Origin of High-Efficiency Near-Infrared Organic Thermally Activated Delayed Fluorescence: The Role of Electronic Polarization

To unveil the underlying mechanisms of high-efficiency near-infrared (NIR) organic thermally activated delayed fluorescence (TADF), we investigated the transition natures of the low-lying excited states for two donor–acceptor structured TADF emitters (TPAAP and TPAAQ) with the best external quantum efficiencies among NIR organic light-emitting diodes by self-consistent quantum mechanics/embedded charge (QM/EC) calculations. The results show that both the first excited singlet (S₁) and triplet (T₁) states of the two emitters are characteristics of hybridized charge transfer (CT) and local excitation but with distinct proportions, thus ensuring both sufficient oscillator strengths for S₁ and considerable spin–orbit couplings between S₁ and T₁. Particularly, owing to a stronger CT component, the S₁ energy is more stabilized by electronic polarization than T₁, leading to a much reduced S₁ excitation energy and small energy difference between S₁ and T₁ in the solid phase. Moreover, the calculations on the J-dimers point out that the intramolecular excited states of the dimers correspond well to the monomer states, and the lowest intermolecular CT states are very close to the monomer S₁ states in the solid phase. These results indicate that electronic polarization will play a crucial role in simultaneously achieving fast fluorescence radiation and reverse intersystem crossing for high-efficiency NIR TADF.