Structure-property relationship of phosphine oxide based thermally activated delayed fluorescence molecules: First-principles study

Abstract

Three phosphine oxide (PO) based blue TADF emitters with weak acceptors and strong donors (WASD) were reported recently by Xu’ group [Chem. Mater. 2016, 28, 5667], which provides a new way for the design of blue TADF emitters. To reveal the relationship between the geometric structures and the photophysical properties, twelve PO based molecules are theoretically studied based on first-principles calculations. It is found that the numbers of donor groups have weak effect on the emission wavelengths since more molecular orbitals are involved in the excitation except the highest occupied molecular orbital and the lowest unoccupied molecular orbital. With the increase of the numbers of donor groups, the energy gap between the first singlet excited state (S1) and the first triplet excited state (T1) becomes narrower. However, the electron donating ability of more donors becomes a little weaker. In addition, the investigation of the types of donor groups on the S1-T1 energy gap indicates that the 9,9-dimethyl-9,10-dihydroacridine (DMAC) group is better than other kinds of donors to narrow the S1-T1 energy gap. In addition, the meta-position is also proven less efficient than the para-position to obtain a small S1-T1 energy gap. Our theoretical study will be helpful to the design of PO based TADF emitters.