Molecular glasses based on 1,8-naphthalimide and triphenylamine moieties as bipolar red fluorescent OLED emitters with conventional versus TADF hosting.

Three new donor–acceptor molecular glasses were designed and synthesized linking 1,8-naphthalimide and triphenylamino groups though the different bridges. The comprehensive characterization of the compounds was carried out using theoretical and experimental approaches. The compounds showed efficient orange-red emission in solid state with photoluminescence intensity maxima in the range of 584–654 nm. The compounds showed extremely high thermal stability with 5 % weight loss temperatures up to 477 °C. They formed molecular glasses with glass-transition temperatures in the range of 161–186 °C. The fabricated organic light-emitting diodes (OLEDs) based on the developed emitters and conventional host showed maximum external quantum efficiency of 2.5 % in the best case. This value was increased up to 4.7 % by the usage of the host exhibiting thermally activated delayed fluorescence (TADF). OLED containing the TADF host displayed orange emission peaking at 589 nm with colour coordinates x of 0.53 and y of 0.45 combined with power efficiency of 6.7 lm·W−1 and current efficiency of 11.8 cd·A-1. Time-resolved electroluminescence technique was used to study the effect of the different guest–host systems on exciton utilization efficiency in devices based on the same emitter exhibiting prompt fluorescence and on the conventional or TADF hosts.