Measurement of the conductance of single conjugated molecules

Electrical conduction through molecules depends critically on the delocalization of the molecular electronic orbitals and their connection to the metallic contacts. Thiolated (-SH) conjugated organic molecules are therefore considered good candidates for molecular conductors(1,2): in such molecules, the orbitals are delocalized throughout the molecular backbone, with substantial weight on the sulphur - metal bonds(1-4). However, their relatively small size, typically similar to 1 nm, calls for innovative approaches to realize a functioning single-molecule device(5-11). Here we report an approach for contacting a single molecule, and use it to study the effect of localizing groups within a conjugated molecule on the electrical conduction. Our method is based on synthesizing a dimer structure, consisting of two colloidal gold particles connected by a dithiolated short organic molecule(12,13), and electrostatically trapping it between two metal electrodes. We study the electrical conduction through three short organic molecules: 4,4' - biphenyldithiol (BPD), a fully conjugated molecule; bis( 4-mercaptophenyl)- ether (BPE)(14), in which the conjugation is broken at the centre by an oxygen atom; and 1,4-benzenedimethanethiol (BDMT), in which the conjugation is broken near the contacts by a methylene group. We find that the oxygen in BPE and the methylene groups in BDMT both suppress the electrical conduction relative to that in BPD.