Computational investigation of diphenyl sulfone based thermally activated delayed fluorescence materials and their...

This study investigates the influence of molecular structure on the thermally activated delayed fluorescence (TADF) mechanism through theoretical calculations. Experimentally verified TADF emitters and their structural derivatives were analyzed using Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT). Ground and excited state geometries (S₀, S₁, T₁) were optimized, and key parameters including vertical and adiabatic singlet-triplet energy gaps (ΔE_ST), reorganization energies (λ), spin-orbit coupling (SOC) matrix elements (SOCME), and reverse intersystem crossing (RISC) rate constants (k_RISC) were determined for a series of DPS (diphenyl sulfone)-based structures employing PXZ (phenoxazine), PTZ (phenothiazine), DMAC (9,9-dimethyl-9,10-dihydroacridine), and Cz (carbazole) as donor units. Our results reveal that introducing stronger donors in symmetric D-A-D systems lowers S₁ and T₁ energies and reduces ΔE_ST, while weaker donors increase both S₁ and T₁ energies with minimal effect on ΔE_ST. Incorporating weaker donors, such as PTZ where the sulfur atom enhances spin-orbit coupling through the heavy atom effect, enhances SOCME and increases k_RISC, presenting a promising design strategy for efficient TADF emitters. These findings highlight the importance of donor type and strength, and substitution pattern (meta or para) in tuning the optoelectronic performance of TADF molecules. This study also represents an investigation of asymmetric D-A-D∗ type structures, aiming to elucidate the role of donor asymmetry in tuning the TADF mechanism.