Aromatic superstructures

Porphyrin chromophores have received much attention, particularly as photoelectric devices and molecular wires. Efficient re-electronic communication between porphyrin macrocycles is pivotal in various complex functions. K.M. Smith et al. showed that neighboring acetylenic units on porphyrins provide a means for the efficient construction of aromatic superstructures triggered by the Bergman cyclization reaction conditions and give rise to novel [n]phenacenoporphyrins, which belong to a new class of highly re-extended porphyrins." ... 

Lignin has a complex structure that varies with the source, growing conditions, etc. This complex and varied structure is typical of many plant-derived macromolecules. Lignin is generally considered as being formed from three different phenylpropanoid alcohols— coniferyl, coumaryl, and sinapyl alcohols, which are synthesized from phenylalanine via various cinnamic acid derivatives and commercially is sometimes treated as being composed of a Cg repeat unit where the superstructure contains aromatic and aliphatic alcohols and ethers, and aliphatic aldehydes and vinyl units. 

As the formation of five-coordinate complexes with nitrogenous bases largely depends on the size of the host cavity, single-face-hindered porphyrins have been developed pocket [40], cyclophane [41-44], bridged [45], crowned [46], capped [47-53], strapped [54-60] and hybrid [61], In these compounds, the size and the chemical nature of the superstructure can be conveniently varied by changing the number of methylene groups and the aromatic substituent incorporated. 

Whereas important progress has been made regarding the use of metalloporphyrins as catalysts for alkene epoxidations and alkane hydroxyla-tions, work concerning the mechanism of hydroxylation of aromatic hydrocarbons has received only limited attention. In fact, the main problem encountered with the design of systems capable of performing such oxidative reactions is in the preparation of superstructured porphyrins for the selective complexation of aromatic compounds. 

Liu Y, Fan Z, Zhang HY, Yang YW, Ding F, Liu SX, Wu X, Wada T, Inoue Y, Supramolecular self-assemblies of y -cyclodextrins with aromatic tethers factors governing the helical columnar versus linear channel superstructures, J. Org. Chem. 2003 68 8345-8352. 

Water-soluble cydophanes are also studied as possible catalysts or artificial enzymes the hydrophobic cav-itiy serves as substrate binding site while appended functional groups act as cofactors. The pyruvate oxidase mimic 20 performed remarkably in the oxidation of aromatic aldehydes to the cossesponding esters (MeOH/ H2O), and it displayed Michaelis-Menten kinetics comparable to those of natural enzymes. The encapsulation of cydophanes in dendrimers creates binding sites within microenvironments, resembling those existing in protein superstructures. 

The various ribbons presented above consist of amphiphilic molecules arranged in bilayers. The long axis of the molecules is perpendicular to the ribbon plane or slightly tilted from this direction. But ribbons or tapes can also be formed from the assembly of molecular rods oriented with their long axis parallel to the width of the ribbon. This is the case for some peptides that form extended /3-sheet tapes which stack to form chiral superstructures (Fig. 3) [73]. It is also the case for numerous gelators consisting of a central aromatic core and chiral cholesteryl saccharidic moieties on the sides, such as the porphyrin derivative shown in Fig. 4 [74]. Chirality effects in these... 

Electrophilic Aromatic Substitution. Micellar SDS has been used as a reaction medium for the chlorination and bromlnatlon of alkyl phenyl ethers T gjj(j phenol by several halogenatlng agents (eq 1). Compared to reactions in H2O alone, theparar.ortho product ratio increased for pentyl, nonyl, and dodecyl phenyl ether, and decreased for anlsole. Enhanced ortho relative to para substitution was obtained with phenol. In each case the observed regios-electivity derived at least in part from alignment of the substrate at the micelle-H20 interface and resultant differential steiic shielding of the para and ortho positions by the micelle superstructure. 

Interestingly, the second and third monolayers of some unsubstituted Pcs still show very good submolecular resolution when observed with low-temperature STM, suggesting a very good orbital overlap between aromatic macrocycles in adjacent layers. In the case of FePc, the unit cell of the formed molecular superstructure of the second layer shifts compared to the unit cell of the first layer. The Fe atoms in the top layer are shifted with respect to the ones underneath, and the plane of the Pc in the second layer was found to be tilted with respect to the plane of the substrate. Multilayers of CoPc (up to three layers) were also studied and it was shown that in this case the positions of both the unit cell and the metal atom are retained from one layer to the next. Nevertheless, the plane of the Pc starts to tilt from the second layer on, being even more tilted in the third monolayer. To date, there is no explanation for this... 

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