(part of Steps Toward Molecular Manufacturing)

- the current status of research (end of 1993):

It recently has become popular to synthesize molecules that could serve in molecular construction sets to build nanometer sized objects and designer solids. There are encouraging attempts which demonstrate some of the design issues discussed above, but all of the constraints have not yet been satisfied simultaneously.

A number of rod-like molecules have been synthesized [KasFriMic92] [YanEtal92], the ends of which could be connected in suitable hubs. But this would lead to the zeolite problem in its most extreme form. The longer the rods, the more humungously large the cavities become. Also, most rod proposals do not have handles, or flexible functionalization possibilities, and so the problem of assembling useful and arbitrary structures with them is still unsolved. Building extended structures and lattices using these building blocks has yet to be demonstrated.

Successful attempts at deliberate lattice building have been accomplished [SimSuWue91] by using tetrahedral MBBs that assemble into a diamond-like lattice. But the crystal is held together only by weak hydrogen bonds, and no actual skeleton-design was pursued. Also, the functionalities forming the links have a rotational degree of freedom, which renders the exact arrangement in the crystal unpredictable.

An interesting approach to MBB skeleton-design has been followed in [WuLeeMoo92], leading to a skeleton construction toolkit with phenylacetylene-units, as mentioned above. This scheme introduces a number of potential functionalization sites, but those have not yet been exploited for achieving specific intermolecular aggregation and thus predictable lattice-formation.

Much work follows more closely the traditional kinds of host-guest chemistry and the self-assembly of compact and "introverted" entities, as surveyed in [Lin91]. For example, [KohMatSto89] describes a number of closed, self-contained structures which are not designed to assemble into larger lattices in a definable way. They do not really live up to the term Molecular LEGO which was used to describe some of the structures, because the proposed molecules lack the extendibility beyond a closed entity of finite size, which would be precisely the feature that gives LEGO its wonderful properties.

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last updated Oct. 5 1996
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