Rational Design of Charge-Neutral, Near-Infrared-Emitting Osmium(II) Complexes and OLED Fabrication

A new series of charge neutral Os(II) isoquinolyl triazolate complexes (1-4) with both tram and cis arrangement of phosphine donors are synthesized, and their structural, electrochemical and photophysical properties are established. In sharp contrast to the ris-arranged complexes 2-4, the trans derivative 1, which shows a planar arrangement of chromophoric Nsubstituted chelates, offers the most effective extended jr-delocalization and hence the lowest excited state energy gap. These complexes exhibit phosphorescence with peak wavelengths ranging from 692-805 nm in degassed CH2CI2 at room temperature. Near-infrared (NIR)-emitting electroluminescent devices employing 6wt % of 1 (or 4) doped in AIq₃ host material are successfully fabricated. The devices incorporating 1 as NIR phosphor exhibit fairly intense emission with a peak wavelength at 814nm. Forward radiant emittance reaches as high as 65.02 μWcm^-2, and a peak EQE of ∼ 1.5% with devices employing Alq₃, TPBi and/or TAZ as electrontransporting/exciton-blocking layers. Upon switching to phosphor 4, the electroluminescence blue shifts to 718 nm, while the maximum EQE and radiance increase to 2.7% and 93.26 (μWcm ^-2) respectively. Their performances are optimized upon using TAZ as the electron transporting and exciton-blocking material. The OLEDs characterized represent the only NIRemitting devices fabricated using charge-neutral and volatile Os(II) phosphors via thermal vacuum deposition.