Phosphorescent multinuclear complexes for optoelectronics: tuning of the excited-state dynamics

Luminescent transition metal complexes have attracted a great deal of attention in the last two decades from both fundamental and application points of view. The majority of the investigated and most efficient systems consist of monometallic compounds with judiciously selected ligand sphere, providing excellent triplet emitters for both lab-scale and real-market light-emitting devices for display technologies. More recently, chemical architectures comprising multimetallic compounds have appeared as an emerging and valuable alternative. Herein, the most recent trends in the field are showcased in a systematic approach, where the different examples are classified by metal center and ligand(s) scaffold. Their optical and electroluminescence properties are presented and compared as well. Indeed, the multimetallic strategy has proven to be highly suitable for compounds emitting efficiently in the challenging red to near-infrared region, yielding metal-based emitters with improved optical properties in terms of enhanced emission efficiency, shortened excited-state lifetime, and faster radiative rate constant. Finally, the advantages and drawbacks of the multimetallic approach will be discussed.

Phospho-rescent mono- and diiridium(III) complexes cyclo-metalated by fluorenyl- or phenyl-pyridino ligands with bulky substituents, as prospective OLED dopants

The crystal structures of tris-[9,9-dihexyl-2-(5-meth-oxy-pyridin-2-yl-κN)-9H-fluoren-3-yl-κC 3]iridium pentane monosolvate, [Ir(C31H38NO)3]·C5H12, (I), di-μ2-chlorido-bis-{bis-[2-(5-fluoro-pyridin-2-yl)-9,9-dihexyl-9H-fluoren-3-yl]iridium} pentane 0.3-solvate, [Ir2(C30H35FN)4Cl2]·0.3C5H12, (II), di-μ2-cyanato-bis-{bis-[9,9-dihexyl-2-(5-meth-oxy-pyridin-2-yl)-9H-fluoren-1-yl]iridium} pentane monosolvate, [Ir2(C31H38NO)4(NCO)2(NCO)2]·C5H12, (III), and {μ-N,N'-bis-[3,5-bis-(tri-fluoro-meth-yl)phen-yl]oxamidato}bis(bis{2-[4-(2,4,6-trimethylphenyl)pyridin-2-yl]phenyl-κ2 C 1,N'}iridium)-chloro-benzene-pentane (1/2.3/0.4), [Ir2(C20H19N)4(C18H6F12N2O2)]·2.3C6H5Cl·0.4C5H12, (IV), synthesized in the quest for organic light-emitting devices, were determined. The bis-μ2-chloro and bis-μ2-cyanato complexes have ΔΔ and ΛΛ configurations of the distorted octa-hedral Ir centres in racemic crystals, whereas the oxamido complex has a centrosymmetric (meso) structure with the ΔΛ configuration. The bridging oxamido moiety has a nearly planar anti geometry. All structures show substantial disorder of both host mol-ecules and solvents of crystallization.

Dinuclear metal complexes: multifunctional properties and applications

Dinuclear metal complexes have enabled breakthroughs in OLEDs, photocatalytic water splitting and CO₂reduction, DSPEC, chemosensors, biosensors, PDT and smart materials.

Exploration of the Structural and Photophysical Characteristics of Mono- and Binuclear Ir(III) Cyclometalated Complexes for Optoelectronic Applications

Intrinsic characteristics possessed and exhibited by Ir(III) cyclometalated complexes need to be further examined, understood, and explored for greater value enhancement and potentiation. This work focuses primarily on the comparative studies of the ligand structures, types, and their substituent influence on the photophysical and optoelectronic properties of typical cyclometalated mono- and binuclear iridium(III) complexes in solution or solid states.

Tetranuclear Iridium Complex with a Self-Host Feature for High-Efficiency Nondoped Phosphorescent Organic Light-Emitting Diodes

A tetranuclear Ir complex 3IrB-IrG was newly designed and synthesized with one green-emitting complex as the core, whose periphery is encapsulated by three blue-emitting complexes via a nonconjugated linkage. In the case of such a multinuclear system, a self-host feature can be formed, showing negligible electron communication, efficient outside-in energy transfer, and unique shielding effect. When using 3IrB-IrG as the emitting layer in the absence of host, the corresponding nondoped device reveals a state-of-art luminous efficiency of 32.6 cd/A (34.2 lm/W, 9.7%) as well as CIE coordinates of (0.38, 0.58). The performance significantly outperforms that of the bare mononuclear complex IrG (8.4 cd/A, 9.2 lm/W, 2.5%), highlighting the potential of self-host multinuclear complexes to realize high-efficiency nondoped PhOLEDs for the first time.

Thorium(IV) and Uranium(IV) Complexes with Cucurbit[5]uril

Tetravalent thorium and uranium complexes with cucurbit[5]uril (Q[5]) were investigated with eight new complexes being synthesized and structurally characterized. [Th(Q[5])(OH)(H₂O)₂]₆·18NO₃· nH₂O (1) has a hexagonal nanowheel structure with each of the six Th⁴⁺ ions being cap-coordinated by a Q[5] and monodentate-coordinated to the nearby Q[5]. [Th(Q[5])(HCOO)(H₂O)₄][Th(NO₃)₅(H₂O)₂]₂[Th(NO₃)₃(HCOO)(H₂O)₂]_0.5·NO₃· nH₂O (2) has a heteroleptic mononuclear structure with a Th⁴⁺ ion cap-coordinated on one side of the Q[5] portal and monodentate-coordinated to a formate anion inside the Q[5] cavity. [KTh_1.5(Q[5])Cl(NO₃)₃][Th(NO₃)₅(H₂O)₂]·2NO₃·2.5H₂O (3) has a heterometallic structure with both Th⁴⁺ and K⁺ ions each occupying one side of the two Q[5] portals forming a capsule. [CsTh(Q[5])Cl(NO₃)₂(H₂O)₃]·2NO₃· nH₂O (4) has a heterometallic 1D polymeric structure with both Th⁴⁺ and Cs⁺ ions each occupying one side of the two Q[5] portals, forming monomers which are linked together by sharing two water molecules and one carbonyl oxygen atom between Th⁴⁺ and Cs⁺ ions. [Th(Q[5])Cl(H₂O)][CdCl₃][CdCl₄]·0.5HCl·4H₂O (5), [Th(Q[5])Cl(H₂O)][Ru₂OCl₉(H₂O)]·0.5HCl·9.5H₂O (6), [Th(Q[5])Cl(H₂O)][IrCl₆]_1.5·3H₂O (7), and [U(Q[5])Cl(H₂O)][ZnCl₃(H₂O)][(ZnCl₄)]·8H₂O (8) have similar 1D polymeric structures with Th⁴⁺/U⁴⁺ ions cap-coordinated on one side of a Q[5] and bidentate coordinated to the nearby Q[5]. The transition metal chlorides act as anions for charge compensation as well as structure inducers via cation-anion interactions forming various anion patterns around the 1D polymers. Actinide contraction has been observed in the early actinide series.

Sky-blue emitting bridged diiridium complexes: beneficial effects of intramolecular π-π stacking

The potential of intramolecular π-π interactions to influence the photophysical properties of diiridium complexes is an unexplored topic, and provides the motivation for the present study. A series of diarylhydrazide-bridged diiridium complexes functionalised with phenylpyridine (ppy)-based cyclometalating ligands is reported. It is shown by NMR studies in solution and single crystal X-ray analysis that intramolecular π-π interactions between the bridging and cyclometalating ligands rigidify the complexes leading to high luminescence quantum efficiencies in solution and in doped films. Fluorine substituents on the phenyl rings of the bridge promote the intramolecular π-π interactions. Notably, these non-covalent interactions are harnessed in the rational design and synthesis of the first examples of highly emissive sky-blue diiridium complexes featuring conjugated bridging ligands, for which they play a vital role in the structural and photophysical properties. Experimental results are supported by computational studies.

Microwave-assisted one-pot synthesis of new ionic iridium complexes of [Ir(bzq)2(N^N)]+A- type and their selected electroluminescent properties

Iridium C,N-cyclometalated complexes with an ionic structure are considered to be promising candidates for application in host/guest solid-state phosphorescent single-layer devices because the employment of such dopants offers the possibility of reducing their concentration in organic matrices as well as allows obtaining organic light emitting devices (OLEDs) with interesting emission parameters. We report herein a methodology enabling the synthesis of cyclometalated ionic iridium(iii) complexes of the type [Ir(C^N)₂(N^N)]⁺A^- according to a three-component one-pot strategy involving the acceleration of the reaction via microwave irradiation. The developed protocol allowed efficient synthesis of a series of new cationic iridium(iii) coordination derivatives, which were isolated and spectroscopically characterized, while the structures of two of them were determined by the X-ray method. Moreover, the iridium(iii) derivatives were subjected to the cyclic voltammetry studies in order to determine the energies of the HOMO and LUMO levels as well as to estimate their electrochemical properties and to predict some electronic properties. Additionally, the ONIOM calculation scheme that was used to predict HOMO-LUMO gaps for the studied Ir(iii) complexes showed a good correlation between the experimental and calculated values. In order to determine the influence of the structure and nature of the ancillary ligand on the location of the maximum emission band, the photophysical properties of the synthesized iridium complexes were characterized. Finally, the selected compounds were used as emitters for the construction of polymer light emitting diodes (PLEDs) based on a poly(N-vinylcarbazole)/2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4-oxadiazole (PVK/PBD) matrix. The highest luminance, above 10 000 cd m^-2, was recorded for the device containing only 1.0 wt% of [Ir(bzq)₂(1,10-phenanthroline)]⁺PF₆^- in the PVK/PBD. The fabricated PLEDs exhibit current efficiency in the range of 1.0 to 2.2 cd A^-1.

Pyrimidine-Based Mononuclear and Dinuclear Iridium(III) Complexes for High Performance Organic Light-Emitting Diodes

Containing two nitrogen atoms, the electron-deficient pyrimidine ring has excellent coordinating capability with transition metal ions. However, compared with the widely used pyridine ring, applications of the pyrimidine ring in phosphorescent Ir(III) complexes are rare. In this research, two highly emissive pyrimidine-based mononuclear Ir(III) complexes and their corresponding dinuclear Ir(III) complexes were prepared with a simple one-pot reaction. The incorporation of the second Ir(III) center can lead to dramatic differences of both photophysical and electrochemical properties between the mono- and dinuclear complexes. Besides, these properties can also be fine-tuned with different substituents. Theoretical calculations have also been performed to understand their photophysical behaviors. The electroluminescent investigations demonstrate that the pyrimidine-based mono- and dinuclear Ir(III) complexes could show impressive device performance. The vacuum-deposited organic light-emitting diode (OLED) based on the mononuclear Ir(III) complex exhibited an external quantum efficiency (EQE) of 16.1% with almost no efficiency roll-off even at 10 000 cd m^-2. More encouragingly, the solution-processed OLED based on the dinuclear Ir(III) complex achieved the outstanding EQE, current efficiency (CE), and power efficiency (PE) of 17.9%, 52.5 cd A^-1, and 51.2 lm W^-1, respectively, representing the highest efficiencies ever achieved by OLEDs based on dinuclear Ir(III) complexes.

From Mononuclear to Dinuclear Iridium(III) Complex: Effective Tuning of the Optoelectronic Characteristics for Organic Light-Emitting Diodes

Phosphorescent dinuclear iridium(III) complexes that can show high luminescent efficiencies and good electroluminescent abilities are very rare. In this paper, highly phosphorescent 2-phenylpyrimidine-based dinuclear iridium(III) complexes have been synthesized and fully characterized. Significant differences of the photophysical and electrochemical properties between the mono- and dinuclear complexes are observed. The theoretical calculation results show that the dinuclear complexes adopt a unique molecular orbital spatial distribution pattern, which plays the key role of determining their photophysical and electrochemical properties. More importantly, the solution-processed organic light-emitting diode (OLED) based on the new dinuclear iridium(III) complex achieves a peak external quantum efficiency (η(ext)) of 14.4%, which is the highest η(ext) for OLEDs using dinuclear iridium(III) complexes as emitters. Besides, the efficiencies of the OLED based on the dinuclear iridium(III) complex are much higher that those of the OLED based on the corresponding mononuclear iridium(III) complex.

Syntheses, Spectroscopic, Electrochemical, and Third-Order Nonlinear Optical Studies of a Hybrid Tris{ruthenium(alkynyl)/(2-phenylpyridine)}iridium Complex

The synthesis of fac-[Ir{N,C1′-(2,2′-NC5H4C6H3-5′-C≡C-1-C6H2-3,5-Et2-4-C≡CC6H4-4-C≡CH)}3] (10), which bears pendant ethynyl groups, and its reaction with [RuCl(dppe)2]PF6 to afford the heterobimetallic complex fac-[Ir{N,C1′-(2,2′-NC5H4C6H3-5′-C≡C-1-C6H2-3,5-Et2-4-C≡CC6H4-4-C≡C-trans-[RuCl(dppe)2])}3] (11) is described. Complex 10 is available from the two-step formation of iodo-functionalized fac-tris[2-(4-iodophenyl)pyridine]iridium(III) (6), followed by ligand-centered palladium-catalyzed coupling and desilylation reactions. Structural studies of tetrakis[2-(4-iodophenyl)pyridine-N,C1′](μ-dichloro)diiridium 5, 6, fac-[Ir{N,C1′-(2,2′-NC5H4C6H3-5′-C≡C-1-C6H2-3,5-Et2-4-C≡CH)}3] (8), and 10 confirm ligand-centered derivatization of the tris(2-phenylpyridine)iridium unit. Electrochemical studies reveal two (5) or one (6–10) Ir-centered oxidations for which the potential is sensitive to functionalization at the phenylpyridine groups but relatively insensitive to more remote derivatization. Compound 11 undergoes sequential Ru-centered and Ir-centered oxidation, with the potential of the latter significantly more positive than that of Ir(N,C′-NC5H4-2-C6H4-2)3. Ligand-centered π–π* transitions characteristic of the Ir(N,C′-NC5H4-2-C6H4-2)3 unit red-shift and gain in intensity following the iodo and alkynyl incorporation. Spectroelectrochemical studies of 6, 7, 9, and 11 reveal the appearance in each case of new low-energy LMCT bands following formal IrIII/IV oxidation preceded, in the case of 11, by the appearance of a low-energy LMCT band associated with the formal RuII/III oxidation process. Emission maxima of 6–10 reveal a red-shift upon alkynyl group introduction and arylalkynyl π-system lengthening; this process is quenched upon incorporation of the ligated ruthenium moiety on proceeding to 11. Third-order nonlinear optical studies of 11 were undertaken at the benchmark wavelengths of 800 nm (fs pulses) and 532 nm (ns pulses), the results from the former suggesting a dominant contribution from two-photon absorption, and results from the latter being consistent with primarily excited-state absorption.

Formylated chloro-bridged iridium(iii) dimers as OLED materials: opening up new possibilities

In this study, a series of four formyl-substituted chloro-bridged iridium(iii) dimers were prepared.

Glycerol as suitable solvent for the synthesis of metallic species and catalysis

This Minireview considers the foremost reported works involving glycerol as a solvent in the synthesis of organometallic complexes and metallic nanoparticles. This analysis highlights their catalytic applications. A special emphasis is devoted to the ability of glycerol to immobilize nanometric species, which, in turn, enables an efficient recycling of the catalytic phase to give metal-free organic products.

Theoretical characterization and design of highly efficient iridium (III) complexes bearing guanidinate ancillary ligand

A density functional theory/time-depended density functional theory was used to investigate the synthesized guanidinate-based iridium(III) complex [(ppy)2Ir{(N(i)Pr)2C(NPh2)}] (1) and two designed derivatives (2 and 3) to determine the influences of different cyclometalated ligands on photophysical properties. Except the conventional discussions on geometric relaxations, absorption and emission properties, many relevant parameters, including spin-orbital coupling (SOC) matrix elements, zero-field-splitting parameters, radiative rate constants (kr) and so on were quantitatively evaluated. The results reveal that the replacement of the pyridine ring in the 2-phenylpyridine ligand with different diazole rings cannot only enlarge the frontier molecular orbital energy gaps, resulting in a blue-shift of the absorption spectra for 2 and 3, but also enhance the absorption intensity of 3 in the lower-energy region. Furthermore, it is intriguing to note that the photoluminescence quantum efficiency (ΦPL) of 3 is significantly higher than that of 1. This can be explained by its large SOC value<T1|HSO|Sn>(n=3-4) and large transition electric dipole moment (μS3), which could significantly contribute to a larger kr. Besides, compared with 1, the higher emitting energy (ET1) and smaller <S0|HSO|T1>(2) value for 3 may lead to a smaller non-radiative decay rate. Additionally, the detailed results also indicate that compared to 1 with pyridine ring, 3 with imidazole ring performs a better hole injection ability. Therefore, the designed complex 3 can be expected as a promising candidate for highly efficient guanidinate-based phosphorescence emitter for OLEDs applications.