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 (S-1) and triplet (T-1) 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 S-1 and considerable spin-orbit couplings between S-1 and T-1. Particularly, owing to a stronger CT component, the S-1 energy is more stabilized by electronic polarization than T-1, leading to a much reduced S-1 excitation energy and small energy difference between S-1 and T-1 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 S-1 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.

Automated summary

The study revealed that the first excited singlet and triplet states of the two emitters mix charge transfer and local excitation, ensuring sufficient oscillator strength and significant spin-orbit coupling. Due to a stronger charge transfer component, the energy of the first excited state is more stabilized by electronic polarization, leading to a reduced excitation energy and small energy difference between the first excited state and triplet state in the solid phase.