From π to σ: Enhanced Charge Transport in Iodine-Substituted Benzene Junctions

Understanding how nonbonded atoms contribute to charge transport offers a pathway to molecular conduction mechanisms beyond conventional π-delocalization. Iodine-substituted benzenes provide a simple and structurally well-defined platform in which halogen-metal anchoring and potential σ-type interactions can be modulated solely by substituent number and topology. Here we investigate p-, m-, and o-diiodobenzene, 1,2,4,5-tetraiodobenzene, and hexaiodobenzene to elucidate how substitution patterns regulate the balance between π-dominated and σ-involved transport in single-molecule junctions. Break-junction measurements reveal conventional π-HOMO transport in the para isomer, destructive quantum interference in the meta isomer, and the absence of stable junction formation in the ortho isomer, where adjacent iodine atoms create a very short and geometrically constrained effective junction length. The tetraiodo derivative shows only modest conductance enhancement, indicating that partial substitution does not generate a continuous σ-framework. In contrast, hexaiodobenzene exhibits a single, narrow, and contact-insensitive conductance peak and positive thermopower, and the corresponding I-V analysis yields a HOMO level only ∼0.9 eV from the Fermi level. Together with molecular-orbital calculations, these results lead us to conclude that σ-involved HOMO-mediated transport emerges only when a complete peripheral iodine ring is present. By establishing a substituent-number-controlled transition from π-dominated to σ-involved transport in a chemically simple series, this work provides a concise design principle for accessing nonbonded σ-delocalized channels in aromatic molecular junctions.