Roles of Ancillary Chelates and Overall Charges of Bis-tridentate Ir(III) Phosphors for OLED Applications

Abstract

A series of charge-neutral bis-tridentate Ir(III) complexes (<b>1</b>, <b>3</b>, and <b>4</b>) were prepared via employing three distinctive tridentate prochelates, that is, (pzp^tBph^FO)H₂, [(phpyim)H₂·(PF₆)], and [(pimb)H₃·(PF₆)₂], which possess one dianionic pzp^tBph^FO, together with a second monoanionic tridentate chelate, namely, (pzp^tBph^FO)H, phpyim, and pimb, respectively. Moreover, a homoleptic, charge-neutral complex <b>2</b> was obtained by methylation of chelating (pzp^tBph^FO)H of <b>1</b> in basic media, while closely related cationic complexes <b>5-7</b> were obtained by further methylation of the remaining pyrazolate unit of previously mentioned neutral complexes <b>2-4</b>, followed by anion metatheses. All of these Ir(III) metal complexes showed a broadened emission profile with an onset at ∼450 nm, a result of an enlarged ligand-centered ππ* transition gap, but with distinct efficiencies ranging from 0.8% to nearly unity. Comprehensive spectroscopic and computational approaches were executed, providing a correlation for the emission efficiencies versus energy gaps and between the metal-to-ligand charge transfer/ππ* emitting excited state and upper-lying metal-centered dd quenching state. Furthermore, Ir(III) complexes <b>3</b> and <b>4</b> were selected as dopant emitters in the fabrication of sky-blue phosphorescent organic light-emitting diodes, affording maximum external quantum efficiencies of 16.7 and 14.6% with CIE_{<i>x</i>,<i>y</i>} coordinates of (0.214, 0.454) and (0.191, 0.404) at a current density of 10² cd/m², respectively. Hence, this research highlights an inherent character of bis-tridentate Ir(III) complexes in achieving high phosphorescence quantum yield at the molecular level.