Porphyrin-based molecular wires as promising candidates for nanoelectronic devices have attracted much attention. Therefore, it is fundamentally important to investigate structure−electrical properties involving such molecules. Herein, a series of 5-ethynyl-2,3dihydrobenzo[b]thiophene-substituted free-metal porphyrins and metalloporphyrins have been synthesized. Rigid, structurally well-defined, and highly conjugated porphyrin-based molecular wires offer a good platform to explore the impact of coordinated metal ions (Cu, Fe, and Zn) on charge transport at a single-molecule scale. Using the scanning tunneling microscope breakjunction (STM-BJ) technique, it is found that single-molecule conductance can change by nearly 500% when the central coordinated metal ions are changed. The theoretically simulated energy-dependent transmission spectra reveal that the spin state of FeTPP can tune charge transport at E = E F , and transmission coefficient T(E) is well correlated with the experimental observation. Our work proves a pronounced metal ion effect on charge transport through a porphyrin plane, providing guidance to design high-performance porphyrin-based nanodevices.

Quantum interference (QI) in single molecular junctions shows a promising perspective for realizing conceptual nanoelectronics. However, controlling and modulating the QI remains a big challenge. Herein, two-type substituents at different positions of meta-linked benzene, namely electron-donating methoxy (-OMe) and electron-withdrawing nitryl (-NO2), are designed and synthesized to investigate the substituent effects on QI. The calculated transmission coefficients T(E) indicates that -OMe and -NO2 could remove the antiresonance and destructive quantum interference (DQI)-induced transmission dips at position 2. -OMe could raise the antiresonance energy at position 4 while -NO2 groups removes the DQI features. For substituents at position 5, both of them are nonactive for tuning QI. The conductance measurements by scanning tunneling microscopy break junction (STM-BJ) show a good agreement with the theoretical prediction. More than two order of magnitude single-molecule conductance on/off ratio could be achieved at the different positions of -NO2 substituent groups at room temperature. The present work proves chemical substituents can be used for tuning QI features in single molecular junctions, which provides a feasible way toward realization of high-performance molecular devices.

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