No abstract

The combination of the drastic reduction in size with the appearance of new properties at the nanoscale have opened new avenues in the use of supramolecular materials, [5][6][7][8] with a number of nanodevices being described and developed for technological [9][10][11] or biological applications. [1,12] In the field of molecular electronics single molecule devices represent the ultimate miniaturization concept. These have evolved from the seminal paper from Aviram and Ratner, [13] which is a foundational work in the field of molecular electronics, and which proposed that molecular devices can act as electrical molecular rectifiers. There has been a renaissance of this area in recent years, [14] which has been boosted by promising applications beyond the initially envisioned one (electrical circuitry at the nanoscale), including single molecular sensing, thermoelectrics, heat transfer, spintronics, switching devices, and biomolecular electronics. [15][16][17][18][19] Despite intense research in the areas of molecular electronics and single molecule devices some key challenges remain unsolved. Reproducibility in the conductance measurement is a recurrent problem in the single-molecule electronics field Future applications of single-molecular and large-surface area molecular devices require a thorough understanding and control of molecular junctions, interfacial phenomena, and intermolecular interactions. In this contribution the concept of single-molecule junction and host-guest complexation to sheath a benchmark molecular wire-namely 4,4′-(1,4-phenylenebis(ethyne-2,1diyl))dianiline -with an insulating cage, pillar[5]arene 1,4-diethoxy-2-ethyl-5-methylbenzene is presented. The insertion of one guest molecular wire into one host pillar[5]arene is probed by 1 H-NMR (nuclear magnetic resonance), whilst the self-assembly capabilities of the amine-terminated molecular wire remain intact after complexation as demonstrated by XPS (X-ray photoelectron spectroscopy) and AFM (atomic force microscopy). Encapsulation of the molecular wire prevents the formation of π-π stacked dimers and permits the determination of the true single molecule conductance with increased accuracy and confidence, as demonstrated here by using the STM-BJ technique (scanning tunneling microscopy-break junction). This strategy opens new avenues in the control of single-molecule properties and demonstrates the pillararenes capabilities for the future construction of arrays of encapsulated single-molecule functional units in large-surface area devices.

We have developed a new strategy for the preparation of a light‐responsive ionic liquid crystal (LC) that shows photo‐switchable proton conduction. The ionic LC consists of a bowl‐shaped calix[4]arene core ionically functionalized with azobenzene moieties. The non‐covalent architectures were obtained by the formation of ionic salts between the carboxylic acid group of an azo‐derivative and the terminal amine groups of a calixarene core. The presence of ionic salts results in a hierarchical self‐assembly process that extends to the formation of a nanostructured lamellar LC arrangement (smectic A phase). In this LC phase, the ionic LC calixarene is able to display proton conductive properties, since the ionic nanosegregated areas (formed by the ionic pairs) generate the continuous channels that favor proton transport. The optical and photo‐responsive properties were studied by UV‐Vis spectroscopy, demonstrating that the azobenzene moieties of the ionic LC undergo reversible (E)‐to‐(Z) isomerization by irradiation with UV light. Interestingly, this (E)‐to‐(Z) photoisomerization results in a decrease of the proton conductivity values since the bent‐shaped (Z)‐isomer disrupts the lamellar LC phase. This isomerization process is totally reversible and leads to an ionic LC material with unique photo‐switchable proton conductive properties.