Theoretical Study of Triphenylphosphine Oxide Derivatives for Blue Thermally Activated Delayed Fluorescence

We designed novel thermally activated delayed fluorescence (TADF) materials by combining the electron donor 10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (DDA) with the electron acceptor triphenylphosphine oxide (PO) unit (mDDA-PO and o-mDDA-PO) and compared their characteristics with those of a reference material using 1,3-Bis(N-carbazolyl)benzene (mCP) as an electron donor (mCP-PO) 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 the lowest unoccupied molecular orbital (LUMO) as well as 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 mDDA-PO (0.16 eV) and o-mDDA-PO (0.07 eV) were smaller than that of mCP-PO (0.48 eV). The results showed that o-mDDA-PO is a suitable blue OLED emitter because it has sufficiently small ΔEST values, which is favorable in a reverse-intersystem process crossing from the T₁ state to S₁ states, as well as an emission wavelength of 446.7 nm.