Hani Amouri #
email: hani.amouri@sorbonne-universite.fr.

Luminescent Complexes of Platinum, Iridium and Coinage Metals Containing N-Heterocyclic Carbene Ligands: Design, Structural Diversity, and Photophysical Properties

Abstract:

The employment of N-heterocyclic carbenes (NHCs) to design luminescent metal compounds have been the focus of recent intense investigations because of their strong σ-donor properties, which bring stability to the whole system and tend to push the d-d dark states so high in energy that it renders them thermally inaccessible, thereby generating highly emissive complexes for useful applications such as organic light-emitting diodes (OLEDs), or featuring chiroptical properties, a field which is still in its infancy. Among the NHC-complexes, those containing organic chromophores such as naphthalimide, pyrene and carbazole exhibit rich emission behavior, and thus have attracted extensive interest in the past five years especially carbene coinage metal complexes with carbazolate ligands. In this review, the design strategies of NHC-based luminescent platinum and iridium complexes with large spin-orbit-coupling (SOC) are described first. Subsequent paragraphs illustrate the recent advances of luminescent coinage metal complexes with nucleophilic and electrophilic based carbenes, based on silver, gold and copper metal complexes which have the ability to display rich excited state emissions and in particular via Thermally Activated Delayed Fluorescence (TADF). The luminescence mechanism and excited state dynamics are also described. We then summarize the advance of NHC metal complexes in the aforementioned fields in recent years. Finally, we propose the development trend of this fast-growing field of luminescent NHC metal complexes.

1-Introduction, purpose and scope

In the past two decades, a great deal of interest has been devoted to the design of luminescent complexes containing N-heterocyclic carbene ligands. [START_REF] Strassner | Phosphorescent Platinum(II) Complexes with C^C* Cyclometalated NHC Ligands[END_REF][START_REF] Elie | N-Heterocyclic carbene transition metal complexes in light emitting devices[END_REF][START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF][START_REF] Bonfiglio | Phosphorescent Tris-Bidentate IrIII Complexes with N-Heterocyclic Carbene Scaffolds: Structural Diversity and Optical Properties[END_REF] These ligands are strong σdonors and can stabilize metal complexes with high or low oxidation states, which make them universal ligands in organometallic and coordination chemistry as well as in material chemistry. [START_REF] Herrmann | Essays on organometallic chemistry. 9. N-Heterocyclic carbenes[END_REF][START_REF] Bourissou | Stable Carbenes[END_REF][START_REF] Diez-Gonzalez | Stereoelectronic parameters associated with N-heterocyclic carbene (NHC) ligands: A quest for understanding[END_REF][START_REF] Crabtree | Recent developments in the organometallic chemistry of N-heterocyclic carbenes[END_REF][START_REF] Lin | Preparation and application of N-heterocyclic carbene complexes of Ag(I)[END_REF][START_REF] Lin | Coinage Metal-N-Heterocyclic Carbene Complexes[END_REF][START_REF] Huynh | Electronic Properties of N-Heterocyclic Carbenes and Their Experimental Determination[END_REF][START_REF] Peris | Smart N-Heterocyclic Carbene Ligands in Catalysis[END_REF][START_REF] Smith | N-Heterocyclic Carbenes in Materials Chemistry[END_REF][START_REF] Danopoulos | N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt[END_REF] They tend to push the non-radiative metal-centered (MC) d-d transitions to higher energy, rendering the target metal-NHC complexes strongly luminescent (Figure 1). [START_REF] Au | Luminescent Cyclometalated N-Heterocyclic Carbene-Containing Organogold(III) Complexes: Synthesis, Characterization, Electrochemistry, and Photophysical Studies[END_REF][START_REF] Yam | Photochromic Diarylethene-Containing Ionic Liquids and N-Heterocyclic Carbenes[END_REF][START_REF] Leung | Luminescent Cyclometalated Alkynylplatinum(II) Complexes with a Tridentate Pyridine-Based N-Heterocyclic Carbene Ligand: Synthesis, Characterization, Electrochemistry, Photophysics, and Computational Studies[END_REF][START_REF] Hong | From Spherical to Leaf-Like Morphologies: Tunable Supramolecular Assembly of Alkynylgold(I) Complexes through Variations of the Alkyl Chain Length[END_REF] Figure 1. Simplified excited states diagram of metal-NHC complexes highlighting the effect of NHCs as strong σ-donor ligands on destabilizing of the 3 dd metal centered states.

Moreover they bring robustness to these compounds and thus may provide long-term functional materials. The great versatility of these ligands arises from their ease in modular synthesis and modifications. [START_REF] Benhamou | Synthetic Routes to N-Heterocyclic Carbene Precursors[END_REF] For instance this can be achieved by functionalization of the imidazolium ring at the nitrogen center with different organic groups or the extension of the πsystem in the backbone of the carbene structure, leading to a variety of NHC ligands with different forms and geometries. [START_REF] Huynh | The Organometallic Chemistry of N-Heterocyclic Carbenes[END_REF][START_REF] Reshi | Recent advances in annellated NHCs and their metal complexes[END_REF] Traditionally NHC ligands are employed in organometallic chemistry and catalysis, and there are several reports devoted to their synthesis, electronic properties, and applications in these fields. [START_REF] Silva | Synthesis, Structure, and Solution Dynamics of Neutral Allylnickel Complexes of N-Heterocyclic Carbenes[END_REF][START_REF] Gusev | Electronic and Steric Parameters of 76 N-Heterocyclic Carbenes in Ni(CO)3(NHC)[END_REF][START_REF] Nelson | Quantifying and understanding the electronic properties of Nheterocyclic carbenes[END_REF][START_REF] Hopkinson | An overview of N-heterocyclic carbenes[END_REF][START_REF] Gaggioli | Modulating the Bonding Properties of N-Heterocyclic Carbenes (NHCs): A Systematic Charge-Displacement Analysis[END_REF] In contrast only a few reviews were reported where such ligands were used to construct luminescent compounds. [START_REF] Strassner | Phosphorescent Platinum(II) Complexes with C^C* Cyclometalated NHC Ligands[END_REF][START_REF] Bonfiglio | Phosphorescent Tris-Bidentate IrIII Complexes with N-Heterocyclic Carbene Scaffolds: Structural Diversity and Optical Properties[END_REF] The aim of this review is to cover the recent developments since 2015 in this fast growing area to demonstrate the thrust, richness and broadness of the field of luminescent complexes with NHC ligands. For instance, the choice of the NHC ligand and the geometry of the metal are important to design the target luminescent complex and to tailor the emission energy which eventually justifies the kind of applications for instance for biological applications where complexes emitting in the NIR region are required. We will also highlight the chiroptical properties of recent examples of luminescent chiral NHC complexes. This field is still in its infancy but is expected to expand greatly in the near future. Moreover we wish to highlight the importance of luminescent carbene complexes in the field of OLEDs. The quantum efficiency of an OLED device depends on the formation of electron-hole pairs known as excitons, Ground state (S 0 ) [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] which upon relaxation release energy in the form of light. However, electrically generated excitons can be either singlets or triplets, created in a 1:3 statistical ratio. While the use of fluorescent materials has limited efficiency because only 25% of the generated excitons can be used for light emission, the use of phosphorescent materials, mainly those based on transitionmetal complexes that enable the access to triplet excited states due to the spin-orbit coupling of the metal luminophore, allows instead the potential use of all generated excitons, which quadruples the internal quantum efficiencies compared to fluorescent emitters. [START_REF] Mao | Recent progress in phosphorescent Ir(III) complexes for nondoped organic light-emitting diodes[END_REF] Moreover one would expect that carbene ligands stabilize luminescent complexes, which make them more amenable as longlived emitters for optical devices. [START_REF] Li | Rational design of phosphorescent iridium(III) complexes for emission color tunability and their applications in OLEDs[END_REF] Furthermore we wish to feature the research activities involving luminescent NHC-metals containing an organic chromophore such as naphthalimide, [START_REF] Nandhikonda | Theme and variation on N-aryl-1, 8naphthalimides: minimal modification to red-shifted fluorescence and applications in fluorescent chemosensors[END_REF][START_REF] Banerjee | Recent advances in the development of 1,8-naphthalimide based DNA targeting binders, anticancer and fluorescent cellular imaging agents[END_REF][START_REF] Kamal | Naphthalimide derivatives with therapeutic characteristics: a patent review[END_REF][START_REF] Jia | Naphthalimides for labeling and sensing applications[END_REF][START_REF] Gopikrishna | Functional 1,8-Naphthalimide AIE/AIEEgens: Recent Advances and Prospects[END_REF][START_REF] Dong | 1,8-Naphthalimide-based fluorescent chemosensors: recent advances and perspectives[END_REF] pyrene [START_REF] Winnik | Photophysics of Preassociated Pyrenes in Aqueous Polymer-Solutions and in Other Organized Media[END_REF][START_REF] Karuppannan | Supramolecular Chemical Sensors Based on Pyrene Monomer-Excimer Dual Luminescence[END_REF][START_REF] Figueira-Duarte | Pyrene-Based Materials for Organic Electronics[END_REF][START_REF] Zhong | Ionics fluorescent chemsensor based on pyrene[END_REF][START_REF] Howarth | Photophysical properties and applications of coordination complexes incorporating pyrene[END_REF][START_REF] Feng | Functionalization of Pyrene To Prepare Luminescent Materials-Typical Examples of Synthetic Methodology[END_REF][START_REF] Qiu | Fluorescent Phosphorus Dendrimers: Towards Material and Biological Applications[END_REF][START_REF] Ohishi | Circularly polarized luminescence from pyrene excimers[END_REF] and carbazole [START_REF] Thomas | Light-emitting carbazole derivatives: Potential electroluminescent materials[END_REF][START_REF] Jiang | A Review on Synthesis of Carbazole-based Chromophores as Organic Light-emitting Materials[END_REF][START_REF] Joseph | Manipulation of Donor-Acceptor Interactions in Carbazole-Based Emitters by Chromophore Choice To Achieve Near-UV Emission[END_REF][START_REF] Konidena | Star-Shaped Asymmetrically Substituted Blue Emitting Carbazoles: Synthesis, Photophyscial, Electrochemical and Theoretical Investigations[END_REF][START_REF] Yin | A versatile small-molecule fluorescence scaffold: Carbazole derivatives for bioimaging[END_REF] in their chemical structures. A special treatment will be given to this type of compound because they have been neglected in previous reviews. In fact, the presence of an organic chromophore in close proximity to an inorganic chromophore with NHC-ligand brings additional properties by promoting rich excited states in this class of compounds. For instance, in the past five years carbene-coinage metal complexes with carbazolate ligands have attracted much attention due to their remarkable luminescent properties. [START_REF] Chotard | Mono-versus Bicyclic Carbene Metal Amide Photoemitters: Which Design Leads to the Best Performance?[END_REF][START_REF] Hamze | Highly efficient deep blue luminescence of 2-coordinate coinage metal complexes bearing bulky NHC benzimidazolyl carbene[END_REF] A comparative presentation between those with N-heterocylic-carbenes and those with electrophilic carbenes (CAAC, MAC and DAC) will be presented, highlighting the need of a timely review to underscore this recent class of luminescent carbene complexes.

Indeed carbene-luminescent complexes displaying an organic chromophore in proximity to a heavy metal ion with large spin-orbit coupling (SOC) should facilitate intersystem crossing (ISC)

to the triplet manifold of the organic dyes. [START_REF] Yersin | The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs[END_REF][START_REF] Valeur | Molecular Fluorescence: Principles and Applications[END_REF][START_REF] Castellano | Altering Molecular Photophysics by Merging Organic and Inorganic Chromophores[END_REF] In fact intersystem crossing is favored by spin-orbit coupling with an efficiency approximately proportional to Z [START_REF] Bonfiglio | Phosphorescent Tris-Bidentate IrIII Complexes with N-Heterocyclic Carbene Scaffolds: Structural Diversity and Optical Properties[END_REF] (where Z is the atomic number). [START_REF] Valeur | Molecular Fluorescence: Principles and Applications[END_REF] As a consequence this type of compound will promote novel excited state properties, which make them ideal molecules for displaying dual emissions for useful potential applications. [START_REF] Geist | Directing Energy Transfer in Panchromatic Platinum Complexes for Dual Vis-Near-IR or Dual Visible Emission from sigma-Bonded BODIPY Dyes[END_REF][START_REF] Herberger | Platinum emitters with dye-based sigma-aryl ligands[END_REF] Thus in this review, we will first present recent advances in the preparation of luminescent NHCplatinum and -iridium complexes with 5d 8 and 5d 6 electronic configuration which possess planar and octahedral coordination geometries and display large spin orbit coupling. They also display very large ligand field splitting due to the good metal-ligand overlap. Eventually these carbene metal compounds tend to emit via [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] MLCT/ 3 LLCT excited states. In addition the planar platinum complexes at high concentration tend to aggregate via π-π or M⋅⋅⋅M interactions, which generate low energy emissions (vide infra). In complexes involving predominantly MLCT transitions, photoexcitation changes the d-electron configuration, leading to Jahn-Teller-type distortions in the excited state, which can enhance non-radiative decay. In others, operating by LLCT mechanisms, geometric distortion in the excited state can still play a role but there is no change in d-electron count and the metal center is much less involved.

The subsequent paragraph describes those with coinage metals (Cu, Ag, Au), which display linear two-coordinate, three coordinate and four coordinate geometries. Coinage metal complexes display closed-shell d 10 ground state configuration. As a consequence the deactivation via MC dark states is absent however other non-radiative mechanisms involving excited state distortion occurs, for instance tetrahedral complexes rearrange towards a more planar geometry. In fact this explains why certain complexes can be luminescent in solid state or in films and why in solution the same complex is not luminescent. The flattening of the excited states renders the coinage ions accessible for coordination to solvent molecules or counter ions and as a consequence the formed excited complexes are non-luminescent. [START_REF] Mara | Interplays of excited state structures and dynamics in copper(I) diimine complexes: Implications and perspectives[END_REF] Moreover the abundant copper complexes display a small energy gap ΔE ( 1 MLCT-3 MLCT) less than 1000 cm -1 which enable them to emit via prompt fluorescence or thermally activated delayed fluorescence (TADF) at room temperature. Therefore copper in particular and other coinage metal complexes are capable of harvesting singlet and triplet excitons for OLEDs applications. [START_REF] Kirchhoff | Temperature-Dependence of Luminescence from Cu(NN)2+ Systems in Fluid Solution -Evidence for the Participation of 2 Excited-States[END_REF][START_REF] Czerwieniec | Cu(I) complexes -Thermally activated delayed fluorescence. Photophysical approach and material design[END_REF] Finally, we propose the development trend of NHC metal complexes, which presents attractive research prospects in the aforementioned fields.

2-Luminescent platinum(II) complexes with N-heterocyclic carbene ligands

Platinum (II) complexes with phosphines

Cyclometalated platinum(II) complexes containing NHC are currently considered as important class of compounds for multiple reasons. [START_REF] Strassner | Phosphorescent Platinum(II) Complexes with C^C* Cyclometalated NHC Ligands[END_REF][START_REF] Lin | Photofunctional platinum complexes featuring N-heterocyclic carbene-based pincer ligands[END_REF] For instance they show rich optical properties with high quantum yields and long lifetimes. [START_REF] Williams | The coordination chemistry of dipyridylbenzene: N-deficient terpyridine or panacea for brightly luminescent metal complexes?[END_REF][START_REF] Lam | Luminescent Platinum(II) Complexes of 1,3-Bis(N-alkylbenzimidazol-2 'yl)benzene-Type Ligands with Potential Applications in Efficient Organic Light-Emitting Diodes[END_REF][START_REF] Li | Highly phosphorescent platinum(II) emitters: photophysics, materials and biological applications[END_REF][START_REF] Puttock | The luminescence properties of multinuclear platinum complexes[END_REF] Moreover these compounds display stable square-planar geometry with a d 8 electronic configuration, as a consequence this allow them to form metal-metal and π-π interactions at the supramolecular level, which impact profoundly the properties of their electronic excited states (Figure 2). [START_REF] Miskowski | Electronic-Spectra and Photophysics of Platinum(II) Complexes with Alpha-Diimine Ligands -Solid-State Effects .2. Metal-Metal Interaction in Double Salts and Linear-Chains[END_REF][START_REF] Wong | Luminescence studies of dinuclear platinum(II) alkynyl complexes and their mixed-metal platinum(II)-copper(I) and -silver(I) complexes[END_REF][START_REF] Yam | Solvent-induced aggregation through metal•••metal/π•••π interactions: Large solvatochromism of luminescent organoplatinum(II) terpyridyl complexes[END_REF][START_REF] Wong | Self-Assembly of Luminescent Alkynylplatinum(II) Terpyridyl Complexes: Modulation of Photophysical Properties through Aggregation Behavior[END_REF][START_REF] Chen | Luminescent Pincer-Type Cyclometalated Platinum(II) Complexes with Auxiliary Isocyanide Ligands: Phase-Transfer Preparation, Solvatomorphism, and Self-Aggregation[END_REF][START_REF] Sesolis | Highly Phosphorescent Crystals of Square-Planar Platinum Complexes with Chiral Organometallic Linkers: Homochiral versus Heterochiral Arrangements, Induced Circular Dichroism, and TD-DFT Calculations[END_REF][START_REF] Sesolis | Dinuclear (N C N) Pincer Pt(II) Complexes with Bridged Organometallic Linkers: Synthesis, Structures, Self-Aggregation, and Photophysical Properties[END_REF][START_REF] Fu | A rational molecular design of triazinecontaining alkynylplatinum(II) terpyridine complexes and the formation of helical ribbons via Pt•••Pt, π-π stacking and hydrophobic-hydrophobic interactions[END_REF][START_REF] Amar | Aggregation Effect on the Luminescence Properties of Phenylbipyridine Pt(II) Acetylide Complexes. A Theoretical Prediction with Experimental Evidence[END_REF][START_REF] Mauro | When self-assembly meets biology: luminescent platinum complexes for imaging applications[END_REF][START_REF] Moussa | New N^C^N-coordinated Pd(II) and Pt(II) complexes of a tridentate N-heterocyclic carbene ligand featuring a 6-membered central ring: synthesis, structures and luminescence[END_REF][START_REF] Pinter | Photophysical properties of phosphorescent mono-and bimetallic platinum(II) complexes with C∧C* cyclometalating NHC ligands[END_REF] changes in the excited state energy and as a consequence changes the color of these attractive complexes. As mentioned previously, cyclometalated platinum(II) complexes display strong phosphorescence with high quantum yields, which make them attractive molecules for variety of applications as optical devices and in OLEDs technology. [START_REF] Bullock | In Search of Deeper Blues: Trans-N-Heterocyclic Carbene Platinum Phenylacetylide as a Dopant for Phosphorescent OLEDs[END_REF] To this end, Sicilia and coworkers described a family of blue phosphorescent platinum(II) emitters, (1-5) in PMMA films (PMMA = Poly(methylmethacrylate)). The authors designed a series of complexes where an NHC-unit is linked to either a naphthyl group (π-extended system) or a phenyl ring to which a variety of functional groups were attached (Figure 3). [START_REF] Fuertes | Heteroleptic Cycloplatinated N-Heterocyclic Carbene Complexes: A New Approach to Highly Efficient Blue-Light Emitters[END_REF][START_REF] Arnal | The Use of Cyclometalated NHCs and Pyrazoles for the Development of Fully Efficient Blue Pt-II Emitters and Pt/Ag Clusters[END_REF] They also varied the ancillary ligands in order to study their effect on the photophysical properties. In solution at room temperature the complexes were not emissive; however in a rigid matrix (CH 2 Cl 2 , 77K), these molecules show bright and long-lived luminescence. Compounds 1-5 acted as blue or greenish-blue emitters in PMMA films with high quantum yields varying from 68% (1) to 93% (2). As mentioned previously square planar Pt(II) complexes are known to aggregate via π-π or M⋅⋅⋅M interactions.

Indeed, the authors demonstrated the complexes with π-extended system (3, 5) in solid-state displayed yellowish-orange emission attributable to π-π* excited states formed by intermolecular ππ interactions. Electroluminescent devices were prepared from neat films of the compounds acting as emitting layers; for instance, complex 2 behaved as blue-light emitter while complex 3 displayed yellow orange emission. Interestingly a mixture of both compounds 2-3 behaved as white light emitters.

Figure 3. Cycloplatinated NHC complexes containing phosphine and diphosphine ligands.

In order to obtain warm white light emitting devices, the authors described the synthesis of cycloplatinated N-heterocyclic carbene compounds 7-12 by exchanging the ancillary ligands for chelating diphosphines (P^P). [START_REF] Sicilia | Highly efficient platinum-based emitters for warm white light emitting diodes[END_REF][START_REF] Jaime | Cyclometalated NHCs Pt(II) Compounds with Chelating P^P and S^S Ligands: From Blue to White Luminescence[END_REF] The presence of two chelating ligands around the Pt(II) center confers robustness and thermal stability to these compounds. Complexes 7-9 displayed different optical properties when compared to those with a naphthyl group 10-12 (π-extended system), suggesting that emissions in these compounds arise from different kinds of excited states. For instance, in solution the emission of complexes 7-9 did not vary with concentration. In contrast complexes 10-12 at higher concentration displayed wavelength-dependent emissions. White light remote phosphor devices were achieved by combinations of platinum complexes 7 or 9 acting as blue emitters with the neutral compound [Pt(bzq)(CN)(CNXyl)] (bzq = benzoquinolate, Xyl = 2,6dimethylphenyl) behaving as a red emitter using a 365nm LED as the pumping source. By changing the blue/red ratio, the authors were able to obtain a warm white light with optimal CRI and D uv values and a great range of nominal CCT (4000-2000K). [START_REF] Sicilia | Highly efficient platinum-based emitters for warm white light emitting diodes[END_REF]

Platinum (II) complexes with bidentate acac ligands

Strassner and coworkers designed a family of neutral cycloplatinated N-heterocyclic carbene compounds containing β-diketone chelates as ancillary ligands (Figure 4). [START_REF] Strassner | Phosphorescent Platinum(II) Complexes with C^C* Cyclometalated NHC Ligands[END_REF][START_REF] Leopold | Phosphorescent Thiazol-2-ylidene Platinum(II) Complexes with β-Ketoiminates: Single Isomer Formation by Ligand Architecture[END_REF][START_REF] Leopold | Photoluminescence of a new material: cyclometalated C^C* thiazole-2-ylidene platinum(II) complexes[END_REF][START_REF] Pinter | The Chameleonic Nature of Platinum(II) Imidazopyridine Complexes[END_REF] Again the choice of a cyclometalation with a strong donating carbene unit (C^C*) is made in order to obtain phosphorescent blue emitters for use in organic-light emitting diodes (OLEDs). This type of compound was thoroughly investigated by the authors, where systematic changes were made to the substituents of the N-heterocyclic carbene unit as well as the functional groups on the chelating βdiketone moiety (Figure 4). For instance the authors demonstrated that modification of the π-character of the NHC ligand improves the quantum yield of the Pt(II) complex. More interestingly a significant improvement of the quantum yield was achieved upon the introduction of a heteroatom in the backbone of Nheterocyclic carbene moiety. Indeed, while complex 14 displayed a 45% quantum yield, the platinum compound 15 with imidazopyridine showed 68% while complex 16 with imidazopyrazine provided 84% quantum yield. [START_REF] Pinter | Sky-Blue Triplet Emitters with Cyclometalated Imidazopyrazine-Based NHC-Ligands and Aromatic Bulky Acetylacetonates[END_REF][START_REF] Pinter | Prediction of the Efficiency of Phosphorescent Emitters: A Theoretical Analysis of Triplet States in Platinum Blue Emitters[END_REF] This increase in quantum yield is accompanied by a decrease in the lifetime from 19.6 µs to 3.6 µs, which is appropriate for applications as OLED devices. The authors attribute the remarkably high quantum yield observed for the imidazopyrazine based complex to the fact that 3 MC dark states become less accessible than those compounds that lack a heteroatom in the backbone of the N-heterocyclic carbene. [START_REF] Pinter | Sky-Blue Triplet Emitters with Cyclometalated Imidazopyrazine-Based NHC-Ligands and Aromatic Bulky Acetylacetonates[END_REF] Moreover, attachment to the carbene unit of organic chromophores with extended π-systems such as dibenzofuran and dibenzothiophene was successfully achieved (Figure 5). Depending on the auxiliary ligand, these complexes displayed emissions in the deep blue or the orange-red part of the visible spectrum. For instance, complex 17 in PMMA at 2 wt % displays vibronic emission bands at 463 nm, 497 nm with high quantum yield (90%) and a lifetime of 23µs. Replacement of the methyl group by a CF 3 affords complex 18, which is non-emissive. Complexes 19 and 21 acted as blue emitters as well with high quantum yields (83%) and (91%) and lifetimes on the order of µs. As for complex 20 containing a phenyl group, a red-shifted structureless emission at 530 nm was observed.

DFT calculations suggested that the phenyl ring rotates in the plane of the acac core (acac = acetylacetonate) in the triplet state and hence extending the π-conjugation. Complex 17 was used as a greenish-blue phosphor for OLED device. The electroluminescence spectra showed λ max = 480 nm with structured emission bands. The maximum luminance depends on the doping concentration, for instance at 4900 cd m -2 (13V, 6% doping) to 6750 cd m -2 (13.2 V, 12% doping. The maximum EQE (6.2%) is achieved at the highest doping concentration of 12%. [START_REF] Unger | Green-Blue Emitters: NHC-Based Cyclometalated [Pt(C^C*)(acac)] Complexes[END_REF] The analogous cycloplatinated NHC-dibenzothiophene complex 22 showed lower performance when compared to the dibenzofuran congener 17: the emission spectra recorded in thin film at 2 wt % emitter displayed vibronic bands at 474, 507 and 537 nm with a quantum yield of 63% and lifetime of 31 µs. [START_REF] Tronnier | A Phosphorescent C^C* Cyclometalated Platinum(II) Dibenzothiophene NHC Complex[END_REF] The authors attribute the high quantum yields observed in these complexes to the extended heterocyclic π-system due to the presence of the organic chromophores.

The cycloplatinated NHC-dibenzofuran containing mesityl-acac as auxiliary ligand was obtained as a mixture of two isomers complex 23 and Iso-23 (NHC = benzimidazolin-2-ylidene). [START_REF] Tronnier | Phosphorescent Platinum(II) Complexes with C^C* Cyclometalated NHC Dibenzofuranyl Ligands: Impact of Different Binding Modes on the Decay Time of the Excited State[END_REF] The optical properties of the regio-isomeric complexes were found to be different. Both compounds displayed

N N Pt S O O N N Pt O O O R R N N Pt O O O R = CH 3 (17) R = CF 3 (18) R = t Bu (19) R = Ph (20) R = Mes (21) 22 N N Pt O O O 23 Iso-23
emissions in the blue region at 475 nm for 23 with vibronic progression, while Iso-23 showed a broader band at 479 nm with comparable quantum yields of 84% for 23 and 80% for Iso 23.

However their lifetimes showed marked differences, with a 10.8 µs lifetime for Iso 23 which is 50% shorter than that obtained for complex 23 with τ = 22.4 µs. [START_REF] Tronnier | Phosphorescent Platinum(II) Complexes with C^C* Cyclometalated NHC Dibenzofuranyl Ligands: Impact of Different Binding Modes on the Decay Time of the Excited State[END_REF] Structure modifications made on substituted β-diketone ancillary ligands brought changes in the optical properties of the related platinum compounds. For instance, increasing the steric hindrance, upon moving from methyl group to larger substituents such as t Bu, phenyl, mesityl and tetramethyl-phenyl provoked an increase in the quantum yields and decrease in the lifetimes. Furthermore, while alkyl-substituted β-diketonates complexes displayed structured emission, those containing aromatic auxiliary ligands displayed a single structureless broad emission.

Platinum (II) complexes with alkynyl ligands

Venkatesan and coworkers designed another class of stable cycloplatinated NHC complexes containing dianionic biphenyls as auxiliary ligands (Figure 6). 88 the HOMO-LUMO gap. TGA measurements carried out on compounds 26 and 29 to evaluate the thermal stability of these compounds revealed the onset of thermal degradation for 26 occurs at 309° C and for 29 at 374°C, highlighting their high robustness.

On the other hand the authors prepared a series of NHC-pyridine platinum complexes 30-33, containing bis-acetylide ligands (Figure 7). [START_REF] Bachmann | Tunable and Efficient White Light Phosphorescent Emission Based on Single Component N-Heterocyclic Carbene Platinum(II) Complexes[END_REF] Such compounds behave as single component white light phosphorescent emitters. It should be mentioned that most of the other materials reported require multicomponent emitters that emit in the red, green and blue regions to cover the entire visible electromagnetic spectrum. However, disadvantages are that such white light emitting systems involve complicated mixing or doping processes and drawbacks related to device engineering. Upon increasing the weight percentage of the complex in PMMA films, a low energy emission band at around 575-595 nm appeared. The maximum emission quantum yield was obtained at 20wt%; higher concentrations induced self-quenching. These compounds showed bright white

N N N Pt R R R = -C 6 H 5 -C 6 H 4 F CF 3 CF 3 30 31 32 33
; ; ;

photoluminescence covering the entire visible spectrum due to the presence of monomeric and excimeric emissions. For instance, complex 33 at a concentration of 20wt% in PMMA, emitted with CIE (Commission Internationale de l'Eclairage) color characteristic coordinates of x = 0.31 and y = 0.33, which are very close to the intended pure white light (x = 0.33, y = 0.33). Variation of the alkyne substituents allows fine-tuning of the emission leading to high quantum yields, thus rendering these compounds as promising candidates for single component triplet emitter based WOLEDs.

On the other hand Schanze and coworkers designed some luminescent square planar Pt(II) alkynyls displaying trans N-heterocyclic carbene ligands 34-44 (Figure 8). [START_REF] Bullock | In Search of Deeper Blues: Trans-N-Heterocyclic Carbene Platinum Phenylacetylide as a Dopant for Phosphorescent OLEDs[END_REF][START_REF] Bullock | Blue Phosphorescent trans-N-Heterocyclic Carbene Platinum Acetylides: Dependence on Energy Gap and Conformation[END_REF][START_REF] Bullock | trans-N-(Heterocyclic Carbene) Platinum(II) Acetylide Chromophores as Phosphors for OLED Applications[END_REF] Such compounds acted as blue phosphorescent emitters from triplet excited states ( 3 π-π * ) centered mainly in the alkynyl-aryl moiety (-C≡C-Ar), with a minor contribution from [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] MLCT transitions. The platinum complexes display emissions at λ max = 431-483 nm and lifetimes in the microsecond regime with quantum yields up to 45%. The authors demonstrated that variations of the N-heterocyclic carbene ligands, whether imidazole or benzimidazole, have little effect on the photophysical properties. Conversely, when the acetylide-aryl ligand was changed, dramatic variations in the emission energy, lifetime and phosphorescence were observed. It should be mentioned that the presence of strong σ-donating ability of the NHCs allows them to push the d-d states to higher energy and make these compounds luminescent, in contrast to their analogues containing phosphine ligands. However, the study on these compounds shows that, despite NHC ligands increasing the d-d state energy, yet it remains thermally accessible (ΔE = 1000 cm -1 ) and therefore contributed significantly to the deactivation process at room temperature. In THF, the quantum yield of platinum complex 34 with imidazolin-2-ylidene was found to be 0.029, which increases by 10-fold when doped into a PMMA matrix along with a longer triplet excited state lifetime (Φ PMMA = 0.3, τ solid = 9.4 µs). Varying the doping concentration over the values 1, 10, 20, 50, 90, 100% shows qualitatively the same emission, suggesting no indication of excimer formation. The authors suggest that lack of excimer formation is likely due to the large cyclohexyl groups oriented orthogonally to the plane of the Pt-ligand bonds. Complex 34 was used as a blue phosphor for OLED devices. The electroluminescence spectrum showed a maximum at λ = 444 nm.

The CIE coordinates of the device emission are (0.2, 0.2) with a maximum EQE of 8%.

Moreover, the platinum complexes displaying N-heterocyclic carbene with benzimidazole backbones display better performance as blue emitters. For instance, the platinum complex 41 with benzimidazolin-2-ylidene in THF displays a quantum yield of 0.08. When doped in PMMA matrix, the quantum yield increases to (Φ PMMA = 0.56, τ solid = 21µs) much higher than that obtained for complex 34. Given the efficient phosphorescence in the solid state, complex 41 was used as a blue emitter for OLED devices. The electroluminescence spectrum exhibits an emission maximum at λ = 439 nm, with 14% EQE, the highest performance within this class of compounds. The CIE color coordinates of the device emission are (0.16, 0.13) which is close to that of the National Television System Committee CIE color coordinate for ideal blue (0.14, 0.08).

Platinum (II) complexes with pincer ligands

Gray and coworkers constructed another family of N-heterocyclic carbene Pt(II) complexes 45-53 displaying a BIMCA pincer ligand (BIMCA = bis-imidazolylcarbazolate) and isonitrile or acetylides as ancillary ligand (Figure 9). The BIMCA ligand contains an organic carbazole chromophore, to which are directly attached two NHC units, generating a rigid (C^N^C) pincer ligand around the Pt(II) center. [START_REF] Liska | Room-Temperature Phosphorescent Platinum(II) Alkynyls with Microsecond Lifetimes Bearing a Strong-Field Pincer Ligand[END_REF][START_REF] Li | Isonitrile)platinum(II) Complexes of an Amido Bis(N-heterocyclic carbene) Pincer Ligand[END_REF] Owing to steric factors around the metal center, the X-ray structures of these complexes showed that the metal is tetra-coordinated displaying a distorted tetrahedral geometry instead of a square-planar structure.

+ BPh 4
In the solid state, complexes 50-53 acted as blue/green emitters with λ em = 496-523 nm. As for complex 51, an intense green emission was observed with CIE 1931-coordinates (0.29, 0.65) and Φ= 93% with τ = 11ms. TD-DFT calculations were performed to rationalize the optical properties of this kind of compound, suggesting that the origin of the excited states is 3 (MLCT/LL) with high contribution from the tBu BIMCA pincer ligand in the HOMO and the LUMO as well.

On the other hand, Peris and Poyatos described the synthesis of some cyclometalated Pt(II) complexes 54-55 containing (C^N^C) pincer ligand, (C^N^C = 2,6-diphenylpyridine) supported by pyrene-based mono-or bis -NHC-ligands. [START_REF] Gonell | Pincer-CNC mononuclear, dinuclear and heterodinuclear Au(III) and Pt(II) complexes supported by mono-and poly-N-heterocyclic carbenes: synthesis and photophysical properties[END_REF] The authors intended to prepare a novel class of emissive complexes by combining the benefits of an organic chromophore and that of an inorganic Pt(C^N^C) luminophore within the frame of stable complexes provided by the NHC ligands (Figure 10). long component 70.9 ns and short contribution 6.5 ns lifetimes for 54 and 42.6 ns and 3.2 ns for 55

with comparable quantum yields Φ = 3.5% and 3.1% respectively. It should be mentioned that the mono-and bis-pyrene-azolium salts displayed similar emissions form but displayed higher quantum yields Φ = 32% and 42% respectively with short lifetime in the nanosecond regime for monoazolium (78.8 ns) and bis-azolium (43.1 ns) suggesting the fluorescence character of the observed emissions. This data suggests a minor contribution from the Pt(II) center on the pyrene-based excited states in the above complexes. Complexes 56-63 were found to be non-emissive in solution at room temperature, in contrast in powder form they exhibited strong emission varying from purple to sky-blue emission with high quantum yields approaching unitary for complexes 56-57 with lifetimes of 29-33 µs respectively.

The authors also noticed that the size of the linkage between the two carbene units affects the emission properties of these compounds. For instance the biscarbene complexes 58 and 60 with propylene linkage are weakly emissive with quantum yields of 6% and 2% respectively. The authors explain this behavior to be a consequence of the propylene spacer which forces the dicarbene chelate to display a perpendicular orientation to the square-planar Pt(II) architecture resulting in destabilization of the emitting excited state and hence quenching the emission. TD-DFT calculations were employed and showed that these compounds display a combination of [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] LLCT/ 3 LMCT/ 3 IL emission excited states, unlike related square planar Pt(II) compounds which usually emit from a combination of 3 MLCT and 3 IL π-π* electronic transition states. Compounds 61-62 were found to be amenable to prepare OLED devices. In particular complex 61 showed better performance, displaying high efficiency (8.9%, 19.4 cd.A -1 , 22.5lm.W -1 ) with a sky blue emission and CIE x,y coordinates of (0.18, 0.32).

On the other hand Che and coworkers designed another class of square planar Pt(II) complexes (64-69) supported by bis-carbene chelates and a combination of bidentate terphenyldiacetylide ligands (Figure 12). [START_REF] Luo | Luminescent Platinum(II) Complexes with Bidentate Diacetylide Ligands: Structures, Photophysical Properties and Application Studies[END_REF] It is believed that such a type of chelating ligands should bring rigidity to the system, in addition the nature of strongly σ-donor ligands tend to enhance their photophysical properties. [START_REF] Li | Highly phosphorescent platinum(II) emitters: photophysics, materials and biological applications[END_REF] Figure 12. Pt(II)-bisacetylides with bis-NHC chelating ligands Compounds 64-69 were found to be emissive in CH 2 Cl 2 solution at room temperature. For instance those containing carbazole substituents at the terphenyldiacetylide 67-69 are more emissive (Φ = 23-27%) with lifetimes (τ = 28-42.6 µs) than the non-substituted compounds 64-66 (Φ = 3.4-5.4%) and displaying shorter lifetimes (2.7-4.1 µs). Moreover all complexes displayed vibronically structured emissions profile and were assigned to originate primarily from metal-perturbed 3 IL (πalkynyl-π*alkynyl excited states. Within the same series 67-69, the authors note a minor variation in emission quantum yields (23-27%), when the spacer length (-CH 2 -)n of the biscarbene ligands is varied (n= 1 to 3). Furthermore in solid state at 298 K, these compounds were also emissive displaying again structured emission bands ranging from blue-green to yellow emission with λ max = 470-533 nm, and at 77K they display emission bands at λ max = 470-530 nm. In glassy media at 77K, (66) complex 65 displays a structureless emission band at λ = 477 nm with full width at half maximum, (FWHM) of 75 nm. In contrast complexes 64, 66-69 exhibited structured emission at λ max = 448-497 nm. The vibrational progressive spacing for complex 67 was estimated to be 1470 cm -1 , which indicates the skeletal vibration is assigned to the diacetylide ligand.

N Pt N N N n n = 1 (67) n = 2 (68) n = 3 (69) N Pt N R 1 N N R 1 R 2 N N R 1 = nBu, R 2 = H (64) R 1 = nBu, R 2 = CF 3 (65) R 1 = Me, R 2 = NPh 2
The upcoming paragraph will present recent advances on luminescent iridium complexes containing N-heterocyclic carbenes.

3-Luminescent iridium(III) complexes with N-heterocyclic carbene ligands

Homoleptic cyclometalated iridium complexes facial versus meridional regioisomers

Octahedral cyclometalated Ir(III) complexes possess a range of important properties such as rigid configurational stability, high emissive quantum yield (φ), microsecond-regime phosphorescence lifetime (τ) and high electrochemical stability. [START_REF] Bonfiglio | Phosphorescent Tris-Bidentate IrIII Complexes with N-Heterocyclic Carbene Scaffolds: Structural Diversity and Optical Properties[END_REF][START_REF] Balzani | Photochemistry and Photophysics: Concepts, Research, Applications[END_REF][START_REF] Zhao | Phosphorescent heavy-metal complexes for bioimaging[END_REF][99][100][101][102][103][104][105] Moreover they act as high efficiency emitters in OLED devices. [START_REF] Yersin | The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs[END_REF][106][107][108][109][110][111][112] This field of study has attracted much attention and continues to be highly investigated; thus we intend to present the most recent work in this area. As mentioned previously, the presence of NHCs, which act as strong σ-donor ligands, can serve as a tool to push the metalcentered d-d transitions to higher energy in order to make them thermally inaccessible. [START_REF] Bonfiglio | Phosphorescent Tris-Bidentate IrIII Complexes with N-Heterocyclic Carbene Scaffolds: Structural Diversity and Optical Properties[END_REF][113][114][115] The first cyclometalated iridium complexes 70-71 containing NHC ligands were reported in 1982 by Lappert and coworkers (Figure 13). 116 Forrest, Thompson and coworkers investigated thoroughly this type of compound in order to construct blue emitters. The choice of NHCs in the coordination sphere of cyclometalated iridium complexes is crucial, because they tend to emit from high-energy triplet states. [117][118][119][120][121] Figure 13. Homoleptic cycloiridated NHC complexes displaying facial and meridional regioisomers.

More recently the group designed a luminescent cyclometalated NHC iridium complex in which the backbone of the imidazole ring contains a fused pyridine moiety. The two geometrical isomers fac-72 and mer-72 were isolated and characterized (Figure 13). 107 The electrochemical properties were investigated for both compounds and allowed to estimate the HOMO-LUMO gaps, for instance the mer-72 isomer displayed the smaller gap. At room temperature in CH 2 Cl 2 both complexes are highly emissive in the blue region at λ = 418 nm, (Φ = 76%, τ = 1.2µs) for fac-70 and at λ = 465 nm, (Φ = 78%, τ = 0.8µs) for mer-72. This result indicates that the emission of complex mer-72 is red-shifted compared to the other geometrical isomer, in line with the electrochemical properties.

The study also shows a more dominant ligand-centered [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] LC transitions relative to a 3 MLCT excited states in fac-72, while the emission in complex mer-72 stems more from the 3 MLCT emission rather than the ligand-centered 3 LC excited state. Due to their high optical performance in solution and solid state, both compounds were employed as blue emitters for OLED devices. The electroluminescence spectra of fac-72 and mer-72, measured at a current density of J = 10 mA cm -2 , displayed emission in the deep blue with CIE coordinates of (0.16, 0.09) and (0.16, 0.15) with maximum external quantum efficiencies EQE = 10.1 and 14.4 respectively.

Zysman-Colman, Samuel and coworkers, prepared the homoleptic meridional cyclometalated iridium complexes 73 and 74 containing NHC ligands (Figure 14). 122 To enlarge the HOMO-LUMO gap, the electron-withdrawing group CF 3 -was attached to the phenyl-rings in the para-and meta

N Ir N N N N N N N N N Ir N N N N N N N N fac-72 mer-72 N Ir N R N N R N N R R = OMe ; (70) (71)
positions relative to the carbene-unit, which permits sufficient stabilization of the HOMO energy of the emissive triplet state. Both complexes act as blue emitters in CH 2 Cl 2 solution and in PMMA film at 10% emitter. At 298K in solution, complex 73 displayed structured emission at λ = 414 nm with moderate quantum yield Φ = 25%, complex 74 behaved similarly (λ = 412 nm) but showed higher quantum yield Φ = 72%.

Both complexes exhibited biexponential decay lifetimes τ with longer τ at 77K in de-aerated 2-MeTHF, when compared to those at 298 K in degassed CH 2 Cl 2 . Due to their interesting optical properties, complexes 73 and 74 were used as blue emitters for OLED devices. The optimal device consisted in using complex 74 as the emitter and complex 73 as an efficient exciton/electron blocker, and the device provided blue emission with CIE coordinates (0.154, 0.052) with external quantum yield EQE of 13.4%; this value decreases slightly to 12.5% at 100 cd m -2 . This data fulfills the requirement for a deep blue OLED application. For instance, the television display criterion is of (0.14, 0.08) as specified by the European Broadcast Union (EBU). It should be mentioned that most cyclometalated iridium complexes do not meet the deep blue chromaticity

N Ir N N N N N mer-73 CF 3 F 3 C CF 3 N Ir N N N N N mer-74 CF 3 CF 3 F 3 C
requirements; they exhibit CIE y coordinates greater than 0.1 because their triplet energies are not sufficiently high (≥ 2.8 eV).

The development of blue triplet emitters for OLED devices with high EQE continues to be a challenging objective. As mentioned previously, cyclometalated iridium complexes containing NHC ligands are among the most promising candidates. To this end, Jin and coworkers prepared a series of homoleptic cyclometalated iridium complexes 75-78, and isolated the meridional kinetic isomers (Figure 15). 123 Complexes 75-77 were designed with different electron-withdrawing groups being introduced at the phenyl ring (-F, O=PPh 2 ) in order to obtain pure blue phosphoresce by elevating the T 1 energy of these emitters. As a consequence, complex 77 with a pyridyl-carbene ligand exhibits pure blue phosphorescence with high quantum yield (95%), and short lifetime (τ = 0.116µs), the electroluminescence (EL) spectrum of 77 displays emission band with λ = 430 nm with the CIE color emission (0.16, 0.08) where the CIEy perfectly matches the NTSC standard blue.

However, complex 77 shows slightly a lower maximum EQE (7.1%) than that of 75 {EQE = 8.6%, CIE (0.16, 0.12)}, but higher than that displayed by complex 76 {EQE = 3.8%, CIE (0.17, 0.13)}.

The authors attribute this downfall to the intermolecular interactions at higher doping concentration.

To overcome this obstacle, the authors prepared the meridional kinetic isomer 78 in 65% yield in which a benzyl unit instead of a methyl was introduced to the pyridyl-carbene unit.

The presence of a benzyl unit suppresses intermolecular interactions and triplet-triplet annihilation (TTA) of the cyclometalated iridium complex 78 at high doping ratios, owing to the steric hindrance. Moreover placement of a benzyl group on the pyridylcarbene unit reduces concentration quenching of the mer-Ir complex 78. The electrochemical and photophysical properties of complex 78 were investigated. At room temperature, complex 78 displays a blue emission at λ = 450 nm in 2-MeTHF. This emission band is sensitive to the solvent polarity in use; for instance in CH 2 Cl 2 , this band is red shifted to λ = 470 nm.

When doped in diphenyl (4-(triphenylsilyl)phenyl)phosphineoxide (TSPO1) film at 10vol% of complex, the emission band appears at λ = 442 nm with very high quantum yield at (Φ) = 95%.

Figure 15. Homoleptic cyloiridated NHC-complexes displaying meridional regioisomers Due to its excellent optical properties, complex 78 was used as a blue emitter for OLED devices.

The manufactured device with high dopant ratio (40 vol%) of complex 78 gives a deep blue emission with CIE coordinates (0.149, 0.085) and a very high EQE of 24.8%. 124 The authors suggest that these important results highlight the role of the N-benzyl substituted pyrimidazole carbenebased Ir(III) complex for the fabrication of high performance deep blue OLEDs.

The impact of the geometrical isomers whether meridional or facial on the optical properties of cyclometalated iridium complexes was investigated by Kang, Son and coworkers. 125 dibenzofuranyl-N-methylimidazole) 3 Ir(III) (mer-79) were prepared and their photophysical properties were investigated and compared (Figure 16). 126 Compounds 79 were originally reported by Sasabe and Kido. 127 These compounds contain in their structure a dibenzofuranyl, which acts as an organic chromophore. (CV) of mer-79 shows a lower oxidation potential (E ox = 0.17 V vs. Fc/Fc + ) than fac-79 (E ox = 0.26 V vs. Fc/Fc + ) while the reduction potentials of fac-79 and mer-79 are almost similar (E red = 2.37 V and 2.64 V vs. Fc/Fc + ). This data indicates that mer-79 has a smaller energy gap than fac-79, which eventually affects their relative photophysical properties. Indeed at 298 K the photoluminescence spectrum of mer-79 showed a bathochromic shift with λ max = 450nm when compared to the emission of fac-79 (λ max = 443 nm), with quantum yields Φ of 53% and 68% and lifetimes τ = 11.0 µs and τ = 11.2 µs respectively. Both isomers displayed structured emissions, suggesting that emissions originate not from pure [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] MLCT states but from the mixed 3 LC/ 3 MLCT transitions, although mer-79 displayed a higher [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] MLCT contribution, as evidenced by solvent polarity dependence emission spectra and lifetime. Due to their high optical properties, these compounds were investigated as dopants for OLED devices. Both compounds act as blue emitters when doped in in diphenyl (4-(triphenylsilyl)phenyl)phosphineoxide (TSPO1) film at 10% of either complex.

The electroluminescence spectra display blue emissions with the CIE chromaticity for fac-79 device (0.14, 0.11) and EQE of 18.5% and for mer-79 device (0.14, 0.14) and EQE of 18.2%.

Heteroleptic cyclometalated iridium complexes

Containing acyclic diamino carbenes (ADC) ancillary ligands

Teets and coworkers prepared some luminescent heteroleptic cyclometalated iridium complexes containing acyclic diaminocarbene (ADC) ancillary ligands (Figure 17). These compounds are formed via metal-mediated nucleophilic addition to a metal-coordinated isocyanide, with subsequent orthometalation of the ADC under mild conditions. [128][129] Figure 17. Cyclometalated iridium complexes containing acyclic diamino carbenes.

Two sets of complexes were prepared, those containing the cyclometalated ligand "F2ppy" (80-87) and those with 2-phenylbenzothiazole "bt" (84-87). Unlike those with NO 2 -functionality (82-83; 86-87) all other compounds were emissive in fluid solutions of de-aerated CH 2 Cl 2 at room temperature. At low temperature when doped into PMMA films, these complexes become more luminescent. Complexes 80-81 with difluorophenylpyridine display blue emission in CH 2 Cl 2 with 87) N S λ max = 498 and 495 nm, and quantum yields (Φ) of 0.22 and 0.18 respectively. The related compounds 84-85 with bt "C^N" (bt = 2-phenylbenzothiazole) ligands exhibit yellow-orange broad emissions at λ max = 588 and 585 nm respectively with significantly lower quantum yields (Φ) of 0.049 (84) and 0.047 (85). All complexes exhibit a biexponential decay lifetime τ with longer weighted average τ in the range of 0.90-1.1 µs for the F2ppy complexes 80-81 and smaller values of 0.50-0.60 µs for bt complexes 84-85. The four CF 3 -substituted emissive complexes (80-81; 84-85) when doped into PMMA thin films (2 wt%) show dramatic increases in the quantum yields; for instance complexes 80-81 act as blue-green emitters with quantum yields of 79%, while compounds 84-85 display quantum yields of 30-38% and behave as orange emitters. It should be mentioned that the acyclic diaminocarbene (ADC) ancillary ligands are even better σ-donor than the usual NHCs as demonstrated by the electrochemical properties of these compounds, where the oxidation potentials are cathodically shifted when compared to related compounds with NHC ligand (NHC = Imidazolin-2-or benzimidazolin-2-ylidene). The authors have compared the X-ray solid-state structures of the acyclic diamino carbene complex 80 and the NHC cyclometalated iridium complex [(F2ppy) 2 Ir(C^C)] (C^C = 4-CF 3 -C 6 H 4 -benzimidazolin-2-ylidene). The comparison revealed a notable difference between the carbene N-C-N angles, for instance in the NHC complex the N-C-N angle is 105.2(4)°, while in complex 80 this angle is wider (116.4(5)°). The authors suggest that ADC complexes display carbenes with higher donor ability because the widening of the carbene bond angle allows for greater N→C carbene π-donation. As a consequence, the iridium-centered HOMO energies in complexes 80-81 are strongly destabilized due to the strong donation character of the ADC ligands. On the other hand, the group prepared other heteroleptic cyclometalated iridium complexes 88-90 with mixed carbene ligation, i.e. containing both NHC-derived cyclometalating ligands as well as the ADC ancillary ligands (Figure 18). 130 Figure 18. Cyclometalated-NHC iridium complexes containing acyclic amino carbenes.

NH Ir NR 1 R 2 X N N F F F F C N N C Ir NH NR 1 R 2 X C N N C Ir Cl C N X NHR 1 R 2 CH 2 Cl 2 , RT HC N N N S X = CF 3 , R 1 = H, R 2 = Pr, (80)
X = CF 3 , R 1 = H, R 2 = 4-OMeC 6 H 4 -CH 2 ,( 81
) X = NO 2 , R 1 = Et, R 2 = Et, (82)
X = NO 2 , R 1 = Pr, R 2 = Pr, ( 83
) NH Ir NH R 2 X F F F F N S X = CF 3 , R 2 = Pr, (84) X = CF 3 , R 2 = 4-OMeC 6 H 4 -CH 2 , (85) X = NO 2 , R 2 = Et, (86) X = NO 2 , R 2 = Pr, (
The electrochemistry and the photophysical properties of 88-90 were investigated. In CH 2 Cl 2 solution at room temperature, these compounds are weakly emissive with emissions observed at λ = 420 nm for 89, and at λ = 511 nm for complex 90 while 88 is non-emissive. However, in PMMA films at 2wt%, they behave as blue emitters with higher quantum yields. For instance, complexes 88 and 89 display emissions at λ = 418 nm for both compounds but with different quantum yields (Φ) of 0.13 and 0.31 respectively. Complex 90 emit at λ = 459 nm, blue-shifted significantly in PMMA film with respect to CH 2 Cl 2 and with a higher quantum yield of (Φ) = 0.48. Complex 90 gives sky blue coloration with the CIE coordinates of (0.16, 0.18), while complexes 88 and 89 display deep blue emissions with CIE coordinates of (0.16, 0.07) and (0.16, 0.10) respectively. The authors suggest that these mixed carbene complexes can be used as dopants for pure blue OLEDs.

Containing naphthalimide chromophore

Naphthalimide (NI) is a versatile organic chromophore and has been widely used in fluorescent molecular probes and as DNA targeting, anticancer and cellular imaging agents. [START_REF] Nandhikonda | Theme and variation on N-aryl-1, 8naphthalimides: minimal modification to red-shifted fluorescence and applications in fluorescent chemosensors[END_REF][START_REF] Banerjee | Recent advances in the development of 1,8-naphthalimide based DNA targeting binders, anticancer and fluorescent cellular imaging agents[END_REF][START_REF] Jia | Naphthalimides for labeling and sensing applications[END_REF][START_REF] Dong | 1,8-Naphthalimide-based fluorescent chemosensors: recent advances and perspectives[END_REF] More recently efforts were devoted to prepare metal complexes containing this organic chromophore. [131][132][133] Amouri and coworkers prepared a series of phosphorescent heteroleptic cyclometalated Ir(III)complexes displaying N-heterocyclic carbenes linked directly to a naphthalimide fluorophore (NI = naphthalimide). 134 Two sets of compounds were prepared containing NHC-derived naphthalimide ligands (NHC = Imidazolin-2-ylidene, (91-93); Benzimidazolin-2-ylidene, (94-96)) (Figure 19). 135

NH Ir

NH Pr N N X N N X C Cl Ir N N X N N X N N N X N N N N N N N CF 3 CF 3 PrNH 2 CH 2 Cl 2 (excess) (88) (89) (90)
These compounds are very stable in solution and in solid state, due to the presence of the NHC-NI (C^C) ligands. was determined for all these compounds and was found to be in the order of 10

Containing helicenes and chiroptical properties

Another important property displayed by cyclometalated NHC Ir(III) complexes containing three chelating ligands is chirality. [137][138][139] Thus efforts were devoted to merge chirality and luminescence in the hopes of establishing a novel class of compounds with dual properties. 136,140 For instance Crassous et al. designed a unique type of chiral meridional cyclometalated Ir III -compounds containing NHC-helicene derived (C^C:) ligands (Figure 22). [141][142] These complexes were obtained in (70:30) yield as four diastereomers, mer-(P, Λ Ir )-97/(M-Δ Ir )-97 and mer-(P, Δ Ir )-98/(M-Λ Ir )-98.

Each pair comprises two enantiomers, with the chirality arising from a combination of the stereochemistry at the iridium center (Δ, Λ) and the helicity of the NHC-helicene derived ligand (P, M). The X-ray molecular structure of the pure diastereomer 97 was obtained. The emission properties of complexes 97 and 98 were studied and showed light green phosphorescence.

Structured vibrational bands are visible with λ max at 525 nm, red-shifted relative to the model complex 99, and quantum yields of 9-13%, suggesting that emissions originate from the mixed 3 LC/ 3 MLCT transitions and not from only [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] MLCT state as observed in the model complex 99

without a helicene moiety (Figure 22). Remarkably complexes 97 and 98 display long lifetimes τ of around 300µs, which are much longer than that observed for the model complex 99 τ = 3.1µs.

These results also confirm the contribution of the 3 LC-centered transitions originated from the NHC-helicene derived ligand. 141 These complexes were successfully resolved using chiral column chromatography into their corresponding single enantiomers. It should be mentioned that the [5] helicenic carbene ligand displayed high configurational stability (inversion barrier > 125 kJ mol -1 ), thus refluxing of the isolated pure samples in chloroform for several hours did not lead to any epimerization. 23). The authors demonstrate that the stereochemistry of the NHChelicene ligand controls the sign of the CPL signal with a value of dissymmetry factor (g lum = +3.7 x 10 -3 ) at 530 nm for (P, Λ Ir )-97, much higher than that obtained for (P, Δ Ir )-98 (g lum = +1.5 x 10 -3 ).

Moreover the presence of the NHC-helicene ligand amplifies the dissymmetry factor when compared with the model complex Δ Ir -99 (g lum = +9x 10 -4 ) at 493 nm. 141 Moreover the authors prepared another type of chiral cyclometalated iridium compound containing NHC-derived helicene ligand (C^C:), which was resolved into its corresponding enantiomers mer-unit as in the preceding example but attached to the nitrogen center and cyclometalated to the iridium (III) (Figure 24). 143 Figure 24. Cyclometalated iridium-NHC complexes containing [4]-helicenes.

Again, the π-extended helicene ligand changes the photophysical properties of complex 100 when compared to the model cyclometalated NHC compound 99 without the helicene moiety. In CH 2 Cl 2 solution at room temperature, complex 100 displays structured emission bands with λ at 510, 549, 660 and 690 nm and quantum yield Φ = 5% with a very long lifetime of 140 µs. The emission of 100 is red-shifted relative to complex 99 which shows a broad band at λ = 483 nm with much shorter lifetime τ = 1.8 µs. The authors suggest that the excited state in 100 is highly centered on the π-system 3 IL with some participation from 3 MLCT, which is in accord with the long luminescence lifetime 140 µs observed for 100. While in complex 99 the short luminescence lifetime as well as the different form of the mission bands is in accord with a strong contribution from the 3 MLCT state. 143 At this stage a comment is required on the effect of NHC-derived organic ligand (C^C:, helicene, naphthalimide, etc.) on the photophysical properties of these cyclometalated iridium complexes. As mentioned previously, cyclometalated iridium complexes commonly emit from a 3 MLCT state. The presence of the organic chromophore or π-extended system directly linked to the NHC dramatically

N N N N Ir F F N N N N Ir F F mer-Λ Ir -100 mer-Δ Ir -100 F F F F
modifies the nature of the excited states from 3 MLCT to mostly 3 LC, triplet manifold centered on the organic dye.

On the other hand the authors investigated the CPL properties of the enantiopure complexes-(Λ Ir )-100 and mer-(Δ Ir )-100 relative to the enantiomers of the model complex 99. The CPL activity was found to be similar in both types of compounds. It should be mentioned that [4]helicene moiety in these complexes is configurationally unstable in solution and therefore do not impact their CPL properties as observed for compounds 97 and 98 described previously.

Cyclometalated iridium complexes with tridentate ligands

Throughout the previous section we have presented several examples of octahedral cyclometalated NHC iridium complexes with a variety of appealing structures, properties and useful applications. However all of these examples displayed tris-bidentate [2+2+2] chelating ligands coordinated to the Ir(III) center. More recently efforts were devoted to another class of octahedral cyclometalated NHC iridium complexes, which display bis-tridentate [3+3] chelating ligands instead. These investigations are motivated by the fact that only one isomer could be isolated, when compared to the previous iridium tris-bidentate family where at least two geometrical isomers are expected to form. Moreover, tridentate chelating ligands tend to strongly chelate the iridium center and hence deliver highly stable complexes for device applications. However due to the nature of the coordination mode of the bis-tridentate ligands around the iridium center, a strong distortion of the ideal octahedral geometry is expected when compared to those containing tris(bidentate) ligands.

As a consequence this tends to stabilize the MC dd excited state and reduces the HOMO-LUMO energy gap. Again the use of ancillary NHC ligands destabilizes the MC centred excited states and provides entry to highly emissive compounds.

Williams and coworkers described a synthetic approach to obtain cyclometalated iridium complexes displaying the (N^C^N) tridentate chelate. The authors chose 1,3-di(2-pyridyl)-4,6-dimethyl benzene (dpyxH), to react with IrCl 3 .3H 2 O, for which two methyl substituents are attached to the central phenyl ring in order to direct the cyclometalation at the C2-position of the central phenyl ring to afford the dimer [Ir(dpyx-N^C^N)(µ-Cl)Cl] 2 . 144 This dimer reagent and analogous compounds served as key-intermediates to prepare neutral cyclometalated bis(tridentate)iridium complexes by coupling monoanionic (N^C^N), whether (C^N^C) or (C^C^C) and dianionic tridentate chelating ligands. [START_REF] Williams | The coordination chemistry of dipyridylbenzene: N-deficient terpyridine or panacea for brightly luminescent metal complexes?[END_REF]145 For instance Chi, Chou and coworkers combined the monoanionic (C^N^C) containing NHC ligand with the dianionic chromophoric tridentate pyrazolyl based ligand around the coordination sphere of the Ir(III) center in order to obtain the neutral bis tridentate [3+3] coordinated iridium complexes 101-103. 146 Their emission properties ranged from cyan 101, green 102 and to orange-red 103 respectively (Figure 25). The authors demonstrated using TD-DFT calculations that the observed emissions largely originate from the [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] MLCT processes (L represents the dianionic chromophoric chelate) with contributions from ligand-centered π-π* transitions. The emission color is also influenced by the dianionic chromophoric chelate pyrazolyl ligand, ranging from 473 to 608 nm. These compounds were later used as OLED devices. For instance, green OLEDs using complex 102 give a peak external quantum efficiency of 18.8%, a luminance efficiency of 58.5 cd/A and a power efficiency of 57.4 lm/W. As for complex 103 the designed OLED device exhibits saturated red emission with maximum efficiencies of 12.5%, 10.4 cd/A and 9.0 lm/W.

On the other hand, the authors prepared another series of complexes by replacing the monoanionic (C^N^C) ligand with the tridentate (C^C^C) ligand bearing an NHC pincer ancillary. 108,[147][148] The modification is inspired by the belief that a greater Ir-C bond strength of the NHC fragment (versus an Ir-N dative bond) would exert the strongest destabilization of the MC dd and hence enhance the luminescent properties of the target complexes 104-107 (Figure 26).

N N N F 3 C R 1 Ir N N Me R 2 N N Me N N N F 3 C R Ir N N Me tBu N N Me R 1 = F, R 2 = CF 3 (104) R 1 = CF 3 , R 2 = CF 3 (105) R 1 = CF 3 , R 2 = t-Bu (106) (107)
with 27.4% (36.9 cd/A, 36.2 lm/W) was obtained. The authors consider that these Ir(III) phosphors display among the highest EL efficiencies. Moreover Chi, Fox and Wu designed other bis-tridentate [3+3] coordinated Ir(III) phosphors 108-112 in order to obtain deep-blue OLED devices (Figure 27). [149][150] In order to achieve their objectives, the designed molecules displayed larger HOMO-LUMO gaps compared to those described previously. The synthetic approach follows that described previously, combining a monoanionic tridentate (C^C^C) ligand bearing two NHC units and the chromophoric dianionic ligand 6-pyrazolyl-2-phenoxy pyridine around the Ir(III) center. A major difference relative to the preceding complexes however resides in supressing the π-conjugation between the F-phenyl group and central pyridine unit by placing an oxygen atom between them. emits at λ = 441 nm, with Φ = 0.67), highlighting the importance of their synthetic strategy to prepare blue emissive [3+2+1] coordinated iridium complexes. Furthermore due to its high emission performance, complex 122 was used as an emitter in the design of blue OLED with maximum external quantum efficiency of 22.94% and CIE coordinates of (0.14, 0.24). Moreover the related Ir(III)-OCN complex [Ir(F2ppy)(C^C^C)(OCN)] ( 123) acted as a sky-blue OLED device at 20wt% in 4,4',4"-tris(carbazol-9-yl)triphenylamine (TCTA) used as host material and showed comparable performance relative to the well studied [2+2+2] sky-blue iridium complex [Ir(F2ppy) 2 (picolinate)] (FIrpic) [153][154][155][156] OLED device under similar experimental conditions. But, at a high luminance of 1000 cd m -2 , complex 123 displayed improved efficiency roll-offs.

Cyclometalated iridium complexes with bis carbene (NHC) chelating ligands

Cyclometalated NHC iridium complexes emitting in the deep-red and NIR region were also developed by the group of Mao for their anticancer properties. For instance the authors designed the heteroleptic cationic cyclometalated Ir(III) complexes [(C^N) 2 Ir(C^C)][Cl] (138-139) containing a bis-NHC ligand and two dibenzo[a,c]phenazine (dbpz) ancillary ligands (Figure 30). 157 nm, while in phosphate-buffered saline (PBS) they behaved as NIR emitters with emissions at λ max = 765 nm and λ max = 812 nm. These complexes displayed lifetimes τ in the nanosecond regime, which also varied from 273ns to 674ns upon moving from PBS buffer medium to CH 2 Cl 2 and CH 3 CN. Under similar conditions the quantum yields Φ also varied from 0.95% to 14.3%, and was found to be dependent on the solvents polarity. These compounds show interesting mitochondriatargeting anticancer properties and potential use as photodynamic agents. The authors indicate that complexes 138-139 show higher cytotoxicity against cancer cells when compared to the benchmark cisplatin and they also display higher photocytotoxicity under 450nm and 630nm LED light.

Che and coworkers designed a novel class of cationic phosphorescent iridium complexes (140-148) supported by bis(NHC) ligands containing a variety of cyclometalated (C^N) ligands for a multiple useful applications spanning from photocataysis driven by radical cyclization, to CO 2 reduction and also for cellular imaging (Figure 31). 100 Again as mentioned previously in this document, it is the kind of emission energies and lifetime durations that justify the kind of applications. The bis(NHC) ligands bring a great stability to these molecules, and variation of the R-group on the nitrogen atom of the carbene ligands ( R = Me, nBu, Glu: Glucose) affects the solubility of these compounds and make them soluble in water ( R = Glu). On the other hand the nature of the (C^N) ligands impacts their photophysical properties and the nature of the excited states whether 3 MLCT, It is noteworthy that such chemical transformation was also performed in aqueous media (H 2 O/MeOH; 3/1) using complex 145 H c (Φ = 66%, τ = 5µs), which is soluble in water thanks to the glucose substituents on the bis-carbene ligands (see Scheme 2). The target product was formed in 87% yield with 99% substrate conversion. The performance of 143 Me b again exceeded that of the well known photocatalyst fac-Ir(ppy) 3 which under similar experimental conditions led to only (TON>900) (CO) and a selectivity of 85%.

Finally the authors also demonstrated that such bis-carbene cyclometalated iridium complexes with a variety of (C^N) ligands can be used as luminescent probes for bioimaging applications and display high cytotoxicity against Hela cancer cells with N-butyl bis-carbene ligands when compared with the related compounds with Me and glucose functional groups. 100 The next section deals with recent advances in luminescent coinage metals containing Nheterocyclic carbenes. 158

4-Luminescent coinage metals with N-heterocyclic carbene ligands

Coinage metal complexes with alkynyl ligands

Coinage metal complexes display low-dimensional coordination geometries. As a consequence they tend to form non-covalent metal-metal interactions, the best known are complexes containing Au(I) metal centers. [159][160] These kinds of interactions, which impact the photophysical properties 161-169 of the desired compounds, have been coined as aurophilicity by Schmidbaur [170][171] and were investigated in theoretical studies performed by Pyykkö. [172][173] Unlike luminescent complexes with iridium and platinum, which depend on strong spin-orbit coupling (SOC) to generate triplet metal to ligand charge transfer [START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] MLCT emission, the two-coordinated coinage metal complexes emit via prompt fluorescence or thermally activated delayed fluorescence (TADF). 174 Their fast radiative lifetimes are mainly related to two factors: (1) the small energy gap between their lowest singlet and triplet excited states (ΔE ST ) and ( 2) spin-orbit coupling displayed by the metal center. These particular properties enable them to be important phosphors to act as optical devices and for a wide range of applications. [175][176] Poyatos and coworkers designed some gold alkynyl complexes 149-152 attached to a central pyrene organic chromophore (Figure 32). 177 Complexes 149-151 were found to be luminescent in solution at room temperature. In particular compounds 149 and 150 display high luminescent quantum yield Φ = 90%-92% and lifetime in the nanosecond regime while complex 151 exhibits a lower quantum yield of 38% and τ = 1ns. For comparison purposes, the monometallic gold complex 152 was also prepared and was found to be nonluminescent. The emission spectra of NHC gold complexes 150-151 recorded in degassed CH 2 Cl 2 display pyrene-centered vibronic bands at λ = 481, 507, 543 nm and λ = 481, 508, 546 respectively. Complexes 150-151 displayed broad band emissions at λ max = 585 nm , (Φ = 2.2%) and at λ max = 541 (Φ < 1%) respectively in the solid state at room temperature. This kind of emissions was assigned to the formation of pyrene-excimers. The authors attribute this behavior to π-π stacking interactions occurring in the solid state, which is responsible for the aggregation quenching of the emission. At low temperature (T = 77K), the emission color of complex 150 switches from blue to yellow, which also might originate from formation of aggregates of 150. The 1 H-NMR spectra of 150 recorded at variable concentration (0.1-20mM) shows that the aromatic protons of the authors suggested that aggregation through π-π stacking is occurring at high concentration. The association constant K = 48 M -1 was determined.

Due to its high optical properties, complex 150 was tested as a bioimaging probe in healthy cheek cells. After 30 min, intense luminescence was observed in the nucleus, proving the efficient uptake of 150 into the cell interior.

Gray and coworkers described the synthesis and photophysical properties of several mononuclear Au(I) complexes (153-155) attached through a Au-C σ-bond to a benzothiazole-fluorenyl chromophore and displaying either a phosphine or NHC as ancillary ligand (Figure 33). For comparison purposes they also prepared the related alkynyl Au complexes (156-158), in which the organic chromophore is now attached to the acetylide unit, acting as a linkage between the organic chromophore and the gold center. 178-179 crystals of the latter displayed dual emission bands at λ = 586nm and λ = 652nm. Upon changing the ratio of both anions in the assembly, the luminescent properties of the resulting triple salt were altered for instance at a ratio of (160-A8/A10: 200:198:2), white light-emitting species with CIE coordinates (0.37, 0.31) and a quantum yield of 73% were obtained. These results highlight the importance of the aurophilic interactions on the luminescent properties of the coordination assemblies of coinage metals. Indeed the authors have used this approach to construct chiral gold complexes featuring aurophilic interactions, which enable them to act as smart luminescent materials. The work is mentioned on section 4.3 of this review.

Tunik and coworkers designed some luminescent gold alkynyl complexes supported by NHC ligands and displaying aurophilic interactions (Figure 35). The authors obtained these compounds starting from the NHC-gold halogenated precursors followed by treatment with the appropriate alkyl-or aryl-acetylene in the presence of a base. The dinuclear gold complexes 166-174 were obtained in a short period of time, however the mononuclear species required 5 hours to obtain pure mononuclear gold species 163-165. When the reaction was left to proceed for only 1 hour followed by crystallization a rare class of cationic trinuclear species 175-176 was isolated in which the anionic [Au(RC 2 ) 2 ] -1 acetylide complex is sandwiched by two cationic bis-carbene [(NHC) 2 Au] +1 species displaying aurophilic Au⋅⋅⋅Au backbones. The crystal structure of 175 shows short Au⋅⋅⋅Au interactions of 2.9375 (7) similar to those reported in the previous section. Perhaps due to steric hindrance around the gold centers discrete cationic species were obtained instead of a 1D supramolecular coordination polymers displaying alternate anionic/cationic motif.

The luminescent properties of these compounds were investigated in fluid media and solid state. In general the NHC-Au-alkylalkynyl complexes whether mononuclear or binuclear were non-emissive in solution, unlike the related phenyl-alkynyl complexes. In solid state, the mononuclear alkylalkynyl complexes 164-165 were also non-emissive, while their dinuclear congeners 167-168, 170-171, 173-174 were luminescent, however in the absence of crystal structures no correlation between their structural organization and emission properties was made. The mononuclear complex phenylalkynyl 163 was isolated in three different polymorph forms and a pseudopolymorph and their crystal structures were determined: a blue emissive polymorph 163 blue , with no aurophilic or π-stacking interactions, two green emissive species denoted as 163 green and 163 greenDCM displaying similar aurophilic interactions (Au⋅⋅⋅Au = 3.3603(4) Å and Au⋅⋅⋅Au = 3.4190(3) Å respectively).

The only difference between two crystals is the presence of 0. 38CH 2 Cl 2 molecule per formula unit in 163 greenDCM . The third yellow-emitting polymorph 163 yellow , showed a long aurophilic interaction distance of 3.6164(3) Å. Furthermore, the molecules are disposed in head-tail fashion with πstaking between two adjacent NHC ligands with distance of 3.350Å. The room temperature emission spectra of complex 163 blue in solution and solid state were similar and displayed wellresolved vibronic structure with spacing of 1100, 1600 and 2100 cm -1 ascribed to phenyl and acetylide -C≡C-stretching modes. Thus the observed emissions is thus assigned to a gold-perturbed structureless suggesting that the excited state may be localized on the Au⋅⋅⋅Au aurophic interactions and not on the phenylalkynyl moiety. It should be mentioned that the complex exhibiting longer Au⋅⋅⋅Au interaction (163 yellow d Au-Au = 3.6164(3) Å) displayed the lower emission energy at λ max = 573 nm, relative to the green polymorphs 163 green , 163 greenDCM possessing shorter Au⋅⋅⋅Au interactions with λ max = 524 nm and λ max = 527 nm. In general, it is expected that complexes displaying shorter Au⋅⋅⋅Au aurophilic interactions usually display lower energy emission bands. 160,166,[185][186][187][188][189][190] The authors ascribed their counterintuitive structure-luminescence behavior to the different extents of distortion of the Au⋅⋅⋅Au interactions triplet in the excited states. Anomalous structure-luminescence behavior has been reported. [191][192][193] Similar behavior was observed for the dinuclear gold phenyl alkynyl complex 172 and also to the cations trinuclear complex 175 as well.

Coinage metal complexes containing naphthalimide chromophores

Amouri et al. set up a novel synthetic approach to obtain coinage metal carbene complexes in which an organic chromophore (naphthalimide) is attached to a carbene unit (Figure 36). Two families of compounds were prepared, one class featuring direct linkage to the NHC-unit [(L 1 )-M-Cl], M = Cu (177), M = Ag, (178) and M = Au (179) while in the second class the naphthalimide fluorophore is distal from the metal center by an alkynyl-arene bridge [(L 2 )-M-Cl], M = Cu (180), M = Au, (181). 194 The purpose of this study is to probe the effect of the chromophore on the optical properties of this kind of compounds. exhibited bathochromic shifts when compared to the starting material L 1 -H + I -(Figure 36). Those with extended π-system [(L 2 )-M-Cl] (180, 181) displayed a similar behavior. Furthermore, they were found to be more emissive, with Φ = (0.01-0.07) for complexes 179, 177 and Φ = (0.21-0.29)

for compounds 180-181 respectively.

Jamali and coworkers described the synthesis of some coinage metals containing four naphathalimide units attached to NHC-ligand through an ethylene chain (Figure 37). 195 metal or the NHC ligand. The emission spectrum of 182 shows a broad structured emission with λ max = 385 nm, which the authors assigned to monomeric and excimeric emissions. While the azolium salt 184 with the amine functional group displayed a broad band at λ = 500 nm. The authors assigned this band to a mixture of excimeric and also to ICT (internal charge transfer from the amine to naphthalimide unit) emissions. To rationalize the observed emission properties, DFT and TD-DFT calculations were used to calculate the optimized geometries of the ground states 185-188, as well as their singlet excited states. Two main parameters were found to have significant effects on the photophysical properties of these NHC-coinage metals 185-188 with naphthalimide chromophores. (i) The intramolecular distance d between the two adjacent naphthalimide rings to assure interchromophoric interaction which is enhanced in solution. (ii) The dihedral angle between the long axes of the two adjacent naphthalimide units. In solid state this interchromophoric dipole moment is at maximum for α angle of 38°. Moreover, these interchromophoric π-interactions increase from solid state to solution and also with metal ion radius on moving from Cu(I) to Ag(I) and Au(I). In solution complex 187 shows in addition strong intramolecular hydrogen bonding between the two amine substituents which bring rigidity to the system in light of the high quantum yield of 92% observed in solution.

Coinage metal complexes with chiral ligands and chiroptical properties

Chen and coworkers prepared some chiral gold (I) dimers [(R, R)-189][A] and [(S, S)-189][A],

displaying a metallophilic interaction. The synthetic approach follows the one described in section 4.1, however in these examples the cationic carbene-Au complexes contain chiral functionalities tethered to the backbone of the NHC)-Au-unit (Figure 38). The optical properties of these complexes have been investigated in crystalline and amorphous forms and provided distinct behavior (Figure 39). For instance when crystals of [(R, R)-189][A] were excited at 365 nm, a vibrant greenish blue emission was observed (CIExy = 0.17, 0.36), the emission spectrum displayed a band at λ max = 500 nm with τ = 1.29 µs, (Φ = 65%) and a shoulder at 450 nm with lifetime in the nanosecond regime. Thus the high energy emission can be ascribed to a fluorescence while the low energy emission with slow decay in the order of µs can be assigned to a phosphorescence emission. When the excitation wavelength was changed from 320 nm to 400 nm, the fluorescence and phosphorescence emission energies remain unchanged as can be expected following Kasha's rule. 199 In contrast, when a single crystal of [(R, R)-189][A] was mechanically ground, the photoluminescence properties were found to be dynamically dependent on the

Au C Au C Aurophilic Interaction N N N N Au N N + (R, R)-189 N N Au N N + (S, S)-189 N C N C Au Cation = C Cation = C Anion = A + + N N N N Au C Au C Aurophilic Interaction N N N N N N 2.998 Å 2.998 Å [(R, R)-189][A] [(S, S)-189][A]
excitation wavelength. Interestingly the emission λ max = 440 nm shifted to λ max = 526 nm, upon changing the excitation wavelength from 300 nm to 400 nm. This interesting phenomenon is known as excitation wavelength-dependent photoluminescence (Ex-De PL) and might have important applications as anti-counterfeiting, in optoelectronics and also in the field of biomedical imaging. Similar behavior was obtained when the luminescent properties of [(S, S)-189][A] were studied in thin film of PMMA (poly(methyl methacrylate). The excitation spectra were red shifted with increasing emission wavelength suggesting the existence of different emission origins in the ground-state. Moreover the emission spectra remain unchanged when the concentration of the complex in PMMA was increased from 5% to 40%. These results preclude the presence of different oligomeric species whose composition is concentration dependent and support the presence of only dimeric species displaying different Au⋅⋅⋅Au aurophilic interactions in amorphous form.

Experimental evidence was provided by the Raman spectra recorded for a single-crystal of [(R, R)-189][A] and the corresponding amorphous powder. In crystalline form the Raman spectrum shows a band at ν value of 88 cm-1, upon grinding the crystalline material to amorphous state, the spectra evolved and showed a broad band that spans from 70 to 91 cm -1 indicative of the presence of myriad Au⋅⋅⋅Au interactions. These various dimeric species are at the origin of the excitation wavelengthdependent photoluminescence. In contrast a uniform Au⋅⋅⋅Au bond distance is displayed in the single-crystal form (Figure 39). investigated and were also excitation wavelength dependent (Figure 40). Owing to its excitation dependent luminescent properties, ground powder of [(R, R)-189][A] was used as solid ink to reveal painted panda and TIPC images on a filter paper (Figure 41). The image color changed from blue to green in a dark room upon changing the excitation wavelength from λ = 245 nm to λ = 365 nm.

Coinage metal complexes containing carbazolate ligands

Carbazole derivatives are important organic chromophores: they display rigid planar structure, strong donor ability and rich photophysical and hole-transporting properties. 46,200 43,45,201 Owing to these properties, extensive attention has been devoted to preparing fluorescent molecules based on carbazoles for variety of applications 202 including bioimaging probes. [START_REF] Jiang | A Review on Synthesis of Carbazole-based Chromophores as Organic Light-emitting Materials[END_REF][START_REF] Yin | A versatile small-molecule fluorescence scaffold: Carbazole derivatives for bioimaging[END_REF] In recent years efforts were devoted to construct novel class of luminescent metal complexes containing carbazolates and amides as ligands in which the anionic N-atom is bonded directly to the metal center. [203][204][205][206] As mentioned in this review, such molecules provide a platform where organic and inorganic chromophores are merged to generate novel excited states and hence it is expected to bring highly important electronic properties.

In 2005, Lin and coworkers prepared several carbazolate-Au(I)-NHC complexes (190-193) (Figure 42). 207 The crystal structure of 190 was reported and shows that the carbene and carbazolate ligands are coplanar with no Au⋅⋅⋅Au interaction (d Au ⋅⋅⋅ Au = 3.94Å) between two adjacent molecules; moreover the individual molecules form aggregates through π-π interactions of 3.42 Å between the carbene ring and the carbazolate ring and they display weak Au---H contacts. The optical properties of these compounds were investigated in fluid and in solid state. In general the gold compounds displayed the higher stability in solution relative to the copper and silver congeners and due to their strong SOC (spin-orbit coupling) intersystem crossing (IST) S 1 →T n occurs with higher efficiency. For instance, in THF solution the silver complex 197 showed dual emission; (i) a fluorescence emission between 380-440 nm assigned to metal perturbed [START_REF] Strassner | Phosphorescent Platinum(II) Complexes with C^C* Cyclometalated NHC Ligands[END_REF] respectively. Due to the fast k ISC > 10 10 s -1 in the gold complexes, the fluorescence emission is quenched in favor of the phosphorescence. On the other hand, their low k phos = 10 -3 s -1 value allows them to act as good photo-catalysts because they possess long-lived triplet states. Thus their properties as photo-catalysts were explored in particular for complex 200 (vide infra).

The emission of the silver complexes 197-199 in the solid-state exhibited dual emissions; a fluorescence broad band from LC 1 CBZ at λ flo = 390 nm with nanosecond life-time regime and phosphorescence emission between λ phos = 480-700 nm (τ = 45µs and 355µs for 197 and 199).

The triplet-state band is structured but broad and the excitation spectra in the range of 400 to 480 nm different from the absorption observed in solution, suggesting that aggregation in solid state affects their luminescent properties.

The gold compounds 198-200 display phosphorescence emissions similar to those observed in solution (λ phos = 424nm) but with shorter lifetimes τ = 74 and 38 µs respectively. Interestingly a second lower energy phosphorescence structured emission is observed at λ = 550 nm with τ = 885 µs and 347 µs respectively. The excitation spectra of the phosphorescence emissions of the gold complexes corroborate with their absorption in solution, however the longer phosphorescence excitation spectra show additional bands between 400-440 nm. These results suggest again that aggregation affects the luminescent properties in the solid state and the change in life-time suggest that aggregation exists already in the ground-state and not only the result of excimer formation in the solid state.

The Au-complex (200) displayed a long-lived triplet state in the blue-green range with the highest photostability, hence it was selected by the authors as a photo-catalyst for the [ 2+ 2] intramolecular cycloaddition of (E, E')-dicinnamyl ether to give the corresponding fused cyclobutane. Upon irradiation at λ = 365 nm with 18 W LED, at 5% catalyst loading, total conversion (> 99%) to the desired product was achieved after 4 hours (see Scheme 3 and Table 1).

These data suggest that complex 200 is highly efficient for the photocatalysis reaction and represent a proof-of-principle where two coordinate NHC-Au-carbazolate complexes with longlived triplet state can be used as an efficient photocatalyst. 209 Scheme 3. Photocatalysis of (E, E')-dicinnamyl ether and formation of fused cylobutane. As will be shown in section 5 the related compounds containing electrophilic carbenes (CAAC = cyclic (alkyl)(amino) carbene), (MAC = monoamido-amino carbenes), (DAC = N, N'-Diamidocarbene) display short triplet lifetimes which make them efficient emitters for OLED application. 212 We feel a review on luminescent carbene complexes will be incomplete without mentioning this novel class of highly emissive compounds. This work is presented in section 5.

Three-and four-coordinate coinage metal complexes.

Thompson and coworkers reported some phosphorescent three-coordinate carbene copper complexes 209-212 containing anionic non-conjugated (N^N) ligand, mainly di (2pyridyl)dimethyl borate (208) (Figure 45). 213 Complexes 209-211 were found to be emissive in solid-state and provided sky-blue, yellow and orange emissions with quantum yields varying from 0.8, 0.7 and 0.16 respectively with life-times in the microsecond regime (11µs, 15 µs and 7.5 µs respectively). consequence for 212 this reduces effectively the interaction with MLCT states responsible for promoting fast radiative decay.

Another class of three coordinate Cu(I) complexes (213-217) was reported by Cisnetti, Steffen and coworkers (Figure 46). 214 The dimer complexes were characterized by X-ray diffraction studies Roesky and coworkers described the synthesis of cationic bimetallic Cu(I) and Ag(I) carbene complexes 218-219 (Figure 47). 215 The mesityl-NHC is tethered to a bipyridyl ligand, which chelates the metal center generating three coordinate metal complexes. The rigidity of the tridentate ligand brings the two metal centers closer, generating a metal---metal interaction as confirmed by X-ray diffraction study, which provides a tetra coordinate geometry around each metal center. Crystallization of the metal complexes provided red crystals for the copper 218 and colorless crystals for the silver compound 219. The X-ray structures of 218 and 219 showed that the Cu----Cu contact is at 3.2168( 9) Å and for Ag---Ag is at 3.1034 (11). The copper complex displayed an absorption band at λ ~ 450 nm in acetonitrile, and exhibited a weak broad band emission in the NIR zone at λ ~ 800 nm. As for the silver compound the authors indicated that the photoluminescence behavior was complicated and unusual. For instance the life-time decay traces were temperature dependent and could be fit with at least three exponential components. At 16 K a long lifetime τ = 23 msec in the millisecond regime was measured which upon increasing the temperature shortens to the tens of microseconds at 295K. The authors suggest that a phosphorescence process is occurring which tentatively involves Ag-bpy groups and also the Ag---Ag argentophilic interaction. At room temperature the quantum yield for 218 was found to be Φ = 0.4%, and increases to 9% at 15 K. In the absence of TD-DFT calculations no details on the nature of the excited states involved were discussed.

Dinda and coworkers described some silver and gold carbene cationic complexes displaying trigonal geometry around the metal center (Figure 48). [216][217] For comparison purposes the tetracoordinate Cu(I) complex 228 was prepared in which the CAAC ligand was replaced by NHC ligand in order to probe the carbene effect on the luminescent properties of the related complexes. Compound 228 exhibited blue phosphorescence emission at λ max = 462 nm, τ = 29µs and Φ = 0.25. DFT calculations performed on 228 demonstrated that E LUMO = -0.37eV while for complex 227 with CAAC ligand, E LUMO = -0.74eV which is highly stabilized providing a smaller HOMO-LUMO gap and eventually the expected yellow phosphorescence but with higher PLQY and lower lifetime when compared to 228.

This example highlights the importance of CAAC ligands and their use in preparing efficient phosphorescent complexes. This type of complex is detailed in the next paragraph.

5-Luminescent coinage metals with CAAC, MAC and DAC carbene ligands

In the previous section we described a variety of luminescent complexes containing N-heterocyclic carbene (NHC) with appealing structures, and demonstrated their useful applications. It is noteworthy to mention that in the past five years there has been a great interest to investigate another class of metal carbene complexes due to their remarkable luminescent properties where the carbenes display more π-accepting character compared to the N-heterocyclic carbenes (NHCs).

These carbenes are designated by Cyclic (alkyl) (amino) carbene (CAAC), N, N'-diamidocarbene (DAC) and monoamido-amino carbenes (MAC). For instance, the classical N-heterocyclic carbene contains two σ-electron-withdrawing and π-donor N-atoms, while in CAAC, a σ-donating carbon center is now visible instead of one N-atom. As a consequence, the CAAC displays only one σelectron-withdrawing and π-donating N-atom, which generates a smaller HOMO-LUMO gap in CAAC when compared to the classical NHC. DFT calculations on CAAC revealed that the LUMO is lowered, concomitant with a rise in the HOMO relative to NHC. [220][221] This kind of carbene was coined by Bertrand and coworkers in 2005. 222 Two other non-conventional carbenes are DAC and MAC which also show high electrophilicity, comparable to CAAC. They include amido groups in their chemical structure, which increase the electron-withdrawing properties of the carbene and consequently lower the energy level of the LUMO. [223][224][225] Bochmann, Romanov, Linnolahti, and Credgington prepared and studied the photophysical properties of some luminescent coinage metal complexes containing carbenes as well as amide ligands and displaying linear two-coordinate geometry (Figure 51). [START_REF] Chotard | Mono-versus Bicyclic Carbene Metal Amide Photoemitters: Which Design Leads to the Best Performance?[END_REF][204][205]212,[226][227][228][229][230][231][232] Au, 236) displaying six-membered rings in the cyclic (alkyl)(amino) carbene (Figure 52).

Interestingly the related bicyclic carbene metal amide complexes displayed higher performance; this was attributed to an increase in the rigidity of the carbene ligand which minimizes the nonradiative deactivation processes, generating CMA complexes with 100% quantum yields and shorter excited state lifetimes on the order of 0.5µs. [START_REF] Chotard | Mono-versus Bicyclic Carbene Metal Amide Photoemitters: Which Design Leads to the Best Performance?[END_REF] Figure 52. Bicyclic carbene coinage metal amide complexes.

Thompson and coworkers also investigated the optical properties of luminescent coinage metal complexes displaying linear coordination geometries with only a two-coordinate arrangement around the metal center. [233][234] The authors took advantage of the steric hindrance displayed by the ligands to bring rigidity to the desired molecules and to avoid structural deformation in solution, which should eventually impact their photophysical properties. These the anionic ligand (Figure 53). [START_REF] Hamze | Highly efficient deep blue luminescence of 2-coordinate coinage metal complexes bearing bulky NHC benzimidazolyl carbene[END_REF]203,206,235 In these compounds, the authors highlight the electrophilic character of the carbenes. [224][225] Hence, as in the previous section, these (carbene)M(amide) compounds display intramolecular charge transfer "ICT" emission, with electron transfer occurring from an amide-N (Donor) to carbene-C (Acceptor). This transition is also known as ligand-to-ligand charge transfer "LLCT". The luminescence color of these complexes to be tuned from deep blue to deep red by changing the electrophilicity of the carbenes as well as the nucleophilic properties of the amides bound to the metal center. Moreover they are highly emissive with quantum yields (Φ PL > 80%). For instance, complexes with BZI carbenes showed similar behavior (structures, redox potentials, and photoluminescence quantum yields, Φ PL = 80%-100%) to those with MAC carbenes, but displayed different excited state dynamics and are reported to have efficient blue photoluminescence and to give good efficiencies as dopants in OLEDs. For instance, the gold carbene 239 in 1,3-bis(triphenylsilyl)benzene host, was used as a dopant for OLED device and displayed an external quantum efficiency of 12% with narrow deep-blue emission with CIE (0.16, 0.06). [START_REF] Hamze | Highly efficient deep blue luminescence of 2-coordinate coinage metal complexes bearing bulky NHC benzimidazolyl carbene[END_REF] The coinage metal complexes with MAC and DAC carbenes display narrower HOMO-LUMO gaps compared to the other series, because of the more electrophilic character of the carbene ligands which contain carbonyl functions in their cyclic structure that tend to lower the LUMO. For instance, the reduction potentials for the DAC complexes are more anodically shifted relative to the analogous MAC compounds. Hence emission in these derivatives 243-245 is bathochromically shifted to the lower energy of the light spectrum. However, substitution of the carbazole ligand with cyano groups tends to stabilize the HOMO and blue-shift the emission as observed for complexes 240 and 241. In general, the absorption spectra of these compounds displays negative solvatochromism, while the emission spectra undergo red shift in polar solvents with decrease in quantum yields and lifetimes. For instance, complex 242 displays (Φ PL = 0.90, τ = 1.55µs) in MeCy (methylcyclohexane) and (Φ PL = 0.18, τ = 0.41µs) in CH 3 CN. At ambient temperature in polystyrene thin film, the observed emission of complex 242 is attributed to thermally activated delayed fluorescence [236][237][238] (TADF), because of the large increase in emission lifetime upon cooling to 77K and to the small energy separation between the singlet 1 LLCT and triplet 3 LLCT transition states. The TADF process often occurs at room temperature with short emission lifetimes. This is due to spin-allowed emission from the S 1 state upon thermal rISC (reverse intersystem crossing) T 1 → S 1 . At lower temperatures however, the emission lifetime becomes much longer up to one or three orders of magnitude due to impediment of rISC and less efficient emission from the T 1 state, which leads to a sigmoidal lifetime curve typical of TADF. [START_REF] Czerwieniec | Cu(I) complexes -Thermally activated delayed fluorescence. Photophysical approach and material design[END_REF]239 Due to its high optical performance the copper complex (MAC)Cu(carbazole) (242) was doped at 40% into 3,3'-di(9H-crabazol-9-yl)-1,1'-biphenyl (mCBP) for green OLED devices and provided a maximum EQE = 19.4%. 235 On the other hand the copper complex (CAAC)Cu(carbazole) (246) was doped into 1,3 bis(triphenylsilyl)benzene (UGH3) at 20 volume % to generate an efficient blue OLED device with EQE = 9% and 16cd/A at 2mA/cm 2 . 203 It should be mentioned that EQE and emission energies of OLEDs strongly depend on the structure of the OLED and the chosen host material. A critical assessment is only possible if these factors are taken into account. [240][241] The importance of luminescent carbene-metal-amide (CMA) complexes has also stimulated theoretical chemists and physicists to perform computational studies in order to support the experimental observations and to rationalize the obtained results. [242][243][244][245][246][247] For instance, the Shuai group employed a hybrid quantum mechanics and molecular mechanics (QM/MM) approach to investigate the TADF pathways of two CMA complexes in solution and solid state. The authors demonstrated that intersystem crossing (ISC) and reverse intersystem crossing (rISC) are enhanced by two to four orders of magnitude upon aggregation i.e. by close proximity of individual molecules in solid state but without formation M-M contacts discussed earlier in this manuscript. The steric hindrance provided by the CAAC ligand employed does not permit close intermolecular contacts. As a consequence this aggregation leads to higher TADF efficiency in the solid state. 248 The Cui group used DFT/MRCI (Multireference Configuration Interaction) methods to investigate the TADF pathways of a CMA copper compound displaying a bicyclic six membered ring in the CAAC ligand (Figure 54). The authors suggest that a model with only three states (S 0 , S 1 and T 1 ) is sufficient to describe the TADF mechanism, because the S2 and T2 states are located at higher energy, and thus play a negligible role. The calculated S1-T1 energy gap is only of 0.1ev which is small and due to spin-orbit couplings, this facilitates the reverse intersystem crossing from T1 to S1 at room temperature. The calculated constant k rISC = 8.81x10 6 s -1 consequently TADF pathway occurs. At 77K the rISC is very slow with k rISC = 6.42 x 10 1 s -1 , which is much slower than the T 1 phosphorescence rate and hence the rISC process, becomes thermodynamically unsupportive. 249 In these compounds the use of CAArC containing an organic π-fluorophore instead of CAAC promotes a red shift in the UV-vis spectra where absorption bands with strong extinction coefficient are displayed at lowest energy of the visible spectrum and mainly for complex 254. At room temperature in THF solution only complex 250 was found to be emissive. The authors suggest that in polar THF solution the nonradiative decay is very efficient in these Cu(I) CAArC complexes. However, in solid state these complexes 249-250 are brightly emissive in the red region with λ em = 621 nm (249) and 651 nm (250) with quantum yields up to 32%. The trigonal complexes 251, 252 as well as the half-sandwich CpCu-complex 253 were emissive with displaying emissions at λ max = 760nm, 772nm and 700 nm respectively. Complex 254 was not emissive. The (CAArC)Cu(carbzole) complex 250 was found be the most emissive in this class of compounds. Linnolahti and Bochmann and coworkers designed another class of two coordinate coinage metal complexes featuring a CAAC carbene and an anionic carbene ligand based on methylmalonatederived NHC (maloNHC). 250 These complexes can be described as zwitterionic mixed carbene coinage compounds 255-257 (Figure 56). This type of anionic carbene ligand was first isolated by Cesar, Lavigne and coworkers, and demonstrated that such carbenes are strongly donor ligands and form stable metal complexes. 251 In contrast to the carbene-metal-amide complexes described earlier, these complexes show low PLQY with long lifetimes (400 -50 µs) highlighting the importance and the nature of the ligands bound to the coinage metal which impact the photophysical properties of the designed complex. In this case, it involves the replacement of a carbazolate ligand by anionic maloNHC.

On the other hand, Ung and coworkers prepared recently the chiral carbene Cu(I) complexes displaying CAAC ligands. 252 Starting with L-menthol as chiral auxiliary, the C 1 -symmetrical Cu(I)-Cl complex LMent CAACCuCl (258) containing cyclic(alkyl)(amino)carbene was obtained following the synthetic procedure reported by Bertrand and coworkers. [253][254][255] The other unknown enantiomer DMent CAACCuCl (258) was obtained from D-menthol (Figure 57). The crystal structures of both enantiomers were obtained and confirmed the formation of the target compounds. The circular dichroism spectra of both compounds were recorded and showed opposite image absorption bands, confirming the enantiomeric relationship (Figure 58) The circularly polarized luminescence (CPL) properties of LMent CAACCuCl (258) and DMent CAACCuCl (258) display opposite polarization emissions with dissymmetry factor (g lum = +1.1 x 10 -3 ) at 555 nm for LMent CAACCuCl and (g lum = -1.2 x 10 -3 ) for the other enantiomer DMent CAACCuCl (Figure 58). It is worth mentioning that the LMent CAACCuCl (258) and DMent CAACCuCl (258) complexes displayed a significant circularly polarized emission despite the fact they lack the presence of a helical group or axial chirality; however the authors believe that such compounds display significant spin-orbit coupling which increases the rotary strength of the transition and eventually increases the chance of observing a circularly polarized emission signal. 252 Thus such compounds with chiral (CAAC) ligands pave the way to the preparation of a new class of enantiopure luminescent complexes displaying interesting CPL properties, in a field which is still in its infancy.

6-Summary, outlook and perspectives

Throughout this document, we have shown via many examples that NHC-based ligands are adequate tools to construct stable luminescent complexes. The fact that the structure of NHCbased ligands can be easily modified and functionalized brings important modifications to the photophysical properties of the final compounds. As mentioned throughout this review, their strong σ-donor properties tend to push the metal-centered d-d transitions to higher energy in order to make them thermally unattainable. Various luminescent carbene-metal complexes with different geometries based on platinum, iridium and coinage metals were included in this review. Moreover the luminescent properties of these compounds were discussed as well as the dynamic of the excited states, which impact their luminescent properties at the molecular and supramolecular level and hence determine the nature of potential applications. Moreover a special treatment was given to coinage complexes with carbazolate ligands; this field has been the focus of intense investigations in the past five years because of their remarkable luminescent properties. For instance coinage complexes with NHC-ligands tend to show long lifetimes and hence they are more appropriate to act as photoredox catalysts. On the other hand, the discovery by Bertrand and coworkers of CAACs--carbenes which display lower HOMO-LUMO gap compared to NHCs and are more electrophilic carbene ligands--with more π-acceptor carbene ligands have enabled the construction of highly emissive compounds with PLQY > 99%. These electrophilic carbenes including DAC and MAC generate luminescent compounds with sub-microsecond lifetimes regime, which make them appropriate for OLED applications. Thus remarkable emitting OLED devices with EQE max of 9.0% (blue) and 26.3%, (green) were achieved. It should be mentioned however, that blue emitting coinage-based OLEDs display lower efficiencies when compared to the yellow-or orange-emitting metal-based OLEDs (EQE >20). In this respect, a novel generation of coinage metal carbene compounds can be engineered to achieve higher EQE with blue emissions to reach the industrially competitive requirements.

We also demonstrated for the iridium complexes that the presence of organic chromophores (naphthalimide, pyrene) in the structure of the NHC-based ligand brings important modifications to the final optical properties of these compounds. For instance, while cyclometalated Ir(C^N) 3 generally emit from 3 MLCT excited states, the above compounds display emissions from 3 LC ( 3 ππ*) centered at the organic chromophore. In other examples dual emissions are observed and the designed complexes behave as white phosphors. Finally interesting examples of chiral luminescent complexes were also presented and displayed interesting CPL properties. This field is still in its infancy and is expected to grow rapidly in the future for variety of devices such as displays, sensing, for bioimaging and information storage. 140 Moreover, the challenge in this area is to master the factors that control structure ↔ bonding ↔ property in order to engineer very stable and highly luminescent systems based on abundant and cheap metals such as copper 162,[256][257][258][259] , silver 260 and more challenging iron complexes. 261 The understanding of these factors will permit us to manipulate the rich and various excited-states energies, which proceed via different radiative decay mechanisms (fluorescence, phosphorescence, TADF mechanisms).

Apart from that goal, iridium, platinum and gold complexes will continue to be explored due to their intrinsic and high spin-orbit coupling that leads to highly luminescent complexes. Perhaps the coinage metal-based examples illustrate the importance of the carbene ligands to prepare efficient luminescent complexes with useful applications. This and many other results summarized in this review hold promise for future developments in the area of luminescent NHC-metal complexes.

All in all, one would expect that luminescent NHC-metal complexes will remain a very important field in which a large panel of functionalized carbene assembling ligands with structural international meetings including several invited lectures. He coauthored the book "Chirality in Transition Metal Chemistry" published in 2008 by Wiley. He is a member of several advisory editorial boards.

Abbreviations

Figure 2 .

 2 Figure 2. Simplified molecular orbital diagram for square planar platinum complexes involving

Figure 4 .

 4 Figure 4. General motif of the cycloplatinated NHC complexes. Boxes indicate relevant

Figure 5 .

 5 Figure 5. Cycloplatinated NHC (acac) containing π-extended organic chromophore.

Figure 6 .

 6 Figure 6. Cycloplatinated NHC containing biphenyl ligands.

Figure 7 .

 7 Figure 7. Cycloplatinated NHC with bisacetylides

Figure 8 .

 8 Figure 8. Pt(II)-bisacetylides containing trans-NHC-ligands.

Figure 10 .

 10 Figure 10. Cyclometalated Pt(II) complexes 54-55 supported by pyrene-based NHC-ligands.

2. 5

 5 Platinum (II) complexes with bis carbene (NHC) chelating ligands Chi, Chen and Hung described the synthesis of some square planar Pt(II) complexes of general formula [Pt(C^C)(X^X)] (56-63) displaying bis-carbene chelating ligand (C^C) associated to three functional dianionic azolate chelates (X^X = 5,5'-di(trifluormethyl-3

Figure 11 .

 11 Figure 11. Cycloplatinated complexes with bis-NHC chelating ligands.

Figure 14 .

 14 Figure 14. Homoleptic cycloiridated (NHC-trifluoromethyl-phenyl) complexes displaying

  Thus the complexes fac-tris(N-dibenzofuranyl-N-methylimidazole) 3 Ir(III) (fac-79) and mer-tris(N-

Figure 16 .

 16 Figure 16. Homoleptic cyloiridated NHC complexes containing organic chromophore, displaying

Figure 19 .

 19 Figure 19. Cyclometalated-NHC iridium complexes containing naphthalimide chromophore.

Figure 20 .

 20 Figure 20. Plot of non-radiative rate constants, k nr , as a function of the energy, E, of the emissive

Figure 22 .

 22 Figure 22. Cyclometalated Ir-NHC complexes with helicene ligands.

Figure 23 .

 23 Figure 23. CPL traces of mer-(P, Λ Ir )-97/(M-Δ Ir )-97 and mer-(P, Δ Ir )-98/(M-Λ Ir )-98 containing

Figure 25 .

 25 Figure 25. [3+3]-Iridium-NHC complexes displaying bis-tridentate ligands.

Figure 26 .

 26 Figure 26. [3+3]-Iridium-bis(NHC) complexes featuring bis-tridentate (C^N^N)/C^C^C) ligands.

Figure 27 .

 27 Figure 27. [3+3]-Iridium-bis(NHC) complexes with bis-tridentate (C^N^N)/C^C^C) ligands and

Figure 29 .

 29 Figure 29. A wide range of [3+2+1] Ir III compounds displaying tridentate bis carbene ligands,

Figure 30 .

 30 Figure 30. Bis(NHC) cyclometalated iridium complexes containing dbpz ancillary ligands

Figure 31 .

 31 Figure 31. An exhaustive range of luminescent bis(NHC)Ir(III)-complexes containing a variety of

3

  Scheme 1. Visible light induced radical cyclization at room temperature in CH 3 CN.

Scheme 2 .

 2 Photocatalyst (2 mol%), Amines (5 equiv.) HCOOH (2.5 equiv.), 10h

Figure 32 .

 32 Figure 32. Gold-NHC complexes 150-151 supported by pyrene-based tetra-acetylide-ligands.

Figure 33 .

 33 Figure 33. Gold NHC-acetylide complexes containing fluorenyl organic chromophores.

Figure 34 .

 34 Figure 34. Supramolecular 1D coordination polymers containing bis-carbene Au(I) cations

  184

Figure 35 .

 35 Figure 35. Monomeric and dimeric Au(I) alkynyl complexes supported by NHC ligands. B)

Figure 36 .

 36 Figure 36. Two classes of metal carbene compounds displaying naphthalimide fluorophore

Figure 37 .

 37 Figure 37. Coinage metal complexes containing four naphthalimide chromophores

  196

Figure. 38

 38 Figure. 38 Chiral Au(I) NHC complexes displaying short Aurophilic inteactions.

Figure 39 .

 39 Figure 39. a) Uniform Au⋅⋅⋅Au interactions displayed in single crystal form of [(R, R)-189][A] or

Figure 40 .

 40 Figure 40. a) CPL spectra of [(R, R)-189][A] and [(S, S)-189][A] recorded in PMMA and varying

Figure 41 .

 41 Figure 41. Photoluminescence images of screen printed panda and TIPC with [(R, R)-189][A] as

Figure 42 .Figure 43 .Figure 44 .

 424344 Figure 42. Au(I)-NHC complexes containing carbazolate chromophore

  CBZ LC state and (ii) a weak phosphorescence emission from the 3 CBZ state at λ = 437 nm with significant longer life time τ = 1-10 µs. The presence of a fluorescence band at λ = 343 (τ= 13.5 ns) which is assigned to free carbazole as a photodecomposition product because the excitation emission does not correspond to the absorption of 197. In contrast to copper and silver the gold compounds 198 and 200 in THF solution displayed only phosphorescence emissions from high[START_REF] Visbal | N-heterocyclic carbene metal complexes: photoluminescence and applications[END_REF] CBZ transition states at λ = 430 nm and (Φ = 0.32) and life-times of τ = 332 and 266 µs

Figure 45 .

 45 Figure 45. Copper NHC-complexes containing di(2-pyridyl)dimethyl borate.

Figure 46 .

 46 Figure 46. Cu(I) NHC-picolyl dimer complexes 213-216 and the related Cu(I) polymer 217.

Figure 47 .

 47 Figure 47. Bimetallic copper and silver carbene complexes supported by dipyridyl ligands.

Figure 48 .

 48 Figure 48. Silver and Gold carbene complexes displaying trigonal geometry.

Figure 49 .

 49 Figure 49. Three coordinate Cu(I) compounds displaying cyclic(alkyl)amino carbene ligands.

Figure 50 .

 50 Figure 50. Tri-and tetra-coordinate carbene Cu(I) complexes containing trispyrazolylborates.

  Unlike thepreceding examples, in this new class of luminescent carbene-metal-amide (CMA) complexes 229-233 the anionic amide ligand is the σ-donor ligand and upon photo-excitation an electron from the π-system of the amide is promoted into the CAAC carbene-based LUMO.222 These carbene-metalamide complexes display high quantum yields (Φ PL up to 98%) and very short excited state lifetimes (< 1µs). The authors highlight the importance of rotational flexibility along the C-M-N axis of the carbene-metal-amide complexes for photoemission efficiency. For instance, the S1-T1 energy gap converges to zero at high torsion angles, as a consequence this accelerates the intersystem crossing and provides luminescence with short lifetimes in the submicrosecond regime.

Figure 51 .

 51 Figure 51. Coinage metal complexes with cyclic (alkyl)(amino) carbene (CAAC) ligands and

  bulky benzimidazolyl carbene (BZI) (237-239), unconventional Nheterocyclic carbenes such as monoamido-amino carbenes (MAC) (240-242) and diamidocarbenes (DAC) (243-245) or cyclic (alkyl)(amino)carbenes (CAAC) (246-248) and carbazolate (Cbz) as

Figure 53 .

 53 Figure 53. A variety of coinage metal complexes with NHC, MAC, DAC and CAAC carbenes and

Figure 54 .

 54 Figure 54. Proposed TADF pathways in CMA copper compound based on DFT/MRCI computational studies. Adapted with permission from ref 249 . Copyright 2022 American Chemical

Figure 55 .

 55 Figure 55. Cu(I) with cyclic(amino) (aryl) carbene (CAArC) ligands.

  On the basis of experimental studies involving VT luminescence, transient absorption, fluorescence upconversion, and theoretical calculations, the authors suggest that in the solid state a very efficient thermally activated delayed fluorescence (TADF) process involving singlet 1 LLCT excited states is taking place in a similar fashion to that reported by Thompson and coworkers in the previous section.

Figure 56 .

 56 Figure 56. Mixed carbene coinage metal complexes

  , 255 ; Ag , 256 ; Au, 257

Figure 57 .

 57 Figure 57. Chiral Cu(I)-Cl with cyclic (alkyl)(amino) carbene (CAAC) ligands and their chiral

Figure 58 .

 58 Figure 58. CD and CPL spectra of LMent CAACCuCl (258) and DMent CAACCuCl (258) complexes.

Table 1 .

 1

	Entry	Catayst Load i [mol%]	Time [h] Conversion ii [%]
	1	50	21	>99%
	2	10	21	>99%
	3	5	21	>99%
	4	10	4	>99%
	5	5	4	>99%
	6	none	21	12
	7 iii	10	21	0

i) Entry 1: catalyst amount was weighed. Entries 3 to 7, a stock solution in THF was employed. ii) Conversions were calculated using 1 H-NMR spectroscopy by determining the ratio of the product and reactant. iii) Control reaction carried out in the dark.

  / 1000cd m -for 229 (8.9%/9.2%) and 231(26.1%/ 25.2%) and 232 (26.6% / 24.5%).212 Moreover, OLEDs based on Ag-carbene complexes 230, 233 were also manufactured and displayed (EQE max) = 4.3% and 13.7% respectively. EQE values at low brightness 100 cd m -2 were also obtained for 230 (2.7%) and for 233 (12.9%).228 At ambient temperature both complexes emit via TADF-type process with short lifetimes in the sub-microsecond regime (230, τ = 358 ns and 233, τ = 368 ns in PVK matrix). The lower EQE observed for the silver complex 233 when compared with the gold complex 231 is consistent with PLQY of the silver complex which is around 74% compared to the PLQY of the gold compound which is around 98%.

232

While these examples featured five-membered rings in the carbene ligands, the Bochmann group reported also several luminescent carbene-metal-amide (CMA) complexes (M = Cu, 234; Ag, 235 ;

[π→π](C≡CPh) state, the phosphorescence character is confirmed by the lifetime τ = 3.9 µs.Unlike complex 163 blue , the emission band forms of 163 green , 163 greenDCM , and 163 yellow were broad

Acknowledgments Notes Biography Abbreviations References

Acknowledgments

The author would like to thank Sorbonne Université, Campus Pierre et Marie Curie and CNRS for supporting this work. HA would like to thank Professors V.W.W. Yam and A. L. Cooksy and Dr. A. Barbieri for fruitful collaborations. HA would like to thank all members of the ARChiteture Molecular group.

Author Information Corresponding Author

Compounds 194 and 195 were found to be luminescent and exhibit dual emission. For instance, at room temperature complex 194 exhibits structured emissive peaks at λ em = 400-550 nm which are insensitive to O 2 , with τ = (16, 24 ns) identified as fluorescence emission and a long-lived emission (τ = 55ms) with vibronic structure at λ = 550-750 nm denoted as phosphorescence.

Similar behavior was obtained for complex 195. The authors suggest that there exist singlet 1 LE and triplet 3 LE excited states localized at the carbazole ligand in the crystal. In order to highlight this behavior, the authors selectively deuterated the carbazole unit in complex 194. The deuterated complex (IPr-Cu-dCbz) (194-deuterated) displayed longer lifetime for the room temperature phosphorescence (t = 140 ms) and enhancement of the phosphorescence PLQY = 22.8% compared to that of the (IPr-Cu-Cbz) (194) which is only 7.9%.

Nolan, Steffen and coworkers described a novel synthetic procedure to prepare a variety of NHC-coinage metals 196-207 containing carbazolate ligands (Figure 44). 209 The synthetic procedure involves the use of the weak base strategy (base = K 2 CO 3 , NEt 3 ) in acetone or ethanol under mild conditions for 24h. [210][211] It should be mentioned that K 2 CO 3 provided better yields and is the base of choice for this reaction procedure. This synthetic approach was also used to obtain the halogenated precursors in good yields. The authors suggest based on DFT calculations that the reaction pathway to the target molecules involves a concerted metalation/deprotonation process. diversity will be prepared and judiciously attached to variety of metal centers to generate novel class of luminescent complexes with intriguing properties to be discovered. However it should be kept in mind that such future compounds should present high stability and EQE efficiency to meet the standard industrial requirements.

AUTHOR INFORMATION

Corresponding Author: Hani Amouri, Sorbonne Université-Faculté des Sciences et Ingénerie Campus Pierre et Marie Curie, CNRS, IPCM (UMR 8232), 4 place Jussieu, 75252 Paris cedex 05, France. orcid.org/0000-0001-6955-8406. E-mail: hani.amouri@sorbonne-universite.fr.

Notes

The author declares no competing financial interest.

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