Synthesis and characterization of a partially quaternized poly(1-vinylimidazole) bound to a ruthenium(II) complex and a viologen moiety

Abstract

Three metallopolymers having both a photosensitizer and an electron acceptor on the same polymer backbone were prepared and characterized by UV-Vis absorption and fluorescence spectroscopies and thermal measurements as differential scanning calorimetry (DSC) and thermogravimetric differential analysis (TG/DTA). These metallopolymers consist of bis(2,2′-bipyridine)ruthenium(II) complexes as a photosensitizer and a viologen species as an electron acceptor linked covalently to the imidazolyl residues on partially quaternized poly(1-vinylimidazole) through an alkyl spacer. DSC curves showed a glass transition temperature at ca. 230°C and an exothermic peak around 170°C. The exothermic peak was assigned to the cleavage of the alkyl spacer between the viologen and the quaternized imidazolium segment. UV-vis absorption spectra showed that these polymers have three absorption maxima, the metal-to-ligand charge transfer band (MLCT) of the ruthenium(II) complex residue being in the visible region and two electronic transition bands deriving from the viologen residue and the imidazolyl ring in the UV region. These absorption maxima are independent of the length of the alkyl group in the viologen residues and the spacer between the imidazolyl and the viologen segment. However, the luminescence spectra depend significantly on the length of alkyl group and the spacer, demonstrating that part of the ruthenium(II) complex residues are quenched by viologen residues. Furthermore, quenching studies revealed that these polymers contain at least four ruthenium(II) complex residues on the same polymer backbone.