Photophysics of Fullerenes

5 Photophysics of Fullerenes - Interest in the synthesis, derivatization, and chemistry of the various fullerenes has almost reached the epidemic stage and these exotic molecules continue to attract the photochemist. There have been numerous publications concerned with the fundamental photophysical properties of fullerenes in solution and the solid state over the past five years or so but there are still some important issues to resolve. Lately, the characterization of fullerenes and related carbon nanotubes has been reviewed, along with the luminescence properties of The prompt and delayed fluorescence spectral 

The nature of the lowest-lying excited states of the fullerenes has been difficult to identify with much certainty. From Shpol skii-type luminescence spectra recorded at 1.5 K it has been concluded that the first-excited singlet state in C70 is of A 2 character. The origins of the lowest energy transitions in Ceo, namely Si(T]g) and S2(Gg), have been assigned on the basis of fluorescence and excitation spectra, supported by theoretical calculations. The luminescence properties and relaxation dynamics of single crystals of Qo have been described while related measurements have been made for solid films of Ceo Similar studies have reported the luminescence spectral properties of 50 trapped inside the cavities of NiY zeolites. An analysis of the fine structure of electron-vibrational spectra has been made for 50 and its derivatives in a solid toluene matrix. The rate of triplet energy transfer between fullerenes in toluene solution has been measured as a function of temperature and used to derive thermodynamic parameters for the transfer process.  

Fullerenes, especially Qo, have been used as electron acceptors in covalently-linked donor-acceptor dyads and triads (see Part IV on Artificial Photosynthesis). A critical comparison has been made between the rates of 

4 Photophysics of Fullerenes - Interest in the synthesis, derivatization, and photochemistry of the various fullerenes continues but perhaps the fervour of earlier years is passing. These materials display important photophysical properties and it is clear that they can serve as unique components in supramolecular assemblies. A special issue has been devoted to the photophysics and photo- 

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The chapter is organized as follows the second section will discuss the photophysics of conjugated polymer/fullerene composites as a standard model for a charge-generating layer in plastic solar cells. Pristine polymer devices will be discussed in the third section while bilayer and interpenetrating network devices are presented in Sections 4 and 5. Section 6 contains some remarks on large area plastic solar cells and Section 7 conclusions. 

In 2000, it was proposed that the regioselectivity of the [3 + 2] cycloaddition of fullerenes could be modified under microwave irradiation. Under conventional heating, N-methylazomethine yhde and fullerene-(C7o) gave three different isomeric cycloadducts because of the low symmetry of C70 vs. Ceo. Using microwave irradiation and o-dichlorobenzene as a solvent, only two isomers were obtained, the major cycloadduct 114 being kinetically favored (Scheme 39) [75]. The same authors had previously reported the 1,3-dipolar cyclo addition of pyrazole nitrile oxides, generated in situ, to Geo under either conventional heating or microwave irradiation. The electrochemical characteristics of the cycloadduct obtained with this method made this product a candidate for photophysical apphcations [76]. 

Foote, C.S. Photophysical and Photochemical Properties of Fullerenes. 169,347-364 (1994). Fossey, J., Sorba, J., and Lefort, D. Peracide and Free Radicals A Theoretical and Experimental Approach. 164, 99-113 (1993). 

Foote CS (1994) Photophysical and photochemical properties of fullerenes. Top Curr Chem 169 347-363. 

Top. Curr. Chem. (Volume Eds. C. A. Schalley, F. Vogtle), 1998, 228, 87-110 cf. also J. Iehl, R. P. de Freitas, B. Dela-vaux-Nicot, J.-F. Nierengarten,/. Chem. Soc., Chem. Comm. 2008, 2450-2452 for photophysical properties of fullerene-rich dendrimers see U. Hahn, J.-F. Nierengarten, F. Vogtle, A. Listorti, F. Monti,... 

In this chapter we have described the photophysics and photochemistry of C6o/C70 and of fullerene derivatives. On the one hand, C6o and C70 show quite similar photophysical properties. On the other hand, fullerene derivatives show partly different photophysical properties compared to pristine C6o and C70 caused by pertuba-tion of the fullerene s TT-electron system. These properties are influenced by (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in case of multiple adducts by the addition pattern. As shown in the last part of this chapter, photochemical reactions of C60/C70 are very useful to obtain fullerene derivatives. In general, the photoinduced functionalization methods of C60/C70 are based on electron transfer activation leading to radical ions or energy transfer processes either by direct excitation of the fullerenes or the reaction partner. In the latter case, both singlet and triplet species are involved whereas most of the reactions of electronically excited fullerenes proceed via the triplet states due to their efficient intersystem crossing. 

Since their discovery photochemists have been attracted to fullerenes and their derivatives. Literature on this topic has exploded during the past decade. In Chapter 11, Mattay and his coauthors summarize the photochemical and photophysical behavior of fullerenes and their derivatives. This chapter, in conjunction with two earlier ones in this series (Maggini and Guldi, Vol. 6, Chap. 4, and Sun, Vol. 1, Chap. 9), should serve readers well for many years to come. 

A possible explanation for the lack of electron-transfer characteristics in the trimer 9d is derived when extrapolating the linear relationship in Fig. 9.8 to the distance of the trimer. As a matter of fact, the charge-separation would not be able to compete with the intrinsic singlet lifetime of C6o (i.e. dashed line). This, in turn, explains the lack of fullerene emission quenching in 9d. Nevertheless, the photophysical assays clearly established that oPPE bridges effectively mediate electron-transfer processes over distances up to 20 A. These findings were further corroborated by quantum mechanical calculations. 

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