The phase behavior of conjugated polymers in bulk and solution state is closely related to the “self-assembly” process. Self-assembly is the fundamental principle which generates structural organization on all scales from molecules to galaxies. (cf.
Figure 1-11) It is defined as a reversible processes in which pre-existing parts or disordered components of a system form structures of patterns. Self-assembly can be classified as either static or dynamic.48 In static self-assembly the ordered state occurs when the system is in equilibrium and does not dissipate energy. Dynamic self-assembly, on the other hand requires dissipation of energy of the ordered state.
Examples of self-assembling systems include weather patterns, solar system, histogenesis and self-assembled monolayers (table 1-1). The most well-studied subfield of self-assembly is molecular self-assembly. Molecular self-assembly is the assembly of molecules without guidance or management from an outside source.
There are two types of such self-assembly, intramolecular self-assembly and intermolecular self-assembly. Intramolecular self-assembling molecules are often complex polymers with the ability to assemble from the random coil conformation into a well-defined stable structure. An example of intramolecular self-assembly is protein folding.49 Intermolecular self-assembly is the ability of molecules to form supramolecular structures. A simple example is the formation of micelles by surfactant molecules in solution. 50 Self-assembly can occur spontaneously in nature, for example in cells (such as the self-assembly of the lipid bilayer membrane51) and other biological systems, as well as in human engineered systems such as a Langmuir monolayer. It usually results in the increase in internal organization of the system.
Biological self-assembling systems, including synthetically engineered self-assembling peptides and other biomaterials, have been shown to have superior
handling, biocompatibility and functionality. These advantages are due to direct self-assembly from biocompatible precursors creating biomaterials engineered at the nano-scale.
Also, self-assembly is a manufacturing method used to construct things at the microscale, which is comprised of structures with at least one dimension that is less than 100 microns. Many systems use self-assembly to assemble various molecules and structures.52-54 Imitating these strategies and creating novel molecules with the ability to self-assemble into supramolecular assemblies is an important technique in nanotechnology. In self-assembly the final structure is ”encoded” in the shape and properties of the molecules that are used, as compared to traditional techniques, such as lithography, where the desired final structure must be carved out from a larger block of matter. Self-assembly is thus referred to as a 'bottom-up' manufacturing technique, as compared to lithography being a 'top-down' technique. An example of self-assembly in nature is the way that hydrophilic and hydrophobic interaction cause molecules to self assemble. Molecular self-assembly is a strategy for nanofabrication that involves designing molecules and supramolecular entities so that shape-complementarity causes them to aggregate into desired structures.
Self-assembly hence has became a rapidly growing part of organic field for two reason: first, it is a concept that is crucial to understand many structures important in biology and second, it is one solution to the problem of synthesizing structures larger than molecules. Self-assembly also poses a number of substantial intellectual challenges. The brief summary of these challenges is that we do not yet know how to do it, and cannot even mimic those processes known to occur in biological systems at other than quite elementary levels. Although there are countless examples of self-assembly all around us – from molecular crystals to mammals - the basic rules that govern these assemblies are not understood in useful detail, and self-assembling
processes cannot, in general, be designed and carried out "to order". Many of the ideas that are crucial to the development of this area – “molecular shape”, the interplay between enthalpy and entropy, nature of non-covalent forces that connect the particles in self-assembled molecular aggregates – are simply not yet under the control of investigators. The design of components that organize themselves into desired final patterns and functions is the key to applications of self-assembly. The components must be able to move with respect to one another. Their steady-state positions balance attractions and repulsions. Although self-assembly originated in the study of molecules, it is a strategy that applicable at all scales. Molecular self-assembly involves non-covalent force or weak covalent interactions (i.e. van der Waals, electrostatic, and hydrophobic interactions, hydrogen and coordination bonds).
In the self-assembly of larger components – meso- or macroscopic objects – interactions can often be selected and tailored, and can include interactions such as gravitational attraction, external electromagnetic fields, and magnetic, capillary, and entropic interactions, which are not important in the case of molecules. Larger molecules, molecular aggregates, and forms of organized matter more extensive than molecules cannot be synthesized bond-by-bond. Self-assembly is one strategy for organizing matter on these larger scales. By utilizing interactions as diverse as aromatic π – π stacking and metal-ligand coordination for the information source for assembly processes, chemists in the last decade have begun to construct nanoscale structures and superstructures with a variety of forms and functions. Therefore, self-assembly necessarily play the important role of nanoscale science in future.
(a) (b)
(c) (d)
Figure 1-11. Example of self-assembly (a) Crystal structure of ribosome. (b) self-assembly of peptide-amphiphile nanofibers. (c) Thin film of a nematic liquid crystal on an isotropic substrate. (d) A school of fish (dymanic self-assembly).
Table 1-1. Examples of self-assembly
(S: static, D: dynamic, T: templated, B: biological)