Unveiling the influence of oxidation state and heavy atom effects in chalcogen group on boron centered D(X)BNA core: a computational study on RTP versus TADF†

The endeavor of utilizing non-radiative triplet excitons in RTP and TADF molecules has garnered significant interest in recent studies, presenting a highly desirable yet challenging pursuit. In this investigation, we utilized DFT and TD-DFT computational approaches to anticipate the photophysical characteristics of multifunctional materials, uncovering their significant reliance on the oxidation state and heavy atom influences of the chalcogen group on boron centered D(X)BNA cores, along with substitutions of weak phenylcarbazole (P-CBZ) and strong phenyldimethylacridine (P-DMAC) donors. The calculations demonstrated that both heavy atom (X = O, S, Se, Te) and oxidation (S, SO, SO₂, and Se, SeO) effects caused a decrease in singlet (S₁) and triplet (T₁) energies. Unexpectedly, the first singlet-triplet energy difference (ΔE_ST) values exhibit a systematic decrease with weak donor-based molecules, while they increase with strong donor unit-based molecules with the heavy atom effects. Moreover, the ΔE_ST values decrease systematically with the oxidation effect in both types of donor unit-based molecules. Conversely, the magnitudes of spin–orbit coupling (SOC) increase with heavy atom effects due to the orbital mixing and screening effects of lone pair electrons and decrease with oxidation effects because of their decreased lone pair electrons in both the S₁–T₁ and T₁–S₀ pathways. The elevated SOC and intersystem crossing (ISC) rates in heavy atom-based molecules, and low ΔE_ST and high reverse intersystem crossing (RISC) in oxidation-based molecules, meet the criteria for multifunctional RTP and TADF molecules, respectively.