Declined S but Constant T 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) and high radiative rate constants (k) 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 is often at the expense of reducing the k. Herein, we demonstrated a new ΔE modulation approach to construct TADF with high k, based on triplet blocking effect, i.e., the extension of π-conjugation of a triplet constrainer (IB) leads to a gradually red-shifted S but a constant T energy and therefore reduced ΔE controlled from monomer (IB), monomer-linker (IB-BF), to dimer of IB-BF-IB. The natural transition orbital analysis indicates that S state is delocalized while T state is localized as confirmed by time resolved electron paramagnetic resonance spectroscopy. Therefore, the ΔE is reduced from 0.60 eV, 0.46 eV to 0.25 eV, while keeping faster radiation rate (around 10 s) than that of conventional donor-acceptor molecules (10∼10 s). As a result, the emission mechanisms are regulated from fluorescence for IB, phosphorescence/TADF dual emissions for IB-BF to TADF for IB-BF-IB. This paper proposed a new approach of ΔE modulation and a new type of TADF molecule with high radiation rate, which is crucial for fundamental photophysics as well as material science.