Polyphenylene Nanostructures - Cluster for Molecular Chemistry

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1770 Chemical Reviews, 1999, Vol. 99, No. 7 Berresheim et al. Scheme 38 Scheme 39 bon 3 from 148 is expected, as discussed above, the formation of the same product from 149 is surprising (Scheme 38). This can be explained by a quantitative 1,2-phenyl migration at the central meta-substituted benzene ring. 163 Even more remarkable is the fact that cyclodehydrogenation of the tetraphenylene 152 occurs by a different route depending on the reaction temperature. Reaction with copper(II) chloride and aluminum(III) chloride in carbon disulfide at 30 °C leads to the quantitative removal of 12 hydrogens providing 154, while the same reaction at 80 °C in 1,1,2,2-tetrachloroethane provides 155 (Scheme 39). This can only be explained by assuming a rearrangement of the central eight-membered ring into a dibenzopyrene unit and subsequent dehydrogenation. 163 Not surprisingly, 155 can also be obtained from the oligophenyl precursor 153, which can readily be made from the Diels-Alder reaction of 2,2′diethynylbiphenyl (151) and 80a. 163 The above examples provide clear evidence that the cyclodehydrogenation is applicable to polycyclic aromatic hydrocarbons much larger than 5. In view of its hexagonal symmetry, 140 is considered as a superbenzene, 228 155 can be regarded as supernaphthalene 162 and higher superacenes, such as the supertriphenylene 163 topology 156, can also be prepared (Chart 7). A related structure is the superbiphenyl 157, 230 which could be synthesized from a bromo-substituted hexa-peri-hexabenzocoronene derivative and which in one dimension contains a sexiphenyl axis corresponding to a length of approximately 2.4 nm. The systematic variation of size and shape of PAHs, i.e., of 2-dimensional nanostructures, can also be used to build up a homologous series, such as the one comprising C24 (158), 231,232 C42 (140), 228 C60 (159a), 163 C78 (3), 163 and C96 (160) 230 (Cx ) C6 + nC18, n g 1) leading to an all-ribbon polymer with C4 (Chart 8). It should be noted that there is a systematic increase in area of the disc-type molecular
Polyphenylene Nanostructures Chemical Reviews, 1999, Vol. 99, No. 7 1771 Chart 7 Chart 8 graphite subunits down the series. The preparation of C60 (159a) 163 from the oligophenylene precursor 94 is similar to the preparation of 140 and 3. A key concern in the above syntheses is the limited solubility of the planar structures in organic solvents, which hampers purification and processing. The presence of alkyl groups renders discs such as 140 and 159 soluble so that they can be subjected to full spectroscopic characterization 161,233 but is also important for the formation of 2- and 3-dimensional supramolecular architectures (Chapter VI.B). However, as mentioned for compound 140, the presence of alkyl chains can create problems during the oxidative cyclodehydrogenation. Nevertheless, the octa-n-alkyl derivative of C60 159c has been prepared, 163 (Scheme 40) although the first oxidative cyclodehydrogenation provides only partially dehydrogenated products, which can then be transformed into the disc structure only by a second reductive cyclodehydrogenation. An important aspect in view of the direct visualization of single disc-type molecules by scanning tunneling microscopy and the recording of currentpotential curves of single molecules (Chapter VI.B) is the fact that modification of the above synthetic procedures also provides functionalized derivatives of hexa-peri-hexabenzocoronene, such as 161, 162, and 163 (Chart 9). 230 This not only allows subsequent chemical transformation but also a significant change in the energy of the frontier orbitals, which is thought to be relevant to the magnitude of the tunneling current seen in an STM experiment. 230 VI. Supramolecular Ordering A. Linear Polyphenylenes The key structural factor in describing the supramolecular ordering of PPPs is their anisotropic shape, which follows from a rodlike architecture that differentiates them from flexible polymers. Rigid-rod polymers are characterized by a stiff backbone, allowing only one single rotation about the chain axis. This class of polymers includes, other than PPPs, polymers such as poly(para-phenylene)-2,6-benzodiimide (164) and poly(para-phenylene)ethynylene (165) (Chart 10). 234,235 However, it should be noted that the design of rigid-rod polymers is not restricted to chain structures consisting of aromatic building blocks but also includes multilayered systems such as the phthalocyanatopolysiloxanes (166). 32 Unfortunately, PPPs are insoluble in many organic solvents, which limits their processability. Therefore, attachment of conformationally mobile alkyl side chains to the backbone has been important because it has allowed the controlled synthesis of soluble and
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Polyphenylene Nanostructures - Cluster for Molecular Chemistry

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