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.