Design and Modulation on the Excited State Properties in Organic Electrofluorescence Materials

Excited state plays an important role in photoluminescence (PL) and electroluminescence (EL) properties of organic light-emitting materials. Charge-transfer (CT) state is beneficial to harvest triplet exciton utilization in fluorescent organic light-emitting diodes (FOLEDs) by efficient reverse intersystem crossing. However, the CT-dominated emissive state seriously decreases PL efficiency in such materials. Our strategy is to combine both locally-excited (LE) state and CT state into hybridized local and charge-transfer (HLCT) state, aiming at a balance between high PL efficiency and high exciton utilization. As a solution, a quasi-equivalent hybridization is obtained in TBPMCN, and its nondoped OLED exhibited a very high performance: a pure blue emission with a CIE (0.156, 0.159), a high EQE of 7.8% and a high exciton utilization of 97% without delayed component. Furthermore, the excited state properties were systematically investigated in donor-acceptor (D-A) system using time-dependent density functional theory (TDDFT). The hybridization and de-hybridization processes between LE and CT states were involved with an increasing distance between donor and acceptor. What is more, HLCT state composition can be finely modulated by D-A strength, linkage, etc. Using HLCT conception, we achieved high-efficiency blue, green, red and even NIR luminescent materials and their FOLED devices. In a word, the excited state modulation could be a practical method in designing low-cost, high-efficiency FOLED materials.