Enhancement of Thermally Activated Delayed Fluorescence (TADF) in Multi-Resonant Emitters via Control of Chalcogen Atom Embedding

Multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters based on heteroatom-embedded organoboron molecules are emerging as interesting candidates for organic light-emitting diode (OLED) applications. This is mainly due to their high photoluminescence quantum yields, thermal and chemical stabilities, and color purity. However, their reverse intersystem crossing (RISC) rates generally remain relatively small. To circumvent that drawback, here, we expand on our recent investigations and design a series of MR-TADF molecules through the incorporation of chalcogen atoms (O, S, or Se) in a nitrogen-centered organoboron emitter consisting of an ADBNA (13b-aza-5,9-diboranaphtho[3,2,1-de]anthracene) core. The results obtained by means of highly correlated wave function-based calculations indicate that these molecules (i) retain small singlet-triplet energy gaps (0.1-0.2 eV) and (ii) have a highly emissive nature (radiative rates around 108 s-1) due to the alternating distribution of electron-rich and electron-poor regions related to multi-resonant effects. Importantly, the RISC rates, especially for emitters containing selenium atoms, can be as high as 108 s-1, which comes from both increased spin-orbit couplings and contributions from the second excited triplet (T2) states. Coupled with the increased core rigidity associated with a larger number of chalcogen atoms that bridge and fuse rings together, these characteristics make these MR-TADF molecules promising strong emitters with high color purity. Our calculations further underline that it is not only the nature of the chalcogen atoms (O, S, or Se) but also their positions within the backbone that have a critical impact on the emission color, which can vary from deep blue to yellow-green.