Blue electroluminescence and hyperluminescence facilitated by Ir(iii) phosphors with 5-cyano-imidazo[4,5-b]pyridin-2-ylidene cyclometalates†

Abstract

Despite the rapid advancements of organic light-emitting diode (OLED) technology, designing suitable blue emitters remains a great challenge in meeting future demands. In this study, two efficient blue-emitting Ir(III) carbene complexes bearing 5-cyano-imidazo[4,5-b]pyridin-2-ylidene cyclometalates were designed, namely, f-ct3a and f-ct3b, instead of 6-cyano-imidazo[4,5-b]pyridin-2-ylidene cyclometalates as shown in f-ct9a–c. With the relocation of the cyano group, both complexes displayed blue emission peaking at ∼468 nm in toluene. Moreover, they demonstrated high quantum yields (Φ_PL ≥ 80%) and high radiative constant (k_r ≥ 3.37 × 10⁵ s⁻¹) in both solution and co-doped thin films, indicating their great potential for the fabrication of OLED devices. Notably, the OLEDs employing f-ct3a or f-ct3b as dopants afforded electrophosphorescence at 476 nm with maximum external quantum efficiencies (max. EQE) of 25.5 and 26.8%, respectively. Furthermore, the hyper-OLED incorporating the sensitizer f-ct3b and terminal emitter ν-DABNA achieved a max. EQE of 29.0% with Commission Internationale de l’Eclairage (CIE_x,y) coordinates of (0.124, 0.130). This narrowband blue emission (λ_max = 468 nm, FWHM = 22 nm) revealed efficient Förster resonance energy transfer (FRET), boosting the max. luminance close to 10 000 cd m⁻², which confirmed the excellent performance of Ir(III) carbene complexes as both the phosphor dopants and FRET sensitizers.