Aromatic Amide Foldamers Show Conformation-Dependent Electronic Properties

Abstract

Electron transport in organic molecules and biomolecules is governed by electronic structure and molecular conformations. Despite recent progress, key challenges remain in understanding the role of intramolecular interactions and three-dimensional (3D) conformations on the electron transport behavior of organic molecules. In this work, the electronic properties of aromatic amide foldamers are characterized that organize into distinct 3D structures, including an extended secondary amide that adopts a trans-conformation and a folded N-methylated tertiary amide that adopts a cis-conformation. Results from single-molecule electronic experiments show that the extended secondary amide exhibits a fourfold enhancement in molecular conductance compared to the folded N-methylated tertiary amide, despite a longer contour length. The results show that extended amide molecules are governed by a through-bond electron transport mechanism, whereas folded amide molecules are dominated by through-space transport. Bulk spectroscopic characterization and density functional theory calculations further reveal that extended amides have a smaller HOMO-LUMO gap and larger transmission values compared to folded amides, consistent with single-molecule electronic experiments. Overall, this work shows that 3D molecular conformations significantly influence the electronic properties of single-molecule junctions.

Keywords: electron transport; electron tunneling; foldamers; molecular electronics; supramolecular chemistry.

Scheme 1

Benzanilide derivative FH (foldamer monomer unit with a hydrogen atom on the amide) and its N‐methylated compound FM (foldamer monomer unit with a methyl group on the amide). The extended secondary amide can be converted to folded N‐methylated tertiary amide via N‐methylation. Upon oligomerization, the folded N‐methylated tertiary amide forms repeats of N‐alkylbenzamide (PFM), which is known to exhibit dynamic helical properties.^{[ 20 ]}

Figure 1

Schematic of experimental setup and foldamer monomer units studied in this work. a) Chemical structures of benzanilide derivative FH and N‐methylbenzanilide derivative FM. b) Schematic of a single‐molecule junction containing FH and FM.

Figure 2

STM‐BJ measurements for benzanilide derivative FH and N‐methylbenzanilide derivative FM. a) Characteristic single‐molecule traces for FH and FM. b) 1D conductance histogram for FH and FM. c) 2D conductance histogram for FH. d) 2D conductance histogram for FM. e) Flicker noise analysis for FH, indicating through‐bond mediated electron transport. f) Flicker noise analysis for FM, indicating through‐space mediated electron transport. All data were obtained using 0.1 mM concentrations of FH and FM in 1,2,4‐trichlorobenzene (TCB) solvent at 250 mV applied bias across ensembles of at least 5000 single molecules.

Figure 3

DFT calculations for FH and FM. a) Geometry optimized structures indicating that FH and FM adopt a trans and cis conformation, respectively. b) Molecular orbital isosurfaces for FH and FM. c) Molecule electrode geometries in nanoscale junctions for NEGF‐DFT calculations. d) Transmission probability values for FH and FM as a function of energy relative to the Fermi energy level.