Efficient near-infrared-emitting cationic iridium complexes based on highly conjugated cyclometalated benzo[g]phthalazine derivatives

Abstract

Two near-infrared- (NIR-) emitting cationic iridium(III) complexes, [Ir(dpbpa)₂(Bphen)]⁺PF₆⁻ (1) and [Ir(dtbpa)₂(Bphen)]⁺PF₆⁻ (2), were rationally designed and synthesized, where dpbpa, dtbpa, and Bphen represent 1,4-diphenylbenzo[g]-phthalazine, 1,4-di(thiophen-2-yl)benzo[g]phthalazine and 4,7-dipheny-1,10-phenanthroline, respectively. By using highly conjugated cyclometalated benzo[g]phthalazine ligands, these two complexes exhibited a significantly large red shift in wavelength to the truly NIR region with maximum peaks at 715 nm for 1 and 775 nm for 2. Complex 1 exhibited unexpectedly improved quantum efficiency up to 6.1% in the solid films. Based on these solution-processable phosphors, NIR organic-light-emitting devices (OLEDs) have been fabricated and demonstrated negligible efficiency roll-off with nearly constant external quantum efficiency around 0.5% over a wide range of current density of 1–100 mA cm⁻². Density functional theory calculations were performed to discover that the newly cyclometalated benzo[g]phthalazine ligands have several areas of superiority over the previous benzo[g]quinoline ligands in views of stronger Ir–N bonds, smaller chelate congestion, higher electron-accepting ability, thus improving the overall phosphorescence of the corresponding iridium complexes in the NIR region.