No abstract

Four diplatinum(II) complexes with the formula [Pt(pypm)(μ-F n )] 2 (2, 3a−c) bearing both a pyridine-pyrimidinate chelate and formamidinate bridge, where (pypm)H and F n H stand for 5-(pyridin-2-yl)-2-(trifluoromethyl)pyrimidine and functional formamidines with various substituents of i Pr (n = 1), Ph (n = 2), C 6 H 4 t Bu (n = 3), and C 6 H 4 CF 3 (n = 4), were synthesized en route from a mononuclear intermediate represented by [Pt(pypm)Cl-(F 1 H)] (1). Single-crystal X-ray diffraction studies confirmed the structure of 1 and 3a comprised of an individual "Pt(pypm)" unit and two "Pt(pypm)" units with a Pt•••Pt distance of 2.8845(2) Å, respectively. Therefore, in contrast to the structured emission of mononuclear 1 with the first vibronic peak wavelength at 475 nm, all other diplatinum complexes with shortened Pt•••Pt separation exhibited greatly red shifted and structureless metal−metal to ligand charge transfer (MMLCT) emission that extended into the near-infrared region in solid states. Their photophysical characteristics were measured under three distinctive morphological states (i.e., crystals, sublimed powders, and vacuum-deposited thin films) by steady-state UV−vis spectroscopy, while retention of Pt•••Pt interactions in deposited thin films of 2 and 3a−c was confirmed using Raman spectroscopy, demonstrating lowered Pt•••Pt stretching at 80−200 cm −1 . Most importantly, complexes 3a−c exhibited a gradual red shift with the trends crystals < sublimed powders < vacuum-deposited thin films, a result of increased intermolecular π−π stacking interactions and Pt•••Pt interactions, while crystalline samples exhibited the highest luminescence among all three morphological states due to the fewest defects in comparison to other morphologies. Finally, 3b was selected as a nondoped emitter for the fabrication of NIR-emitting OLEDs, giving an electroluminescence peak at 767 nm and a maximum external quantum efficiency of 0.14% with negligible roll-off.

Near‐infrared organic light‐emitting diodes (NIR OLEDs) enable many unique applications ranging from night‐vision displays and photodynamic therapies. However, the development of efficient NIR OLEDs with a low efficiency roll‐off is still challenging. Here, a series of new heteroleptic Pt(II) complexes (1–4) flanked by both pyridyl pyrimidinate and functional azolate chelates are synthesized. The reduced ππ* energy gap of the pyridyl pyrimidinate chelate, and strong intermolecular interaction and high crystallinity in vacuum‐deposited thin films engender strong intermolecular charge transfer transition including metal–metal‐to‐ligand charge transfer; thereby, exhibiting efficient photoluminescence within 776–832 nm and short radiative lifetimes of 0.52–0.79 µs. Consequently, nondoped NIR‐emitting OLEDs based on these Pt(II) complexes are fabricated, to which Pt(II) complexes 2 and 4 give record high maximum external quantum efficiency of 10.61% at 794 nm and 9.58% at 803 nm, respectively. Moreover, low efficiency roll‐off is also observed, among which the device efficiencies of 2 and 4 are at least four times higher than that of the best NIR‐emitting OLEDs recorded at current density of 100 mA cm−2.

Novel fluorene-based bis-tridentate Ir(iii) phosphors are capable of exhibiting both improved stability and efficient deep-blue emission in solution.

Near‐infrared (NIR) organic‐light emitting devices (OLEDs) with high radiance are useful for applications including invisible marking, communication, and biomedical imaging. However, performances of NIR OLEDs are typically limited by their severe efficiency roll‐offs at high current density. Herein, three isoquinolinyl azolate based Os(II) complexes (Isq‐1–3) with short radiative decay lifetime (in hundreds of ns), and photoluminescence with peak wavelengths > 745 nm and quantum yield up to 48% as doped thin films, are reported. Upon concomitant employment of exciplex‐forming co‐host (tris(4‐carbazoyl‐9‐ylphenyl)amine and 2,4,6‐tris(biphenyl‐3‐yl)‐1,3,5‐triazine), efficiency roll‐off is greatly reduced, giving external quantum efficiency of 9.66% at a current density of 300 mA cm−2. A maximum radiance over 170 W sr−1 m−2 is also achieved in devices based on Isq‐2 and Isq‐3.