Pursuing the holy grail of thermally activated delayed fluorescence emitters: a molecular strategy for reducing the energy gap and enhancing spin–orbit coupling

The beauty and complexity of organic materials stem from the intricate interplay of their structural effects, where the same substituent can prove highly beneficial in one position yet entirely redundant in another. In this study, we demonstrate how a single halogen atom, strategically positioned, can significantly enhance the emissive properties of thermally activated delayed fluorescence (TADF) emitters. To address the inherent "low singlet-triplet energy gap (Δ<i>E</i> _ST) equals low spin-orbit coupling (SOC)" tradeoff in donor-acceptor (DA) emitters, we propose leveraging heavy-atom substitution to achieve two simultaneous effects: (1) reducing rotational heterogeneity and thereby lowering Δ<i>E</i> _ST at the macroscopic level due to the substituent's steric bulk, and (2) increasing SOC through the substituent's high atomic number (<i>Z</i>). In pursuit of efficient blue emitters, our investigation focused on DMAC-DPS derivatives with halogens positioned in the linker fragment at the <i>ortho</i> position to the donor. Fluorine substitution stabilized the charge-transfer state in polar media, resulting in a remarkably low activation energy of 20 meV. Bromine substitution enhanced SOC by over 20-fold in a nonpolar medium, while chlorine, striking a balance between the two, emerged as a "golden mean," offering both low activation energy and sufficiently high SOC. Molecular dynamics analysis revealed that molecular vibrations which disrupt the linker benzene ring symmetry promote excited-state mixing. The resulting hybrid nature of the T₁ state, involving electron density on the halogen, is particularly important for SOC enhancement in the chlorine derivative. These findings highlight that combining a mild heavy-atom effect (HAE) with the control of selective molecular vibrations offers a promising strategy for the design of efficient blue TADF emitters. Our results provide valuable insights for advancing the development of high-performance organic light-emitting materials.