Terminal redox-site effect on the long-range electron conduction of Fe(tpy)2 oligomer wires on a gold electrode.

This article completes our comprehensive understanding of the electron transport properties of our original π-conjugated redox-active molecular wires comprising Fe bridged by p-phenylene linkers (tpy=2,2':6',2''-terpyridine). The Fe(tpy)2 oligomer wires comprise three types of tpy ligands: the anchor tpy ligand (A series) makes a junction between the wire and electrode, the bridging bis-tpy ligand (L series) connects the Fe(tpy)2 units, and the terminal tpy ligand (T series) possesses a redox site as a probe for the long-range electron transport ability. Taking advantage of the precise tunability of the composition of the Fe(tpy)2 oligomer wires, thus far we investigated how A and L impacted on the electron-transport ability. The excellent long-range electron transport ability with ultrasmall attenuation constants (β(d), 0.002 Å(-1) as the minimum) depends on L significantly [Chem. Asian J. 2009, 4, 1361], whereas A is unrelated to the β(d) value, but influences the zero-distance electron-transfer rate constant, k(et)(0) [J. Am. Chem. Soc. 2010, 132, 4524]. Herein we study the influence of terminal ligand T(x) (x=1-3). β(d) is independent of T, however, T(3), with a cyclometallated Ru complex as the redox site, gives rise to a k(et)(0) value greater than T(1) and T(2) with ferrocene. This series of simple but definitive conclusions indicates that we have reached the stage of being able to precisely design molecular wires to attain desirable single-molecule electron conduction.