A Novel Design of Thermally Activated Delayed Fluorescence Molecules: Experimental and Computational Studies.*

In an organic light emitting diode (OLED), electrically injected charge carriers form singlet and triplet excitons in a 1:3 ratio. The goal of OLED research is to overcome the limitation imposed by forbidden decay from triplet states. One approach is to enhance T₁ to S₁ reverse intersystem crossing (ISC) by carefully designing organic molecules that exhibit a small energy gap (E_ST) between S₁ and T₁ levels. This allows for non-radiative triplet states to up-convert to radiative singlet states, a process called thermally activated delayed fluorescence (TADF). One strategy to minimize E_ST is to configure molecules in a donor-acceptor (D-A) electronic alternation. However, this may cause geometric changes in molecular conformation, resulting in low quantum yields and a broad emission. By synergistically combining experiments and simulation, we were able to design novel TADF emitting molecules based on the donor-acceptor-donor (D-A) concept, but where the in molecular conformation change is successfully suppressed, leading to higher electroluminescence efficiencies and sharp emissions. This design can guide the development of future TADF organic molecules.