Molecular assembly is crucial in functional molecular materials and devices. Among the molecular interactions that can form assemblies, stacking among π-conjugated molecular backbones plays an essential role in charge transport through organic materials and devices. The single-molecule junction technique allows for the application of an electric field of approximately 10 8 V/m to the nanoscale junctions and to investigate the electric field-induced assembly at the single-stacking level. Here, we demonstrate an electric field-induced stacking effect between two molecules using the scanning tunneling microscope break junction (STM-BJ) technique and we found an increase in the stacking probability with increasing intensity of the electric field. The combined density functional theory (DFT) calculations suggest that the molecules become more planar under the electric field, leading to the energetically preferred stacking configuration. Our study provides a new strategy for tuning molecular assembly by employing a strong electric field.

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The emerging of molecular spintronics offers a unique chance for the design of molecular devices with different spin-state, and the control of spin-state becomes essential for molecular spin switches. However, the intrinsic spin switching from lowspin to high-spin state is a temperature-dependent process with a small energy barrier that low temperature is required to maintain the low-spin state, and thus the roomtemperature operation of single-molecule devices have not yet been achieved. Here, we present a reversible single-molecule conductance switch by manipulating the spin states of the molecule at room temperature using the scanning tunneling microscope break-junction (STM-BJ) technique. The manipulation of the spin states between S = 0 and S = 1 is achieved by complexing or decomplexing the pyridine derivative molecule with the square planar nickel(II) porphyrin. The bias-dependent conductance evolution proved that the strong electric field between the nanoelectrodes plays a crucial role in the coordination reaction. The DFT calculations further revealed that the conductance changes come from the geometry change of the porphyrin ring and spin-state switching of Ni(II) ion. Our work provides a new avenue to investigate room-temperature spin-related sensors and molecular spintronics.

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