Donor-only Triads Exhibiting Room Temperature Phosphorescence as Hosts for Red Electroluminescent Devices

Aiming to explore donor-based host materials for phosphorescent organic light emitting diodes (PhOLEDs), four donor*-donor-donor* type compounds were designed using typical electron-donating moieties with the different electron-donating abilities (phenothiazine, phenoxazine, 9,9-dimethylacridine or dibenzo[b,f]azepine). The molecular design strategy resulted in the synthesis of the compounds with relatively low ionization energies of 5.1-5.45 eV, which are favourable for efficient hole injection. The synthesized compounds demonstrate relatively good hole transporting properties with hole drift mobilities exceeding 1 × 10^- 4 cm² V^- 1 s^- 1 at electric field of 1.6 × 10⁵ V/cm. Their triplet energies of the compounds are in the range of 2.61-3.08 eV ensuring effective triplet exciton confinement and preventing reverse energy transfer from red phosphorescent emitter to the host. Red PhOLED with the developed 9,9-dimethylacridine-carbazole-9,9-dimethylacridine as the host shows the increase of external quantum efficiency by 35% when compared to that of the device with the previously reported host as a reference. It is higher than that of the reference device with previously reported 9'-(2-ethylhexyl)-9'H-9,3':6',9''-tercarbazole as the host. The molecular mixture of the phenothiazine-carbazole-phenothiazine derivative in Zeonex show room temperature phosphorescence. The triplet emission of the system is significantly quenched upon interaction with picric acid, indicating its sensitivity toward nitroaromatic compounds.