Quantum coherence and spin-vibronic dynamics in thermally activated delayed fluorescence emitters

Thermally activated delayed fluorescence (TADF) materials are emerging as a new and promising class of organic and environmentally friendly emitters capable of achieving very high internal quantum efficiency by harvesting triplet excitons through reverse intersystem crossing (rISC) into the radiative singlet state. This type of emitter is gaining significant attention due to its purely organic structure, low cost, and color tunability, which together eliminate the need for expensive rare-earth transition metals. However, the rISC rate is mainly governed by the singlet–triplet energy gap and spin–vibronic coupling, which are not yet fully understood. Recent advances in TADF have shown that the spin–vibronic mechanism (SVM) plays a significant role in coupling the singlet and triplet states and enhancing the rISC rate, but direct experimental validation is still lacking. With this motivation, we have developed ultrafast coherent laser spectroscopy using broadband pump–probe techniques to study how vibrational and electronic states interact in TADF emitters. By probing femtosecond–picosecond vibrational superposition states, we resolve how the spin–vibronic mechanism (SVM) facilitates spin conversion and modulates the singlet–triplet energy gap. We aim to present and provide experimental evidence of spin–vibronic coupling in TADF materials, especially in charge-transfer-type donor–acceptor (D–A) and donor–acceptor–donor (D–A–D) molecular designs, which helps us to understand how coherent vibrational motion drives efficient spin mixing through coupled electronic and nuclear dynamics.