Fullerenes, hetero

Hydrogen-Bonded Phthalocyanine-Fullerene Hetero-Arrays. 190... 

Heteroatom fullerene-type clusters — The possibility of incorporation of hetero atoms into C clusters has excited the attention of both theoreticians and experimentalists since the earliest days of fullerene chemistry, particularly in view of the known stability and ubiquity of organic heterocycles. The structural relationship between Ceo and /3-rhombohedral boron has already been alluded to (p. 142). 

Hirsch and co-workers calculated NICS values for tetrahedral clusters of N, P, As, Sb, and Bi, as well as for the corresponding tetra-anions composed of Si, Ge, Sn, or Pb atoms, finding diatropic values for 2,n(n 1) jr-systems.296a b It was postulated by Hirsch, Schleyer, and their co-workers that for icosahedral fullerenes and their hetero-analogues the Hiickel rule, involving 4/2+2 //-electrons, should be replaced by the 2(/2+l)2 electron rule.296... 

A few examples of hetero-Diels-Alder adducts have been reported [75-81]. A thio-chroman-fused fuUerene adduct was synthesized by the reaction of o-thioquinone with CgQ in o-dichlorobenzene at 180 °C [77]. The obtained cyclic sulfide 84 (Figure 4.4) can be oxidized to the corresponding sulfoxide and sulfone with m-chloroperoxybenzoic acid. Reaction of azadienes with Cgg leads to hetero Diels-Alder adducts such as, for example, 85 (Figure 4.4) [79]. The tetrahydropyrido[60]-fullerene 85 is formed in refluxing o-dichlorobenzene. 

So far, all efforts to generate, isolate and characterize heterofuUerenes via Kratsch-mer-Huffman vaporization of graphite in the presence of hetero-element-containing compounds such as boron nitride (BN) or cyanogen (CN)2 have failed. An alternative route for the direct formation of heterofuUerenes is cluster rearrangement within exohedral fullerene derivatives such as iminofullerenes and azafuUeroids. The first hints of success by this approach were obtained from mass spectrometry investigations of the cis-l-diazabishomo[60]fullerene 3 [12], the n-butylamine adduct 4 [12] the 1,2-epiminofullerene 5 [11] and the cluster opened ketolactam 6 [2]. 

In analogy to olefins, Cjq undergoes a broad variety of cycloadditions (see Chapter 4 and Scheme 14.3). In many cases cycloadducts of Cjq exhibit the same stability as the corresponding non-fullerene based adducts. These reactions are very useful for the introduction of fimctionat groups. Among the most important cycloadditions are [4-1-2] cycloadditions such as Diels-Alder and hetero-Diels-Alder reactions, where Cjq reacts always as dienophile, [3-1-2] cycloadditions with 1,3 dipoles, thermal or photochemical [2-1-2] cycloadditions, [2-t-l] cycloadditions and others, for example, [8-1-2] cycloadditions. Among these general reactions several examples deserve special attention, since they reflect characteristic chemical properties of Cjq [36] ... 

ESI-MS of fulleroid 1,2-oxazetidine 34 (Equation 2), obtained by a hetero-Diels-Alder reaction between buck-minsterfullerene Cgo and nitrosobenzene, provided a useful method for the characterization of small neutral organic molecules, otherwise not detectable, as fullerene derivatives <1997USP5635404>. 

Other results with unsaturated thioamides include a nice reversal [233] of the endo selectivity in favour of the exo adduct by adjustment (Table 4, entry 9) of the thermal conditions (-30 °C vs 80 °C). Unexpected dimerisa-tions of enethioamides (entries 10 and 11) were observed [234, 235]. Thiocarbonyl chemistry has already been used in fullerene chemistry Acyl thio-acrylamides react [236] with [60]fullerene to yield hetero-Diels-Adler products (Table 4, entry 12). 

Fig. 5.5. Luminescence quenching (bullets, right hand, axis) and short-circuit current Ac (black squares, left hand axis) vs. molar fullerene concentration in a bulk hetero junction composite. The different onsets for percolation for the two phenomena (exciton diffusion versus ambipolar carrier transport) can be clearly seen...

This view of Voc generation is additionally supported by the fact that the values of the temperature coefficient dUoc/dT = -(1.40-1.65) mVK-1 for the cells under the present study (with bilayer LiF/Al and ITO/PEDOT contacts) coincide with those for polymer/fullerene bulk heterojunction solar cells of the previous generation (with the same components of the active layer but without LiF and PEDOT contact layers) [156]. In this picture, the temperature dependence of Voc is directly correlated with the temperature dependence of the quasi-Fermi levels of the components of the active layer under illumination, i.e., of the polymer and the fullerene. Therefore, the temperature dependence of Voc over a wide range, and in particular V),c(0 K), are essential parameters for understanding bulk hetero junction solar cells. 

Hirsch and co-workers developed a different synthetic route to hetero-fullerenes which, in the case of (Cs9N)2, involves intermediates of type 52 (cf. Section IV.A.l.d and Scheme 1.7) bearing acid-labile /V-substituents 47,158,159... 

The (semi)conductive properties of fullerene derivatives and carbon nanotubes have been a topics of intense recent interest. Unfortunately, at the time of writing it is still not possible to obtain well-defined samples of nano-tubes in sufficient amounts to carry out PR-TRMC measurements. Relatively pure, powder samples of C60 were however available in the mid-1990s and large, if short-lived, PR-TRMC transients could be observed. More recently attention has focused on the methyl ester of phenyl-C61-butyric acid (PCBM) which is a soluble C60 derivative that has found applications as the electron accepting component in hetero-junction materials for solar energy conversion. 

Fig. 1. Left graph shows the chemical structure of the fullerene derivative - PCBM. Right graph shows a schematic cross section of the Al/p-Si/PCBM/Al hetero-junction with an sample photograph [2],...

In summary, it has been shown that by infrared radiation charge-carriers can be directly excited across the interface of a silicon/fullerene hybrid hetero-junction. Besides its scientific relevance, the simple fabrication process as well as its compatibility with the well established silicon technology makes the presented hybrid approach a promising candidate for widespread applications. 

As wUl be seen later, there are also fuUerenes with one or more carbon atoms being substituted by other elements. In fuUerene nomenclature these molecules are described by prefixing to the name the designating syllable for the respective hetero atom, for ejample, aza[70]fullerene C69N, bis(aza[60]fullerene) (C59N)2, bora[60]fuUerene C59B, or phospha[60]fuUerene C59P. 

In certain compounds, the so-caUed endohedral fuUerenes, atoms or small molecules are incorporated into the cage without forming a covalent bond to the fullerene scaffold. The notation M C (pronounced M at C ) is used to indicate the position within the fuUerene, with M representing the atom or molecule inside the cage. In this way, hetero- and endofuUerenes can clearly be distinguished, for... 

Volume 1 contains 15 chapters on the recent developments in charge-transfer salts, fullerenes, photoconductors and molecular conductors. The topics covered include electron acceptor molecules, photo-induced intermolecular electron transfer systems, perylene based conductors, tetrachalcogenafulvalenes, metal 1,2-dichaIcogenoIenes and their conductive salts, conductive hetero-TCNQs (tetracyano-p-quinodi-methane derivatives), molecular metals and superconductors based on transition metal complexes. 

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