Homoleptic Facial Ir(III) Complexes via Facile Synthesis for High-Efficiency and Low-Roll-Off Near-Infrared Organic Light-Emitting Diodes over 750 nm

Despite the great potential for applications spanning from military night-vision displays and information-secured devices to civilian medical diagnostics and phototherapy, the development of highly efficient, stable, and low-cost near-infrared (NIR) emitting lumophores is still a formidable challenge. Herein, we report two novel NIR-emitting homoleptic facial Ir(III) complexes based on extended π-conjugated benzo[g]phthalazine ligands, namely, tris[1,4-di(thiophen-2-yl)benzo[g]phthalazine] iridium(III) (Ir(dtbpa)3, 1) and tris[1-(2,4-bis(trifluoromethyl)phenyl)-4-(thiophen-2-yl)benzo[g]phthalazine] iridium(III) (Ir(Ftbpa)3, 2). Actually, these two ligands not only enable simple one-pot synthesis of homoleptic Ir(III) complexes without any catalyst under mild conditions, but also contribute to intense NIR-emission with high photoluminescence quantum yield up to 5.2% at 824 nm for 1 and 17.3% at 765 nm for 2, respectively. Single-crystal structure of 1 demonstrates desired facial form with short Ir–N and Ir–C bonds because of strong coordination and small steric hindrance of those highly conjugated C^N═N ligands. Importantly, the incorporation of CF3 groups in 2 further leads to high thermal stability and a good ability to sublime, thus resulting in ultrapurity for highly efficient NIR-organic light-emitting diodes (NIR-OLEDs) with a high maximum external quantum efficiency of 4.5% at 760 nm and small efficiency roll-off remaining of 3.5% at 100 mA cm–2, values which rank with those of the most efficient NIR-OLEDs with small roll-off and peak emission over 750 nm. Notably, the content percentages of the noble metal in these two complexes (∼10% Ir) are markedly lower by about two-thirds than that of typical green-emitting tris(2-phenylpyridine)iridium (∼30% Ir). The findings may provide a new strategy to develop robust NIR emitters and achieve high efficiency, small roll-off, and low cost simultaneously in NIR-OLEDs for practical applications.