The characteristics of molecular electronic devices are critically determined by metal-organic interfaces, which influence the arrangement of the orbital levels that participate in charge transport.Studies on self-assembled monolayers (SAMs) show (molecule-dependent) level shifts as well as transport-gap renormalization, suggesting that polarization effects in the metal substrate play a key role in the level alignment with respect to the metal's Fermi energy. Here, we provide direct evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport in single porphyrin-type molecules using electrically gateable break junctions.In this set-up, the position of the occupied and unoccupied levels can be followed in situ and with simultaneous mechanical control. When increasing the electrode separation, we observe a substantial increase in the transport gap with level shifts as high as several hundreds of meV for displacements of a fewÅngstroms. Analysis of this large and tunable gap renormalization with image-charge calculations based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions.

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Different bridging geometries can explain the observation that porphyrin molecules with added thiol end groups and pyridine axial groups form more‐stable single‐molecule junctions with an increased spread in low‐bias conductance. The stability of these geometries is demonstrated by time‐dependent conductance measurements. In contrast, rodlike molecules show one preferential binding geometry.

SummaryWe have investigated charge transport in ZnTPPdT–Pyr (TPPdT: 5,15-di(p-thiolphenyl)-10,20-di(p-tolyl)porphyrin) molecular junctions using the lithographic mechanically controllable break-junction (MCBJ) technique at room temperature and cryogenic temperature (6 K). We combined low-bias statistical measurements with spectroscopy of the molecular levels in the form of I(V) characteristics. This combination allows us to characterize the transport in a molecular junction in detail. This complex molecule can form different junction configurations, having an observable effect on the trace histograms and the current–voltage (I(V)) measurements. Both methods show that multiple, stable single-molecule junction configurations can be obtained by modulating the interelectrode distance. In addition we demonstrate that different ZnTPPdT–Pyr junction configurations can lead to completely different spectroscopic features with the same conductance values. We show that statistical low-bias conductance measurements should be interpreted with care, and that the combination with I(V) spectroscopy represents an essential tool for a more detailed characterization of the charge transport in a single molecule.

Unterschiedliche Brückengeometrien können die Beobachtung erklären, dass Porphyrinmoleküle mit Thiolendgruppen und axialen Pyridingruppen stabilere Einzelmolekülkontakte mit einer größeren Spanne der Ruheleitfähigkeit bilden. Die Stabilität dieser Geometrien wurde in zeitabhängigen Leitfähigkeitsmessungen gezeigt. Stäbchenförmige Moleküle dagegen haben eine bevorzugte Bindungsgeometrie.