Evaluation of the 5-ethynyl-1,3,3-trimethyl-3H-indole ligand for molecular materials applications

The modification of conjugated organic compounds with organometallic moieties allows the modulation of the electronic and optoelectronic properties of such compounds and lends them to a variety of material applications. The organometallic complexes [M(Cp')(L)(n)] (M = Ru or Fe; Cp' = cyclopentadiene (Cp) or pentamethylcyclopentadiene (Cp*); (L)(n) = (PPh3)(2) or 1,2-bi (diphenylphosphino)ethane (dppe)) and [M(L)(n)] (M = Ru; (L)(n) = (dppe)(2) or (P(OEt)(3))(4); or M = Pt; (L)(n) = (PEt3)(2), (PPh3)(2) or tricyclohexylphosphine, (PCy3)(2)) modified with a 5-ethynyl-1,3,3-trimethyl-3H-indole ligand were prepared and characterised by NMR spectroscopy, IR and single-crystal X-ray diffraction. Cyclic voltammetry and IR spectroelectrochemistry of the ruthenium systems showed a single-electron oxidation localised over the M-C=C-aryl moiety. The N-heteroatom of the indole ligand showed Lewis base properties and was able to extract a proton from a vinylidene intermediate as well as coordinate to CuI. Examples from the wire-like compounds were also studied by single-molecule break junction experiments but molecular junction formation was not observed. This is most likely attributable to the binding characteristics of the substituted terminal indole groups used here to the gold contacts.