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

Abstract

Listen

Translate article

In attempting to perturb the electronic properties of the spacer, we now describe an interesting example of Ru 2+–tpy (tpy = terpyridine) complexes with 1,1′-bis(ethynyl)polyferrocenyl moiety attached directly to the 4′-position of the tpy ligand (tpy–C C–(fc) n –C C–tpy; fc = ferrocenyl; n = 2–3). Complexes of Ru 2+–tpy have room-temperature luminescence in H 2O/CH 3CN (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–C C–(fc) n –C C–tpy–Ru II(tpy)] 4+ complex is dominated by the Ru 2+/Ru 3+ redox couple ( E 1/2 from 1.35 to 1.39 V), Fe 2+/Fe 3+ redox couples ( E 1/2 from 0.4 to 1.0 V) and tpy/tpy −/tpy 2− 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 H 2O/CH 3CN (4/1) solution at 25 °C, the oxidized complex of [(tpy)Ru II–tpy–C C–(fc) 2–C C–tpy–Ru II(tpy)] 5+ is nonemissive. The presence of lower energy ferrocenium-centered [ Ru a II – tpy - – fc III – fc III – tpy – Ru b II ] 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.