Impact of Donor Substitution Pattern on the TADF Properties in the Carbazolyl-Substituted Triazine Derivatives

In this work, we report on the synthesis and photophysical investigation of a new star-shaped triazine-carbazole derivative 2,4,6-tris(3-(3,6-di- tert -butyl-9 H -carbazol-9-yl)phenyl)-1,3,5-triazine. Comparative study of the photophysical properties of the newly synthesized emitter along with its para -substituted isomer 2,4,6-tris(4-(3,6-di- tert -butyl-9 H -carbazol-9-yl)phenyl)-1,3,5-triazine was performed. While para -linkage caused higher oscillator strength of the lowest energy absorption band and high fluorescence quantum yield, the meta -linkage resulted in stronger charge transfer character as well as higher triplet energy. Delayed emission of meta -isomer was found to be 3 orders of magnitude more intense than that of para -isomer. Temperature dependent measurements of meta -isomer confirmed the thermally activated delayed fluorescence origin of its delayed fluorescence with the activation energy of 0.07 eV. Organic light emitting diode containing this emitter dispersed in bis[2-(diphenylphosphino)phenyl] ether oxide with emission spectrum peak at 475 nm was fabricated. Commission Internationale de l’Éclairage color coordinates corresponded to a sky-blue emission color (0.16, 0.23). The turn-on voltage of the electroluminescent device was found to be in the range of 5–6 V with a maximum external quantum efficiency of 9.5%. These results confirm the importance of the linking pattern between donor and acceptor moieties in the molecular design of thermally activated delayed fluorescence emitters.