A series of five BODIPY dyes substituted with carbazole(s) (Cz) in different positions and one with an iodine atom in Cz were synthesized by the Suzuki-Miyaura cross-coupling reaction. Their photophysical and electrochemical properties were studied in order to correlate the chemical modulation with intersystem crossing (ISC) from singlet to triplet states, which is a key aspect for their potential use as photosensitizers in antitumor photodynamic therapy (PDT). From the combined experimental analysis and DFT and STEOM-DLPNO-CCSD theoretical calculations, electron transfer, which is a pathway for ISC, occurs from Cz to the BODIPY core only when Cz is substituted either in 2- or 2,6-positions, whereas in meso-phenyl position, both chromophores are electronically isolated because of their orthogonality. The iodine substituted molecule can yield the triplet state, instead, through spin orbital coupling (SOC) produced by the iodine heavy atom effect. Because a larger singlet oxygen yield was obtained by UV-Vis spectroscopy for this latter derivative, its PDT performance was evaluated in vitro on breast and lung cancer against healthy human cells and murine fibroblasts. Positive effect was obtaining under green irradiation. In particular, the cells of breast cancer MDA-MB-231 were the most sensitive to the photosensitizer.

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