Effect of Fluorination on the Properties of 9,9′-(Sulfonylbis(4,1-Phenylene))bis(3-(tert-Butyl)-6-Fluoro-9H-Carbazole) as Host for Thermally Activated Delayed Fluorescent Emitters

Abstract

To enhance organic light emitting diode (OLED) performance, host materials with high triplet energies are crucial for confining excitons, despite increasing driving voltages due to the singlet-triplet energy gap. We synthesized sulfonylbis(4,1-phenylene)bis(3,6-disubstituted-9H-carbazole) derivatives as donor-acceptor-donor host materials, namely compounds 3, 5 and 7, with varying fluorination levels. These compounds show moderate singlet-triplet energy splitting and molecular dipole moments, allowing for fine-tuning of hole-transport mobilities, deeper frontier orbital energies, and a red shift in singlet emission while maintaining high triplet energy levels. These adjustments impact a range of physical, electronic and optical properties. The materials exhibit exceptional thermal stability, with decomposition starting above 400 °C and glass transition temperatures over 130 °C. Used with the green TADF emitter DACT-II, these hosts enable reverse intersystem crossing rates between 7.43×10⁴ s⁻¹ and 1.77×10⁵ s⁻¹. While OLEDs using mCP as a reference host achieve a maximum quantum efficiency of 18.5 %, those with host 5 show lower efficiency roll-off, leading to higher external quantum efficiency at brightness levels above 2000 cd/m² without colour shift. The reduced roll-off in devices with host 5 compared to mCP is attributed to effective Förster and Dexter energy transfers to DACT-II at high currents, enhancing light emission pathways.