(353a) Molecular Electronics of Organic Foldamers

The electronic properties of organic molecules and biomolecules are governed by intramolecular interactions and 3D molecular conformations. Non-covalent interactions between monomer units such as H-bonding play a key role in conformational transitions, but the structure-function relations for these materials are not yet understood. Here, we use single molecule experiments to characterize the electronic properties of an extended secondary amide and a folded N-methylated tertiary amide, which are monomers of synthetic organic foldamers that can organize into distinct 3D structures. Foldamer monomer units containing amide bonds (FH) and N-methylated amide bonds (FM) are studied, and our results show that FH exhibits a 4x enhancement in molecular conductance compared to FM, despite longer molecular length. The two molecules adopt different molecular conformations, with FH adopting a trans conformation and FM a cis conformation. Interestingly, FH molecules are governed by a through bond-electron transport mechanism, whereas FM molecules rely on through-space electron transport. Bulk UV-vis absorption and fluorescence spectroscopy experiments illustrate longer conjugation lengths and smaller energy gaps for FH compared to FM. Density functional theory (DFT), molecular orbital visualization, and non-equilibrium Green’s function-density functional theory (NEGF-DFT) calculations further reveal that FH has a smaller HOMO-LUMO gap and higher transmission values compared to FM, consistent with bulk spectroscopy and single-molecule electronic experiments. Overall, our results show that distance is not the sole determinant of electron tunneling, rather additional factors such as chemistry (molecular composition), 3D folds (molecular conformation), and electron transport pathways significantly influence electron tunneling currents in molecular junctions.