Systematic investigation via controlling the energy gap of the local and charge-transfer triplet state for enabling high efficiency thermally activated delayed fluorescence emitters

Thermally activated delayed fluorescence (TADF) emitters have evolved as a certified candidate in light generation technologies for producing efficient organic light-emitting diodes (OLEDs) on account of their 100% internal quantum efficiency (IQE) via reverse intersystem crossing (RISC) and toxic metal-free design. The fast rate of RISC (k(RISC)) is the ultimate requirement of an efficient TADF emitter, which can be achieved by minimizing the singlet-triplet energy gap (Delta E-ST). Here, four donor-acceptor type TADF emitters namely 3BPy-mDCz, 3BPy-mDTA, 3BPy-mDMAC, and 3BPy-mDPT were synthesized based on benzoyl pyridine (3BPy) as an unaltered acceptor and varying the donor strength ranging from carbazole to phenothiazine. These emitters show low Delta E-ST values forecasting their TADF nature. The Delta E-ST values decreased from 0.22 to 0.14 eV upon increasing the donor strength. The maximum external quantum efficiency (EQE) of 18.7% for 3BPy-mDCz, 22.5% for 3BPy-mDTA, 13.8% for 3BPy-mDMAC and 2.1% for 3BPy-mDPT was obtained. These drastic differences in the performances of 3BPy-mDTA and 3BPy-mDPT are due to the locally excited (LE)-L-3(T-2) intermediate state between the lowest singlet (S-1) and triplet (T-1). Among 3BPy-mDMAC and 3BPy-mDPT, the efficiency of 3BPy-mDMAC is superior due to less CT character and high photoluminescence quantum yield (PLQY). This work paves a new direction for efficient TADF molecular design by indicating the role of the intermediate triplet state ((LE)-L-3) despite possessing high Delta E-ST values.