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Journal of the American Chemical Society
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Figure 1. (a) Schematic of an NHC-bound single-molecule junction created via in situ electrochemical conversion of a metal NHC complex. (b)
Synthetic strategies employed to prepare NHC-bound SAMs for subsequent STM-BJ measurements in air (solution, vapor). These proved
unsuccessful, in contrast to the use of metal NHC complexes to form junctions directly from solution (in situ).
the electronic structure and transmission spectra, respectively.
Transmission eigenchannels were computed at the center of the
Brillouin zone.29
decay of conductance, indicating a coherent nonresonant
transport mechanism. These results establish our approach as a
robust yet flexible strategy to form NHC-linked molecular
junctions.
RESULTS AND DISCUSSION
■
EXPERIMENTAL SECTION
Our first objective was to measure the transport properties of a
linear aromatic molecule utilizing an NHC as the contacting
group. To achieve this goal, we exploited a new in situ
electrochemical-reduction approach to form the single-
molecule junction using the STM-BJ technique (Figure 1a).
For this method, the molecule is terminated by an aurophilic
thiomethyl group (SMe) on one end and a benzannulated
NHC on the other end. The NHC is capped with one of a
series of metal chloride complexes; the resulting air-stable
metal NHC precursors are labeled NHC1−MCl (M = Au, Ag,
Cu). Synthetic details are included in the Experimental
Section. A single-molecule junction is established when the
SMe binds to the electrode surface via an Au−S bond, and the
metal NHC complex is reduced at the STM tip. We have
developed this innovative strategy because we were unable to
characterize the single-molecule transport characteristics of
NHCs bound in SAMs prepared using traditional methods,
shown in Figure 1b. Indeed, STM-BJ measurements of
monolayers assembled on Au surfaces from solutions of free
NHCs, or from the vapor phase using NHC−CO2 adduct
precursors,12,30 did not yield reproducible conductance results.
Figure 2a presents the molecular structure of the precursors
NHC1−CuCl, NHC1−AgCl, and NHC1−AuCl, as deter-
mined by single-crystal X-ray diffraction (SCXRD). Details of
the STM-BJ measurements in an ionic environment have been
described previously31 and are outlined in the Experimental
Section. Briefly, the measurements are performed using an
insulated Au STM tip, with an exposed area of ∼1 μm2, and an
Au substrate with an area larger than 1 cm2. When a bias is
applied, a dense double layer of charge builds up around the
small area of the coated tip while a sparse double layer is
formed on the large-area, uncoated substrate. Previous studies
have demonstrated that, under these conditions, redox-active
compounds such as ferrocene32 or molecular clusters33 can be
oxidized or reduced in the junction under positive or negative
tip biases, respectively.
■
STM-BJ Details. Conductance measurements were carried out
using the STM-BJ technique, which has been described in detail
elsewhere.23 Conductance measurements for all complexes were
performed using 10 μM solutions in PC. The Au tip was coated with
an insulating layer to suppress background ionic current. The
insulated tips were created by driving a mechanically cut gold tip
through Apiezon wax.24 1D conductance histograms were constructed
using logarithmic bins (100 per decade) without any data selection.
Synthesis of NHC1−AuCl. The NHC1−AuCl complex was
synthesized using a modified literature procedure.25 NHC1−HI (45
suspended in 8 mL of THF. KOtBu (12 mg, 110 μmol) was added to
the suspension, and the reaction was stirred for 1 h. The mixture was
then filtered through Celite, and (SMe)2AuCl (31 mg, 105 μmol) was
added to the filtrate. The reaction was stirred for 3 h, protected from
light. Activated carbon (∼100 mg) was added to the reaction, which
was stirred for 1 h and then filtered through Celite. The solvent was
then removed in vacuo. The solid residue was redissolved in DCM
and filtered, and the solvent was removed in vacuo to yield the pure
complex NHC1−AuCl as a light-yellow powder (yield = 48 mg,
86%). Analogous procedures were used to prepare the other NHCn−
MCl complexes.
Computational Details. The electronic and transport properties
of molecular junctions were calculated using first-principles methods
based on DFT-NEGF.26,27 We modeled the junction by attaching the
molecular backbone on the carbene side to an Au adatom for NHCn−
AuCl, an Ag adatom for NHC1−AgCl, and a Cu adatom for NHC1−
CuCl (without including the Cl atom). To simplify geometry
optimization, the isopropyl groups were replaced with methyl groups,
resulting in negligible changes in conductance. The SMe side was
attached to an Au adatom for all junctions. The supramolecular
structure was placed between two electrodes that consisted of a 4 × 4
face-centered cubic (fcc) Au(111) surface. We optimized the junction
structure by relaxing the coordinates of the molecular backbone and
the Au atoms to a force threshold smaller than 0.02 eV/Å, also
optimizing the interelectrode vertical separation. In the calculations,
the exchange correlation was described using the GGA approximation
in the PBE implementation.28 We used a double-ζ polarized local-
orbital basis for molecular atoms and a single-ζ basis for Au atoms.
We performed subsequent transport calculations at optimized
geometries by adding additional Au(111) layers. We used
Monkhorst−Pack grids of 5 × 5 × 1 and 15 × 15 × 1 for calculating
When we use this approach with NHC1−MCl in propylene
carbonate (PC), we can form electrically conducting NHC-
bound single-molecule junctions by applying a negative (i.e.,
B
DOI: 10.1021/jacs.8b05184
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX