Declined S1 but Constant T1 Energy: Thermally Activated Delayed Fluorescence with Triplet Blocking Effect

Thermally activated delayed fluorescence (TADF) molecules have been widely investigated in organic light emitting diodes (OLED), organic lasing, etc. Small singlet-triplet energy gap (Delta EST) and high radiative rate constants (kF) are highly desired to utilize triplet excitons efficiently and are beneficial to reduce efficiency roll-off of devices of OLED devices. The prevalent TADF molecules are via donor-acceptor molecular design, for which the decreasing of the Delta EST is often at the expense of reducing the kF. Herein, we demonstrated a new Delta EST modulation approach to construct TADF with high kF, based on triplet blocking effect, i.e., the extension of pi-conjugation of a triplet constrainer (IB) leads to a gradually red-shifted S1 but a constant T1 energy and therefore reduced Delta EST controlled from monomer (IB), monomer-linker (IB-BF2), to dimer of IB-BF2-IB. The natural transition orbital analysis indicates that S1 state is delocalized while T1 state is localized as confirmed by time resolved electron paramagnetic resonance spectroscopy. Therefore, the Delta EST is reduced from 0.60 eV, 0.46 eV to 0.25 eV, while keeping faster radiation rate (around 108 s-1) than that of conventional donor-acceptor molecules (106 similar to 107 s-1). As a result, the emission mechanisms are regulated from fluorescence for IB, phosphorescence/TADF dual emissions for IB-BF2 to TADF for IB-BF2-IB. This paper proposed a new approach of Delta EST modulation and a new type of TADF molecule with high radiation rate, which is crucial for fundamental photophysics as well as material science.