Strongly Luminescent Tetradentate Palladium(II)-TADF Emitters. Blue TADF And TADF-Sensitized OLEDs with External Quantum Efficiencies Over 23%

Abstract

Due to the low-lying Pd(4d)-orbitals, luminescent palladium(II) complexes usually show 3IL phosphorescence (IL = intraligand) with excited state lifetimes in the range of hundreds of microseconds, which limits their application in blue organic light-emitting diodes (OLEDs). Here, a molecular design strategy for the development of strongly luminescent Pd(II)-based thermally activated delayed fluorescence (TADF) emitters with fast radiative decay rate constants and reverse intersystem crossing rate (kRISC) constants is presented. The synthesized tetradentate [Pd(N*C*C^N)] TADF emitters show sky-blue to yellow (λmax = 484–565 nm) emission with excited state lifetimes in the range of 0.9−8.7 µs and high photoluminescence quantum yield of up to 77% in CH2Cl2 solution. In 5 wt.% 2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) thin films, these [Pd(N*C*C^N)] TADF complexes display blue to sky-blue (476−496 nm) emission with quantum yields close to unity. Variable-temperature emission lifetime measurements, femtosecond time-resolved spectroscopy, and density functional theory (DFT) and time-dependent DFT (TDDFT) calculations together reveal ultrafast kRISC rate constants of these [Pd(N*C*C^N)] TADF emitters. Using these Pd(II)-TADF emitters as dopants, sky-blue TADF and blue TADF-sensitized OLEDs with small efficiency roll-offs achieved high maximum external quantum efficiencies (EQEs) of 24.8% and 23.1%, respectively.