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-tert-butyl-9H-carbazole) (pDTCz-DPmS) and 9,9′-(sulfonylbis­(pyrazine-5,2-diyl))­bis­(3,6-di-tert-butyl-9H-carbazole) (pDTCz-DPzS). 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- (pDTCz-DPS) and pyridine-based analogues
(pDTCz-3DPyS and pDTCz-2DPyS). Conformational
modulation of the donor groups as a function of the bridge results
in high photoluminescence quantum yields (ΦPL >
68%) and small energy gaps between singlet and triplet excited states
(ΔEST < 160 meV). OLEDs using pDTCz-DPmS and pDTCz-DPzS as emitters exhibit
blue and green electroluminescence, respectively, with higher maximum
external quantum efficiencies (EQEmax of 14% and 18%, respectively)
and a reduced efficiency roll-off as compared to the reference devices
using pDTCz-DPS, pDTCz-3DPyS, and pDTCz-2DPyS 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.