High-Efficiency Blue TADF Palladium(II) Complexes with Ligand-to-Ligand Charge Transfer Excited State

Electroluminescent complexes of the second-row transition metals remain underdeveloped due to the challenge of simultaneously achieving fast radiative decay and slow nonradiative decay, particularly in the blue spectral region. Herein, we report a design strategy for strongly blue-emitting pincer-type Pd(II) complexes based on the thermally activated delayed fluorescence (TADF) mechanism. By paring a terdentate trifluoromethyl-substituted bis-N-heterocyclic carbene (NHC) with a cyano-substituted carbazolide ligand, the lowest-lying singlet excited states of the resulting Pd(II) complexes exhibit dominant ligand-to-ligand charge transfer (LLCT) character. Electrochemical and theoretical calculations confirm that the oxidation and reduction processes are ligand-controlled with minor metal involvement. Through tuning of the electron-accepting strength and the triplet locally-excited (³LE) energy of the pincer bis-NHC ligand via a subtle structural change, a blue emitting TADF Pd(II) complex (PyPdCN) is achieved, exhibiting an emission maximum of 472 nm and a quantum yield of 86% in doped film. The energy gap between ¹LLCT and ³LE states proves crucial for regulating the excited state dynamics. Organic light-emitting diodes employing PyPdCN as the emitter show blue electroluminescence with peaks ranging from 460 to 473 nm and maximum external quantum efficiencies exceeding 20%, representing a significant advancement in luminescent Pd(II) complexes.