Pyridine-, Pyrimidine-, and Triazine-Based Thermally Activated Delayed Fluorescence Emitters.

Novel thermally activated delayed fluorescence (TADF) materials with pyridine (Pd), pyrimidine (Pm), and triazine (Trz) as electron acceptors and carbazole (Cz) as an electron donor (TmCzPd, TmCzPm, and TmCzTrz) were designed, and the effect of the number of nitrogen (N) atoms with acceptor units was investigated by comparing their electronic and optical properties for blue organic light-emitting diodes (OLEDs). Using density functional theory (DFT) and time-dependent DFT calculations, we obtained the electron distributions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and the energies of the lowest singlet (S1)and lowest triplet (T1) excited states. The calculated energy difference (ΔEST) between the S1 and T1 states of TmCzPm (0.200 eV) and TmCzTrz (0.186 eV) were smaller than that of TmCzPd (0.395 eV). We show that TmCzPm would be a suitable blue OLED emitter because it has sufficiently small ΔEST values, which is favorable for a reverse intersystem crossing process from the T1 to S1 states, and an emission wavelength of 473.1 nm with sufficiently large oscillator strength for fluorescence.