  Devices formed from single molecules attached to noble-metal electrodes
exhibit large conductance fluctuations, which inhibit their development as
reproducible functional units. We demonstrate that single molecules with planar
anchor groups attached to carbon-based electrodes are more resilient to
atomic-scale variation in the contacts and exhibit significantly lower
conductance fluctuations. We examine the conductance of a 2,6-dibenzylamino
core-substituted naphthalenediimide chromophore attached to carbon electrodes
by either phenanthrene anchors or more extended anchor groups, which include
oligophenylene ethynylene spacers. We demonstrate that for the more spatially
extended anchor groups conductance fluctuations are significantly reduced. The
current-voltage characteristic arising from long-range tunneling is found to be
strongly nonlinear with pronounced conductance suppression below a threshold
voltage of approximately 2.5 V.
