Synthesis, electrochemistry, and photophysical properties of binuclear ruthenium(II)-terpyridine complexes comprising redox-active ferrocenyl spacer

In attempting to perturb the electronic properties of the spacer, we now describe an interesting example of Ru²⁺-tpy (tpy = terpyridine) complexes with 1,1′-bis(ethynyl)polyferrocenyl moiety attached directly to the 4′-position of the tpy ligand (tpy-CC-(fc)_n-CC-tpy; fc = ferrocenyl;n = 2-3). Complexes of Ru²⁺-tpy have room-temperature luminescence in H₂O/CH₃CN (4/1) solution. The ground-state HOMO and LUMO energies were probed by electrochemical measurements and the excited-state photophysical properties were probed by UV-Vis absorption spectroscopy and luminescence spectroscopy. The redox behavior of (tpy)Ru^II-tpy-CC-(fc)_n-CC-tpy-Ru^II(tpy)]⁴⁺ complex is dominated by the Ru²⁺/Ru³⁺ redox couple (E_1/2 from 1.35 to 1.39 V), Fe²⁺/Fe³⁺ redox couples (E_1/2 from 0.4 to 1.0 V) and tpy/tpy⁻/tpy²⁻ redox couples (E_1/2 from −1.3 to −1.5 V). Electrochemical data, UV absorption and emission spectra indicate that the π-delocalization in the spacer is enhanced by the insertion of ethynyl unit. Interestingly, the insertion of ethynyl unit into the main chain causes a dramatic increase of phosphorescence yield (1.48 × 10⁻⁴ for n = 2; 1.13 × 10⁻⁴ for n = 3), triplet lifetime (67 ns for n = 2; 24 ns for n = 3), and emission intensity. The biferrocenyl spacer can be converted into mixed-valence biferrocenium spacer, which gives a more effective π-delocalization along main chain, by selective chemical oxidation of ferrocenyl unit. In deoxygenated H₂O/CH₃CN (4/1) solution at 25 °C, the oxidized complex of (tpy)Ru^II-tpy-CC-(fc)₂-CC-tpy-Ru^II(tpy)]⁵⁺ is nonemissive. The presence of lower energy ferrocenium-centered excited-state provides an additional channel for excited-state decay. The mixed-valence biferrocenium center acts as an efficient quencher for the MLCT excited-state.