Halogenated silicon(IV) phthalocyanines with axial poly(ethylene glycol) chains. Synthesis, spectroscopic properties, complexation with bovine serum albumin and in vitro photodynamic activities

A new series of unsubstituted and halogenated silicon(IV) phthalocyanines with two axial poly(ethylene glycol) (PEG) chains having an average molecular weight of 550 or 750 (PEG₅₅₀ or PEG₇₅₀) have been synthesised by treating the corresponding silicon phthalocyanine dichloride with PEG methyl ether in the presence of NaH. The compounds have been unambiguously characterised with ¹H NMR and MALDI-TOF mass spectrometry. With two bulky polymeric substituents, the compounds are essentially non-aggregated in common organic solvents. The longer PEG ₇₅₀ chain enhances the hydrophilicity of the phthalocyanine ring and is more effective to prevent aggregation and fluorescence quenching by Cu(OAc)₂. Substitution with heavier halogen atoms on the periphery of the ring leads to a reduction in fluorescence emission and an increase in singlet oxygen quantum yield, as a result of heavy atom effect. The compounds Si(PcX₈)(PEG₇₅₀)₂ [X = H (4b), Cl (4c), Br (4d)] are photocytotoxic towards HepG2 human hepatocarcinoma cells and J774 mouse mammary tumour cells. Although halogenation results in an increase in singlet oxygen quantum yield, the general photocytotoxicity follows the order 4b > 4d > 4c. This can be attributed to the opposite effect of aggregation, which follows the order 4a <4b <4c in the growth medium. The interactions of 4b-d with bovine serum albumin (BSA) have also been investigated by a fluorescence quenching method and a non-covalent conjugate of 4b and BSA has been prepared. Conjugation with BSA leads to a higher photocytotoxicity against J774 cells, which have a BSA-loving macrophage origin.