Synthesis, electrochemistry, and photophysical properties of binuclear ruthenium(II)–terpyridine complexes comprising redox-active ferrocenyl spacer
作者:Teng-Yuan Dong、Mei-Ching Lin、Shu-Wei Chang、Chih-Chien Ho、Shu-Fan Lin、Liangshiu Lee
DOI:10.1016/j.jorganchem.2007.02.007
日期:2007.5
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-C=C-(fc)(n)-C=C-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-C=C-(fc)(n)-C=C tpy-Ru-II(tpy)](4+) complex is dominated by the Ru2+/Ru3+ redox couple (E-1/2 from 135 to 1.39 V), Fe2+/Fe3+ 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 pi-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 x 10(-4) for n = 2; 1.13 x 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 pi-delocalization along main chain, by selective chemical oxidation of ferrocenyl unit. In deoxygenated H2O/CH3CN (4/1) solution at 25 degrees 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 ferroceanium-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. (C) 2007 Elsevier B.V. All rights reserved.