Abstract:
Charge-transfer (CT) states govern the performance of optoelectronic and biomedical materials but suffer from an intrinsic dilemma: enhancing CT absorption through orbital overlap inevitably widens the singlet–triplet gap (ΔEST), destroying thermally activated delayed fluorescence (TADF) and crippling emission efficiency. This long-standing absorption-TADF trade-off restricts the brightness of CT-based fluorophores and the efficiency of organic light-emitting diodes (OLEDs), while no molecular class has yet achieved dual TTA–TADF functionality across the entire visible spectrum. Here, a new paradigm is proposed by elevating vibronic coupling (VC) from a secondary phenomenon to a central molecular design principle. Through the Herzberg–Teller mechanism, weak CT transitions borrow oscillator strength from nearby locally excited (LE) states, redistributing intensity without increasing HOMO-LUMO overlap. This approach decouples absorption enhancement from geometric constraints, enabling simultaneous strong absorption and a small ΔEST, which is essential for efficient TADF. By directionally aligning the transition dipole moments (TDMs) of the LE and CT states, intensity borrowing enhances both excitation and radiative emission, thereby retaining orthogonal donor-acceptor geometries that are compatible with triplet harvesting. This strategy is further extended to dual TTA-TADF systems, providing a unified framework for achieving broadband fluorescence with inherent triplet management. Harnessing built-in VC allows computational pre-screening of promising structures before synthesis, unlocking universal design flexibility for OLEDs and biolabels. The proposed approach transforms an unavoidable photophysical compromise into a tunable parameter, providing a pathway toward high-efficiency, low-phototoxic materials for next-generation optoelectronics and biomedical imaging.