An impact of the aryl substituent in position 4 of dicyanopyridines on efficiency of thermally activated delayed fluorescence and electroluminescence

Our research is on the side modification of the compound exhibiting efficient thermally activated delayed fluorescence (TADF) by different aryl acetylene moieties. The modification results in the synthesis of unique 2,6-bis-(3,6-di- tert -butyl-carbazol-9-yl)-4-[4-(4-substituted ethynyl) phenyl]-pyridine-3,5-dicarbonitriles. Organic light-emitting diodes with these compounds exhibit a wide range of external quantum efficiencies from 9.8 to 12.5–17.3 %. This wide variation in efficiency, which even surpassed that of the device with the reference emitter, is primarily attributed to the significant influence of the aromatic acetylenes on the TADF properties of the compounds. The presence of ferrocene moiety results in completely quenched emission of the compound. The attachment of phenyl ethynyl group induces efficient TADF due to the narrowing of singlet-triplet energy gap (ΔE ST ) down to 0.07 eV. The spatial alignment of the singlet and triplet states is studied by quantum chemical computations demonstrating the effect of the modifications on the reduction of ΔE ST values. It is shown that the different efficiency of reverse intersystem crossing is caused by the different spin-orbit coupling values between singlet and triplet states. The proposed acetylene modifications offer a chemical approach to finely tune TADF properties and control conformer formations of donor-acceptor type emitters in the solid state, providing a practical tool for the future research and applications of TADF emitters.