Dihexadecyl Phosphate Vesicles: Difference in the Spectroscopic Properties of Tris(2,2′-bipyridine)ruthenium(II) Adsorbed on the Inner and Outer Surfaces

Abstract

The visible absorption spectrum of tris (2,2′-bipyridine)ruthenium(II) [ Ru ( bpy )₃²⁺] adsorbed on the outer surface of dihexadecyl phosphate vesicles is indistinguishable from the spectrum of free Ru ( bpy )₃²⁺ in aqueous solution. By contrast, the absorption spectrum of Ru ( bpy )₃²⁺ that is entrapped in vesicles by adsorption on inner surfaces exhibits a red shift in its visible absorption peak, and the extinction coefficient for this peak is increased. The emission spectrum for outer-adsorbed Ru ( bpy )₃²⁺ is red-shifted whilst that for inner-adsorbed Ru ( bpy )₃²⁺ is blue-shifted, compared with the spectrum for free Ru ( bpy )₃²⁺ in solution. The excited state of Ru ( bpy )₃²⁺ was longer lived, and was much less sensitive to quenching by oxygen, when the complex was adsorbed on inner vesicle walls. The above spectral variations indicate that the adsorption of Ru ( bpy )₃²⁺ on inner surfaces occurs in a way which leads to more spatial restriction on Ru ( bpy )₃²⁺ ions than does adsorption on outer surfaces of the bilayer . This asymmetry may be due to differences in the packing of dihexadecyl phosphate molecules as well as in the ionic strength of inner and outer aqueous environments.