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 S₀, S₁, T₁, and T₂ states are involved in the luminescence. Both the S₁ → T₁ and S₁ → T₂ intersystem crossing (ISC) processes are more efficient than the S₁ fluorescence and insensitive to temperature. However, the direct T₁ → S₁ and T₂-mediated T₁ → T₂ → S₁ reverse ISC (rISC) processes change remarkably with temperature. At 300 K, these two processes are more efficient than the T₁ phosphorescence and therefore enable TADF. Importantly, the T₁ → S₁ rISC and T₁ 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 T₁ phosphorescence is recorded experimentally.