Supramolecular oligomers

The method of choice to determine whether or not synthetic ion channels and pores act as monomers or supramolecular oligomers (Fig. 11.13, A vs B) is Hill... 

Polymers involving CyD moieties can be either regular polymers or supramolecu-lar ones. The latter ones are novel and promising future applications while the former, more traditional ones have already found numerous applications. Since supramolecular oligomers and polymers seem to be more interesting they will be presented first. It should be stressed that most these systems have the rotaxane structure, which together with the structure of catenanes involving CyDs are discussed in Chapter 14. 

An atypical rotaxane structure is that involving a thread covalently attached to the CyD ring. Such a system acts as both host and guest and it has to be specially designed so that the thread is induded spontaneously in a cavity other than the one bearing it intermolecular inclusion), whereas intramolecular indusion is prevented. The resulting assembly, named a daisy chain by J.F. Stoddart, is either an acyclic supramolecular polymer 8 or a cyclic supramolecular oligomer 9 [50] (Fig. 12.4). Acyclic cyclodextrin polymers were first observed in the solid state in... 

CiO-a-CD and 6-CiO-p-CD form supramolecular oligomers and polymers. When guest molecules which bind strongly in a CD cavity are added, the modified cyclodextrins bind the competitive guest to give host-guest monomers. 

METAL-IONS A SELF-ASSEMBLY MOTIF IN SUPRAMOLECULAR OLIGOMERS... 

Mesoscopic supramolecular assemblies are also obtainable in organic media from amphiphilic linear networks of complementary hydrogen-bonds. An equimolar mixture of bw-barbituric acid derivative 18 and 1 gave helical superstructures with a minimum width of 50 A (Figure 12(c), note that the components are achiral, but compound 18 displays molecular asymmetry) [86]. The observed thickness corresponds to the three-layered membrane structure schematically depicted in Figure 12(d). Soluble, tapelike supramolecular oligomers can be also prepared, by the covalent preorganization of melamine units [87]. 

The presumed supramolecular oligomers were subjected to concentration-dependent absorption spectroscopic analysis in solution. Fits of the extinction coefficients obtained as a function of concentration to an isodesmic model allowed association constants to be derived (cf. Table 12.1). On this basis, it was noted that... 

Table 12.1 Association constants for the supramolecular oligomers built up from TTE-C4Ps and DNP-C4PS, as inferred from fits of concentration-dependent absorption spectroscopic data to an isodesmic model [86]...

Overall, this initial report by Sessler and collaborators served to demonstrate that TTF-C4Ps could be used to create heterocomplementary supramolecular oligomers, both in solution and in the solid state, and that stimulus-induced responsive behavior was an inherent feature of the resulting self-assembled systems. 

Fig. 12.35 Solution-based analyses of the supramolecular oligomeric system [42 47] . (a) UV-vis-NIR absorption spectra of 42, 47, and 42 -l- 47 recorded in chloroform at 298 K. (b) Partial H NMR spectra (600 MHz, 298 K) of 1) 42 (0.7 mM), II) 42 (0.7 mM), ID) 42 + 47 (0.7 mM of 42 and 47, respectively) and IV) 42 -l- 47 (7.0 mM in both 42 and 47) all in CDCI3. c 2D NOESY NMR spectroscopic analysis of a mixture of 42 (7 mM) and 47 (7 mM) in CDCI3. (Reproduced from Bahring, S. Kim, D. S. Duedal, T. Lynch, V. M. Nielsen, K. A. Jeppesen, J. O. Sessler, J. L. Use of solvent to regulate the degree of polymerisation in weakly associated supramolecular oligomers. Reprinted from Chem. Commun. 2014, 50, 5497-5499 [90], with the permission from the Royal Society of Chemistry)...

D NOESY NMR spectral analyses (cf. Fig. 12.35c) revealed cross peaks between the aromatic DNP-protons of 47 and the wso-methyl protons of 42, as well as coupling between the pyrrole NH protons of 42 and the 4-H proton of the aromatic DNP moiety of 47. Such findings are consistent with the formation of a supramolecular oligomer in solution. 

The monomeric subunits in 50, esters of 2,5,7-trinitrodicyanomethylene-fluorene-4-carboxylate, has previously been used to create complexes with TTE-C4P 42 [91]. This led to the consideration that ditopic systems, such as 50, that incorporates an adequate glycol-linker between the two moieties would promote the formation of supramolecular oligomers when exposed to 42. Initial evidence of a supramolecular oligomer formation came from H NMR spectroscopic studies (cf. Eig. 12.38a). As expected based on the previous studies described above, the proton resonances of the aromatic TNDCE moiety 50 were shifted upheld, while the NH proton resonances of 42 were shifted to lower held. 

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