Modulation of OLED efficiency via a combination of aromatic electrophilic directing and intramolecular charge transfer

Abstract

Thermally activated delayed fluorescence (TADF) materials utilizing purely organic compounds have become a promising and more eco-friendly alternative to phosphorescent organometallic emitters in organic light-emitting diode (OLED) devices. However, the modulation of luminescence properties for molecules with a high degree of structural complexity remains challenging. Herein, by combining the aromatic electrophilic directing (AED) and intramolecular charge transfer (ICT) effects, two OLED emitters containing an electron donor (triphenylamine, TPA) and acceptor (benzophenone) covalently linked via a carbazole ring were synthesized and compared. The design concomitantly allows for spatial separation of origin/destination orbitals involved in the lowest singlet excited state and increased density of charge carriers in contrast to a non-conjugated linker. More conveniently, we demonstrate that increased acceptor strength by cyano-substitution results in a significant decrease of the singlet–triplet energy gap from 0.23 to 0.09 eV. The latter exhibits a TADF photoluminescence quantum yield of up to 63% with a lifetime of 7.67 μs in the OLED matrix. Notably, the OLED device using the cyano-TADF molecules as the emitting layer possesses an enhanced external quantum efficiency of 15.6%, compared to 10.3% efficiency obtained from the non-substituted version, and small efficiency roll-off, demonstrating the modulation versatility of purely organic molecule-based OLED devices.