(part of Steps Toward Molecular Manufacturing)

Molecular Building-Blocks

In recent years there has been a rapidly rising interest in synthesizing large assemblies of organic molecules that might be able to serve as scaffolding structures in efforts to construct molecular objects of nanometer sized dimensions. An overview of the current direction of these investigations can be found in [Lehn93] and [Ama93]. Those molecular aggregates might find applications in molecular electronic and computing devices, or simply as novel materials with special chemical, optical, and electrical properties. These efforts have been an outgrowth of the field of supramolecular chemistry, which started with the serendipitous discovery that certain crown ethers are able to specifically complex (and so to "recognize") alkali metal cations [Ped67]. The resulting complexes were soluble in non-aqueous solvents, which seemed unusual for a compound that essentially is a salt. Analysis of the causes of this intriguing phenomenon has led to the synthesis of innumerable variations of these host-guest complexes [Cram88]. A very useful theoretical inquiry into the nature of self-assembly and molecular recognition of many similar complexation arrangements has been presented in [Lin91].

This growing research field is moving towards establishing an important enabling technology for the technological direction that has been outlined in [Dre81] [Dre86] and [Dre92], namely of eventually attaining manufacturing capabilities at the molecular level, leading to products which obtain their big utility from having all their atoms in precisely specifiable positions (as opposed to most of today's engineering materials like metals, ceramics, plastics, and wood, which have mesoscopically amorphous structures). It is useful to outline what molecular design issues in the field of supramolecular chemistry have to be systematically considered to help establish a focused effort to boot-strap molecular manufacturing.

Previous attempts directed towards finding a toolkit of elementary molecular units that can be stacked together in a flexible manner have been named largely according to brandnames of children's toys. Because the terms LEGO, Meccano, Tinkertoys, etc. already have their connotations of specific chemical implementations, I will refrain from using these occupied names and will simply call the abstract devices analyzed in this discussion "molecular building-blocks", or "MBBs" for short.

The largest amount of insight into finding relevant design criteria for MBBs is gained if one would like to employ them in a demanding application, like the construction of molecular machinery. Not only do the MBBs have to be able to stack in three dimensions potentially infinitely (to obtain scaffolding of large dimensions), but one also has to be able to specify distinct 3D stacking patterns and sequences, to obtain the highly idiosyncratic patterns present in machinery as well as in computers. As explained in [Mer93], achieving complex long range order is necessary.

For building machinery, it is necessary additionally to be able to specify the surfaces of interacting mechanical components in atomic detail, to construct required sliding interfaces (as discussed in chapt.10 of [Dre92]) and to place functional groups which can act as specific binding receptors or as catalytic sites similar to enzymes. In mechanical components, the ratio of interior volume to surface area is much smaller than in an infinitely extended regular crystal, because in these components mainly the surface gives a part its desirable characteristics. This being so, it follows that the slim interior of mechanical parts is best held together by strong interactions, preferably through the use of covalent bonds, to avoid the part from falling apart during usage. Most attempts in designing crystals and solids [Ama93] have so far only used ionic or even weaker interactions which would be unsuitable for achieving the declared goal of building molecular machinery. That is why covalent connections between the MBBs are assumed here.

Starting from a set of clearly stated questions [Kru91], subsequent analysis has found the following issues to be important in designing MBBs:

- division into two sub-problems

- rasters and lattices

- functional group classification

- positional control

- link chemistry

- storage of MBBs

- MBB skeletons

- almost no skeleton

- the zeolite-effect

- the current status of research (end of 1993)

- MBB conclusions

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last updated Oct. 5 1996
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