An anthracene-containing bistable [2]rotaxane featuring color and fluorescence changes

Abstract

We report an anthracene-containing bistable [2]rotaxane that can respond to the acidic/basic reagents, the change of the solvent polarity, and the counteranions, etc. The movement can generate remarkable, high-contrast fluorescent intensity change, along with an obvious color change.

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Due to the potential applicability in mesoscale molecular electronic devices, the design and synthesis of molecular switches that function through host–guest recognition has been a topic of growing interest.¹ Bistable [2]rotaxanes² have become suitable candidates for the construction of functional molecular switches because of their feasible synthesis strategies and easy modification. A bistable [2]rotaxane employs a macrocycle component as a shuttling unit, which can reversibly move between two stations on a thread component, triggered by chemical or electrical energies,³ light,⁴ ions⁵ or solvent polarity,⁶ generating a series of recognizable signal outputs, such as UV-vis absorption,⁷ fluorescence⁸ or electrochemical signals.⁹ Here we report a bistable [2]rotaxane, which can respond to acid–base inputs to induce the shuttling motion of an anthracene-containing crown ether macrocycle between a dibenzyl-ammonium (DBA) and a 4,4′-bipyridinium (Bipy²⁺) recognition sites (Scheme 2). The molecular shuttle can also respond to the changes of the solvent polarity and the counteranions. The movement can cause a change in charge transfer (CT) interaction between the π-electron anthracene donor and the π-electron-acceptor 4,4′-bipyridinium unit, which can generate remarkable, high-contrast fluorescent intensity change, along with obvious color change. The kind of system paves the way for designing multi-stimuli responsive smart materials with easily detectable features.

The syntheses, molecular structures, and schematic representations of [2]rotaxane 1-H, dumbbell 3-H and the anthracene-containing crown ether 2 are shown in Schemes 1 and 2. The azide 10, incorporating a dialkylammonium (DBA) unit as a primary recognition site for macrocycle 2 in the middle, terminated at one end by 3,5-di-tert-butylphenyl stopper and at the other end by a terminal alkyne group, was obtained in five steps in 18% yield, as shown in Scheme 1a. Compound 11 was prepared according to a previous work,¹⁰ which was then reacted with 4-bromo-1-butyne in DMF, followed by a counterion exchange to obtain alkyne 13 in 80% yield (Scheme 1b) purified using silica gel column chromatography. 3,5-Di-tert-butylphenyl group was chosen as the stoppers, which is big enough to prevent the macrocycle dissociating from the thread component. The synthesis of the macrocyclic 2 is depicted in Scheme 1c. Bis-phenolate intermediate 14 was prepared in three steps according to a reported procedure.¹¹ Compound 14 was treated with bis-tosylate 15 in the presence of Cs₂CO₃ in CH₃CN, which generated anthracene-containing macrocycle 2 in 67% yield. The anthracene was chosen as a fluorescent unit because of its high fluorescent quantum yield, and desirable spectroscopic properties, as well as its planar conjugated backbone.

Scheme 1 Synthetic strategies of (a) compound 10, (b) compound 13 and (c) macrocycle 2.

Scheme 2 (a) A bistable [2]rotaxane with an acid–base driven performance. (b) The chemical structures and schematic representations of [2]rotaxane 1-H and thread component 3-H.

The [2]rotaxane 1-H and dumbbell 3-H were successfully obtained from the initially prepared alkyne 13 containing a Bipy²⁺ moiety and the azide ammonium 10, by using the “click chemistry”, namely, Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC).¹² The CuAAC reaction was chosen as the end-capping method due to its functional group tolerance. As shown in Scheme 2b, crown ether 2 was stirred in dry CH₂Cl₂ with azide 10 at room temperature, after complete solubilization of 10 by 2, 1.2 equiv. of alkyne 13 were added to the reaction mixture, followed by a catalytic amount of Cu(CH₃CN)₄PF₆. The pure [2]rotaxane 1-H and its corresponding free dumbbell 3-H were isolated by silica gel chromatographic column in 25% and 38% yields, respectively. [2]Rotaxane 1-H, and the thread component 3-H were well characterized by ¹H NMR, ¹³C NMR spectroscopies and HR-ESI mass spectrometry (ESI†). The reversible shuttling motion of the macrocycle between the two different recognition sites in rotaxane 1-H was also confirmed by the ¹H NMR spectroscopy, as discussed below.

Comparison between the ¹H NMR spectrum of 1-H with those of macrocycle 2 and thread component 3-H can give direct evidences of the interlocked structure of 1-H and the localization of the ring component. As shown in Fig. 1c, the spectrum of rotaxane 1-H contains all the peaks for the protons in both macrocycle 2 and thread 3-H, however, the protons H₁₈ and H₁₉ on the DBA site exhibit obvious upfield shifts (Δδ = −0.91 and −0.55 ppm, respectively), due to a shielding effect of macrocycle 2, moreover, H₂₇, H₂₈, H₂₉, and H₃₀ on the Bipy²⁺ unit only have a little change in comparison with those of 3-H. Moreover, the hydrogens H₁₇ and H₃₁ experience an upfield shift (Δδ = −0.28 ppm) and a slightly upfield shift (Δδ = −0.08 ppm), respectively, indicating that the macrocycle 2 is closed to hydrogen H₁₇. These observations revealed that the DBA recognition site was encircled by macrocycle 2 in the original state. Furthermore, the HR-ESI mass spectrum of the [2]rotaxane 1-H (SI) also confirmed the structure, which gave two peaks at m/z 502.9527 [M-3PF₆]³⁺ and 826.9189 [M-2PF₆]²⁺, respectively, in excellent agreement with the calculated value (502.9586 and 826.9201, respectively).

Fig. 1 Partial ¹H NMR spectra (400 MHz, CD₃COCD₃, 298 K) of (a) macrocycle 2, (b) thread component 3-H, (c) [2]rotaxane 1-H, (d) deprotonation with addition of 2 equiv. of Bu₃N to sample c and (e) reprotonation with addition of 3 equiv. of TFA to sample d. The assignments correspond to the structures as shown in Scheme 2.

Next we investigated the shuttling motion of macrocycle 2 between the two distinguishable recognition sites in the rotaxane system using ¹H NMR spectroscopy. It was found that tributylamine (Bu₃N) is ideal base to drive the switching process because it is strong enough to deprotonate the NH₂⁺ center while behaving concurrently as very weak nucleophiles. Addition of 2 equiv. Bu₃N to a solution of 1-H in CD₃COCD₃ can induce the movement of macrocycle 2 to the Bipy²⁺ unit, and the solution was changed from colorless to yellow (Fig. S1†), indicating the creation of CT interaction between anthracene moiety in 2 and the Bipy²⁺ dication.¹³ As shown in Fig. 1d, addition of Bu₃N can result in the deprotonation of the DBA unit, with a characteristic of an upfield chemical shift of ammonium proton H₁₈ (Δδ = −0.25), meanwhile the Bipy²⁺ protons H₂₇, H₂₈, H₂₉ and H₃₀ were also shifted upfield (Δδ = −0.39, −0.45, −0.52 and −0.56 ppm, respectively) due to the association with macrocycle 2. Moreover, protons H₁₇ and H₃₁ were shifted with Δδ of 0.21 and −0.25 ppm, also suggesting that macrocycle 2 interacted with the Bipy²⁺ unit. All these changes revealed that addition of excess Bu₃N can drive macrocycle 2 moving from the DBA station to the Bipy²⁺ recognition site. Upon addition of 3 equiv. CF₃COOH (TFA), macrocycle 2 shuttled back completely to the DBA recognition site, which was evidenced by the regeneration of the original ¹H NMR spectrum (Fig. 1e).

Similar ¹H NMR measurements performed in DMSO-d₆ proved that macrocycle 2 resided exclusively on the Bipy²⁺ recognition site (Fig. S2†). The chemical shifts H₁₈ and H₁₉ on DBA station almost have no changes compared with those in thread component 3-H. On the other hand, the peaks for aromatic protons H₂₇, H₂₈, H₂₉ and H₃₀ on the Bipy²⁺ recognition site shows dramatic chemical shift changes (Δδ = −0.53, −0.60, −0.70 and −0.85 ppm) compared to those of reference compound 3-H, which is due to the shielding effect of the macrocycle.

It has been reported that the nature of the counteranions can affect the intermolecular interactions between the DBA station and DB24C8.¹⁴ Addition of 3.5 equiv. tetrabutylammonium fluoride (TBAF) to the CD₃COCD₃ solution of compound 1-H can generate a ¹H NMR spectrum (Fig. S3b†) similar to that in Fig. 1d, suggesting that the macrocycle 2 moved to the Bipy²⁺ recognition site. The shuttling movement was attributed to the fact that the fluoride anion has a relatively stronger disruption to the [N⁺-H⋯O] hydrogen bonding between the DBA station and the macrocycle.¹⁵ Upon addition of 4 equiv. Ca(PF₆)₂, the fluoride anions was removed from the solution by generating a precipitation of CaF₂, providing a spectrum (Fig. S3c†) almost the same as that of the original [2]rotaxane 1-H. We have also studied the ¹H NMR spectra upon addition of TBACl, TBABr, and TBAI (Fig. S4†), and they are in the same pattern as that of rotaxane 1-H upon addition of TBAF, which prove that all of the three halide anions can also induce the shuttling movement of macrocycle 2. Thus, by ¹H NMR spectroscopic measurements, we have demonstrated that this rotaxane can respond to acidic/basic reagents, the solvent polarity, and the counteranions.

Then we focused on the physical properties of [2]rotaxane 1-H and macrocycle 2 in response to external stimuli. Upon titration of the macrocycle component 2 with Bu₃N or TBAF, almost no absorption and emission spectra change were observed (Fig. S5 and S6†). The absorption spectrum of macrocycle 2 in DMSO was a little lower than that in dichloromethane, and the emission intensity decreases about 5% (Fig. S7†). Upon addition of 2 equiv. Bu₃N to the dichloromethane solution of 1-H, the absorption spectrum was accompanied by a small blue shift and a decrease in the anthracene based band (Fig. 2a). Meanwhile, there was an increase in the absorption band at 405–425 nm, which was owing to the CT process from the excited state of anthracene unit to the electron acceptor Bipy²⁺ unit. Similar results were also obtained when change the dichloromethane solution to dimethylsulfoxide solution. Addition of 3.5 equiv. TBAF to the dichloromethane solution of 1-H could also cause a small blue shift in the anthracene based band.

Fig. 2 (a) Absorption and (b) fluorescence spectra of 1-H upon addition of 2 equiv. of Bu₃N (CH₂Cl₂), 3.5 equiv. TBAF (CH₂Cl₂) and in DMSO. The concentration was 2 × 10⁻⁵ M and excitation wavelength was 375 nm.

Moreover, after addition of 2 equiv. Bu₃N to the dichloromethane solution of 1-H, the emission intensity of anthracene unit decreased 89% compared with the original spectrum (Fig. 2b), and the absolute fluorescence quantum yield decreased from 0.52 to 0.01. The fluorescence quenching of the anthracene moiety was mainly due to the strong CT process from the excited state of anthracene to the electron acceptor Bipy²⁺ unit, along with a low PET process from the newly created amino group to anthracene unit.¹⁶ Importantly, the shuttling motion of the macrocycle could be repeated many times by alternate acid–base stimuli (Fig. S8†). Similarly, the fluorescence intensity of 1-H in DMSO or upon addition of 3.5 equiv. TBAF in dichloromethane is remarkably lower than the original spectrum (Fig. 2b). To further study the influence of counteranions, other halide anions are added to the solution of rotaxane 1-H and macrocycle 2, respectively. There were no obvious changes in the absorption and fluorescence spectra of macrocycle 2 upon addition of 3.5 equiv. of TBACl, TBABr, and TBAI (Fig. S9†). However, both of TBACl and TBABr can induce a little blue shift of the absorption of 1-H, and change the color to light yellow. TBAI can result in a blue shift of anthracene's absorption along with an increase in an absorption band at 420–600 nm, and the color turn to red, which is due to CT complexes of the Bipy²⁺ unit and I⁻.¹⁷ The fluorescence intensity decreased 80% compared with the original spectrum upon addition of excess TBACl, TBABr, and TBAI (Fig. S10†). Thus, we have demonstrated the movement of this system using UV-vis absorption and fluorescence spectroscopies.

In conclusion, a novel bistable [2]rotaxane has been synthesized and characterized, and the shuttling movement of the anthracene-containing macrocycle relative to the rotaxane thread can be achieved by changing the pH. In addition, the molecular switch can also respond to the solvent polarity and the counteranions. Importantly, the stimulus-induced movement is accompanied by an obvious color change and a high-contrast fluorescent intensity change observed by the naked eye, generated by altering the charge transfer process of the anthracene and Bipy²⁺ functional units. This system represents as a molecular switch that can realize signal-responsive color and fluorescence sensing, which provides insight for constructing advanced responsive smart materials.

We thank the NSFC/China (21272073, 21072058 and 21190033), the National Basic Research 973 Program (2011CB808400), and the Foundation for the Author of National Excellent Doctoral Dissertation of China (200957), the Fok Ying Tong Education Foundation (121069), the Fundamental Research Funds for the Central Universities, the Innovation Program of Shanghai Municipal Education Commission.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and characterizations for all new compounds, the absorption and fluorescence spectra of [2]rotaxane 1-H and macrocycle 2. See DOI: 10.1039/c3ra46517a

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