STM CONTRAST, ELECTRON-TRANSFER CHEMISTRY, AND CONDUCTION IN MOLECULES

We have used a series of metalloporphyrin compounds to test for a rela tionship between the: contrast of STM images and the electrochemical p roperties of the molecules. Molecules were tethered to a gold (111) su rface by means of an isothiocyano linkage and both images and current- voltage (I-V) curves were obtained with the sample submerged in oxygen -free mesitylene. The contrast of the reducible molecules changed stro ngly with bias, and the corresponding I-V curves were highly asymmetri c. The derivative of the se curves (d/dV) had a Gaussian-shaped peak a t a voltage characteristic of the compound, although local measurement s showed that there was considerable variation in this value from mole cule to molecule of a given compound. These bias-dependent features we re not observed in the less electroactive molecules, so the STM is cap able 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 t hese observations.