Fullerene derivatives functionalization

Fullerene derivatives functionalized with pyridine or imidazole substituents can coordinate with several metal porphyrins, Pc, and Nc in solution by means of self-assembly. Such electron-rich macrocyclic dyes are good electron donors as they absorb light in the visible near-infrared (IR) region and exhibit a usable redox potential. 

Figure 23-5. Examples of fullerene derivatives functionalized with a trialkoxysilyl moiety used in sol-gel processing of hybrid, organic-inorganic compounds (upon ref Kordatos, 2001).

The most important classes of functionalized [60]fullerene derivatives, e.g. methanofullerenes [341, pyrrolidinofullerenes [35], Diels-Alder adducts [34i] and aziridinofullerene [36], all give rise to a cancellation of the fivefold degeneration of their HOMO and tlireefold degeneration of their LUMO levels (figure Cl.2.5). This stems in a first order approximation from a perturbation of the fullerene s 7i-electron system in combination with a partial loss of the delocalization. 

This behaviour also stands for functionalized [60]fullerene derivatives, with, however, a few striking differences. The most obvious parameter is the negative shift of the reduction potentials, which typically amounts to -100 mV. Secondly, the separation of the corresponding reduction potentials is clearly different. Wlrile the first two reduction steps follow closely the trend noted for pristine [60]fullerene, the remaining four steps display an enlianced separation. This has, again, a good resemblance to the ITOMO-LUMO calculations, namely, a cancellation of the degeneration for functionalized [60]fullerenes [31, 116, 117]. 

Guldi D M and Asmus K-D 1997 Photophysical properties of mono- and multiply-functionalized fullerene derivatives J. Phys. Chem. A 101 1472-81... 

The ability of a dendritic shell to encapsulate a functional core moiety and to create a specific site-isolated microenvironment capable of affecting the molecular properties has been intensively explored in recent years [19]. A variety of experimental techniques have been employed to evidence the shielding of the core moiety and to ascertain the effect of the dendritic shell [19, 20]. Dendrimers with a fullerene core appear to be appealing candidates to evidence such effects resulting from the presence of the surrounding dendritic branches. Effectively, the lifetime of the first triplet excited state of fullerene derivatives... 

Fullerene derivative 34 substituted with two long alkyl chains (solubilizing groups) and a carboxylic function was used as peripheral subunit for the constructions of the dendrons (Fig. 14). 

In addition, the use of appropriate hydrophilic constituents on the aldehyde or glycine reactants can result in excellent water solubility of the (To derivative. Two such modification arms can be added simultaneously to the pyrrolidine ring, thus providing a functional group for further conjugation and a hydrophilic arm for increased water solubility. PEG derivatives have been formed in this manner, which create highly soluble fullerene derivatives. 

Various commercial suppliers now offer fullerene derivatives with functionalities available for bioconjugation, including carboxylic and poly-hydroxylic derivatives, which are very hydrophilic and water-soluble (BuckyUSA, NanoLab, NanoNB, Nano-C, and Aldrich). 

Redox molecules are particularly interesting for an electrochemical approach, because they offer addressable (functional) energy states in an electrochemically accessible potential window, which can be tuned upon polarization between oxidized and reduced states. The difference in the junction conductance of the oxidized and the reduced forms of redox molecules may span several orders of magnitude. Examples of functional molecules used in these studies include porphyrins [31,153], viologens [33, 34,110,114,154,155], aniline and thiophene oligomers [113, 146, 156, 157], metal-organic terpyridine complexes [46, 158-163], carotenes [164], nitro derivatives of OPE (OPV) [165, 166], ferrocene [150, 167, 168], perylene tetracarboxylic bisimide [141, 169, 170], tetrathia-fulvalenes [155], fullerene derivatives [171], redox-active proteins [109, 172-174], and hydroxyquinones [175]. 

Quick K, Dugan L (2004) Fullerene derivative (C ) functions as a SOD mimetic by reducing age-related increase in superoxide levels and prevention of age-related loss of mitochondrial membrane potential in brain. Free Rad. Biol. Med. 37 S163-S163. 

The two most commonly used derivatization methods for exohedral functionalization are the nucleophilic cyclopropanation with malonates (Bingel, 1993) and the formation of fulleropyrrolidines (Maggini et al., 1993). Both of these protocols have been used extensively to produce water-soluble fullerenes for biomedical applications. Other stable water-soluble fullerene adducts have also been reported (Hirsch and Brettreich, 2005). Sections 3.2.2-3.2.5 will give a short overview on the state-of-the-art of water-soluble fullerene derivatives and outline some general trends for designing such molecular structures. 

C60 is also a highly versatile synthetic scaffold that can easily be functionalized by the methods of synthetic organic chemistry. The formation of C60 derivatives (i.e., covalently modified C60) nearly always involves the addition of a functional group (addend) across one or more of its 30 double bonds. When only one addend is attached, the fullerene derivative is called a monoadduct, with two, a bisadduct, etc. The ability to sensitize molecular oxygen in the presence of visible light is retained in the simple derivatives of C60 (i.e., mono-, bis-, and trisadducts). 

Hamano T, Okuda K, Mashino T, et al. (1997) Singlet oxygen production from fullerene derivatives effect of sequential functionalization of the fullerene core. Chem Commun. 1 21-22. 

Prat F, Stackow R, Bernstein R et al. (1999) Triplet-state properties and singlet oxygen generation in a homologous series of functionalized fullerene derivatives. J Phys Chem. 103 7230-7235. 

Although there have been great advances in covalent functionalization of fullerenes to obtain surface-modified fullerene derivatives or fullerene polymers, the application of these compounds in composites still remains unexplored, basically because of the low availability of these compounds [132]. However, until now, modified fullerene derivatives have been used to prepare composites with different polymers, including acrylic [133,134] or vinyl polymers [135], polystyrene [136], polyethylene [137], and polyimide [138,139], amongst others. These composite materials have found applications especially in the field of optoelectronics [140] in which the most important applications of the fullerene-polymer composites have been in the field of photovoltaic and optical-limiting materials [141]. The methods to covalently functionalize fullerenes and their application for composites or hybrid materials are very well established and they have set the foundations that later were applied to the covalent functionalization of other carbon nanostructures including CNTs and graphene. 

Among all the carbon nanomaterials, fullerenes are by far the most studied systems in terms of chemical modification. It is safe to say that this field of research has been one of the most active for more than 20 years [113,114]. Therefore, it is not surprising to find several examples of well-known dyes that have been functionalized with fullerene derivatives and have been tested in DSSCs. The first report in this regard was given in 2007 by Kim et al. [115]. They described a route to attach C60 to N3 dye (cis-bis(4,4 -dicarboxy-2,2 -bipyridinejdithiocyanato ruthenium(II)) via diaminohydorcar-bon linkers with different alkyl chains (Fig. 18.7). While the photocurrents were almost equal for all devices, Vocs increased up to 0.70 V with the functionalized ruthenium(II) complex from 0.68 V with pristine N3. In terms of efficiency, the values were 4.0 and... 

Abstract More than a decade has passed since fullerenes became avaUahle to researchers in almost all fields of science. The explosive development of study on the chemical functionalization of fullerenes has led to a wide variety of fullerene derivatives. However, most of these reactions have been carried out in the liquid phase, and curiously enough the solid-state reaction (or solid-solid reaction) of fullerenes has been developed only in recent years. This chapter focuses on the solid-state reaction of fullerenes, particularly the reaction which was conducted under what is called high-speed vibration milling conditions. It will be shown how this reaction technique is pertinent for the creation of fullerene derivatives with novel structures, and how efficient this method is for certain reactions compared with the liquid-phase reaction. 

In cycloaddition reactions the [6,6] double bonds of Cjq exhibit a dienophilic character. A large variety of cycloadditions have carried out with Cjq and the complete characterization of the products, mainly monoadducts, has greatly increased our knowledge of fullerene chemistry. These chemical transformations also provide a powerful tool for the functionalization of the fullerene sphere. Almost any functional group can be covalently linked to Cjq by the cycloaddition of suitable addends. Some types of cycloadducts exhibit a remarkable stability for example, they can be thermally treated up to 400 °C without decomposition. This is an important requirement for further side-chain chemistry as well as for possible applications of the new fullerene derivatives, which may be of interest due to their biological activity or as new materials. 

The synthetically most valuable intermediate in heterofullerene chemistry so far has been the aza[60]fulleronium ion C59N (28). It can be generated in situ by the thermally induced homolytic cleavage of 2 and subsequent oxidation, for example, with O2 or chloranil [20-24]. The reaction intermediate 28 can subsequently be trapped with various nucleophiles such as electron-rich aromatics, enolizable carbonyl compounds, alkenes and alcohols to form functionalized heterofullerenes 29 (Scheme 12.8). Treatment of 2 with electron-rich aromatics as nucleophilic reagent NuH in the presence of air and excess of p-TsOH leads to arylated aza[60]fullerene derivatives 30 in yields up to 90% (Scheme 12.9). A large variety of arylated derivatives 30 have been synthesized, including those containing cor-annulene, coronene and pyrene addends [20, 22-25]. 

Heterogeneous mixing of fullerenes and fullerene derivatives with Ji-conjugated polymers has been used to produce excellent materials for photovoltaic devices [141], Upon irradiation of fullerene/polymer blends, charge transfer from the polymer to occurs, resulting in efficient photoconductivities. Better behavior of fullerene derivatives than with pristine Cgg has been observed, and attributed to the improved miscibility of the functionalized species. 

The covalent chemistry of fullerenes has developed very rapidly in the past decade in an effort to modify fuUerene properties for a number of applications such as photovoltaic cells, infrared detectors, optical limiting devices, chemical gas sensors, three-dimensional electroactive polymers, and molecular wires [8, 25, 26, 80-82]. Systematic studies of the redox properties of Cgo derivatives have played a crucial role in the characterization of their unique electronic properties, which lie at the center of these potential applications. Furthermore, electrochemical techniques have been used to synthesize and separate new fullerene derivatives and their isomers as well as to prepare fullerene containing thin films and polymers. In this section, to facilitate discussion of their redox properties, Cgo derivatives have been classified in three groups on the basis of the type of attachment of the addend to the fullerene. In group one, the addends are attached via single bonds to the Cgo surface as shown in Fig. 6(a) and are referred to as singly bonded functionalized derivatives. The group includes... 

In comparison with pristine C6o and C70, the fullerene derivatives (Fig. 2) show partly different photophysical properties due to the pertubation of the fullerene s TT-electron system. The degree of variation is dependent on (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in the case of multiple adducts on the addition pattern at the fullerene core [59-112],... 

The emission band position is also influenced by the electron-withdrawing and electron-donating effect of the addend, respectively, and by the type of chemical functionalization (e.g., various size of the rings fused to the fullerene core). For example, it has been reported for three different fullerene derivatives that an enhanced electron-withdrawing effect causes an increasing red-shift of the emission band [108], Another example is a C6o derivative 12 (Fig. 10). The emission maxima appears at 705 nm, but if there are methoxy groups additionally attached to the anthryl moiety the emission maxima is shifted to 712 nm [92],... 

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