Isomeric thermally activated delayed fluorescence emitters based on a quinolino[3,2,1-de]acridine-5,9-dione multiple resonance core and carbazole substituent

Abstract

The color purity of the pixels is an essential indicator in organic light-emitting diode (OLED) commercial displays. Since the two important parameters of high color purity and efficiency can be achieved simultaneously, multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters have achieved rapid development. Recently, there has been a lot of research work on connecting various donor (D) moieties to the existing MR core, but few studies on how the linking moiety affects the efficiency of MR-TADF emitters. To figure out the influence of substituents on the MR-TADF system, we developed three isomers QAOCz1, QAOCz2, and QAOCz3, which are constructed with the same moieties of 3,11-diphenylquinolino[3,2,1-de]acridine-5,9-dione and 9-phenyl-9H-carbazole (PhCz) by different site connections. Through reasonable adjustments of the substitution site, the donor–acceptor (D–A) interaction of the isomers gradually weakened and molecular rigidity gradually increased. As a result, their singlet–triplet energy gap (ΔE_ST) gradually decreased and their photoluminescence quantum yield (PLQY) gradually rose. QAOCz3 with the weakest D–A interaction successfully achieves a much higher PLQY of 98.9% and a smaller ΔE_ST of 0.16 eV. The QAOCz3 based OLED not only realizes the best maximum external quantum efficiency (EQE) of 21.1% but also has the narrowest full-width at half maximum (FWHM) of 40 nm. This work shows that weakening the D–A interaction between the substituents and the MR core by a spacer group is of great significance for the construction of efficient MR-TADF emitters.