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

Molecular self-assembly is considered as a promising way to control the manufacture of new materials and their integration into hybrid devices with novel properties. In this work we have synthesized a fulleropyrrolidine bearing a phenylacetylene moiety via the Prato reaction. The characterization of the fulleropyrrolidine by nuclear magnetic resonance and optical spectroscopy is reported, and its self-assembly by crystallization study has been investigated according to the used solvents. If the solvent that effectively solubilizes fullerene derivative is tetrahydrofurane, the nano-square plates with 1–3 μm in length and 50–100 nm in thickness are formed, while if the solvent is toluene, 5 μm diameter ‘nano-flowers’ are obtained.


Introduction
During the last fifteen years, fullerene-based supramolecular assemblies have been extensively investigated. C 60 is a highly hydrophobic molecule with a perfect icosahedral symmetry. It also possesses exceptional photophysical properties and low reorganization energy [1]. All these properties make fullerene an ideal candidate for a large number of applications such as solar cells [2,3], organic light-emitting diode (OLED) [4], organic field-effect transistor (OFET) [5,6]. The high performance of these optoelectronic devices is often conditioned by supramolecular organized structures of fullerenebased materials. So it is the key to mastering the aspects of supramolecular organization in organic optoelectronic devices.
In this paper we explore the self-assembling properties of a fulleropyrrolidine functionalized with a phenylacetylene moiety via the Prato reaction. The characterization of this compound by nuclear magnetic resonance (NMR), optical spectroscopy, x-ray diffraction as well as microscopy allowed identifying the interactions that lead to self-assembly properties.

Synthesis of fulleropyrrolidine
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 C 60 to form fulleropyrolidines. The synthesis of fulleropyrrolidine 2 is described in scheme 1; it is obtained by condensation of 4-(2trimethylsilylethynyl)benzaldehyde 1 [40] and N-methylglycine onto C 60 .
Fulleropyrrolidine 2: C 60 (50 mg, 0.069 mmol) was dissolved in dry toluene 50 mL and then 4-(2-trimethylsilylethynyl)benzaldehyde (14 mg, 0.069 mmol) and Nmethylglycine (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 CH 2 Cl 2 and precipitation by pouring the solution into MeOH) gave pure 2 as a brown powder.

Techniques
Absorption spectra were recorded in quartz cuvettes on a Perkin-Elmer Lambda 900 UV-Vis-NIR spectrophotometer. 1 H 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 Persepti-veBiosystems 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.

UV-visible absorption spectra
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].

1 H NMR spectra
Due to the low solubility of compound 2 in CDCl 3 , the signalto-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 1 H 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 NCH 3 resonate at 2.76 ppm and the protons of Si (CH 3 ) 3 at 0.2 ppm.

Self-assembly and SEM photographs
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.
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.

X-ray diffraction and discussion on the formation of precipitate
To understand the organization of molecules in the nanoplates and in the nano-flowers, the precipitates were studied by x-ray diffraction. The diffraction pattern of 2 in the nanoplates ( figure 5(a)) shows three reflections, d 1 = 21.675 Å; d 2 = 10.77 Å and d 3 = 7.085 Å. These reflections, with a spacing ratio 1:2:3, indicate a long-range lamellar organization of the C 60 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, d 1 = 24.4 Å, d 2 = 12.18 Å and d 3 = 8.16 Å ( figure 5(b)). These three reflections show once again the lamellar organization of the fulleropyrrolidine in the nanostructures.
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 nanoplates and 22°within the layers of the nano-flowers (cosα = d/26,4) (figure 6(c)).
'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.

Conclusion
We have described herein the synthesis and characterization of fulleropyrrolidine monoadduct bearing a phenyltrimethylsilylacethylene moiety formed by Prato reaction. The  self-assembly properties of the fulleropyrrolidine derivative have been investigated. The formation of the nano-plates and 'nano-flowers' seem more dependent on the interactions of the molecules with the solvent than the interactions between molecules themselves. Indeed, the organization is governed by the careful balance between 'good' and 'bad' solvents for the fullerene derivatives.

Acknowledgment
This work has been partially supported by ANR (project f-DNA ANR-09-NANO-005-01). The authors warmly thank Benoit Heinrich and Dr Bertrand Donnio from the IPCMS in Strasbourg (France) for performing small-angle x-ray diffraction experiments.