Blue Phosphorescence and Hyperluminescence Generated from Imidazo[4,5‐b]pyridin‐2‐ylidene‐Based Iridium(III) Phosphors

Four isomeric, homoleptic iridium(III) metal complexes bearing 5‐(trifluoromethyl)imidazo[4,5‐b]pyridin‐2‐ylidene and 6‐(trifluoromethyl)imidazo[4,5‐b]pyridin‐2‐ylidene‐based cyclometalating chelates are successfully synthesized. The meridional isomers can be converted to facial isomers through acid induced isomerization. The m‐isomers display a relatively broadened and red‐shifted emission, while f‐isomers exhibit narrowed blue emission band, together with higher photoluminescent quantum yields and reduced radiative lifetime relative to the mer‐counterparts. Maximum external quantum efficiencies of 13.5% and 22.8% are achieved for the electrophosphorescent devices based on f‐tpb1 and m‐tpb1 as dopant emitter together with CIE coordinates of (0.15, 0.23) and (0.22, 0.45), respectively. By using f‐tpb1 as the sensitizing phosphor and t‐DABNA as thermally activated delayed fluorescence (TADF) terminal emitter, hyperluminescent OLEDs are successfully fabricated, giving high efficiency of 29.6%, full width at half maximum (FWHM) of 30 nm, and CIE coordinates of (0.13, 0.11), confirming the efficient Förster resonance energy transfer (FRET) process.