The Taming of the Shrew: Using Mild Heavy-Atom Effect for Constraining Molecular Vibrations in a Triphenylamine Donor-Acceptor System for Enhanced Photo- and Electroluminescence

Achieving efficient triplet harvesting remains a key challenge in the development of all-organic red and near-infrared (NIR) thermally activated delayed fluorescence (TADF) emitters. In this study, we demonstrate how strategic ortho-chlorine substitution in a triphenylamine (TPA)-based donor-acceptor (D-A) system significantly improves reverse intersystem crossing (rISC) efficiency by simultaneously reducing the singlet-triplet energy gap (ΔEST) and enhancing spin-orbit coupling (SOC). Through combined spectroscopic and theoretical analysis, we reveal that chlorine-induced molecular distortion of two side phenyl rings of TPA and increased rigidity lead to lower reorganization energies, while collectively these factors facilitate an over 20-fold enhancement in rISC rates. Notably, the modified emitter (diClDa-CNBPz) exhibits higher photoluminescence quantum yield, reduced sensitivity to external media, and optimized triplet harvesting via both 3CT-1CT and 3LE-1CT pathways. The latter is responsible for the enhanced SOC owing to a heavy-atom effect. When incorporated into OLEDs, diClDa-CNBPz achieves twice the external quantum efficiency and a fourfold reduction in efficiency roll-off compared to the unsubstituted counterpart. Additionally, its application in NIR hyperfluorescent OLEDs further underscores the benefits of enhanced triplet harvesting. These findings highlight halogen substitution as a powerful tool for fine-tuning TADF materials, paving the way for next-generation high-performance optoelectronic devices.