Theoretical perspective on the electronic structure and photophysical properties for a series of mixed-carbene tris-cyclometalated iridium(iii) complexes

The electronic structure and photophysical properties of four mixed-carbene tris-cyclometalated iridium(iii) complexes have been theoretically investigated by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The effect of varying the main ligand by introducing different ring structures on the photophysical properties of the studied complexes has been explored. All studied complexes have slightly distorted octahedral geometries. The complex with a rigid skeletal structural main ligand possesses the smallest difference between the recombination energy of hole transport and recombination energy of electron transport among these complexes, enhancing the charge transfer balance. The lowest energy emission wavelength calculated is in very good agreement with the available experimental value. This study will provide useful information for the design of new phosphorescent organic light-emitting diode (OLED) materials.

Two platinum(ii) complexes with a 4-phenyl-4H-1,2,4-triazole derivative as an ancillary ligand for efficient green OLEDs

Two new cyclometalated platinum(ii) complexes ((TN₃T)Pt(dptp), (4tfmppy)Pt(dptp)) with 2',6'-bis(trifluoromethyl)-2,3'-bipyridine (TN₃T) and 4-trifluoromethylphenylpyridine (4tfmppy) as the cyclometalated ligands and the nitrogen heterocycle 2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenol (dptp) as the ancillary ligand were developed. Both complexes are green phosphors with high photoluminescence quantum efficiency yields (λ_max = 532 nm, Φ_p = 65% for (TN₃T)Pt(dptp) and λ_max = 502 nm, Φ_p = 71% for (4tfmppy)Pt(dptp)) in CH₂Cl₂ solutions at room temperature, respectively. Organic light-emitting diodes (OLEDs) with single-emitting layer and double-emitting layer structures were fabricated with good performances. Particularly, the double-emitting layer device D2 based on the complex (4tfmppy)Pt(dptp) with 6 wt% doped concentration shows superior performances with a maximum EQE of 26.90% with mild efficiency roll-off. This study demonstrates that the ancillary ligand attached with a 4-phenyl-4H-1,2,4-triazole group could facilitate charge trapping across the bulk of the device for efficient OLEDs.

Emissive Iridium(III) Complexes with Phosphorous-Containing Ancillary

Ir(III) metal complexes and related emitters bearing all kind of cyclometalated chromophoric chelates and non-chromophoric ancillary are extensively studied during the past three decades. Many of them have been found to display bright room temperature phosphorescence from triplet excited states in both solution and solid states, offering a possible application in contemporary optoelectronic technologies, including organic light emitting diodes, electrochemiluminescence, biological imaging and chemical sensing. Among reported materials, there are Ir(III) complexes with at least one phosphorus (P)-containing ligand and/or ancillary chelate, together with cyclometalates or equivalents that are in control of the actual emission energy. Particularly, possession of P-based donor can lead to divergent structural and photophysical properties compared to the traditional designs. This review aims to provide a literature overview as well as the authors' personal account to the development of relevant Ir(III) based phosphors bearing these P-donors. To the readers' convenience, the contents are subdivided into six sessions, according to whether or not they are charge natural, or with mono- or dianionic electronic character, and in accordance to their divergent bonding modes, i. e. monodentate, bidentate and tripodal coordination. In many cases, the P-based ancillaries offer an easy accessible route to the formation of efficient sky-blue and true-blue emitters due to their π-accepting property, together with enlarged emission energy gap and destabilized upper lying quenching state.