Severin T. Schneebeli

Functionalizing Pillar[n]arenes

Macrocyclic chemistry has relied on the dominance of some key cavitands, including cyclodextrins, calixarenes, cyclophanes, and cucurbiturils, to advance the field of host-guest science. Very few of the many other cavitands introduced by chemists during these past few decades have been developed to near the extent of these four key players. A relatively new family of macrocycles that are becoming increasingly dominant in the field of macrocyclic chemistry are the pillar[n]arenes composed of n hydroquinone rings connected in their 2- and 5-positions by methylene bridges. This substitution pattern creates a cylindrical or pillar-like structure that has identical upper and lower rims. The preparation of pillar[n]arenes is facile, with pillar[5]- through pillar[7]arene being readily accessible and the larger macrocycles (n = 8-14) being accessible in diminishing yields. The rigid pillar[n]arene cavities are highly π-electron-rich on account of the n activated aromatic faces pointing toward their centers, allowing the cavities to interact strongly with a range of π-electron-deficient guests including pyridiniums, alkylammoniums, and imidazoliums. The substitution pattern of pillar[n]arenes bestows chirality onto the macrocycle in the form of n chiral planes. The absolute configuration of the chiral planes in pillar[n]arenes can be either fixed or rapidly undergoing inversion. The future of pillar[n]arenes is going to be dependent on their ability to fulfill specific applications. Chemical modification of the parent pillar[n]arenes lets us create functionalized hosts with anticipated chemical or physical properties. The featured potential applications of pillar[n]arenes to date are far reaching and include novel hosts with relevance to nanotechnology, materials science, and medicine. Pillar[n]arenes have an overwhelming advantage over other hosts since the number of ways available to incorporate handles into their structures are diverse and easy to implement. In this Account, we describe the routes to chemically modified pillar[n]arenes by discussing the chemistry of their functionalization: monofunctionalization, difunctionalization, rim differentiation, perfunctionalization, and phenylene substitution. We assess the synthetic complications of employing these functionalization procedures and survey the potential applications and novel properties that arise with these functionalized pillar[n]arenes. We also highlight the challenges and the synthetic approaches that have yet to be fully explored for the selective chemical modification of these hosts. Finally, we examine a related class of macrocycles and consider their future applications. We trust that this Account will stimulate the development of new methods for functionalizing these novel hosts to realize pillar[n]arene-containing compounds capable of finding applications.

In situ formation of highly conducting covalent Au-C contacts for single-molecule junctions

Charge transport across metal-molecule interfaces has an important role in organic electronics. Typically, chemical link groups such as thiols or amines are used to bind organic molecules to metal electrodes in single-molecule circuits, with these groups controlling both the physical structure and the electronic coupling at the interface. Direct metal-carbon coupling has been shown through C60, benzene and π-stacked benzene, but ideally the carbon backbone of the molecule should be covalently bonded to the electrode without intervening link groups. Here, we demonstrate a method to create junctions with such contacts. Trimethyl tin (SnMe(3))-terminated polymethylene chains are used to form single-molecule junctions with a break-junction technique. Gold atoms at the electrode displace the SnMe(3) linkers, leading to the formation of direct Au-C bonded single-molecule junctions with a conductance that is ∼100 times larger than analogous alkanes with most other terminations. The conductance of these Au-C bonded alkanes decreases exponentially with molecular length, with a decay constant of 0.97 per methylene, consistent with a non-resonant transport mechanism. Control experiments and ab initio calculations show that high conductances are achieved because a covalent Au-C sigma (σ) bond is formed. This offers a new method for making reproducible and highly conducting metal-organic contacts.

An artificial molecular pump

Carrier proteins consume fuel in order to pump ions or molecules across cell membranes, creating concentration gradients. Their control over diffusion pathways, effected entirely through noncovalent bonding interactions, has inspired chemists to devise artificial systems that mimic their function. Here, we report a wholly artificial compound that acts on small molecules to create a gradient in their local concentration. It does so by using redox energy and precisely organized noncovalent bonding interactions to pump positively charged rings from solution and ensnare them around an oligomethylene chain, as part of a kinetically trapped entanglement. A redox-active viologen unit at the heart of a dumbbell-shaped molecular pump plays a dual role, first attracting and then repelling the rings during redox cycling, thereby enacting a flashing energy ratchet mechanism with a minimalistic design. Our artificial molecular pump performs work repetitively for two cycles of operation and drives rings away from equilibrium toward a higher local concentration.

Probing the conductance superposition law in single-molecule circuits with parallel paths

According to Kirchhoffs circuit laws, the net conductance of two parallel components in an electronic circuit is the sum of the individual conductances. However, when the circuit dimensions are comparable to the electronic phase coherence length, quantum interference effects play a critical role1, as exemplified by the Aharonov–Bohm effect in metal rings2,3. At the molecular scale, interference effects dramatically reduce the electron transfer rate through a meta-connected benzene ring when compared with a para-connected benzene ring4,5. For longer conjugated and cross-conjugated molecules, destructive interference effects have been observed in the tunnelling conductance through molecular junctions6,7,8,9,10. Here, we investigate the conductance superposition law for parallel components in single-molecule circuits, particularly the role of interference. We synthesize a series of molecular systems that contain either one backbone or two backbones in parallel, bonded together cofacially by a common linker on each end. Single-molecule conductance measurements and transport calculations based on density functional theory show that the conductance of a double-backbone molecular junction can be more than twice that of a single-backbone junction, providing clear evidence for constructive interference. Kirchhoffs conductance superposition law is investigated in single-molecule circuits. A single-molecule junction with two backbones in a parallel configuration can exhibit more than twice the conductance of a single-molecule junction with one backbone, a demonstration of constructive quantum interference.

Highly Conducting π-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes

We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ, resulting in formation of a direct covalent σ bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated π system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the π system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G(0) = 2e(2)/h). Junctions formed with methylene-terminated oligophenyls with two to four phenyl units show a 100-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling, as they exhibit an exponential dependence of conductance on oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission, with a crossover to tunneling for the longer oligomers.

Single-Molecule Conductance through Multiple π-π-Stacked Benzene Rings Determined with Direct Electrode-to-Benzene Ring Connections

Understanding electron transport across π-π-stacked systems will help to answer fundamental questions about biochemical redox processes and benefit the design of new materials and molecular devices. Herein we employed the STM break-junction technique to measure the single-molecule conductance of multiple π-π-stacked aromatic rings. We studied electron transport through up to four stacked benzene rings held together in an eclipsed fashion via a paracyclophane scaffold. We found that the strained hydrocarbons studied herein couple directly to gold electrodes during the measurements; hence, we did not require any heteroatom binding groups as electrical contacts. Density functional theory-based calculations suggest that the gold atoms of the electrodes bind to two neighboring carbon atoms of the outermost cyclophane benzene rings in η(2) fashion. Our measurements show an exponential decay of the conductance with an increasing number of stacked benzene rings, indicating a nonresonant tunneling mechanism. Furthermore, STM tip-substrate displacement data provide additional evidence that the electrodes bind to the outermost benzene rings of the π-π-stacked molecular wires.

Electron Sharing and Anion–π Recognition in Molecular Triangular Prisms

Stacking on a full belly: Triangular molecular prisms display electron sharing among their triangularly arranged naphthalenediimide (NDI) redox centers. Their electron-deficient cavities encapsulate linear triiodide anions, leading to the formation of supramolecular helices in the solid state. Chirality transfer is observed from the six chiral centers of the filled prisms to the single-handed helices.

Synthesis and Structural Data of Tetrabenzo[8]circulene

In 1976, the first attempted synthesis of the saddle-shaped molecule [8]circulene was reported. The next 37 years produced no advancement towards the construction of this complicated molecule. But remarkably, over the last six months, a flurry of progress has been made with two groups reporting independent and strikingly different strategies for the synthesis of [8]circulene derivatives. Herein, we present a third synthetic method, in which we target tetrabenzo[8]circulene. Our approach employs a Diels-Alder reaction and a palladium-catalyzed arylation reaction as the key steps. Despite calculations describing the instability of [8]circulene, coupled with the reported instability of synthesized derivatives of the parent molecule, the addition of four fused benzenoid rings around the periphery of the molecule provides a highly stable structure. This increased stability over the parent [8]circulene was predicted by using Clars theory of aromatic sextets and is a result of the compound becoming fully benzenoid upon incorporation of these additional rings. The synthesized compound exhibits remarkable stability under ambient conditions-even at elevated temperatures-with no signs of decomposition over several months. The solid-state structure of this compound is significantly twisted compared to the calculated structure primarily as a result of crystal-packing forces in the solid state. Despite this contortion from the lowest-energy structure, a range of structural data is presented confirming the presence of localized aromaticity in this large polycyclic aromatic hydrocarbon.

Electron Delocalization in a Rigid Cofacial Naphthalene‐1,8:4,5‐bis(dicarboximide) Dimer

Investigating through-space electronic communication between discrete cofacially oriented aromatic π-systems is fundamental to understanding assemblies as diverse as double-stranded DNA, organic photovoltaics and thin-film transistors. A detailed understanding of the electronic interactions involved rests on making the appropriate molecular compounds with rigid covalent scaffolds and π-π distances in the range of ca. 3.5 Å. Reported herein is an enantiomeric pair of doubly-bridged naphthalene-1,8:4,5-bis(dicarboximide) (NDI) cyclophanes and the characterization of four of their electronic states, namely 1) the ground state, 2) the exciton coupled singlet excited state, 3) the radical anion with strong through-space interactions between the redox-active NDI molecules, and 4) the diamagnetic diradical dianion using UV/Vis/NIR, EPR and ENDOR spectroscopies in addition to X-ray crystallography. Despite the unfavorable Coulombic repulsion, the singlet diradical dianion dimer of NDI shows a more pronounced intramolecular π-π stacking interaction when compared with its neutral analog.

Assembly of Supramolecular Nanotubes from Molecular Triangles and 1,2-Dihalohydrocarbons

Precise control of molecular assembly is a challenging goal facing supramolecular chemists. Herein, we report the highly specific assembly of a range of supramolecular nanotubes from the enantiomeric triangular naphthalenediimide-based macrocycles (RRRRRR)- and (SSSSSS)-NDI-Δ and a class of similar solvents, namely, the 1,2-dihalo-ethanes and -ethenes (DXEs). Three kinds of supramolecular nanotubes are formed from the columnar stacking of NDI-Δ units with a 60° mutual rotation angle as a result of cooperative [C-H···O] interactions, directing interactions of the [X···X]-bonded DXE chains inside the nanotubes and lateral [X···π] or [π···π] interactions. They include (i) semiflexible infinite nanotubes formed in the gel state from NDI-Δ and (E)-1,2-dichloroethene, (ii) rigid infinite nonhelical nanotubes produced in the solid state from NDI-Δ and BrCH2CH2Br, ClCH2CH2Br, and ClCH2CH2I, and (iii) a pair of rigid tetrameric, enantiomeric single-handed (P)- and (M)-helical nanotubes formed in the solid state from the corresponding (RRRRRR)- and (SSSSSS)-NDI-Δ with ClCH2CH2Cl. In case (i), only the electron-rich C═C double bond of (E)-1,2-dichloroethene facilitates the gelation of NDI-Δ. In cases (ii) and (iii), the lengths of anti-DXEs determine the translation of the chirality of NDI-Δ into the helicity of nanotubes. Only ClCH2CH2Cl induces single-handed helicity into the nanotubes. The subtle interplay of noncovalent bonding interactions, resulting from the tiny structural variations involving the DXE guests, is responsible for the diverse and highly specific assembly of NDI-Δ. This research highlights the critical role that guests play in constructing assembled superstructures of hosts and offers a novel approach to creating supramolecular nanotubes.