Three-centers models for electron transfer through a bridge. 1. Potential energy surfaces.

Three centers models adapted to the description of electron transfer through a bridge are discussed, with a special emphasis on potential energy surfaces. A short historical review of the available models is given, with a particular interest on the Bersuker-Borshch-Chibotaru model (1989) and the Lambert-Nöll-Schelter model (2002). We propose our own model, inspired from the Bersuker-Borshch-Chibotaru model, but with a more physical discussion of the parameters and coordinates. The diabatic surfaces, before the intervention of electronic couplings between external site and bridge, consist of three revolution paraboloids of equal radii. The bottoms of the paraboloids do not form in general an equilateral triangle; they form an isosceles one. At this stage, the basic parameters are the ones describing the position of the third paraboloid (corresponding to a redox process on the bridge) with respect to the other two. We define in particular an energy shift parameter (Delta) and a depth parameter (d), the latter corresponding to the position of this paraboloid in the third dimension, i.e., along a coordinate of reaction perpendicular to the usual reaction coordinate. The topology of diabatic and adiabatic surfaces is discussed. As an application, we explain the contrasted behavior of two mixed valence systems bridged by anthracene and dimethoxybenzodithiophene, which differ by the value of the d parameter.

An approach to long and unsubstituted molecular wires: synthesis of redox-active, cationic phenylethynyl oligomers designed for self-assembled monolayers.

Various oligo(phenyleneethynylene)s (OPEs) have been synthesized in the past, as they are considered as prototypes of molecular wires. When the oligomers are capped by a redox site at one end and a thiol at the other end, the resulting molecules can be grafted as a self-assembled monolayer on a gold electrode and fully studied by electrochemical techniques. Unfortunately, such molecules are usually poorly soluble and require the incorporation of solubilizing pendant groups. In this paper, we show that the replacement of the classically used redox group ferrocene by a cationic organometallic ruthenium complex, namely, [Ru(bipy)(2)(ppH)](+) (bipy, 2,2'-bipyridine; ppH, 2-(2'-yl-phenyl)pyridine), allows a concise synthesis of an unsubstituted thioacetate-capped OPE up to four repetitive units long. The positive charge does not interfere with the conventional organic chemistry used to elongate, purify, or characterize the hexafluorophosphate salts of the molecules. To our knowledge, this represents the first family of long, poorly substituted OPEs designed for self-assembly.

A new redox site as an alternative to ferrocene to study electron transfer in self-assembled monolayers.

The cyclometallated ruthenium complex [Ru(bpy)2(pp)]+ (bpy: 2,2'-bipyridine; pp: 2-(2'-ylphenyl)pyridine) was easily grafted to a omega-alkanethiol and the resulting compound was coadsorbed with 11-hydroxyundecanethiol on gold yielding a Self-Assembled Monolayer (SAM) in an analogous manner as for a ferrocene derivative, as shown by impedance spectroscopy; the kinetics of the heterogeneous electron transfer were shown to be very fast, compared to ferrocene, which makes this new redox site a promising candidate for further studies about molecular wires.