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-0, S-1, T-1 and T-2 states are involved in the luminescence. Both the S-1 -> T-1 and S-1 -> T-2 intersystem crossing (ISC) processes are more efficient than the S-1 fluorescence and insensitive to temperature. However, the direct T-1 -> S-1 and T-2-mediated S-1 -> T-2 -> S-1 reverse ISC (rISC) processes change remarkably with temperature. At 300 K, these two processes are more efficient than the T-1 phosphorescence and therefore enable TADF. Importantly, the T-1 -> S-1 rISC and T-1 phosphorescence rates are comparable at 300 K, which leads to dual emissions of TADF and RTP, whereas these two channels become So blocked at 100 K so that only the T-2 phosphorescence is recorded experimentally.