Chiral Nanotubes

Abstract

Organic nanotubes, as assembled nanospaces, in which to carry out host-guest chemistry, reversible binding of smaller species for transport, sensing, storage or chemical transformation purposes, are currently attracting substantial interest, both as biological ion channel mimics, or for addressing tailored material properties. Nature's materials and machinery are universally asymmetric, and, for chemical entities, controlled asymmetry comes from chirality. Together with carbon nanotubes, conformationally stable molecular building blocks and macrocycles have been used for the realization of organic nanotubes, by means of their assembly in the third dimension. In both cases, chiral properties have started to be fully exploited to date. In this paper, we review recent exciting developments in the synthesis and assembly of chiral nanotubes, and of their functional properties. This review will include examples of either molecule-based or macrocycle-based systems, and will try and rationalize the supramolecular interactions at play for the three-dimensional (3D) assembly of the nanoscale architectures.

Keywords: anisotropic materials; chirality; nanotubes; supramolecular polymers; three-dimensional (3D) assembly.

Figure 1

Classification of tubular objects, and a schematic representation of a biological tubule (micrometer scale), and of a single-walled carbon NT (SWCNT) (nanometer scale).

Figure 2

Molecular structure of the cyclopeptide-polymer conjugate 1 and illustration of pH responsiveness in the formation of NTs. Partially reproduced with permission from Ref. [35] (Copyright American Chemical Society, 2016).

Figure 3

Comparison between triazole and amide functionalities, and diastereoisomeric rotamers of macrocycle 2. For the comparison between dipole moments, see Ref. [37].

Figure 4

R-Δ and S-Δ macrocycles 3a,b (top left). Schematic view of the [C–H···O] interactions between the NDI units (top right): (A) bottom: top view showing that the coaxial DBA chain is stabilized through latitudinal [Br···π] interactions; and (B) bottom schematic illustration of nanotubular structure. Partially reproduced with permission from Ref. [45] (Copyright American Chemical Society, 2014).

Figure 5

Comparison of single-crystal X-ray superstructures: nonhelical tetrameric unit of DBA&R-Δ (on left) and (P)-helical tetrameric DCA&R-Δ (on right). Reproduced with permission from Ref. [45] (Copyright American Chemical Society, 2014).

Figure 6

Amino acid functionalized NDIs 4a–c. Schematic representation of the equilibria between anti and syn conformations of the NDI monomer (on top) and possible NDI aggregates (above). Partially reproduced with permission from Ref. [46] (© American Chemical Society).

Figure 7

¹H-NMR spectrum of the NDI protons of a solution of 4a in TCE, at different temperatures. Partially reproduced with permission from Ref. [46] (Copyright American Chemical Society, 2012).

Figure 8

Monomer 9 with two orthogonal 2H-bonding sites (marked as red and blue). Molecular models of 2H-bonded tetramer 9₄ and the corresponding polymeric tube, represented as 9₈. Schematic representation of: (A) tetrameric cyclic aggregate in CDCl₃; (B) nanotubular structure in toluene-d₈; and (C) nanotubular structure in CDCl₃ with C₇₀ as guest. Partially reproduced with permission from Ref. [48] (Copyright Nature Publishing Group, 2017).

Figure 9

Monomer 10; schematic representation of: (A) tetrameric cyclic aggregates in CDCl₃; (B) capsule-like insertion complex C₆₀@9₄ and tetramer 10₄, in CDCl₃; and (C) capsule-like insertion complex C₆₀@10₄ and nanotubular structure 9_n, in toluene-d₈. Partially reproduced with permission from Ref. [48] Copyright Nature Publishing Group, 2017).

Figure 10

(A) Building blocks used in self-assembly of noncovalent NTs; (B) NTs section formed from 12a with reversible dilatation-contraction in function of thermal response; and (C) stimuli response on NTs in function of the temperature in sensing of C₆₀. Reproduced with permission from Ref. [50] (Copyright the American Association for the Advancement of Science, 2012).

Figure 11

PDI-based building blocks for construction of chiral NTs. Partially reproduced with permission from Ref. [52] (Copyright Wiley-VCH, 2016).

Figure 12

Top: Structure of amphiphile 15a and 15b. Bottom: Schematic model for the observed behavior of NTs consisting of 15a and 15b as a function of the amount of 15b: (a) pure 15a; long, isolated achiral nanotubes; (b) <25% 15b; long, isolated achiral nanotubes; (c) 25–50% 15b; long, isolated chiral nanotubes; (d) >50% 15b; short, bundled chiral nanotubes; and (e) pure 15b; short, bundled chiral nanotubes. Partially reproduced with permission from Ref. [53] (Copyright Royal Society of Chemistry, 2017).

Figure 13

(A) Building blocks used for construction of NTs; and (B) schematic illustration of possible noncovalent structure upon complexation with Ag(I). Partially reproduced with permission from Ref. [60] (Copyright the American Association for the Advancement of Science, 2014).

Figure 14

TEM micrographs of air-dried MeCN/water dispersions of: (A) 16; (B) 17; (C) CD active NT formed by 16 in presence of (+/−)-MS; and (D) CD spectrum of NTs obtained using (+)-MS (red line) and (−)-MS (blue line). Partially reproduced with permission from Ref. [60] (Copyright the American Association for the Advancement of Science, 2014) and [61] (Copyright the American Chemical Society, 2015).

Figure 15

(A) Cartoon representation of the design; (B) chemical structure of the D₂ symmetrical macrocycle (RR)-18; (C) CD spectra of the macrocycle, and of the aggregates in MeCN solutions upon addition of two equivalents of the bidentate Pd²⁺; and (D) AFM images of fibers formed by (RR)-18, C60 and PdCl₂(MeCN)₂, with cross section of ca. 1 nm. Partially reproduced with permission from Ref. [62] (Copyright Royal Society of Chemistry, 2015).

Figure 16

Molecular structure of 19 and fabrication of hierarchical chiral nanostructures, which was based on the self-assembly of 3 and regulated by solvents and metal ions: (A) nanotubes (scale bar = 100 nm); and (B) flower-like structures (scale bar = 1 μm). Partially reproduced with permission from Ref. [73] (Copyright Wiley-VCH, 2016).

Figure 17

Structures of m-PE macrocycles 20a–l (top left), QMD simulation of a helical stack of the macrocycles (top right), and variable-temperature CD spectra of 20i and 20k measured in CCl₄ (10 µM) (bottom). Partially reproduced with permission from Ref. [80] (Copyright Nature Publishing Group, 2012) and [81] (Copyright the American Chemical Society, 2016).