198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

100 R.E. Meléndez · A.D. Hamilton
a
b
c
d
Fig. 1. Representative supramolecular synthons
directionality. For example, the top components in Fig. 1c have, in left to right
order, acceptor-donor-acceptor, acceptor-acceptor-donor, and acceptor-acceptoracceptor
hydrogen bonding orientation. Their bottom counterparts have complementary
functionalities. Figure 1d shows the formation of cyclic trimers based on
strong hydrogen bonds or polarization interactions. The synthons described by
Desiraju have been observed repeatedly in solid state structures and have been
used in the design of crystals, as well as in other areas of supramolecular chemistry
(see also the article by A. Nangia and G.R. Desiraju in this volume).
2.2
Patterns in Crystal Engineering: Tapes, Ribbons, and Sheets
In discussing tapes, ribbons, and sheets we should not ignore the competing
formation of cyclic aggregates in many of the designed systems. There has been
extensive discussion of the enthalpic and entropic influences on cyclic vs linear
aggregate formation [19]. As a consequence, different approaches have been
used to promote formation of cyclic aggregates rather than linear aggregates.
These approaches will be presented throughout the sections of this article.
The terms tape and ribbon have not yet been defined in a way that allows
them to be easily distinguished. In general, ribbons or tapes have been used to