Electronic Modulation through Substituent Engineering in Conventional and Multiresonance TADF Emitters

Since its introduction in 2012, thermally activated delayed fluorescence (TADF) has become a pivotal concept in organic optoelectronics, particularly for high-efficiency organic light-emitting diodes (OLEDs). TADF enables harvesting of nonradiative triplet excitons through reverse intersystem crossing (rISC), leading to external quantum efficiencies (EQEs) comparable to those of phosphorescent systems but without the need for heavy metals. Molecular design strategies have therefore focused on optimizing rISC kinetics and minimizing singlet–triplet energy gaps (ΔEST). Donor–acceptor (D–A) charge-transfer architectures have been extensively investigated as TADF platforms due to their inherent structural tunability and ability to modulate electronic coupling between frontier orbitals. By controlling the electron-donating and -withdrawing abilities of D–A units, charge-transfer character and exciton dynamics can be systematically modulated, allowing precise control over ΔEST. Recently, multiresonance thermally activated delayed fluorescence (MR-TADF) emitters, which deviate from the conventional D–A molecular design, have gained significant attention. The MR-TADF strategy has emerged as a highly effective approach for realizing efficient and spectrally stable deep-blue emission, thereby addressing one of the long-standing challenges in the advancement of next-generation display and solid-state lighting technologies. This review emphasizes the role of substituent engineering in tailoring both D–A and MR-TADF systems. Substituents strongly influence the orbital distribution, charge-transfer strength, and excited-state dynamics, providing a rational pathway to improve photophysical properties and device stability. Recent advances demonstrate that targeted functionalization of D and A moieties can regulate ΔEST, spin–orbit coupling, and emission profiles.