Reversible Quinoid‐Diradical Inter‐Conversion in Single‐Molecule Junctions

A profound understanding of the reversible regulation mechanisms among multiple redox states of organic molecules is essential for further development of molecular switching devices. In this study, an oligo-aniline derived quinoidal molecular wire was designed and synthesized. The reversible inter-conversion processes between its initial (quinoid) and protonated (diradical) states were comprehensively investigated with optical measurements, and the EPR experiments confirmed the formation of radical species upon protonation. The single-molecule charge transport properties were then investigated using scanning tunneling microscopy break junction (STM-BJ) technique. It was found that the molecular wire O-ANI can serve as a reversible molecular switching process with ≈ 6.5-fold conductance variation through acid/base adjustments. Additionally, theoretical analyses elucidated the mechanism of the quinoid-diradical inter-conversion. The enhanced comprehension of the reversible quinoid-diradical inter-conversion at the single-molecule level provides new strategies for advancing the molecular switching materials and devices.