  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.
