Os(II) Phosphors with Near-Infrared Emission Induced by Ligand-to-Ligand Charge Transfer Transition

Heating of Os₃(CO)₁₂ with 6 equiv of 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl) pyridine (fptzH) in refluxing diethylene glycol monomethyl ether, followed by sequential treatment with stoichiometric Me₃NO and addition of PPhMe₂, afforded two isomeric mixtures of red-emitting [Os(fptz)₂(PPhMe₂) ₂] (1T and 1C), for which the notations T and C stand for the trans and cis-oriented fptz chelates, respectively. Alternatively, preparation of Os(II) complex using a 1:1 mixture of 5,5′-di(trifluoromethyl)-3,3′- di-1,2,4-triazole (dttzH₂) and 2,2′-bipyridine (bpy), instead of fptzH, gave isolation of a mononuclear Os(II) complex [Os(bpy)(dttz)(CO) ₂] (2) in moderate yield. Replacement of CO with PPhMe₂ on 2 afforded near-infrared (NIR)-emitting Os(II) complex [Os(bpy)(dttz) (PPhMe₂)₂] (3). The single-crystal X-ray structural analyses were executed on 1C, 2, and 3 to reveal the structural influence imposed by the various chelates. The photophysical and electrochemical properties were measured and discussed using the results of density functional theory (DFT) and time-dependent DFT calculations. Complex 3 is selected as the dopant to probe its electroluminescent properties by fabrication of the NIR emitting organic light-emitting diodes.