Horizontally oriented pyrido[2,3-b]pyrazine-based thermally activated delayed fluorescence emitters exhibiting bipolar charge transport for efficient organic light-emitting diodes

Abstract

Four pyrido[2,3-b]pyrazine-based donor–acceptor emitters containing carbazole or phenothiazine donor moieties are synthesized via a cyclization reaction followed by Buchwald–Hartwig cross-coupling. The solid-state films containing developed compounds emit conformer-assisted blue-to-red light with photoluminescence quantum yields of up to 82%. Their thermally activated delayed fluorescence (TADF) properties suggest efficient triplet-to-singlet reverse intersystem crossing under electrical excitation. Their applicability in organic light emitting diodes (OLEDs) are also enhanced by bipolar charge transport with hole mobilities in the range from 2.5 × 10⁻⁶ to 3.8 × 10⁻⁴ cm² V⁻¹ s⁻¹ and electron mobilities in the range from 1.4 × 10⁻⁶ to 4.6 × 10⁻⁵ cm² V⁻¹ s⁻¹ at electric field of 1 × 10⁶ V cm⁻¹. Hole and electron injections into the light-emitting layers are ensured by ionisation energies of 5.26–5.55 eV and electron affinities of 2.64–2.91 eV, as determined by photoelectron emission spectroscopy. The compounds demonstrate the required tendency towards horizontal molecular orientation in thin films (horizontal orientation order parameter of up to 0.90), thereby enhancing internal light outcoupling efficiency. OLEDs with the developed compounds exhibit high internal quantum efficiency of up to 82% and external quantum efficiency of up to 21.3%.