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 triazinecarbazole 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 photo physical properties of the newly synthesized emitter along with its para-substituted isomer 2,4,6-tris (4- (3,6-di-tert-butyl-9H-carbazol-9-71)phenyl)-1,3,5-triazine was performed. While paralinkage 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'Eclairage 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.