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 (ΔE_ST) and high radiative rate constants (k_F) 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 ΔE_ST is often at the expense of reducing the k_F. Herein, we demonstrated a new ΔE_ST modulation approach to construct TADF with high k_F, based on triplet blocking effect, i.e., the extension of π-conjugation of a triplet constrainer (IB) leads to a gradually red-shifted S₁ but a constant T₁ energy and therefore reduced ΔE_ST 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 ΔE_ST 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∼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 ΔE_ST modulation and a new type of TADF molecule with high radiation rate, which is crucial for fundamental photophysics as well as material science.