Thermally activated delayed fluorescence tetradentate ligand-containing gold(iii) complexes with preferential molecular orientation and their application in organic light-emitting devices

Abstract

A new class of thermally activated delayed fluorescence (TADF) pyridine-/pyrazine-containing tetradentate C^C^N^N gold(III) complexes have been designed and synthesized. Displaying photoluminescence quantum yields (PLQYs) of up to 0.77 in solid-state thin films, these complexes showed at-least a six-fold increase in the radiative decay rate constant (k_r) in toluene upon increasing temperature from 210 to 360 K. Using variable-temperature (VT) ultrafast transient absorption (TA) spectroscopy, the reverse intersystem crossing (RISC) processes were directly observed and the activation parameters were determined, in line with the results of the Boltzmann two-level model fittings, in which the energy separation values between the lowest-lying singlet excited state (S₁) and the lowest-lying triplet excited state (T₁), ΔE(S₁–T₁), of these complexes were estimated to be in the range of 0.16–0.18 eV. Through strategic modification of the position of the electron-donating –^tBu substituent in the cyclometalating ligand, the permanent dipole moments (PDMs) of these tetradentate gold(III) emitters could be manipulated to enhance their horizontal alignment in the emitting layer of organic light-emitting devices (OLEDs). Consequently, the resulting vacuum-deposited OLEDs demonstrated a 30% increase in the theoretical out-coupling efficiency (η_out), as well as promising electroluminescence (EL) performance with maximum external quantum efficiencies (EQEs) of up to 15.7%.