Sterically Hindered Tetradentate [Pt(O^N^C^N)] Emitters with Radiative Decay Rates up to 5.3 × 105 s-1 for Phosphorescent Organic Light-Emitting Diodes with LT95 Lifetime over 9200 h at 1000 cd m-2

Described here are sterically hindered tetradentate [Pt(O^N^C^N)] emitters (Pt-1, Pt-2, and Pt-3) developed for stable and high-performance green phosphorescent organic light-emitting diodes (OLEDs). These Pt(II) emitters exhibit strong saturated green phosphorescence (λmax = 517-531 nm) in toluene and mCP thin films with emission quantum yields as high as 0.97, radiative rate constants (kr) as high as 4.4-5.3 × 105 s-1 and reduced excimer emission, and with a preferential horizontally oriented transition dipole ratio of up to 84%. Theoretical calculations show that p-(hetero)arene substituents at the periphery of the ligand scaffolds in Pt-1, Pt-2, and Pt-3 can i) enhance the spin-orbit coupling (SOC) between the lower singlet excited states and the T1 state, and S0→Sn (n = 1 or 2) transition dipole moment, and ii) introducing additional SOC activity and the bright 1ILCT[π(carbazole)→π*(N^C^N)] excited state (Pt-2 and Pt-3), which are the main contributors to the increased kr values. Utilizing these tetradentate Pt(II) emitters, green phosphorescent OLEDs are fabricated with narrow-band electroluminescence (FWHM down to 36 nm), high external quantum efficiency, current efficiency up to 27.6% and 98.7 cd A-1, and an unprecedented device lifetime (LT95) of up to 9270 h at 1000 cd m-2 under laboratory conditions.

Deep-blue light-emitting cationic iridium(III) complexes featuring diamine ancillary ligands: Experimental and theoretical investigations

Deep-blue emitters based on combining two approaches are reported here. In the first approach, the nitrogen content of the cyclometallated ligand is increased using 2,4'-bpy ligands, leading to a low highest occupied molecular orbital energy. In the second approach, 2,2'-bpy is replaced with a lower electron donating ligand, leading to a high lowest unoccupied molecular orbital (LUMO) energy. Thus, three new ionic transition metal complexes of [Ir(2,4'-bpy)2(NN)]PF6 [where NN is 2,2'-bpy (1), o-phenylenediamine (2), and 4-methoxy-o-phenylenediamine (3)] were synthesized, and their electronic and photophysical features were studied. In solution, [Ir(2,4'-bpy)2bpy]PF6 emits blue light centered at 396 nm, which is blue-shifted compared to [Ir(ppy)2bpy]PF6. The low electron donation of the diamine ancillary ligands introduces the contribution of the cyclometallated ligand to the LUMO, changing the nature of the emission and leading to different photophysical features. Density functional theory calculations indicate the long bond distances of Ir-Ns at the diamine ligands, suggesting weak metal-ligand interactions and low quantum yields.1.

Orange Light-Driven C(sp2)-C(sp3) Cross-Coupling via Spin-Forbidden Ir(III) Metallaphotoredox Catalysis

We report the development and characterization of a library of Ir(III) photocatalysts capable of undergoing spin-forbidden excitation (SFE) under orange light irradiation (595 nm). These catalysts were successfully applied to the construction of synthetically valuable C(sp2)-C(sp3) bonds inaccessible with existing methods of low-energy light-driven dual nickel/photoredox catalysis, demonstrating the synthetic utility of this photocatalyst family. The photocatalysts are capable of accessing both oxidatively and reductively activated coupling partners, illustrated through deaminative arylation and potassium alkyl trifluoroborate cross-coupling reactions with aryl halides. We demonstrate diverse substrate scopes of both cross-coupling paradigms under mild conditions in the first example of low-energy light-driven C(sp2)-C(sp3) metallaphotoredox coupling.

Low-cost machine learning prediction of excited state properties of iridium-centered phosphors

Prediction of the excited state properties of photoactive iridium complexes challenges ab initio methods such as time-dependent density functional theory (TDDFT) both from the perspective of accuracy and of computational cost, complicating high-throughput virtual screening (HTVS). We instead leverage low-cost machine learning (ML) models and experimental data for 1380 iridium complexes to perform these prediction tasks. We find the best-performing and most transferable models to be those trained on electronic structure features from low-cost density functional tight binding calculations. Using artificial neural network (ANN) models, we predict the mean emission energy of phosphorescence, the excited state lifetime, and the emission spectral integral for iridium complexes with accuracy competitive with or superseding that of TDDFT. We conduct feature importance analysis to determine that high cyclometalating ligand ionization potential correlates to high mean emission energy, while high ancillary ligand ionization potential correlates to low lifetime and low spectral integral. As a demonstration of how our ML models can be used for HTVS and the acceleration of chemical discovery, we curate a set of novel hypothetical iridium complexes and use uncertainty-controlled predictions to identify promising ligands for the design of new phosphors while retaining confidence in the quality of the ANN predictions.

Theoretical insight into the photophysical properties of five phosphorescent heteroleptic iridium(iii) complexes bearing oxadiazol-substituted amide ligands

A DFT/TDDFT (density functional theory/time-dependent density functional theory) investigation on the geometries in the ground and lowest triplet excited states, the frontier molecular orbitals, the absorption spectra and the phosphorescent emission properties of five heteroleptic iridium(iii) complexes have been performed to obtain a better understanding of structure-property relationships. The key aim was to investigate the effect of π-conjugation on the photophysical properties of these studied complexes. The lowest energy absorption wavelengths are located at 461 nm for 1, 418 nm for 2, 422 nm for 3, 426 nm for 4 and 417 nm for 5. The lowest energy emissions of complexes 1-5 are localized at 572, 463, 583, 707 and 553 nm, respectively, simulated in acetonitrile medium at M052X level. These findings could be useful to provide good guidance for the further design of new potential phosphorescent organic light-emitting diode (OLED) materials.

Second-order nonlinear optical response of phenyl-substituted cationic BIS-cyclometalated iridium(III) complexes: Effect of different position

In this paper, a series of cationic iridium complexes [(2-phenylpyridine)2(2,2[Formula: see text]-bipyridine)Ir][Formula: see text] which substituted phenyl on different ligands position have been systematically investigated by density functional theory (DFT) method. Significantly, the first hyperpolarizability [Formula: see text] values can be enhanced by introducing phenyl on 2-phenylpyridine ligands R1 or R2, whereas substituting phenyl on 2,2[Formula: see text]-bipyridine ligands R3 result in a decreasing [Formula: see text] values. The [Formula: see text] values exhibit obvious connection with the corresponding HOMO and LUMO energy gap. Furthermore, the time-dependent (TD) DFT calculations suggest that the enhanced [Formula: see text] values are related to obvious charge transfer from 2-phenylpyridine ligands to 2,2[Formula: see text]-bipyridine ligands. The investigation of frequency-dependent first hyperpolarizability [Formula: see text] ([Formula: see text]; [Formula: see text], 0) and [Formula: see text] ([Formula: see text]; [Formula: see text], [Formula: see text]) shown less dispersion effect at the low-frequency region for all of the studied complexes. Overall, tuning phenyl on the different ligands position can be seen as an effective strategy to modulate the second-order nonlinear optical response for these iridium complexes, which is benefit to theoretical and experimental further investigation.

DFT/TDDFT computational study of the structural, electronic and optical properties of rhodium (III) and iridium (III) complexes based on tris-picolinate bidentate ligands

The electronic structures and spectroscopic properties of two complexes [M(pic)₃] (M = Ir, Rh) containing picolinate as bidentate ligands have been calculated by means density functional theory (DFT) and time-dependent DFT/TD-DFT using three hybrid functionals B3LYP, PBE0 and mPW1PW91. The PBE0 and mPW1PW91 functionals, which have the same HF exchange fraction (25%), give similar results and do not differ drastically from B3LYP results. Calculated geometric parameters of the complexes are in good agreement with the available experimental data. The UV absorptions observed in acetonitrile were assigned on the basis of singlet state transitions. The most intense band observed in the UV-C region corresponds to ligand-to-ligand charge transfer states (LLCT) in both complexes. The theoretical spectrum of the rhodium complex is characterized by a large degree of mixing between metal-to-ligand-charge-transfer (MLCT), LLCT and metal centered (MC) states in the UV-A region. The presence of low-lying excited states with MC character affects the absorption spectrum under spin-orbit coupling (SOC) effects and play important roles in the photochemical properties. Graphical abstract Frontier molecular orbital diagram of mer-M(pic)₃ (M=Ir, Rh).

Quantum chemical investigation on the Ir(iii) complexes with an isomeric triazine-based imidazolium carbene ligand for efficient blue OLEDs

Ir(iii) complexes of isomeric triazine-based imidazolium carbene with phenylpyridine or bipyridine as an ancillary ligand show blue phosphorescence with high quantum efficiency.

A theoretical study on the electronic structures and photophysical properties of phosphorescent Iridium(iii) complexes with –CH3/H and t-Bu substituents

We present the electronic structures, absorption and emission spectra of a series of Ir(iii) complexes to shed light on the effect of different substituents on the photophysical properties.

The effect of substituted 1,2,4-triazole moiety on the emission, phosphorescent properties of the blue emitting heteroleptic iridium(III) complexes and the OLED performance: a theoretical study

A series of neutral heteroleptic mononuclear iridium(III) complexes was investigated using the density functional theory/time-dependent density functional theory approach to determine the effect of the substituted 1,2,4-triazole moiety on the electronic structures, emission, and phosphorescent properties and the organic light emitting diode (OLED) performance. The results reveal that substitution of the free position in the triazole ring by -PhOCH3 (2) provides a higher emission energy and a lower oscillator strength, leading to longer radiative lifetime values mainly due to the ligand-to-ligand charge transfer transition character. The evaluation, based on one-center spin-orbit coupling, results in higher kr values for the substituent -F5Ph (5) and a lower ΔE(S-T) value. Furthermore, we also investigated the performance of the OLED device, including the charge injection/transport/balance ability, increases in the Förster energy transfer rate and triplet exciton confinement for host and guest materials of blue emitting Ir(III) complexes. Finally, we hope that our investigations will help in the design of highly efficient phosphorescent materials.

Tuning the electronic and phosphorescence properties of blue-emitting iridium(iii) complexes through different cyclometalated ligand substituents: a theoretical investigation

A DFT/TDDFT investigation was performed to estimate the OLED performance of six reported and seven newly designed iridium(iii) complexes.

Enhancing the electronic properties and quantum efficiency of sulfonyl/phosphoryl-substituted blue iridium complexes via different ancillary ligands

The influence of the ancillary ligands for iridium complexes on the electronic structures, absorption and emission spectra, and quantum efficiency was investigated. The results reveal that they not only tune the energy gap but also enhance the quantum efficiency.

Modification of the emission colour and quantum efficiency for oxazoline- and thiazoline-containing iridium complexes via different N^O ligands

We present the electronic structures, absorption and emission spectra, the quantum efficiency of Ir(iii) complexes to shed light on the effect of different N^O ligands on the emitting colour and quantum efficiency.

The reasons for ligand-dependent quantum yields and spectroscopic properties of platinum(II) complexes based on tetradentate O^N^C^N ligands: a DFT and TD-DFT study

DFT and TD-DFT methods have been employed to theoretically investigate the properties of three recently synthesized green-emitting platinum(II) complexes (1-3) bearing tetradentate O^N^C^N ligands (O^C^N^C = 2-(4-(3,5-di-tert-butylphenyl)-6-(3-(pyridin-2-yl)phenyl)-pyridin-2-yl)phenolate and its derivatives) that have been testified to be good emitters in organic light-emitting diodes (OLEDs), especially for complex 3. The effect of the variation of the substituents on the electronic and optical properties is emphatically explored. Our calculation results reveal that the introduction of an electron-releasing group on one phenyl ring of the O^C^N^C ligand (complex 2) does not result in a distinct alteration of the spectra. However, the incorporation of the norbornane group to the O^C^N^C ligand (complex 3) leads to a blue-shift in the absorption and emission spectra as compared with 1. In addition, how the absorption and emission spectra are affected by the solvent polarity is studied. Both the absorption and the emission spectra display red-shifts of various degrees with the decrease of the solvent polarity. The different phosphorescent quantum yields of the three complexes are compared. It is reasonable to believe that the high (3)MLCT (metal-to-ligand charge transfer) contribution and high percentage of the metallic character (M(c), %) in the emission process, as well as the largest vertical transition energy for 3, result in the highest quantum efficiency.

Theoretical study on the effect of different substituents on the electronic structures and photophysical properties of phosphorescent Ir(iii) complexes

The high quantum yield of 1 compared to 4 is explained by the S₁–T₁ splitting energy, the transition dipole moment and the energy gap between ³MLCT/π–π* and ³MC d–d states. Complexes 2 and 3 are expected to be the potential phosphorescence emitters in OLEDs with high quantum efficiency.