Self-assembly: What Does it Mean ?

1.1 Introduction

Supramolecular chemistry – broadly the chemistry of multicomponent molecular assemblies in which the component structural units are typically held together by a variety of weaker (non-covalent) interactions – has developed rapidly over recent years. 'Typically' is used since, in a considerable number of systems, metal-donor bonds – often essentially covalent in nature – have also been employed to 'stitch' together organic components into larger assemblies. Such metal-linked assemblies will be treated as part of the supramolecular realm in the present work (although not employed here, perhaps 'supermolecular' is a better term for this category).

With the development of supramolecular chemistry, there has been a concomitant shift in the mind-set of chemists working in the area. This has involved a change in focus from single molecules, often constructed step by step via the formation of direct covalent linkages, towards molecular assemblies, with their usual (see exception above) non-covalent weak intermolecular contacts. This change in focus is nicely encapsulated in Lehn's description of supramolecular chemistry as 'the designed chemistry of the intermolecular bond'.

As a consequence of the intense interest in the field, a very large number of synthetic supramolecular systems have now been synthesised, with many of the (non-polymeric) systems ranging in size from around a nanometre or so to tens of nanometres. Quite often, innovative design features have been required to achieve the desired structures – with the design and synthesis of individual systems often representing a very considerable intellectual and practical achievement. The field remains an exciting and fast moving one that continues to produce a range of new materials; many of which are endowed with aesthetically pleasing structures as well as unusual properties. The latter, for example, may include novel redoxactive, photoactive, conductive (including superconductive) and non-linear optical behaviour. Clearly, the area is one that continues to show considerable promise for underpinning the development of molecular scale components and devices, including opto-electronic devices. The promise of useful molecular devices remains a motivation for the continuing widespread interest in the field.

Much of the work in supramolecular chemistry has focused on molecular design for achieving complementarity between single molecule hosts and guests. Besides complementarity, recognition, self-assembly, preorganisation and even self-replication represent important 'key words' in the armoury of the supramolecular chemist. As a consequence, the practice of supramolecular chemistry tends to be a somewhat interdisciplinary activity, often requiring knowledge of a range of appropriate chemical, physical and biochemical procedures and techniques. Indeed, aspects of supramolecular chemistry now impinge on virtually all of the chemistry sub-disciplines.

Apart from the special case where metal ions are used as the 'glue', central to the supramolecular field is the use of a variety of weaker (non-covalent) interactions – including hydrogen bonding, π — π stacking, dipolar interactions, van der Waals forces and hydrophobic interactions – to hold molecular components together. These are the same forces that Nature uses to bind its molecular assemblies. Indeed, much of the activity in the area aims to mimic (but not necessarily copy directly) the way that Nature goes about things.

Creativity and challenge are, by necessity, key ingredients in any effort to devise and synthesise totally synthetic molecular systems that function like biological systems. To achieve such an aim, the elements of molecular recognition, self-assembly and (ultimately) self-synthesis, all ubiquitous in biology, need to be mastered. Further, the product of such a synthesis should be capable of being functionally active if it is truly to match the behaviour of a natural system. The work discussed in this and subsequent chapters documents the progress made, across a broad front, towards this goal. While some quite beautiful examples of self-assembled synthetic systems have now been produced (very often in good yield and under mild conditions), in general there is still a long way to go before individual systems match the biological ones in both subtlety and function. Therein lies the challenge! Indeed, the entire synthetic supramolecular enterprise so far tends to be dominated by the interaction of relatively simple molecular components that are associated with a limited number of bonding contacts on forming the aggregated product. In contrast, for larger biological assemblies, such as DNA, the tobacco mosaic virus, the enzymes or the respiratory proteins, the respective components are of high molecular weight and are of a quite complex nature. Indeed, the resulting assemblies typically incorporate hundreds, if not thousands, of intermolecular contacts. Many systems of this type are able to reassemble from their separated components; the amount of steric and electronic information stored in the latter, and which must be 'read out' during reassembly, is thus very large indeed. Such levels of information storage (and processing) have not yet been approached in the synthetic systems investigated so far. Inevitably, a move towards greater complexity will represent one direction for future development.

How might higher molecular weight assemblies be produced? One (of many possible) modus operandi would be to mimic Nature by stringing together complementary molecular units in predetermined sequences such that two matched strands are produced that will induce self-assembly over many tens, or even hundreds of nanometers of strand length. In such an approach, the characteristics of the final assembly would be set by the nature, positioning in the strand sequence, and frequency of incorporation of the individual complementary molecular units together with their relative 'cross-strand' orientations. So far, the use of such a 'modular unit' approach for the construction of larger synthetic assemblies has been little exploited.

Of course, the above suggestion ignores the problem of possible supramolecular functionality. Whereas the natural systems are invariably characterised by high functionality in terms of their biochemical roles, in contrast, the functionality of the majority of synthetic assemblies so far investigated has very often been either minimal or, indeed, absent altogether. The incorporation of designed functionality into supramolecular systems will thus undoubtedly continue to attract increased attention in future studies.

1.2 Self-assembly

In the present context, self-assembly may be defined as the process by which a supramolecular species forms spontaneously from its components. For the...