Modulating Ballistic Transport Across Organic Radical Molecular Wires

Integrating open-shell organic molecules with radical character into nanoscale junctions has been shown to promote more efficient quantum transport, particularly in longer molecule wires, with potential for next-generation electronic and spintronic devices. While much effort has been devoted to identifying potential molecular candidates, a fundamental and comprehensive understanding of how bridging conformations and external environmental factors, such as electric field and solvents, can modulate ballistic transport in such quantum systems is lacking. Here, we use the scanning tunneling microscope-based break junction (STMBJ) method to probe binding and electron transport through a class of oligophenylene group-bridged double triarylamine sulfide molecules (dTMeS) with two intrinsic radical centers within molecular backbone at room temperature. Measurements of neutral and diradical molecules synthesized <i>ex-situ</i> suggest that open-shell systems possess higher conductance than their original neutral analogs and can be generated <i>in situ</i> under an electric field in a non-polar solvent, such as 1-bromonaphthalene (BNP), without the addition of chemical oxidizing reagent. Unique binding behaviors are also observed, where radical character is quenched at early stages of junction evolution when the molecule is bound through the nitrogen embedded in the polycyclic end group but then regenerated under electric field with further junction elongation when binding switches to the thioether linker further from the radical. The polarity of solvents also plays a critical role in modulating ballistic transport by shifting the energetic alignments between molecular resonances and the gold Fermi level. Our study demonstrates the feasibility of rational tuning of transport properties in the single open-shell molecular quantum system through geometry modulation and environment control.