Bottom-up supramolecular assembly in two dimensions

Abstract

The self-assembly of molecules in two dimensions (2D) is gathering attention from all disciplines across the chemical sciences. Attracted by the interesting properties of two-dimensional inorganic analogues, monomers of different chemical natures are being explored for the assembly of dynamic 2D systems. Although many important discoveries have been already achieved, great challenges are still to be addressed in this field. Hierarchical multicomponent assembly, directional non-covalent growth and internal structural control are a just a few of the examples that will be discussed in this perspective about the exciting present and the bright future of two-dimensional supramolecular assemblies.

Fig. 1. General design strategy to direct monomer self-assembly in 2D. The orientation of several attractive interactions, both directional (e.g. H-bonding, π–π stacking, etc.) and non-directional (e.g. hydrophobic effects), on synthetic monomers – depicted here as cubes – determines the supramolecular propagation in the different dimensions of space. (A) One single attractive interaction (red) can be used to achieve one-dimensional elongation. (B) Two orthogonal interactions (red and green) in perpendicular orientation can propagate the 2D assembly. (C) Incorporation of a third attractive interaction (grey) allows for 2D bilayers to form. In all cases, repulsive interactions (e.g. electrostatic, in blue) prevent further aggregation of the assembly.

Fig. 2. Hydrophobic packing of triblock (A and B) and diblock (C) peptide amphiphiles as mono- and bilayers, respectively. (A) Janus nanosheets obtained from peptides with segregated hydrophobic blocks (Phe₆vs. C10), which favour parallel monomer alignment by Phe H-bonding and π–π stacking in two perpendicular axes. Reprinted with permission from ref. . Copyright 2017 American Chemical Society. (B) Steric zipping of complementary hydrophobic interfaces between adjacent β-sheets. Reprinted with permission from ref. . (C) Amphiphilic cyclic peptide with alternating (d/l) chirality with 2D self-assembly via inter-backbone axial H-bonding (3) and lateral Leu zippers between nanotubes (4). Adapted with permission from ref. . Copyright 2019 American Chemical Society.

Fig. 3. (A and B) Aromatic scaffolds allow precise control over the geometrical disposition of their substituents (A) and the establishment of complementary multivalent interfaces (e.g. nucleobases, B). (C–F) Examples of rigid aromatic monomers capable of 2D self-assembly and their minimised 3D structures (Avogadro v.1.2.0): seesaw (C), attractor–repeller (D), curved (E) and boat-shaped (F); green substituents represent polar pendants.

Fig. 4. (A) Illustration of geometric control attainable from trigonal multivalent host monomers and cucurbituril CB[8] cavitands as porous hexagonal 2D networks. (B) 2D self-assembly topologically encoded on the outer surface of pre-folded protein four-helix bundles: Two high-affinity Zn²⁺ coordination sites allow for the elongation of this protein in helical 1D assemblies, while a low affinity site provides a perpendicular elongation point for 2D growth. Polar contacts at interfacing protein domains (cyan) contribute to the stabilisation of the assembly. Reprinted with permission from ref. . Copyright 2012 Springer Nature. (C) Woven 2D assembly obtained from oligoproline monomers elongated though π–π stacking of their aromatic pendants. Each individual 1D suprapolymer (red, blue and orange) interlocks by top-down alternation of monomers, thus leaving periodically spaced voids filled by the other 1D chains. Adapted with permission from ref. . Copyright 2017 Springer Nature.

Fig. 5. Edge applications of supramolecular 2D assemblies. (A) Enantiomeric molecular recognition and pumping of racemic guests on chiral 2D porous nanosheets. Adapted with permission from ref. . Copyright 2018 Springer Nature. (B) Immobilisation of Pd nanoparticles on 2D nanosheets and subsequent catalytic Suzuki–Miyaura coupling. X = I or Br, R = H, CH₃, CH₃O, NO₂, CHO or CN, (i) K₂CO₃, ethanol, 65 °C. (C) Solvent-dependent self-assembly results in different supramolecular π-stacking and photoconductive anisotropy. MCH = methylcyclohexane. (D) Bacterial agglutination mediated by galactose-decorated nanosheets. Adapted from ref. . Copyright 2020 Wiley-VCH. (E) DNA-functionalised enzymes anchored selectively to each side of a DNA–peptide–DNA hybrid Janus nanosheet. GOx = glucose oxidase; HRP = horseradish peroxidase; TMB = 3,3′,5,5′-tetramethylbenzidine. Adapted from ref. . Copyright 2021 Wiley-VCH.

Ignacio Insua, Julian Bergueiro, Alejandro Méndez-Ardoy, Irene Lostalé-Seijo and Javier Montenegro