Large planar molecules

Earlier chapters in this volume have dealt with the molecular properties and crystal structures of organic metals. The large planar molecules under consideration here are usually stacked face to face in chains sometimes the molecular planes are perpendicular to the stacking axis, as in the case of the Bechgaard salts, or they may be tilted by as much as 30°, as in TTF-TCNQ. Because the overlap of the partially occupied tt orbitals is much better along the stacking axis, their electronic band structures are often quasi-one-dimensional. 

The scientific interest in porphyrin ligands (Fig. 5) derives in part from their ability to accommodate a large series of different elements, often in various oxidation states. On the other hand porphyrins are planar molecules with a delocalized 18 Ti-electron system and a diatropic ring current [25], which makes them interesting for the design of new materials with applications in photochemistry [25-27]. 

Zareba et al. [165] described the crystal structure of the chiral 4-(l-methyl-heptyloxycarbonyl)-phenyl 4-heptyloxytolane-4 -carboxylate (C7-tolane) which shows monotropic antiferroelectric and ferroelectric phases. The single-crystal X-ray analysis of this compound shows that the crystal has a smectic-like layer structure composed of largely bent molecules where the chain of the chiral group is almost perpendicular (86°) to the core moiety. Within the layers, the molecules are tilted. The central tolane group of the molecule is roughly planar. 

A large set of results obtained in recent years for various carotenoids (see, e.g., Simonyi et al. (2003) for review) suggests that planarity of the carotenoid molecule is crucial for aggregation. This hypothesis is supported by the observation that zeaxanthin and astaxanthin, both fairly planar molecules, form aggregates more readily than other carotenoids. Moreover, zeaxanthin and astaxanthin are the only two carotenoids studied so far that can, depending on preparation conditions, form exclusively either H- or J-aggregates (Billsten et al. 2005, Kopsel et al. 2005, Avital... 

Three dimensional X-ray diffraction data point to a largely planar structure in all of the molecules studied this ensures optimum conjugation (overlapping of the TT-electrons). Intramolecular hydrogen bonds probably contribute considerably towards supporting the planar conformation. 

In the medium scale are various substructures of a molecule such as delocalized bonds, rings and fused rings, and their conformations—whether they are planar or shaped like a boat or a chair. Large biological molecules have secondary and tertiary structures, such as the a helix and p sheets of proteins, and the double helix of DNA molecules. 

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