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

We report the synthesis and characterization of two chiral binuclear iridium(III) complexes (ΛΛ and ΔΔ) prepared from enantiopure building blocks [μ-Cl2(Δ-Ir(C^N)2)2] and [μ-Cl2(Λ-Ir(C^N)2)2]. These building blocks have been obtained by chiral preparative high-performance liquid chromatography of the neutral iridium(III) complex Irpiv (piv = 2,2,6,6-tetramethylheptane-3,5-dionate) followed by selective degradation of the ancillary ligand. For comparison purposes, we also synthesized a monomer (IrL1) and a dimer (Ir2L2, mixture). All the complexes exhibit similar emission properties, emitting in the orange-red region of the spectra with a good photoluminescence quantum yield (λmax = 610-625 nm, Φ ∼ 25%, τ ∼ 800-900 ns). However, the ΛΛ and ΔΔ complexes are optically active, indicating that no isomerization occurred during the different synthetic steps, as evidenced by both the circular dichroism spectra and their circularly polarized luminescence (CPL). The capital gain of the dimers (Ir2L2, ΛΛ, and ΔΔ) is a 4-fold brightness (B380 = ε380 nm × Φ) compared to the monomer (IrL1) and the CPL brightness (BCPL = B380 × glum/2) of the binuclear complexes being among the highest reported to date for chiral iridium(III) complexes.

Chiral Organometallic Complexes Derived from Helicenic N-Heterocyclic Carbenes (NHCs): Design, Structural Diversity, and Chiroptical and Photophysical Properties

ConspectusRecently, helicene derivatives have emerged as an important class of molecules with potential applications spanning over asymmetric catalysis, biological activity, magnetism, spin filtering, solar cells, and polymer science. To harness their full potential, especially as emissive components in circularly polarized organic light-emitting diodes (CP-OLEDs), generating structural chemical diversity and understanding the resulting photophysical and chiroptical properties are crucial. In this Account, we shed light on chemical engineering combining helicene and N-heterocyclic carbene (NHC) chemistries to create transition-metal complexes with unique architectures and describe their photophysical and chiroptical attributes. The σ-donating and π-accepting capabilities of the helically chiral π-conjugated NHCs endow the complexes with remarkable structural and electronic features. These characteristics manifest in phenomena such as chirality induction, very long-lived phosphorescence, and strong chiroptical signatures (electronic circular dichroism and circularly polarized luminescence).We describe the different classes of ligands primarily developed in our group by classifying them according to their connection between the helicenic moiety and the imidazole precursor. This connection is essential in determining the degree of π-conjugation and characterizing the emissive state. We comprehensively discuss 6-coordinate, 4-coordinate, and 2-coordinate complexes, delving into their structural nuances and examining how the interplay between metals and auxiliary ligands shapes their photophysical properties, with interpretations enriched by DFT calculations. Helicenes are known to promote intersystem crossing thanks to strong spin-orbit coupling, while metals offer robust frameworks leading to a variety of molecular architectures with specific topologies together with distinct excited-state properties. The electronic configurations and energy levels of the ligand and metal orbitals thus significantly modulate the photophysical and chiroptical behaviors of these complexes. In-depth analysis of chiroptical properties, notably electronic circular dichroism and circularly polarized luminescence, emphasizes the influence of different stereogenic elements on the chiroptical responses across various energy ranges with appealing "match-mismatch" effects. Finally, we describe future prospects of helicene NHCs, particularly in the context of emerging research on cost-effective and abundant transition metals for materials science and for photocatalysis. Indeed, the inherent long-lived MLCT, excited-state delocalization, structural rigidity, and intrinsic chirality of these complexes present intriguing avenues for future investigations.

Enantiopure [6]-Azairidahelicene by Dynamic Kinetic Resolution of a Configurationally Labile [4]-Helicene

A pair of enantiopure [6]-azairidahelicenes incorporating chirality at the metal center and on the helicenic ligand were synthesized by dynamic kinetic resolution (dkr) of a configurationally labile [4]-helicenic ligand (4-(2-pyridyl)-benzo[g]phenanthrene, L1H) using bis-cyclometalated chiral-at-metal only iridium(III) precursors as chiral inductors. The origin of the observed dkr is attributed to the different conformation and stability of diastereomeric reaction intermediates formed during the cyclometalation process. The isolated enantiomers exhibited circularly polarized phosphorescence (CPP), with |gphos| values of 1.8×10-3.

Circularly polarized phosphorescence with a large dissymmetry factor from a helical platinum(II) complex

A chiral platinum(II) complex with a helical Schiff-base [4]helicene ligand exhibits intense red circularly polarized phosphorescence (CPP) with a glum of 0.010 in the dilute solution state. The intense CPP was caused by a change in the electronic transition character based on the induction of the helical structure.

Enantiopure cycloplatinated pentahelicenic N-heterocyclic carbenic complexes that display long-lived circularly polarized phosphorescence

The preparation of the first enantiopure cycloplatinated complexes bearing a bidentate, helicenic N-heterocyclic carbene and a diketonate ancillary ligand is presented, along with their structural and spectroscopic characterization based on both experimental and computational studies. The systems exhibit long-lived circularly polarized phosphorescence in solution and in doped films at room temperature, and also in a frozen glass at 77 K, with dissymmetry factor g_lum values ≥10^-3 in the former and around 10^-2 in the latter.

The acid-mediated isomerization of iridium(III) complexes with cyclometalated NHC ligands: kinetic vs. thermodynamic control

The isomerization of iridium(III) complexes with metalated N-heterocyclic carbene (NHC) ligands was studied. The fac isomers of complexes with 1-phenyl-3-methylbenzimidazolin-2-ylidene or 1-phenyl-3-benzylbenzimidazolin-2-ylidene ligands are transformed cleanly into the mer isomers when solutions of the complexes are treated with first HNTf₂ and then NEt₃. The transformation can be accomplished within a few minutes and the side product (NEt₃H)(NTf₂) is easy to separate. Spectroscopic and structural analyses indicate that the isomerization proceeds by protonation of the carbene ligand at the metalated phenyl group, accompanied by a fac → mer rearrangement of the carbene donors. An iridium complex with a 1-phenyl-1,2,4-triazolo[4,3-f]phenanthridine-based carbene ligand could not be isomerized under similar conditions, most likely because of its reduced conformational flexibility.

Helicenic N-heterocyclic carbene copper(I) complex displaying circularly polarized blue fluorescence

Enantiopure copper(I) chloride complexes bearing a monodentate N-(carbo[6]helicenyl)-NHC ligand have been prepared and characterized experimentally and computationally. Their high stability enables the stereochemistry to be probed by X-ray crystallography and NMR spectroscopy. The resolved enantiomeric complexes emit circularly polarized blue fluorescence with g_lum ∼1.3 × 10^-3 in solution. The photophysical and chiroptical properties of these systems, with their helicene-centred origin, are similar to those of the organic helicene-benzimidazole precursor proligand, although the reverse axial chirality configuration is preferentially observed for the complex compared to the ligand.

Optical Activity of Spin-Forbidden Electronic Transitions in Metal Complexes from Time-Dependent Density Functional Theory with Spin-Orbit Coupling

The calculation of magnetic transition dipole moments and rotatory strengths was implemented at the zeroth-order regular approximation (ZORA) two-component relativistic time-dependent density functional theory (TDDFT) level. The circular dichroism of the spin-forbidden ligand-field transitions of tris(ethylenediamine)cobalt(III) computed in this way agrees very well with available measurements. Phosphorescence dissymmetry factors g lum and the corresponding lifetimes are evaluated for three N-heterocyclic-carbene-based iridium complexes, two of which contain helicene moieties, and for two platinahelicenes. The agreement with experimental data is satisfactory. The calculations reproduce the signs and order of magnitude of g lum , and the large variations of phosphorescence lifetimes among the systems. The electron spin contribution to the magnetic transition dipole moment is shown to be important in all of the computations.

Helical Chiral N-Heterocyclic Carbene Ligands in Enantioselective Gold Catalysis

The first chiral helicene-NHC gold(I) complexes efficient in enantioselective catalysis were prepared. The L-shaped chiral ligand is composed of an imidazo[1,5-a]pyridin-3-ylidene (IPy) scaffold laterally substituted by a configurationally stable [5]-helicenoid unit. The chiral information was introduced in a key post-functionalization step of a NHC-gold(I) complex bearing a symmetrical anionic fluoreno[5]helicene substituent, leading to a racemic mixture of complexes featuring three correlated elements of chirality, namely central, axial and helical chirality. After HPLC enantiomeric resolution, X-ray crystallography and theoretical calculations enabled structural and stereochemical characterization of these configurationally stable NHC-gold(I) complexes. The high potential in asymmetric catalysis is demonstrated in the benchmark cycloisomerization of N-tethered 1,6-enynes with up to 95 : 5 er.

Enantiopure, luminescent, cyclometalated Ir(III) complexes with N-heterocyclic carbene-naphthalimide chromophore: design, vibrational circular dichroism and TD-DFT calculations

A series of chiral cyclometalated iridium complexes of the type [Ir(C^N)₂(C^C:)], {(C^N) = ppy (2); dfppy (3)} featuring a naphthalimide N-heterocyclic carbene ligand (C^C:) = (Naphthalimide-NHC) are described and fully characterized. The racemic complexes rac-2 and rac-3 were resolved via chiral column chromatography techniques into their corresponding enantiopure species Δ-2, Λ-2, Δ-3, Λ-3 as confirmed by their CD curves. This unique class of molecules containing organic and inorganic chromophores might be used as a platform to probe the stereochemical effect on the photophysical properties. Vibrational circular dichroism (VCD) was used as an important tool to assign successfully the stereochemistry of the enantiomers. TD-DFT calculations are also advanced to support the experimental studies and to rationalize the observed results.