Fullerenes host-guest complexes

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... 

The nanotubular cavities (mean diameter 12.4 A) were found to be effective hosts for C6o molecules (van der Waals radius 10.3 A), and were capable of solubilising Ceo in solvents such as chloroform, where the fullerene has poor solubility. UV-vis, CD, 13C-NMR and molecular modelling were used to characterize this NDI nano-tube-C6o host-guest complex [26]. 

Increasing the amount of XIV Cl in a solution containing 2 + C60 complex also resulted in the rapid decrease of absorption at 452 nm (attributed to fullerene-fullerene interactions in a closed-packed one-dimensional array of C6o inside the nanotubular cavity), indicating that the encapsulation of ammonium ions led to partial disruption of the close-packed C60 array resulting in the formation of a mixed complex ion-pair/C6o host-guest complex, where the ion pair is intercalating between the fullerenes. 

Only a few studies investigating the pressure effect on the formation of host-guest complexes have been carried out hitherto. A large effect of pressure has been observed for reactions in which atoms or molecules are compressed into the cavity of a container molecule without releasing other molecules out of the cavity. The volume of such a reaction is highly negative so that high pressure favors complex formation. One example is the incorporation of noble gas atoms into the cavity of buckminster fullerene Cgo which proceeds at 250 MPa and 600 °C [10]. 

Fullerene-porphyrin supramolecular discrete host-guest complexes 05ACR235. 

Fig. 2 The types of arrangements of fullerenes (van der Waals contacts) in their host-guest complexes (host molecules not shown). These cover finite structures, (a) monomeric (encapsulated), (b) dimeric, and (c) the proposed trimeric complex involving p-Bu -calixa[8]arene, and higher aggregates, and continuous structures, all of which were established in parts (d-j), with scope for structures of higher complexity. (View this a t in color at www.dekker.com,)...

The association of porphyrin and fullerene electron acceptors in LB films can also proceed via a stepwise, noncovalent formation of the assembly, as demonstrated by Shinkai and Ikeda (Figure 13.88). The host-guest interaction of Ceo with homooxa-calix[3]arene is the noncovalent interaction that assists film formation The fullerene absorption in the host-guest complex is used to determine a surface concentration of 1.4 x 10 ° molcrn" in the Ceo layer, which is then further covered with the polymer-sustained porphyrins 165 or 166. Porphyrin surface concentrations are then controlled by UV-visible absorption and are found to be 7.6 X 10 and 5.5 x 10 for 165 and 166, respectively. Photocurrent measurements performed under irradiation at 420 nm, with a 1.36 mW cm for a coating of 165, and at 430 nm, with a 1.53mWcm for a coating of 166, in... 

The first and only example of a host-guest complex with cycloparaphenylene was assembled by Yamago and coworkers [37]. In this work, solid fullerene Ceo was ingeniously added to an NMR sample of the product mixture from the shotgun synthesis of [8-13]CPP in CDCI3. The monitored proton spectrum showed the downfield shift of only the signal corresponding to [10]CPP (Fig. 37). 

Unusually for CTV, it also binds Cgo in solution, forming a 1 1 host/guest complex." Fullerenes are highly colored in solution, and hence electronic spectroscopy is ideal for establishing solution binding constants with them. Functionalization of the CTV core can result in... 

The fullerene-binding abihties of CTVs have also been exploited in order to solubilize fullerenes, to immobilize them onto surfaces, and to incorporate them into liquid crystals. The hexa-substituted 12, for example, forms a liquid-crystalline phase. In the presence of C o, a 2 1 host/guest complex is formed, which also displays liquid-crystalline behavior, with a fluid birefringent phase at temperatures below 70 °C." The thio-derived CTV 13 forms self-assembled monolayers (SAMs) on gold surfaces." SAMs of 13 on modified gold beads can bind Ceo from solution when soaked in 1,2-dichIorobenzene solutions of Ceo-... 

In the next paragraph, the aromatic 7t-cation interaction in host-guest complex in C32 fullerenes will be analyzed. It is well known that in jt-cation interactions there are strong forces between the cation and the 7t-face of an aromatic structure. Electrostatic forces, polarization, and the charge distribution of aromatic species appear to play a dominant role in this kind of interactions [22,23]. 

A helical array of the [60]fuUerene molecules in the solid state has been obtained with the aid of calix[5]arene complexation [62]. p- Butyl caltx[5]arene 4 was used as a host molecule instead of 3, giving rise to a 1 1 host-guest complex and linear striations of [60]fullerenes within bundled nanofibers (Figure 9.7). Detailed analysis of the powder X-ray diffraction pattern of the [60]fuUerene complex with... 

Figure 9.9 Permethoxylated hexa-peri-hexabenzocoronene (5) and [60]fullerene, and the crystal structure of the host-guest complex. (Reproduced with permission from Reference [63], Copyright Wiley-VCH Verlag GmbH.)...

M. Makha, A. Purich, C. L. Raston, A. N. Sbolev, Strucutral diversity of host-guest and intercalation complexes of fullerene C60. Eur. J. Inorg. Chem. 2006, 507. 

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