an opposite sign compared to experimental data. As expected, the <sup>7</sup>F<sub>1</sub> ← <sup>5</sup>D<sub>0</sub> transitions are predominantly governed by magnetic dipole moments, as indicated by the magnitudes of the squared transition moments (Tables S5 and S6). Interestingly, for the <sup>7</sup>F<sub>2</sub> ← <sup>5</sup>D<sub>0</sub> transitions, the electric dipole and magnetic dipole contributions are of similar magnitude, both playing a significant role in determining the overall CPL signal. For the <sup>7</sup>F<sub>3</sub> ← <sup>5</sup>D<sub>0</sub> bands, the lower-energy components align well with experimental observations, while the higher-energy transitions exhibit positive signals instead of negative ones. This slight discrepancy is even more pronounced in the <sup>7</sup>F<sub>4</sub> ← <sup>5</sup>D<sub>0</sub> transitions, where multiple overlapping transitions contribute to the CPL bands, making a direct comparison with the experimental spectra more complex. Nonetheless, all simulated transitions, especially those at lower energies, are mirror images of one another, as expected for enantiomers (Figure S18). This strong correlation between theoretical predictions and experimental results further validates the reliability of the computational approach.

<figure><img src="image_16.png" /><figcaption>**Figure 4.** Simulated CPL spectra of Eu(4R,7S-2,2) (left) and Eu(4S,7R-2,2) (right) at the SA-CAS(6,7)SCF/RASSI-SO level of theory with an energy shift of -3464 cm⁻¹ to match the transition at 615 nm. The spectra were normalized and the respective experimental spectra are shown as black dashed lines, while the bar plots correspond to the calculated rotatory strengths.</figcaption></figure>

These findings also enhance our confidence in the CPL data simulation for Eu(4S,7R-1,2), for which no X-ray structure was available. To address this, several possible structures were considered, varying the arrangement of the ligands around the Eu(III) ion based on the known structures of the other characterized complexes. The best agreement between the simulated and experimental CPL was achieved for the structure depicted in Figure 5 (see also Figure S19). As observed previously, the absence of <sup>7</sup>F<sub>0</sub> ← <sup>5</sup>D<sub>0</sub>, along with the well-reproduced main bands corresponding to the <sup>7</sup>F<sub>1</sub> ← <sup>5</sup>D<sub>0</sub>, <sup>7</sup>F<sub>2</sub> ← <sup>5</sup>D<sub>0</sub> and <sup>7</sup>F<sub>3</sub> ← <sup>5</sup>D<sub>0</sub> transitions, further supports the accuracy of the structural model. Thus, the regioselectivity of the linkage between the chiral camphor-pyrazole units and the pyridine ring, i.e. N1 versus N2, induces a profound change in the coordination sphere of the Ln ion which triggers a strong modulation of the CPL activity.
<figure><img src="image_17.png" /><figcaption>**Figure 5.** DFT optimized structure of Eu(4S,7R-1,2) (left). Corresponding simulated CPL spectrum at the SA-CAS(6,7)SCF/RASSI-SO level of theory with an energy shift of -3464 cm⁻¹ to match the transition at 615 nm (right). The spectrum has been normalised and has a Gaussian full width at half maximum of 80 cm⁻¹ centred in each transition. The corresponding experiment are shown as black dashed lines, while the bar plot corresponds to the calculated rotatory strength.</figcaption></figure>

## CONCLUSIONS

This study highlights the ability of the bis.camphor)-bpp family of ligands to act as efficient sensitizers for the CPL of europium(III)-based complexes and unveils an unprecedented switch of the polarization of the emission with the linkage position of the pyrazoles to the pyridine unit. Indeed, due to tautomerism in the (4S,7R)-camphor-pyrazole precursor, two isomeric ligands, bis.camphor)-2,2-bpp and bis.camphor)-1,2-bpp, were obtained during the synthesis. Although both share the same absolute configuration of their asymmetric carbon centres, their corresponding Eu(III) complexes exhibit mirror-image CPL spectra. A similar inversion in CPL was observed between the complexes derived from the enantiomeric ligands, bis(4R,7S)camphor-2,2-bpp and bis(4S,7R)camphor-2,2-bpp. Thus, we demonstrated throughout this study, that CPL inversion can be obtained not only by using opposite enantiomeric pairs of ligands, but also by playing with the connectivity between pyridine and pyrazoles, while keeping the same absolute configuration of the chiral units. Notably, all three investigated complexes display high glum factors of ±0.2 for the magnetically-allowed transition, further demonstrating their strong chiroptical activity. Ab initio calculations provided a detailed interpretation of the CPL signals for all complexes and unequivocally enabled the characterization of the structural organization of Eu(4S,7R-1,2). Therefore, using this family of bis.camphor)-bpp ligands, both left-handed and right-handed CPL can be achieved from the (4S,7R)-camphor-pyrazole enantiomer. Derived from the natural (R)-(+)-camphor, the bis.camphor)-2,2-bpp and bis.camphor)-1,2-bpp isomers can be readily separated by standard chromatography. This straightforward and cost-effective synthesis of chiral ligands and their corresponding lanthanide-based CPL emitters presents a significant advantage for developing optoelectronic devices with strong circularly polarized electroluminescence performance, for which the handedness of the polarization can be tuned not only with the absolute configuration of the camphor unit but also with the regioisomerism of the pyrazole ligand. Ongoing investigations in our groups involve the use of other emissive lanthanide ions, such as near infrared emitters, and variation of the hfac ancillary ligands.

## ASSOCIATED CONTENT
**Supporting Information**

The Supporting Information is available free of charge. Additional crystallographic data and figures, 1H NMR spectra, chiral HPLC separation, photophysical figures and photographs, computational details (PDF).

**Accession Codes**

CCDC 2430296-2430301 contain the supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

# AUTHOR INFORMATION

**Corresponding Authors**

**Alexandre Abhervé** – Univ Angers, CNRS, MOLTECH-Anjou, SFR MATRIX, F-49000 Angers, France. E-mail: [alexandre.abherve@univ-angers.fr](mailto:alexandre.abherve@univ-angers.fr)

**Boris Le Guennic** – Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, F-35000 Rennes, France. E-mail: [boris.leguennic@univ-rennes.fr](mailto:boris.leguennic@univ-rennes.fr)

**Narcis Avarvari** – Univ Angers, CNRS, MOLTECH-Anjou, SFR MATRIX, F-49000 Angers, France. E-mail: [narcis.avarvari@univ-angers.fr](mailto:narcis.avarvari@univ-angers.fr)

**Authors**

**Justin Cado** – Univ Angers, CNRS, MOLTECH-Anjou, SFR MATRIX, F-49000 Angers, France.

**Maxime Grasser** – Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, F-35000 Rennes, France. Present address: Department of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.

**Nicolas Vanthuyne** – Aix Marseille Univ, CNRS, Centrale Med, UAR 1739, FSCM, Chiropole, Marseille, France

**Francesco Zinna** – Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via G. Moruzzi 13, 56124, Pisa, Italy

**Lorenzo di Bari** – Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via G. Moruzzi 13, 56124, Pisa, Italy

**Author Contributions**

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

**Notes**

The authors declare no competing financial interest.

# ACKNOWLEDGMENTS
This work was supported in France by the CNRS and the University of Angers. MG and BLG thank the French National Research Agency (ANR) through the project SMMCPL (ANR-19-CE29-0012). LDB acknowledges the University of Pisa for financial support through PRA_2022_12.

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