Unidirectional Threading Synthesis of Isomer‐Free [2]Rotaxanes

Abstract

The threading of an alpha-cyclodextrin (alpha-CyD) by an unsymmetrical dumbbell generally results in two isomeric [2]rotaxanes differing in the orientation of the alpha-CyD. In this work, two methods have been developed for the unidirectionally threading an alpha-CyD to obtain isomer-free [2]rotaxanes. These methods use the Suzuki coupling of a boronic acid derivative and a halide in aqueous alkaline solution. The conformations of the two unidirectional [2]rotaxanes-R3 and R4 were determined by 2D 1H ROESY NMR spectra. The optical spectral studies revealed that each of the two [2]rotaxanes can proceed with E/Z photoisomerization and shuttling motions of the alpha-CyD ring on the thread under alternating irradiation at 330 and 275 nm, accompanied by fluorescence intensity changes at 530 nm. The induced circular dichroism (ICD) spectra of another two analogous [2]rotaxanes R1 and R2 were also studied. Distinctive ICD signal changes resulting from the photoisomerization with respect to the movements of alpha-CyD were detected. This demonstrates that, besides the fluorescence, ICD signal is another way to identify the shuttling motions of alpha-CyD in these [2]rotaxanes.

Cyclodextrin-based rotaxanes

“…For synthesis of unidirectional [2]rotaxanes the Suzuki coupling of iodides and boronic acid derivatives in an aqueous a-CD solution has been performed. The obtained unidirectional [2]rotaxanes 35-38 upon irradiation show E/Z photoisomerization and shuttling motion of the CD ring [73].…”

Orientational Isomers of Cyclodextrin Semirotaxanes and Rotaxanes With Organic and Transition Metal Complex Stoppers

“…Park and Song have prepared the two orientational isomers of a a-CD [2]rotaxane with carbazole and bulky viologen stoppers [30]. The assemblies of a-CD [2]-and [3]rotaxanes with asymmetric threads that exhibit only one orientational isomer have also been prepared, by the research groups of Anderson [31][32][33], Tian [34,35] and Park [36,37]. Park has recently re-evaluated the rate and equilibrium constants for the formations and dissociations of CD inclusion complexes to incorporate bidirectional inclusion pathways involving orientational isomers [38].…”

A novel sensitive and selective nanocomposite sensor for Doxorubicin based on Graphene Oxide and fluorescent [2]Rotaxane

“…23 Seok Seo et al discussed the fluorescence resonance energy transfer efficiency between GO and Cy3.5 dye by controlling the donor-acceptor distance using double-stranded DNA and demonstrated that the GO serves as an acceptor rather than a donor in this FRET system. 24 Rotaxane, a typical interlocked supramolecular system, 25-26 is described as a molecular system in which the macrocycle interlocks with a dumbbell structure by noncovalent interaction, [27][28][29] i.e., two bulky stoppers at the end of the axle are prepared to prevent dethreading of the cyclic component. [30][31] Because of their unique structural architectures, some supramolecular rotaxanes display increasing potential for use in applications such as molecular switches, molecular logic gates, molecular wires, and information storage.…”