Hydrogen bonding supramolecular building blocks

Fig. 2 Classification of supramolecular building blocks functionalized with hydrogen-bonding groups (HBGs) and a few examples of self-complementary and heterocomplementary binding motifs. BTA benzene-1,3,5-tricarboxyamide, UPy ureidopyrimidinone, DATdiamino triazine. Thy thymine, Napy 2,7-diaminonaphtyriine, Ba barbituric acid, HR hamilton receptor...

Supramolecular polymers can be classified based on the dominant noncovalent interaction that brings the building blocks together. In this way, it is possible to distinguish between hydrogen-bonded supramolecular polymers, n—Tt stacked supramolecular polymers, coordination polymers, and so on. However, in many cases, it is a combination of noncovalent interactions that determines the structure and properties of the resulting polymer. 

In its natural state, cellulose is highly crystalline in structure. It is a polydisperse linear stiff-chain homopolymer composed of the glucose building blocks which form hydrogen-bonded supramolecular structures. These strong hydrogen bonds are responsible for the stiff, linear shape of the cellulose polymer chains [18]. 

C. Examples of Multiply Hydrogen-Bonding Building Blocks Hydrogen-Bonded, Supramolecular Liquid-Crystalline Polymers... 

Coco, S., Espinet, E., Espinet, P. and Palape, I. (2007) Functional isocyanide metal complexes as building blocks for supramolecular materials hydrogen-bonded liquid crystals. Dalton Transactions, (30), 3267-3272. 

Strong intermolecular interactions between active SCO mononuclear building blocks stem from the presence of efficient hydrogen-bonding networks or 7i-7i stacking interactions and have led to abrupt spin transitions [1], sometimes with associated hysteresis [2-4]. Despite the important efforts made by crystal engineers in establishing reliable connections between molecular and supramolecular structures on the basis of intermolecular interactions, the control of such forces is, however, difficult and becomes even more complicated when uncoordinated counter-ions and/or solvent molecules are present in the crystal lattice. 

Because of the vastness of the subject matter, we shall focus our attention on hydrogen bonding interactions between ions and on the possibilities and limitations of their use in the design and construction of molecular materials of desired architectures and/or destined to predetermined functions. Obviously, the crystal engineer (or supramolecular chemist) needs to know the nature of the forces s/he is planning to master, since molecular and ionic crystals, even if constructed with similar building blocks, differ substantially in chemical and physical properties (solubility, melting points, conductivity, mechanical robustness, etc.). 

A dendritic two-component gelator was synthesised by Smith et al. on the basis of self-assembling acid-base/hydrogen bond interactions. Dendritic lysine building blocks serve as dendrons, and an aliphatic diamine as core (Fig. 2.12). Depending upon the choice of building blocks, the supramolecular complex forms fibrous gel phases by hierarchic self-organisation. 

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