Cyclodextrin-based ordered rotaxane-monolayers at gold surfaces

Abstract

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.

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Introduction

Rotaxanes, mechanically interlocked molecules with rodlike dumbbell subunits threading into macrocycles, have emerged as important well-defined architectures that can offer fascinating applications in nanostructural functional materials.¹ Introducing these functional structures into solid states to transform fashion properties such as stimuli response and structural controllability from supramolecular chemistry to nanomaterials has become a hot research topic.^2,3 In particular, nanoconstructions based on rotaxanes grafted onto gold surfaces have revealed a significant effect in optoelectronics.⁴ Self-assembled monolayers (SAMs) at gold surface containing well-defined rotaxane structures provide an ideal system for studying and utilizing functional compounds readily grafted 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.

Cyclodextrin (CD), a type of natural macrocyclic rings, has a huge range of employments in preparation of functional materials.^5–7 It is also an attractive building block for supramolecular pseudorotaxanes and rotaxanes owing to its rigid structure and ability to form various stable host–guest architectures. The unique complexation effect with CD cavity was found to provide assistance in area of dyes, photochemistry, catalysis and molecular mechanics, etc.⁷ Nevertheless, the example of preparing CD-based rotaxane-monolayers at Au surface is rare.⁸ In terms of the size of the CD-based host–guest system and the complexation-to-deaggregation effect, we anticipate that the orderliness of surface behavior and the leading electrochemical properties with non, one and two cyclodextrin included may be of great difference. Thus we herein focus on the preparation, assembly and investigation of a class of CD-based rotaxane-monolayers at gold surface.

Experimental

Characterization

¹H NMR spectra and the ¹³C 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 electronic 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 on a Renishaw Invia system (Renishaw, UK), equipped with 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 auxiliary electrodes, and saturated calomel electrodes as the reference. The experiments were carried out in aqueous solution containing 0.2 M of NaClO₄ as supporting electrolyte. The scanning electron microscopy (SEM) was tested on JEOL (JSM-6360LV) electron microscope (JEOL Ltd., Japan).

Materials

α-Cyclodextrin (α-CD), 1,2-dibromoethane, (±)-α-lipoic acid, 4,4′-bipyridine, dicyclohexyl carbodiimide (DCC), 4-dimethylamino pyridine (DMAP) and the inorganic reagents were commercially available and used as received. Acetone were dried with anhydrous magnesium sulfate. THF was refluxed over sodium particles and distilled before used.

Synthesis of compound R1

This compound was prepared as described by a related literature.⁹ Namely, 4-nitrophenol (7.8 g, 0.0561 mol), KOH (40 g, 0.714 mol) were mixed together in 30 mL H₂O (30 mL). The mixture was stirred and heated to 180 °C for 45 min. And then the mixture was cooled to room temperature and acidified with hydrochloric acid. The precipitate was filtered and washed with water. Crystallization from ethanol/water = 3 : 1 gives pure R1 (5.7 g, 56% yield) as a yellow solid, mp 212–214 °C.

Synthesis of compound R2

A mixture of (±)-α-lipoic acid (2.0 g, 9.7 mmol), R1 (2.1 g, 9.8 mmol), DCC (2.0 g, 9.7 mmol) and DMAP (59 mg, 0.48 mmol) in 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 g, 64.9%). Mp 128–129 °C. ¹H NMR (400 MHz, CDCl₃, 298 K, TMS): δ = 7.93 (d, J = 8.8 Hz, 2H), 7.89 (d, J = 8.4 Hz, 2H), 7.24 (d, J = 8.8 Hz, 2H), 6.96 (d, J = 8.8 Hz, 2H), 5.22 (s, 1H), 3.65 (m, 1H), 3.22 (m, 1H), 3.15 (m, 2H), 2.64 (t, J = 7.2 Hz, 2H), 2.50 (m, 1H), 1.90 (m, 1H), 1.82 (m, 2H), 1.60 (m, 2H), 1.29 (m, 2H). ¹³C NMR (400 MHz, CDCl₃, 298 K, TMS): δ = 172.31, 158.69, 152.03, 150.42, 146.87, 124.98, 123.78, 122.15, 115.81, 56.32, 40.27, 38.54, 34.61, 34.24, 28.72, 24.62. HRMS (ESI): m/z: 401.0997 [R2 − H]⁻.

Synthesis of compound R3

Compound R2 (0.5 g, 1.24 mmol) was added with magnetic stirring to a solution of 1,2-dibromoethane (2.4 g, 12.9 mmol) in acetone (ca. 12 mL). The mixture was then added upon K₂CO₃ (355 mg, 2.57 mmol). The solution was stirred refluxing for 8 h under Ar protection. The mixture was filtered. After the filtrate was concentrated under vacuum, the residue was applied to silica gel chromatography (petroleum ether : ethyl acetate = 9 : 2) to afford yellow compound R3 (0.395 g, 62.7%). Mp 107–108 °C. ¹H NMR (400 MHz, CDCl₃, 298 K, TMS): δ = 7.97 (d, J = 8.8 Hz, 2H), 7.90 (d, J = 8.8 Hz, 2H), 7.25 (d, J = 8.8 Hz, 2H), 7.04 (d, J = 8.8 Hz, 2H), 4.40 (t, J = 6.0 Hz, 2H), 3.71 (t, J = 6.0 Hz, 2H), 3.65 (m, 1H), 3.22 (m, 1H), 3.16 (m, 1H), 2.64 (t, J = 7.2 Hz, 2H), 2.50 (m, 1H), 1.90 (m, 1H), 1.80 (m, 2H), 1.60 (m, 2H), 1.29 (m, 2H). ¹³C NMR (400 MHz, CDCl₃, 298 K, TMS): δ = 171.69, 152.73, 152.29, 150.14, 147.34, 124.83, 124.10, 122.23, 114.94, 68.04, 56.31, 40.26, 38.54, 34.62, 34.20, 28.81, 28.73, 24.61. HRMS (ESI): m/z: 509.0565 (⁷⁹Br), 511.0553 (⁸¹Br) [R3 + H]⁺.

Synthesis of compound R4

A solution of R3 (0.36 g, 0.709 mmol) and 4,4′-bipyridine (1.1 g, 7.09 mmol) in acetonitrile (20 mL) was stirred for 2 days at 80 °C. The solvent was removed under vacuum, and the residue was dissolved in some acetonitrile to applied to silica gel chromatography (dichloromethane : methanol = 50 : 5) to afford white compound R4 (0.33 g, 70.2%). Mp > 250 °C. ¹H NMR (400 MHz, CDCl₃, 298 K, TMS): δ = 9.73 (d, J = 5.6 Hz, 2H), 8.85 (d, J = 5.2 Hz, 2H), 8.20 (d, J = 6.0 Hz, 2H), 7.83 (d, J = 8.4 Hz, 2H), 7.80 (d, J = 8.0 Hz, 2H), 7.57 (d, J = 5.6 Hz, 2H), 7.15 (d, J = 8.4 Hz, 2H), 6.93 (d, J = 8.8 Hz, 2H), 5.60 (m, 2H), 4.69 (m, 2H), 3.55 (m, 1H), 3.10 (m, 2H), 3.16 (m, 2H), 2.54 (t, J = 7.6 Hz, 2H), 2.41 (m, 1H), 1.88 (m, 1H), 1.74 (m, 2H), 1.54 (m, 2H), 1.25 (m, 2H). ¹³C NMR (400 MHz, DMSO-d₆, 298 K, TMS): δ = 171.53, 160.12, 152.80, 152.29, 150.97, 149.51, 146.55, 146.01, 140.78, 125.22, 124.56, 123.47, 122.84, 121.94, 115.33, 66.46, 59.39, 56.01, 38.11, 34.00, 33.28, 27.99, 24.00. HRMS (ESI): m/z: 585.2042 [R4 − Br]⁺.

Synthesis of compound G

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, 3.15 mmol) was added into the solution and the mixture was stirred at 60 °C for 10 h. After cooling to room temperature, the mixture was filtered. And then the solid was washed with acetonitrile and gave a yellow compound ca. 254 mg. Mp > 250 °C. ¹H NMR (400 MHz, DMSO-d₆, 298 K, TMS): δ = 9.58 (d, J = 6.0 Hz, 2H), 9.46 (d, J = 6.0 Hz, 2H), 8.79 (m, 4H), 8.58 (s, 2H), 8.52 (s, 1H), 7.89 (m, 4H), 7.34 (d, J = 8.4 Hz, 2H), 7.15 (d, J = 8.4 Hz, 2H), 6.09 (s, 2H), 5.19 (s, 2H), 4.17 (s, 2H), 3.91 (s, 6H), 1.96 (m, 2H), 1.65 (m, 2H), 1.48 (m, 2H), 1.24 (m, 2H). m/z = 396.2 [M − 2Br]²⁺, 792.4 [M − 2Br]⁺. Elemental analysis calcd for G (C₄₃H₄₄Br₂N₄O₇S₂) (H₂O)₂: C 52.23, H 4.89, N 5.67; found: C 52.22, H 4.72, N 5.34.

Preparation the monolayers at lab-built screen-printed three-electrodes for Raman test

The fabrication of screen-printed three-electrode (SPE) was achieved according to the previous report.¹⁰ The SPE was first washed with distilled water and dried by stream, and then pre-activated in a 0.2 M PBS (pH = 7.0) by applying an anodic potential of 2.0 V (versus Ag/AgCl) for 100 s. Further, SPE was utilized for the electrodeposition of AuNPs. AuNPs were electrodeposited at −0.3 V vs. Ag/AgCl for 300 s, by immersing the SPE into a 0.5 M H₂SO₄ solution containing 0.1 mM HAuCl₄. The modified electrode was denoted as AuNPs/SPE. The AuNPs/SPE was then electrochemically cleaned in 5 mL fresh 0.5 M H₂SO₄, by potential scanning between −0.1 V and 1.5 V vs. Ag/AgCl at 100 mV s⁻¹, until a typical voltammogram for gold was obtained. Finally, the clean AuNPs/SPE were modified with SAMs by immersion in 2 mM water solutions of G, G-CD and G-2CD for 36 h at room temperature, and then rinsed with copious 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 100× objective (nominal aperture 0.9) providing scattering areas of ca. 0.25 μm².

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 electrode. Screen-printed three-electrodes with Au plated are of benefit 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. = 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-dithiolane endgroup (depicted in Fig. 4).

Lab-built elbow gold electrodes were prepared by melting a 50 μm Au wire fixed into soft glass that was then polished with 0.05 μm alumina slurry, and then cleaned by soaking in hot Piranha etching solution (H₂SO₄ : H₂O₂ = 3 : 1) for 10 min. Finally, they were sonicated in millipore H₂O. Each electrode was inspected by light microscopy to ensure that the Au electrode 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 = 10 s) of potassium ferricyanide in aqueous 1.0 M KCl solution. Then each electrode undergo potential scanning from −0.1 to 1.6 V during 10 cycles at 100 mV s⁻¹ in fresh 0.1 M H₂SO₄. And electrochemical roughness factor (R_f = 1.42 ± 0.11) was assessed by cyclic voltammetry between −0.1 V and 1.5 V vs. SCE, at 100 mV s⁻¹ (performed in fresh 0.1 M H₂SO₄) taking into account the relationship of reduction a layer of adsorbed oxygen (386 μC cm⁻²) on gold.¹¹

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 first designed and synthesized a linear water-dissolved guest compound G (shown in Fig. 1) as expected to be effectively complexed with α-cyclodextrin (α-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 or two α-CD rings except the viologen.¹² Isophthalate moiety was introduced into one end of the guest as a stopper. The other stopper was conceived to be generated via the self-assembly of the 1,2-dithiolane endgroup to the gold surface.

Fig. 1 The synthetic route to the guest compound G and the representation of G and macrocyclic host α-cyclodextrin (α-CD).

The guest compound G and other precursors were fully characterized by ¹H NMR, ¹³C 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 α-CD for ca. 15 min.^6,13 A relatively exhaustive conversion to host–guest structures can be expected based on high association constants between the azobenzene compounds and α-CD (ca. 4000–10 000 M⁻¹),^5,6 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-CD, peak at m/z = 882.2872, corresponding to [G-CD − 2Br]²⁺, is observed.¹⁴ The NOE patterns, observed from the azobenzenyl protons to the internal protons of α-CD, confirm that the macroring dominantly located on the azobenzene group (see Fig. 2). When G was encapsulated by two α-CD rings together to form G-2CD, the complex was also evidenced by ESI-MS (Fig. S2†).¹⁴ The peak at 1368.9521 is assigned to [G-2CD − 2Br]²⁺. NOEs can be found from the protons of both the azobenzene and lipoic acid to the internal protons of α-CD. These results suggest that two of the rings located on the azobenzene and lipoic acid part respectively (Fig. 2).

Fig. 2 The two-dimensional ¹H NOESY NMR spectra (500 MHz in D₂O at 298 K) of G-CD and G-2CD. The inserts shows their corresponding proposed geometries.

Cyclodextrins are prone to encapsulate hydrophobic part of a water-soluble compound. In this case, the guest molecule 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 measurement (Fig. 3) and ¹H 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 μm as shown in its SEM spectra, whereby the hydrophobic part stacked up to each other whereas hydrophilic viologen group exposed to the aqueous environment.¹⁵ As shown in ¹H NMR spectra, the proton resonances of G could be fully revealed. However, the peak shapes of these resonances are inconsistent among each other. Here protons H_h–H_k and H_d–H_g signals, which belong to the azobenzene group and the lipoic acid part, respectively, are found to be slightly broad. But the signals H_o–H_u of hydrophilic end are well splitted. Such phenomenon should be originated from different geometries of flexible structure of G when dispersed in water, especially that it can self-organize via the intermolecular hydrophobic effect.

Fig. 3 SEM images of (A) G, (B) G-CD (G + 1.0 eq. of α-CD), (C) G-2CD (G + 2.0 eq. of α-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.

Fig. 4 (A) The representation of the hemi-rotaxanes; (B) the representation 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.

The sample after complexation of one α-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 μm wide. The encapsulation of the macroring also makes the azobenzenyl proton resonances H_h–H_k splitting, but the resonances of lipoic acid part still keep broad (Fig. 3B). This phenomenon shows that the complexation of the α-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 causes the reproduction of the peaks of H_h–H_k and H_d–H_g in ¹H NMR spectrum (Fig. 3C). In this case, the complexation with two of α-CD cavity thoroughly avoids the aggregation effect of the flexible groups.

Preparation and study of CD-based ordered rotaxane-monolayers at gold surface

Upon the obtained SAMs followed by the “Synthesis/complexation precedes assembly” process, we try to involve X-ray photoelectron spectroscopy (XPS) measurement to investigate the construction of monolayers with one or two α-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 film 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 other interactions.¹⁶ The XPS spectrum of S2p region of monolayer shows significant 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†).

Raman scattering spectroscopy is able to provide a molecular fingerprint of the molecules under study. However, Raman scattering is an intrinsically weak effect, only the compound in every 10⁶ to 10⁸ photons involves a change in energy.¹⁷ A phenomenon known as the surface-enhanced Raman scattering (SERS) was widely used, for that in the presence of noble metal nanostructures such as gold or silver surface, Raman scattering of molecules that are on or in close proximity to the nanostructure surface is significantly enhanced.¹⁷ It has been applied to the chemically specific analysis of hybrid layer on gold surface. Furthermore, SERS shows strong distance dependence of the nanoparticle surface structural information on the layer which fits our rigid system very well.

SERS spectra recorded from the three monolayers G@Au, G-CD@Au and G-2CD@Au exhibit a peak at 1138 cm⁻¹ due to the stretch of azobenzenyl C–N (ν_–C–N=) (Fig. 5).¹⁸ 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†).

Fig. 5 Normalized Raman spectra of the monolayers (A) G@Au; (B) G-CD@Au and (C) G-2CD@Au. The strongest peak (ν_–C–N=) at 1138 cm⁻¹ was set as the internal standard.

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 of C–N at 1138 cm⁻¹ 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 standard. By comparing the intensities of other group with C–N in the same system, peak intensity changes within different monolayers with non, one or two α-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 Fig. 5B have a little increase as compared to those in Fig. 5A. However, the enhancement of the relative intensity of these peaks is very remarkable in Fig. 5C. A reasonable explanation for the phenomenon that the peaks grow from spectrum (a) to (c) is the increasing orderliness with the complexation of α-CD. With two of the macrocyclic rings encapsulated to the guest component, the ordered array will be favorable for the freedom and the vibrations of the guest component,¹⁹ so as to strengthen the most of the vibrational forms in this system.

Viologen moiety is a typical redox-active group that can be performed as an electrochemical probe to directly give feedback of the geometric information.²⁰ Cyclic voltammogram (CV) curves of the comparison of the three monolayers are shown in Fig. 6. In this work, the scan range covers the first monoelectronic redox process of the viologen unit. The full width at half maximum (FWHM) of both the oxidation and the reduction peaks are reduced gradually from curve A to C. These experimental results suggest a straightforward enhancement of the uniformity of the modified Au surface in accordance with monolayers G@Au, G-CD@Au and G-2CD@Au in turn.¹⁷ This phenomenon can be deduced as the complexation-to-deaggregation effect of α-CD encapsulation to the guest, which has promoted the surface quality of the prepared SAMs, for the foldings or interactions among the monomers were interrupted. Meanwhile, we also find that the potential differences between the oxidation and the reduction of viologen moiety reveals a decreased tendency from curve (A) to (C) (this parameter is 0.140, 0.128 and 0.074 V for curve (a), (b) and (c), respectively). It is indicated that with the improvement of the uniformity of the monolayers, the redox of the viologen unit in the monomer would become easier.

Fig. 6 Cyclic voltammograms of (A) G@Au, (B) G-CD@Au and (C) G-2CD@Au in water at 298 K. The curves were recorded at a scan rate of 120 mV s⁻¹. Two vertical lines for each curve are set for representation of the potential differences.

Conclusions

Immobilization of mechanically interlocked molecules has attracted a considerable interest for the possible usage as basic components of future molecular electronic devices. Involvement of well-defined rotaxane with CD offers conformational changes of SAMS monolayers with increased orderliness and electronic properties. In this context, SAMS monolayers based on rotaxane with one or two CD was constructed, its surface 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 gold surface using approaches of NMR, ESI-MS, SERS and CV, etc.

The major contribution in this work is to utilize multi-cyclodextrin complexation to build ordered functional monolayers. The complexation-to-deaggregation effect of CD ring is advantageous since it can limit the different number of geometries that a flexible counterpart can adopt at gold surface. In this case, the improvement of the uniformity of the monolayers was also observed. Such solid-state self-assembled materials with better uniformity and multi-function might be found application in fields of advanced molecular switches or digital information processing.²¹

Acknowledgements

This work was supported by the Research Grant for Talent Introduction of Fudan University (JIH1717006) and National Program for Thousand Young Talents of China. Ruyi Sun thanks the NSFC/China (21402053), China Postdoctoral Science Foundation (2014M560318, 2015T80414). The authors also thank Prof. H. Tian and Prof. Y.-T. Long for helpful discussions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ESI-MS spectra of G-2CD; ¹H 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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