FULLERENES

Fullerene-based Nanohybrids

We are interested on the organic functionalization of fullerenes in order to create novel nanohybrids potentially useful for diverse nano- and bio-technological applications. Representative examples include: i) donor-acceptor hybrids for mimicking natural photosynthesis, for photovoltaics, for conversion of light into electric current and for energy storage, ii) optical limiting materials, for sol-gel processes and liquid crystals, and iii) biologically active fullerene-based hybrid materials for biomedical applications.

Azafullerenes

Substitution of a carbon atom from a fullerene cage by a heteroatom leads to the formation of heterofullerenes. Azafullerenes, in which a carbon atom has been replaced by a nitrogen atom are the only heterofullerenes synthesized so far by rational organic synthesis. Due to the difference in valence between carbon and nitrogen, azafullerene is an open-shell and as such it rapidly dimerises to (C59N)2 or abstracts a hydrogen to form C59HN. We are interested on the organic transformations of azafullerenes as well as on their electronic, magnetic and photophysical properties.

Endohedral Metallofullerenes

Introducing metal ions, atoms, carbides or clusters within the empty space of the hollow fullerene cage leads to endohedral metallofullerenes. We are interested on this novel class of materials and more specifically on their electronic, magnetic and photophysical properties as well as their utility as guest nanomaterials within the empty space of carbon nanotubes.

CARBON NANOTUBES

In the bottom-up approach to molecular nanotechnology, fabrication and manipulation takes advantage of novel nanomaterials possessing intriguing potential. Carbon Nanotubes (CNT) as effectively long, thin cylinders of graphite seems to be ideal building blocks in nanotechnology. However, high molecular weights and strong intertube forces keep CNT together in bundles, making their manipulation, characterization and analytical investigation very difficult. The organic functionalization offers the great advantage of producing soluble and easy-to-handle CNT. Consequently, compatibility of CNT with other materials, such as polymers, should be strongly improved.

Covalent Functionalization

The chemical modification of CNT is an emerging area in materials science. The 1,3-dipolar cycloaddition of azomethine ylides generated in-situ by thermal condensation of aldehydes and -aminoacids gives rise to soluble functionalized CNT materials. Novel nanohybrids consisting CNT and electron donors or amine functionalities amenable to further transformations, can be generated.

Non-covalent Functionalization / Supramolecular Interactions

In another approach, electrostatic interactions can be used for the generation of novel CNT-based donor-acceptor nanoensembles. With this approach, CNT are first brought into solution upon supramolecular and/or van-der-waals interactions and then are coupled via coulombic attractive forces with the corresponding electron donors.

Characterization Techniques

The solubilization of CNT provides opportunities for spectroscopic characterization of these unique structures. Solution phase absorption spectroscopy offers useful information about electronic transitions of CNT. Proton nuclear magnetic resonance spectroscopy (1H NMR) is a very important tool for the structural assignment of organic and inorganic substances. However, proton signals appear to be broad as a result of the statistical distribution of the addends on the CNT surface. Raman spectroscopy is an extremely useful tool for characterizing CNT. Pristine CNT exhibit two main characteristic absorptions: the diameter-dependent radial breathing mode (RBM) at 250 cm-1 and the higher frequency tangential mode at 1580 cm-1 respectively. In addition, functionalized CNT show a third mode at 1295 cm-1, the so-called disorder or sp3 mode and its relative intensity corresponds to the degree of disruption (functionalization) of the CNT framework. Thermal gravimetric analysis (TGA) gives information about functionalized CNT because most of the covalent attachments can be thermally eliminated.
Microscopy techniques such as TEM (transmission electron microscopy), SEM (scanning electron microscopy) and AFM (atomic force microscopy) offer much insight for the presence of CNT in solution. Thus, in such a way, CNT samples can be monitored qualitatively obtaining evidence about dimensions, purity and aggregation. However, in order to observe species attached on the surface of CNT, high-resolution instrumentation is required.

CARBON NANOHORNS

Carbon nanohorns (CNHs), a new carbon allotrope within the family of elongated CNTs, has recently emerged as an ideal and promising material for a multitude of technological as well as biomedical applications. The production of CNHs involves laser ablation of a graphite target, at room temperature, in high yield and purity. Importantly, the total absence of any catalyst in the as-produced CNHs is considered their major advantage as compared with the metal and/or amorphous carbon contaminated CNTs. Pristine CNHs aggregate in a dahlia-like shape, with an average diameter of 80 nm, while a large number of conical-shaped single-layered tips sticking out in all directions of the spherical superstructure. Importantly, these tips are capped by highly stretched five-membered rings. Therefore, it comes without surprise that the unique conical structure of CNHs has great influence on the electronic properties of this material, as it was shown for example, that the existence of a localized electron state at the Fermi level results in enhanced charge density near the conical-shaped tip. Numerous key-challenges arise around the yet virgin field of CNHs allotropes. Can these spherical secondary superstructures serve donor- or acceptor-functions in novel nanoconjugate and/or nanohybrid systems? Can this enhanced charge density near the conical-shaped tip of CNHs lead to new paradigms in electron-transfer processes? How this unique morphology of CNHs, possessing structures that resemble fullerenes at the conical tips (i.e. due to the presence of 5 five-membered rings) and CNTs at the side-walls, affect the chemistry of CNHs in terms of not only their skeleton modification by surface grafting but also their hollow empty space that expands as extending from the conical tips? Our group is deeply involved in the study of this novel material and more specifically on the chemical functionalization of CNHs by diverse means, covalently at the side-walls or conical tips as well as non-covalently via supramolecular interactions, trying to answer the above mentioned questions.