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.

Towards understanding π-stacking interactions between non-aromatic rings

“…-Stacking of aromatic rings is a well known type of intermolecular interaction which has been studied extensively over the last few decades (Wheeler & Bloom, 2014, and references therein) and applied in supramolecular chemistry (Steed & Atwood, 2009) and crystal engineering (Desiraju et al, 2011;Tiekink & Zukerman-Schpector, 2012). It plays a significant role in the crystal packing of aromatic compounds (Steed & Atwood, 2009;Groom et al, 2016) and the properties of functional materials (Bredas et al, 2011;Carini et al, 2017). These interactions are of great significance in molecular recognition in biological systems (Salonen et al, 2011;Riley & Hobza, 2013;Madhusudan Makwana & Mahalakshmi, 2015;Neel et al, 2017).…”

“…1), the total interaction is slightly positive, sinceattraction is stronger thanrepulsion. The -polar model (Hunter et al, 2001) of aromatic interactions has been refined over time, taking into consideration direct substituent interactions and solvation/desolvation effects (Hunter & Sanders, 1990;Janiak, 2000;Hunter et al, 2001;Salonen et al, 2011;Martinez & Iverson, 2012;Wheeler & Bloom, 2014;Carini et al, 2017). Face-to-face stacking is possible between electronrich aromatics and electron-depleted ones such as hexafluorobenzene (also an example of double aromaticity arising from -orbital and -orbital interactions; Furukawa et al, 2018) and is often referred to as an aromatic donor-acceptor interaction; some level of -orbital mixing occurs and the donor-acceptor term better describes a situation in which relatively electron-deficient and electron-rich aromatic molecules stack in an alternating fashion (Hunter & Sanders, 1990;Martinez & Iverson, 2012).…”

“…In addition to classical stacking of two aromatic systems (including heteroaromatics), there are numerous examples of hetero stacking involving a non-aromatic stacking partner (Neel et al, 2017): (i) XH pointing towards the centroid of the aromatic ring (X = B, C, N, O, halogen) (Hunter & Sanders, 1990;Bloom et al, 2012;Neel et al, 2017), (ii) ions (Quiñ onero et al, 2006;Neel et al, 2017) and (iii) a lone pair (Carini et al, 2017;Neel et al, 2017;Newberry & Raines, 2017). One recent result based on experimental and theoretical evidence has revealed dimer stacking through a -pyrroleÁ Á Á-(N 2 ) interaction that energetically overpowers hydrogen bonding (Ramanathan et al, 2017).…”

Tunneling Probability Increases with Distance in Junctions Comprising Self-Assembled Monolayers of Oligothiophenes

“… 22 When a molecular tunneling junction is treated as a rectangular energy barrier, β is related to the average height of that barrier φ ( eq 2 ), which is, in turn, related to the offset between dominant frontier orbital and E f such that The magnitude of β for n -alkanethiolates is a benchmark in ME because β = (0.7 ± 0.1) Å –1 [or (1.0 ± 0.1) n CH 2 –1 ] across many experimental platforms. 11 , 12 , 23 , 24 Through clever molecular design, β can approach zero, 25 enabling efficient tunneling transport over tens of nanometers. 26 Unusually small values of β occur in disparate molecular motifs and experimental platforms; 27 , 28 there are no design rules beyond general observations like β ≈ 0.3 Å –1 for oligophenyleneethynylenes (OPEs), oligoacenes, and oligophenylenes.…”

Hooking Together Sigmoidal Monomers into Supramolecular Polymers

“…Apart from the self-assembling structural features described above,a nthracene and acridine units were selected because of their excellent structural and electronic complementarity since they are constituted by three linearly fused rings and because of their well-balanced properties in terms of charge transport, stability and solubility.A lso,t riisopropylsilyl (TIPS) groups have been introduced in the pyridine units to increase the solubility.T oe nsure that the Ac-A-Ac monomers are able to self-assemble into desired (Ac-A-Ac) n polymers,aseries of model experiments were carried out with A-A and Ac-A tweezer-like compounds (Figure 1a nd Figure 2) to understand the interactions between the different units and motifs. A-A has been prepared following ar oute described previously by some of us, [7] while all details of the synthesis and characterization of the Ac-A and Ac-A-Ac are given in the supporting information.…”