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.