Electronic Conductance and Thermopower of Cross-Conjugated and Skipped-Conjugated Molecules in Single-Molecule Junctions

Abstract

We report a combined experimental and theoretical study of a series of thiomethyl (SMe) anchored cross-conjugated molecules featuring an acyclic central bridging ketone and their analogous skipped-conjugated alcohol derivatives. Studies of these molecules in a gold|single-molecule|gold junction using scanning tunneling microscopy-break junction techniques reveal a similar conductance (G) value for both the cross-conjugated molecules and their skipped-conjugated partners. Theoretical studies based on density functional theory of the molecules in their optimum geometries in the junction reveal the reason for this similarity in conductance, as the predicted conductance for the alcohol series of compounds varies more with the tilt angle. Thermopower measurements reveal a higher Seebeck coefficient (S) for the cross-conjugated ketone molecules relative to the alcohol derivatives, with a particularly high S for the biphenyl derivative 3a (−15.6 μV/K), an increase of threefold compared to its alcohol analog. The predicted behavior of the quantum interference (QI) in this series of cross-conjugated molecules is found to be constructive, though the appearance of a destructive QI feature for 3a is due to the degeneracy of the HOMO orbital and may explain the enhancement of the value of S for this molecule.

Hydroxyl Group as the ‘Bridge’ to Enhance the Single-Molecule Conductance by Hyperconjugation

“…As shown in Figure 1 , the most probable conductance of molecule 1 is 10 −4.85 G 0 (1.09 nS, G 0 represents the quantum conductance and equals 77,500 nS) [ 23 ], which is lower than the conductance of molecule 2 (10 −4.29 G 0 , 3.97 nS) and higher than the conductance of molecule 3 (10 −5.10 G 0 , 0.62 nS). This implies that, compared to the fully conjugated molecule 2 , breaking the conjugated system with a saturated carbon atom at the central site of molecule 1 suppresses the conductance effectively.…”

“…Here we report a hyperconjugation system based on a skipped-conjugation structure [ 23 ] with a non-conjugated hydroxyl group as a ‘bridge’ to connect the conjugated fragments. The distinct enhancement of conductance for the hydroxyl-contained hyperconjugation systems was confirmed by comparison with that for the corresponding hydroxyl-free derivatives.…”

Exploring the Impact of the HOMO–LUMO Gap on Molecular Thermoelectric Properties: A Comparative Study of Conjugated Aromatic, Quinoidal, and Donor–Acceptor Core Systems

“…They can be used in a variety of applications, ranging from power generation in automobiles and industrial processes via the Seebeck effect to cooling of electronics and other devices via the Peltier effect. − Current inorganic thermoelectric materials, such as bismuth telluride (Bi 2 Te 3 ), lead telluride (PbTe), and silicon germanium (SiGe), have been widely studied and utilized, but their applications are restricted due to toxicity and limited availability, as well as a generally low conversion efficiency at room temperature. Also, in most semiconductors, the increase in conductance is accompanied by a decrease of the Seebeck coefficient, which is not desirable and makes improving the thermoelectric efficiency a difficult task. − …”

How substituents tune quantum interference in meta-OPE3 molecular junctions to control thermoelectric transport

“…To gain deeper insight into QI phenomena, recent studies have explored the impact of intramolecular structural variations on the destructive or constructive interference in SMJs which includes geometrical isomerism, 20,21 heteroatom substitution within the molecular backbones, 17,18,22–25 different anchoring groups to the contacted electrodes instead of traditional sulfur containing ones, 21,26 addition of different repeating molecular units into the backbone 27 such as oligo(phenyleneethynylene)s, 28 distinct molecular bridging schemes, 29 as well as design of organic radial molecules. 30 In addition, it has been theoretically predicted that introducing different substituents to a current-carrying molecular backbone could tune the destructive QI.…”