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

Area of Science:

• Molecular Electronics
• Organic Chemistry
• Quantum Transport

Background:

• Conventional molecular charge transport relies on π-delocalization.
• Nonbonded atoms offer alternative pathways for charge transport.
• Iodine-substituted benzenes are model systems for studying substituent effects.

Purpose of the Study:

• To investigate how iodine substitution patterns influence charge transport mechanisms in single-molecule junctions.
• To elucidate the transition from π-dominated to σ-involved transport.
• To establish design principles for accessing nonbonded σ-delocalized channels.

Main Methods:

• Fabrication and measurement of single-molecule junctions using break-junction techniques.
• Systematic variation of iodine substitution on benzene rings (di-, tetra-, and hexaiodobenzene).
• Analysis of conductance, current-voltage (I-V) characteristics, and thermopower.
• Supporting molecular-orbital calculations.

Main Results:

• Para-diiodobenzene shows conventional π-HOMO transport.
• Meta-diiodobenzene exhibits destructive quantum interference.
• Ortho-diiodobenzene does not form stable junctions due to geometric constraints.
• Tetraiodobenzene shows limited conductance enhancement, lacking a continuous σ-framework.
• Hexaiodobenzene displays contact-insensitive conductance and positive thermopower, indicating σ-involved HOMO-mediated transport.

Conclusions:

• σ-involved HOMO-mediated transport requires a complete peripheral iodine ring.
• Substitution pattern critically controls the balance between π and σ transport pathways.
• Provides a strategy for designing molecular junctions with tunable charge transport properties.