Molecular structures of fullerenes

Figure 58 Molecular structures of fullerene derivatives forming a library of acceptor materials investigated in solar cells in combination with P3HT. Solubility values determined in chlorobenzene are given in brackets near numbers of the compounds. (Reproduced from Ref. 180. Wiley-VCH, 2009.)...

FIGURE 2.35 Molecular structure of fullerene-free base porphyrin-Zn porphyrin-ferrocene tetrad and Au electrode modified with a self-assembled monolayer of fullerene-Zn porphyrin-ferrocene triad. 

Electronic Structure of Fullerenes. As discussed above, the molecular structures of fullerenes show some very conspicuous features which will be certainly determinant in the electronic structure of this form of carbon. The principal of such structural features is certainly their spheroidal geometry. 

The most extensive calculations of the electronic structure of fullerenes so far have been done for Ceo- Representative results for the energy levels of the free Ceo molecule are shown in Fig. 5(a) [60]. Because of the molecular nature of solid C o, the electronic structure for the solid phase is expected to be closely related to that of the free molecule [61]. An LDA calculation for the crystalline phase is shown in Fig. 5(b) for the energy bands derived from the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for Cgo, and the band gap between the LUMO and HOMO-derived energy bands is shown on the figure. The LDA calculations are one-electron treatments which tend to underestimate the actual bandgap. Nevertheless, such calculations are widely used in the fullerene literature to provide physical insights about many of the physical properties. 

Calculations for Ceo in the LDA approximation [62, 60] yield a narrow band (- 0.4 0.6 eV bandwidth) solid, with a HOMO-LUMO-derived direct band gap of - 1.5 eV at the X point of the fee Brillouin zone. The narrow energy bands and the molecular nature of the electronic structure of fullerenes are indicative of a highly correlated electron system. Since the HOMO and LUMO levels both have the same odd parity, electric dipole transitions between these levels are symmetry forbidden in the free Ceo moleeule. In the crystalline solid, transitions between the direct bandgap states at the T and X points in the cubic Brillouin zone arc also forbidden, but are allowed at the lower symmetry points in the Brillouin zone. The allowed electric dipole... 

Figure 28 shows the molecular structure of [Fe2( -S)2(772-C60)(CO)6], in which again there is no direct bond between the metal(s) and the fullerene ligand.40... 

We report an electron spin resonance (ESR) study on a C60 anion and a metal (M) encapsulated in fullerene (C ) (a metallofullerene M C ). The anisotropy components of the g-factor of Cg0 were determined accurately from the analysis of angular-dependent ESR spectra of single crystal Cg0 salt. The evaluation of the g-factor was performed according to the classification of symmetry of the C60 geometry. It was found out from the evaluation that the molecular structure of Cg0 should he distorted to lower symmetry, C2h or C,. The variety of ESR spectra of metallofullerenes of La C s was obtained in terms of a g-factor, a hyperfine coupling constant, and a line width. In the case of the isomer I of La C80 and the isomer II of La C84, an abnormally large line width was measured. The molecular structure with high symmetry would reflect on the specific spin dynamics. 

Figure 5.32 Molecular structure of the surface active thiol-terminated fullerene, reported by Neusch and co-workers [66]...

In the next example, a mixed SAM is discussed which aims to utilize photoinduced energy and electron transfer processes to create a photocurrent in an approach which is reminiscent of the natural photosynthetic process. Figure 5.33 illustrates the molecular structures of the components of interest, i.e. the molecular triad ferrocene-porphyrin-fullerene (Fc-P-C6o) and a boron dipyrrin thiol (BoDy) [67]. Mixed monolayers were generated by coadsorption onto vacuum-deposited gold... 

Structure of fullerene-Cy,o (a) molecular shape, (b) valence-bond formula, and (c) planar formula with carbon atom-numbering scheme. 

The first discovered solid phase of fullerenes C6o represents typical molecular crystal. Later it was established that high pressure applied to solid C6o at high temperature induces polymerization of C6o [1-2]. Using the computer modeling methods allows confirming the existence of at least three different planar polymerized structures of fullerene Cgo with coordination numbers 2, 4, 6, and besides the values 4 and 6 are more probable ones. 

FIGURE 14 Schematic molecular structure of the (Sc2C2 C84) carbide metallofullerene based on the synchrotron X-ray powder diffraction and NMR experiments. The two (top and bottom) spheres in the fullerene correspond to Sc atoms, whereas the C2 molecules are depicted between the Sc atoms. 

A few years ago, a novel molecular structure of carbon was detected. Globular C o molecules (or C70 and other sizes) form a molecular lattice. The Ceo buckyballs have a diameter of 1 nm. Within the moieties, the polyaromatic structure sp is fully retained in annealed five- and six-membered rings. The fullerene molecule resembles a football in design. The close analogy to graphite, with sp -structures of 100% C, justifies a treatment immediately following Section 3 on graphite. 

Solid fullerene displays interesting properties. After condensation, the Cgo molecules forms a face centered cubic (f.c.c.) structure fullerite. This is the only material which consists of quasi-spherical molecules, all atoms of which are of one kind. X-ray dispersion experiments show that fullerite forms a closely packed f.c.c. crystal in which the distance between the nearest molecules is 10.04 A [2]. The least distance between two molecular surfaces is 2.9 A, and the distance between the nearest atoms in a crystal is 1.42 A. Thus, the experiments specify that the molecular structure of Ceo is preserved in the solid. Strong orientational disorder is observed at room temperature [64] and this disorder decreases as the temperatures decreases. 

Figure 17 Molecular structures of the positively charged fullerene derivatives and negatively charged porphyrins undergoing facile...

Figure 47 Schematic layout of organic fullerene/polymer solar cell, (a) Phase segregation on the nanoscale creates bulk heterojunction in the active layer, which allows for efficient photon harvesting and charge generation and (b) molecular structures of classical fullerene-based and pol3mer-based materials most widely investigated in organic bulk heterojunction solar cells. (Reproduced from Ref. 156. American Chemical Society, 2004.)...

Figure 78 Molecular structure of polymerizable fullerene derivative C-PCBSD.

Fig. 1.2 Molecular structure of liquid crystalline fullerene molecules...

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