ABSTRACT
The covalent functionalization of fullerenes has over the last few years developed to such an extent that now Cr.,. by far the most abundant and most well-studied fullerene, can he considered a versatile building block in organic chemistry [ 1]. In contrast to graphite and diamond, Cm and the higher fullerenes display a considerable chemical reactivity that has stimulated a systematic modification of their structure [I ,2]. The extensive investigation of the addition reactions to Cw revealed that the basic principles of its enhanced reactivity stem from the release of strain associated with the bent geometry of the C-C double bonds. Nonchemists might wonder why anybody would break the aesthetically pleasing geometry and perfect symmetry of the fullercnes. One simple answer is because it is challenging. A major problem, connected with the addition reactions to the fullerenes, is the high number of potential products. The second answer is more practical. Functionalized fullerencs arc more soluble than their pristine counterparts in solvents of common usc and less prone to aggregate. Decreased aggregation is a fundamental prerequisite to exploit and combine the unusual electronic and redox prope11ies of the fullcrencs with those of the appended functional groups for applications in materials science f3,4] and medicinal chemistry [5,6]. Thus, deter-
Among the most striking properties of C00 is its ability to reversibly accept up to six electrons in solution [7] and its low energy of the first singlet excited state compared to other electron acceptors [8]. These properties caused many to believe that Coo could be a good partner in photo-induced redox processes. To this end, several research groups have prepared a wide variety of elaborate molecular systems in which C00, acting as an electron acceptor unit, is covalently linked to electron donors such as aromatics [9-18], porphyrins [8,19-28], phthalocyanine [29,30], ferrocene [31-36], tetrathiafulvalene [31,37-40], carotene [41-43], rotaxanes [44,45], and ruthenium(II) polypyridine complexes [46-51]. The study of these systems is concerned with the fundamental understanding of their photophysical properties and of those factors that govern energy and electron transfer processes in relation to natural photosynthesis [52] and to practical applications such as photovoltaic devices for solar energy conversion [53-57]. The C60 chromophore is a particularly interesting electron acceptor not only for the abovementioned reasons, but also because its three-dimensional structure, and its larger size than that of conventional planar acceptors, such as aromatics, quinones, or pyromellitic imides, accelerates the forward electron transfer while retarding the back electron transfer [58]. The distribution of the negative charge over 60 carbon atoms reduces the energy contribution to solvent reorganization. Also the contribution to the internal reorganization energy is probably small due to the rigid nature of the C60 framework [58].
