The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Maximizing the [c2]daisy chain to lasso ratio through competitive self-templating clipping reactions

Self-templating two-component coupling reactions allowed the isolation of two threaded products with different molecular sizes: a lasso-type [1]rotaxane and a [c2]daisy chain rotaxane. Their distribution in the final reaction mixture varies as a factor of the concentration of the reactants. Through this methodology we obtained a large 84-membered cyclic multistation [2]rotaxane.

Supramolecular Properties of a Monocarboxylic Acid-Functionalized "Texas-Sized" Molecular Box

A new carboxylic acid-functionalized "Texas-sized" molecular box TxSB-CO₂H has been prepared by combining two separate building blocks via an iodide-catalyzed macrocyclization reaction. A single-crystal X-ray diffraction analysis revealed a paired "clip-like" dimer in the solid state. Concentration-dependent behavior is seen for samples of TxSB-CO₂H as prepared, as inferred from ¹H NMR spectroscopic studies carried out in DMSO- d₆. However, in the presence of excess acid (1% by weight of deuterated trifluoracetic acid; TFA- d₁), little evidence of aggregation is seen in DMSO- d₆ except at the highest accessible concentrations. In contrast, the conjugate base form, TxSB-CO₂^-, produced in situ via the addition of excess triethylamine to DMSO- d₆ solutions of TxSB-CO₂H acts as a self-complementary monomer that undergoes self-assembly to stabilize a formal oligomer ([TxSB-CO₂^-] _n) with a degree of polymerization of approximately 5-6 at a concentration of 70 mM. Evidence in support of the proposed oligomerization of TxSB-CO₂^- in solution and in the solid state came from one- and two-dimensional ¹H NMR spectroscopy, X-ray crystallography, dynamic light scattering (DLS), and scanning electron microscopy (SEM). A series of solution-based analyses carried out in DMSO and DMSO- d₆ provide support for the notion that the self-assembled constructs produced from TxSB-CO₂^- are responsive to environmental stimuli, including exposure to the acetate anion (as its tetrabutylammonium, TBA⁺, salt), and changes in overall concentration, temperature, and protonation state. The resulting transformations are thought to reflect the reversible nature of the underlying noncovalent interactions. They also permit the stepwise interconversion between TxSB-CO₂H and [TxSB-CO₂^-] _n via the sequential addition of triethylamine and TFA- d₁. The present work thus serves to illustrate how appropriately functionalized molecular box-type macrocycles may be used to develop versatile stimuli-responsive materials. It also highlights how aggregated forms seen in the solid state are not necessarily retained under competitive solution-phase conditions.

A novel extended N-methyl monopyrrolotetrathiafulvalene based on 2-methylene-4,5-bis(methylthio)-1,3-dithiole

The title compound was prepared via a cross-coupling reaction and its crystal structure has been determined. It crystallized in the triclinic space group P-1 with cell parameters: a = 8.552(2) Å, b = 11.310(2) Å, c = 16.150(3) Å, α = 109.55(3)°, β = 91.45(3)°, γ = 91.28(3)°, V = 1470.6(5) ų, Z = 2 at 296 K. There is one molecule in the asymmetric unit. In the crystal structure, the neighboring molecules from dimers by weak intermolecular π···π interactions between the pyrrole and tetrathiafulvalene units. The dimers are further linked through C-H···π interactions to generate one-dimensional chains along the [100] direction. The arrangement of the molecules corresponds to an overlap between the HOMO and LUMO.

Healable Supramolecular Polymeric Materials

This chapter details the design, synthesis and evaluation techniques required to produce healable supramolecular materials. Key developments in supramolecular polymer chemistry that laid down the design concepts necessary to produce responsive materials are summarized. Subsequently, select examples from the literature concerning the synthesis and analysis of healable materials containing hydrogen bonding, π−π stacking and metal–ligand interactions are evaluated. The last section describes the most recent efforts to produce healable gels for niche applications, including electrolytes and tissue engineering scaffolds. The chapter also describes the design criteria and production of nano-composite materials that exhibit dramatically increased strength compared to previous generations of supramolecular materials, whilst still retaining the key healing characteristics.