Pd(II)-TADF Complexes With Metal-Perturbed Intraligand Charge Transfer State: Emitters and Sensitizers for...

The metal-perturbed intraligand charge transfer (MPICT) excited state has proven to be a viable design strategy for thermally activated delayed fluorescence (TADF) complexes featuring a metal-donor-acceptor configuration. However, the structure-photophysics correlation and the applications of such complexes for organic light-emitting diodes (OLEDs) remain largely unexplored. Herein, we report a series of Pd(II) complexes (Pd3-Pd6) having dibenzo[a,c]phenazine (DBPZ)-substituted carbazolyl ligands with varied torsion angles. The energy gap between the lowest singlet and triplet excited states is tuned from 0.05 eV to 0.26 eV, which governs the emission mechanism and radiative decay rate. Complex Pd3 exhibits strong red TADF with a photoluminescence quantum yield of 82% and a lifetime of 1.13 µs. In bottom-emitting OLEDs, Pd3 affords red electroluminescence with a peak at 617 nm, a maximum external quantum efficiency (EQE_max) of 30.15%, and a small efficiency roll-off of 2.32% at 1000 cd m^-2. In top-emitting ITO-free OLEDs, Pd3 shows pure-red emission with CIE_x > 0.68 and an EQE_max of 38.68%. Pd3 is also demonstrated as an efficient sensitizer for a red-emitting multi-resonance emitter, enabling hyperfluorescence OLEDs with an EQE_max of 27.06% and suppressed efficiency roll-off. These results highlight the potential of MPICT-type Pd(II) complexes for high-performance OLED displays.