The main issue concerning the use of carbon-based nanomaterials is their poor solubility in organic and aqueous solvents. To solve this problem, chemists have developed methods which facilitate their manipulation in solution and permit the incorporation of new functionalities on their surface. These methods of functionalization are based on: (i) the covalent grafting of molecules onto the sp2 carbon of the CNTs or (ii) the non-covalent functionalization by adsorption of polycyclic aromatic and/or amphiphilic derivatives via hydrophobic interactions. These methods have been widely used to combine carbon nanotubes and graphene with photo- and electroactive materials (in particular porphyrin derivatives). Porphyrins and phthalocyanines are very attractive molecules for optoelectronic, spintronic or catalytic applications. The fabrication of nanohybrids combining such molecules with nanotubes and graphene thus holds great promises for the development of new electronic, light harvesting or catalytic systems.
The fabrication of molecules exhibiting tailor-made properties often requires several synthetic steps and therefore efficient reactions must be used to combine them with nanotubes. In this context, we demonstrated that CuAAC (copper-mediated azide-alkyne cycloaddition) can provide an elegant solution to functionalize efficiently SWNTs with chromophores.
Upon illumination, strong electronic interactions between the photo- and electroactive constituents were observed, leading to rapid excited-state deactivation of the chromophores in the presence of the nanotubes suggesting that the nanoconjugates can be used for the fabrication of photoactive electrodes.
It is well established that the covalent grafting of molecules onto the nanotube sidewalls gives rise to robust conjugates since the nanotubes and the addends are linked through covalent bonds. However, the transformation of carbon atoms hybridized sp2 into sp3 in the nanotube framework induces a sizeable loss of their electronic properties. On the contrary, the non-covalent functionalization permits to better preserve the electronic properties of the nanotubes. So, for a number of applications, the non-covalent approach is preferred. However, the major limitation of this method is the lack of stability of the resulting assemblies. Indeed, molecules adsorbed onto the nanotube sidewall can desorb, more or less easily (depending on the size and/or conformation of the grafted molecules/polymers), when for example the solvent changes or the nanotubes are filtered.
We are now working on new methods of non-covalent functionalization which combine the advantages of both covalent and non-covalent methods without their respective drawbacks. Our method is based on the controlled polymerization of hydrophobic molecules on nanotubes dispersed in micelles. The nanotube hybrids can be purified, manipulated and dispersed without loss of their functionality. This method is currently used to fabricate new catalytic and light harvesting nanohybrids.
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