Tuning the electrical conductance of metalloporphyrin supramolecular wires

In contrast with conventional single-molecule junctions, in which the current flows parallel to the long axis or plane of a molecule, we investigate the transport properties of M(II)-5,15-diphenylporphyrin (M-DPP) single-molecule junctions (M=Co, Ni, Cu, or Zn divalent metal ions), in which the current flows perpendicular to the plane of the porphyrin. Novel STM-based conductance measurements combined with quantum transport calculations demonstrate that current-perpendicular-to-the-plane (CPP) junctions have three-orders-of-magnitude higher electrical conductances than their current-in-plane (CIP) counterparts, ranging from 2.10−2 G0 for Ni-DPP up to 8.10−2 G0 for Zn-DPP. The metal ion in the center of the DPP skeletons is strongly coordinated with the nitrogens of the pyridyl coated electrodes, with a binding energy that is sensitive to the choice of metal ion. We find that the binding energies of Zn-DPP and Co-DPP are significantly higher than those of Ni-DPP and Cu-DPP. Therefore when combined with its higher conductance, we identify Zn-DPP as the favoured candidate for high-conductance CPP single-molecule devices.


Theory
To accurately calculate the binding energies , basis set superposition errors were avoided by retaining 'ghost states' as prescribed in the counterpoise method [23,24], using the formula In this expression the total bound state energy of entity A bound to B is , the total energy of B in the presence of the ghost states is and the total energy of A in the presence of the ghost states is .

Experiments.
All glassware and PTFE STM-cells were cleaned with piranha solution (1:1 H2SO4/H2O2 by volume) before usage followed by rinsing with 18 MΩ cm−1 Milli-Q water (Millipore). An Au (111) single crystal substrate (10 mm x 1 mm) of 99.9999% purity and orientation accuracy < 0.1 degrees was purchased from MaTeck (Germany). Before each experiment, the single crystal Au (111)  Single-molecule conductance experiments in the absence of porphyrin molecules and in the presence of the empty DPP are presented in Figure S3a and b respectively. The former show no molecular junction events whereas the latter show no high conductance peak, displaying the two low conductance features associated of the interaction between PY and porphyrin backbone27.

Materials and Methods
The 5,15-diphenylporphyrin (DPP) was purchased from Frontier Scientific. The Ni(OAc)2·4H2O and Cu(OAc)2·H2O salts were purchased from Sigma-Aldrich and used as received. The IR spectra were recorded on a Shimazu FTIR-8300 spectrophotometer. The 1H-NMR (300 MHz) spectra were recorded on a BRUKER ARX 300 spectrometer. MALDI-TOF-MS mass spectra were recorded in the positive ion mode using a Bruker Ultraflex MALDI-TOF/TOF spectrometer. UV-Vis spectra were recorded on a Varian Cary 5000 UV-Vis-NIR spectrophotometer.

Technical details of the single-molecule transport measurements
Dynamic STM-break junction approach. Details about the STM-break junction (STM-BJ) technique have been published elsewhere.S1 All the conductance measurements were carried out with a mechanically and electronically isolated PicoSPM II microscope head controlled by a Picoscan 2500 electronics (all from Agilent, USA) and using a homemade PTFE-STM cell. Data captures were acquired using a NI-DAQmx/BNC-2110 National Instruments (LabVIEW data acquisition System, USA) and analyzed with LabVIEW code. The procedure of a typical break-junction experiment is based on first bringing the STM tip to a tunneling distance over a flat Au (111) surface area. The STM feedback is then turned off and the tip is driven into and out of contact with the substrate at a speed of 1-2V/s. These 2-points feedback loop is used to collect thousands of current decays (5000-6000). Single molecule conductance (G) was determined using the expression G=Istep/UBIAS, where I is the current and U is the voltage potential difference between the two electrodes. The current decays are accumulated to semi-logarithmic conductance histograms. The observed plateaus in the individual current decays result in the observed peaks in the conductance histograms and provide an averaged value of the single-molecule conductance. Transient curves that are either noisy or that showed smooth exponential decay due to the absence of molecular bridge formation were rejected when building the histograms using an automatic selection procedure driven by a LabVIEW code. The histograms were compiled by applying the same automated selection criteria to each set of all the recorded decay curves.
The percentage of decay curves that showed plateau features were typically 10-15 % and were all selected to build the histograms. This selection process make peaks in the conductance histograms protrude above the tunneling background and allow a quantitative measure of the yield of molecular junction formation in all of the measured series.
Static blinking approach. In order to study the interaction of the PY with the metal or with the porphyrin skeleton, current transient captures were carried out at a fixed distance such that spontaneous formation of molecular junctions is attained. Briefly, imposing a set point tunneling current first sets an initial distance between the functionalized electrodes. The STM feedback is then turned off and the tunneling current is recorded as a function of time. When the molecular junction is spontaneously formed, the current abruptly increases S2, S3. The observed current "jumps" are referred to as "blinks". The blinks typically last for a short period of time, after which the current suddenly drops returning to the set point tunneling current due to the spontaneous breakdown of the molecular junction. A representative blink for the Co-DPP capture is shown in Figure S3b.

Synthesis of Ni-DPP
To a solution of 100 mg (216 mmol) of DPP in dry DMF (30 mL) was added 128 mg (707 mmol) of Ni(OAc)2·4H2O and the mixture was stirred at 120 oC for 6h and then at room temperature overnight.
After this time, the solvent was removed and the residue was extracted with brine/hexane and the organic phase was dried with MgSO4. Evaporation of the volatiles at reduced pressure gave a crude product, which was crystallized with CH2Cl2 and MeOH, to obtain a red solid.  Control blinking experiments also shows that only when the three components (the two pyridinyl and the Porphyrin) are present a molecular electrical connection (bridge) can be established.  Figure S7. 2D blinking map in absence of the porphyrin molecules with both tip and surface functionalized with pyridinyl groups. All traces were set into a common time origin and baseline.