Correlated Energy-Level Alignment Effects Determine Substituent-Tuned Single-Molecule Conductance

TL;DR

This study uncovers how correlated energy-level effects influence conductance in single-molecule junctions, providing new principles for designing molecules with tunable electrical properties.

Contribution

It introduces a novel understanding of correlated energy-level effects as design principles for predicting and controlling single-molecule conductance.

Findings

Correlated energy-level effects significantly impact conductance.

Proposed design principles are broadly applicable across molecular series.

The principles explain conductance variability better than previous models.

Abstract

The rational design of single molecule electrical components requires a deep and predictive understanding of structure-function relationships. Here we explore the relationship between chemical substituents and the conductance of metal-single molecule-metal junctions, using functionalized oligophenylenevinylenes as a model system. Using a combination of mechanically controlled break-junction experiments and various levels of theory including non-equilibrium Green's functions, we demonstrate that the connection between gas-phase molecular electronic structure and in-junction molecular conductance is complicated by the involvement of multiple mutually correlated and opposing effects that contribute to energy level alignment in the junction. We propose that these opposing correlations represent powerful new "design principles," because their physical origins make them broadly applicable, and they are capable of predicting the direction and relative magnitude of observed conductance trends. In particular, we show that they are consistent with the observed conductance variability not just within our own experimental results, but also within disparate molecular series reported in literature and, crucially, with the trend in variability across these molecular series, which previous simple models fail to explain. The design principles introduced here can therefore aid in both screening and suggesting novel design strategies for maximizing conductance tunability in single-molecule systems.