Supramolecular interactions types

Fig. 1. Schematic representation of a receptor—substrate (host—guest) complex involving cavity inclusion of the substrate and the formation of different types of weak supramolecular interactions between receptor (hatched) and substrate (dotted).

The examples cited in this chapter are but a rather small and arbitrary selection from the richly varied possibilities for supramolecular bonding. Recognition of the intrinsic chemical (partially covalent, exchange-type) character of supramolecular interactions leads inevitably to an extended definition of chemistry that includes many aspects of nanoscale aggregation, structure, and function in the biophysical and material-science domains. From this viewpoint, the molecule is seen to be... 

Furthermore, C-H - - O hydrogen bonds are formed between some of the polyether oxygen atoms and the a-bipyridinium protons. A second type of hydrogen bonding interaction, C-H- -77, is observed between the 1,4-dioxybenzene protons and thep-phenylene spacers. In most systems it is very difficult to measure their individual strength and importance in the assembly process. However, it is usually assumed that rr-stacking between complementary aromatic species is the main supramolecular interaction in these systems. 

Another type of supramolecular interaction of DNA is the intercalation of fused aromatic compounds into the stacked base pairs in double-stranded DNA (see Figure 6). Intercalation induces not only dehydration from the polar groups in intercalator but also concomitant unwinding, lengthening, dehydration, and stiffening of the DNA double helix. 

However, the particular synthetic requirements in the preparation of conjugated polymers have thus far severely limited the number of similarly hierarchically structured examples. Pu et al. reported different types of conjugated polymers with fixed main-chain chirality containing binaphthyl units in their backbone which exhibited, for example, nonlinear optical activity or were used as enantioselective fluorescent sensors [42—46]. Some chirally substituted poly(thiophene)s were observed to form helical superstructures in solution [47-51], Okamoto and coworkers reported excess helicity in nonchiral, functional poly(phenyl acetylenejs upon supramolecular interactions with chiral additives, and they were able to induce a switch between unordered forms as well as helical forms with opposite helical senses [37, 52, 53]. 

The second article also deals with PET in arranged media, however, this time by discussing comprehensively the various types of heterogeneous devices which may control supramolecular interactions and consequently chemical reactions. Before turning to such applications, photosynthetic model systems, mainly of the triad type, are dealt with in the third contribution. Here, the natural photosynthetic electron transfer process is briefly discussed as far as it is needed as a basis for the main part, namely the description of artificial multicomponent molecules for mimicking photosynthesis. In addition to the goal to learn more about natural photosynthetic energy conversion, these model systems may also have applications, which, for example, lie in the construction of electronic devices at the molecular level. 

Figure 6.4. Molecules of Type A, B, and C containing anchor groups for supramolecular interaction.

Ultimate progress in the control of the primary structure of synthetic macromolecules might be expected if it were possible to develop a template type synthesis, in analogy to the polymerase chain reaction [27]. In spite of many efforts, no successful concept has yet been developed, which is no surprise regarding the complicated supramolecular interactions involved in the necessary steps, i.e. preorganization of the template, binding of the monomer, initiation and termination of the polymerization, and the release of the formed raacromolecule from the template [28,29]. 

Over recent decades, a couple of intermolecular interaction patterns have been identified and refined in order to facilitate understanding of supramolecular processes. The decomposition of a whole interaction pattern into individual contributions is somewhat artificial - only the total interaction energy is well defined - but it is required for our classical, macroscopic view and understanding of these processes. The following list of interaction types contains a few important ones but cannot be considered complete (for instance, magnetic fields or reversibly built and broken covalent bonds are completely neglected) ... 

---