Recent progress and future challenges in the supramolecular polymerization of metal-containing monomers

Abstract

The self-assembly of discrete molecular entities into functional nanomaterials has become a major research area in the past decades. The library of investigated compounds has diversified significantly, while the field as a whole has matured. The incorporation of metal ions in the molecular design of the (supra-)molecular building blocks greatly expands the potential applications, while also offering a promising approach to control molecular recognition and attractive and/or repulsive intermolecular binding events. Hence, supramolecular polymerization of metal-containing monomers has emerged as a major research focus in the field. In this perspective article, we highlight recent significant advances in supramolecular polymerization of metal-containing monomers and discuss their implications for future research. Additionally, we also outline some major challenges that metallosupramolecular chemists (will) have to face to produce metallosupramolecular polymers (MSPs) with advanced applications and functionalities.

Fig. 1. Schematic representation of commonly investigated aspects of MSPs.

Fig. 2. (a) Molecular structures of Pd^II complex 1 and reference OPE 2. (b) Schematic depiction of self-assembled fibres of 1 and the molecular packing around the metal centre. (c) Photographs of solutions of 1 showing the color change upon reversible self-assembly through temperature variation. Panels (b) and (c) adapted with permission from ref. . Copyright 2013 American Chemical Society.

Fig. 3. (a) Pt^II and Pd^II complexes 3–7 investigated by our group, unravelling N–H⋯X interactions of different strength leading to the competition of a slipped and a parallel molecular packing. (b) The relative magnitude of these interactions in comparison to other driving forces has a significant impact on the energy landscape of self-assembly, engendering hidden aggregates and supramolecular packing polymorphism. Panel (b) adapted with permission from ref. . Copyright 2021 American Chemical Society.

Fig. 4. (a) Molecular structure of Zn^II chlorine 8 investigated by the group of Würthner. (b) Energy landscape of the self-assembly of 8 illustrating the supramolecular polymerization into two different J-type aggregate species. Panel (b) adapted with permission from ref. . Copyright 2018 Royal Society of Chemistry.

Fig. 5. (a) Molecular structures of V-shaped complex 9 and linear complex 10. (b) Time-resolved photoluminescence decay of Agg9 at 298 K. (c) Schematic energy landscape illustrating the distinct supramolecular polymerization behaviour induced by a variation of geometry with luminescence micrographs of Agg9 and Agg10. Panels (b) and (c) adapted with permission from ref. . Copyright 2021 Royal Society of Chemistry.

Fig. 6. (a) Chemical structures of pincer-isocyanide Pd^II complexes 11–13. (b) Top: time-dependent absorption spectra of 11PF6 and photographs of the solution. Bottom: photographs showing the corresponding changes of the emission of 11PF6 over time. (c and d) Crystal structures of 11PF6 and 11OTF6 illustrating the two possible aggregation modes with (11PF6) and without (11OTF6) metal contacts. Panel (b)–(d) adapted with permission from ref. . Copyright 2018 John Wiley and Sons.

Fig. 7. (a) Closed and open forms of Pt^II molecular hinge 14 investigated by the group of Yam. (b) Qualitative energy landscape illustrating the conversion processes between the different species. Adapted with permission from ref. . Copyright 2019 National Academy of Sciences.

Fig. 8. (a) Molecular structures of Zn^II porphyrins 15 investigated by the groups of Sugiyasu and Takeuchi. (b) Complex energy landscape of the self-assembly into different morphologies. (c) Schematic representation of the self-assembled Archimedean spirals formed by 15FF. Panels (b) and (c) adapted with permission from ref. and , respectively. Copyright 2020 John Wiley and Sons and Copyright 2020 Springer Nature.

Fig. 9. (a) Structure of complex double salts 16–18. (b) TEM, (c) SEM and (d) AFM images of the aggregate of 16 formed in water. (d–f) TEM images of the aggregates formed by 17 (e) and 18 (f and g). Panels (b)–(g) adapted with permission from ref. . Copyright 2018 American Chemical Society.

Fig. 10. Representative examples of living crystallization driven self-assembly (LCDSA) in 1 (a) and 2 dimensions (b). Adapted with permission from ref. . Copyright 2021 Royal Society of Chemistry.

Fig. 11. (a) Structure of complexes 19 and 20 which statistically co-assemble into nanofibres or nanodiscs when one component (which dictates the morphology) is in large excess, or into a mixture of Pd-enriched, co-assembled fibres and Pt-only nanodiscs at intermediate ratios (b). Panel (b) adapted with permission from ref. . Copyright 2019 John Wiley and Sons.

Fig. 12. (a and b) Molecular structure of BTA 21 (a) and the halogen bonding donors (b) used for orthogonal secondary binding for graftable supramolecular (co)polymers. (c) Schematic representation of the self-assembly of 21 in the absence (left) and presence (right) of orthogonal halogen bonding donors. Adapted with permission from ref. . Copyright 2020 Royal Society of Chemistry.

Fig. 13. (a) Molecular structures of complex series 22 & 23. (b and c) Confocal fluorescence microscopy images of: (b) 1D 5-block yellow-green-red ([Pd : Pt 2 : 1]–Pd–Pt segments), (c) flower-shaped triblock green-red-green (Pd–Pt–Pd segments) copolymers. (d) Photograph comparing the emission of Agg2 of pure Pt-complex (left), Pt-doped Pd-copolymer (middle) and pure Pd complex (right). Scale bars: 5 μm. Panel (b)–(d) adapted with permission from ref. . Copyright 2020 Elsevier.

Fig. 14. (a) Molecular structures of the investigated metal-porphyrins 24-Cu and 24-Zn. (b) Kinetic profiles after the addition of DMAP to SPs of pure 24-Zn (i), SPs of pure 24-Cu (ii), a mixture of SPs of pure 24-Zn and 24-Cu (iii), the 24-Cu–Zn–Cu block-copolymer (iv). (c) Schematic illustration of the depolymerization of 1D aggregates of Zn^II porphyrin 24-Zn through DMAP coordination and its suppression by “end-capping” the 24-Zn SPs with blocks of SPs of 24-Cu. Panel (b) and (c) adapted with permission from ref. . Copyright 2018 American Chemical Society.

Fig. 15. Comparison of the mono-component self-assembly of complex 25 and its co-assembly with complex 26. Whereas the SP stacks of isolated 25 lack metal–metal interactions, the co-assembly induces heteromeric Pt⋯Pt interactions, giving rise to an MMLCT transition. Adapted with permission from ref. . Copyright 2018 Royal Society of Chemistry.

Fig. 16. (a) Molecular structure of Pt^II complexes 27 & 28 whose co-assembly renders a reversible supramolecular wrapping of fibres formed by 27 through aggregates of 28. The lower panel (b and c) shows the confocal microscopy images visualizing the time-progression of the co-assembly and a schematic illustration of the molecular processes (d). Adapted with permission from ref. . Copyright 2021 John Wiley and Sons.

Fig. 17. (a) Schematic representation of the insulin amyloid directed self-assembly of 29 with its molecular structure depicted below. (b) Corrected emission spectra of 29 in PBS buffer upon addition of different amounts of insulin amyloid. (c) Fluorescence microscopy image of aggregates of 29 formed on amyloid insulin fibrils. Adapted with permission from ref. . Copyright 2019 American Chemical Society.

Fig. 18. (a) Molecular structure of triplet emitter Pt^II complex 30. (b) Schematic illustration of the self-assembly of 30 (blue labels) at fluorescein-labelled (green labels) BSA. Panel (b) adapted with permission from ref. . Copyright 2018 American Chemical Society.

Fig. 19. Schematic representation of a photocatalytically active co-assembly based on donor–acceptor interactions, exhibiting strong Pt–Pt interactions. Adapted with permission from ref. . Copyright 2018 Royal Society of Chemistry.

Fig. 20. (a–d) Schematic representations of the potentially viable properties of supramolecular assemblies for applications in catalysis.

Nils Bäumer

Jonas Matern

Gustavo Fernández