Current and Recent Projects
- Porphyrin Wires For Nanoelectronics.
- The effect of environment, notably water, on molecular conductance.
- Single Molecule Conductance and Electrochemistry.
- Oligo-yne Single Molecule Wires.
- Thermal Effects in Single Molecule Bridges.
- Peptides Single Molecule Wires.
1. Porphyrin Wires For Nanoelectronics.
Collaborators: Harry Anderson (Oxford), Colin Lambert (Lancaster), Víctor M. García-Suárez (Universidad de Oviedo), Gita Sedghi (Liverpool), Martin Elliott and Emyr MacDonald (Cardiff).
Oligo-porphyrins can mediate electron transmission over unusually long distances with low attenuation, making them attractive targets as molecular wires. Thus they could be utilised in the electrical "wiring" of sub-10-nm electronic devices and also in photo-voltaic systems for charge transport and separation. We have been examining charge transport in oligo-porphyrins and the mechanism of conduction.
2. The effect of environment, notably water, on molecular conductance.
Collaborators: Colin Lambert and Iain Grace (Lancaster), Ed Leary (Madrid).
The medium is an integral component of electron transfer in condensed matter environments, such as biomolecules and conjugated polymers. It has recently become possible to assess the influence of medium in single molecule conductance experiments and to compare these experiments with detailed ab-initio computations which include the medium. We have demonstrated orders of magnitude effects associated with surrounding media for short thiophene oligomers and key results are shown in the figure below. This clearly demonstrates that single molecule conductance can be extremely sensitive to the environment and also highlights the convergence of experiment and theory.
Figure. Experimental (left) and theoretical (right) results for the single molecule conductance of thiophene oligomers in water and in the absence of water. The central pictures illustrate the oligothiophene (x = 1, 2, 3, 5) molecular bridge between gold contacts. This is a large effect. For x = 3, the presence or absence of environmental water changes the conductance by two orders of magnitude. By contrast, changing the redox state in molecules HS(CH2)6-RG-(CH2)6SH (RG = 4,4'–bipyridinium, pyrrolo-tetrathiafulvalene) changed the conductances by ca. a factor of 6, although larger changes have been seen for other fully-conjugated redox-active molecules. Theory reveals that H2O significantly increases the conductance of the longer molecules (1; x = 3, 5) by interacting with the p–clouds of the thiophene rings and shifting orbital energies, such that the frontier orbitals move closer to the Au Fermi energy. By cycling between 'wet' and dry argon we were able to switch the conductance for this molecule reversibly.
3. Single Molecule Conductance and Electrochemistry.
Collaborators: Jens Ulstrup (DTU, Denmark), Jan Jeppesen (Uni. Southern Denmark), Ed Leary (Madrid).
We pioneered the measurement of single molecule conductance in electrochemical environments. This has enabled detailed investigation of charge transfer through molecules in electrochemical environments. Large changes in molecule conductance can occur with electrode potential, but the form of these changes is very sensitive to the molecule structure and redox properties of the molecular wire. An example is shown below.
4. Oligo-yne Single Molecule Wires.
Collaborators: Martin Bryce (Durham), Santiago Martin (Zaragoza), Colin Lambert and Iain Grace (Lancaster).
Oligo-ynes can be considered as archetypal molecular wires due to their "simplicity" as they consist of a linear conjugated chain of carbon atoms. We have been studying the electrical properties of these "wires".
5. Thermal Effects in Single Molecule Bridges.
Collaborators: Wolfgang Haiss, Santiago Martin (Zaragoza), Richard Whitby (Southampton)
Temperature is a powerful variable in studies of charge transmission through molecular junctions since it gives access to quantitative determination of charge transport mechanisms. We have a number of projects in which we are examining thermal effects in single molecule conductance in condensed media, these include:
• Conformational gating of the molecular conductance in alkanedithiol.
• Temperature dependence of the conductance of porphyrin molecular wires (see "Porphyrin Wires" section above).
6. Peptides Single Molecule Wires.
Collaborators: Thomas Doneux (Brussels), Lisa Scullion (Liverpool), Laurent Bouffier (CRNS, Bordeaux).
We have been looking at the conductance of defined oligo-peptide molecular bridges. In one study we show a large conductance switching upon changes the pH which resulting from conformational changes to the molecule.
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