Rodlike Bimetallic Ruthenium and Osmium Complexes Bridged by Phenylene Spacers. Synthesis, Electrochemistry, and Photophysics

In the search for light-addressable nanosized compounds we have synthesized 10 dinuclear homometallic trisbipyridyl
complexes of linear structure with the general formula [M(bpy)3-BL-M(bpy)3]4+ [M = Ru(II) or Os(II); BL =
polyphenylenes (2, 3, 4, or 5 units) or indenofluorene; bpy = 2,2‘-bipyridine]. By using a “chemistry on the complex”
approach, different sizes of rodlike systems have been obtained with a length of 19.8 and 32.5 Å for the shortest
and longest complex, respectively. For one of the ruthenium precursors, [Rubpy-ph2-Si(CH3)3][PF6]2, single crystals
were obtained by recrystallization from methanol. Their photophysical and electrochemical properties are reported.
All the compounds are luminescent both at room and low temperature with long excited-state lifetimes due to an
extended delocalization. Nanosecond transient absorption showed that the lowest excited state involves the chelating
unit attached to the bridging ligand. Electrochemical data indicated that the first reduction is at a slightly more
positive potential than for the reference complexes [M(bpy)3]2+ (M = Ru, Os). This result confirms that the best
acceptor is the bipyridine moiety connected to the conjugated spacers. The role of the tilt angle between the
phenylene units, in the two series of complexes, for the ground and excited states is discussed.