High conductance values in π-folded molecular junctions

Abstract

Folding processes play a crucial role in the development of function in biomacromolecules. Recreating this feature on synthetic systems would not only allow understanding and reproducing biological functions but also developing new functions. This has inspired the development of conformationally ordered synthetic oligomers known as foldamers. Herein, a new family of foldamers, consisting of an increasing number of anthracene units that adopt a folded sigmoidal conformation by a combination of intramolecular hydrogen bonds and aromatic interactions, is reported. Such folding process opens up an efficient through-space charge transport channel across the interacting anthracene moieties. In fact, single-molecule conductance measurements carried out on this series of foldamers, using the scanning tunnelling microscopy-based break-junction technique, reveal exceptionally high conductance values in the order of 10^-1 G₀ and a low length decay constant of 0.02 Å^-1 that exceed the values observed in molecular junctions that make use of through-space charge transport pathways.

Figure 1. π-Folded molecular junctions.

Schematic representation of the folding and anchoring processes needed to obtain π-folded molecular junctions from a representative member of the foldamer family studied in this work.

Figure 2. Chemical structures of the foldamers.

Structures of the complete -NHBoc and -NH₂ foldamer series studied in this work: dimer-NHBoc, dimer-NH₂, trimer-NHBoc, trimer-NH₂, tetramer-NHBoc, tetramer-NH₂, pentamer-NHBoc, and pentamer-NH₂. Arrows indicate the transformation of the -NHBoc series into the -NH₂ series.

Figure 3. Structural characterization of the foldamers.

(a) Solid-state structure of trimer-NHBoc with two DMSO molecules (highlighted in yellow). (b) Chemical structure of A-NHBoc. (c) NMR spectra of the foldamer NHBoc series at 25 °C. (d) Two-dimensional DOSY NMR of pentamer-NHBoc at 25 °C. (e) VT-NMR of pentamer-NHBoc. (f) Absorption and (g) PL spectra of the NHBoc foldamer series at 25 °C. Infrared ion dip spectrum of (h) dimer-NHBoc and (i) trimer-NHBoc. The triangles in b–e indicate the signals that correspond to anthracene protons.

Figure 4. Computed structure of the foldamers.

(a) Representative CH–π and π–π structures for the different foldamers. (b) HOMO and LUMO orbitals for CH–π and π–π conformers of pentamer-NH₂.

Figure 5. Single-molecule conductance measurements.

(a) Individual pulling traces and (b) conductance histograms of the different foldamers. All conductance values were extracted from Gaussian fits. (c) Calculated ß value for the π–π foldamers. Error bars capture the variability in peak position. (d) Conductance histograms for #-trimer-NH₂. (e) Structure of #-trimer-NH₂. (f) Conductance histograms for the pentamer-NH₂ at variable temperatures. (g) Arrhenius plot of the single-molecule conductance for the pentamer-NH₂. Error bars represent the s.d. of the experimental conductance values extracted from the full width at half maximum of the Gaussian fits. Applied voltage bias were 5 mV for the trimer-NH₂–pentamer-NH₂ series and 50 mV for dimer-NH₂.