Electron tunneling through fluid solvents

Despite the biological, chemical, and physical importance of electron tunneling across noncovalent matrices, relatively little is known about the ability of the various nonbonding interactions (hydrogen-bonding and van der Waals forces) to mediate charge transfer. Herein, we report the steady-state current-voltage (I-V) profiles of nanometer junctions filled with water and a variety of organic solvents. The maximum currents for the solvents studied span 6 orders of magnitude. The I-V data can be reasonably fit to a simple electron tunneling model with a rectangular energy barrier representing the solvent. Protic solvents provide the smallest barrier heights (greatest tunneling currents), and nonpolar solvents exhibit the largest energy barriers (lowest currents). Trends in the barrier heights with the strength of the solvent-solvent interactions (hydrogen-bonding < dipole-dipole < dispersion interactions) indicate that the solvent's cohesive energy largely determines/limits the barrier heights of the fluid systems rather than the electronic structure of the solvent molecule (e.g., electron affinity or ionization potential). These results demonstrate that facile electron tunneling through nonbonding media must be accompanied by relatively strong intermolecular interactions.