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Tapes and Hydrogen-bonded Sheets

Besides such helical aggregates also a variety of linear dimers with a tape or ladder-like structure have been designed. These are mostly based on rather rigid aromatic molecules with a distinct alternating H-bond donor and acceptor pattern. Some excellent reviews on this class of H-bonded dimers have been written [15, 34, 46]. One recent example is an ureido-naphthyridine dimer introduced by Zimmermann [35] (Fig. 4.10). This molecule presents a self-complementary AADDAADD H-bond acceptor and donor pattern at the edge of a rigid aromatic

Fig. 4.10 Eight hydrogen-bonds in a self-complementary AADDAADD ureido-naphtyridine monomer lead to a strong dimerization in chloroform (A: schematic representation, B: calculated structure of the dimer) [35].

scaffold. The monomer adopts a helical structure. In chloroform dimerization via eight hydrogen bonds can occur as could be demonstrated by concentration dependent NMR experiments. Due to the fact that the JH NMR spectra remained unchanged over a concentration range from 423 mM to 13.5 mM in 10% DMSO in CDCl3, a dimerization constant Kdim of 4.5 x 105 M_1 could be set as a lower limit. Upon increasing the DMSO content to 20%, Kdim dropped dramatically to a value of 40 M_1. This clearly demonstrates the significant effect that the polarity of the solvent has on H-bonded complexes (see below).

Therefore, H-bonds alone are not sufficient to achieve strong association in more polar solvents. But in combination with other types of interactions such as ion pairs H-bonds can still be quite important for self-assembly. For example, Schmuck [36] and coworkers designed a self-complementary guanidiniocarbonyl pyrrole-carboxylate zwitterion which dimerizes in DMSO with an association constants of K > 109 M_1 and even in pure water has a K = 170 M_1. The ion pair interaction is crucial as could be shown by comparison with a neutral "knock-out" analog which has the same H-bond pattern but lacks the charges (Fig. 4.11). This neutral analog forms isostructural dimers but which are stable only in chloroform (K > 104 M_1). Already the addition of small amounts of

Fig. 4.11 Dimer formation (A) of a zwitterionic guanidiniocarbonyl pyrrole-carboxylate zwitterion (left) and a neutral amidopyridine-carboxylic acid "knock-out" analog (right). Both dimers are isostructural as the solid state structures reveal (B), but differ significantly in their stability [36].

Fig. 4.11 Dimer formation (A) of a zwitterionic guanidiniocarbonyl pyrrole-carboxylate zwitterion (left) and a neutral amidopyridine-carboxylic acid "knock-out" analog (right). Both dimers are isostructural as the solid state structures reveal (B), but differ significantly in their stability [36].

DMSO dramatically reduces the stability of the neutral dimer (K a 10 in 5% DMSO in CHCl3). However, as a theoretical study of systematically varied ''knock-out'' analogs showed, the charge interaction alone is not sufficient either to explain the large stability of the zwitterionic dimer [37]. Other zwitterions with different H-bond patterns form much less stable dimers. Hence, the stability of the guanidiniocarbonyl pyrrole-carboxylate zwitterions stems from a combination of both the formation of a directed ion pair and this specific H-bond pattern.

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