Modulation of Chiroptical and Photophysical Properties in Helicenic Rhenium(I) Systems: The Use of an N-(Aza[6]helicenyl)-NHC Ligand.

The photophysical and chiroptical properties of a novel, chiral helicene-NHC-Re(I) complex bearing an N-(aza[6]helicenyl)-benzimidazolylidene ligand are described, showing its ability to emit yellow circularly polarized luminescence. A comparative analysis of this new system with other helicene-Re(I) complexes reported to date illustrates the impact of structural modifications on the emissive and absorptive properties.

Platinum(IV) Complexes with Tridentate, NNC-Coordinating Ligands: Synthesis, Structures, and Luminescence.

Platinum(II) complexes of NNC-cyclometalating ligands based on 6-phenyl-2,2'-bipyridine (HL1) have been widely investigated for their luminescence properties. We describe how PtL1Cl and five analogues with differently substituted aryl rings, PtL2-6Cl, can be oxidized with chlorine and/or iodobenzene dichloride to generate Pt(IV) compounds of the form Pt(NNC-Ln)Cl3 (n = 1-6). The molecular structures of several of them have been determined by X-ray diffraction. These PtLnCl3 compounds react with 2-arylpyridines to give a new class of Pt(IV) complex of the form [Pt(NNC)(NC)Cl]+. Elevated temperatures are required, and the reaction is accompanied by competitive reduction processes and generation of side-products; however, four examples of such complexes have been isolated and their molecular structures determined. Reaction of PtL1Cl3 with HL1 similarly generates [Pt(NNC-L1)2]2+, which we believe to be the first example of a bis-tridentate Pt(IV) complex. The lowest-energy bands in the UV-vis absorption spectra of all the PtLnCl3 compounds are displaced to higher energy relative to the Pt(II) precursors, but they red-shift with the electron richness of the aryl ring, consistent with predominantly 1[πAr → π*NN] character to the pertinent excited state. A similar trend is observed for the [Pt(NNC)(NC)Cl]+ complexes. They display phosphorescence in solution at room temperature, centered around 500 nm for [PtL1(ppy)Cl]+ and [Pt(L1)2]2+, and 550 nm for methoxy-substituted derivatives. The lifetimes are in the microsecond range, rising to hundreds of microseconds at 77 K, consistent with triplet excited states of primarily 3[πAr → π*NN] character with relatively little participation of the metal.

Platinum(II) Complexes of Nonsymmetrical NCN-Coordinating Ligands: Unimolecular and Excimeric Luminescence Properties and Comparison with Symmetrical Analogues.

A series of seven new platinum(II) complexes PtLnCl have been prepared, where Ln is an NCN-coordinating ligand comprising a benzene ring 1,3-disubstituted with two different azaheterocycles. In PtL1-5Cl, one heterocycle is a simple pyridine ring, while the other is an isoquinoline, a quinoline, a pyrimidine (L1, L2, L3), or a p-CF3- or p-OMe-substituted pyridine (L4 and L5). PtL6Cl incorporates both a p-CF3 and a p-OMe-substituted pyridine. The synthesis of the requisite proligands HLn is achieved using Pd-catalyzed cross-coupling methodology. The molecular structures of six of the Pt(II) complexes have been determined by X-ray diffraction. All the complexes are brightly luminescent in deoxygenated solution at room temperature. The absorption and emission properties are compared with those of the corresponding symmetrical complexes featuring two identical heterocycles, PtLnsymCl, and of the parent Pt(dpyb)Cl containing two unsubstituted pyridines [dpybH = 1,3-di(2-pyridyl)benzene]. While the absorption spectra of the nonsymmetrical complexes show features of both PtLnsymCl and Pt(dpyb)Cl, the emission generally resembles that of whichever of the corresponding symmetrical complexes has the lower-energy emission. PtL1Cl differs in that─at room temperature but not at 77 K─it displays emission bands that can be attributed to excited states involving both the pyridine and the isoquinoline rings, despite the latter being unequivocally lower in energy. This unusual behavior is attributed to thermally activated repopulation of the former excited state from the latter, facilitated by the very long-lived nature of the isoquinoline-based excited state. At elevated concentrations, all the complexes show an additional red-shifted emission band attributable to excimers. For PtL1Cl, the excimer strikingly dominates the emission spectra at all but the lowest concentrations (<10-5 M). Trends in the energies of the excimers and their propensity to form are compared with those of the symmetrical analogues.

Insights into the Antibacterial Mechanism of Action of Chelating Agents by Selective Deprivation of Iron, Manganese, and Zinc.

Bacterial growth and proliferation can be restricted by limiting the availability of metal ions in their environment. Humans sequester iron, manganese, and zinc to help prevent infection by pathogens, a system termed nutritional immunity. Commercially used chelants have high binding affinities with a variety of metal ions, which may lead to antibacterial properties that mimic these innate immune processes. However, the modes of action of many of these chelating agents in bacterial growth inhibition and their selectivity in metal deprivation in cellulo remain ill-defined. We address this shortcoming by examining the effect of 11 chelators on Escherichia coli growth and their impact on the cellular concentration of five metals. The following four distinct effects were uncovered: (i) no apparent alteration in metal composition, (ii) depletion of manganese alongside reductions in iron and zinc levels, (iii) reduced zinc levels with a modest reduction in manganese, and (iv) reduced iron levels coupled with elevated manganese. These effects do not correlate with the absolute known chelant metal ion affinities in solution; however, for at least five chelators for which key data are available, they can be explained by differences in the relative affinity of chelants for each metal ion. The results reveal significant insights into the mechanism of growth inhibition by chelants, highlighting their potential as antibacterials and as tools to probe how bacteria tolerate selective metal deprivation. IMPORTANCE Chelating agents are widely used in industry and consumer goods to control metal availability, with bacterial growth restriction as a secondary benefit for preservation. However, the antibacterial mechanism of action of chelants is largely unknown, particularly with respect to the impact on cellular metal concentrations. The work presented here uncovers distinct metal starvation effects imposed by different chelants on the model Gram-negative bacterium Escherichia coli. The chelators were studied both individually and in pairs, with the majority producing synergistic effects in combinations that maximize antibacterial hostility. The judicious selection of chelants based on contrasting cellular effects should enable reductions in the quantities of chelant required in numerous commercial products and presents opportunities to replace problematic chemistries with biodegradable alternatives.

Donor-Acceptor Boron-Ketoiminate Complexes with Pendent N-Heterocyclic Arms: Switched-on Luminescence through N-Heterocycle Methylation.

A series of intramolecular, donor-stabilized BF2 complexes supported by phenanthridinyl-decorated, ß-ketoiminate chelating ligand scaffolds is described, along with their characterization by spectroscopy and X-ray diffraction. In solution, the relative orientation of the pendent phenanthridinyl arm is fixed despite not coordinating to the boron center, and a well-resolved through-space interaction between a phenanthridinyl C-H and a single fluorine atom can be observed by 19F-1H NOE NMR spectroscopy. The neutral compounds are nonetheless only weakly luminescent in fluid solution, ascribed to nonradiative decay pathways enabled by rotation of the N-heterocyclic unit. Methylation of the phenanthridinyl nitrogen restricts this rotation, "switching on" comparably strong emission in solution. Modeling by density functional theory (DFT) and time-dependent DFT (TDDFT) indicates that the character of the lowest energy excitation changes upon methylation, with shallow calculated potential energy surfaces of the neutral complexes consistent with their lack of significant radiative decay.

Yellow-Emitting, Pseudo-Octahedral Zinc Complexes of Benzannulated N

A series of yellow-emitting, pseudo-octahedral Zn(II) complexes supported by monoanionic, tridentate acetylacetone-derived N^N-^O ligands incorporating phenanthridine (benzo[c]quinoline) units is presented. These species emit weakly in solution but exhibit extended millisecond luminescence lifetimes in the solid state at room temperature, and in a frozen glass at 77 K, indicative of phosphorescence from low-lying triplet excited states. Excitation spectra indicate a role for aggregation in enhancing emission in the solid state. In contrast to four-coordinate phenanthridinyl amide-supported tetradentate Zn(II) complexes which are nonemissive in fluid solution, solid-state X-ray crystallographic structures, solution IR spectroscopy, and computational analysis all indicate a delocalized character for the central deprotonated NH which tempers the amido character of the ligand. This design provides a mechanism for "turning on" long-lived luminescence from N-heterocycle/amido-supported Zn(II) coordination compounds.

Synthesis and Coordination Chemistry of a Benzannulated Bipyridine: 6,6'-Biphenanthridine.

The synthesis, characterization, and coordination chemistry of a doubly π-extended bipyridine analogue, 6,6'-biphenanthridine (biphe), is presented. The structure of the molecule has been determined in the solid state by X-ray diffraction, showing an angle of 72.6° between the phenanthridine planes. The free, uncoordinated organic molecule displays blue fluorescence in solution. It can be singly protonated with strong acids, and the protonated form displays more intense yellow emission. The effect of acid on the excited states is interpreted with the aid of TDDFT calculations. Two Ru(II) coordination complexes, tris(6,6'-biphenanthridine)ruthenium(II) dichloride, [Ru(biphe)3]Cl2, and bis(2,2'-bipyridine)(6,6'-biphenanthridine)ruthenium(II) tetraphenylborate, [Ru(bpy)2(biphe)](BPh4)2, are also reported and their structures determined in the solid state by X-ray diffraction. Both complexes display emission at 77 K that is strongly bathochromically shifted by almost 200 nm compared to that of the archetypal 3MLCT emitter [Ru(bpy)3]2+. Such a red shift is consistent with the more extended conjugation and lower-energy π* orbitals associated with the biphe ligand, lowering the energy of the 3MLCT excited state, as revealed by TDDFT calculations. The efficient non-radiative decay that is typical of such low-energy emitters renders the phosphorescence extremely weak and short-lived at ambient temperature, and rapid ligand photodissociation also competes with radiative decay, especially in the heteroleptic complex. Electrochemical analysis illustrates the effect of biphe's stabilized vacant π* manifold, with multiple reversible reductions evident at much less negative potentials than those observed for [Ru(bpy)3]2+.

Brightly Luminescent Platinum Complexes of N∧C-∧N Ligands Forming Six-Membered Chelate Rings: Offsetting Deleterious Ring Size Effects Using Site-Selective Benzannulation.

Brightly emissive platinum(II) complexes (λemission,max = 607-612 nm) of the type RLPtCl are reported, where RL is a cyclometalated N∧C-∧N-coordinating ligand derived from 1,3-di(2-trifluoromethyl-4-phenanthridinyl)benzene (CF3LH) or 1,3-di(2-tert-butyl-4-phenanthridinyl)benzene (tBuLH). Metathesis of the chlorido ligand can be achieved under mild conditions, enabling isolation of ionic compounds with the formula [CF3LPtL']PF6 where L' = pyridine or (4-dimethylamino)pyridine (DMAP), as well as the charge-neutral species tBuLPt(C≡C─C6H4─tBu) (C≡C─C6H4─tBu = 4-tert-butylphenylacetylido). Compared with N∧N∧N-ligated Pt(II) complexes that form 5-membered chelates, these compounds all contain 6-membered rings. Expanding the chelate ring size from 5 to 6 has been previously demonstrated to enhance emission in some N∧N∧N-coordinated Pt(II) species─for example, in complexes of 2,6-di(8-quinolinyl)pyridine vs those of 2,2':6',2″-terpyridine─but in related N∧C-∧N-coordinated species, luminescence quantum yields are significantly lower for the 6-membered chelate ring complexes. Here, we demonstrate that site-selective benzannulation of the quinolinyl side-arms can offset the deleterious effect of changing the chelate ring-size and boost photophysical properties such as the quantum yield. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations suggest that benzannulation counterintuitively destabilizes the emissive triplet states compared to the smaller π-system, with the "imine-bridged biphenyl" form of the phenanthridinyl arm helping to buffer against larger molecular distortions, enhancing photoluminescence quantum yields up to 0.09 ± 0.02. The spontaneous formation under aerated conditions of a Pt(IV) derivative (CF3LPtCl3) is also reported, together with its molecular structure in the solid state.

Enantioenriched Ruthenium-Tris-Bipyridine Complexes Bearing One Helical Bipyridine Ligand: Access to Fused Multihelicenic Systems and Chiroptical Redox Switches.

The synthesis and photophysical and chiroptical properties of novel aza[n]helicenes (6a-d, 10a,b, n = 4-7) substituted with one or two 2-pyridyl groups are described. The preparation was performed via an adapted Mallory reaction using aromatic imines as precursors. The obtained novel class of helical 2,2'-bipyridine ligands was then coordinated to Ru(bipy)22+ units, thus affording the first diastereomerically and enantiomerically pure [RuL(bipy)2]2+ (11a,c, L = 6a,c) or [Ru2L'(bipy)4]4+ (12, L' = 10b) complexes. The topology and stereochemistry of these novel metal-based helical architectures were studied in detail, notably using X-ray crystallography. Interestingly, the coordination to ruthenium(II) enabled the preparation of fused multihelical systems incorporating aza- and ruthena-helicenes within the same scaffold. The photophysical, chiroptical, and redox properties of these complexes were examined in detail, and efficient redox-triggered chiroptical switching activity was evidenced.

Deep-Red Luminescence from Platinum(II) Complexes of N

A synthetic methodology for accessing narrow-band, deep-red phosphorescence from mononuclear Pt(II) complexes is presented. These charge-neutral complexes have the general structure (N^N-^N)PtCl, in which the Pt(II) centers are supported by benzannulated diarylamido ligand scaffolds bearing substituted quinolinyl and/or phenanthridinyl arms. Emission maxima ranging from 683 to 745 nm are observed, with lifetimes spanning from 850 to 4500 ns. In contrast to the corresponding proligands, benzannulation is found to counterintuitively but markedly blue-shift emission from metal complexes with differing degrees of ligand benzannulation but similar substitution patterns. This effect can be further tuned by incorporation of electron-releasing (Me, tBu) or electron-withdrawing (CF3) substituents in either the phenanthridine 2-position or quinoline 6-position. Compared with symmetric bis(quinoline) and bis(phenanthridine) architectures, "mixed" ligands incorporating one quinoline and one phenanthridine unit present a degree of charge transfer between the N-heterocyclic arms that is more pronounced in the proligands than in the Pt(II) complexes. The impact of benzannulation and ring-substitution on the structure and photophysical properties of both the proligands and their deep-red emitting Pt(II) complexes is discussed.

Long-Lived Circularly Polarized Phosphorescence in Helicene-NHC Rhenium(I) Complexes: The Influence of Helicene, Halogen, and Stereochemistry on Emission Properties.

The first enantiopure chiral-at-rhenium complexes of the form fac-ReX(CO)3 (:C^N) have been prepared, where :C^N is a helicene-N-heterocyclic carbene (NHC) ligand and X=Cl or I. These have complexes show strong changes in the emission characteristics, notably strongly enhanced phosphorescence lifetimes (reaching 0.7 ms) and increased circularly polarized emission (CPL) activity, as compared to their parent chiral models lacking the helicene unit. The halogen along with its position within the dissymmetric stereochemical environment strongly affect the photophysics of the complexes, particularly the phosphorescence quantum yield and lifetime. These results give fresh insight into fine tuning of photophysical and chiroptical properties of Re-NHC systems.

A Highly Luminescent Tetrahydrocurcumin IrIII Complex with Remarkable Photoactivated Anticancer Activity.

Curcumin has chemopreventative properties against a variety of tumours, but has poor bioavailability. Here, two new bis-cyclometallated iridium(III) complexes have been prepared, featuring the natural product curcumin (CUR) or its reduced form, tetrahydrocurcumin (THC), as bidentate, anionic O O-binding ligands. The iridium THC complex is highly luminescent in deoxygenated solution and efficiently generates singlet oxygen under aerated conditions, whereas in the CUR analogue, other non-radiative decay pathways are competitive. The complexes are rapidly taken up by a variety of human tumour cell lines from solutions of micromolar concentration. They show negligible cytotoxicity in the absence of irradiation. When briefly irradiated with visible light, Ir-THC becomes highly phototoxic, inducing rapid apoptosis within 2 h. The results show the high potential of such complexes as sensitizers in photodynamic therapy (PDT).

Luminescent Platinum(II) Complexes of N

A platform for investigating the impact of π-extension in benzannulated, anionic pincer-type N^N-^N-coordinating amido ligands and their Pt(II) complexes is presented. Based on bis(8-quinolinyl)amine, symmetric and asymmetric proligands bearing quinoline or π-extended phenanthridine (3,4-benzoquinoline) units are reported, along with their red-emitting, phosphorescent Pt(II) complexes of the form (N^N-^N)PtCl. Comparing the photophysical properties of complexes of (quinolinyl)amido ligands with those of π-extended (phenanthridinyl)amido analogues revealed a counterintuitive impact of site-selective benzannulation. Contrary to conventional assumptions regarding π-extension, and in contrast to isoenergetic lowest energy absorption bands and a red shift in fluorescence from the organic proligands, a blue shift of nearly 40 nm in the emission wavelength is observed for Pt(II) complexes with more extended bis(phenanthridinyl) ligand π-systems. Comparing the ground state and triplet excited state structures optimized from density functional theory (DFT) and time-dependent-DFT calculations, we trace this effect to a greater rigidity of the benzannulated complexes, resulting in a higher energy emissive triplet state, rather than to a significant perturbation of orbital energies caused by π-extension.

Tuning Mg(II) Selectivity: Comparative Analysis of the Photophysical Properties of Four Fluorescent Probes with an Alkynyl-Naphthalene Fluorophore.

Four alkynyl-1-naphthyl fluorophores have been synthesised with tri- or pentadentate ligating groups suited to the binding of magnesium. Their photophysical and binding properties for magnesium, calcium and zinc ions have been assessed using absorption, emission and excitation spectroscopy. Each compound has a pKa value between 6.2 and 5.1 and exhibits no significant pH response around pH 7.2. Enhanced selectivity for Mg2+ over Ca2+ was observed with a pentadentate phosphinate substituted system, compared to its carboxylate analogue, due to a 10-fold lowering of affinity for the Ca2+ ion. In each case, the overall dissociation constants for Mg2+ fall in the low mm range.

APTRA-Based Luminescent Lanthanide Complexes Displaying Enhanced Selectivity for Mg2.

A series of three europium(III) complexes has been created in which an APTRA moiety has been integrated into the sensitising chromophore (APTRA=o-aminophenol-N,N,N-triacetate). The constitutionally isomeric complexes EuL1 and EuL2 feature the APTRA unit linked to a metal-bound pyridine ring through an alkynyl unit, differing according to the disposition of the APTRA substituents relative to the C≡C unit (para-N and para-O). In EuL3 , the APTRA ring is directly bonded to the Eu-coordinated pyridine (para-O). The metal binding affinities for magnesium, calcium and zinc ions have been measured by using emission and excitation spectroscopy. The pyridylalkynylaryl systems, EuL1 and EuL2 , offer superior affinity and selectivity for Mg2+ . The Mg2+ affinities are surprisingly very different from prior studies on structurally related systems that incorporate organic fluorophores as reporters, as opposed to the macrocyclic Eu complex moiety. A much-reduced affinity for calcium and zinc-possibly arising from the lower donor ability of the aryl N or O atoms arising from extended conjugation-means that magnesium ion concentrations can be measured directly in serum for the first time, by using such an approach. An apparent dissociation constant for magnesium binding of Kd =2.4 mm was calculated in the serum background.

Synthesis, Mesomorphism, and Photophysics of 2,5-Bis(dodecyloxyphenyl)pyridine Complexes of Platinum(IV).

It has been shown for the first time that the PtIV complex cis-[Pt(N^C-tolpy)2 Cl2 ] (tolpy=2-(4-tolyl)pyridinyl) can be prepared in a one-pot reaction from K2 [PtCl4 ], although analogous complexes containing 2,5-bis(4-dodecyloxyphenyl)pyridine (=HL) could be prepared using existing routes. The resulting complexes cis-[Pt(N^C-L)2 Cl2 ] are liquid crystals and small-angle X-ray scattering suggests formation of a lamellar mesophase. Surprisingly, heating [Pt(κ2 -N^C-L)2 Cl(κ1 -N^C-LH)] also leads to a mesomorphic compound, which results from thermally induced oxidation to cis-[Pt(N^C-L)2 Cl2 ] and what is presumed to be another geometric isomer of the same formula. The PtIV complexes are quite strongly luminescent in deoxygenated solution, with φ≈10 % and show vibrationally structured emission spectra, λmax (0,0)=532 nm, strongly displaced to the red compared to cis-[Pt(N^C-tolpy)Cl2 ]. Long luminescence lifetimes of 230 µs are attributed to a lower degree of metal character in the excited state accompanying the extension of conjugation in the ligand. There is no significant difference between the emission properties of the bromo- and chloro-complexes, in contrast with the known complexes cis-[Pt(N^C-ppy)X2 ], where the quantum yield for X=Br is some 30 times lower than for X=Cl (ppyH=2-phenylpyridine). The lower energy of the excited state in the new complexes probably ensures that deactivating LLCT/LMCT states remain thermally inaccessible, even when X=Br.

Site-Selective Benzannulation of N-Heterocycles in Bidentate Ligands Leads to Blue-Shifted Emission from [( P

Benzannulated bidentate pyridine/phosphine ( P^N) ligands bearing quinoline or phenanthridine (3,4-benzoquinoline) units have been prepared, along with their halide-bridged, dimeric Cu(I) complexes of the form [( P^N)Cu]2(µ-X)2. The copper complexes are phosphorescent in the orange-red region of the spectrum in the solid-state under ambient conditions. Structural characterization in solution and the solid-state reveals a flexible conformational landscape, with both diamond-like and butterfly motifs available to the Cu2X2 cores. Comparing the photophysical properties of complexes of (quinolinyl)phosphine ligands with those of π-extended (phenanthridinyl)phosphines has revealed a counterintuitive impact of site-selective benzannulation. Contrary to conventional assumptions regarding π-extension and a bathochromic shift in the lowest energy absorption maxima, a blue shift of nearly 40 nm in the emission wavelength is observed for the complexes with larger ligand π-systems, which is assigned as phosphorescence on the basis of emission energies and lifetimes. Comparison of the ground-state and triplet excited state structures optimized from DFT and TD-DFT calculations allows attribution of this effect to a greater rigidity for the benzannulated complexes resulting in a higher energy emissive triplet state, rather than significant perturbation of orbital energies. This study reveals that ligand structure can impact photophysical properties for emissive molecules by influencing their structural rigidity, in addition to their electronic structure.

Metal Complexes for Two-Photon Photodynamic Therapy: A Cyclometallated Iridium Complex Induces Two-Photon Photosensitization of Cancer Cells under Near-IR Light.

Photodynamic therapy (PDT) uses photosensitizers (PS) which only become cytotoxic upon light-irradiation. Transition-metal complexes are highly promising PS due to long excited-state lifetimes, and high photo-stabilities. However, these complexes usually absorb higher-energy UV/Vis light, whereas the optimal tissue transparency is in the lower-energy NIR region. Two-photon excitation (TPE) can overcome this dichotomy, with simultaneous absorption of two lower-energy NIR-photons populating the same PS-active excited state as one higher-energy photon. We introduce two low-molecular weight, long-lived and photo-stable iridium complexes of the [Ir(N^C)2 (N^N)]+ family with high TP-absorption, which localise to mitochondria and lysosomal structures in live cells. The compounds are efficient PS under 1-photon irradiation (405 nm) resulting in apoptotic cell death in diverse cancer cell lines at low light doses (3.6 J cm-2 ), low concentrations, and photo-indexes greater than 555. Remarkably 1 also displays high PS activity killing cancer cells under NIR two-photon excitation (760 nm), which along with its photo-stability indicates potential future clinical application.

Synthesis and Chiroptical Properties of Hexa-, Octa-, and Deca-azaborahelicenes: Influence of Helicene Size and of the Number of Boron Atoms.

Four members of a new class of cycloborylated hexa-, octa-, and deca-helicenes (1 a-d) have been prepared in enantiopure form, along with two cycloplatinated deca-helicenes (1 d', 1 d1 ), further extending the family of cycloplatinated hexa- and octa-helicenes reported previously. The azabora[n]helicenes display intense electronic circular dichroism and large optical rotations; the dependence of the optical activity on the size of the helix (n=6, 8, 10) and the number of boron atoms (1 or 2) has been examined in detail both experimentally and theoretically. The photophysical properties (nonpolarized and circularly polarized luminescence) of these new fluorescent organic helicenes have been measured and compared with the corresponding organometallic phosphorescent cycloplatinated derivatives (1 a1 -d1 ).

Enantiopure Cycloiridiated Complexes Bearing a Pentahelicenic N-Heterocyclic Carbene and Displaying Long-Lived Circularly Polarized Phosphorescence.

A fused π-helical N-heterocyclic carbene (NHC) system was prepared and examined through its diastereoisomerically pure cycloiridiated complexes. The latter display light-green phosphorescence with unusually long lifetimes and circular polarization that depends on both the helical NHC P/M stereochemistry and the iridium Δ/Λ stereochemistry. These unprecedented features are attributed to extended π conjugation within the helical carbenic ligand and efficient helicene-NHC-Ir interaction.