Hierarchically Ordered Self‐Assembly of Amphiphilic Bifullerenes

Abstract

A series of novel functionalised dumbbell‐shaped bifullerenes in which two [5.0] pentakis‐adducts of C60 are covalently connected by cyclic bismalonates were synthesised. These dimeric compounds, carrying various combinations of hydrophilic and hydrophobic addends, self‐assemble in aqueous solution towards supramolecular architectures of different structural complexity as observed by cryogenic transmission electron microscopy (cryo‐TEM). The detailed analysis of the image data revealed an unprecedented hierarchical aggregation behaviour. Whereas completely hydrophilic substituted bifullerenes formed profoundly monodisperse populations of small oligomeric elementary micelles consisting of only three or four bifullerene molecules in a supposedly bent conformation, their amphiphilic equivalents underwent a hierarchical two‐step assembly process towards larger spherical and even rod‐like structures. The data suggest that the hierarchical assembly process is driven by hydrophobic interactions of preformed tetrameric elementary micelles.

Designed Enclosure Enables Guest Binding Within the 4200 Å3 Cavity of a Self‐Assembled Cube

“…[21] Theg uest-binding properties of 2 were investigated by screening the same neutral and anionic guests explored previously with 1a [9a] and 1b.Although no neutral guests were found to undergo complexation, several large,anionic species were observed to bind inside the cavity of 2,i ncluding hexamolybdate (Mo 6 À ;see Sections 3.1-3.5, respectively,inthe Supporting Information). In all the studies,the binding processes were followed by 1 HNMR titrations in [D 3 ]acetonitrile,w hereby the pyridine protons closest to the Pt II centers on the faces of 2 (H 1 )w ere observed to shift upfield upon saturation of the solution with ag uest, thus indicating fast exchange on the NMR time scale;the imine peak of 2 also underwent spectral shifts consistent with guest binding.S uch shifts were not observed during host-guest investigations with 1a or 1b,and we infer they were caused by binding of the guest to the cavity of 2.Downfield chemical shifts in the 19 FNMR spectra of the fluorinated species B 12 F 12 2À and B(C 6 F 5 ) 4 À were observed (Figures S18 and S27, respectively) in the presence of 2, consistent with their binding.N OESY correlations between BPh 4 À and the Pt II -bound pyridine protons from 2 (H 1 and H 2 ) were observed ( Figure S21), which suggested aC oulombic attraction of the negatively charged guest to the center of the faces of the host;t he DOSY spectrum of the host-guest complex also revealed aslower diffusion coefficient for BPh 4 À when compared to free BPh 4 À in solution ( Figure S22). Job's plots by UV/Vis spectroscopy identified the stoichiometry of binding to be 1:1i na ll cases,a nd the titration data were, therefore,fitted to 1:1binding isotherms (see Section 3inthe Supporting Information);the low solubility of the host-guest complexes required the stoichiometry experiments to be ,was also observed to bind more strongly than as maller guest, B 12 F 12 2À .T his observation is consistent with higher occupancyl eading to better binding,d espite the low occupancyf actors observed with all anions (Table 1);w ei nfer at least part of the remaining cavity volume to be occupied by solvent molecules.…”

“…Thea pproximately O-symmetric solid-state structure of 1b (Figure 2a,c) is consistent with the high-symmetry NMR spectra recorded in solution. Although X-ray analysis identified eight BF 4 À ions occupying the cavity of 1b (Figure 2a), no evidence of encapsulation was observed in solution by 19 FNMR spectroscopy.T he eight tris(pyridylimine)iron(II) vertices in 1b all have the same D or L stereochemistry;both enantiomers are present in the crystal lattice.A lthough the eight tris(pyridylimine)cadmium(II) vertices in 2 also have facial stereochemistry,X -ray analysis identified four metal centers on one face to have L handedness,with four D metal centers opposite,t hus giving the cube approximate C s point symmetry, which has not previously been observed for M 8 L 6 structures [17] (Figure 2b,d). As the observed NMR spectra of 2 in solution indicated higher symmetry, we posit that the solid-state arrangement of 2 resulted from maximization of CÀ H···p interactions between neighboring anthracenes around the corners, thereby rendering the C s diastereomer the most energetically favorable conformation in the solid state.…”

“…[19] We employed cryogenic transmission electron microscopy (cryo-TEM) imaging to visualize directly the structure of 2 in a1 :1 water/acetonitrile solution (3 10 À6 m;F igure 3). The cryo-TEM images displayed lamellar structures consistent with the formation of aggregates of 2.Whenthese structures Figure 1.…”

Two-step self-assembly of hierarchically-ordered nanostructures

“…83 In this example, small particles 3-4 nm in diameter were first formed from hydrophilic substituted bifullerene derivatives and continued to aggregate towards higher architectures in the case of the amphiphilic substituted bifullerenes. The second aggregation step is directed by the exposure of the hydrophobic surfaces due to the reduced number of hydrophilic substituents.…”

Designed Enclosure Enables Guest Binding Within the 4200 Å3 Cavity of a Self‐Assembled Cube

“…Thea pproximately O-symmetric solid-state structure of 1b (Figure 2a,c) is consistent with the high-symmetry NMR spectra recorded in solution. Although X-ray analysis identified eight BF 4 À ions occupying the cavity of 1b (Figure 2a), no evidence of encapsulation was observed in solution by 19 FNMR spectroscopy.T he eight tris(pyridylimine)iron(II) vertices in 1b all have the same D or L stereochemistry;both enantiomers are present in the crystal lattice.A lthough the eight tris(pyridylimine)cadmium(II) vertices in 2 also have facial stereochemistry,X -ray analysis identified four metal centers on one face to have L handedness,with four D metal centers opposite,t hus giving the cube approximate C s point symmetry, which has not previously been observed for M 8 L 6 structures [17] (Figure 2b,d).…”