Halogenation effects on the structure, spectral properties, and cytotoxicity of BOIMPYs.

This article presents the targeted synthesis and comprehensive investigation of novel dihalogenated Cl2-BOIMPY and Br2-BOIMPY dyes. DFT/TDDFT calculations, in high agreement with experimental data, reveal that halogenation at the β'-positions enhances structural rigidity and significantly modulates excited-state dynamics. The introduction of bromine atoms facilitates a narrowing of the S1-T2 energy gap and a substantial increase in spin-orbit coupling (SOC), leading to efficient intersystem crossing. Consequently, Br2-BOIMPY exhibits a singlet oxygen quantum yield of 30% (in CS2), outperforming the chlorinated analogue (16%). In vitro confocal imaging demonstrates successful cytosolic internalization and robust intracellular ROS generation in HeLa cells. With a photocytotoxicity IC50 of 2.5 μM, Br2-BOIMPY shows superior PDT efficiency compared to previously reported brominated BOIMPYs. These findings expand the library of available photosensitizers and provide a fundamental basis for the further optimization of BOIMPY-based phototherapeutic agents.