Theoretical study on the differences in donor-acceptor interaction and electron transition mechanism in Pd(II) and Pt(II) complexes

The relativistic effect enhances spin-orbit coupling (SOC), making metal complexes potential candidates for phosphorescent OLED emitters. However, the relativistic effect profoundly influences the donor and acceptor interactions (D-A), resulting in unique electron transition processes. By stabilizing the s orbitals and destabilizing the d orbitals of the center metal atom, the relativistic effect enhances donation, back donation, and the trans effect in PtN1N more than in PdN1N. Particularly, the back donation in PtN1N is approximately four times greater than that in PdN1N, contributing to the greater stability and rigidity in PtN1N. Furthermore, the relativistic effect enhances the SOC and reduces the excitation energy and stabilizes the excited states of PtN1N. Consequently, the radiative decay rate $k_p$ and non-radiative rate $k_{\mathrm{nr}}$ are accelerated simultaneously. The reverse intersystem crossing rate $k_{\text{RISC}}\left(T_3\to S_1\right)$ in PdN1N is accelerated by high temperature, which is responsible for thermally activated delayed fluorescence (TADF).