Luminescent iridium(III) complexes with N^C^N-coordinated terdentate ligands: dual tuning of the emission energy and application to organic light-emitting devices

Tuning Excited State Character in Iridium(III) Photosensitizers and Its Influence on TTA-UC

A series of mixed ligand, photoluminescent organometallic Ir(III) complexes have been synthesized to incorporate substituted 2-phenyl-1H-naphtho[2,3-d]imidazole cyclometalating ligands. The structures of three example complexes were categorically confirmed using X-ray crystallography each sharing very similar structural traits including evidence of interligand hydrogen bond contacts that account for the shielding effects observed in the 1H NMR spectra. The structural iterations of the cyclometalated ligand provide tuning of the principal electronic transitions that determine the visible absorption and emission properties of the complexes: emission can be tuned in the visible region between 550 and 610 nm and with triplet lifetimes up to 10 μs. The nature of the emitting state varies across the series of complexes, with different admixtures of ligand-centered and metal-to-ligand charge transfer triplet levels evident. Finally, the use of the complexes as photosensitizers in triplet-triplet annihilation energy upconversion (TTA-UC) was investigated in the solution state. The study showed that the complexes possessing the longest triplet lifetimes showed good viability as photosensitizers in TTA-UC. Therefore, the use of an electron-withdrawing group on the 2-phenyl-1H-naphtho[2,3-d]imidazole ligand framework can be used to rationally promote TTA-UC using this class of complex.

Competition between N,C,N-Pincer and N,N-Chelate Ligands in Platinum(II)

Replacement of the chloride ligand of PtCl{κ3-N,C,N-[py-C6HR2-py]} (R = H (1), Me (2)) and PtCl{κ3-N,C,N-[py-O-C6H3-O-py]} (3) by hydroxido gives Pt(OH){κ3-N,C,N-[py-C6HR2-py]} (R = H (4), Me (5)) and Pt(OH){κ3-N,C,N-[py-O-C6H3-O-py]} (6). These compounds promote deprotonation of 3-(2-pyridyl)pyrazole, 3-(2-pyridyl)-5-methylpyrazole, 3-(2-pyridyl)-5-trifluoromethylpyrazole, and 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole. The coordination of the anions generates square-planar derivatives, which in solution exist as a unique species or equilibria between isomers. Reactions of 4 and 5 with 3-(2-pyridyl)pyrazole and 3-(2-pyridyl)-5-methylpyrazole provide Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[R'pz-py]} (R = H; R' = H (7), Me (8). R = Me; R' = H (9), Me (10)), displaying κ1-N1-pyridylpyrazolate coordination. A 5-trifluoromethyl substituent causes N1-to-N2 slide. Thus, 3-(2-pyridyl)-5-trifluoromethylpyrazole affords equilibria between Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[CF3pz-py]} (R = H (11a), Me (12a)) and Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N2-[CF3pz-py]} (R = H (11b), Me (12b)). 1,3-Bis(2-pyridyloxy)phenyl allows the chelating coordination of the incoming anions. Deprotonations of 3-(2-pyridyl)pyrazole and its substituted 5-methyl counterpart promoted by 6 lead to equilibria between Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[R'pz-py]} (R' = H (13a), Me (14a)) with a κ-N1-pyridylpyrazolate anion, keeping the pincer coordination of the di(pyridyloxy)aryl ligand, and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[R'pz-py]} (R' = H (13c), Me (14c)) with two chelates. Under the same conditions, 3-(2-pyridyl)-5-trifluoromethylpyrazole generates the three possible isomers: Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[CF3pz-py]} (15a), Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N2-[CF3pz-py]} (15b), and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[CF3pz-py]} (15c). The N1-pyrazolate atom produces a remote stabilizing effect on the chelating form, pyridylpyrazolates being better chelate ligands than pyridylpyrrolates. Accordingly, reactions of 4-6 with 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole yield Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[(CF3)2C4(py)HN]} (R = H (16), Me (17)) or Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[(CF3)2C4(py)HN]} (18), displaying κ1-N1-pyrrolate coordination. Complexes 7-10 are efficient green phosphorescent emitters (488-576 nm). In poly(methyl methacrylate) (PMMA) films and in dichloromethane, they experience self-quenching, due to molecular stacking. Aggregation occurs through aromatic π-π interactions, reinforced by weak platinum-platinum interactions.

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

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

Cyclometalated Rhodium and Iridium Complexes Containing Masked Catecholates: Synthesis, Structure, Electrochemistry, and Luminescence Properties

Two neutral cyclometalated rhodium and iridium coordination assemblies [(F2ppy)₂M(η-Cat)], M = Rh, (2) and M = Ir, (3) (F2ppy: 2,4-difluorophenylpyridine), displaying a masked catecholate (η-Cat = η-O∧O) are described. The catecholate ligand is π-bonded to an organometallic Cp*Ru(II) moiety. The latter brings stability to the whole system in solution and suppresses the formation of the related paramagnetic semiquinone complex. The determination of the molecular structure of the iridium complex [(F2ppy)₂Ir(η-Cat)] (3) corroborates the formation of the target compound and reveals the generation of a rare two-dimensional (2D) honeycomb supramolecular architecture in the solid state, in which the Δ-enantiomer self-assembles with the Λ-enantiomer through encoded π-π interactions among individual units. The electrochemistry of complexes 2 and 3 was investigated and showed that reduction occurs at very negative potentials (∼-2.2 V versus saturated calomel electrode (SCE)), while oxidation of the cyclometalated Rh and Ir centers occurs at 0.8 and 0.86 V. In contrast to complexes with 1,2-dioxolene chelates, which are nonemissive, the heterodinuclear diamagnetic complexes 2 and 3 were found to be emissive at room temperature both in solution and in the solid state. Moreover, at 77 K in a solid state, both compounds display opposite emission behavior, for instance, complex 3 displays a blue-shifted emission, while rhodium compound 2 exhibits red-shifted emission to lower energy.

Fine Emission Tuning from Near-Ultraviolet to Saturated Blue with Rationally Designed Carbene-Based [3 + 2 + 1] Iridium(III) Complexes

We designed and synthesized a new class of six phosphorescent [3 + 2 + 1] iridium(III) complexes [(pbib)Ir(C^C)CN] bearing a tridentate 1,3-bis(1-butylimidazolin-2-ylidene) phenyl N-heterocyclic carbene (NHC)-based pincer ligand (pbib), bidentate imidazole-based NHC ligands (C^C), and a monodentate cyano group and investigated their photophysical, electrochemical, and thermal stabilities and electroluminescent properties. The extended π-conjugation of the imidazole-based C^C ligand is found to be the key to fine-tune the emission energies from ultraviolet blue (λ = 378 nm) to saturated blue (λ = 482 nm), as shown by electrochemical and photophysical studies, which is also revealed by the density functional theory (DFT) and time-dependent DFT calculations. Vacuum-deposited organic light-emitting diode devices have been fabricated with these newly synthesized emitters and exhibited the best external quantum efficiency of 6.4% and Commission International de L'Éclairage (CIE) coordinates of (0.163, 0.096), where the CIE y is very similar to the National Television System Committee standard blue CIE (x, y) coordinates of (0.149, 0.085). These results indicate that the novel [3 + 2 + 1] coordinated iridium(III) complexes [(pbib)Ir(C^C)CN], having a saturated blue emission, not only could alleviate the photodegradation of the emitters when compared to [(pbib)Ir(pmi)CN] but also provide new design strategies of saturated-blue-emitting iridium(III) complexes.

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

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

Ligand-Mediated Photophysics Adjustability in Bis-tridentate Ir(III) Complexes and Their Application in Efficient Optical Limiting Materials

A family of novel bis-tridentate Ir(III) complexes (Ir1-Ir5) incorporating both functional N^∧C^∧N-type ligands (L1-L5) and N^∧N^∧C-type ligand (L0) were synthesized attentively and characterized scientifically. The crystalline structures of Ir1, Ir3 and Ir4 were resoundingly confirmed by XRD. With the aid of experimental and theoretical methods, their photophysical properties at transient and steady states were scientifically investigated. The broadband charge-transfer absorption for these aforementioned Ir(III) complexes is up to 600 nm as shown in the UV-visible absorption spectrum. The emission lifetimes of their excited states are good. Between the visible and near-infrared regions, Ir1-Ir5 possessed powerful excited-state absorption. Hence, a remarkably robust reverse saturable absorption (RSA) process can occur once the complexes are irradiated by a 532 nm laser. The RSA effect follows the descending order: Ir3 > Ir5 > Ir4 ≈ Ir1 > Ir2. To sum up, modifying electron-donating units (-OCH₃) and large π-conjugated units to the pyridyl N^∧C^∧N-type ligands is a systematic way to markedly raise the RSA effect. Therefore, these octahedral bis-tridentate Ir(III) complexes are potentially state-of-the-art optical limiting (OPL) materials.

Phosphorescent [3 + 2 + 1] coordinated Ir(iii) cyano complexes for achieving efficient phosphors and their application in OLED devices

A series of neutral [3 + 2 + 1] coordinated iridium complexes bearing tridentate bis-NHC carbene chelates (2,6-bisimidazolylidene benzene), bidentate chelates (C^N ligands, e.g. derivatives of 2-phenylpridine), and monodentate ions (halides and pseudo-halides, such as Br, I, OCN and CN ions) have been systematically designed and synthesized. X-ray single crystal structure characterization revealed that the nitrogen atom in C^N ligands is located trans to the carbon atom in the benzene ring in tridentate chelates, while the coordinating carbon atom in C^N ligands is located trans to the monodentate ligands. Photophysical studies reveal that the C^N ligands play a vital role in tuning the UV absorption and emission properties, while the tridentate bis-NHC carbene chelates influence the lowest absorption band and emission energy when compared to heteroleptic Ir(ppy)2(acac) [i.e. molar absorptivities at ∼450 nm for ppy-OCN and Ir(ppy)2(acac) are 350 M⁻¹ cm⁻¹ and 1520 M⁻¹ cm⁻¹ and emission maximum peaks are at 465 nm and 515 nm respectively]. Among monodentate ligands that the complexes bear, the group containing the cyanide ligand displays higher emission energy, higher photophysical quantum yields, longer triplet lifetimes and better electrochemical and thermal stabilities than those of cyanate and bromide. Particularly, a blue organic light-emitting diode (OLED) based on dfppy-CN exhibited a maximum external quantum efficiency of 22.94% with CIE coordinates of (0.14, 0.24). Furthermore, a small efficiency roll-off of 5.7% was observed for this device at 1000 cd m⁻².

Construction of [3 + 2 + 1] coordinated iridium(iii) cyano complexes for achieving high-efficiency phosphors and their application in blue OLEDs with low efficiency roll-off.

Dual emission from an iridium(III) complex/counter anion ion pair

[Ir(tpy)2](PF6)3 (tpy = 2,2':6',2''-terpyridine) dissolved in CH3CN was found to exhibit dual color luminescent emission depending on the excitation wavelength. Specifically, blue and green emissions were obtained with excitation at 350 and 410 nm, respectively. Because the associated emission spectra were consistent with those of [Ir(tpy)2]Cl3 in water and [Ir(tpy)2](PF6)3 in the crystalline state, respectively, this dual emission is attributed to emissions from the [Ir(tpy)2]3+ cation and its ion pair [Ir(tpy)2]3+·PF6-. The emission is assigned to the 3π-π* transition of the ligands based on time-dependent density functional theory (TD-DFT) calculations. Conversely, [Ir(tpy)2]I3 in CH3CN shows emission due to [Ir(tpy)2]3+ but not [Ir(tpy)2]3+·I-, while crystalline [Ir(tpy)2]I3 emits red luminescence at 77 K that is inconsistent with that from [Ir(tpy)2]3+. Since the emission energies of crystalline [Ir(tpy)2]X3 (X- = Cl-, Br- or I-) show a good correlation with the electron affinity of X, the emissions are assigned to a counter anion to complex ion charge-transfer transition. This hypothesis is supported by TD-DFT calculations regarding [Ir(tpy)2]3+·X-.

Deep-Red Luminescence from Platinum(II) Complexes of N^N-^N-Amido Ligands with Benzannulated N-Heterocyclic Donor Arms

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

Mono and dinuclear iridium(iii) complexes featuring bis-tridentate coordination and Schiff-base bridging ligands: the beneficial effect of a second metal ion on luminescence

Ditopic bis-N^N^O-coordinating ligands, prepared by Schiff base chemistry, lead to dinuclear iridium complexes that emit much more brightly than their mononuclear counterparts.

Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs

The light-emitting efficiency of luminescent materials is invariably compromised on moving to the red and near-infrared regions of the spectrum due to the transfer of electronic excited-state energy into vibrations. We describe how this undesirable "energy gap law" can be sidestepped for phosphorescent organometallic emitters through the design of a molecular emitter that incorporates two platinum(II) centers. The dinuclear cyclometallated complex of a substituted 4,6-bis(2-thienyl)pyrimidine emits very brightly in the red region of the spectrum (λ_max = 610 nm, Φ = 0.85 in deoxygenated CH₂Cl₂ at 300 K). The lowest-energy absorption band is extraordinarily intense for a cyclometallated metal complex: at λ = 500 nm, ε = 53 800 M^-1 cm^-1. The very high efficiency of emission achieved can be traced to an unusually high rate constant for the T₁ → S₀ phosphorescence process, allowing it to compete effectively with nonradiative vibrational decay. The high radiative rate constant correlates with an unusually large zero-field splitting of the triplet state, which is estimated to be 40 cm^-1 by means of variable-temperature time-resolved spectroscopy over the range 1.7 < T < 120 K. The compound has been successfully tested as a red phosphor in an organic light-emitting diode prepared by solution processing. The results highlight a potentially attractive way to develop highly efficient red and NIR-emitting devices through the use of multinuclear complexes.

Experimental and theoretical investigation of cyclometalated phenylpyridine iridium(iii) complex based on flavonol and ibuprofen ligands as potent antioxidant

An Ir(iii) complex was synthesized using mixed ligands of biological importance, namely ibuprofen, flavonol and 2-phenylpyridine. The compound was characterized by ¹H-NMR, ¹³C-NMR and TOF-MS spectroscopies and elemental analysis. Structures of the complex and its ligands were also calculated by density functional theory using B3LYP/Lanl2dz//6-31G(d) level of theory. Analyses of electrostatic potential, natural population, and frontier orbitals of the molecules as well as the calculation of intrinsic thermochemical properties such as bond dissociation enthalpy, ionization potential, electron affinity and proton affinity in the gas phase and in solvents (water and pentylethanoate) give the first indication that the complex is a potential antioxidant. The latter even shows better antioxidant capacity than the parent ligands. The antioxidant properties of the complex and its ligands were experimentally evaluated by studying the free radical scavenging activity towards HO˙, NO˙, DPPH˙ and ABTS˙⁺ radicals. Further computational work on the antioxidant processes such as the single electron transfer, the proton loss, the formal hydrogen transfer (FHT) and the radical adduct formation reactions was conducted. Results show that the FHT reaction is the mechanism responsible for the radical scavenging activity of the complex towards HO˙, HOO˙, NO˙ and DPPH˙ radicals while ABTS˙⁺ seems to be scavenged by an electron-donating mechanism. The FHT was further determined as a hydrogen-atom transfer but not a proton-couple electron transfer mechanism.

A cyclometalated phenylpyridine iridium(iii) complex based on flavonol and ibuprofen was designed and its antioxidant activity was evaluated via experimental and theoretical studies.

Efficient green photoluminescence and electroluminescence of iridium complexes with high electron mobility

Aiming to balance the injection and transport of electrons and holes, nitrogen heterocycle and 1,3,4-oxadiazole derivatives were introduced in iridium(iii) complexes to obtain organic light-emitting diodes (OLEDs) with high performances. Thus, two novel Ir(iii) complexes (Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop)) with green emissions using 2-(3,5-bis(trifluoromethyl)phenyl)pyrimidine (tfmphpm) and 2-(2,6-bis(trifluoromethyl)pyridin-4-yl)pyrimidine (tfmppm) as cyclometalating ligands, and 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol (pop) as an ancillary ligand were synthesized. Both emitters show high photoluminescence efficiencies up to 94% and good electron mobility. The devices using two emitters with the structure of ITO (indium-tin-oxide)/MoO3 (molybdenum oxide, 5 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, 30 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 5 nm)/Ir(iii) complexes (6 wt%) : PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl) benzene, 40 nm)/LiF (1 nm)/Al (100 nm) display good electroluminescence performances with a maximum luminance of 48 981 cd m-2, a maximum current efficiency of 92.79 cd A-1 and a maximum external quantum efficiency up to 31.8%, respectively, and the efficiency roll-off ratio is low, suggesting that they have potential application in OLEDs.

Cyclometalated N-heterocyclic carbene iridium(iii) complexes with naphthalimide chromophores: a novel class of phosphorescent heteroleptic compounds

A series of cyclometalated N-heterocyclic carbene complexes of the general formula [Ir(C^N)₂(C^C:)] has been prepared. Two sets of compounds were designed, those where (C^C:) represents a bidentate naphthalimide-substituted imidazolylidene ligand and (C^N) = ppy (3a), F2ppy (4a), bzq (5a) and those where (C^C:) represents a naphthalimide-substituted benzimidazolylidene ligand and (C^N) = ppy (3b), F2ppy (4b), bzq (5b). The naphthalimide-imidazole and naphthalimide-benzimidazole ligands 1a,b and the related imidazolium and benzimidazolium salts 2a,b were also prepared and fully characterized. The N-heterocyclic carbene Ir(iii) complexes have been characterized by NMR spectroscopy, cyclic voltammetry and elemental analysis. Moreover, the molecular structures of one imidazolium salt and four Ir(iii) complexes were determined by single-crystal X-ray diffraction. The structures provide us with valuable information, most notably the orientation of the naphthalimide chromophore with respect to the N-heterocyclic carbene moiety. All compounds are luminescent at room temperature and in a frozen solvent at 77 K, exhibiting a broad emission band that extends beyond 700 nm. The presence of the naphthalimide moiety changes the character of the lowest excited state from ³MLCT to ³LC, as corroborated by DFT and TD-DFT calculations. Remarkably, replacing imidazole with a benzimidazole unit improves the quantum yields of these compounds by decreasing the k_nr values which is an important feature for optimized emission performance. These studies provide valuable insights about a novel class of N-heterocyclic carbene-based luminescent complexes containing organic chromophores and affording metal complexes emitting across the red-NIR range.

An electron poor iridium pincer complex for catalytic alkane dehydrogenation

A novel electron deficient 4,6-bis(trifluoromethyl)-1,3-phenylene diphosphinite ligand 4 was developed and synthesized. Reaction of Ir precursors with ligand 4 gave chloro(hydride) pincer complex 5, which demonstrated a higher TON in alkane dehydrogenation reactions compared to similar phosphinite based pre-catalysts. The formation of cyclooctene (COE) and tert-butylethylene adducts of the 14e catalysts was also studied and the COE adduct is implicated as the resting state of the catalyst. All compounds were characterized by NMR spectroscopy and, in addition, the molecular structures of key complexes were confirmed by X-ray analysis.

Efficient yellow electroluminescence of four iridium(iii) complexes with benzo[d]thiazole derivatives as main ligands

Four yellow iridium(iii) complexes with benzo[d]thiazole derivatives as the main ligands display good device performances with a maximum current efficiency of up to 69.8 cd A⁻¹ and a maximum external quantum efficiency of up to 24.3%.

Efficient electroluminescence of bluish green iridium complexes with 2-(3,5-bis(trifluoromethyl)phenyl)pyrimidine and 2-(3,5-bis(trifluoromethyl)phenyl)-5-fluoropyrimidine as the main ligands

Four bluish green iridium complexes with high photoluminescence quantum efficiencies were applied in OLEDs, and they showed a maximum current efficiency of 74.70 cd A⁻¹ and a maximum external quantum efficiency of 35.0% with mild efficiency roll-off.

Efficient bluish green electroluminescence of iridium complexes with good electron mobility

Two efficient bluish green iridium complexes with good electron mobility were applied in efficient OLEDs, showing a maximum current efficiency of 71.18 cd A⁻¹ and a maximum external quantum efficiency of 27.7%.

Synthesis and Characterization of a Series of Bis-homoleptic Cycloruthenates with Terdentate Ligands as a Family of Panchromatic Dyes

A series of six homoleptic bis-cyclometalated ruthenium complexes, Ru(N^N^C)₂, is reported where N^N^C is a 6-(2,4-difluoro-3-R³-phenyl)-4-R²-4'-R¹-2,2'-bipyridine with R³ = -H or -CF₃ and R² and R¹ = -COOEt or -CF₃. An effective synthesis of the ligands and the complexes is described. The UV-visible absorption studies demonstrate that these complexes are panchromatic dyes absorbing up to 900 nm. Importantly, the onset of absorption depends only on the substitution on the metalated phenyl, whereas the intensity of absorption throughout the spectra is a function of substituents on both the phenyl and the bipyridine moieties. The same trend is observed in electrochemistry as the redox gap depends only on the substitution on the metalated phenyl, whereas the oxidation and reduction potentials are a function of substituents on both the phenyl and the bipyridine moieties. Preliminary tests as sensitizer for dye-sensitized solar cells demonstrate that the number of anchoring groups on the dye has a major influence on the device efficiency.

An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand

A new family consisting of three luminescent neutral Ir(III) complexes with the unprecedented [Ir(C^N^C)(N^N)Cl] architecture, where C^N^C is a bis(six-membered) chelating tridentate tripod ligand derived from 2-benzhydrylpyridine (bnpy) and N^N is 4,4'-di-tert-butyl-2,2'-bipyridine (dtBubpy), is reported. X-ray crystallography reveals an unexpected and unusual double C-H bond activation of the two distal nonconjugated phenyl rings of the bnpy coupled with a very short Ir-Cl bond trans to the pyridine of the bnpy ligand. Depending on the substitution on the bnpy ligand, phosphorescence, ranging from yellow to red, is observed in dichloromethane solution. A combined study using density functional theory (DFT) and time-dependent DFT (TD-DFT) corroborates the mixed charge-transfer nature of the related excited states.

Photoluminescence and electroluminescence of an iridium(iii) complex with 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 2-(5-phenyl-1,3,4-thiadiazol-2-yl)phenol ligands

Efficient greenish yellow OLEDs based on an iridium(iii) complex containing electron transport ligands of 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 2-(5-phenyl-1,3,4-thiadiazol-2-yl)phenol show a maximum current efficiency of 54.5 cd A⁻¹and a maximum external quantum efficiency of 16.1%.

Fluorinated cyclometalated iridium(iii) complexes as mitochondria-targeted theranostic anticancer agents

Cyclometalated iridium(iii) complexes bearing different numbers of fluorine atoms were developed to induce apoptosis via mitochondrial pathways and demonstrated much better anticancer activities than the widely used clinical chemotherapeutic agent cisplatin.

Photoluminescence and electroluminescence of iridium(iii) complexes with 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 1,3,4-oxadiazole/1,3,4-thiadiazole derivative ligands

Highly efficient OLEDs based on green and orange iridium(iii) complexes based on 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 2-(5-(4-(trifluoromethyl)phenyl)-1,3,4-oxadiazol-2-yl)phenol show peak current efficiencies of 74.8 and 41.0 cd A⁻¹, respectively, with low efficiency roll-off.

Photoluminescence and electroluminescence of deep red iridium(iii) complexes with 2,3-diphenylquinoxaline derivatives and 1,3,4-oxadiazole derivatives ligands

OLEDs using efficient deep red iridium(iii) complexes display good electroluminescence performances with maximum current efficiency and external quantum efficiency of up to 14.0 cd A⁻¹and 17.8%, respectively, and the efficiency roll-off is mild.

Room temperature blue phosphorescence: a combined experimental and theoretical study on the bis-tridentate Ir(iii) metal complexes

A series of new bis-tridentate Ir(iii) complexes (1/1b, 2/2b and 3/3b) incorporating both bis(imidazolylidene)benzene and dianionic functional pyrazolyl (or phenyl) pyridine chelates have been synthesized, among which complexes 2 and 2b exhibit intense and structural sky-blue emission in both solution and solid states. In stark contrast, 1/1b is non-emissive in solution, while 3/3b reveals highly red-shifted emission with a featureless spectral profile. This variation in photophysics is associated with the interchange of metal-chelate bonding in the selected tridentate chelate, which affects both the crystal field stabilization energy and the ππ* transition character of the resulting Ir(iii) metal complexes. In 1/1b, the stabilized metal-centered (MC) dd excited states induce a dominant radiationless channel that accounts for the lack of emission in solution. The appreciable ligand-to-ligand charge transfer (LLCT) in 3/3b rationalizes its broad and featureless emission, which is different from the dominant intraligand ππ* transition in 2/2b. The combination of experimental and theoretical approaches thus provides fundamental insight into the influence of chelates as well as the metal-chelate interaction, which is beneficial for the future design of efficient and robust Ir(iii) phosphors for OLED applications.

When two are better than one: bright phosphorescence from non-stereogenic dinuclear iridium(III) complexes

A new family of eight dinuclear iridium(iii) complexes has been prepared, featuring 4,6-diarylpyrimidines L(y) as bis-N^C-coordinating bridging ligands. The metal ions are also coordinated by a terminal N^C^N-cyclometallating ligand L(X) based on 1,3-di(2-pyridyl)benzene, and by a monodentate chloride or cyanide. The general formula of the compounds is {IrL(X)Z}2L(y) (Z = Cl or CN). The family comprises examples with three different L(X) ligands and five different diarylpyrimidines L(y), of which four are diphenylpyrimidines and one is a dithienylpyrimidine. The requisite proligands have been synthesised via standard cross-coupling methodology. The synthesis of the complexes involves a two-step procedure, in which L(X)H is reacted with IrCl3·3H2O to form dinuclear complexes of the form [IrL(X)Cl(μ-Cl)]2, followed by treatment with the diarylpyrimidine L(y)H2. Crucially, each complex is formed as a single compound only: the strong trans influence of the metallated rings dictates the relative disposition of the ligands, whilst the use of symmetrically substituted tridentate ligands eliminates the possibility of Λ and Δ enantiomers that are obtained when bis-bidentate units are linked through bridging ligands. The crystal structure of one member of the family has been obtained using a synchrotron X-ray source. All of the complexes are very brightly luminescent, with emission maxima in solution varying over the range 517-572 nm, according to the identity of the ligands. The highest-energy emitter is the cyanide derivative whilst the lowest is the complex with the dithienylpyrimidine. The trends in both the absorption and emission energies as a function of ligand substituent have been rationalised accurately with the aid of TD-DFT calculations. The lowest-excited singlet and triplet levels correlate with the trend in the HOMO-LUMO gap. All the complexes have quantum yields that are close to unity and phosphorescence lifetimes - of the order of 500 ns - that are unusually short for complexes of such brightness. These impressive properties stem from an unusually high rate of radiative decay, possibly due to spin-orbit coupling pathways being facilitated by the second metal ion, and to low non-radiative decay rates that may be related to the rigidity of the dinuclear scaffold.

TD-DFT Benchmark on Inorganic Pt(II) and Ir(III) Complexes

We report in the present paper a comprehensive investigation of representative Pt(II) and Ir(III) complexes with special reference to their one-photon absorption spectra employing methods rooted in density functional theory and its time dependent extension. We have compared nine different functionals ranging from generalized gradient approximation (GGA) to global or range-separated hybrids, and two different basis sets, including pseudopotentials for 4 iridium and 7 platinum complexes. It turns out that hybrid functionals with the same exchange part give comparable results irrespective of the specific correlation functional (i.e., B3LYP is very close to B3PW91 and PBE0 is very close to MPW1PW91). More recent functionals, such as CAM-B3LYP and M06-2X, overestimate excitation energies, whereas local functionals (BP86 -GGA-, M06-L -Meta GGA-) strongly underestimate transition energies with respect to experimental results. As expected, basis set effects are weak, and the use of a triple-ζ polarized (def2-TZVP) basis set does not significantly improve the computed excitation energies with respect to a classical double-ζ basis set (LANL2DZ) augmented by polarization functions, but it significantly raises the computational effort.