Polyfullerene

Polyfullerenes C qH with n reaching from 18 to 44 were observed imder Birch conditions (for n > 44 see Section 5.3.4). Isomers with more than 36 hydrogens could not be obtained with the usual Birch procedure. Much milder conditions were necessary and were found with the Benkeser reduction [43]. CgoHjg, obtained with Birch reduction, was subjected to a reduction with li in refluxing ethylene-diamine and yielded four new polyhydrofullerenes, C qH with n = 38, 40, 42 and 44 [43]. These derivatives could be separated by preparative HPLC and characterized by mass spectrometry. 

The dimers may then be linked through further [2+2] cycloadditions into polyfullerenes. Although all-carbon polyfiillerenes have been obtained (see below), fullerene dimers as the species that are most important to the proposed [24-2] cycloaddition mechanism for fullerene photopolymerization are yet to he isolated and characterized. 

Preparations of polyfullerene materials using other methods have also been reported [168-176]. A quantitative comparison of the polyfullerene polymers obtained photochemically with the materials from other methods will prove to be very interesting. 

Structural studies of polymer surfaces materials that have been studied include polymethyl methacrylate (PMMA) [22], PMMA-polypyrrole composites [23], poly(chloromethyl styrene) bound 1,4,8,11 tetrazacrylotetra decane poly(chloromethyl styrene) bound theonyl trifluoroacetone [24], polydimethyl siloxane-polyamide copolymers [25], PS [26], ion-implanted PE [27], monoazido-terminated polyethylene oxide [28], polyurethanes [29], polyaniline [30], fluorinated polymer films [31], poly(o-toluidine) [32], polyetherimide and polybenzimidazole [33], polyfullerene palladium [34], imidazole-containing imidazolylethyl maleamic acid - octadecyl vinyl ether copolymer [35], polyphenylene vinylene ether [36], thiphene oligomers [37], fluorinated styrene-isoprene derivative of a methyl methacrylate-hydroxyethyl methacrylate copolymer [38], polythiophene [39], dibromoalkane-hexafluorisopropylidene diphenol and bisphenol A [40], and geopolymers [41],... 

Since then, a huge number of very different Ca)-polymers have been prepared, and their literature has been widely reviewed [11]. Furthermore, as expected, several polymers showed outstanding properties such as optical limiting [12] or photoinduced electron transfer [13] to name just a few. Notably, however, several polyfullerenes have been employed as active materials in electroluminescent devices [14], non-volatile flash devices [15], as well as in one of the most realistic applications of fullerenes, photovoltaic solar cells [16]. 

In this introductory chapter we summarize the various types of polyfullerenes, resuming the main synthetic strategies employed for their preparation. Moreover, for each family, the first examples have been emphasized as well as some more recently reported remarkable examples, to give a more precise idea about where the field is moving to. 

As a criterion for the classification, we have ordered the many families of polyfullerenes taking into account both their increasing chemical complexity and the difficulty in synthesizing them. In this sense, all-Cso and organometallic polymers are presented first, followed by the random crosslinked ones, since their synthesis... 

The first example was described by Rao in 1993 who, after exposure of Cgo to visible light, obtained an insoluble photopolymerized film [18]. Soon thereafter, the first piezopolymerized polyfullerene was also reported [19]. Beside polymerizations carried out in the presence of light or with pressure, electron beam-induced [20] and plasma-induced polymerization [21] are also known. 

More recently, a new frontier was opened up by filling single-walled carbon nanotubes with Cso to form the so-called fullerene peapods [23]. In 2003, Terrones et al. studied fullerene coalescence to a polyfullerene induced by electron irradiation on pristine nanotube peapods [24], paving the way to new highly conducting and semiconducting tubular structures with specific electronic characteristics (Figure 1.3). 

Nevertheless, two groups have strongly helped the development of this class of polyfullerenes Goh s, which has focused mainly on the mechanical behavior, miscibility properties and interpolymer complexes of C )-end-capped polymers (Chapter 4) [33], and Tam s, which has emphasized the aggregation behavior and self-assembly of endopped polymers under appropriate conditions (Chapter 3)... 

Owing to the low control of the double-addition to C ) that leads to eight possible isomers, and the difficult preparation of pure bis-adducts, this class of compounds has been perhaps the least explored among all the polyfullerenes (Chapter 2). 

Although the supramolecular chemistry of fullerene derivatives is a well explored and established field [53], surprisingly, fuUerene-containing supramolecular polymer systems have been scarcely studied. In fact, this family can be considered as the most recently explored among the other polyfullerenes, and it is now experiencing growing interest... 

Figure 8.1 A doubleoble polyfullerene consisting of fullerene pendants attached to a polythiophene conjugated chain. (Reproduced with permission of Nature Publishing from Reference [18a].)...

In addition, polyfullerenes such as doubleintimate contact between the D and the A by preventing the phase separation often observed in blended layers. Provided that the driving forces are actually established in these devices (as discussed below), in doublepositive electrode by inter-chain hopping, while the electrons jump between the pending A units in the opposite direction. 

We first, briefly, introduce the concept of bulk heterojunchon solar cells, with the aim of pointing out the importance of the nanomorphology and the degree of D-A separation within the photoactive layer. Then, with no claims of completeness, we select examples of polyfullerene double-cables from the recent literature. 

The above results demonstrated that in this class of polyfullerenes (i) the ground-state electronic properties of the D backbone and the tethered fullerene A can retain their individual properties and (ii) a photoinduced electron transfer from the D to the A leads to metastable and mobile charges, that is, the covalent linkage between them does not forcedly enhance geminate monomolecular recombination. 

Electrochemically or Chemically Synthesized Double-Cable Polyfullerenes... 

Interesting polyfullerenes such as 8 and 9 have been described by the group of RoncaU [44]. These polymers were obtained by electrosynthesis of electron donor 3,4-ethylenedioxythiophene and 3-alkylsulfanylthiophene, carrying fullerene moieties via alkyl spacers of different length. This type of precursor allows for a ready and efficient electropolymerization at low oxidation potentials. Note that 9a, b are... 

Recently, the group of li have described solar cells based on double-cable polyfullerenes [54]. The PT-F (see 16 in Figure 8.14) is based on a polythiophene chain... 

As shown in Figure 8.15, under AMI.5 solar simulated illumination (lOOmWcm ), solar cells based on this polyfullerene double-cable provided an open circuit voltage of 750 mV and a short circuit current density of 2.41 mAcm values that together with a filling factor of 0.29 determine a power conversion efficiency of 0.52% [6]. 

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