Photoexcitation and Electron Transfer Reactions of Zinc Lipidporphyrins in DMSO

The photophysical and photochemical properties of 5,10,15,20-tetrakis{α,α,α,α-o-[2′,2′- dimethyl-20′-((2″-(trimethylammonio)ethyl)phosphonatoxy)alkanamido] phenyl}porphinatozinc(II) (zinc lipid-porphyrins, ZnLPs (C₁₀, C₁₈)) have been studied in homogeneous DMSO solution and compared with those of 5,10,15,20-tetrakis{α,α,α,α-o-pivalamidophenyl} porphinatozinc(II) (ZnTpivPP) and tetrakis-phenylporphin-atozinc(II) (ZnTPP). The fluorescence quantum yields of the ZnLPs were lower than that of ZnTPP, but their fluorescence lifetimes were relatively long. The electron transfer reactions from the photoexcited states of these Zn porphyrin complexes to several quinone derivatives were evaluated by fluorescence spectroscopy and laser flash photolysis. The efficiences of oxidative quenching of the excited porphyrins via a dynamic process were significantly decreased by the presence of the bulky substituents on one side of the porphyrin macrocycle. This steric effect of the porphyrin side-chains was quantitatively examined by the Marcus classical treatment.