Theoretical investigation on bisarylselanylbenzo-2,1,3-selenadiazoles as potential photosensitizers in photodynamic therapy.

Density functional theory and time-dependent (TDDFT) calculations were carried out for recently reported bisarylselanylbenzo-2,1,3-selenadiazoles derivatives capable of producing singlet oxygen (O) under UV-Vis irradiation. Conformational behaviors, excitation energies, singlet-triplet energy gaps, and spin-orbit coupling constants were evaluated. The conformational analysis evidences that two different conformers have to be taken into consideration to completely describe the photophysical properties of this class of molecules. TDDFT results show that these compounds, though possessing absorption wavelengths that fall in the violet region, are characterized by singlet-triplet energy gaps greater than the energy required to excite the molecular oxygen, thus being able to produce the cytotoxic species, spin-orbit coupling constants large enough to ensure efficient singlet-triplet intersystem spin crossing, and even the highly reactive superoxide anion O by autoionization and subsequent electron transfer to molecular oxygen in its ground state.