Double-cable polymers

In this subclass of side-chain polymers, the main chain always consists of a n-conjugated backbone with electron-donating characteristics, the so-called p-type cable, to which several electron-accephng fullerene cages, or an n-type cable, are covalently linked. Owing to its intrinsic electronic properties, numerous double-cable-polymers (D-C) have been employed in electro-optical devices, namely photovoltaic devices (Chapters 7 and 8) [50]. 

The chemical strategies followed for the synthesis of D-C-polymers are the same as those described for side-chain C )-polymers. In addition, they can be prepared by electropolymerization of a suitable monomer. 

The first example was reported in 1994 by Diederich, who electropolymerized a bis(l,3-dialkyne)methanofullerene to obtain conductive films [51]. Nowadays, several research groups are engaged in the synthesis and study of D-C polymers, especially in the search for more efficient organic photovoltaic solar devices [52]. 

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The majority of the synthetic studies towards the construction of novel photodiodes of fullerene-based double-cable polymers concern either electropolymerization of suitable aromatic monomeric units or copolymerization of two monomeric units, one carrying the fullerene moiety and one designed to improve solubility. Most of the electropolymerized conjugated polymeric materials bearing fullerenes that have been obtained consist of bithiophene units with low oxidation potential that favor electropolymerization [162-164]. On the other hand, the preparation of some photodiodes based on novel chemically synthesized double-cable polymers was recently reported and studied using their photophysical properties [165-170]. For example, the structures of some conju-gated-backbone hybrids covalently linked with organofullerene moieties are shown in Scheme 8. 

Novel hybrid materials have been realized in which fullerenes participate in composite films with 7r-conjugated-polymer electron donors such as oligothio-phenes. Established studies have already shown that the photoinduced electron transfer is rather enhanced between 7r-conjugated polymers and fullerenes, while back electron transfer is considerably slower [145,149,171,172].Electrosynthe-sized polythiophene with pendant fullerene substituents was recently obtained from the corresponding biothiophene-fulleropyrrolidine dyad [173]. The novel material described has the potential of a double-cable polymer, heavily loaded with fullerene electron-conducting moieties. 

Cravino, A., G. Zerza, M. Maggini, S. Bucella, M. Svensson, M.R. Andersson, H. Neugebauer, and N.S. Sariciftci. 2000. A novel polythiophene with pendant fullerenes Toward donor/acceptor double-cable polymers. Chem Commun 2487-2488. 

Giacalone, R, J.L. Segura, N. Martin, M. GateDani, S. Luzzati, and N.-O. Lupsac. 2003. Synthesis of soluble donor-acceptor double-cable polymers based on polythiophene and tetracyanoanthraqui-nodimethane (TCAQ). Org Lett 5 1669-1672. 

A. Cravino and N. S. Sariciftci, Double-cable polymers for fuUerene based organic optoelectronic applications, J. Mater. Chem., 12, 1931-1943 (2002). 

FIGURE 2.40 (a) Most used fullerene derivative in solar cells PCBM, (b) example of double-cable polymer, and (c) supramolecular polymer assembled through hydrogen bonding. 

For recent and more complete coverage on double-cable polymers, including those containing A moieties other than fullerenes, see Reference [36] and references therein. 

Finally, double-cable polymers could be ambipolar materials for photomodu-lated p-n type organic field effect transistors (OFETs). 

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