Synthesis and electronic structure investigations of α,ω-bis(arylthio)oligothiophenes:: Toward understanding wire-linker interactions in molecular-scale electronic materials

Several oligothiophenes with 2-mesitylthio (MesS) substituents have been prepared and studied by UV-visible spectroscopy and cyclic voltammetry. These compounds can be considered as models for thiol-terminated conjugated oligomers, which have attracted intense interest as materials for molecular-scale electronics. Three types of oligomers were prepared: alpha,omega-bis(mesitylthio)oligothiophenes 4-7, alpha,omega-bis(mesitylthio)oligo(3,4-ethylelendioxythiophene)s 12-14, and alpha,omega-bis(mesitylthio)oligomers 15-19 containing both thiophene and 3,4-ethylenedioxythiophene rings. The mesitylthio groups were introduced via nucleophilic attack of lithiated thiophenes on mesitylenesulfenyl chloride. The oligomers were assembled by oxidative coupling or palladium-catalyzed Stille coupling of 2-stannylthiophenes with 2-bromothiophenes, The solution electronic spectra of all oligomers display a red-shift in the lowest-energy transition maximum (lambda(max)) relative to oligothiophenes lacking the MesS- group. The red-shift arises from conjugative overlap of a mesitylthio sulfur lone pair with the oligothiophene pi system. Cyclic voltammetry studies indicate that the MesS group significantly lowers the first and second oxidation potentials of the oligomers and improves the stability of the incipient radical cations and dications relative to alkyl-capped oligothiophenes. Additionally, the difference between first and second oxidation potentials in the Mess-substituted oligomers is much lower than known alkyl-substituted oligomers. This effect is due to the terminal MesS groups which cause charge density in the radical cations to concentrate at the chain ends, thereby lowering the Coulombic barrier to introduction of a second charge. The electronic structure perturbations caused by the MesS- group are discussed in the context of single-molecule conduction in thiol-terminated conjugated oligomers bound to gold electrodes.