Enhanced acidity, photophysical properties and liposome binding of perfluoroalkylated phthalocyanines lacking C-H bonds

The acid-base, spectroscopic, photophysical and liposome-binding properties of the recently synthesized free base, 29H,31H,1,4,8,11,15,18,22,25-octafluoro-2,3,9,10,16,17,23, 24-octakisperfluoro(isopropyl) phthalocyanine, F ₆₄PcH ₂, are reported. The perfluoroalkylation of the phthalocyanine core renders the hydrogen atoms acidic, with a pK _a = 6. The F ₆₄Pc ^-2 dianion is detected already at pH 3, by singular-value decomposition analysis of electronic spectra. F ₆₄Pc ^-2 generates ¹O ₂ with quantum yields Φ _Δ = 0.252 (in MeOH) and 0.019 in liposomes. Metallation of the Pc macrocycle to yield F ₆₄PcZn increases Φ _Δ to 0.606 and 0.126 in MeOH and liposomes, respectively. Surprisingly, F ₆₄Pc ^-2 (but not F ₆₄PcH ₂ or F ₆₄PcZn) binds strongly to liposomes, with a binding constant K _b = 25 (mg/mL) ^-1. The fully protonated F ₆₄PcH ₂, but not the zwitterionic F ₆₄Pc ^-2, might favor hydrogen bonding, thus reducing its lipophilicity. Similarly, the Lewis acidity of Zn in F ₆₄PcZn, and thus its ability to bind water within a hydrophobic perfluoroalkyl pocket, is significantly enhanced by the fluorinated substituents.