  We demonstrate that the conductance switching of benzo-bis(imidazole)
molecules upon protonation depends on the lateral functional groups. The
protonated H-substituted molecule shows a higher conductance than the neutral
one (Gpro>Gneu), while the opposite (Gneu>Gpro) is observed for a molecule
laterally functionalized by amino-phenyl groups. These results are demonstrated
at various scale lengths : self-assembled monolayer, tiny nanodot-molecule
junction and single molecules. From ab-initio theoretical calculations, we
conclude that for the H-substituted molecule, the result Gpro>Gneu is correctly
explained by a reduction of the LUMO-HOMO gap, while for the amino-phenyl
functionnalized molecule, the result Gneu>Gpro is consistent with a shift of
HOMO, which reduces the density of states at the Fermi energy.
