Biasing the Hierarchy Motifs of Nanotoroids: from 1D Nanotubes to 2D Porous Networks

Abstract

Hierarchical organization of self-assembled structures into superstructures is omnipresent in Nature but has been rarely achieved in synthetic molecular assembly due to the absence of clear structural rules. We herein report on the self-assembly of scissor-shaped azobenzene dyads which form discrete nanotoroids that further organize into 2D porous networks. The steric demand of the peripheral aliphatic units diminishes the trend of the azobenzene dyad to constitute stackable nanotoroids in solution, thus affording isolated (unstackable) nanotoroids upon cooling. Upon drying, these nanotoroids organize at graphite surface to form well-defined 2D porous networks. The photoirradiation with UV and visible light enabled reversible dissociation and reconstruction of nanotoroids through the efficient trans↔cis isomerization of azobenzene moieties in solution.

Keywords: azobenzene; hierarchical organization; nanotoroids; photoresponsive systems; supramolecular polymers.

Figure 1

a) Molecular structures of scissor‐shaped azobenzene dyads (S)‐1, (S)‐2 and 2. b),c) Representations of the self‐assembly of b) (S)‐1 and c) (S)‐2 and 2.

Figure 2

a) VT‐UV/Vis spectra of (S)‐2 in MCH (c _T=300 μM) at different temperatures. The temperature interval between spectra was 5 °C. b) Plots of the variation of the molar extinction coefficient (ϵ) of (S)‐2 at 342 nm versus temperature extracted from VT‐UV/Vis spectra. c) Representations of the self‐assembly process of (S)‐2.

Figure 3

a),b) AFM images of nanotoroids of (S)‐2 in MCH (c _T=100 μM) obtained by cooling a hot MCH solution to 10 °C. Inset in (a) shows a wider range AFM image. Inset in (b) shows a cross‐sectional analysis along the pink line in (b).

Figure 4

a),b) AFM images of a) stackable nanotoroids of (S)‐1 and b) unstackable nanotoroids of (S)‐2 obtained by spin‐coating of these MCH solutions onto HOPG substrates. c) AFM cross‐sectional analysis of the nanotoroids formed by (S)‐1 (along the blue line in (a)) and by (S)‐2 (along the pink line in (b)). d) Representations of the proposed toroidal structures of (S)‐1 and (S)‐2, respectively. Bulky wedge groups located inside the nanotoroid of (S)‐2.

Figure 5

a),b) AFM image of nanotoroids of (S)‐2 in a) n‐octane and b) n‐dodecane (c _T=100 μM) obtained by cooling a hot MCH solution to 10 °C. Inset in (a) shows a magnified AFM image of nanotoroids. c),d) Plots of the variation of the molar extinction coefficient (ϵ) of (S)‐2 at 342 nm versus temperature in c) n‐octane and d) n‐dodecane extracted from VT‐UV/Vis spectra.

Figure 6

a) UV/Vis spectra of (S)‐2 in MCH (c _T=300 μM) at 20 °C recorded at as‐prepared (blue spectrum), PSS_UV (purple spectrum) and PSS_Vis (sky‐blue spectrum), respectively. b) Plots of the change in the mole fraction of trans‐isomer as a function of UV‐ (purple circles) and visible‐light (sky‐blue circles) irradiation time, respectively. c)–e) AFM images of (S)‐2 in MCH (c _T=300 μM) obtained by irradiation with c),d) UV‐ and e) visible light to the as‐prepared MCH solution. These solutions were diluted to 100 μM before spin‐coating onto HOPG substrates.