Thioxanthen-Based Blue Thermally Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes

Novel thermally activated delayed fluorescence (TADF) materials with 9,9-dimethyl-9H-thioxanthene 10,10-dioxide (SB) as an electron acceptor and andspiro[acridine-9,9'-fluorene] (D1) and spiro[acridine-9,9'-xanthene] (D2) as electron donors (2D1-SB and 2D2-SB) and their characteristics were compared with those of a reference material using 9,9-dimethylacridin (DMAC) as an electron donor (2DMAC-SB) for blue organic light-emitting diodes (OLEDs). We obtain the energies of the first singlet (S₁) and first triplet (T₁) excited states of TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the ground state using the dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. We show that 2D2-SB would be a suitable blue OLED emitter because it has sufficiently small value (0.064 eV) of energy difference (ΔE_ST) between the S₁ and T₁ states, which is favorable for a reverse inter system crossing process from the T₁ to S₁ states and an emission wavelength of 439.5 nm with sufficiently large oscillator strength (F) value.