Photophysics and electron transfer in poly(3-octylthiophene) alternating with Ru(II)- and Os(II)-bipyridine complexes

A series of soluble metal-organic polymers that contain Ru(II)- and Os(II)-polypyridine complexes interspersed within a pi-conjugated poly(3-octylthiophene) backbone are prepared. Detailed electrochemical and photophysical studies are carried out on the polymers and two model complexes to determine the extent that the metal-polypyridine units interact with the pi-conjugated system. The results indicate that there is a strong electronic interaction between the metal-based chromophores and the pi-conjugated organic segments, and consequently the photophysical properties are not simply based on the sum of the properties of the individual components. In the Ru(II) polymers, the metal-to-ligand charge-transfer (MLCT) excited state is slightly higher in energy than the 3 pi,pi* state of the poly(3-octylthiophene) backbone. This state ordering results in a material that displays only a weak MLCT luminescence and a long-lived transient absorption spectrum that is dominated by the 3 pi,pi* state. In the Os(II) polymer the MLCT state is lower in energy than the polythiophene-based 3 pi,pi* state and the "unperturbed" MLCT emission is observed. Finally, all of the metal-organic polymers undergo photoinduced bimolecular electron-transfer (ET) reactions with the oxidative quencher dimethyl viologen. Transient absorption spectroscopy reveals that photoinduced ET to dimethyl viologen produces the oxidized polymers, and in most cases, the transient spectra are dominated by features characteristic of a poly(3-octylthiophene) polaron.