Synthesis and self-assembly properties of fulleropyrrolidine prepared by Prato reaction

C₆₀ (99.9%) was purchased from MER Corporation. Chemicals were purchased from Aldrich and were used as-received. Solvents were purchased form Aldrich or VWR and were used as-received. For synthesis, CH₂Cl₂ (CaH₂, N₂), toluene (K/benzophenone, N₂), tetrahydrofuran (THF) (K/benzophenone, N₂) were distilled before use.

The Prato reaction [39] is an example of cycloaddition [3 + 2] from the azomethine ylides which are highly reactive 1,3 dipoles. The ylide is generated in situ after decarboxylation of iminium salts obtained by condensation of amino acids and aldehydes. These ylides react with the C₆₀ to form fulleropyrolidines. The synthesis of fulleropyrrolidine 2 is described in scheme 1; it is obtained by condensation of 4-(2-trimethylsilylethynyl)benzaldehyde 1 [40] and N-methylglycine onto C₆₀.

Zoom In Zoom Out Reset image size

4-(2-trimethylsilylethynyl)benzaldehyde 1 was synthesized by Sonosashira coupling of 4-bromobenzaldehyde and trimethylsilylacetylene: To a stirred mixture of 4-bromobenzaldehyde (9.25 g, 50.0 mmol), CuI (380 mg, 2.0 mmol), and Pd(PPh₃)₂Cl₂ (700 mg, 1.0 mmol) in 50 mL of THF was added triethylamine (10.1 g, 75.0 mmol). A solution of trimethylsilylacetylene (5.15 g, 52.5 mmol) in 10 mL of THF was then added over one hour. The solvent was evaporated, and the residue was treated with pentane. The filtration through Celite and evaporation of the solvent yielded compound 1.

Fulleropyrrolidine 2: C₆₀ (50 mg, 0.069 mmol) was dissolved in dry toluene 50 mL and then 4-(2-trimethylsilylethynyl)benzaldehyde (14 mg, 0.069 mmol) and N-methylglycine (62 mg, 0.693 mmol) were added. The mixture was stirred overnight at reflux, and then evaporated to dryness. The purification of the residue by column chromatography (eluent toluene) and precipitation (dissolution in CH₂Cl₂ and precipitation by pouring the solution into MeOH) gave pure 2 as a brown powder.

Absorption spectra were recorded in quartz cuvettes on a Perkin–Elmer Lambda 900 UV-Vis-NIR spectrophotometer. ¹H NMR spectra were recorded with a Bruker ac-300 (300 MHz) spectrometer with solvent used as internal reference; MS (MALDI-TOF) spectra were recorded with a PerseptiveBiosystems Voyager DE-STR spectrometer. Scanning electron microscopy (SEM) measurements were performed using a Hitachi S4500 microscope. Molecular modeling was performed using the HyperChem software in conjunction with the MM+ method.

The UV-visible absorption spectra of fulleropyrrolidine 2 in toluene shows two-band characteristics: a narrow and intense peak at 430 nm and a broad band around 700 nm, as shown in figure 1. These peaks are characteristic of fulleropyrrolidine monoadducts [39].

Zoom In Zoom Out Reset image size

Due to the low solubility of compound 2 in CDCl₃, the signal-to-noise ratio of the NMR spectrum is not very good, but can be observed in the region between 4–5 ppm three protons belonging to the pyrrolidine (enlarged view of ¹H NMR spectra shown in 4–5 ppm) (figure 2). We note that the four aromatic protons resonate at 7.50 and 7.73 ppm. The signals of these protons should be doublets any time; because of the presence of the fullerene, they appear as a broad signal. The protons of NCH₃ resonate at 2.76 ppm and the protons of Si(CH₃)₃ at 0.2 ppm.

Zoom In Zoom Out Reset image size

We started by investigating the self-assembly properties of fulleropyrrolidine 2. The sample was dissolved in THF at a concentration of 1 mM, 500 μl of this solution was filled into NMR tube and acetonitrile (AcCN) was slowly added to the top of the tube. This tube was capped and the solution was left two days to allow the slow diffusion of AcCN in THF. After two days at room temperature, the formation of precipitate is observed. The suspension was then homogenized and a drop was deposited on a silicon substrate to be imaged by SEM (scheme 2). Figure 3 shows a typical example of the images obtained: nano-square plates with 1–3 micrometers in length and 50–100 nm in thickness.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Nakanishi has extensively studied the different architectural aspects of fullerene self-assembly as a function of the solvents. They showed that very simple molecules can give rise to a wide variety of assemblies according to the solvents used [12]. The difference of organization results from the balance between the 'good' and 'bad' solvents for fullerene in the mixture. Therefore, we decided to explore the difference between two 'good' solvents of the fulleropyrrolidine 2. We solubilized compound 2 in toluene and then AcCN was added. After seven days, the suspension was imaged by SEM, figure 4 shows the type of assemblies obtained: 'nano-flowers' with about 5 microns in diameter. It is worth mentioning that toluene is a good solvent for both unfunctionalized fullerene and for compound 2 while THF is able to solubilize only the fulleropyrrolidine derivative. The different interactions of the toluene and THF with the fullerene part of 2 are certainly responsible for the difference of supramolecular organization.

Zoom In Zoom Out Reset image size

To understand the organization of molecules in the nano-plates and in the nano-flowers, the precipitates were studied by x-ray diffraction. The diffraction pattern of 2 in the nano-plates (figure 5(a)) shows three reflections, d₁ = 21.675 Å; d₂ = 10.77 Å and d₃ = 7.085 Å. These reflections, with a spacing ratio 1:2:3, indicate a long-range lamellar organization of the C₆₀ moieties with an average lamellar periodicity of d = 21.5 Å. Other reflections, typically (hkl) with h, k or l indices simultaneously non-zero, reflect three-dimensional extension of the supramolecular nanostructure. In the same way, the diffraction pattern of the nano-flowers shows three reflections, d₁ = 24.4 Å, d₂ = 12.18 Å and d₃ = 8.16 Å (figure 5(b)). These three reflections show once again the lamellar organization of the fulleropyrrolidine in the nanostructures.

Zoom In Zoom Out Reset image size

By molecular modeling, we estimated that the length of a molecule of 2 is 17.7 Å (figure 6(a)). The interlamellar distance of about 21.5 Å and 24.4 Å in the nano-plates and the nano-flowers, respectively, do not correspond to either the length of a molecule or the length of an interdigitated bilayer of 2 (26.4 Å—figure 6(b)). It is therefore inferred that the molecules are inclined about 35° in the layers of the nano-plates and 22° within the layers of the nano-flowers (cosα  = d/26,4) (figure 6(c)).

Zoom In Zoom Out Reset image size

'Nano-flowers' assemblies have been obtained by Nakanishi et al by slow cooling of a solution of fulleropyrrolidine containing long alkyl chains in 1,4-dioxane at 60 °C to 5 °C [41]. They explained the formation of these objects by the successive folding of a very thin film of molecules.