C O M M U N I C A T I O N S
Scheme 2. Synthesis of (3‚5)⊂γ-CD, Showing Part of the 1H
NMR ROESY Spectrum of the [3]Rotaxane (d6-DMSO, 500 MHz,
343 K)
Figure 2. Absorption and fluorescence spectra of rotaxanes 3⊂γ-CD (blue),
5⊂γ-CD (red), and (3‚5)⊂γ-CD (green) in aqueous sodium phosphate buffer
(pH 11.4). The areas of the fluorescence spectra (dotted lines) are scaled in
proportion to the fluorescence quantum yields.
the [3]rotaxane. However excitation of the stilbene component of
(3‚5)⊂γ-CD results in quantitative energy transfer to the cyanine
dye, and emission from the cyanine, as demonstrated by the lack
of stilbene-type emission at 430 nm when (3‚5)⊂γ-CD is excited
at 350 nm and by the fact that the excitation spectrum of (3‚5)⊂γ-
CD is superimposable with its absorption spectrum (see Supporting
Information). The fluorescence quantum yield of the [3]rotaxane
(3‚5)⊂γ-CD (Φf ) 0.56) is substantially higher than that of the
[2]rotaxane 5⊂γ-CD (Φf ) 0.12) presumably because of restricted
conformational freedom
In conclusion, the high affinities of [2]rotaxanes such as 3⊂γ-
CD and 5⊂γ-CD for a second threaded guest provides an efficient
route to [3]rotaxane synthesis. It should be possible to synthesize
a variety of homo- and hetero-[3]rotaxanes and polyrotaxanes using
this chemistry. The binding behavior of 3⊂γ-CD and 5⊂γ-CD also
suggests that they may be useful in sensors.
Acknowledgment. We thank EPSRC and Avecia Ink Jet
Limited for support.
Supporting Information Available: Details of synthesis, UV-
vis titrations, 2D NMR analysis, and fluorescence spectra. This material
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NOEs between the three components (Scheme 2) shows that one
end of the cyanine dye resides near the narrow 5/6-rim of the
cyclodextrin. Thus protons H5/6 of the γ-CD show NOEs to protons
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