Effect of Axial Ligands on the Molecular Configurations, Stability, Reactivity, and Photodynamic Activities of Silicon Phthalocyanines

To demonstrate the effect of axial ligands on the structure–activity relationship, a series of axially substituted silicon phthalocyanines (SiPcs) have been synthesized with changes to the axial ligands. The reactivity of the axial ligand upon shielding by the phthalocyanine ring current, along with their stability, photophysical, and photodynamic therapy (PDT) activities were compared and evaluated for the first time. As revealed by single‐crystal XRD analysis, rotation of the axial OMe ligands was observed in SiPc 3, which resulted in two molecular configurations coexisting synchronously in both the solid and solution states and causing a split of the phthalocyanine α protons in the 1H NMR spectra that is significantly different from all SiPcs reported so far. The remarkable photostability, good singlet oxygen quantum yield, and efficient in vitro photodynamic activity synergistically show that compound 3 is one of the most promising photosensitizers for PDT. Minor adjustment: Through systematic changes to the axial ligands, the effect of these ligands on structure–activity relationships in silicon phthalocyanines (SiPc) was studied. As revealed by single‐crystal XRD analysis, rotation of the axial OMe ligands was only observed in the SiPc depicted, and resulted in two molecular configurations coexisting synchronously.