We report on the experimental analysis of the charge transport through single-molecule junctions of the open and closed isomers of photoswitching molecules. Sulfur-free diarylethene molecules are developed and studied via electrical and optical measurements as well as density functional theory calculations. The single-molecule conductance and the current-voltage characteristics are measured in a mechanically controlled break-junction system at low temperatures. Comparing the results with the single-level transport model, we find an unexpected behavior of the current-dominating molecular orbital upon isomerization. We show that both the side chains and end groups of the molecules are crucial to understand the charge transport mechanism of photoswitching molecular junctions.

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A combined experimental and theoretical analysis of the charge transport through single-molecule junctions is performed to define the influence of molecular end groups for increasing electrode separation. For both amine-ended and thiol-ended octanes contacted to gold electrodes, we study signatures of chain formation by analyzing kinks in conductance traces, the junction length, and inelastic electron tunneling spectroscopy. The results show that for amine-ended molecular junctions no atomic chains are pulled under stretching, whereas the Au electrodes strongly deform for thiol-ended molecular junctions. This advanced approach hence provides unambiguous evidence that the amine anchors bind only weakly to Au.

Here we present a theoretical study of the thermoelectric transport through [2,2]paracyclophane-based singlemolecule junctions. Combining electronic and vibrational structures, obtained from density functional theory (DFT), with nonequilibrium Green's function techniques allows us to treat both electronic and phononic transport properties at a first-principles level. For the electronic part, we include an approximate self-energy correction, based on the DFT+ approach. This enables us to make a reliable prediction of all linear response transport coefficients entering the thermoelectric figure of merit ZT . Paracyclophane derivatives offer a great flexibility in tuning their chemical properties by attaching different functional groups. We show that, for the specific molecule, the functional groups mainly influence the thermopower, allowing us to tune its sign and absolute value. We predict that the functionalization of the bare paracyclophane leads to a largely enhanced electronic contribution Z el T to the figure of merit. Nevertheless, the high phononic contribution to the thermal conductance strongly suppresses ZT . Our work demonstrates the importance to include the phonon thermal conductance for any realistic estimate of the ZT for off-resonant molecular transport junctions. In addition, it shows the possibility of a chemical tuning of the thermoelectric properties for a series of available molecules, leading to equally performing hole-and electron-conducting junctions based on the same molecular framework.