Design, synthesis and application of tetraphenylbenzene-based blue organic electroluminescent materials with aggregation-induced emission and hot exciton properties

Abstract

A pair of deep blue isomers, TPB-CN-CZ1 and TPB-CN-CZ2, with both aggregation-induced emission (AIE) and hot exciton properties have been designed and synthesized by introducing carbazole (CZ) on the transverse axis and cyano (CN) on the longitudinal axis of AIE-active group tetraphenylbenzene (TPB). Changing the connection of CZ groups and TPB can adjust the degree of conjugation on the transverse axis of the molecule to adjust the wavelength of luminescence. The introduction of CN in the longitudinal axis facilitates high-level charge transfer (CT) state characteristics to weaken the binding energy of excitons during the electroluminescence process. TPB-CN-CZ1 and TPB-CN-CZ2 possess distorted molecular configurations, which are not only beneficial to induce the AIE phenomenon, but also reduce the carrier injection barrier. In TPB-CN-CZ1 and TPB-CN-CZ2, a high-level reverse intersystem crossing (RISC) process through a high-level T_CT → S_CT or T_HLCT → S_CT pathway, and then an internal conversion (IC) process to the S₁ state radiation transition of locally excited (LE) properties can occur to achieve both high exciton utilization and high photoluminescence quantum efficiency (PLQY). Of the two isomers, TPB-CN-CZ2 can use the spin–orbit coupling (SOC) between the LE state and the CT state to further promote a high-level RISC process. Finally, the Commission Internationale de l’Eclairage (CIE) coordinates of a non-doped OLED prepared with TPB-CN-CZ2 as the light-emitting layer are (0.16, 0.06), and the maximum external quantum efficiency (EQE_max) is 11.8% with an efficiency roll-off of only 6%.