Use of Pyrimidine and Pyrazine Bridges as a Design Strategy To Improve the Performance of Thermally Activated Delayed Fluorescence Organic Light Emitting Diodes.

We present a study of two isomeric thermally activated delayed fluorescence (TADF) emitters 9,9'-(sulfonylbis(pyrimidine-5,2-diyl))bis(3,6-di--butyl-9-carbazole) () and 9,9'-(sulfonylbis(pyrazine-5,2-diyl))bis(3,6-di--butyl-9-carbazole) (). The use of pyrimidine and pyrazine as bridging units between the electron donor and acceptor moieties is found to be advantageous compared to the phenyl- () and pyridine-based analogues ( and ). Conformational modulation of the donor groups as a function of the bridge results in high photoluminescence quantum yields (Φ > 68%) and small energy gaps between singlet and triplet excited states (Δ < 160 meV). OLEDs using and as emitters exhibit blue and green electroluminescence, respectively, with higher maximum external quantum efficiencies (EQE of 14% and 18%, respectively) and a reduced efficiency roll-off as compared to the reference devices using , , and as the emitters. Our results provide a more complete understanding on the impact of the bridge structure in D-A-D TADF systems on the optoelectronic properties of the emitter and how the balance between color purity and EQE in the devices can be controlled, advancing the design strategies for TADF emitters.