Length-dependent conductance of oligothiophenes.

We have measured the single-molecule conductance of a family of oligothiophenes comprising 1-6 thiophene moieties terminated with methyl-sulfide linkers using the scanning tunneling microscope-based break-junction technique. We find an anomalous behavior: the peak of the conductance histogram distribution does not follow a clear exponential decay with increasing number of thiophene units in the chain. The electronic properties of the materials were characterized by optical spectroscopy and electrochemistry to gain an understanding of the factors affecting the conductance of these molecules. We postulate that different conformers in the junction are a contributing factor to the anomalous trend in the observed conductance as a function of molecule length.