Determining Charge Transport Pathways through Single Porphyrin Molecules Using Scanning Tunneling Microscopy Break Junctions

Charge transport in a porphyrin with four identical pyridyl substituents, 5,10,15,20-tetra­(4-pyridyl)-21<i>H</i>,23<i>H</i>-porphine (TPyP), was investigated using the scanning tunneling microscopy break junction method. To determine the dominant pathway, we studied two structurally similar porphyrins, <i>o</i>-DPyP and <i>p</i>-DPyP. Our experiments reveal that charge transport through TPyP in a break junction configuration does not follow the traditional assumption, i.e., the shortest path between the neighboring side groups. Instead, the charge transport pathway was dominated by the farthest anchoring groups. Furthermore, these single molecule experiments can distinguish between the two structural isomers, which is important in molecular discrimination, porphyrin chemistry, and molecular electronics

Effect of Anchoring Groups on Single Molecule Charge Transport through Porphyrins

Controlling charge transport through individual molecules and further understanding the effect of anchoring groups on charge transport are central themes in molecule-based devices. However, in most anchoring effect studies, only two, or at most three nonthiol anchoring groups were studied and compared for a specific system, i.e., using the same core structure. The scarcity of direct comparison data makes it difficult to draw unambiguous conclusions on the anchoring group effect. In this contribution, we focus on the single molecule conductance of porphyrins terminated with a range of anchoring groups: sulfonate (−SO₃^–), hydroxyl (−OH), nitrile (−CN), amine (−NH₂), carboxylic acid (−COOH), benzyl (−C₆H₆), and pyridyl (−C₆H₅N). The present study represents a first attempt to investigate a broad series of anchoring groups in one specific molecule for a direct comparison. It also is the first attempt, to our knowledge, to explore single molecule conductivity with two novel anchoring groups sulfonate (−SO₃^–) and hydroxyl (−OH). Our experimental results reveal that the single molecule conductance values of the porphyrins follow the sequence of pyridyl > amine > sulfonate > nitrile > carboxylic acid. Electron transport calculations are in agreement that the pyridyl groups result in higher conductance values than the other groups, which is due to a stronger binding interaction of this group to the Au electrodes. The finding of a general trend in the effect of anchoring groups and the exploration of new anchoring groups reported in this paper may provide useful information for molecule-based devices, functional porphyrin design, and electron transfer/transport studies

From Redox Gating to Quantized Charging

Electron transport characteristics were studied in redox molecule-modified tunneling junctions Au(111)|6-thiohexanoylferrocene (Fc6)|solution gap|Au STM tip in the absence and in the presence of gold nanoclusters employing an electrochemical STM setup. We observed transistor- and diode-like current−voltage responses accounted for by the redox process at the ferrocene moiety. We demonstrate that the reorganization energy of the redox site decreases with decreasing gap size. As a unique new feature, we discovered the formation of uniform (size ∼2.4 nm) gold nanoparticles, upon multiple oxidation/reduction cycles of the Fc6 adlayer. The immobilized nanoparticles modify the electron transport response of the Fc6 tunneling junctions dramatically. On top of embedded single nanoparticles we observed single-electron Coulomb charging signatures with up to seven narrow and equally spaced energy states upon electrochemical gating. Our results demonstrate the power of the electrochemical approach in molecular electronics and offer a new perspective toward two-state and multistate electronic switching in condensed media at room temperature

Quasi-Ohmic Single Molecule Charge Transport through Highly Conjugated <i>meso</i>-to-<i>meso</i> Ethyne-Bridged Porphyrin Wires

Understanding and controlling electron transport through functional molecules are of primary importance to the development of molecular scale devices. In this work, the single molecule resistances of <i>meso</i>-to-<i>meso</i> ethyne-bridged (porphinato)­zinc­(II) structures (<b>PZn</b>_{<b><i>n</i></b>} compounds), connected to gold electrodes via (4′-thiophenyl)­ethynyl termini, are determined using scanning tunneling microscopy-based break junction methods. These experiments show that each α,ω-di­[(4′-thiophenyl)­ethynyl]-terminated <b>PZn</b>_{<b><i>n</i></b>} compound (<b>dithiol-PZn</b>_{<b><i>n</i></b>}) manifests a dual molecular conductance. In both the high and low conductance regimes, the measured resistance across these metal–<b>dithiol-PZn</b>_{<b><i>n</i></b>}–metal junctions increases in a near linear fashion with molecule length. These results signal that <i>meso</i>-to-<i>meso</i> ethyne-bridged porphyrin wires afford the lowest β value (β = 0.034 Å^–1) yet determined for thiol-terminated single molecules that manifest a quasi-ohmic resistance dependence across metal–<b>dithiol-PZn</b>_{<b><i>n</i></b>}–metal junctions

Single-Molecule Charge Transport and Electrochemical Gating in Redox-Active Perylene Diimide Junctions

A series of redox-active perylene tetracarboxylic diimide (PTCDI) derivatives have been synthesized and studied by electrochemical cyclic voltammetry and electrochemical scanning tunnelling microscopy break junction techniques. These PTCDI molecules feature the substitution of pyrrolidine at the bay (1,7-) position of perylene and are named pyrrolidine-PTCDIs. These moieties exhibit a small bandgap (2.1 eV) compared with the “normal” (unsubstituted) PTCDI molecule (2.5 eV). Pyrrolidine-PTCDIs were functionalized with different anchoring groups (thiol, amine, pyridine) for building metal–molecule–metal (m–M–m) junctions. The single-molecule conductance values of pyrrolidine-PTCDIs have been determined by analyzing a large number of molecular (m–M–m) junctions created between an STM tip and substrate using a statistical method. Furthermore, we studied the gate dependence of the single-molecule conductance by trapping a molecule between the two electrodes and recording the current as a function of electrochemical gate potential. The experimentally determined conductance values for these bay-substituted pyrrolidine-PTCDI molecules are about twice as much as the unsubstituted PTCDI molecules. The present work shows that single-molecule conductance can be tuned by the bandgap of a molecular system without significantly altering the conductance pathway

Nickel tungstate (NiWO4) nanoparticles/graphene composites: preparation and photoelectrochemical applications

Nickel tungstate/graphene composite was synthesized in various compositions with application of a hydrothermal method. Chemical composition and morphology of each sample was studied via application of x-ray diffraction and transmission electron microscopy techniques. In the continuous, a photosystem was obtained by deposition of composite sample on a fluorine-doped tin oxide electrode with application of electrophoretic method. Electrode morphology was studied by employment of atomic force microscopy and SEM techniques. Eventually, light conversion properties and involved mechanism of fabricated photosystem was studied with application of the Mott–Schottky method. Our results confirmed that the optimum ratio between graphene and nickel tungstate is in the regime of 1:1