Rotaxane and Catenane Structures Involving Cyclodextrins

These noncovalent supramolecular systems are stabilized via a mechanical interlocking that prevents the components from disassembling. A common precursor 

Cyclodextrins and Their Complexes. Edited by Helena Dod2iuk Copyright 2006 WILEY-VCH Verlag GmbH Co. KGaA, Wemheim ISBN 3-527-31280-3 

Being in essence CyD complexes, [2]pseudorotaxanes are characterized by medium stability, unless a particular structural host-guest fit has been achieved. Con- 

In the next larger cydodextrin, j8-CyD, aliphatic inserts are less popular, although the crystal structures of [3]pseudorotaxanes with aliphatic mono- [25] and diacids [18, 26, 27], diols [28], and bis-imidazolyl hydrocarbons [29] have been reported. j8-CyD/monocarboxylic acid complexes pack in channel structures and create supramolecular polymers, whereas 8-CyD/diacid complexes break the channel structure, contrary to what could be anticipated based on the propensity of acids to dimerize. With the wide y-CyD and aromatic guests, such as Congo Red [14] and ( )-stilbene [15] dimerization of the guest inside y-CyD is observed to form [4] and [3]pseudorotaxanes. Irradiation of crystalline complexes of the latter yielded syn-tetraphenylcyclobutane as a single isomer. 

An atypical rotaxane structure is that involving a thread covalently attached to the CyD ring. Such a system acts as both host and guest and it has to be specially designed so that the thread is induded spontaneously in a cavity other than the one bearing it intermolecular inclusion), whereas intramolecular indusion is prevented. The resulting assembly, named a daisy chain by J.F. Stoddart, is either an acyclic supramolecular polymer 8 or a cyclic supramolecular oligomer 9 [50] (Fig. 12.4). Acyclic cyclodextrin polymers were first observed in the solid state in 

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