Triphenylamino or 9-phenyl carbazolyl-substituted pyrimidine-5-carbonitriles as bipolar emitters and hosts with triplet harvesting abilities

To obtain highly efficient organic semiconductors exhibiting fast emission decays, triplet-harvesting abilities and good bipolar charge-transporting properties for optoelectronic applications, compounds containing triphenylamine or 9-phenylcarbazole as donor moieties and pyrimidine-5-carbonitrile as electron-withdrawing unit were synthesised. Toluene solutions of the compounds demonstrated high photoluminescence quantum yields reaching 98%. As required for electroluminescent device applications, compound containing triphenylamino moiety showed high mobilities of both electrons and holes, which reached 4.4 × 10⁻⁴ cm²/V × s and 7.3 × 10⁻³ cm²/V × s, respectively at electric field of 3.6 × 10⁵ V/cm. This triplet-harvesting mechanism was confirmed by the theoretical and experimental studies including a femtosecond transient absorption pump−probe technique and time-resolved electroluminescence spectroscopy. Pure-blue and greenish-blue fluorescent organic light-emitting diodes (OLEDs) with external quantum efficiency (EQE) reaching 7% and 6%, corresposndingly, were obtained using the newly synthesised compounds as emitters. The operation time (T50) of ca. 650 h were observed for blue OLED and of ca. 3800 h for greenish-blue OLED until reaching the half initial brightness (100 cd/m²). EQE of more than 20% and T50 exceeding 20,000 h were observed for electroluminescent devices based on emitter characterised by triplet−triplet annihilation and thermally activated delayed fluorescence which was utilised to test hosting properties of the differently donor-substituted pyrimidine-5-carbonitriles.