Correlation between ¹H Nuclear Magnetic Resonance Chemical Shifts and Tunneling Transport in Self-Assembled Monolayer-Based Molecular Junctions.

Abstract

We investigate the correlation between solution phase ¹H NMR chemical shifts δ and tunneling conductance <i>G</i> in molecular junctions based on self-assembled monolayers (SAMs). The SAM-forming molecules are a series of four substituted oligophenylene dithiols in which π-electron delocalization is systematically varied. Molecular junctions are formed using a conducting probe atomic force microscope (CP-AFM) to make soft contact to the SAMs on Au or Ag. We observe that <i>G</i> exhibits an exponential correlation with the chemical shift δ_β of the β-protons for each dithiol molecule, consistent with recently reported single-molecule studies. This <i>G</i>-δ_β sensitivity is qualitatively supported by highest occupied molecular orbital (HOMO) distribution calculations, which reflect the degree of π-electron delocalization. To further explore the underlying causes, we employ an analytical off-resonance tunneling model to extract key electronic density of states parameters from the junction current-voltage (<i>I</i>-<i>V</i>) characteristics. The extracted HOMO-Fermi level offset ε_h is nearly constant across the molecular series, consistent with commonly observed HOMO pinning in dithiol systems, and thus there is no correlation with δ_β. In contrast, the metal-HOMO electronic coupling Γ exhibits a strong exponential correlation with δ_β. Thus, we establish that the cause of the exponential <i>G</i>-δ_β correlation is the exponential Γ-δ_β correlation. We also find a linear correlation between δ_β and SAM work function change ΔΦ measured with a Kelvin probe. Combining the Γ-δ_β and ΔΦ-δ_β correlations demonstrates that Γ is exponentially correlated with ΔΦ, which measures interfacial charge transfer. First-principles calculations are necessary for thorough understanding of these correlations, but our results demonstrate that NMR chemical shifts, which reflect local atomic structure and electron densities in molecules, are a potentially powerful tool to understand connections between molecular structure, electron density, interfacial charge transfer, and electronic coupling in molecular junctions.