Mechanistic Study on Dual-Donor Modified Distyryl-BODIPY Photosensitizers

This work systematically investigates how different electron-donating groups modulate the photophysical properties and singlet-oxygen quantum yields of donor-acceptor (D-A) distyryl boron-dipyrromethene (BODIPY) photosensitizers, with the donors attached at the para positions of both styryl arms. The study focuses on three compounds that have been previously reported in the literature (M1 with a dimethylacridine donor, M2 with a phenoxazine donor, and M3 with a phenothiazine donor), yet only experimental characterizations are available for them. Using computational chemistry methods, we comprehensively analyzed their geometric configurations, spectral features, spin-orbit coupling (SOC) effects, and electron-hole orbital distributions. The calculated results reveal that M2 possesses the lowest oxidation potential and thus the strongest electron-donating ability among the three donors. Compared with M1 and M3, M2 exhibits larger SOC values, a smaller energy gap between the excited singlet and triplet states, and the highest intersystem crossing (ISC) rate constant. Notably, the energy gaps ΔE (T₁ → S₀) for all three molecules exceed 0.98 eV, which is thermodynamically favorable for sensitizing triplet oxygen to singlet oxygen. Moreover, each of M1-M3 can form dual charge-transfer singlet states, affording two distinct singlet charge-transfer (¹CT) → triplet locally excited (³LE) ISC pathways. This theoretical work demonstrates that dual-donor modification can generate dual charge-transfer states and donor strength serves as an important regulating factor for the intersystem-crossing performance of the three investigated distyryl-BODIPY derivatives. The presence of such dual ISC channels offers a new strategic avenue for the rational design of highly efficient photosensitizers from a theoretical perspective, although further experimental validation in biological systems is required to confirm their therapeutic applicability.