Embedding and Positioning of Two FeII4 L4 Cages in Supramolecular Tripeptide Gels for Selective Chemical Segregation

Abstract

An unreported d,l-tripeptide self-assembled into gels that embedded Fe^II₄ L₄ metal-organic cages to form materials that were characterized by TEM, EDX, Raman spectroscopy, rheometry, UV/Vis and NMR spectroscopy, and circular dichroism. The cage type and concentration modulated gel viscoelasticity, and thus the diffusion rate of molecular guests through the nanostructured matrix, as gauged by ¹⁹ F and ¹ H NMR spectroscopy. When two different cages were added to spatially separated gel layers, the gel-cage composite material enabled the spatial segregation of a mixture of guests that diffused into the gel. Each cage selectively encapsulated its preferred guest during diffusion. We thus present a new strategy for using nested supramolecular interactions to enable the separation of small molecules.

Keywords: chemical separation; host-guest systems; low-molecular-weight gelators; metal-organic cages; self-assembly.

Figure 1

a) Self‐assembly of (p‐aminobenzoyl)‐l‐Phe‐D‐Ala‐l‐Phe‐NH₂ into gel‐forming fibrils. b,c) Synthesis of cages 1 and 2 by subcomponent self‐assembly and encapsulation of ReO₄ ⁻ or TFA⁻ in 1 and FA in 2.

Figure 2

a,b) Peptide and either cage 1 or 2 yield 1⊂Gel and 2⊂Gel, respectively. c,d) TEM images show the fibrils of the gels and EDX spectra (insets) show the presence of Fe from MOCs at 1 mm in 1⊂Gel (c) and 2⊂Gel (d). Scale bar=100 nm. e,f) Gelation kinetics depict G′ (dark blue and dark purple) and G′′ (light blue and light pink) for 1⊂Gel (e) and 2⊂Gel (f).

Figure 3

a) ReO₄ ⁻ uptake kinetics by 1 in solution (light‐blue diamonds) and by 1⊂Gel (dark‐blue dots) and diffusion of ReO₄ ⁻ in 1⊂Gel (black circles). b) FA uptake kinetics by 2 in solution (pink diamonds) and by 2⊂Gel (purple dots) and diffusion of FA in 2⊂Gel (black circles). Dotted lines represent the asymptotic fitting from which the initial rates (k _ini) were deduced.

Figure 4

a) Photograph of the three‐layered gel and b) 2D mapping of the ¹H NMR spectra for the three‐layered gel, showing the presence of MOC 1 in layer 1 (1⊂Gel), the peptide alone in the buffer gel layer, and MOC 2 in layer 2 (2⊂Gel). c) ¹⁹F NMR spectra of layer 1 (top, blue) and layer 2 (bottom, purple) showing, respectively, the decrease in the peak for encapsulated TFA⁻ (▴) and the increase in both the encapsulated (▪) and free FA (•) over time after the addition of mixed FA and ReO₄ ⁻ (1 equiv each).