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

Thermally activated delayed fluorescence (TADF) molecules have been widely investigated in organic light emitting diodes, etc. Small singlet-triplet energy gap and high radiative constants are desired. The prevalent TADF molecules are via donor-acceptor molecular design, which is at the expense of reducing radiative constants. Herein, we demonstrated a new singlet-triplet energy gap modulation approach to construct TADF with high radiative constant, based on triplet blocking effect, i.e., the extension of conjugation of a triplet constrainer (IB) leads to a gradually red-shifted singlet but a constant triplet energy, and therefore reduced singlet-triplet energy gap controlled from monomer (IB), monomer-linker (IB-BF2), to dimer of IB-BF2-IB. The natural transition orbital analysis indicates that singlet state is delocalized while triplet state is localized as confirmed by time resolved electron paramagnetic resonance spectroscopy. Therefore, the singlet-triplet energy gap is reduced from 0.60 eV, 0.46 eV to 0.25 eV, while keeping faster radiation rate (around 10^8/s) than that of conventional donor-acceptor molecules (10^6 ~ 10^7/s). 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 singlet-triplet energy gap modulation, which is crucial for fundamental photophysics and material science.