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

Abstract

Abstract 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 ( 3 LE) 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 1 LLCT and 3 LE 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.