Nature of the Lowest Singlet and Triplet Excited States of Organic Thermally Activated Delayed Fluorescence Emitters: A Self-Consistent Quantum Mechanics/Embedded Charge Study

Thermally activated delayed fluorescence (TADF) is dictated by the properties of the lowest singlet (S-1) and triplet (T-1) excited states. Both small energy difference (Delta E-ST) and large spin-orbit coupling (SOC) between S-1 and T-1 are desired to increase the rate for reverse intersystem crossing (RISC). In this work, we investigated the ground- and excited-state electronic properties of three representative D-(pi)-A type TADF molecules in solid phase by means of a self-consistent quantum-mechanics/embedded-charge (QM/EC) approach, which consists of a series of iterative QM/EC single-point computations to account for the solid-state polarization effect. The results show that, unless the D and A units are perpendicular to each other, both the S-1 and T-1 states are characteristic of mixed charge transfer (CT) and local excitation (LE). Thereby, the LEST values are relatively large in gas phase. Importantly, the CT contribution is relatively larger in the S-1 state than in the T-1 state; thus, the S-1 energy is more stabilized by electronic polarization, leading to smaller REST in solid phase. At the same time, the SOC can be considerable due to the difference in the nature of the S-1 and T-1 states. These results shed light on the origin of fast RISC in efficient organic TADF systems.