A conducting atomic force microscope was used to measure the electrical properties of carotenoid molecules attached to a gold electrode. The thiolated carotene molecules were embedded in insulating n-alkanethiol self-assembled monolayers. At a contact force of a few nanoNewtons, a carotenoid molecule behaves ohmically with a resistance of approximately 4.2 ± 0.7 GΩ, over a million times more conductive than an alkane chain of similar length. Modes of electron transport are discussed.

We have used a series of metalloporphyrin compounds to test for a relationship between the contrast of STM images and the electrochemical properties of the molecules. Molecules were tethered to a gold (111) surface 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 strongly with bias, and the corresponding I−V curves were highly asymmetric. The derivative of these curves (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.

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