Columnar aggregation

It has been shown frequently that without the presence of strong intermolecular interactions, discotic molecules are highly mobile in the liquid crystalline state.1 They undergo both lateral as well as rotational translations, resulting in the absence of positional order. Similarly, such discotics also freely rotate in the columnar aggregates they form in solution. This lack of positional order in the columns accounts for the absence of chiral or helical supramolecular order. We will demonstrate this characteristic using results obtained for triphenylenes. 

Metal phthalocyanines functionalized with four helicenes (62) have also been reported to form chiral columnar aggregates.76 In chloroform solutions of these metal phthalocyanines aggregation into columns occurred upon addition of ethanol, as was observed by UV-Vis spectroscopy. CD spectroscopy revealed that the chromophores within the columnar aggregates are in a chiral environment, implying that the chirality of the peripheral helicenes has been transferred to the supramolecular aggregates. These phthalocyanines stack with a typical intermolecular distance of 3.4 A, and calculations have indicated that to allow this distance the two phthalocyanine moieties have to be rotated because of the bulkiness of the helicenes. It can easily be imagined that a phthalocyanine provided with both R and S helicenes cannot stack in such a defined manner because of the steric interactions between the nonconform helicenes. 

There are a great many methods by which small molecules can be assembled to form nanotubes and other similar columnar aggregates [50]. Unlike the stacked disc methodology employed by Fenniri s group, Russell and co-workers took a completely different approach, which consisted of nanotubes made by way of rolled-up bilayers [48], Their study of biocidal nanotubes from a self-assembled diacetylene salt employed a hydrophobically driven bilayer type of self-assembly step. This bilayer formation was based on the alignment of the hydrophobic chains, including aligned Ji-systems of the acetylenes on the interior of the bilayer and the polar hydrobromide salts of an amine on the exterior of the bilayer (Fig. 7.6). 

Katzfs columnar aggregates of helicenes have relatively modest g-values, e.g. for 1 mM solutions of enantiopure derivative of [7]thiahelicene 11, gabs IgcppiJ =... 

Similar helical columnar aggregates have also been synthesized through the combination of n - n interactions and hydrogen bonding and/or ion-... 

Lyotropic (in solvent) cholesteric mesophases have been observed for self-assembled guanosine derivatives. In water, the compounds shown in Fig. 10 generate left-handed columnar aggregates [91]. When the concentration is sufficiently high, cholesteric phases are formed which also have a left-handed twist, as determined by CD spectroscopy. The same tetrameric motif is present in the lyomesophases formed by more lipophilic guanosine deriva-... 

The monophthalocyanines 106 and 107 show a weak aggregation tendency in chloroform. The latter has a self-dimerization constant of 1,175 M-1. By contrast, the donor-acceptor bis(phthalocyanine) 99 exhibits a much stronger aggregation tendency with a dimerization constant of 1.1 x 106 M-1 in chloroform. It is believed that in addition to the hydrophobic effect, the two phthalocyanine halves of compound 99 may be considered as donor and acceptor subunits that interact with each other. As revealed by electron microscopy, 99 forms one-dimensional nanoaggregates through intermolecular interactions between its complementary donor and acceptor phthalocyanine units as shown in Fig. 8. The dimerization constant of 99 is about one order of magnitude lower than that observed for the hetero-dimerization of 106 and 107, which may be due to the cyclophane step that hinders the formation of columnar aggregates of double phthalocyanine dimer. 

Schouten PG (1994) Charge carrier dynamics in pulse-irradiated columnar aggregates of mesomorphic porphyrins and phthalocyanines. Delft University of Technology, The Netherlands. ISBN 90-73861-22-5 (available from Warman JM on request)... 

Warman JM, de Haas MP, van der Pol JF, Drenth W. (1989) Charge separation and recombination in pulse-irradiated columnar aggregates of peripherally octa- -alkoxy-substituted phthalocyanines. Chem Phys Lett 164 581-586. 

Providing phthalocyanines with chiral bulky heli-cenes at the periphery (29) also results in helical aggregates.165 Aggregation in chloroform occurs upon addition of ethanol as was observed with UV—Vis spectroscopy, while CD spectroscopy revealed the helical nature of the columnar aggregates. Calculations have indicated that two phthalocyanines need to be rotated to allow a favorable intermolecular distance of 3.4 A, because of the bulky helicenes. As such, phthalocyanines provided with racemic helicenes cannot stack in such a defined manner, because of the steric hindrance of the racemic side groups. Aggregation of a smaller analogue of 29, a triphen-ylene-based porphyrazine, has been shown to occur as well.166... 

Charge delocalization and h q)ersensitivity of the conductivity to packing and lateral order may hold other clues to the observed remarkable insensitivity of interfacial ET of DNA-based molecules. Other notions such as columnar aggregation and cationic condensation " can be other crucial factors in these intriguing observations. 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

Scheme 17. A representation of the chiral columnar aggregates formed by d(pG), d(GpG), d(GpGpG), and d(GpGpGpGpGpG). The filled circles represent the sugar units (for clarity, only one sugar per tetramer is shown). The screw thread is obtained by joining the filled circles the screw pitches are in the order d(GpG) > d(GpGpG) > d(GpGpCpGpG), indicating parallel unwinding of the helical structure. Reproduced from ref. 362 (Gottarelli et al., Comprehensive Supramolecular Chemistry 1996, Vol. 9, p. 483) with permission from Elsevier Science.

J.M. Warman, M.P. de Haas, J.F. van der Pol, and W. Drenth, Charge Separation and Recombination in Pulse Irradiated Columnar Aggregates of Peripherally Octa-n-alkoxy-Substituted Phthalocyanines, Chem. Phys. Lett., 164 (1989) 581. 

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