Understanding electron transfer (ET) from a single molecule to another single molecule holds essential importance to realize bottom-up molecular devices in which constituent molecules are self-assembled via noncovalent interactions between each other. However, rather little is currently known about the ET properties at the single-molecule interface. Here we employ molecular tips to quantify the ET through a H-bond between single molecules. We found that a H-bond conducts electrons better than a covalent σ bond at short-range. Its conductance, however, decays steeply as the chain length of the H-bonded molecules increases. First-principle calculations were performed to reveal the electronic origin of the facile ET through the H-bond. Our results demonstrate that H-bonding in a molecular junction significantly affects its transport property.

Electron transfer through a noncovalent interaction bears essential relevance to the functions of bottom-up supramolecular assembly. However, rather little knowledge regarding such phenomena at the single-molecule level is currently available. Herein we report the direct quantification of electron-transfer processes for a single noncovalently linked porphyrin-fullerene dyad. Facilitated electron transfer via a charge-transfer interaction in-between was successfully measured by utilizing a fullerene molecular tip. The rectification property of the supramolecular assembly was determined and quantitatively assessed. The present study opens up a way to explore quantitatively the rich electronic properties of supramolecules at the single-molecule level.

Molecular tips were used to investigate electron transfer through metal-coordination bonds between single molecules. Coordination of a single metal ion to two carboxylate-terminated thiolate molecules formed a sandwich-type single molecular junction. It was found that a favorable charge transfer is induced through such molecular junctions. The electron transfer facilitated by metal coordination was utilized to implement conductance switching in a molecular junction of a head-to-head pyridine dimer. The present research offers a method to control electronic functions required in the construction of functional electronic molecular devices.

Single-stranded DNA was utilized as a probe tip for single-molecule DNA detection. Hybridization of the DNA tip and target DNA induces electron tunneling through the resulting DNA duplex. It is demonstrated that the DNA tip allows not only genetic detection but also discovery of single-nucleotide polymorphisms at the single-molecule level.

A DNA molecule was utilized as a probe tip to achieve single-molecule genetic diagnoses. Hybridization of the probe and target DNAs resulted in electron tunneling along the emergent double-stranded DNA. Simple stationary monitoring of the tunneling current leads to single-molecule DNA detection and discovery of base mismatches and methylation.