  The conductance of a single metal-molecule-metal junction depends critically
on the conformations of the molecule. In the simple case of a biphenyl, two
phenyl rings linked together by a single C-C bond, the conductance is expected
to depend on the relative twist angle between the two rings, with the planar
conformation having the highest conductance. A number of different techniques
have measured the conductance of metal-molecule(s)-metal junctions. However,
the conductance variation from junction to junction has made it difficult to
verify even the simplest predictions about how molecules should behave in
unimolecular devices. Here, using amine link groups to form single molecule
junctions, we show a clear correlation between molecule conformation and
junction conductance in a series of seven biphenyl molecules with different
ring substitutions that alter the twist angle of the molecules. We find that
the conductance for the series decreases with increasing twist angle,
consistent with a cosine squared relation predicted theoretically for transport
through pi-conjugated systems.
