Achieving Efficient Triplet Exciton Utilization with Large ΔEST and Nonobvious Delayed Fluorescence by Adjusting Excited State Energy Levels

Enhancing the rate of reverse intersystem crossing ( k_risc) and the rate of radiative transition ( k_r) has been regarded as the key to improve molecular design strategy in the field of thermally activated delayed fluorescence (TADF) materials. Herein, two sky-blue donor-acceptor (D-A)-type TADF materials, namely, CzDCNPy and tBuCzDCNPy, were designed following a strategy of controlling the energy difference among the charge-transfer singlet state (¹CT), local exciton triplet state (³LE), and charge-transfer triplet state (³CT). Significantly different from most previously reported TADF materials, large values of k_r and k_risc and a nearly 100% exciton utilization efficiency were simultaneously achieved despite nonobvious delayed fluorescence and a large value of the singlet-triplet energy difference (Δ E_ST) being observed. This work presents a view that photoinduced delayed fluorescence and a small Δ E_ST are sufficient but not necessary for TADF materials. It also provides a reference that the high-energy ³LE state plays a key role in the RISC process in electroluminescence.