Metal-Perturbed Through-Space Charge Transfer Excited State: A Platform for Strongly Emissive Pd(II) Complexes via...

The excited states of transition metal complexes generally involve an electron shuttle between the metal center and ligand or between ligands in the first coordination sphere. Herein, a series of pincer-type C^N^C Pd(II) complexes containing an electron-accepting but non-coordinating dibenzo[a,c]-phenazine (DBPZ) moiety on the ancillary carbazolyl ligand is developed. The face-to-face orientation of the C^N^C ligand and DBPZ moiety elicits lowest-lying singlet metal-perturbed through-space charge transfer (¹TSCT) excited states. Spectroscopic measurements and theoretical calculations unveiled their lowest triplet excited states possess mixed ³TSCT, through-bond charge transfer and locally excited character. The resulting Pd(II) complexes exhibit strong thermally activated delayed fluorescence (TADF) with efficiencies of 68%-93%. The through-space electronic coupling is found closely associated with the strength of noncovalent interactions between the tridentate C^N^C unit and the lateral acceptor. Orange-red organic light-emitting diodes doped with selected Pd(II) complex as an emitter achieve maximum external quantum efficiencies exceeding 30%. Its use as a sensitizer for narrowband multiple-resonance emitter is also validated by increased device efficiency and reduced roll-off. This work not only demonstrates the realization of strong TSCT-TADF emission in Pd(II) complexes but also broadens the molecular design landscape of molecular design for photoactive transition metal complexes toward diverse applications.