Theoretical Insight into the Effect of Phosphorus Oxygenation on Nonradiative Decays: Comparative Analysis of P-Bridged Stilbene Analogs

Heteroarene-Fused Benzo[b]arsoles: Structure, Photophysical Properties, and Effects of the Bridging Element

Heteroarene-fused heteroles have attracted considerable attention owing to their unique electronic and photophysical properties. The bridging element plays a crucial role in determining the electronic characteristics of the resulting π-conjugated molecules. In this study, we synthesized a series of heteroarene-fused benzo[b]arsoles and investigated their structures and photophysical properties. All the synthesized arsoles exhibited phosphorescence at 77 K, whereas arsole oxide did not. The phosphorescence intensities of the pnictogen-containing heteroles (phosphorus and arsenic) were significantly higher than those of the tetrel-containing heteroles (silicon and germanium). This trend was analyzed through theoretical calculations, focusing on the energy levels of the excited singlet and triplet states as well as the spin-orbit coupling matrix elements. Furthermore, the indole-fused benzoarsole oxide exhibited a mechanically induced emission color change. The removal of crystalline water triggered a change in the molecular packing that reduced the excimer emission.

Short-Range Coulomb Interaction Is a Key to Switch the Utilization of Higher Triplet Excitons in Multiresonance Thermally Activated Delayed Fluorescence Doped Film

Multiresonance thermally activated delayed fluorescence (MR-TADF) materials have attracted widespread attention due to ultrahigh definition display standards. However, many MR-TADF materials lack TADF properties in both the solution and solid states. Interestingly, the TADF characteristics appear once these MR-TADF compounds are doped in a suitable host film, but the precise mechanism involved in the host-guest interaction remains uncertain. Herein, we systematically investigated the role of host-guest interactions employing doped films (DABNA-1@mCBP and DABNA-1@DPEPO) with opposite phenomena. The results indicate that mCBP with a V-shape and enhanced rigidity could facilitate the formation of an energy spacing layer by employing short-range Coulomb energy through the MR luminescent core, which could offset the sensitivity of the stacking distance, enhancing the coupling between T1 and T2, and thus switch the reverse internal conversion and the higher T2 reverse intersystem crossing process. This work is a further development of luminescence mechanisms and an update of the host-guest interaction criteria for the targeted design of doped films.

Extended theoretical modeling of reverse intersystem crossing for thermally activated delayed fluorescence materials

Thermally activated delayed fluorescence (TADF) materials and multi-resonant (MR) variants are promising organic emitters that can achieve an internal electroluminescence quantum efficiency of ~100%. The reverse intersystem crossing (RISC) is key for harnessing triplet energies for fluorescence. Theoretical modeling is thus crucial to estimate its rate constant (kRISC) for material development. Here, we present a comprehensive assessment of the theory for simulating the RISC of MR-TADF molecules within a perturbative excited-state dynamics framework. Our extended rate formula reveals the importance of the concerted effects of nonadiabatic spin-vibronic coupling and vibrationally induced spin-orbital couplings in reliably determining kRISC of MR-TADF molecules. The excited singlet-triplet energy gap is another factor influencing kRISC. We present a scheme for gap estimation using experimental Arrhenius plots of kRISC. Erroneous behavior caused by approximations in Marcus theory is elucidated by testing 121 MR-TADF molecules. Our extended modeling offers in-depth descriptions of kRISC.