Self-complementary species

The mixed sites species TPU would represent a self-complementary component capable of homo-self-assembly into a (TPU) supramolecular entity. 

Fig. 47. X-Ray structure of the hydrogen-bonded, ribbon-like species formed by self-assembly of the self-complementary component 168. In the schematic representation (right) the two DMF molecules contained in each cavity between the octyl chains are omitted [9.122].

Structure 5 is one example of a number of dipyridones that incorporate different spacer groups.Since 5 was designed to be self-complementary, it was anticipated that it would self-associate to produce a dimer of type 6. Indeed, this was shown to be so in chloroform (> 90% dimer) by means of vapour pressure osmometry. X-Ray crystallography also confirmed that the dimer persists in the solid state. The behaviour of 5 contrasts markedly with that of 7 which was designed to be complementary only in an offset manner, such that linear polymerisation might be promoted. Under the dilute conditions of measurement, vapour pressure osmometry indicated that this species remains predominantly monomeric in chloroform however. X-ray diffraction confirmed that 7 adopts the linear polymeric structure illustrated by 8 in the solid state. As anticipated, since self-association involves hydrogen bonding, both 5 and 7 were shown to exist only as monomers in the pro-tic solvent methanol. 

Fig. 2 The development of a supramolecular system from molecular building blocks, (a) Host-guest complexation and (b) self-assembly between complementary species (circles represent binding sites). View this art in color at www.dekker.com.)...

Figure 3.3 Self-complementary hydrogen bonding to yield a helical structure (a) and butterfly-type motion of a bis(porphyrin)terpyridine complex to give a species capable of through-space energy transfer (b).

Classical coordination compounds are formed by interactions between acceptor species (Lewis acids) and donor species (Lewis bases). When potential acceptor and donor sites are present simultaneously in the same molecule these can be described as self-complementary and their reciprocal recognition can lead to self-assembly (or self-organization). This often occurs with organometallic compounds. For self-assembly to occur an additional steric fitness or geometric complementarity is required, as shown schematically in Figure 1.1. 

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