Fullerene arrays

N. Armaroli (2003) From metal complexes to fullerene arrays exploring the exciting world of supramolecular photochemistry fifteen years after its birth, Photochem. Photobiol. Sci., 2 73-87. 

Apart from zinc(II) (na)phthalocyanines, the ruthenium(II) analogs have also been used to complex with pyridyl fullerenes. Arrays 33-35 show electronic coupling between the two electroactive components in the ground state as shown by UV-Vis spectroscopy and electrochemical measurements [45], The use of ruthe-nium(II) instead of zinc(II) phthalocyanines reduces the undesired charge recombination, increasing the lifetime of the radical ion pair state (on the order of hundreds of nanoseconds). 

In addition to axial coordination, hydrogen bonding is another important approach to construct various supramolecular systems. This strategy, however, has not been widely used to assemble phthalocyanines and other functional units. To our knowledge, examples are so far confined to several phthalocyanine-fullerene arrays and self-assembled phthalocyanine aggregates, which are described in this section. 

Khlobystov AN, Britz DA, Wang J, O Neil SA, Poliakoff M, Briggs GAD. Low temperature assembly of fullerene arrays in single-walled carbon nanotubes using supercritical fluids. J Mater Chem 2004 14 2852-7. 

The occupation of each tetraliedral and octaliedral site in tliese regularly oriented arrays of cavities by, for example, alkali atoms results in tire transfer of a single electron to tire fullerene s conduction band (ti ) [58]. Consequently,... 

An alternative approach envisages the stimulating idea to produce an all-carbon fullerene polymer in which adjacent fullerenes are linked by covalent bonds and align in well characterized one-, two- and tliree-dimensional arrays. Polymerization of [60]fullerene, with the selective fonnation of covalent bonds, occurs upon treatment under pressure and relatively high temperatures, or upon photopolymerization in the absence of a triplet quencher,... 

Figure C 1.2.9. Schematic representation of photo induced electron transfer events in fullerene based donor-acceptor arrays (i) from a TTF donor moiety to a singlet excited fullerene and (ii) from a mthenium excited MLCT state to the ground state fullerene.

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

The hexaadducts [C60 M(PEt3)2 6] (M = Ni, Pd, Pt) involve an octahedral array of metal moieties in a similar fashion to C60 C(CO2Et)2 6 and possess the very rare point group Th (16). It is believed that each metal fragment binds to one fullerene C=C bond and sterically blocks the neighboring four, so that six moieties will block all 30 C=C bonds of C60. Hence, 7r-adducts involving more than six addends are likely to be difficult to prepare. 

In 1991, scientists at AT T Bell Laboratories discovered a new class of high-temperature superconductors based on fullerene, the allotrope of carbon that contains Cgo molecules (Sections 10.10 and 19.6). Called "buckyballs," after the architect R. Buckminster Fuller, these soccer ball-shaped Cgo molecules react with potassium to give K3C6o- This stable crystalline solid contains a face-centered cubic array of buckyballs, with K+ ions in the cavities between the Cgo molecules (Figure 21.16). At room temperature, K3Q,o is a metallic conductor, but it becomes a superconductor at 18 K. The rubidium fulleride, Rb C o, and a rubidium— thallium-Cfio compound of unknown stoichiometry have higher Tc values of 30 K and 45M8 K, respectively. 

The Fullerenes form particularly strong complexes with porphyrins as exemplified by the X-ray crystal structure of the covalent Fullerene-porphyrin conjugate 15.8 (Figure 15.29).48 This property allows fullerenes and porphyrins to form extended supramolecular arrays (even when not covalently linked) and has been used to engineer host-guest complexes in which a Fullerene is sandwiched in between a pair of porphyrins, and ordered arrays involving interleaved porphyrins and Fullerenes. Applications include the use of porphyrin solid phases in the chromatographic separation of Fullerenes and potential applications in porous frameworks and photovoltaic devices.49... 

As an example of the range of chemical species that can be inserted into nanotubes, researchers have filled nanotubes with fullerenes [965], which could be a step towards making nanoscale arrays of magnetically active molecules that could be used to create extremely small computing devices. 

Ferrocene is composed of a pair of 6-7r-electron carbon arrays and a 6-d-electron iron(II) atom. Ferrocene-fullerene donor-acceptor dyads carry all the requisites for electron-transfer phenomena. However, data for the formation of ferrocene-fullerene hybrids are not abundant. Some such dyads have already been synthesized following the methodology of 1,3-dipolar cycloaddition of the appropriate azome-thine ylides to C60, with either variable-spacing building blocks or a rigid-bridge all-cj-bonded framework, in order to tune the redox properties of the system [40,234, 248-251]. Another novel dyad that contained two covalently bound ferrocene units was recently synthesized via cyclopropanation of the fullerene core [252]. 

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