3,5-Dicyanopyridine motifs for electron-transporting semiconductors: from design and synthesis to efficient organic light-emitting diodes

Proposing 3,5-dicyanopyridine motifs for the design of electroactive materials for optoelectronic devices, three electron-transporting semiconductors are synthesised. The compounds are characterised by high triplet energies of 2.68-2.79 eV, high glass-transition temperatures reaching 185 & DEG;C, ionisation potentials of ca. 6 eV and electron affinities of ca. 2.7-2.9 eV. Two compounds show deep-blue emissions caused by the relaxation of hybridised local and charge-transfer states. In contrast, another compound with the additional carbazole donor unit demonstrates pure charge-transfer emission. The compounds are characterised by different hosting properties causing very different efficiencies of thermally activated delayed fluorescence (TADF) of the emitter 4,6-di(9,9-dimethylacridan-10-yl)isophthalonitrile (DAcIPN). The different reverse intersystem crossing (RISC) rates and RISC activation energies of DAcIPN are in the ranges of 1 x 10(5)-1.7 x 10(6) s(-1) and 26-34 meV, respectively. An organic light-emitting diode based on the same TADF emitter and the newly synthesised host demonstrates a maximum external quantum efficiency of 21.9% which is considerably higher than that (12.9%) of devices based on the conventional host 3,3 & PRIME;-di(9H-carbazol-9-yl)-1,1 & PRIME;-biphenyl. Time-resolved electroluminescence study proves efficient emissive harvesting of triplets when a 3,5-dicyanopyridine-based host is used.