  If the factors controlling the decay in single-molecule electrical
conductance G with molecular length L could be understood and controlled, then
this would be a significant step forward in the design of high-conductance
molecular wires. For a wide variety of molecules conducting by phase coherent
tunneling, conductance G decays with length following the relationship G =
Aexp-\b{\eta}L. It is widely accepted that the attenuation coefficient \b{\eta}
is determined by the position of the Fermi energy of the electrodes relative to
the energy of frontier orbitals of the molecular bridge, whereas the terminal
anchor groups which bind to the molecule to the electrodes contribute to the
pre-exponential factor A. We examine this premise for several series of
molecules which contain a central conjugated moiety (phenyl, viologen or
{\alpha}-terthiophene) connected on either side to alkane chains of varying
length, with each end terminated by thiol or thiomethyl anchor groups. In
contrast with this expectation, we demonstrate both experimentally and
theoretically that additional electronic states located on thiol anchor groups
can significantly decrease the value of \b{eta}, by giving rise to resonances
close to EF through coupling to the bridge moiety. This interplay between the
gateway states and their coupling to a central conjugated moiety in the
molecular bridges creates a new design strategy for realising
higher-transmission molecular wires by taking advantage of the
electrode-molecule interface properties.
