C O M M U N I C A T I O N S
reversible extension-contraction motions, triggered by temperature.
It is also notable that this dynamic conformational change leads to
a fluorescence switching between the fluorescent stretched and
nonfluorescent compressed states of the supramolecular springs
(Figure 4a, inset). Such mechanical motions of the supramolecular
spring may offer intriguing potentials for dynamic nanodevices,
optical modulators, and fluorescent thermometers.
Acknowledgment. This work was supported by the National
Creative Research Initiative Program. E.L. thanks the Seoul Science
Fellowship Program, and H.-J.K. and E.L. acknowledge a fellowship
of the BK21 program from the Ministry of Education and Human
Resources Development. We thanks the Center for Bioactive
Figure 4. (a) Fluorescence spectra and reversible switching cycles of
emission intensity in aqueous solution of C2 (0.1 wt %, excited at 280 nm)
with temperature variation. (b) Fluorescence spectra of C3 (1 wt % aqueous
solution) in the absence (black, solid line) and in the presence (red, dashed
line) of 5 equiv of perylene at 40 °C (excited at 280 nm).
1
Molecular Hybrids for H NMR experiments.
of both coordination polymers revealed bundles of the elementary
cylindrical objects aligned in a parallel fashion, indicating that the
1-D structure in shape remains unchanged upon heating. However,
there is a significant decrease in the cross-sectional dimensions of
the cylindrical objects upon heating. The density profile of C2 taken
perpendicular to the long axis of the bundle showed the diameter
to be about 5.5 ( 0.4 nm (Figure 3d), indicative of 15% reduction
in cylindrical width compared to that taken at room temperature.11
Similarly, C1 also showed 14% reduction in cylindrical width upon
heating (Figure S9). These results suggest that the helical strands
are elongated accompanied by a decrease in the cross-sectional
width. The fluorescence spectra of C2, upon heating, showed a
significant increase in the emission intensity (Figure 4a), indicative
of separation between the adjacent aromatic units within the helical
fiber due to breakup of π-π stacking interactions.12 To provide
further evidence of the helical spring motion, fluorescence resonance
energy transfer (FRET) and 2-D NMR experiments were performed
with model complex C3.13 The FRET experiments with C3 and
perylene dye showed efficient energy transfer to occur only above
lower critical solution temperature (LCST), clearly indicating that
the π-stacked helical chains are extended upon heating (Figure 4b).
This extension of the helical chains was further evidenced by
preliminary 2-D NMR experiments with C3, showing the extended
π-stacking NOE correlations above LCST, again demonstrating the
extension of the helical chains.
These results can be explained by considering a LCST behavior
of the ethylene oxide chains in aqueous medium.14 At room
temperature, the ethylene oxide chains are fully hydrated and thus
adopt a random coil conformation. Above the LCST, however, the
ethylene oxide chains would be dehydrated to collapse into a
molecular globule due to the loss of the hydrogen bonding between
ether oxygens and water molecules. This dehydration of the ethylene
oxide dendritic chains was confirmed by 1H NMR measurements.
Upon heating above the LCST, the resonances associated with the
ethylene oxide segments were shifted upfield (Figure S8), clearly
demonstrating the loss of hydrogen-bonding interactions between
ether oxygens and water. Consequently, the conformational trans-
formation of the side ethylene oxide dendritic chains into a de-
hydrated molecular globule drives the π-stacked helical structure
to be unstable due to steric crowdings between the globules with
greater cross-sectional area. To relieve the steric crowdings at the
interface, the helical strands would be extended to allow a larger
interfacial area, thus lowering total free energy, as evidence by
TEM.
Supporting Information Available: Synthetic procedures, char-
acterization, SLS, DLS, TEM, NMR, and calculated helical conforma-
tion of C2. This material is available free of charge via the Internet at
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The unique feature of the coordination polymers investigated
here is their ability to adopt a helical structure that displays
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