Jean-François Nierengarten

Chemical modification of C60 for materials science applications

Fullerene derivatives have shown a wide range of physical and chemical properties that make them attractive for the preparation of supramolecular assemblies and new materials. As a part of this research, we are interested in the chemical modification of C60 for various applications. These results are summarized in the present account to illustrate the current state-of-the-art of fullerene chemistry for the development of new advanced materials. In the first part, some of the fundamental architectural requirements needed for the design of amphiphilic fullerene derivatives capable of forming stable Langmuir films are reported. In particular, we have shown that the encapsulation of C60 in a cyclic addend surrounded by long alkyl chains, cholesterol subunits or dendritic branches is an efficient strategy to prevent the irreversible aggregation resulting from the strong fullerene-fullerene interactions usually observed for amphiphilic C60 derivatives at the air–water interface. In the second part, water-soluble C60 derivatives and fullerene-functionalized dendrimers are described. These compounds are easy to process, owing to their high solubility, and thus easily incorporated in mesoporous silica glasses for optical limiting applications. Finally, our recent developments on organic photovoltaic cells using covalently linked fullerene-(π-conjugated oligomer) ensembles as the active layer are summarized. This molecular approach appears to be particularly interesting since the behavior of a unique molecule in a photovoltaic cell and the study of its electronic properties means one can easily obtain the structure-activity relationships leading to a better understanding of the photovoltaic conversion.

Fullerenes : principles and applications

Preface Production, Isolation and Purification of Fullerenes Basic Principles of the Chemical Reactivity of Fullerenes Three Electrodes and a Cage: an Account of Electrochemical Research on C60, C70 and their Derivatives Light Induced Processes in Fullerene Multicomponent Systems Dendritic Encapsulation of Fullerenes to Facilitate their Nanoscopic Organization Hydrogen Bonding Donor-Acceptor Carbon Nanostructures Fullerenes for Material Science Plastic Solar Cells Using Fullerene Derivatives in the Photoactive Layer Fullerene Modified Electrodes and Solar Cells Biological Applications of Fullerenes Covalent and Non-covalent Approaches towards Multifunctional Carbon Nanotube Materials Subject Index

Glycosidase Inhibition with Fullerene Iminosugar Balls: A Dramatic Multivalent Effect†

The electronic and structural properties of fullerene derivatives make them very attractive candidates for the construction of nanostructures that are potentially useful for applications in materials science and biological chemistry. In particular, the C60 hexakis adducts with a Th-symmetrical octahedral addition pattern initially developed by Hirsch and co-workers are unique organic molecules with an appealing compact spherical scaffold for the construction of multifunctional nanomaterials. However, the synthesis of functionalized fullerene hexakis adducts from malonates and C60 is difficult. 4] This major problem limits the applications of such systems and has been recently solved by the development of synthetic methodologies based on the postfunctionalization of easily accessible building blocks of fullerene hexakis adducts. 6] It has been shown that fullerene hexakis adducts that bear 12 peripheral carbohydrate moieties can be prepared in excellent yields by grafting unprotected sugar derivatives onto the fullerene core. Although these fullerene sugar balls are obviously perfectly suited for applications in the field of carbohydrate–lectin interactions, the evaluation of carbohydrate-processing enzyme inhibition with such multivalent derivatives is less obvious. Indeed, among the possible strategies to attain specific potent glycosidase inhibition, the concept of multivalent design has been clearly overlooked. Most enzymes actually have a single, deep active site that is usually less accessible than the shallow binding pockets or grooves on the lectin surfaces. Consequently, a limited number of binding mechanisms, including statistical rebinding, are possible, whereas multivalent ligands may interact with multiple receptors by additional mechanistic options (e.g., the chelate effect, receptor clustering). It is likely that these factors may have hampered interest in projects directed towards the design of multivalent glycosidase inhibitors. In addition, the experimental results obtained to date were not particularly encouraging. Dito tetravalent analogues of 1-deoxynojirimycin, which is a well-known glycosidase inhibitor, generally displayed comparable if not decreased inhibition compared with their monomeric counterparts. The best result reported to date was found for a trivalent iminosugar that showed a sixfold affinity enhancement towards Jack bean a-mannosidase. Herein we report the synthesis of a fullerene hexakis adduct decorated with 12 iminosugar residues. The inhibition profile of this fullerene iminosugar ball has been systematically evaluated against various glycosidases, and dramatic multivalent effects have been observed for the first time. In order to explore the potential of multivalency on glycosidase inhibition with a globular polytopic ligand constructed around the fullerene scaffold, an N-alkyl analogue of 1-deoxynojirimycin was selected as the peripheral ligand. This class of compounds is indeed poorly selective and displays modest to good glycosidase inhibition. It was thus anticipated that these compounds could be excellent models for the examination of the influence of multivalency on inhibition selectivity over a large range of glycosidases. In addition, the alkyl chain on the endocyclic nitrogen atom of the iminosugar is an ideal spacer that may allow for easy grafting onto the central C60 core by means of a cycloaddition reaction. [16] The synthesis of the azide building block is based on the optimization of a strategy reported independently by Overkleeft et al. and Vasella and co-workers. As shown in Scheme 1, the d-hydroxy amide 2 was obtained directly from commercially available tetra-O-benzyl d-glucopyranose (1) in 78% yield by oxidative amidation with iodine in 30% aqueous ammonia (30%). The main advantage of this onepot process is that aldehyde oxidation and C N bond formation are performed in a single synthetic step. Oxidation of the hydroxy group at C5 followed by intramolecular [*] Prof. P. Compain, C. Decroocq, Dr. D. Hazelard Laboratoire de Synth se Organique et Mol cules Bioactives Universit de Strasbourg et CNRS (UMR 7509) Ecole Europ enne de Chimie, Polym res et Mat riaux 25 rue Becquerel, 67087 Strasbourg (France) Fax: (+ 33)3-6885-2754 E-mail: philippe.compain@unistra.fr

Fullerene sugar balls

Fullerene hexakis-adducts bearing 12 peripheral carbohydrate moieties have been prepared by grafting sugar derivatives onto the fullerene core through the copper mediated Huisgen 1,3-dipolar cycloaddition of azides and alkynes.

Electrophosphorescent homo- and heteroleptic copper(I) complexes prepared from various bis-phosphine ligands

Homo- and heteroleptic copper(I) complexes obtained from various chelating bis-phosphine ligands and Cu(CH3CN)4BF4 have been used for the preparation of light emitting devices.

Artificial light-harvesting arrays: electronic energy migration and trapping on a sphere and between spheres.

A sophisticated model of the natural light-harvesting antenna has been devised by decorating a C(60) hexa-adduct with ten yellow and two blue boron dipyrromethene (Bodipy) dyes in such a way that the dyes retain their individuality and assist solubility of the fullerene. Unusually, the fullerene core is a poor electron acceptor and does not enter into light-induced electron-transfer reactions with the appended dyes, but ineffective electronic energy transfer from the excited-state dye to the C(60) residue competes with fluorescence from the yellow dye. Intraparticle electronic energy transfer from yellow to blue dyes can be followed by steady-state and time-resolved fluorescence spectroscopy and by excitation spectra for isolated C(60) nanoparticles dissolved in dioxane at 293 K and at 77 K. The decorated particles can be loaded into polymer films by spin coating from solution. In the dried film, efficient energy transfer occurs such that photons absorbed by the yellow dye are emitted by the blue dye. Films can also be prepared to contain C(60) nanoparticles loaded with the yellow Bodipy dye but lacking the blue dye and, under these circumstances, electronic energy migration occurs between yellow dyes appended to the same nanoparticle and, at higher loading, to dye molecules on nearby particles. Doping these latter polymer films with the mixed-dye nanoparticle coalesces these multifarious processes in a single system. Thus, long-range energy migration occurs among yellow dyes attached to different particles before trapping at a blue dye. In this respect, the film resembles the natural photosynthetic light-harvesting complexes, albeit at much reduced efficacy. The decorated nanoparticles sensitize amorphous silicon photocells.

Fullerodendrimers: A New Class of Compounds for Supramolecular Chemistry and Materials Science Applications

Encapsulation of a fullerene sphere in the middle of a dendritic structure prevents unfavorable effects of the C60 unit, such as aggregation or steric hindering. Such fullerodendrimers appear to be promising compounds for materials science applications. On the other hand, fullerodendrons with peripheral C60 subunits or containing a C60 sphere at each branching unit appear to be versatile building blocks for the preparation of fullerene-rich macromolecules with intriguing properties.

Molecular and supramolecular C60–oligophenylenevinylene conjugates

Fullerene derivatives are attractive building blocks for the preparation of molecular and supramolecular photoactive devices. As a part of this research, combination of C60 with oligophenylenevinylene (OPV) subunits has generated significant research efforts. These results are summarized in the present account to illustrate the current state-of-the-art of fullerene chemistry for the development of new photoactive materials.

A new pyridyl-substituted methanofullerene derivative. Photophysics, electrochemistry and self-assembly with zinc(II) meso-tetraphenylporphyrin (ZnTPP)

A new methanofullerene derivative with a pyridine residue at the methano bridge (PC60) has been synthesized. Its electrochemical and photophysical properties have been investigated in toluene solution along with those of the analogous fullerene mono-malonate adduct RC60, which does not contain the pyridyl moiety. PC60 and RC60 display almost identical steady state absorption and luminescence properties and transient absorption spectra, as shown by means of nano- and picosecond laser flash photolysis. Their markedly different spectroscopic features with respect to plain C60 are discussed, with the aid of semi-empirical calculations. PC60 binds zinc(II) meso-tetraphenylporphyrin (ZnTPP) through coordination of the pyridyl moiety to the zinc ion giving, to the best of our knowledge, the first non-covalent assembly of a porphyrin–fullerene diad. The association constant of the 1:1 complex between PC60 and ZnTPP has been determined at 300 K by 1H NMR (Ka=3600±150 L mol-1, C6D6 solution) and fluorescence titrations (Ka=3000±400 L mol-1, toluene solution). Mixtures of RC60 and ZnTPP have also been investigated, giving no evidence of association. Extremely fast ZnTPP luminescence quenching in the PC60–ZnTPP complex was observed by time-resolved luminescence spectroscopy (k>5×1010 s-1), tentatively attributed to an energy transfer mechanism. The bimolecular quenching constant of the lowest triplet excited state of ZnTPP by PC60 was determined via a Stern–Volmer analysis (kq=6.7×109 s-1).

Fullerodendrimers: Fullerene-Containing Macromolecules with Intriguing Properties

Fullerenes possess electronic and photophysical properties which make them natural candidates for the preparation of functional dendrimers. The attachment of a controlled number of dendrons on a C(60) core provides a compact insulating layer around the carbon sphere, and the triplet lifetimes of the C(60) chromophore can be used to evaluate its degree of isolation from external contacts. The fullerene core can also act as a terminal energy receptor in dendrimer-based light-harvesting systems. When a fullerodendrimer is further functionalized with a suitable electron donor, it may exhibit the essential features of a multicomponent artificial photosynthetic system in which photo-induced energy transfer from the antenna to the C(60) core is followed by electron transfer. On the other hand, the preparation of dendrons with peripheral C(60) subunits or containing a C(60) sphere at each branching unit has been achieved. These fullerodendrons are not only interesting building blocks for the synthesis of monodisperse fullerene-rich macromolecules with intriguing properties, but they are also amphiphilic compounds capable of forming stable Langmuir films at the air-water interface.