Room-Temperature Phosphorescence and Thermally Activated Delayed Fluorescence in the Pd Complex: Mechanism and Dual Upconversion Channels.

The Pd complex PdN3N exhibits an unusual dual emission of room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), but the mechanism is elusive. Herein, we employed both density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to explore excited-state properties of this Pd complex, which shows that the S, S, T, and T states are involved in the luminescence. Both the S → T and S → T intersystem crossing (ISC) processes are more efficient than the S fluorescence and insensitive to temperature. However, the direct T → S and T-mediated T → T → S reverse ISC (rISC) processes change remarkably with temperature. At 300 K, these two processes are more efficient than the T phosphorescence and therefore enable TADF. Importantly, the T → S rISC and T phosphorescence rates are comparable at 300 K, which leads to dual emissions of TADF and RTP, whereas these two channels become blocked at 100 K so that only the T phosphorescence is recorded experimentally.