Azolate-Based Osmium(II) Complexes with Luminescence Spanning Visible and Near Infrared Region

Mononuclear osmium(II) complexes bearing azolate chromophoric chelates constituted a new class of emitters that are isoelectronic to the iridium(III) complexes, i. e., all with low-spin d⁶ electronic configuration. The high reactivity of azole toward the osmium reagent Os₃(CO)₁₂ via N−H oxidative addition gives eventual formation of various mononuclear osmium(II) complexes showing bright emission ranging from sky-blue to near infrared. Tuning of luminescence was executed by variation of the nature and π-conjugation of chelates, as well as changing the metal-based electron density by further ligand substitution. One important result is the successful isolation of two classes of NIR emissive osmium(II) complexes; those with the ligand-to-ligand charge transfer process at the excited state seems to be less efficient than those facilitated by the distinctive intra-ligand charge transfer process. In addition, fabrication and characterization of OLED devices were elaborated, confirming their potential to serve as efficient saturated-red and NIR phosphors.