(Self) Assembled News: Recent Highlights from the Supramolecular Chemistry Literature (Quarter 1, 2024)

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• supramolecular chemistry

Fishing sulfate out of water

Anion recognition in water is a challenging objective attracting significant research interest due to potential applications in biology, sensing and hazardous waste remediation. Sulfate has a particularly high hydration energy [1] and is therefore notoriously difficult to bind selectively and strongly in aqueous solution. Deplazes, Wu and colleagues have reported a neutral [2.2.2] urea cryptand that can achieve sulfate encapsulation in water via the formation of up to 12 hydrogen bonds (Figure 1) [2].

Figure 1. A neutral [2.2.2] urea cryptand reported by Deplazes and Wu that can bind SO₄^2- in water [2].

The cryptand was synthesised via a remarkably simple, sulfate-templation strategy in just 2 steps and 30% isolated yield. NMR titrations in 9:1 H₂O/D₂O revealed 1:1 complex formation and K_a = 66 M⁻¹, while isothermal titration calorimetry revealed that sulfate binding is both enthalpically and entropically favoured. The authors also investigated the effect of binding within a hydrophobic microenvironment. In the presence of cetyltrimethylammonium bromide (CTAB) micelles, the binding affinity increased to 990 M⁻¹. The cryptand could also be used to quantify anion concentrations in river, lake and tap water along with iced tea and beer.

Photo-triggered disassembly of supramolecular polymers caught on (AFM) camera

Controlled depolymerisations underpin key cellular processes [3] and can yield responsive function in synthetic systems. However, directly observing dynamic disassembly in real time is a real challenge, with analysis tending to be limited to before and after the disassembly. A new study from Feringa and Roos has captured the photo-triggered disassembly of a helical, molecular motor-based supramolecular polymer using high speed atomic force microscopy (AFM) [4]. They observed that UV-irradiation triggered the disintegration of the polymer only from the free ends, with no disruption of the structure in the middle of the fibre. The authors noted the similarity between this phenomenon and actin chain-end depolymerisation. Additionally, a time lag between irradiation and the onset of disassembly suggested that the thermal helix inversion of the motor may be the cause of the disassembly, rather than the initial photo-switch from the stable cis to metastable trans motor; thus key mechanistic insight was derived from these experiments.

Multicolour circularly polarised luminescence switched by anions and temperature

Helical, light-emitting polymers have attracted significant interest due to their potential applications in generating circularly polarised luminescence (CPL). Materials that can emit a range of colours of light with controlled/switchable handedness are highly sought after for applications in encrypted information storage and 3D display technology. Full colour CPL materials with both right- and left-handedness tend to be derived from the self-assembly of monomers of opposite chirality [5]. Now, work from Liu and co-workers has developed CPL materials derived from a single homochiral monomer that can emit both left-handed and right-handed luminescence [6].

The polyvinylphospocarbochlorine monomers are based on a chiral cholesterol scaffold appended with a pyridine metal-binding site. Co-assembly with a zinc salt and achiral fluorescent dyes of various colours gives rise to helical aggregates with CPL across the colour spectrum. Interestingly, introducing chloride anions caused a morphological transformation from nanorods to nanofibers, yielding an inversion of the CPL handedness, while changing the temperature also led to a chirality and CPL inversion. Doping the co-assemblies into polymer films at varying temperatures yielded materials with tuneable emission wavelength and controlled CPL directionality.

In brief

Quadruple amide receptor surrounds multivalent anions

A recent paper published by Shi, Liu and co-workers reports a new V-shaped quadruple amide receptor with the ability to selectively bind multivalent anions (H₂PO₄⁻ and HSO₄⁻) over a selection of monovalent anions (CF₃SO₃⁻, Cl⁻ and Br⁻) using a complex hydrogen-bonding network [7]. DFT analysis suggested that the receptor forms a bowl-shaped cavity, with multiple hydrogen bonds selectively encapsulating the multivalent anions. In contrast, complexes with smaller, monovalent anions resulted in the receptor twisting and a smaller overall number of hydrogen bonds forming.

Salt-tuned conductivity in hydrogels formed at physiological pH

Supramolecular gels have potential applications as biomaterials, but methods to trigger gelation such as exposure to pH extremes or solvent switching are incompatible with biological applications. Draper and co-workers have found that hydrogels formed from amino-acid appended perylene bisimides can be prepared at physiological pH [8]. The gelators initially formed negatively charged worm-like micelles in aqueous solution. These could be cross-linked by divalent metal salt coordination, yielding conductive hydrogel materials. One of the gelators yielded hydrogels with similar mechanical properties when cross-linked with different salts, yet possessed markedly different reduction potentials. This demonstrated that simply tuning the salt composition can fine-tune the properties of the final materials.

Dynamic covalent photoswitches with long-lived Z-isomers

Greenfield and co-workers have studied arylaminopyrazoles as a new class of small molecule photo-switch [9]. These scaffolds can be formed quantitatively via dynamic covalent imine formation in CDCl₃. Irradiation of the stable E-isomer with visible light leads to >95% conversion to the Z-isomer, which has a half-life of up to 19 hours at room temperature. This is a significant improvement over previously reported arylamine photo-switches, which required UV wavelengths for switching and had short-lived Z-isomers [10]. This was achieved with a simple scaffold that seems well suited to further development within supramolecular chemistry and beyond.

Disclosure statement

No potential conflict of interest was reported by the authors.