Manipulating the LUMO distribution of quinoxaline-containing architectures to design electron transport materials: Efficient blue phosphorescent organic light-emitting diodes

• Four new quinoxaline-containing compounds with highly twisted structures were designed as electron transport materials.
• The LUMO distributions of these compounds can be manipulated by adjusting the locations of quinoxaline moieties.
• Using them as ETLs, the Firpic-based electrophosphorescent devices achieved a ηc,max of 30.2 cd A−1 and a ηext,max of 14.2%.
• These efficiencies reveal smaller roll-offs at high luminance in contrast to the TmPyPB based device.

A series of new quinoxaline-containing compounds, namely, 2,3,6,7-tetrakis(3-(pyridin-3-yl)phenyl)quinoxaline (Tm3PyQ), 2,3,6,7-tetrakis(3-(pyridin-4-yl)phenyl)quinoxaline (Tm4PyQ), 1,4-bis(2,3-dimethyl-7-(pyridin-3-yl)quinoxalin-6-yl)benzene (3PyDQB), and 1,4-bis(2,3-dimethyl-7-(pyridin-4-yl)quinoxalin-6-yl)benzene (4PyDQB) were designed and synthesized as electronic transporting materials. The lowest unoccupied molecular orbital (LUMO) distributions of these compounds vary with the locations of quinoxaline moieties, which result in adjustable intermolecular charge-transfer integrals. All the compounds exhibit favorable electron affinity (2.73–2.88 eV) and good thermostability (glass transition temperatures in the range of 112–148 °C). Using these compounds as electron transport layers, the bis(4,6-(difluorophenyl)pyridinato-N,C2′)picolinate iridium(Ⅲ) (Firpic)-based blue phosphorescent organic light emitting diodes (PhOLEDs) achieve good performances with a maximum current efficiency (ηc,max) of 30.2 cd A−1 and a maximum external quantum efficiency (ηext,max) of 14.2%. Moreover, these efficiencies reveal small roll-offs at high luminance.

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