# Phosphorescent Chiral Cationic Binuclear Iridium(III) Complexes: Boosting the Circularly Polarized Luminescence Brightness

Pierre-Henri Lanoë, Christian Philouze, Florian Molton, Nicolas Vanthuyne, Debsouri Kundu, Mathias Delporte-Pebay, Jeanne Crassous, Camille Latouche, Frédérique Loiseau

► To cite this version:

Pierre-Henri Lanoë, Christian Philouze, Florian Molton, Nicolas Vanthuyne, Debsouri Kundu, et al. Phosphorescent Chiral Cationic Binuclear Iridium(III) Complexes: Boosting the Circularly Polarized Luminescence Brightness. Inorganic Chemistry, 2024, 63 (52), pp.24855-24866.
10.1021/acs.inorgchem.4c04271. hal-04861456

HAL Id: hal-04861456

https://hal.science/hal-04861456v1

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# Phosphorescent chiral cationic binuclear iridium(III) complexes: boosting the Circularly Polarized Luminescence brightness

Pierre-Henri Lanoë,<sup>a,*</sup> Christian Philouze,<sup>a</sup> Florian Molton,<sup>a</sup> Nicolas Vanthuyne,<sup>b</sup> Debsouri Kundu,<sup>c</sup> Mathias Delporte--Pebay,<sup>c</sup> Jeanne Crassous,<sup>c</sup> Camille Latouche,<sup>d,e</sup> and Frédérique Loiseau<sup>a</sup>

**ABSTRACT:** We report the synthesis and characterization of two chiral binuclear iridium(III) complexes ($\Lambda\Lambda$ and $\Delta\Delta$), prepared from enantiopure building blocks [$\mu$-Cl$_2$(Δ-Ir(C$^N$N)$_2$)$_2$] and [$\mu$-Cl$_2$(Λ-Ir(C$^N$N)$_2$)$_2$]. These building blocks have been obtained by chiral preparative HPLC of the neutral iridium(III) complex Irpiv (piv = 2,2,6,6-tetramethylheptane-3,5-dionate) followed by a selective degradation of the ancillary ligand. For comparison purposes, we also synthesized a monomer (IrL$^1$) and a dimer (Ir$_2$L$^2$, mixture). All the complexes exhibit similar emission properties, emitting in the orange-red region of the spectra with good photoluminescence quantum yield ($\lambda_{max}$ = 610-625 nm, $\Phi$ ~25%, $\tau$~800-900 ns). However, the $\Lambda\Lambda$ and $\Delta\Delta$ complexes are optically active, indicating no isomerization occured during the different synthetic steps, as evidenced by both the circular dichroism (CD) spectra and their circularly polarized luminescence (CPL). The capital gain of the dimers (Ir$_2$L$^2$, $\Lambda\Lambda$ and $\Delta\Delta$) is a fourfold brightness ($B_{380} = \varepsilon_{380nm} \times \Phi$) compared to the monomer (IrL$^1$) and the CPL brightness ($B_{CPL} = B_{380} \times g_{lnm}/2$) of the binuclear complexes being among the highest reported to date for chiral iridium(III) complexes.

## Introduction

Since the early 2000s, phosphorescent iridium(III) complexes have become a cornerstone in modern photochemistry, due to their successful application as emissive materials in optoelectronics,$^{1-6}$ sensors and luminescent biological probes for confocal microscopy, for example.$^{7-13}$ These applications are possible thanks to their exceptional physicochemical properties, such as high photoluminescence quantum efficiency, good electrochemical stability and reversibility, and availability

through reliable and robust methods. The emission color can be tuned from sky blue to near-infrared by judicious modification of the ligands.$^{14-26}$ Moreover, Circularly Polarized Luminescence (CPL) of materials have garnered increasing interest. Given the luminescent properties of Ir(III) complexes, it is highly desirable to obtain enantiopure Ir(III) complexes. These complexes might play a key role in several potential applications, including photodynamic therapy, catalysis, and high-resolution 3D display.$^{27-29}$

<footnote><sup>a</sup>Dr. P.-H. Lanoë, Dr. C. Philouze, F. Molton and Pr F. Loiseau. Univ. Grenoble Alpes, CNRS, DCM, 38000 Grenoble, France, E-mail: pierre-henri.lanoe@univ-grenoble-alpes.fr</footnote>

<footnote><sup>b</sup>Aix Marseille University, CNRS, Centrale Marseille, FSCM, Chiropole, Marseille, France</footnote>

<footnote><sup>c</sup>Univ Rennes, CNRS, ISCR-UMR 6226, ScanMAT-UMS 2001, F-35000 Rennes, France</footnote>

<footnote><sup>d</sup>Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000, Nantes, France</footnote>

<footnote><sup>e</sup>Institut Universitaire de France (IUF), 75005, Paris, France</footnote>
<figure><img src="image_2.png" /><figcaption>**Scheme 1:** Left: Synthesis of ligand L¹ and the Rac complex: i) DMSO, n-butylamine, 100 °C; ii) Na₂S₂O₅, EtOH/H₂O reflux, iii) MeOH/DCM, L¹, Ar, reflux and iv) KPF₆ (aq., Sat.). Right: Synthesis of ligand L² and the Diast complexes, ΔΔ and ΛΛ: i) DMF, 1-bromobutane, NaH, 0 °C to 60 °C; ii) Na₂S₂O₅, DMF/H₂O reflux, iii) MeOH/DCM, L², Ar, reflux and iv) KPF₆aq (Sat.).</figcaption></figure>

Considering Ir(III) complexes, the D₃ or C₂ local symmetry of complexes containing bidentate ligands of general structures *fac/mer*-Ir(N^A C)₃ or [Ir(N^A C)₂(L)]⁺ formed as racemic mixtures of Δ and Λ isomers, makes such complexes chiral.³⁰ Most often, racemic mixtures are reported, which does not have a deleterious impact since enantiomers have identical linear optical properties. The situation is different when dealing with homo-bimetallic complexes with the generation of diastereomers (Λ,Λ/Δ,Δ and Λ,Δ isomers) induces different properties.²⁴,³¹⁻³⁵ Nonetheless, the synthesis of bimetallic, and thus multimetallic, complexes as pure stereoisomers is difficult and requires chiral resolution, a process that can often be laborious and time-consuming. Such complexes with a variety of bridging ligands also typically have photoluminescence quantum yields lower than their mononuclear analogues, leading to the reputation that homo-binuclear iridium complexes are inferior emitters, irrespective of the desired end application.³²,³⁵ The nature of the bridging ligand is crucial to maintain or to enhance the luminescence properties.³⁵ Wong *et al* have reported the synthesis of a red-emitting dinuclear iridium complex, derived from [Ir(ppy)₂acac] (ppy = 2-phenylpyridine, acac = acetylacetonate), featuring a bridging 2-(4-tert-butylphenyl)-pyrimidine (ppyr) ligand.³⁶ The authors were able to separate by column chromatography the *meso* complex (Δ,Λ) in 15% yield from the inseparable isomeric mixture (Δ,Δ and Λ,Λ) in 13% yield. Sauvage *et al* have reported cationic chiral dinuclear complexes of the form [(ppy)₂Ir(μ-L)Ir(ppy)₂]²⁺ with L being 3,8-dipyridyl-4,7-phenanthroline.³¹ They obtained a mixture of complexes consisting of a *meso* form Δ,Λ and a racemic form (enantiomeric pair Δ,Δ and Λ,Λ). They were able to separate the mixture by ion pairing chromatography with Δ-TRISPHATE anion (tris(tetrachlorobenzenediolato)phosphate(V)) "in extremely modest chemical yield (≤10%)".³¹ Chiral multimetallic assemblies have been recently synthesized through the use of chiral building blocks [μ-Cl₂(Δ-Ir(ppy)₂)] and [μ-Cl₂(Λ-Ir(ppy)₂)], which were obtained by chiral resolution of the diastereomeric
mixture of neutral intermediates. This method has been in-
spired by the seminal works of Meggers et al, in the field of
chirality at metal catalysts.<sup>37,38</sup> The key diastereomer interme-
diates contain an easily removable ancillary ligand: a L- or D-
serine. The method has been used by Hosseini et al, adapted
from Lusby et al.<sup>39,40</sup>, and consists in the synthesis of the di-
astereomers Δ/Λ-[Ir(ppy)<sub>2</sub>(L-serine)] or Δ/Λ-[Ir(ppy)<sub>2</sub>(D-
serine)]. The diastereomers can be separated by conventional
flash column chromatography in high yield (up to 52%). It is
worth noting that the faster running diastereoisomer is (Δ-
[Ir(ppy)<sub>2</sub>(L-serine)] or Λ-[Ir(ppy)<sub>2</sub>(D-serine)] and the slower
eluting compound, Δ-[Ir(ppy)<sub>2</sub>(D-serine)] or Λ-[Ir(ppy)<sub>2</sub>(L-
serine)]. The second compound is always contaminated with
the other diastereomer. The first fraction is then treated in
acidic condition in order to remove the chiral ancillary ligand,
leading to the precipitation of the [μ-Cl<sub>2</sub>(Δ-Ir(ppy)<sub>2</sub>)] and [μ-
Cl<sub>2</sub>(Λ-Ir(ppy)<sub>2</sub>)] enantiomer. The method has been success-
fully used to obtain homochiral multimetallic assemblies<sup>39,40</sup>
and binuclear iridium complexes.<sup>32</sup> A powerful, but not that
common method, is the use of preparative HPLC with chiral
stationary phase that allows high enantiomeric purity. The
method has been reported with the separation of the neutral
iridium complexes allowing to study the chiroptical properties
of the individual enantiomers.<sup>41-47</sup> For example, Bernhard et
al<sup>41</sup> used Supercritical Fluid Chromatography (SFC) separa-
tions for resolving racemates of a series of neutral iridium
complexes of the form [(C<sup>^N</sup>)<sub>2</sub>Iracac] and fac-[(C<sup>^N</sup>)<sub>3</sub>Ir] with
enantiomeric excess (ee) in the range 95-100%. Recently,
Gauthier et al synthesized a series of four stereoisomers of
tris-helicenic architectures, formed of one central Ir(III) ion
surrounded by three hexahelicenic N-heterocyclic carbene
(NHC) ligands resulting in unique triskelion (triple-spiral)-
shaped complexes.<sup>46</sup> The use of chiral HPLC separation al-
lowed the authors to obtain each stereoisomer with ee's higher
than 99%.

Herein, we describe the luminescence, circular dichroism (CD) and circularly polarized luminescence (CPL) properties of the two isomers of a cationic homo-bimetallic iridium(III) (ΛΛ and ΔΔ). The enantiomers were prepared starting from the separation based on a preparative chiral HPLC technique of a racemic mixture of **Rac-Irpiv**<sup>48</sup> (namely, [2-(2-pyridinyl)phenyl](2,2,6,6-tetramethylheptane-3,5-dionato)iridium(III)) yielding to enantiopure Δ-Irpiv and Λ-Irpiv isomers which were successfully converted to the chiral

building blocks [μ-Cl₂(Δ-(ppy)₂Ir)₂] and [μ-Cl₂(Λ-(ppy)₂Ir)₂]
(the building block will be referred as μ-Cl₂ΔΔ and μ-Cl₂ΛΛ
here after), respectively. These building blocks were used in
the synthesis of two isomers, with the ΛΛ and ΔΔ isomerism.
For comparison purposes, a diastereomeric mixture of the
homo-bimetallic iridium(III) complexes and the corresponding
monomer were also synthesized. All the complexes were fully
characterized (¹H & ¹³C NMR, HRMS), and the standard
photophysical properties were thoroughly studied, as well as
the chiroptical properties, demonstrating the great interest of
dimers regarding overall luminescence efficiency, i.e. the
brightness both B (B = ε x Φ) (eq. 1) and B<sub>CPL</sub> (B<sub>CPL</sub> = B x
g<sub>lum</sub>/2) (eq. 2). DFT and TD-DFT simulations were also con-
ducted to get more insights into the ground and excited states
properties.

**Results and discussion**

**Synthesis and Characterizations.**

All the compounds were characterized by NMR (¹H and ¹³C), mass spectrometry and elemental analysis. Both ligand L¹ and ditopic ligand L² were synthesized by adapting known procedures and the synthetic routes are described in Scheme 1 for the monomer and the dimers respectively.<sup>49,50</sup> First, L¹ was synthesized starting from the aromatic substitution of 1-bromo-4-methyl-2-nitrobenzene by n-butylamine in hot dimethyl sulfoxide giving 1 in high yield (89%). The benzimidazole ring was formed by the reduction of 1 with 5-bromo-2-pyridinecarbaldehyde in presence of Na₂S₂O₅ in a refluxing mixture of water and ethanol. The crude material was purified by column chromatography over silica gel giving L¹ in moderate yield (49%). Ligand L² has been synthesized by a slightly different route than L¹, starting from the alkylation of the two aromatic amines from the 3,3'-dinitroazidine with 1-bromobutane in presence of potassium carbonate in hot DMSO, yielding to 2 in moderate yield (44%). The two benzimidazole rings were formed in the same above-mentioned conditions and yielded, after column chromatography, L² in moderate yield (51%). The dichlorobridged dimer (μ-Cl₂Ir₂ hereafter) has been synthesized according to well-known procedures,<sup>51,52</sup> it is worth noting that because of steric interactions due to the proximity of the iridium cores (Ir-Ir < 4 Å), the dimer resulting from the reaction of the

<figure><img src="image_3.png" /><figcaption>**Figure 1:** Ellipsoid representation of the single crystal diffraction with 50% thermal probability, hydrogens atoms and counterions have been omitted for clarity. **A)** Image from the Diast (meso diastereomer), **B)** asymmetric unit, representing half of the molecule, pointing out the special position in the lattice occupied by the complex.</figcaption></figure>
cyclometallating ligand with IrCl₃·nH₂O is a racemate of blocks **μ-Cl₂ΔΔ** and **μ-Cl₂ΛΛ** with the pyridines in *trans*.⁵³⁻⁵⁸ In order to perform the preparative chiral HPLC, we synthesized a neutral racemic complex **Irpiv**⁴⁸ and the racemate was successfully resolved in high yield (63-65%) with enantiomeric excesses over 98.5%. A schematic overview of the strategy for obtaining enantiopure building block is presented in figure S1. Chiral HPLC separation for compound **Irpiv** and absolute configuration determination for enantiomers by comparison of experimental and TD-DFT calculated ECD spectra are presented in the supporting materials. It is worth noting that the yields using preparative chiral chromatography are higher than the ones obtained by the classical route using the chiral aminoacid ancillary ligands leading to the separation of the diastereomers in maximum yields around 50-52%.³⁹,⁴⁰,⁵⁹

The next step is the obtention of the enantiopure [μ-Cl₂(Δ/Λ -Ir)₂]. **Λ-Irpiv** and **Δ-Irpiv** were dissolved in EtOH and they were converted to the corresponding **μ-Cl₂ΔΔ** and **μ-Cl₂ΛΛ**, which precipitated, by the removal of the ancillary ligand (piv) with the addition of hydrochloric acid (2 M).³⁹,⁴⁰,⁶⁰ The final complexes, IrL¹, Ir₂L² (mixture), ΛΛ and ΔΔ, were synthesized in a refluxing mixture of methanol and dichloromethane overnight in presence of the adequate number of equivalents of the corresponding chelating ligands (L¹ or L²).⁶¹,⁶² The complexes were obtained by precipitation through anion metathesis with an aqueous solution of KPF₆ (sat.) with yields from moderate to high (IrL¹ 93%, Ir₂L² 79%, ΛΛ 58% and ΔΔ 56%), without further purification.

<table><caption>Table 1: Selected bond lengths and angles</caption><thead><tr><th>Bond lengths (Å)</th><th>Angles (°)</th><th>Torsion angles (°)</th></tr></thead><tbody><tr><td>Ir1-N4<br>2.132(9)</td><td>C2^Ir1^N2A<br>83.1(9)</td><td>C29,<br>C34,C34',C33' i= 1-x,2-y,1-z<br>1.2(19)</td></tr><tr><td>Ir1-N3<br>2.147(8)</td><td>C1^Ir1^N1<br>81.2(5)</td><td>N3,C27,C28,N4</td></tr><tr><td>Ir1-C1<br>2.029(12)</td><td>N4^Ir1^N3,<br>75.1(3)</td><td>-0.5(14)(1)</td></tr><tr><td>Ir01-N1<br>2.044(10)</td><td>N2A^Ir1^N1<br>170.7(7)</td><td>N2A,C18A,C17,<br>C2</td></tr><tr><td>Ir1-C2<br>1.988(10)</td><td>C2^Ir1^N3<br>174.0(4)</td><td>-9(2)</td></tr><tr><td>Ir1-N2A<br>2.05(1)</td><td>C1^Ir1^N4<br>169.6(4)</td><td>C1,C7,C8,N1<br>-4.5(15)</td></tr></tbody></table>

## X-ray Crystal Structures

Single crystals of Ir₂L² suitable for single X-ray diffraction were obtained by slow vapor diffusion of diisopropylether in an acetonitrile solution of the complex. Unfortunately, all attempts of crystallization of enantiopure ΛΛ and ΔΔ in different sets of solvents lead to amorphous materials. Complex Ir₂L² crystalized in monoclinic Bravais lattice (space group C2/c), bond lengths and ligands bite angles are summarized in Table 1, and packing is presented in supporting materials (Figure S1-4). The complex occupies a special position in the lattice, as a consequence the asymmetric unit is reduced to the half of the molecule (Figure 1) and the meso diastereomer is solely found in the lattice which can be observed in the three plans (ab, ac, and bc) of the packing projections. A quasi octahedral coordination around the metal core is observed with slight distortions and the angles with the ligands in *trans* of the metal center are of 169.6(4)° and 174.0(4)°. The pyridines of the cyclometallating ligands are in *trans* position, as it is expected in such cyclometallated iridium(III) complexes forming an angle of 170.7(7)°.³,⁶³ The carbon-metal bonds are of the order of 2.00 Å and the nitrogen-metal bond lengths of the cyclometallating ligands are 2.05 Å. In comparison, the coordination bond lengths of the bridging ligands are longer (~2.13 Å), which is due to the *trans* effect induced by the strong σ-donating character of the cyclometallating carbons. In addition, the bite angles of the cyclometallating ligands and the bridging ligand are around 82.2° and 75.1°, respectively. Those values of distances and angles are very close to those encountered in similar Ir(III) complexes.³,⁶³⁻⁶⁵ The phenyl and pyridine rings of cyclometallating ligands are not coplanar and display slight torsion angles from 4.5° to 8.6°. In contrast, the bridging ligand is almost planar (dihedral angle of ~1°, between the pyridine and the benzimidazole moiety) which is comparable to the corresponding angles in [Ir(ppy)₂bpy]PF₆ and in other similar complexes.³,⁶³⁻⁶⁵ The two coordination sites of the bridging ligand are quasi co-planar with a dihedral angle of only 1(2)°, and the metal cores are in the plane of the bridging ligand.
<table><caption>Table 2: Steady state and transient photophysical and electrochemical properties.</caption><thead><tr><th rowspan="2">Complex</th><th rowspan="2">Absorption<br>(x10<sup>4</sup> L.mol<sup>-1</sup>.cm<sup>-1</sup>)</th><th rowspan="2">λ<sub>em</sub><br>/nm</th><th rowspan="2">Φ<sup>[a]</sup>, [b]<br>[air]</th><th rowspan="2">τ<sup>[b]</sup>/ns<br>[air]</th><th rowspan="2">77 K<sup>[c]</sup>/nm<br>[τ/ns]</th><th rowspan="2">k<sub>0</sub>/s<sup>-1</sup><br>(x 10<sup>5</sup>)</th><th rowspan="2">Σk<sub>0</sub>/s<sup>-1</sup><br>(x 10<sup>5</sup>)</th><th rowspan="2">B<sup>[d]</sup><br>x 10<sup>3</sup><br>/mol<sup>-1</sup>.cm<sup>-1</sup></th><th rowspan="2">B<sub>CPL</sub><sup>[e]</sup><br>/mol<sup>-1</sup>.cm<sup>-1</sup></th><th colspan="2">Electrochemical properties<sup>[f]</sup> (V)</th></tr><tr><th>E<sup>red</sup></th><th>E<sup>ox</sup></th></tr></thead><tbody><tr><td>IrL<sup>1</sup><br>(racemate)</td><td>257 (4.7), 302 (2.6),<br>340 (2.6), 355 (2.7),<br>388 (1.1), 416 (0.5),<br>470 (0.1),</td><td>610</td><td>0.23<br>[0.08]</td><td>918<br>[228]</td><td>526*,<br>567, 622<br>[5200]</td><td>2.5</td><td>8.4</td><td>3.0</td><td>-</td><td>-1.56<sup>rr</sup></td><td>0.95</td></tr><tr><td>Ir<sub>2</sub>L<sup>2</sup><br>(mixture)</td><td>267 (9.8), 303 (5.1),<br>335 (3.3), 378 (5.5),<br>470 (0.2)</td><td>625</td><td>0.24<br>[0.07]</td><td>783<br>[242]</td><td>530*,<br>572, 621<br>[6820]</td><td>3.1</td><td>9.7</td><td>13.0</td><td>-</td><td>-1.55<sup>rr</sup></td><td>0.96</td></tr><tr><td>ΛΛ</td><td>266 (9.8), 302 (5.2),<br>332 (3.4), 380 (5.3),<br>470 (0.2)</td><td>624</td><td>0.25<br>[0.08]</td><td>793<br>[241]</td><td>532*,<br>569, 614<br>[6180]</td><td>3.2</td><td>9.5</td><td>13.3</td><td>-2.4</td><td>-1.55<sup>rr</sup></td><td>0.96</td></tr><tr><td>ΔΔ</td><td>265 (10.5), 301 (5.3),<br>337 (3.4), 379 (5.7),<br>467 (0.2)</td><td>624</td><td>0.24<br>[0.08]</td><td>755<br>[240]</td><td>532*,<br>570, 620<br>[6250]</td><td>3.2</td><td>10.0</td><td>13.7</td><td>+2.4</td><td>-1.55<sup>rr</sup></td><td>0.96</td></tr></tbody></table>

[a] Ru[bpy]<sub>3</sub><sup>2+</sup> in CH<sub>3</sub>CN was used as standard, [b] deaerated, [c] recorded in benzonitrile, [d] molecular brilliance B = ε x Φ (eq. 1), ε at 380 nm; [e] CPL brightness B<sub>CPL</sub> = B x g<sub>lum</sub>/2 (eq. 2) [f] Potentials obtained by cyclic voltammetry using Bu<sub>4</sub>NPF<sub>6</sub> (0.1 M) as the supporting electrolyte in CH<sub>3</sub>CN. Potentials are given relative to the Ag<sup>+</sup>/Ag couple (Reference electrode ANE2, AgNO<sub>3</sub> = 10<sup>-2</sup> M) measured under the same conditions. * denotes the most intense band

<figure><img src="image_4.png" /><figcaption>Figure 2: Absorption and deaerated normalized emission spectra in CH<sub>2</sub>Cl<sub>2</sub> of complexes IrL<sup>1</sup>, Ir<sub>2</sub>L<sup>2</sup>, ΛΛ and ΔΔ. * Meso was simulated (see details in the text). Inset zoom of the absorption spectra in the 440 to 640 nm.</figcaption></figure>

## Electrochemistry

The electrochemical properties of IrL<sup>1</sup> and the dimers complexes have been studied in CH<sub>3</sub>CN (Table 2, Figure S5 and S6). The redox potentials of both IrL<sup>1</sup> and the dimer complexes are very closed. First in reduction, an irreversible peak is observed around -1.56 V and -1.55 V, for IrL<sup>1</sup> and the dimers respectively. These peaks are less negative than the

reduction potential of -1.68 V for [Ir(C<sup>^N</sup>)<sub>2</sub>bpy<sup>+</sup>]<sup>66</sup> (C<sup>^N</sup> = 2,2'-phenylpyridine, and bpy = 2,2'-bipyridine) in which the reduction occurs on the ancillary ligand (bpy). Secondly, the potential is also very closed to the ones observed in similar complexes (E<sup>red</sup> = -1.67 - -1.57 V).<sup>67,68</sup> Thus, we ascribe the reduction peak in IrL<sup>1</sup> and the dimers to the reduction of the 2-2'-(4'-bromopyridyl)benzimidazole moiety with the presence of the electron withdrawing bromine atom that impacts the potential. In oxidation, both complexes display a peak within the range 0.95 V - 0.96 V, which can be ascribed to the couple Ir<sup>III</sup>/Ir<sup>IV</sup>.<sup>63,69</sup> The iridium(III) cores are not electronically coupled within the bimetallic complexes, as the dimers display only one oxidation wave and it is in agreement with previously reported dinuclear complexes.<sup>70-73</sup> The oxidation potential is significantly shifted in comparison with the ones observed in similar complexes lacking the bromine atom (E<sup>ox</sup> = 0.76-0.88 V).<sup>67,68</sup> The electron density over the HOMO (i.e. Ir<sup>III</sup> core) is impacted by the electron withdrawing group in *meta* of the N<sub>py</sub>-Ir bond.

## Photophysical properties

The absorption and photoluminescence spectra of the series in CH<sub>2</sub>Cl<sub>2</sub> are displayed in Figure 2 (individual spectra are displayed in Figure S7) and the summary of the photophysical characteristics is given in Table 2.
The absorption spectra of iridium complexes could be divided into three spectral domains. In the UV region of the spectrum, in the range 250-340 nm, intense absorption bands can be assigned to the spin-allowed π-π* ligand centered transitions (¹LC) in both cyclometallating and chelating ligands (i.e. L¹ and L²).

In the second region ranging from 320 nm to 440 nm moderate to low absorption bands can be assigned to mixing of spin allowed metal-to-ligand charge transfer (MLCT) admixed with ligand-to-ligand charge transfer transitions (¹LLCT), from the cyclometallating ligand to the π* of L¹/L². The third region is above 440 nm with very low absorption tail, which can be assigned to the direct spin forbidden absorption from ground state to the ³MLCT/²LLCT excited states. The latter is partially allowed thanks to the high spin-orbit coupling constant (ζ = 3909 cm⁻¹) induced by the iridium.¹⁴ In addition, the bimetallic complexes display a bit more than double of the absorbance of complex IrL¹, below 350 nm. For example, the highest absorption band has a molar extinction coefficient of 46.3 × 10³ M⁻¹ cm⁻¹ for IrL¹ and 98.7 x 10³ M⁻¹ cm⁻¹, 97.7 x 10³ M⁻¹ cm⁻¹ and 100.5 x 10³ M⁻¹ cm⁻¹ for Ir₂L², ΛΛ and ΔΔ respectively. One can also observe that the bimetallic complexes display, certainly, higher molar absorptivity in the visible region of the spectrum, and the bands corresponding to the (M/L)LCT are red-shifted (Δλmax = 1853 cm⁻¹), from 355 nm to roughly 380 nm, for IrL¹ and bimetallic complexes respectively, such a behavior has been observed previously in similar complexes.⁷³ Finally, the absorption tail above 440 nm displays a molar absorptivity that corresponds almost exactly to the addition of the absorbance spectra of two IrL¹ units, pointing out that there is no electronic coupling in the ground state between the two parts of the molecules, which would be the case with a rigid bridging ligand.²¹,⁷⁴

<figure><img src="image_5.png" /><figcaption>**Figure 4: ECD spectra.** The simulated one (black dash) is obtained via the VG vibronic approach on top of the TDDFT electronic computations. The simulated spectrum is shifted by 0.12 eV to better fit experimental values.</figcaption></figure>

The emission spectra have been recorded in dilute CH₂Cl₂ solutions under air equilibrated and deaerated conditions at room temperature. All the complexes display an orange-red emission according to the CIE diagram (Figure 3), with an emission maximum in the range 610-625 nm. It is worth noting that the photoluminescence quantum yields (~24%) of the bimetallic complexes are similar to that of monometallic counterpart (φ = 23%). In comparison with previously reported complexes bearing 2-2'-pyridylbenzimidazole, the introduction of a bromine atom in position 5' of the pyridyl moiety

<figure><img src="image_6.png" /><figcaption>**Figure 3: CIE 1931 diagram of the complexes obtained from the integrated emission spectra.**</figcaption></figure>

leads to a somewhat strong bathochromic shift of 30 nm (556 cm⁻¹) and 45 nm (950 cm⁻¹), for the monometallic and bimetallic complexes respectively.⁶⁸,⁷⁵ The bimetallic complexes display a slightly redshifted emission (λmax ~ 625 nm) in comparison with IrL¹ (λmax = 610 nm), but the difference in energy is almost meaningless with 0.05 eV (394 cm⁻¹). It is quite striking that the radiative and non-radiative constants (kᵣ and Σkᵣₙₚ respectively) are almost identical within the bimetallic complexes, kr ~ 3 x 10⁵ s⁻¹ and Σkᵣₙₚ ~ 10 x 10⁵ s⁻¹. In comparison with complex IrL¹, kᵣ and Σkᵣₙₚ are of the same order of magnitude, 2.5-3 x 10⁵ s⁻¹ and 8-10 x 10⁵ s⁻¹, respectively. The slight decrease of the Σkᵣₙₚ and increase of the kᵣ in the bimetallic series could be attributed to the flattening of the bridging ligand in the excited state and to the increase of the spin-orbit coupling with the presence of two Ir cores. The emission of the complexes could be reasonably assigned to the radiative deactivation of the ³MLCT* state to the ground state, considering the broad shapes and the lifetimes of the room temperature emission spectra, the rigidochromism effect (77 K spectra displayed in Figure S8), and this family of cationic iridium(III) complexes.¹⁴,⁷⁶ Luminescence spectroscopy and microscopy have emerged as powerful tools for real-time, noninvasive monitoring of biomolecules of interest in their native environments with high spatial and temporal resolution.⁷⁷ Phosphorescent dyes allow to increase the imaging resolution *via* using a delay between the excitation and the acquisition, allowing the background fluorescence to significantly decrease.⁷⁸,⁷⁹ The imaging could be also enhanced by increasing the molecular brightness (B = ε x Φ) (eq. 1), where ε is the molar absorptivity at the excitation wavelength and Φ the quantum yield.⁸⁰⁻⁸² Within the series, the bimetallic derivatives display a four times higher brightness value, with ε at 380 nm, than the monometallic, with an average value of B = 13.3 x 10³ for the bimetallic series and B = 3 x 10³ for IrL¹. Such a difference is explained by the advantageous redshift of the MLCT absorption band observed in the dimers in comparison with the monomer. Thus, such results demonstrate the strong potential of multi-metallic assemblies as imaging agents, adding to that the possibility of the discrimination of substrates via the controlled chirality at the metal.⁸³

The circular dichroism spectra of the three bimetallic complexes have been registered in dichloromethane solution (10⁻⁵ M) and they are depicted in Figure 4. As expected the CD spectrum of complex Ir₂L² does not display any absorbance
variation. In stark contrast, both CD mirror spectra of the ΛΛ and ΔΔ were found rich and the Δε values quite intense. For instance, for complex ΛΛ, Cotton effects were observed at 257 (Δε = +74 M⁻¹ cm⁻¹), 283 (−91), 326 (+52), 343 (+45), 366 (+38), 312 (−28) and 495 nm (−5.3). The other enantiomer, ΔΔ, displayed opposite Cotton effects at the same wavelengths.

Being efficient chiral phosphors, their circularly polarized luminescence (CPL) was then examined. The circular polarized luminescent spectra have been recorded in dilute solution of dearerated CH₂Cl₂ at r.t (Figure 5). The two enantiomeric ΔΔ and ΛΛ complexes were found to display positive and negative CPL signal, respectively, at 613 nm, with emission dissymmetry factors g<sub>lum</sub> of around 3.5 × 10⁻⁴. Although the signals are low and approaching instrument sensitivity, these responses fall within the already published complexes with metal centered chirality.³⁰,⁸⁴ Furthermore, it is appealing to consider the CPL brightness,⁸⁵ defined as B<sub>CPL</sub> = B × g<sub>lum</sub>/2 (eq. 2) which gives a value of 2.4. These B<sub>CPL</sub> values are among the highest than the average values around 0.8 for iridium(III) complexes already reported.⁸⁵ These results highlight again the added value of the present enantiopure bimetallic iridium complexes.

## Conclusion

We report four cationic iridium(III) complexes among which two enantiopure homo-bimetallic complexes ΛΛ and ΔΔ, with its diastereomeric mixtures. The enantiopure complexes have been synthesized using an original strategy that consists in the chiral preparative chromatography resolution to obtain the two neutral iridium(III) complexes Λ-Irpiv and Δ-Irpiv which were used to generate enantiopure µ-Cl₂ΔΔ and µ-Cl₂ΛΛ dimers, in acidic conditions. The chiral chromatography allowed us to obtain the desired complexes with a remarkably high enantiomeric excess, over 98.5, and with higher yield (63-65%) than the ones obtained with the classical route using a chiral ancillary ligand, such as proline or serine for example. We used the chiral building blocks to synthesize the homo bimetallic complexes ΔΔ and ΛΛ. The retention of chirality through the synthesis has been assessed by circular dichroism. All the complexes are phosphorescent with relatively high emission quantum yields, over 20%, in the orange-red region of the visible spectra. The dimers reveal to be a very attractive strategy to obtain bright phosphor emitters as these complexes demonstrate a brightness four times higher than the corresponding monomer and CPL brightness among the highest of the iridium(III) reported in the literature up to now.

## Experimental details: general consideration

**Synthesis:** Commercially available reagents were purchased from Sigma-Aldrich, Alfa Aesar, Acros Organics, TCI Chemical, Merck, Strem or Fluorochem and used as received unless otherwise specified. Solvents were obtained from same commercial sources and used without further purification. For moisture sensitive reactions, glassware was oven-dried prior to use. ¹H NMR spectra were recorded on a 400 MHz and on a 500 MHz in deuterated solvent (CDCl₃, DMSO-d₆ or CD₂Cl₂) and data are reported as follows: chemical shift in ppm from

<figure><img src="image_7.png" /><figcaption>Figure 5: CPL spectrum ΛΛ (red line) and ΔΔ (blue line) in deoxygenated CH₂Cl₂ (C ~ 1. 10⁻⁵ M). The average of 9 scanscans for the (ΔΔ)- and (ΛΛ)-isomers are represented by solid and dashed black lines, respectively (λ<sub>exc</sub> : 375</figcaption></figure>

tetramethylsilane with the solvent as an internal indicator (CDCl₃ 7.26 ppm, DMSO-d₆ 2.50 ppm, CD₂Cl₂ 5.32 ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, p = pentet, hept = heptuplet, m = multiplet or overlap of non-equivalent resonances), J-coupling constants (Hz), and proton integration. ¹³C{¹H} NMR spectra were recorded either at 101 MHz or at 126 MHz in suitable deuterated solvent and data are reported as follows: chemical shift in ppm from tetramethylsilane with the solvent as an internal indicator (CDCl₃ 77.16 ppm, DMSO-d₆ 39.52 ppm, CD₂Cl₂ 53.84 ppm). ¹⁹F NMR spectra were recorded at 376 MHz and at 470 MHz in the suitable deuterated solvent. ³¹P NMR spectra were recorded at 202 MHz in the suitable deuterated solvent.

**Electrochemistry:** Cyclic voltammograms were recorded with a BioLogic SP-300 potentiostat. All electrochemical measurements were performed in a standard three-compartment electrochemical cell under an argon atmosphere using tetra-n-butylammonium hexafluorophosphate (TBAPF₆, Aldrich) as supporting electrolyte (0.1 M) in dry CH₃CN. Silver/silver nitrate in CH₃CN (Ag/AgNO₃ 10 mM in CH₃CN, ANE2) was used as reference, counter electrodes was a Pt wire, and the working electrode a 3 mm diameter glassy carbon disk.

**Absorption and emission spectroscopies:** Absorption spectra were recorded on a Cary 300 UV-visible spectrophotometer (Varian) and emission spectra (in solution at room temperature and in rigid matrix at 77 K) were recorded on a Horkiba Fluoromax 4 or on FLS-1000 Edinburgh Instrument equipped with a photomultiplier (PMT) detector cooled by a Peltier. Quartz cuvettes with 1 cm optical path were used. Lifetimes were measured using LP900 spectrometer with a Flashlamp pumped by Q-switched Nd:Yag laser operating at 355 nm and with a photomultiplier (PMT) detector, or with a picosecond laser diode operating at 405 nm and using a time-correlated single photon counting detection (TCSPC, PicoHarp 300).

**Emission quantum yields** in solution were determined in dearerated CH₂Cl₂ or CH₃CN solutions at room temperature, and calculated with the following equation.

$$QY_s = QY_{\text{ref}} \times \frac{I_s}{I_{\text{ref}}} \times \frac{A_{\text{ref}}}{A_s} \times \left( \frac{\eta_s}{\eta_{\text{ref}}} \right)^2$$
**QY**<sub>s</sub> corresponds to the quantum yield of the sample to be analysed, **QY**<sub>ref</sub> is the quantum yield of the reference compound (in this work is Ru(bpy)<sub>3</sub><sup>2+</sup> in air equilibrated CH<sub>3</sub>CN solution with value of 0.018±0.002)<sup>86</sup>, **I** corresponds to the intensity of the emission (area of the spectrum), **A** is the absorption at the excitation wavelength, **η** corresponds to the refractive index of the solvents.

The two properties that provide information about the excited state beside the shape of the emission are the **QY** (Φ) and lifetime (τ), represented in the following equations.

$$ \Phi = \frac{k_r}{k_r + k_{nr}} \quad \tau = \frac{1}{k_r + k_{nr}} $$

The radiative (k<sub>r</sub>) and non-radiative (k<sub>nr</sub>) constants can be obtained from combination and rearrengment of the **QY** (Φ) and τ equations to give equations:

$$ k_r = \frac{\Phi}{\tau} \quad k_{nr} = \frac{1}{\tau} - k_r $$

**Electronic Circular Dichroism** (ECD) spectra were acquired on a JASCO J-810 spectropolarimeter (Jasco corporation, Tokyo, Japan). Data were recorded at room temperature with a spectral resolution of 1 nm.

**Circularly polarized luminescence** (CPL) measurements were performed using a home-built CPL spectrofluoropolarimeter (constructed with the help of the JASCO Company). The samples were excited using a 90° geometry with a xenon ozone-free lamp 150 W LS.

**Crystal Structures Determinations and Refinements:** Single crystals of complex Ir<sub>2</sub>L<sup>2</sup> (CCDC-2312516) was picked up from the mother liquor, coated with a parafin mixture, collected with a nylon loop and mounted on goniometer head. Measurement was made at 200 K on an Enraf-Nonius 4 circles kappa goniometer equipped with an Incoatec high brilliance microsource with Montel optics monochromated Mo-Kα radiation (λ = 0.71073 Å). The detector was a Bruker APEXII and an Oxford Cryosystem cryostream cooler was used. The crystal data and details of the data collections are given in SI. The data were integrated and corrected for Lorentz and polarization effects using Eval14<sup>87</sup>, corrected for absorption using SADABS<sup>88</sup> and finally merged using Xprep<sup>89</sup>. Crystallographic structures were solved using direct methods implemented by Superflip.<sup>90</sup> Refinement was performed using ShelxL-2019<sup>91</sup> run under Olex2<sup>92</sup>. C, Cl, F, Ir, N, O, and Br atoms were refined anisotropically by the full matrix least-squares method on F<sup>2</sup>. H atoms were set geometrically.

Supplementary data are available on request from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, quoting the deposition number 2312516. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or fax: +44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk.

## Computational approach

DFT and TDDFT computations were performed using the global hybrid B3PW91 associated with the LANL2DZ basis set augmented by polarization functions of all atoms except hydrogen since it gave very good results in previous studies.<sup>93</sup>

<sup>103</sup> Solvent effects (CH<sub>2</sub>Cl<sub>2</sub>) were also included using the PCM model.<sup>104,105</sup> All ground and excited states structures were checked to be true minima on their respective potential energy surfaces. All the computations were performed using the Gaussian16 package.<sup>106</sup>

The simulated ECD and luminescence spectra were obtained including the vibrational contributions to the electronic transitions. For the ECD spectrum, more than 280 calculations were performed on the ground state structure of LL to first get the dipole and velocity strength until 140 excited states (TDDFT with the force keyword to generate the required chk files) and 140 vibronic calculations were performed using the VG (vertical gradient) approach.<sup>107</sup> The luminescence spectrum of the meso conformer was obtained by the very robust AH approach that we already used in other studies.<sup>76,103</sup> In all vibronic computation, we ensured to get a sufficient vibronic progression (>90%). More details concerning the vibronic approach used are available in the literature.<sup>108</sup> All the post-treatments were done using the VMS software and in-house Python 3 programs.<sup>109,110</sup>

## Synthetic procedures

Compound 1 N-butyl-3-aminobenzidine: In a round bottom flask, DMSO was added to a mixture of 4-fluoro-3-nitrotoluene (2 g, 12.9 mmol) and n-butylamine (3.4 mL, 34.8 mmol). The flask was sealed with a rubber stopper and the mixture was heated at 60 °C overnight. The mixture colour turned rapidly from yellow to orange. At r.t., to the orange solution was added 30 mL of ethylacetate and the organic phase was extracted three times with water and washed with brine. The organic layer was dried over MgSO<sub>4</sub> and the product was isolated as a light orange translucent solid (2.2 g, 88%). The product was used without further purification. <sup>1</sup>H NMR (400 MHz, CDCl<sub>3</sub>) δ 8.04 (d, J = 8.8 Hz, 2H), 6.61 (d, J = 1.8 Hz, 2H), 6.43 (dd, J = 8.8, 1.9 Hz, 2H), 3.28 (t, J = 7.1 Hz, 2H), 2.33 (s, 3H), 1.71 (p, J = 7.3 Hz, 2H), 1.48 (h, J = 14.6 Hz, 2H), 0.98 (t, J = 7.3 Hz, 3H). <sup>13</sup>C NMR (101 MHz, CDCl<sub>3</sub>) δ 147.8, 145.8, 130.0, 126.9, 117.0, 113.5, 42.8, 31.1, 22.3, 20.4, 13.9. HRMS (ESI) calcd m/z 209.12845 for [C<sub>11</sub>H<sub>16</sub>N<sub>2</sub>O<sub>2</sub>+H]<sup>+</sup>; found m/z 209.12747.

L<sup>1</sup>: In a round bottom flask, a mixture of methoxyethanol/water (25 mL, 4:1) was added on compound 1 (1.0 g, 4.8 mmol) and 5-bromo-2-pyridinecarboxaldehyde (0.9 g, 4.8 mmol). When solids were dissolved, Na<sub>2</sub>S<sub>2</sub>O<sub>4</sub> (7.7 g, 44.2 mmol) was added and the solution was heated at 100 °C overnight. At r.t., water (30 mL) was added, followed by 30 mL of ethylacetate. The phases were separated and the organic layer was washed 3 times with water, once with brine, dried over Na<sub>2</sub>SO<sub>4</sub>. After solvent evaporation, the compound was obtained as a light beige solid (814 mg, 49%) and used without further purification. <sup>1</sup>H NMR (400 MHz, CDCl<sub>3</sub>) δ 8.70 (d, J = 2.4 Hz, 1H), 8.31 (d, J = 8.5 Hz, 1H), 7.91 (dd, J = 8.5 Hz, J = 2.4 Hz, 1H), 7.69 (d, J = 8.2 Hz, 1H), 7.19 (s, 1H), 7.12 (dd, J = 8.3 Hz, J = 1.8 Hz, 1H), 4.73 (t, J = 7.6 Hz, 2H), 2.52 (s, 3H), 1.89 – 1.77 (m, 2H), 1.37 (h, J = 7.4 Hz, 2H), 0.94 (t, J = 7.4 Hz, 3H). <sup>13</sup>C NMR (101 MHz, CDCl<sub>3</sub>) δ 149.6, 149.3, 148.4, 140.8, 139.4, 137.0, 133.7, 125.7, 124.5, 121.0, 119.7,
110.1, 45.4, 32.3, 22.1, 20.2, 13.8. HRMS (ESI) calcd m/z 344.07437 for [C<sub>17</sub>H<sub>18</sub>BrN<sub>3</sub>+H]<sup>+</sup>; found m/z 344.07569.

**Compound 2:** N,N-dibutyl-3,3'-diaminobenzidine: From an adapted procedure,¹¹¹ to an ice-cold solution of 3,3'-diaminoc-4,4'-dinitrobiphenyl (1 g, 3.65 mmol) in dry DMF (20 mL) was added sodium hydride (60% in oil, 438 mg, 10.95 mmol) in portion over 10 min, followed by 1-bromobutane (1.2 mL, 11.12 mmol). The mixture was warmed at 60 °C during one night and, at room temperature, water was added. The solution was extracted 3 times with ethylacetate and the organic phase was washed once with water and with brine, and finally dried over MgSO₄. The latter was filtered off with cotton and the solvent was evaporated under reduced pressure. A column chromatography (SiO₂) with cyclohexane/ethylacetate as eluent (6:4) gives the title compound as a dark orange solid (620 mg, 44%). ¹H NMR (400 MHz, CDCl₃) δ 8.34 (t, *J* = 2.1 Hz, 2H), 8.09 (t, *J* = 5.3 Hz, 2H), 7.68 (dd, *J* = 9.0, 2.6 Hz, 2H), 6.93 (d, *J* = 9.0 Hz, 2H), 3.35 (td, *J* = 7.2, 5.1 Hz, 4H), 1.74 (p, *J* = 7.4 Hz, 4H), 1.50 (h, *J* = 7.3 Hz, 4H), 1.00 (t, *J* = 7.3 Hz, 6H).¹³C NMR (101 MHz, CDCl₃) δ 144.8, 134.3, 132.0, 126.2, 123.6, 114.8, 43.0, 31.2, 20.4, 13.9. HRMS (ESI) calcd m/z 387.20268 for [C₂₀H₂₆O₄N₄+H]<sup>+</sup>; found m/z 387.20183.

**L²:** To DMF (18 mL) at rt was added N,N-dibutyl-3,3'-diaminobenzidine (1 g, 2.6 mmol), followed by 5-bromo-2-pyridinecarboxaldehyde (967 mg, 5.2 mmol). After complete dissolution upon stirring, solid Na₂S₂O₄ (8.4 g, 48 mmol) was added followed immediately by addition of water (4.5 mL). The reaction mixture was stirred at 100 °C overnight to give a solution containing a pale-yellow suspension. At rt, the reaction was quenched by adding more water and the precipitate was filtered off with a Büchner fitted with Célite®. The solid was heavily washed with water and the top layer of the Célite® cake was scratched off and dissolved/suspended in CH₂Cl₂. The solution was washed 3 times with water, then once with brine and finally the organic phase was dried over NaSO₄ and, after filtration, the solvent was evaporated. Purification was achieved by column chromatography (SiO₂) with a tertiary mixture for eluent (CH₂Cl₂/Acetone/Et₃N, 9.49:0.5:0.01). The product was obtained as a light beige solid (885 mg, 52%). Elemental analysis for C₃₂H₃₀Br₂N₆, calcd. C, 58.36%, H, 4.60%, N, 12.77%; found C, 58.14%, H, 4.54%, N, 12.68%. ¹H NMR (400 MHz, CDCl₃) δ 8.76 (dd, *J* = 2.4 Hz, *J* = 0.8 Hz, 2H, 8.38 (dd, *J* = 8.5 Hz, *J* = 0.8 Hz, 2H), 8.10 (d, *J* = 1.6 Hz, 2H), 7.98 (dd, *J* = 8.5 Hz, *J* = 2.4 Hz, 2H), 7.68 (dd, *J* = 8.5 Hz, *J* = 1.6 Hz, 2H), 7.53 (d, *J* = 8.5 Hz, 2H), 4.84 (t, *J* = 7.5 Hz, 6H), 1.90 (p, *J* = 7.5 Hz, 4H), 1.42 (h, *J* = 7.3 Hz, 7H), 0.97 (t, *J* = 7.3 Hz, 13H).¹³C NMR (101 MHz, CDCl₃) δ 149.9, 149.6, 149.3, 143.5, 139.7, 137.2, 136.3, 126.1, 124.0, 121.4, 118.8, 110.7, 45.8, 32.6, 20.4, 14.0. HRMS (ESI) calcd m/z 659.09537 for [C₃₂H₃₀Br₂N₆+H]<sup>+</sup>; found m/z 659.09361.

**General procedure for synthesis of complexes Rac, Diast, ΔΔ and ΛΛ**

The suitable µ-Cl₂ building block (µ-Cl₂Ir₂, µ-Cl₂ΔΔ or µ-Cl₂ΛΛ) (1 eq.) was suspended in a mixture of CH₂Cl₂/MeOH (1:1) and the ligand (2 eq. in the case of Rac and 1-0.9 eq. for the bimetallic derivatives) was added. The yellow solution became rapidly orange after few minutes and the reaction was

refluxed overnight under Ar. At r.t., the solvent was concentrated to the 1/3rd, an excess of aqueous KPF₆ (sat.) was added drop-by-drop and an orange precipitate appeared rapidly and the suspension was stirred an additional 10 min. The solution was filtrated off with a millipor® apparatus. The solid was washed with water, ice-cold methanol (5 mL) and finally with ice cold diethylether. To the exception of Rac and Diast, the latter being an inseparable mixture of meso, ΔΔ and ΛΛ, the reaction gave a clean solution and the complex was isolated as a dark yellowish to orange-red solid.

**IrL¹:** The µ-Cl₂Ir₂⁵¹,⁵² (50 mg, 0.05 mmol) was suspended in a mixture of CH₂Cl₂/MeOH (10 mL, 1:1) and the ligand L¹ (32 mg, 0.09 mmol) was added. The refluxing yellow solution became rapidly orange after few minutes and the reaction was stirred overnight under Ar. At r.t., the solvent was concentrated to the 1/3rd, an excess of aqueous KPF₆ (sat.) was added drop-by-drop and an orange precipitate appeared rapidly and the suspension was stirred an additional 10 min. The solution was filtrated off with a Millipor® apparatus. The solid was washed with water (10 mL), ice-cold methanol (5 mL) and finally with ice cold diethylether (15 mL). IrL¹ was obtained pure after recrystallization by vapor diffusion of diethylether to a concentrated CH₂Cl₂ solution as a yellow powder. (87 mg, 93%). ¹H NMR (500 MHz, CDCl₃) δ 8.34 (dd, *J* = 8.8 Hz, *J* = 2.2 Hz, 1H), 8.26 (d, *J* = 8.8 Hz, 1H), 8.06 (d, *J* = 2.1 Hz, 1H), 7.98 (d, *J* = 8.2 Hz, 1H), 7.89 (d, *J* = 8.4 Hz, 1H), 7.81 – 7.70 (m, 4H), 7.62 (dd, *J* = 5.7 Hz, *J* = 1.6 Hz, 1H), 7.52 (dd, *J* = 5.7 Hz, *J* = 1.8 Hz, 1H), 7.35 (s, 1H), 7.13-7.09 (m, *J* = 7.6 Hz, *J* = 4.5 Hz, 1.2 Hz, 2H), 7.05 – 6.91 (m, 4H), 6.89 (dd, *J* = 8.7 Hz, *J* = 1.5 Hz, 1H), 6.37 (dd, *J* = 7.7, 1.2 Hz, 1H), 6.33 (dd, *J* = 7.6 Hz, *J* = 1.2 Hz, 1H), 6.23 (d, *J* = 8.6 Hz, 1H), 4.81 – 4.67 (m, 2H), 2.45 (s, 3H), 2.03 – 1.90 (m, *J* = 6.7 Hz, 2H), 1.47 (h, *J* = 7.5 Hz, 2H), 0.99 (t, *J* = 7.3 Hz, 3H).¹³C NMR (126 MHz, CDCl₃) δ 168.2, 168.1, 153.5, 151.0, 150.6, 149.8, 149.0, 146.8, 146.4, 144.5, 144.4, 142.8, 138.6, 138.3, 138.1, 138.0, 137.3, 132.2, 132.0, 131.3, 130.5, 128.0, 125.6, 125.5, 125.0, 124.7, 123.8, 123.6, 123.3, 123.0, 120.3, 119.8, 118.3, 111.4, 46.3, 32.2, 22.2, 20.4, 13.9. Elemental analysis for [C₃₉H₃₄N₅F₆BrPIr(CH₂Cl₂)₀.₅], calcd. C, 45.96%, H, 3.42%, N, 5.98%; found C, 46.05%, H, 3.09%, N, 5.98%. HRMS (ESI) calcd m/z 844.16072 for [C₃₉H₃₄BrIrN₅]⁺; found m/z 844.15908.

**Ir₂L²:** µ-Cl₂Ir₂ (50 mg, 47 mmol), ligand L² (31 mg, 47 mmol) and solvent 10 mL. The Diast mixture was isolated after recrystallisation by vapor diffusion of diethylether to a CH₂Cl₂ solution, as a dark yellowish solid (72 mg, 79%). ¹H NMR (400 MHz, (CD₃)₂CO) δ 8.76 (d, *J* = 8.7 Hz, 2H), 8.57 (d, *J* = 8.9 Hz, 3H), 8.28 (t, 1H, *J* = 8.0 Hz), 8.21 (s, 1H), 8.16 (dd, *J* = 8.2 Hz, 2H), 8.10 – 8.03 (m, 3H), 8.02 – 7.93 (m, 8H), 7.90 (t, *J* = 7.7 Hz, 2H), 7.26 (dd, *J* = 7.4 Hz, 2H), 7.19 – 6.95 (m, 10H), 6.83 (d, *J* = 8.2 Hz, 3H), 6.52 (t, *J* = 6.0 Hz, 2H), 6.41 (d, *J* = 7.7 Hz, 2H), 5.10 (d, *J* = 5.1 Hz, 2H), 1.56-1.40 (m, 4H), 0.98 (t, *J* = 7.4 Hz, 6H). ¹³C NMR (101 MHz, (CD₃)₂CO) δ 172.9, 172.7, 157.8, 157.5, 156.1, 155.3, 155.1, 155.0, 152.3, 151.4, 150.1, 149.6, 147.6, 145.3, 143.9, 143.6, 141.8, 141.5, 137.7, 136.8, 135.7, 135.1, 131.7, 130.7, 130.3, 130.0, 129.4, 128.8, 128.0, 127.8, 125.1, 124.7, 117.8, 51.1, 36.9, 24.8, 18.3.
**Complex ΛΛ: μ-Cl₂ΔΔ** (20 mg, 19 mmol), ligand L² (12 mg, 19 mmol) and solvent 5 mL. The complex ΛΛ was isolated after recrystallisation by vapor diffusion of diethylether to a CH₂Cl₂ solution, as an orange-red solid (21 mg, 58%). [α]D²₀ = + 276 ° (CH₂Cl₂, c = 10⁻⁴ M).¹H NMR (400 MHz, (CD₃)₂CO) δ 8.73 (d, *J* = 8.8 Hz, 2H), 8.54 (d, *J* = 8.6 Hz, 2H), 8.26 (d, *J* = 8.2 Hz, 2H), 8.18 (s, 2H), 8.12 (d, *J* = 8.2 Hz, 2H), 8.02 (d, *J* = 5.8 Hz, 4H), 7.96 (t, *J* = 9.4 Hz, 2H), 7.86 (t, *J* = 7.8 Hz, 2H), 7.23 (t, *J* = 7.5 Hz, 2H), 7.16 – 7.03 (m, 7H), 6.97 (t, *J* = 7.5 Hz, 4H), 6.79 (s, 2H), 6.49 (d, *J* = 7.7 Hz, 2H), 6.38 (d, *J* = 7.5 Hz, 2H), 5.07 (hept, *J* = 8.2 Hz, 3H), 1.46 (qt, *J* = 13.8, 7.1 Hz, 3H), 0.95 (t, *J* = 7.2 Hz, 6H).¹³C NMR (101 MHz, Acetone) δ 168.7, 168.5, 153.8, 153.3, 151.9, 151.3, 150.8, 148.1, 147.2, 146.0, 145.4, 143.4, 141.1, 139.7, 139.4, 137.6, 137.3, 133.5, 132.6, 131.6, 130.9, 127.5, 126.8, 126.1, 125.8, 125.2, 124.6, 124.6, 123.8, 123.6, 121.0, 120.6, 116.7, 113.6, 46.9, 32.7, 20.6, 14.2.

**ΔΔ: μ-Cl₂ΔΔ** (21 mg, 20 mmol), ligand L² (13 mg, 20 mmol) and solvent 5 mL. Complex ΛΛ was isolated after recrystallisation by vapor diffusion of diethylether to a CH₂Cl₂ solution, as an orange-red solid (22 mg, 56%). [α]D²₀ = - 258 ° (CH₂Cl₂, c = 10⁻⁴ M)¹H NMR (400 MHz, (CD₃)₂CO) δ 8.75 (d, *J* = 8.8 Hz, 2H), 8.56 (d, *J* = 8.8 Hz, 2H), 8.29 (d, *J* = 8.3 Hz, 2H), 8.15 (d, *J* = 8.2 Hz, 2H), 8.04 (d, *J* = 5.8 Hz, 4H), 7.98 (dd, *J* = 12.9, 5.7 Hz, 9H), 7.89 (t, *J* = 7.9 Hz, 2H), 7.26 (t, *J* = 7.7 Hz, 2H), 7.18 – 7.05 (m, 8H), 6.99 (t, *J* = 7.3 Hz, 4H), 6.81 (s, 2H), 6.51 (d, *J* = 7.7 Hz, 2H), 6.40 (d, *J* = 7.5 Hz, 2H), 5.09 (q, *J* = 7.2 Hz, 4H), 1.48 (qt, *J* = 14.0, 7.0 Hz, 4H), 0.97 (t, *J* = 7.2 Hz, 6H).¹³C NMR (101 MHz, Acetone) δ 168.6, 168.4, 153.7, 153.2, 151.7, 151.1, 150.7, 148.0, 147.1, 145.9, 145.3, 143.3, 141.0, 139.6, 139.3, 137.5, 137.2, 133.4, 132.5, 131.4, 130.8, 127.4, 126.6, 126.0, 125.7, 125.1, 124.5, 123.7, 123.5, 120.9, 120.5, 116.6, 113.5, 46.8, 32.6, 20.5, 14.0.

## ASSOCIATED CONTENT

**Supporting Information** material is available free of charge via the Internet at http://pubs.acs.org. The reader will find synthesis and chiral resolution of the chiral building blocks μ-Cl₂ΔΔ and μ-Cl₂ΔΔ scheme overview, information about the Structural characterisation, and the NMR spectra.

## Author Contributions

The manuscript was written through contributions of all authors.

## ACKNOWLEDGMENT

This work is supported by the French National Research Agency in the framework of the "Investissements d'avenir" program (ANR-15-IDEX-02) and ANR-PRC (ANR-21-CE29-0015-03). This work has been partially supported by CBH-EUR-GS (ANR-17-EURE-0003). The authors would like to thank Kamilla Mammadzada and Dr. Maria V. Vlasova who participated with enthusiasm to the project during their internships. The NanoBio ICMG (UAR 2607), is acknowledged for providing facilities for mass spectrometry (A. Durand, L. Fort and R. Gueret), elemental analyses (M. Fayolle) and single crystal X-ray diffraction (N. Altounian). D.K. and J.C. thank The European Commission Research Executive Agency (Grant Agreement Number 859752 – HEL4CHIROLED – H2020-MSCA-ITN-2019) for financial support. This work was supported by the computing facilities of the CRCMM, 'Centre

Régional de Compétences en Modélisation Moléculaire de Marseille'.

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<figure><img src="image_8.png" /></figure>

Chiral binuclear Ir(III) complexes (ΛΛ and ΔΔ) are described and they exhibit similar emission properties, emitting in the orange-red region of the spectra with good luminescence quantum yield (λmax = 610-625 nm, Φ ~25%, τ~900 ns) and are optically active. The dimers display a higher brightness compared to the monomer and the Circular Polarized Luminescence brightness (BCPL) of the binuclear complexes is among the highest reported to date for chiral Ir(III) complexes.