tailieunhanh - Ebook Supramolecular chemistry (2/E): Part 2

(BQ) Part 2 book “Supramolecular chemistry” has contents: Crystal engineering, network solids, self-assembly, molecular devices, biological mimics and supramolecular catalysis, interfaces and liquid assemblies, supramolecular polymers, gels and fibres, and other contents. | 8 Crystal Engineering ‘Whether on crystal rocks ye rove, / Beneath the Bosom of the sea Wand’ring in many a coral grove, / Fair nine, forsaking Poetry How have you left the ancient love / That bards of old enjoy’d in you! The languid strings do scarcely move! / The sound is forc’d, the notes are few.’ William Blake (1757–1827), To the Muses. Crystal Engineering 442 Concepts Introduction Braga, D., Brammer, L., Champness, N. R., ‘New trends in crystal engineering’, Cryst. Eng. Comm. 2005, 7, 1–19. The crystallisation process is, by definition, a self-assembly process in the sense that the component molecules or ions encounter and recognise one another, usually in solution, and pack together as close as possible to their optimum orientation in the time available, according to their intrinsic steric and electronic properties. The resulting aggregate, as an assembly, then goes on to accrete further molecules or ions faster than the dissociation of those already comprising the aggregate, ultimately resulting in an ordered nucleus of more than transient stability. This nucleus continues to grow by adding molecules from solution to produce the whole crystal. We must be clear, however, about the distinction between crystal self-assembly and solution self-assembly (which is discussed fully in Chapter 10). • Crystallisation is fundamentally a non-equilibrium phenomenon in which both kinetic and thermodynamic aspects contribute to the eventual structure. The final structure is a function in many cases of crystallisation conditions, and polymorphs (crystals made from the same molecules but with different packing arrangements) are common. Structures that form faster may well predominate over structures that are most stable. • Solution self-assembly is a thermodynamically controlled equilibrium reaction in which components are able to sift through a variety of possible structures until they fi nd the one of the maximum stability. There is effectively infi nite .