Photo-oxidation of 1,3-cyclopentadiene using partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complexes

Abstract

Photo-oxidation of cyclopenta-1,3-diene (CP) with singlet oxygen using tris(2,2′-bipyridine)ruthenium(II) [Ru(bpy)₃²⁺] and polymer-bound ruthenium(II) complexes as photosensitizers was investigated in oxygen-saturated ethanol. The polymer-bound ruthenium(II) complexes consist of a bis(2,2′-bipyridine)ruthenium(II) complex coordinated with two imidazolyl residues on partially quaternized poly(1-vinylimidazole) with hexyl groups (C₆RuQPIm) and hexadecyl groups (C₁₆RuQPIm) as the alkyl side chains. The photo-oxidation of CP using the polymer photosensitizers effectively occurred in a comparable manner to the Ru(bpy)₃²⁺ system: the degree of reaction, being given by the ratio of reacted CP and initial CP concentrations, was high. In particular, all of the CP added was oxidized in the C₁₆RuQPIm system even when the CP concentration was low. This was attributed to the concentration of CP species into the heterogeneous reaction field formed by the polymer photosensitizers. The Ru(bpy)₃²⁺ photosensitizer showed excellent stability and was repeatedly able to be used for the photo-oxidation reaction. During the repeated experiments, the reaction activity for the polymer photosensitizer systems gradually decreased because the polymer photosensitizers changed to monochloro complexes [CnRu(Cl)QPIm (n=6 or 16)] in which one imidazolyl residue was substituted by a chloride ion. However, these polymer photosensitizers also had excellent stability. Compared with other photosensitizers, such as Rose Bengal and zinc(II) phthalocyaninetetrasulfonic acid, the stability of the ruthenium(II) complex photosensitizers was excellent.