Rational control of π-conjugation and CT component in hybridized local and charge transfer molecules for high performance deep blue emitters

The ratio of locally excited (LE) and charge transfer (CT) components plays a key role in harvesting the "hot" triplet excitons through high-lying reverse intersystem crossing (hRISC) in organic light-emitting materials. Herein, two blue carbazole derivative isomers 2,7-Cz(DOFCP)(2) and 3,6-Cz(DOFCP)(2) were designed with A-pi-D-pi-A structures. The localized pi-conjugation and intramolecular charge transfer components ratio was rationally varied by the substitute position of 9,9-dioctylfluorenyl (DOF) pi-bridge on the carbazolyl donor unit. Both of the emitters exhibited hybridized local and charge transfer (HLCT) characteristics, and 2,7-Cz(DOFCP)(2) with prolonged localized pi-conjugation nature realized a smaller energy splitting (Delta ES2-T3) and higher fluorescence quantum yield. The OLEDs based on the emitters achieved deep blue and nearly standard blue electroluminescence with the CIE coordinates of (0.153, 0.063) and (0.153, 0.091), and maximum EQEs of 5.57% and 7.37% that was beyond the conventional fluorescence devices, respectively. This work would provide insights for promoting the luminescence performance of the HLCT molecules by adjusting the LE/CT proportion in the excited states.