A Direct Arylation Approach toward Thermally Activated Delayed Fluorescence-Active Benzo[c][1,2,5]thiadiazole Emitters for Near-Infrared Solution-Processed OLEDs

Abstract

An isomeric emitter (2TPA-iCNBT) is designed and synthesized, displaying enhanced thermally activated delayed fluorescence (TADF) properties as compared to the reference near-infrared (NIR) emitter TPACNBz (hereafter referred to as 2TPA-CNBT). Its modified benzo[c][1,2,5]thiadiazole-4,7-dicarbonitrile (iCNBT) acceptor (A) core positions the two triphenylamine (TPA) donor (D) units adjacently, thereby increasing the D-A torsion angle. Synthesis is realized through the use of an unexploited direct arylation strategy, which, besides offering the desired materials in an efficient and straightforward way, can also yield monofunctionalized emitters (1TPA-CNBT and 1TPA-iCNBT). In total, four emitters are synthesized, characterized, and subsequently compared in terms of their spectroscopic and device properties. Density functional theory is applied to simulate their relative molecular geometry and the arrangement of their (emissive) excited states. Steady-state and time-resolved emission spectroscopy reveal strongly contrasting TADF properties, with 2TPA-iCNBT exhibiting the largest increase in the photoluminescence quantum yield on removal of oxygen (from 27 to 55%), and the fastest TADF emission kinetics in doped films (k RISC similar to 105 s-1). In solution-processed organic light-emitting diodes, decent maximum external quantum efficiency (EQE) values are obtained for 2TPA-iCNBT (2.49%), 1TPA-CNBT (2.91%), and 1TPA-iCNBT (2.76%), in clear contrast to 2TPA-CNBT (1.16%), highlighting the decisive role of the D-A substitution pattern (and the number of D groups) on the performance of NIR-TADF emitters. Furthermore, 2TPA-iCNBT is shown to maintain the highest EQE at larger current densities (EQE = 1.98% at 10 mA cm-2) within the investigated series, a consequence of its standout TADF behavior.