When Dihedral Angles Mask Denticity in Molecular Conductance.

Stronger molecule-electrode coupling is associated with higher conductance in single-molecule junctions. This has been taken to imply that more coordination-what will be referred to here as higher denticity-between the molecule and the electrode is expected to impart higher conductance to the overall junction. Herein, this assumption using a single molecule construct, a rigid N-heterohexacene molecule with tetradentate ethyl sulfide (-SEt) anchors, is examined. Thus, rather than comparing a series of molecules with different anchoring groups, it is investigated how variations in effective denticity arise naturally within one molecule. Using the nonequilibrium Green's function technique in conjunction with density functional theory and mechanically controlled break-junction (MCBJ) experiments, it is found that increasing the denticity between the molecule and the electrode does not yield the expected higher conductance. Instead, simulated break-junction traces reveal a strong correlation between conductance and the dihedral angle between the electrode and the molecular core, with changes to dihedral angles providing far more variation in conductance values than denticity alone. In fact, it is shown that counter to naïve expectations, different denticities cannot be distinguished by conductance, merging instead into a single conductance feature. This is supported by MCBJ experiments on this molecule, where only a single conductance state is identified, suggesting that the expected denticity-dependent multistate conductance behavior is dominated by the effect of dihedral angles. By restricting dihedral angles to more favorable values by molecular design, the calculations show that significantly higher conductance values can still be achieved despite the limitations imposed by dihedral-denticity coupling. The work demonstrates that mere denticity may not be sufficient to design highly conductive molecular junctions, and that the association of conductance features with different denticities should be treated with caution.