Fullerene solubility enhancement

Because monofunctionalization of fullerenes shows that a single hydrophilic addend is insufficient to prevent the strong hydrophobic interactions among the compounds [89, 90, 93, 94], multiple functionalized derivatives were examined as water-soluble probes. In particular, introduction of a second hydrophilic ligand (e.g., pyrrolidinium salts or carboxylates) to the fullerene core enhances the surface coverage of the hydrophobic fullerene surface. In turn, it was expected that fullerene aggregation might be suppressed. It should be stated that these water-soluble derivatives are important alternatives to the y-CD-incorporated and surfactant-embedded fullerenes. 

C(io(OH)i8 (10) Poly hydroxy lation (Scheme 4) of the hydrophobic [60]fullerene core enhances the water solubility of this carbon allotrope up to 4.0 x 10 M (67). The tt-radical anion, (Ceo )(OH)ig, generated by electron transfer from hydrated electrons and (CH3)2 C0H radicals, absorbs with maxima at 870, 980 and 1050 nm. The bimolecular rate constant for a reaction with hydrated electrons is 4.5 x 10 M s . Based on electron transfer studies with suitable electron donor / acceptor substrates, the reduction potential of the C6o(OH)ig/(C6o )(OH)i8 couple was estimated to be in the range between -0.358 V and -0.465 V versus NHE. 

Given the interest in extended carbon systems in recent years, it seemed useful to study the solubility of C60 (fullerene) in various organic liquids.54 55 It was now for the solvents that the molecular surface properties were computed. The resulting Eq. (14) shows that, for this large nonpolar solute, solubility is enhanced by solvent molecule surface area and by the latter having somewhat... 

Copolymers of [60] fullerene and in situ generated bis-o-quinodimethanes were prepared by Gtigel and colleagues114. In order to get soluble polymers, it proved necessary to introduce flexible groups on the bis-o-quinodimethanes. A maximum of 10 [60]fullerene units were incorporated into oligomers when [60]fullerene was reacted with a 7 3 mixture of 157 and 158 (i.e. with o-quinodimethanes 159 and 160). Monosulfone 158 was added to induce the formation of triple cycloadducts of [60]fullerene. This prevented polymerization of the oligomer (quadruple cycloadditions to [60]fullerene are hard to accomplish) and enhanced its solubility. 

Beside fulleropyrrolidines and malonate adducts, a variety of other water-soluble fullerene derivatives with entirely different addition patterns have been synthesized within the past years (Fig. 3.6). These investigations have enhanced the available... 

Zhang S, Sun D, Li X, Pei F, Liu S (1997) Synthesis and solvent enhanced relaxation property of water-soluble endohedral metallofullerenes. Fullerene Sci. Tech. 5 1635-1643. 

Example Isotopic enrichment is a standard means to enhance the response of an analyte in nuclear magnetic resonance (NMR). Such measures gain importance if extremely low solubility is combined with a large number of carbons, as is often the case with [60]fullerene compounds. [19] The molecular ion signals, IVT, of Qo with natural isotopic abundance and of C-enriched Cgo are shown below (Fig. 3.11 for EI-MS of [60]fullerenes cf. Refs. [20-22]). From these mass spectra, the enrichment can be determined by use of Eq. 3.1. For Qo of natural isotopic abundance we obtain Mrceo = 60 x 12.0108 u = 720.65 u. Applying Eq. 

A similar approach was used in grafting Cjq onto a pregenerated lithiated polyethylene surface [121]. A polyethylene film with terminal diphenylmethyl groups was deprotonated with BuLi to yield an anionic polyethylene surface that was treated with Cg0 and quenched with methanol. The incorporation at the polyethylene surface was determined by XPS, UV/Vis and fluorescence spectroscopy. This reaction also works for polyisopropene, polybutadiene [69], poly(vinylbenzyl chloride) or poly(N-vinylcarbazole) PVK [54] with BuLi or NaH as a base. Charge-transfer interactions in the soluble fullerene-PVK derivative between the positively charged carbazole and Cjq lead to an enhanced photoconductivity compared with PVK [54]. 

It is clear that a variety of solvents commonly used in electrochemistry is available for low-temperature studies. Particularly noteworthy are the solvent mixture butyronitrile/ethyl chloride, which can be used down to about 100 K [25,47], and the inclusion of the low-polarity cosolvent, toluene, to enhance the solubility of a substrate that is insoluble in many polar solvents, in this case the fullerene, [50,51]. When low solution resistance is a priority and only moderately low temperatures are needed (above ca. -50°C), polar solvents such as acetonitrile and A Af-dimethylformamide are preferred. 

Films of fullerenes (C60 and C70) have shown interesting electrochemical behavior (45,46). Neither the C60 nor the completely reduced C60 him is a good electronic conductor, although a partly reduced him displayed enhanced conductivity (47). Recently, the same group reported investigations on a him of tm-butylcalix[8]arene-C60. As a consequence of the reduction of the inner fullerene, the latter escaped from the calixarene basket and soluble C6 0 species entered the acetonitrile solution, as observed in an SECM collection experiment with simultaneous measurements with a quartz crystal microbalance. 

The ball-and-chain polymer has been proposed to be made by attaching, for example, a fullerene (Cj, see Sect. 2.5.3) to a polymer chain. This shows that there are no limits to the structures to which chains can be attached. Adding flexible chains to rather rigid structures can enhance the solubility of the often poorly soluble rigid molecules and change the processing and the physical properties. 

The authors also found that their rod-coil systems proved to be feasible to encapsulate fullerens into the cavities. Compared to usual solvents for 50 and C70, such as dichloromethane or toluene, the solubility was enhanced by up to 1000 times when the molecules were encapsulated into rod-coil micelles. The solubilization of up to 10 billion fullerenes into one micelle increased the aggregation number to 10 and the obtained diameters up to 30 fim [136]. Such micro containers could also be used for an encapsulation process of other substances and so one can think of applications, such as drug delivery, emulsions, etc. 

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