Conjugated fullerene system

N.V. Kamanina, Mechanisms of optical limiting in p-conjugated organic system fullerene doped polyimide, Synthetic Metals, vol. 127, pp. 121-128, 2002. 

Within the large number of multiredox arrays containing metalloporphyrins/covalently bound (conjugate) fullerene-metalloporphyrin dyads have gained enormous interest in the last ten years, mainly due to their potential application as artificial antennae Due to the multiredox behaviour of the fullerenes (up to six reversible one-electron reductions and at least one reversible one-electron oxidation), the porphyrin ligands and the incorporated metals, the assignment of electron-transfer steps in such systems is difficult. Recently, spectroelectrochemical characterisation has been carried out on a number of fullerene-[(TPP)Co] dyads shown in Scheme 4.3, which exhibit rather complex redox behaviour (Figure 4.19). 

Yet another scheme exists besides the principles mentioned above. This is mainly encountered in the addition of radicals and certain nucleophiles like lithium or copper organyls. Upon reaction with an excess of the reagent, the favorable products are a hexakis- or a pentakis-adduct, respectively, that adopts the structure of a 1,4,11,14,15,30-hexahydrofullerene (Figure 2.38). The adduct s characteristic feature is a cyclopentadienyl unit whose double bonds are isolated from the conjugated n-system of the remaining fullerene skeleton. This structure is... 

It was also more recently that the chemistry of the less abundant and less symmetrical higher fullerenes like C70 and C74 started to develop. Aspects such as the inherent chirality of some of these carbon cages or the search for the principles of the regioselectivities of addition reactions to their conjugated TT-system make these investigations most attractive. 

Complexation of metals to curved carbon n surfaces has attracted continuous interest since the discovery of fullerenes. So far, fullerenes have been found to act as an -coordination ligand to various transition metals from its eYO- r-surface [1, 2], Other coordination modes in exo complexes were reported in the modified fullerene it systems [3], such as buckyferrocene [4] Fe(CgoMe5)Cp (Cp = cyclopentadienyl). Fullerenes are also known to form the endohedral metal complexes to provide potential materials in carbon chemistry [5, 6], On the other hand, fullerene fragments, termed as n bowls (we use this term in the present chapter, whereas they have been also called open geodesic polyarenes [7] or buckybowls [8]), also possess curved carbon n surfaces, n Bowls have attracted interest in the science of nonplanar -conjugated carbon systems [9-14], From the viewpoint of coordination chemistry, such molecules are quite unique because they can provide not only a convex surface but also an open concave surface for binding (a schematic illustration is shown in Fig. 35.1), which was first addressed by the computational study of hemifullerene (C30H12) in 1993 [15],... 

In this chapter we concentrate on reduction processes of carbon-rich systems. The formation of anions and radical anions of -conjugated monocyclic systems, cyclophanes, bowls and fullerenes is described. Carbon-rich compounds can be reduced directly by contact with alkali metals Li, Na, K, Rb and Cs, which have a low reduction potential. Proton, carbon and lithium NMR and EPR spectroscopies are the main methods used to gain a better understanding of the mono- and polycyclic systems in solution. Special attention will be given to modes of electron delocalization, aromaticity, anti-aromaticity, as well as aggregation, bond formation and bond cleavage processes of diamagnetic electron transfer products. Electrochemical reductions will be briefly discussed. 

The aim of this chapter is to give a state-of-the-art report on the plastic solar cells based on conjugated polymers. Results from other organic solar cells like pristine fullerene cells [7, 8], dye-sensitized liquid electrolyte [9], or solid state polymer electrolyte cells [10], pure dye cells [11, 12], or small molecule cells [13], mostly based on heterojunctions between phthaocyanines and perylenes [14], will not be discussed. Extensive literature exists on the fabrication of solar cells based on small molecular dyes with donor-acceptor systems (see for example [2, 3] and references therein). 

Comparison of the spectral response and of the power efficiency of these first conjugated polymer/fullerene bilayer devices with single layer pure conjugated polymer devices showed that the large potential of the photoinduced charge transfer of a donor-acceptor system was not fully exploited in the bilayers. The devices still suffer from antibatic behavior as well as from a low power conversion efficiency. However, the diode behavior, i.e. the rectification of these devices, was excellent. 

The first realization of a conjugated polymer/fullerene diode [89] was achieved only recently after the detection of the ultrafasl phoioinduced electron transfer for an lTO/MEH-PPV/CW)/Au system. The device is shown in Figure 15-18. Figure 15-19 shows the current-voltage characteristics of such a bilayer in the dark at room temperature. The devices discussed in the following section typically had a thickness of 100 nm for the MEH-PPV as well as the fullerene layer. Positive bias is defined as positive voltage applied to the 1TO contact. The exponential current tum-on at 0.5 V in forward bias is clearly observable. The rectification ratio at 2 V is approximately l()4. 

In contrast to the high stability of anionic species, cations possessing a positive charge on the -conjugated system of a fullerene cage are uncommon. The chemistry of such cations have remained unexplored for nearly a decade, since C6o became available in macroscopic quantities in 1990 (70), in spite of their potential significance in both fundamental and application studies. 

The RC60+ cations are carbenium ions (trivalent carbocations), which are formally conjugated with the entire 7i-system of the fullerene cage. This chapter focuses on recent developments in the author s laboratory regarding the prepara-... 

Several effects can influence the electronic structure of Cjq upon metal complex formation. One is the removal of one double bond from the remaining 29 fullerene double bonds. As in any polyene system, this decreased conjugation is expected to raise the energy of the LUMO and therefore decreases the electron affinity of the system. Conversely, the d-orbital backbonding transfers electron density from the metal into n orbitals of the remaining double bonds, which also decreases the electron affinity. 

Their isolation by flash chromatography on silica gel was comparatively easy. The CD spectra of related pairs of diastereomers whose addition pattern represent pairs of enantiomers, reveal pronounced Cotton effects and mirror image behavior. It is the chiral arrangement of the conjugated Jt-electron system within the fullerene core that predominantly determines the chiroptical properties. Adducts with a C2-... 

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