Impact of Molecular Symmetry on Single-Molecule Conductance

Abstract

We have measured the single-molecule conductance of a family of bithiophene derivatives terminated with methyl sulfide gold-binding linkers using a scanning tunneling microscope based break-junction technique. We find a broad distribution in the single-molecule conductance of bithiophene compared with that of a methyl sulfide terminated biphenyl. Using a combination of experiments and calculations, we show that this increased breadth in the conductance distribution is explained by the difference in 5-fold symmetry of thiophene rings as compared to the 6-fold symmetry of benzene rings. The reduced symmetry of thiophene rings results in a restriction on the torsion angle space available to these molecules when bound between two metal electrodes in a junction, causing each molecular junction to sample a different set of conformers in the conductance measurements. In contrast, the rotations of biphenyl are essentially unimpeded by junction binding, allowing each molecular junction to sample similar conformers. This work demonstrates that the conductance of bithiophene displays a strong dependence on the conformational fluctuations accessible within a given junction configuration, and that the symmetry of such small molecules can significantly influence their conductance behaviors.

Electronic structure of α-oligothiophenes with various substituents

“…LUMO) can be attributed to charge transfer. It is interesting to note that a recent experimental investigation by Campos et al [34] on the impact of molecular symmetry on single molecule conductance also corroborate this observation. They have found that reduced symmetry of bithiophene restricts the inter-ring torsion rotations when confined in a junction.…”

Correlating Structure and Function in Organic Electronics: From Single Molecule Transport to Singlet Fission

“…Recently, we reported that the 5-fold symmetry of thiophene results in wider conductance histograms than those of biphenyl due to higher rotational barriers when bound in the break junction. 44 Extending the study to higher oligomers of thiophene (1−6 units long), we also found that the 4-unit molecule (T4) unexpectedly shows a higher peak conductance value than its 3-unit counterpart (T3), as seen in Figure 3. 43 This is because the two central thiophene rings of T4 can rotate to planarize without significantly altering the junction length, in contrast to T3, which can get locked in a non-planar, less conductive, conformation.…”

“…42 As the planarity of a molecule increases, conjugation is enhanced, and conductance increases ( Figure 2B); a similar effect is seen in the conductance of bithiophenes. 44 This mirrors how the twisting between neighboring π systems in a conjugated polymer greatly reduces the transfer integral between them and hinders intrachain transport. Recently, we reported that the 5-fold symmetry of thiophene results in wider conductance histograms than those of biphenyl due to higher rotational barriers when bound in the break junction.…”

Molecular length dictates the nature of charge carriers in single-molecule junctions of oxidized oligothiophenes

“…3b). The relatively narrow breadth of the conductance histogram peaks is indicative of a rather rigid backbone in this class of molecules 28,33 . A 2D histogram for TDO3 that retains the displacement information and is created from the same data (Fig.…”