STM Contrast, Electron-Transfer Chemistry, and Conduction in Molecules

We have used a series of metalloporphyrin compounds to test for a relationship between the contrast of STM imagesandtheelectrochemicalpropertiesofthemolecules. Moleculesweretetheredtoagold(111)surface by means of an isothiocyano linkage and both images and current-voltage (I-V) curves were obtained with thesamplesubmergedinoxygen-freemesitylene. Thecontrastofthereduciblemoleculeschangedstrongly withbias,andthecorrespondingI-Vcurveswerehighlyasymmetric. Thederivativeofthesecurves(dI/dV) had a Gaussian-shaped peak at a voltage characteristic of the compound, although local measurements showed that there was considerable variation in this value from molecule to molecule of a given compound. These bias-dependent features were not observed in the less electroactive molecules, so the STM is capable of distinguishing electroactive molecules from non-electroactive molecules as we demonstrate with images of mixed films. We discuss one- and two-step electron-transfer mechanisms which are consistent with these observations.