Two-dimensional supramolecular spring: Coordination driven reversible extension and contraction of bridged half rings

Article abstract of DOI:10.1039/c4cc02120g

A tetraethylene glycol ether bridged derivative 9 has been designed and synthesized, and its two-dimensional (2D) self-assembled behavior has been investigated at the single-molecule level. Our results revealed that 9 generally adopted the fully extended state but changed to the contracted state when triggered by K₂CO₃, and recovered the original fully extended conformation after subsequent addition of 18-crown-6. Such a coordination-controlled reversible assembly reveals supramolecular springs in response to chemical stimuli, which is of great interest in bionics and materials science.

Full text of DOI:10.1039/c4cc02120g

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Two-dimensional supramolecular spring:
coordination driven reversible extension and
contraction of bridged half rings†
Cite this: Chem. Commun., 2014,
50, 9369
Received 21st March 2014,
Accepted 18th June 2014
Dapeng Luo,‡a Xuemei Zhang,‡b Yongtao Shen,b Jing Xu,b Lijin Shu,*a
Qingdao Zeng*b and Chen Wang*b
DOI: 10.1039/c4cc02120g
www.rsc.org/chemcomm
A tetraethylene glycol ether bridged derivative 9 has been designed
and synthesized, and its two-dimensional (2D) self-assembled behavior
has been investigated at the single-molecule level. Our results revealed
that 9 generally adopted the fully extended state but changed to the
contracted state when triggered by K2CO3, and recovered the original
fully extended conformation after subsequent addition of 18-crown-6.
Such a coordination-controlled reversible assembly reveals supra-
molecular springs in response to chemical stimuli, which is of great
interest in bionics and materials science.
During the past few years, reversible molecular self-assembly
technologies have attracted significant attention in the creation of
functional nanomaterials.1–6 The building blocks in these systems
are generally able to undergo reversible molecular motions under
the effect of external physical or chemical stimuli.7–10 As it is easily
processed, the ion triggered reversible assembly has flourished
Scheme 1 Chemical structures of 9 and 18-crown-6.
in preparing novel nanostructures with increasing structural organic solvents including toluene, dichloromethane, alcohols
complexity, ranging from one-dimensional (1D) polymers, over and acetone.16 Especially, being acyclic analogs of crown
two-dimensional (2D) frames, and to three-dimensional (3D) ethers, PEGs behave as ‘crown-like’ agents with complexation
folded architecture.11,12 One of the most interesting structures ability towards alkali-metal salts to generate flexible helical
are biological springs which can store the energy of conformation conformations or cavities with variable sizes, which depends on
in certain chemical bonds and act as latches.13 Recently, the both the nature of the cation and of the anion as well as on the
dynamic motion of supramolecular springs in solution has been PEG molecular weight.17 The tetraethylene glycol bridged derivative
achieved,14,15 but the direct observation of contraction-to-extension 9 with five ether units in our present work also exhibits coordina-
transformation at surfaces or interfaces is still challenging.
In this communication, the surface-confined extension and
tion ability towards alkali-metal ions.
As shown in Fig. 1a and b, transmission electron microscopy
contraction of a tetraethylene glycol ether derivative (9, Scheme 1) (TEM) revealed that compound 9 revealed a layered regular
has been probed. In general, polyethylene glycols (PEGs) are texture in which individual multilamellar layers were aggregated to
amphiphilic polymers with a high solubility in water and in many a width of around 0.2 mm, indicating the possible crystal structure
at the solid state (a sharp diffraction pattern typical for the single
crystal structure was recorded, see ESI†). However, for sample
9+K2CO3, a wrinkled topology was observed, suggesting the
occurrence of a structural change (diffraction halo rings were
observed, suggesting an amorphous structure). To gain more
information of the morphology, XRD measurement of both
samples 9 and 9+K2CO3 were performed (Fig. 1c and d). For 9, a
sharp intense diffraction peak in the small-angle region (2y = 3.961)
was detected, corresponding to the fully extended interdigitated
a Key laboratory of Organosilicon Chemistry and Material Technology of Ministry of
Education, Hangzhou Normal University, Hangzhou 310012, China.
E-mail: shulj@hznu.edu.cn
b CAS Key Laboratory of Standardization and Measurement for Nanotechnology,
National Centre for Nanoscience and Technology, No. 11 Zhongguancun Beiyitiao,
Beijing 100190, China. E-mail: zengqd@nanoctr.cn, wangch@nanoctr.cn
† Electronic supplementary information (ESI) available: Experimental details,
characterization data and additional STM images. See DOI: 10.1039/c4cc02120g
‡ These authors contributed equally to this work.
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Fig. 2 (a) High-resolution STM image of the self-assembled structure of
compound 9 on HOPG; Iset = 320.2 PA, Vbias = 710.5 mV. (b) A suggested
molecular model corresponding the structure in (a). (c) High-resolution
STM image of the self-assembled structure of 9+K2CO3 on HOPG, after
addition of potassium carbonate; Iset = 290.1 PA, Vbias = 740.7 mV.
(d) A suggested molecular model corresponding to the structure in (c).
1.1 Æ 0.1 nm, the V-shaped bright spots, belonging to different
molecules, were parallel to each other along the green arrow,
corresponding to the ‘‘beads’’ in the abacus. The rows of beads
were bridged by bright pillars whose length (d1) was measured
to be 1.8 Æ 0.1 nm. The length of each bright pillar was similar
Fig. 1 (a, b) Transmission electron microscope (TEM) images of thin areas
of specimen 9 and 9+K2CO3, respectively; (c, d) XRD plots of specimen 9
and 9+K2CO3, respectively.
intercolumn distance of d = 22.3 Å. Besides the fundamental to that of the tetraethylene glycol ether and also equal to the
peak, a peak centered at 2y = 5.721 (d = 15.4 Å) was also observed, substituted dodecyl chains,18–20 demonstrating that the bright
corresponding to the distance of the opposite phenylacetylene pillar may correspond to the bridged tetraethylene glycol ether
moieties. In comparison, for 9+K2CO3, peaks at 2y = 4.398 and and dodecyl chains. Therefore, compound 9 should be composed
6.01 were recorded, corresponding to distances d = 20.1 Å (the of two bright beads and three bright pillars. For each molecule,
intercolumn distance of 9+K2CO3) and d = 14.5 Å (the molecular the distance between two half rings was measured to be L1 = 3.3 Æ
width of opposite phenylacetylene moieties for 9+K2CO3). In 0.1 nm. At this condition, as displayed in Fig. 2b, the substituted
addition, a distance of d = 22.3 Å can be attributed to incompletely dodecyl chains of adjacent molecules are assembled via inter-
complexed 9 in the sample of 9+K2CO3. These results suggest that digitation21 and the tetraethylene glycol ether was fully extended.
the bridged 9 contracted with the incorporation of K2CO3. The Here, we refer to the two half rings combined with the bridged
distances of 0.40 and 0.43 nm (Fig. 1c) as well as 0.39 and 0.44 nm tetraethylene glycol ether as the core part of molecule 9 and also
(Fig. 1d) can be assigned to the distances of adjacent alkyl chains. define the core part in the uncomplexed state (without any
Based on these TEM and XRD measurements, it can be concluded treatment) as the extended conformation.
that the introduction of potassium carbonate in 9 brought about
Upon addition of K2CO3 aqueous solution, the structural
the change in morphology.
changes of the compound 9 can be identified by STM observa-
To have a further insight of the interaction of K2CO3 tions. As shown in Fig. 2c, there appears a zigzag architecture
with bridged 9, potassium salts of trifluoromethanesulfonate, along the blue arrow. Carefully inspecting this novel structure,
tetraphenylborate and carbonate were employed in 1H NMR we found that the zigzag part was assembled by the periodic
analysis. As shown in Fig. S22–S24 (ESI†), notable chemical unit consisted of two V-shaped half rings and one bright dot.
shifts were observed in the bridged tetraethylene parts after the The width of each half ring (W2) was determined to be 1.1 Æ
addition of the potassium salts for all the three salts, revealing 0.1 nm, the whole length (L2) of the periodic unit was 2.7 Æ
the formation of coordination complexes of K2CO3 and 9.
0.1 nm, and the distance (d2) between the two adjacent zigzag
More structural details have been probed by scanning lamellae was 1.6 Æ 0.1 nm. On the basis of these phenomena,
tunneling microscopy (STM). As shown in Fig. 2a, compound we attribute the zigzag architecture to the coordinated 9+K2CO3
9 assembled into an abacus-shaped architecture on the highly complex, and the periodic unit in zigzag lamella should belong to
oriented pyrolitic graphite (HOPG) surface. With a width (W1) of the coordinated parts as well as the whole V-shaped conjugated
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in response to chemical stimuli, which is of great interest in bionics
and material science.
This work was supported by the National Basic Research
Program of China (No. 2011CB932303, 2013CB934200), Zhejiang
Provincial Natural Science Foundation (LY12B04004) and Hangzhou
normal university start-up fund project (2012ZX032). Also the
National Natural Science Foundation of China (No. 21073048,
51173031, 91127043) is also gratefully acknowledged.
Fig. 3 An illustration model for the transformation of specimen 9 from
extension to contraction and back to extension (the purple ball represents K+).
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closer movement of the two V-shaped half rings, and more
importantly, the length of core part in 9 becomes shorter (from
3.3 Æ 0.1 to 2.7 Æ 0.1 nm). In other words, the target molecule 9
adopted a contracted conformation when treated with potassium
carbonate.
Interestingly, the extended assembly of compound 9 can be
obtained again if a solution containing 18-crown-6 was added
to the 9-K2CO3 sample (Fig. S25, ESI†). It is well known that the
cyclic crown ether is able to combine with alkali metals through
strong metal-coordination interaction,22 which can extract the
alkali-metal ions from the other ligands. In our present system,
as shown in Fig. 3, the added K2CO3 was first combined with
the tetraethylene glycol bridged compound 9 (to form the
9+K2CO3 complex), and then reformed 9 (and K(18-crown-6))
¨
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Chem. Commun., 2014, 50, 9369--9371 | 9371