Achieving Efficient Triplet Exciton Utilization with Large Delta E-ST 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 ((CT)-C-1), local exciton triplet state ((LE)-L-3), and charge-transfer triplet state ((CT)-C-3). 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 (Delta E-ST) being observed. This work presents a view that photoinduced delayed fluorescence and a small Delta E-ST are sufficient but not necessary for TADF materials. It also provides a reference that the high-energy (LE)-L-3 state plays a key role in the RISC process in electroluminescence.