Cyclodextrin-based ordered rotaxane-monolayers at gold surfaces

Article abstract of DOI:10.1039/c6ra13764d

Decorating metal surfaces with well-defined architectures can be categorized as one of the currently interesting fields of nanomaterials and supramolecular chemistry, since the leading hybrid materials assembled in specific patterns exhibit unprecedented excellent properties which is different from their individual components and subunits. In this work a class of cyclodextrin based redox-active hemi-rotaxane structures were synthesized. After an investigation of their complexation properties in solution, the successful preparation of corresponding rotaxane monolayers with different orderliness on gold surfaces was demonstrated. Due to the complexation-to-deaggregation effect of the macrocyclic ring, construction of the monolayers with each unit encircled by one or two α-cyclodextrin rings has been accompanied with an increased orderliness at the surface. Such a system was suggested to be potentially useful as components for redox driven molecular electronics and optical controlling devices.

Full text of DOI:10.1039/c6ra13764d

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Cyclodextrin-based ordered rotaxane-monolayers
at gold surfaces†
Cite this: RSC Adv., 2016, 6, 73527
Ruyi Sun and Liangliang Zhu*a
b
Decorating metal surfaces with well-defined architectures can be categorized as one of the currently
interesting fields of nanomaterials and supramolecular chemistry, since the leading hybrid materials
assembled in specific patterns exhibit unprecedented excellent properties which is different from their
individual components and subunits. In this work a class of cyclodextrin based redox-active hemi-
rotaxane structures were synthesized. After an investigation of their complexation properties in solution,
the successful preparation of corresponding rotaxane monolayers with different orderliness on gold
surfaces was demonstrated. Due to the complexation-to-deaggregation effect of the macrocyclic ring,
construction of the monolayers with each unit encircled by one or two a-cyclodextrin rings has been
accompanied with an increased orderliness at the surface. Such a system was suggested to be
potentially useful as components for redox driven molecular electronics and optical controlling devices.
Received 27th May 2016
Accepted 26th July 2016
DOI: 10.1039/c6ra13764d
www.rsc.org/advances
architectures. The unique complexation effect with CD cavity
Introduction
was found to provide assistance in area of dyes, photochemistry,
7
Rotaxanes, mechanically interlocked molecules with rodlike catalysis and molecular mechanics, etc. Nevertheless, the
dumbbell subunits threading into macrocycles, have emerged example of preparing CD-based rotaxane-monolayers at Au
8
as important well-dened architectures that can offer fasci- surface is rare. In terms of the size of the CD-based host–guest
1
nating applications in nanostructural functional materials.
system and the complexation-to-deaggregation effect, we
Introducing these functional structures into solid states to anticipate that the orderliness of surface behavior and the
transform fashion properties such as stimuli response and leading electrochemical properties with non, one and two
structural controllability from supramolecular chemistry to cyclodextrin included may be of great difference. Thus we
2,3
nanomaterials has become a hot research topic. In particular, herein focus on the preparation, assembly and investigation of
nanoconstructions based on rotaxanes graed onto gold a class of CD-based rotaxane-monolayers at gold surface.
4
surfaces have revealed a signicant effect in optoelectronics.
Self-assembled monolayers (SAMs) at gold surface containing
well-dened rotaxane structures provide an ideal system for Experimental
studying and utilizing functional compounds readily graed
and behaving coherently. Although gold–thiol binding has been
proven as an effective immobilization fashion for building this
sort of SAMs, to create ordered array based on the mechanically
interlocked molecules is generally a difficult issue.
Characterization
1
13
H NMR spectra and the C NMR were measured on a Bruker
AV-400 or AV-500 spectrometer (Bruker Corporation, Germany),
and the 2D-NOESY NMR spectra were recorded on a Bruker AV-
500 spectrometer (Bruker Corporation, Germany). The elec-
Cyclodextrin (CD), a type of natural macrocyclic rings, has
a huge range of employments in preparation of functional
tronic spray ionization (ESI) mass spectra were tested on
a HP5989 mass spectrometer (Hewlett-Packard Development
Company, US). Elemental analysis was performed on a vario
EL III instrument (Elementar, Germany). Melting points
were determined by using an X-6 micro-melting point
apparatus (Boteng, China). The Raman spectra were recorded
5–7
materials. It is also an attractive building block for supra-
molecular pseudorotaxanes and rotaxanes owing to its rigid
structure and ability to form various stable host–guest
a
State Key Laboratory of Molecular Engineering of Polymers, Department of on a Renishaw Invia system (Renishaw, UK), equipped with
Macromolecular Science, Fudan University, Shanghai 200433, China. E-mail:
Peltier charge-coupled device (CCD) detectors and a Leica
microscope (Leica, Germany). Samples were excited with a 785
nm (diode) laser line. Cyclic voltammetry (CV) experiments were
performed with a CHI 660C electrochemical workstation
(Chenhua, China) using a 1 cm quartz cell with the lab-built
elbow gold electrodes as the working electrodes, Pt wire
zhuliangliang@fudan.edu.cn
b
School of Chemistry and Molecular Engineering, East China Normal University,
Shanghai 200241, China
†
Electronic supplementary information (ESI) available: ESI-MS spectra of G-2CD;
1
H NMR spectra of G, G-CD and G-2CD; scan rates study; XPS measurement; the
assignments of the SERS bands. See DOI: 10.1039/c6ra13764d
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auxiliary electrodes, and saturated calomel electrodes as the 152.73, 152.29, 150.14, 147.34, 124.83, 124.10, 122.23, 114.94,
reference. The experiments were carried out in aqueous solu- 68.04, 56.31, 40.26, 38.54, 34.62, 34.20, 28.81, 28.73, 24.61.
7
9
81
+
tion containing 0.2 M of NaClO as supporting electrolyte. The HRMS (ESI): m/z: 509.0565 ( Br), 511.0553 ( Br) [R3 + H] .
4
scanning electron microscopy (SEM) was tested on JEOL (JSM-
6
360LV) electron microscope (JEOL Ltd., Japan).
Synthesis of compound R4
A solution of R3 (0.36 g, 0.709 mmol) and 4,4 -bipyridine (1.1 g,
0
Materials
7
.09 mmol) in acetonitrile (20 mL) was stirred for 2 days at 80
a-Cyclodextrin (a-CD), 1,2-dibromoethane, (ꢀ)-a-lipoic acid,
ꢁ
C. The solvent was removed under vacuum, and the residue
was dissolved in some acetonitrile to applied to silica gel
chromatography (dichloromethane : methanol ¼ 50 : 5) to
0
4,4 -bipyridine, dicyclohexyl carbodiimide (DCC), 4-dimethyla-
mino pyridine (DMAP) and the inorganic reagents were
commercially available and used as received. Acetone were
dried with anhydrous magnesium sulfate. THF was reuxed
over sodium particles and distilled before used.
ꢁ
1
afford white compound R4 (0.33 g, 70.2%). Mp > 250 C. H
NMR (400 MHz, CDCl
3
, 298 K, TMS): d ¼ 9.73 (d, J ¼ 5.6 Hz, 2H),
8
.85 (d, J ¼ 5.2 Hz, 2H), 8.20 (d, J ¼ 6.0 Hz, 2H), 7.83 (d, J ¼ 8.4
Hz, 2H), 7.80 (d, J ¼ 8.0 Hz, 2H), 7.57 (d, J ¼ 5.6 Hz, 2H), 7.15 (d,
J ¼ 8.4 Hz, 2H), 6.93 (d, J ¼ 8.8 Hz, 2H), 5.60 (m, 2H), 4.69 (m,
Synthesis of compound R1
2
H), 3.55 (m, 1H), 3.10 (m, 2H), 3.16 (m, 2H), 2.54 (t, J ¼ 7.6 Hz,
This compound was prepared as described by a related litera-
ture. Namely, 4-nitrophenol (7.8 g, 0.0561 mol), KOH (40 g,
9
2H), 2.41 (m, 1H), 1.88 (m, 1H), 1.74 (m, 2H), 1.54 (m, 2H), 1.25
1
3
(m, 2H). C NMR (400 MHz, DMSO-d , 298 K, TMS): d ¼ 171.53,
6
0.714 mol) were mixed together in 30 mL H
2
O (30 mL). The
ꢁ
160.12, 152.80, 152.29, 150.97, 149.51, 146.55, 146.01, 140.78,
mixture was stirred and heated to 180 C for 45 min. And then
the mixture was cooled to room temperature and acidied with
hydrochloric acid. The precipitate was ltered and washed with
1
5
5
25.22, 124.56, 123.47, 122.84, 121.94, 115.33, 66.46, 59.39,
6.01, 38.11, 34.00, 33.28, 27.99, 24.00. HRMS (ESI): m/z:
+
85.2042 [R4 ꢂ Br] .
water. Crystallization from ethanol/water ¼ 3 : 1 gives pure R1
ꢁ
(5.7 g, 56% yield) as a yellow solid, mp 212–214 C.
Synthesis of compound G
Synthesis of compound R2
R4 (0.3 g, 0.45 mmol) was dissolved in acetonitrile (12 mL) at 60
C. Dimethyl-5-bromomethyl-1,3-benzenedicarboxylate (0.9 g,
ꢁ
A mixture of (ꢀ)-a-lipoic acid (2.0 g, 9.7 mmol), R1 (2.1 g, 9.8
3
.15 mmol) was added into the solution and the mixture was
mmol), DCC (2.0 g, 9.7 mmol) and DMAP (59 mg, 0.48 mmol) in
ꢁ
ꢁ
stirred at 60 C for 10 h. Aer cooling to room temperature, the
mixture was ltered. And then the solid was washed with
acetonitrile and gave a yellow compound ca. 254 mg. Mp > 250
anhydrous THF (80 mL) was stirred at 60 C for 24 h. The solvent
was removed under vacuum, and the residue was recrystallized
twice with industrial alcohol to give yellow compound R2 (2.53
1
ꢁ
1
ꢁ
C. H NMR (400 MHz, DMSO-d , 298 K, TMS): d ¼ 9.58 (d, J ¼
g, 64.9%). Mp 128–129 C. H NMR (400 MHz, CDCl , 298 K,
6
3
6
8
.0 Hz, 2H), 9.46 (d, J ¼ 6.0 Hz, 2H), 8.79 (m, 4H), 8.58 (s, 2H),
.52 (s, 1H), 7.89 (m, 4H), 7.34 (d, J ¼ 8.4 Hz, 2H), 7.15 (d, J ¼ 8.4
TMS): d ¼ 7.93 (d, J ¼ 8.8 Hz, 2H), 7.89 (d, J ¼ 8.4 Hz, 2H), 7.24
(
1
1
d, J ¼ 8.8 Hz, 2H), 6.96 (d, J ¼ 8.8 Hz, 2H), 5.22 (s, 1H), 3.65 (m,
Hz, 2H), 6.09 (s, 2H), 5.19 (s, 2H), 4.17 (s, 2H), 3.91 (s, 6H), 1.96
H), 3.22 (m, 1H), 3.15 (m, 2H), 2.64 (t, J ¼ 7.2 Hz, 2H), 2.50 (m,
13
(
m, 2H), 1.65 (m, 2H), 1.48 (m, 2H), 1.24 (m, 2H). m/z ¼ 396.2 [M
H), 1.90 (m, 1H), 1.82 (m, 2H), 1.60 (m, 2H), 1.29 (m, 2H).
, 298 K, TMS): d ¼ 172.31, 158.69, 152.03,
50.42, 146.87, 124.98, 123.78, 122.15, 115.81, 56.32, 40.27,
C
2+ +
ꢂ 2Br] , 792.4 [M ꢂ 2Br] . Elemental analysis calcd for G
) (H O) : C 52.23, H 4.89, N 5.67; found: C
2.22, H 4.72, N 5.34.
NMR (400 MHz, CDCl
1
3
3
(
5
C
43
H
44Br
2
N
4
O
7
S
2
2
2
8.54, 34.61, 34.24, 28.72, 24.62. HRMS (ESI): m/z: 401.0997
ꢂ
[R2 ꢂ H] .
Preparation the monolayers at lab-built screen-printed three-
electrodes for Raman test
Synthesis of compound R3
Compound R2 (0.5 g, 1.24 mmol) was added with magnetic The fabrication of screen-printed three-electrode (SPE) was
10
stirring to a solution of 1,2-dibromoethane (2.4 g, 12.9 mmol) in achieved according to the previous report. The SPE was rst
acetone (ca. 12 mL). The mixture was then added upon K CO washed with distilled water and dried by stream, and then pre-
2
3
(
355 mg, 2.57 mmol). The solution was stirred reuxing for 8 h activated in a 0.2 M PBS (pH ¼ 7.0) by applying an anodic
under Ar protection. The mixture was ltered. Aer the ltrate potential of 2.0 V (versus Ag/AgCl) for 100 s. Further, SPE was
was concentrated under vacuum, the residue was applied to utilized for the electrodeposition of AuNPs. AuNPs were elec-
silica gel chromatography (petroleum ether : ethyl acetate ¼ trodeposited at ꢂ0.3 V vs. Ag/AgCl for 300 s, by immersing the
9
1
8
2 4 4
: 2) to afford yellow compound R3 (0.395 g, 62.7%). Mp 107– SPE into a 0.5 M H SO solution containing 0.1 mM HAuCl .
ꢁ
1
08 C. H NMR (400 MHz, CDCl
3
, 298 K, TMS): d ¼ 7.97 (d, J ¼ The modied electrode was denoted as AuNPs/SPE. The AuNPs/
.8 Hz, 2H), 7.90 (d, J ¼ 8.8 Hz, 2H), 7.25 (d, J ¼ 8.8 Hz, 2H), 7.04 SPE was then electrochemically cleaned in 5 mL fresh 0.5 M
(
2
d, J ¼ 8.8 Hz, 2H), 4.40 (t, J ¼ 6.0 Hz, 2H), 3.71 (t, J ¼ 6.0 Hz, H SO , by potential scanning between ꢂ0.1 V and 1.5 V vs. Ag/
2
4
ꢂ1
H), 3.65 (m, 1H), 3.22 (m, 1H), 3.16 (m, 1H), 2.64 (t, J ¼ 7.2 Hz, AgCl at 100 mV s , until a typical voltammogram for gold
H), 2.50 (m, 1H), 1.90 (m, 1H), 1.80 (m, 2H), 1.60 (m, 2H), 1.29 was obtained. Finally, the clean AuNPs/SPE were modied with
2
1
3
(m, 2H). C NMR (400 MHz, CDCl
3
, 298 K, TMS): d ¼ 171.69, SAMs by immersion in 2 mM water solutions of G, G-CD and G-
73528 | RSC Adv., 2016, 6, 73527–73533
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CD for 36 h at room temperature, and then rinsed with copious
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2
amounts of ethanol and deionized water.
For SERS measurement, the spectra were recorded by
focusing the 785 nm diode laser on the surface of the AuNPs/
SPEs with a total accumulation time of 20 s per spectrum.
Power at the sample was varied between 60 and 150 mW. The
laser beam was focused onto the sample in backscattering
geometry using a 100ꢃ objective (nominal aperture 0.9)
2
providing scattering areas of ca. 0.25 mm .
Fig. 1 The synthetic route to the guest compound G and the repre-
sentation of G and macrocyclic host a-cyclodextrin (a-CD).
Preparation the monolayers at elbow gold electrode for CV test
The employment of Au surface in this work was based on a set of
lab-built screen-printed three-electrodes and elbow gold elec-
trode. Screen-printed three-electrodes with Au plated are of
benet to the optical investigations such as SERS, while elbow
gold electrodes can be used to do some in situ functional
testing. These lab-built gold electrodes were immersed in
aqueous solution of G, G-CD and G-2CD (conc. ¼ 2.0 mM),
respectively, for self-assembly in water environment at room
temperature for 3 days. The corresponding monolayers G@Au,
G-CD@Au and G-2CD@Au were formed with these monomers
self-assembled to the Au (111) surface through the 1,2-dithio-
lane endgroup (depicted in Fig. 4).
stopper was conceived to be generated via the self-assembly of
the 1,2-dithiolane endgroup to the gold surface.
The guest compound G and other precursors were fully
1
13
characterized by H NMR, C NMR, and HR-MS. The hemi-
rotaxanes G-CD and G-2CD were prepared from the guest
compound G by co-grinding of the guest with 1 or 2 eq. of solid
6,13
a-CD for ca. 15 min.
A relatively exhaustive conversion to
host–guest structures can be expected based on high associa-
tion constants between the azobenzene compounds and a-CD
ꢂ1 5,6
(
ca. 4000–10 000 M ), and the uniformity of the well mixed
sample can help enhance the complexation rate of the host–
guest structure in the aqueous solution. The complex G-CD was
well evidenced by ESI-MS (Fig. S1†). In the mass spectrum of G-
Lab-built elbow gold electrodes were prepared by melting
a 50 mm Au wire xed into so glass that was then polished with
2
+
CD, peak at m/z ¼ 882.2872, corresponding to [G-CD ꢂ 2Br] , is
0
.05 mm alumina slurry, and then cleaned by soaking in hot
Piranha etching solution (H SO : H
¼ 3 : 1) for 10 min.
Finally, they were sonicated in millipore H O. Each electrode
14
observed. The NOE patterns, observed from the azobenzenyl
protons to the internal protons of a-CD, conrm that the mac-
roring dominantly located on the azobenzene group (see Fig. 2).
When G was encapsulated by two a-CD rings together to form G-
2
4
2 2
O
2
was inspected by light microscopy to ensure that the Au elec-
trode surface was smooth and an effective seal was made
between the glass and the Au.
The geometric area of the electrodes applied was calibrated
by chronocoulometric studies (time scale ¼ 10 s) of potassium
ferricyanide in aqueous 1.0 M KCl solution. Then each electrode
1
4
2
CD, the complex was also evidenced by ESI-MS (Fig. S2†). The
2
+
peak at 1368.9521 is assigned to [G-2CD ꢂ 2Br] . NOEs can be
found from the protons of both the azobenzene and lipoic acid
to the internal protons of a-CD. These results suggest that two
of the rings located on the azobenzene and lipoic acid part
respectively (Fig. 2).
undergo potential scanning from ꢂ0.1 to 1.6 V during 10 cycles
ꢂ1
at 100 mV s
2 4
in fresh 0.1 M H SO . And electrochemical
Cyclodextrins are prone to encapsulate hydrophobic part of
roughness factor (R
tammetry between ꢂ0.1 V and 1.5 V vs. SCE, at 100 mV s
performed in fresh 0.1 M H SO ) taking into account the
f
¼ 1.42 ꢀ 0.11) was assessed by cyclic vol-
ꢂ1 a water-soluble compound. In this case, the guest molecule
(
2
4
relationship of reduction a layer of adsorbed oxygen (386 mC
ꢂ2
11
cm ) on gold.
Results and discussion
Preparation and study of the hemirotaxanes
A “Synthesis/complexation precedes assembly” strategy was
applied here to realize a high-efficient fabrication of rotaxane-
monolayers. We rst designed and synthesized a linear water-
dissolved guest compound G (shown in Fig. 1) as expected to
be effectively complexed with a-cyclodextrin (a-CD). Functional
moieties viologen was combined into the guest as a redox active
unit. This group also acted as a hydrophilic group. It was
attached with the azobenzene and the lipoic acid group. All
these groups could be performed as the binding stations for one
1
Fig. 2 The two-dimensional H NOESY NMR spectra (500 MHz in D
2
O
12
or two a-CD rings except the viologen. Isophthalate moiety was
at 298 K) of G-CD and G-2CD. The inserts shows their corresponding
introduced into one end of the guest as a stopper. The other proposed geometries.
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normally turns to self-aggregated forms in water and thereby
cannot be immobilized ordered. However, the complexation-to-
deaggregation effect could be revealed when the CD ring was
assembled into such a guest. To analysis these properties for
our system, the study of the assembly structures of the CD-
based hemi-rotaxane in solution was done via SEM measure-
1
ment (Fig. 3) and H NMR (Fig. S3†). The aggregation change of
G with non, one and two CD can be clearly observed from SEM
image. When G was dispersed in water, the coexistence of
hydrophobic 1,2-dithiolane and hydrophilic viologen dication
portion resulted in a globular micelle with the width of 0.8–0.9
mm as shown in its SEM spectra, whereby the hydrophobic part
stacked up to each other whereas hydrophilic viologen group
15
1
exposed to the aqueous environment. As shown in H NMR Fig. 4 (A) The representation of the hemi-rotaxanes; (B) the repre-
spectra, the proton resonances of G could be fully revealed. sentation of fabrication structure of G@Au, G-CD@Au and G-2CD@Au
in water; (C) the representation of the monomers assembled to Au
surface through the 1,2-dithiolane unit in water.
However, the peak shapes of these resonances are inconsistent
among each other. Here protons H –H and H –H signals,
h k d g
which belong to the azobenzene group and the lipoic acid part,
respectively, are found to be slightly broad. But the signals Ho–
Hu of hydrophilic end are well splitted. Such phenomenon
should be originated from different geometries of exible
structure of G when dispersed in water, especially that it can
self-organize via the intermolecular hydrophobic effect.
The sample aer complexation of one a-CD ring (G-CD) can
be more easily dispersed in water. Compared with that of G, the
spectra of G-CD showed weaken aggregation with reduced
vesicle-size of about 0.5 mm wide. The encapsulation of the
macroring also makes the azobenzenyl proton resonances Hh–
cyclodextrin rings employed by host–guest interactions, and
employ the methods of surface-enhanced Raman scattering
(SERS) and cyclic voltammogram (CV), etc. to comprehensively
study the orderliness of the functional rotaxane-monolayers at
gold surface.
The lm thicknesses was determined as less than 10 nm by
ellipsometry. Moreover, a good linear relation between the peak
current and the scan rate is observed in the scan rate study of
these monolayers (Fig. S4–S6†), further featuring pure SAMs
formed on the Au electrode surface rather than adsorption or
k
H splitting, but the resonances of lipoic acid part still keep
16
other interactions. The XPS spectrum of S2p region of mono-
layer shows signicant peaks at 161.7 eV and 163.7 eV. It can be
assigned to the thiol chemisorbed on the Au surface (S–Au
bond, Fig. S7†).
broad (Fig. 3B). This phenomenon shows that the complexation
of the a-CD ring to the azobenzene unit only did not effectively
interrupt the interactions of the guest. In SEM image of G-2CD,
however, the aggregated vesicles were well disappeared. And it
1
Raman scattering spectroscopy is able to provide a molecular
causes the reproduction of the peaks of H –H and H –H in H
h
k
d
g

ngerprint of the molecules under study. However, Raman
NMR spectrum (Fig. 3C). In this case, the complexation with two
of a-CD cavity thoroughly avoids the aggregation effect of the
scattering is an intrinsically weak effect, only the compound in
6
8
17
every 10 to 10 photons involves a change in energy.
phenomenon known as the surface-enhanced Raman scattering
SERS) was widely used, for that in the presence of noble metal
A

exible groups.
(
Preparation and study of CD-based ordered rotaxane-
monolayers at gold surface
nanostructures such as gold or silver surface, Raman scattering
of molecules that are on or in close proximity to the nano-
17
structure surface is signicantly enhanced. It has been applied
to the chemically specic analysis of hybrid layer on gold
surface. Furthermore, SERS shows strong distance dependence
of the nanoparticle surface structural information on the layer
which ts our rigid system very well.
Upon the obtained SAMs followed by the “Synthesis/
complexation precedes assembly” process, we try to involve X-
ray photoelectron spectroscopy (XPS) measurement to investi-
gate the construction of monolayers with one or two a-
SERS spectra recorded from the three monolayers G@Au, G-
ꢂ1
CD@Au and G-2CD@Au exhibit a peak at 1138 cm due to the
18
stretch of azobenzenyl C–N (n–C–N]) (Fig. 5). Other peaks
should be assigned to other mode of azobenzene, as well as the
vibrations of viologen and other groups (the frequency and full
assignments of these SERS bands vibrations of the SAMs are
shown in ESI†).
Interesting phenomenon was observed in SERS spectra
related to the peak intensities which is caused by the stretching
and vibration of different group and affected by the changes of
structures or surrounding conditions. Since the strongest peak
Fig. 3 SEM images of (A) G, (B) G-CD (G + 1.0 eq. of a-CD), (C) G-2CD
(G + 2.0 eq. of a-CD). The samples for SEM images were prepared by
spin-coating a water solution (comprising 0.03 mM monomers) onto
a freshly cleaned silica surface.
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Fig. 5 Normalized Raman spectra of the monolayers (A) G@Au; (B) G-
ꢂ1
CD@Au and (C) G-2CD@Au. The strongest peak (n–C–N]) at 1138 cm
was set as the internal standard.
ꢂ1
of C–N at 1138 cm shown in the normalized Raman spectra
located at the middle of the guest has a minimum sensitivity to
the intermolecular interaction, it was set as the internal stan-
dard. By comparing the intensities of other group with C–N in
Fig. 6 Cyclic voltammograms of (A) G@Au, (B) G-CD@Au and (C) G-
2
CD@Au in water at 298 K. The curves were recorded at a scan rate of
ꢂ1
the same system, peak intensity changes within different 120 mV s . Two vertical lines for each curve are set for representation
of the potential differences.
monolayers with non, one or two a-cyclodextrin rings can be
obviously shown.
Except for the internal standard (highlighted by dash lines),
those of the other peaks (highlighted by ellipses in Fig. 5) in phenomenon can be deduced as the complexation-to-
Fig. 5B have a little increase as compared to those in Fig. 5A. deaggregation effect of a-CD encapsulation to the guest,
However, the enhancement of the relative intensity of these which has promoted the surface quality of the prepared SAMs,
peaks is very remarkable in Fig. 5C. A reasonable explanation for the foldings or interactions among the monomers were
for the phenomenon that the peaks grow from spectrum (a) to interrupted. Meanwhile, we also nd that the potential differ-
(
c) is the increasing orderliness with the complexation of a-CD. ences between the oxidation and the reduction of viologen
With two of the macrocyclic rings encapsulated to the guest moiety reveals a decreased tendency from curve (A) to (C) (this
component, the ordered array will be favorable for the freedom parameter is 0.140, 0.128 and 0.074 V for curve (a), (b) and (c),
19
and the vibrations of the guest component, so as to strengthen respectively). It is indicated that with the improvement of the
the most of the vibrational forms in this system.
uniformity of the monolayers, the redox of the viologen unit in
Viologen moiety is a typical redox-active group that can be the monomer would become easier.
performed as an electrochemical probe to directly give feedback
20
of the geometric information. Cyclic voltammogram (CV)
curves of the comparison of the three monolayers are shown in
Conclusions
Fig. 6. In this work, the scan range covers the rst mono- Immobilization of mechanically interlocked molecules has
electronic redox process of the viologen unit. The full width at attracted a considerable interest for the possible usage as basic
half maximum (FWHM) of both the oxidation and the reduction components of future molecular electronic devices. Involve-
peaks are reduced gradually from curve A to C. These experi- ment of well-dened rotaxane with CD offers conformational
mental results suggest a straightforward enhancement of the changes of SAMS monolayers with increased orderliness and
uniformity of the modied Au surface in accordance with electronic properties. In this context, SAMS monolayers based
17
monolayers G@Au, G-CD@Au and G-2CD@Au in turn. This on rotaxane with one or two CD was constructed, its surface
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immobilization and derived surface property combining with
electronic functions were illustrated. A comprehensive study of
the host–guest systems was conducted both in solution and at
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21
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