We began by studying the molecular conductance properties of Rn (Fig. ) using a custombuilt scanning tunneling microscope break-junction (STM-BJ) instrument, as previously described. STM-BJ has been used in prior work to study single-molecule charge transport as a function of backbone length, chemical substituents, molecular conformation, and anchor-electrode contacts. Prior work has shown that alkyl side chains alter backbone conformation, thereby affecting molecular conductance. Here, we used STM-BJ to systematically understand the role of side chain length and composition in the molecular library generated by automated synthesis. Using this approach, we determined the molecular conductance of Rn in a nonpolar solvent (1 mM solution in 1,2,4-trichlorobenzene) at 0.25 V applied bias. Onedimensional (1D) (Fig. ) and two-dimensional (2D) (Fig. and Supplementary Fig. ) molecular conductance histograms were determined for Rn, such that each histogram is generated from a large ensemble of > 4000 individual traces. Interestingly, molecular conductance in these terphenyl derivatives shows an unexpected dependence on alkyl chain length. In particular, R0-R2 only show a single prominent conductance peak, and molecular conductance decreases upon increasing the alkyl chain length (Fig. ). The average molecular displacement for R0-R2 is constant (≈ 0.8 nm) because the alkyl side chain does not significantly affect molecular end-to-end distance for terphenyl derivatives with short side chains (Fig. and Supplementary Fig. ). In contrast, R3-R12 exhibit two dominant and well-spaced conductance states (high G and low G).