Hapticity-Dependent Charge Transport through Carbodithioate-Terminated [5,15-Bis(phenylethynyl)porphinato]zinc(II) Complexes in Metal–Molecule–Metal Junctions

Single molecule break junction experiments
and nonequilibrium Green’s
function calculations using density functional theory (NEGF-DFT) of
carbodithioate- and thiol-terminated [5,15-bis­(phenylethynyl)-10,20-diarylporphinato]­zinc­(II)
complexes reveal the impact of the electrode-linker coordination mode
on charge transport at the single-molecule level. Replacement of thiolate
(−S–) by the carbodithioate (−CS2–) anchoring motif leads to an order of
magnitude increase of single molecule conductance. In contrast to
thiolate-terminated structures, metal–molecule–metal
junctions that exploit the carbodithioate linker manifest three distinct
conductance values. We hypothesize that the magnitudes of these conductances
depend upon carbodithoate linker hapticity with measured conductances
across Au-[5,15-bis­(4′-(dithiocarboxylate)­phenylethynyl)-10,20-diarylporphinato]­zinc­(II)-Au
junctions the greatest when both anchoring groups attach to the metal
surface in a bidentate fashion. We support this hypothesis with NEGF-DFT
calculations, which consider the electron transport properties for
specific binding geometries. These results provide new insights into
the origin of molecule-to-molecule conductance heterogeneity in molecular
charge transport measurements and the factors that optimize electrode–molecule–electrode
electronic coupling and maximize the conductance for charge transport.