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Organic light-emitting diodes with external quantum efficiency of 38.8% are realized using a Pt-based thin-film emitting layer with photoluminescence quantum yield of 96% and transition dipole ratio of 93%. The emitting dipole orientation of the thin films fabricated using Pt complexes is investigated and the structural relationship between X-ray structural analysis and the structures in thin films are discussed based on quantum chemical calculations.

Tremendous effort has been devoted to developing novel near-infrared (NIR) emitters and to improving the performance of NIR organic light-emitting diodes (OLEDs). Os(II) complexes are known to be an important class of NIR electroluminescent materials. However, the highest external quantum efficiency achieved so far for Os(II)-based NIR OLEDs with an emission peak wavelength exceeding 700 nm is still lower than 3%. A new series of Os(II) complexes (1-4) based on functional pyrazinyl azolate chelates and dimethyl(phenyl)phosphane ancillaries is presented. The reduced metal-to-ligand charge transfer (MLCT) transition energy gap of pyrazinyl units in the excited states results in efficient NIR emission for this class of metal complexes. Consequently, NIR OLEDs based on 1-4 show excellent device performance, among which complex 4 with a triazolate fragment gives superior performance with maximum external quantum efficiency of 11.5% at peak wavelength of 710 nm, which represent the best Os(II)-based NIR-emitting OLEDs with peak maxima exceeding 700 nm.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201906738. zation because of the strong spin-orbit coupling exerted by the thrid-row transition metal elements. [3,18,19] Among these, Pt(II)-based NIR OLEDs represent the state-of-the-art results in the range of 700-900 nm. [18,[20][21][22][23] In particular, in 2017, our group achieved a milestone in NIR OLEDs research, and realized NIR OLEDs with EQE up to 24 ± 1% at 740 nm by using the pyrazinyl pyrazolate Pt(II)-based emitter. [22] Although the efficiencies of these Pt(II)-based NIR OLEDs are remarkable, which also demonstrated a promising way to reduce the efficiency roll-off of the Pt(II)-based NIR OLEDs by using the metal-metal-to-ligand charge transfer (MMLCT) transition. [22][23][24] But in general, NIR OLEDs based on squareplanar Pt(II) emitters, particularly for porphyrin-based Pt(II) phosphors, typically suffer from serious efficiency roll-off at high current density, due to the long-lived triplet exciton generated and corresponding self-quenching. [18,21,25] In comparison Adv. Funct.

Preparation and characterization of a new class of emissive Os(II) complexes ( 1 -5 ) have been achieved using 3-(thiazol-2-yl), 3-(benzothiazol-2-yl), 3-(imidazol-2-yl) and 3-(benzimidazol-2-yl) azole chelates, together with the incorporation of one diphosphine ancillary for satisfying the octahedral coordination arrangement. The resulting Os(II) complexes, except for 5 , all show bright emission spanning visible region from green to saturated red, and their structural versus spectroscopic properties have been comprehended by absorption/emission together with computational approaches. Subsequently, a series of solution-processed OLEDs using 1 -4 as the dopant were successfully fabricated, demonstrating excellent device performances with external quantum effi ciency (EQE) of 15%, current effi ciency exceeding 48 cd/A, and power conversion effi ciency up to 50 lm/W. Complex 4 is the only Os(II) based phosphor so far suitable for the fabrication of highly effi cient green-emitting OLEDs.

Heteroleptic Ir(III) complexes with 3-tert-butyl-5-(2-pyridyl)-1,2,4-triazolate chromophore (bptz) and cyclometalating benzyldiphenylphosphine (bdp) or phenyl diphenylphosphinite (pdpit) ancillary (i.e.[Ir(bptz) 2 (bdp)] (1) and [Ir(bptz) 2 (pdpit)] (2)) are synthesized upon treatment of [IrCl 3 (tht) 3 ] (tht ¼ tetrahydrothiophene) with the relevant phosphine, followed by the addition of 2 equiv. of bptz chelate at elevated temperature. Their photophysical properties in solution were measured, along with the characteristics detected as dopants in thin solid films. For application, organic light emitting diodes (OLEDs) were also fabricated using 1 and 2 as dopants, achieving respective maximum efficiencies of 17.8% (44.8 cd A À1 and 46.3 lm W À1 ) and 9.1% (22.8 cd A À1 and 23.6 lm W À1 ). In addition, sky blue iridium complex 1 was used with red osmium complex [Os(bpftz) 2 (PPhMe 2 ) 2 ] (3) to fabricate phosphorescent OLEDs with a sophisticated red/blue/red emitting layer architecture, attaining a stable warm white color with CIE coordinates of (0.397, 0.411). This white OLED attained an electroluminescence efficiency of up to 18.1%, 39.6 cd A À1 , and 35.7 lm W À1 for the forward direction.