Alkyl side chains are known to change the twist angle between planar conjugated units in molecular backbones, which affects electronic properties. To understand the role of backbone conformation on molecular conductance, we performed molecular modeling of conductance in terphenyl junctions with varying twist angles. Terphenyl junctions with different conformations (Fig. ) were modeled using DFT calculations performed on Spartan'16 Parallel Suite using the B3LYP functional with a 6-31G (d,p) basis set. Following determination of geometry-optimized structures, transmission functions were calculated using nonequilibrium Green's function-density functional theory (NEGF-DFT) via the Atomistix Toolkit package (Fig. and Supplementary Figs. ). Our results show that the transmission values close to the Fermi energy decrease with cos 2 θ (between θ = 0 o and 90 o ), where θ is the dihedral angle between the first two phenyls. The simulated conductance is ≈10 -2.9 G0 for terphenyl junctions with full planar conformations and ≈10 -4.5 G0 when the dihedral angle is close to 90 o , which is in good agreement with the experimentally observed high G and low G states, respectively (Fig. ). To further validate these results, we synthesized two control molecules with planar (R1-planar) and twisted (R1-twisted) conformations (Fig. ). The experimentally measured conductance values for these molecules are in reasonable agreement with results from NEGF-DFT simulations (Fig. and Supplementary Fig. ). These results suggest that the low G state derives from twisted conformations and depends on backbone dihedral angles, whereas the high G state is likely associated with a planarized backbone but is only observed for long alkyl side chains (n ≥ 3). However, the molecular displacement corresponding to the high G state is » 0.4 nm, which is significantly smaller than the expected displacement of an extended molecular backbone for a terphenyl derivative. To fully understand the molecular origin of the high G state, we performed a series of singlemolecule conductance experiments using R3-R12 at different concentrations (100 nM -1 mM) (Fig. and Supplementary Fig. ). Surprisingly, our results show that the emergence of the high conductance state depends on the solution concentration of R3-R12. R3 exhibits a single peak (low G state) at relatively low concentrations (< 1 mM) and dual peaks (high G and low G states) at high concentrations (1 mM). Interestingly, the onset concentration for the emergence of the high G state decreases from 1 mM to 0.1 mM upon increasing the length of the alkyl side chain (Supplementary Fig. ). Moreover, R3-R12 do not exhibit dual peaks at very low concentrations at 0.01 mM (Fig. and Supplementary Fig. ), whereas R2 and R3-iPr (containing the isomeric isopropyl side chain) do not exhibit a high G state at very high concentrations (10 mM) (Supplementary Fig. ), suggesting that a minimum linear alkyl side chain length is required for the emergence of the high conductance state. In addition, we analyzed correlations between the high G and low G states for R3-R12 by determining a 2D covariance histogram (Fig. and Supplementary Fig. ). These results show a clear negative correlation between the high G and low G states, indicating that these two conductance states occur independently.