Recent Advances in Substituent Effects of Blue Thermally Activated Delayed Fluorescence Small Molecules

As the third-generation emitters for organic light-emitting diodes (OLED), thermally activated delayed fluorescence (TADF) materials have attracted widespread attention from both academic and industrial communities in recent years. In TADF molecules, the triplet excitons can be upconverted to the singlet state with the aid of ambient thermal energy by virtue of the reverse intersystem crossing (RISC) owing to the small singlet-triplet splitting energy (Delta E-ST). Therefore, 100% exciton utilization efficiency can be theoretically realized by harvesting both singlet and triplet excitons. Consequently, the external quantum efficiencies of OLEDs can be significantly improved compared with the first-generation devices based on conventional fluorophores. TADF materials are regarded as an effective potential solution to break through the bottleneck of highly efficient and stable blue organic electroluminescence (EL). Generally, TADF molecules is a purely organic push and pull system containing at least an electronic donor and acceptor. The Delta E-ST, frontier orbital distributions, aggregation structures, EL colors and device performance can be effectively tuning by changing the structures and quantities of the donor and the acceptor, the substituents and their substituted positions. Meanwhile, the substituents of TADF molecules play a crucial role in the regulation of the strengths of donor and acceptor, molecular configurations, aggregation structures, stabilities and other physical and chemical properties. The substituent effects of purely organic blue TADF molecules from D-A type and multiple-resonance type blue TADF molecules are summarized, in the hope of providing effective reference for the design and synthesis of high-efficiency and stable blue light TADF molecules.