Synthesis of linear and V-shaped carbazolyl-substituted pyridine-3,5-dicarbonitriles exhibiting efficient bipolar charge transport and E-type fluorescence.

Aiming to develop all-organic bipolar semiconductors with high charge mobility and efficient E-type fluorescence (so-called TADF) as environmentally-friendly light-emitting materials for optoelectronic applications, four noble metals-free dyes with linear and V-shapes were designed using accepting pyridine-3,5-dicarbonitrile and donating carbazole units. Exploiting donor-acceptor design strategy and using moieties with different donating and accepting abilities, TADF emitters with wide variety of molecular weights were synthesized for getting optimum combination of charge-transporting and fluorescent properties in one TADF molecule. Depending on molecule structures, different TADF emitters capable of emitting in the range from 453 to 550 nm with photoluminescence quantum yields up to 98% for the solutions in oxygen-free toluene were obtained. All compounds showed bipolar charge-transport. Hole mobility of 2.8×10-3 cm2/Vs at 7×105 V/cm was observed for the compound containing two di-tert-butyl-substituted carbazole moieties. The compounds were tested in both non-doped and doped organic light-emitting diodes using different hosts. It was shown that the developed TADF emitters are suitable for different colour devices with electroluminescence ranging from blue to yellow and with brightness, maximum current and external quantum efficiencies exceeding 10000 cd/m2, 15 cd/A, and 7%.