Stimuli-Responsive Supramolecular Assemblies between Twisted

Article Cite This: J. Phys. Chem. B 2017, 121, 11119−11123

pubs.acs.org/JPCB

Stimuli-Responsive Supramolecular Assemblies between Twisted Cucurbit[14]uril and Hemicyanine Dyes and Their Analysis Application Jing Zhang,†,‡ Qing Tang,§ Zhong-Zheng Gao,‡ Ying Huang,*,†,‡ Xin Xiao,‡ and Zhu Tao‡ †

The Engineering and Research Center for Southwest Bio-Pharmaceutical Resources of National Education Ministry of China, ‡Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, and §College of Tobacco of Guizhou University, Guizhou University, Guiyang 550025, China S Supporting Information *

ABSTRACT: Two supramolecular assemblies between twisted cucurbit[14]uril and hemicyanine dyes have been successfully constructed on the basis of host−guest recognition. These supramolecular assemblies could be reversibly switched under acidic and neutral conditions. Furthermore, they responded to selected chemical stimuli such as methyl violet, thereby exhibiting potential analysis application.

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INTRODUCTION Cucurbit[n]urils (Q[n]s) are composed of n glycoluril units linked through 2n methylene bridges. These receptors have highly polar carbonyl-fringed portals with hydrophobic cavities and can form remarkably stable complexes with a variety of guest molecules.1−6 Among the cucurbit[n]urils, twisted tQ[14] with 14 normal glycoluril units linked by 28 methylene bridges has two kinds of cavities (a central cavity and two side cavities) and adopts a folded, figure-of-eight conformation.7 As a result, it shows structural flexibility and presents a high density of portal carbonyl groups (Figure 1). Thus, tQ[14] shows unusual binding behavior with various guest molecules.8−11 2-(4-(Dimethylamino)styryl)-1-methylpyridinium iodide (2-ASP) and trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (4-ASP) are common cationic hemicyanine dyes in which the dimethylamino group is an electron donor and the methylpyridinium group is an electron acceptor. Thus, multibond rotation due to intramolecular charge transfer (ICT)12,13 is found to occur in the dyes, which is dependent on both polarity and microviscosity of the medium. Moreover, the interactions between some hemicyanine dye derivatives or styryl dyes with similar structures and Q[n]s have been investigated, and the results demonstrated that Q[n]s could bind to the dyes through ion−dipole interactions. Supramolecular self-assemblies modulated by external stimuli are currently attracting great interest for their applications,14−16 such as switches,17 pseudorotaxanes,18 sensors,19 valves,20 molecular switches,21 and so forth. In particular, cucurbituril− guest supramolecular systems can exhibit controlled self© 2017 American Chemical Society

assembly in response to external stimuli, including pH change or electrochemical or photochemical stimuli.22 In the present work, we report two controllable supramolecular assemblies between twisted cucurbit[14]uril and hemicyanine dyes, the host−guest binding behavior of which could be modulated by chemical stimuli. The self-assembly behaviors have been investigated by 1H NMR, UV−vis absorption, and fluorescence spectroscopies. These systems provide the possibility of producing a logic gate with a fluorescence signal response and potential analysis application.

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EXPERIMENTAL SECTION

Materials. tQ[14] was synthesized according to a procedure developed in our laboratory and characterized by 1H NMR spectrometry. 2-(4-(Dimethylamino)styryl)-1-methylpyridinium iodide (2-ASP) and trans-4-[4-(dimethylamino)styryl]1-methylpyridinium iodide (4-ASP) were obtained from SigmaAldrich (Shanghai, China); methyl violet (MV2+) was obtained from Aladdin (Shanghai, China). All reagents were of analytical reagent grade and were used without further purification. Doubly distilled water was used throughout. Measurement of Absorption and Fluorescence Spectra. All UV−visible spectra were recorded on an Agilent 8453 spectrophotometer (Agilent Technologies, Santa Clara, CA). Received: October 17, 2017 Revised: November 17, 2017 Published: November 22, 2017 11119

DOI: 10.1021/acs.jpcb.7b10285 J. Phys. Chem. B 2017, 121, 11119−11123

Article

The Journal of Physical Chemistry B

Figure 1. Structures of the host and guests.

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RESULTS AND DISCUSSION Binding of Hemicyanine Dyes to tQ[14]. First, the pKa values of (2-(4-(dimethylamino)styryl)-1-methylpyridinium iodide) 2-ASP and (trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide) 4-ASP were measured by pH titration on the basis of absorption signals. The pKa values of 2-ASP and 4ASP were evaluated as 3.7 and 4.0, respectively (Figures S1 and S2, Supporting Information (SI)), in good agreement with those reported previously.23,24 From the UV absorption spectra, it can be inferred that the guests are protonated in Tris−HCl buffer (pH 2) and the absorption maxima of 2-ASPH+ and 4ASPH+ are at 325 nm. At pH 6, the guests are in their nonprotonated forms and the absorption maxima of 2-ASP and 4-ASP are at 438 nm. Upon complexation, the pKa values of 2ASP/tQ[14] and 4-ASP/tQ[14] are 5.9 and 6.0, respectively (Figure S3, SI). Thus, the complexation-induced increases in pKa values amount to 2.2 units for 2-ASP and 2.0 units for 4ASP. To investigate the binding behavior between 2-ASP/2ASPH+ and tQ[14], UV−vis absorption and fluorescence spectra were measured. 2-ASP shows an absorption maximum at 438 nm in Tris−HCl buffer (pH 6). Significant spectral changes were observed upon the gradual addition of tQ[14] (as shown in Figure 2A). The absorption band at 438 nm showed a decrease in intensity and gradually red-shifted to 457 nm, resulting in a well-defined isosbestic point at 365 nm. Interestingly, this was accompanied by the appearance of a new absorption band at 335 nm, corresponding to the protonated form of 2-ASP, which gradually intensified, suggesting that 2-ASP is protonated when encapsulated as a guest in tQ[14]. Furthermore, 2-ASP in Tris−HCl buffer (pH 6) showed only a weak fluorescence at 580 nm, whereas its interaction with tQ[14] resulted in a dramatic enhancement of its emission intensity. Figure 2B shows that the fluorescence intensity increased upon the addition of tQ[14] and the emission wavelength hypsochromically shifted from 580 to 563 nm, demonstrating the interaction of the dye with tQ[14]. An approximately 12-fold fluorescence enhancement was observed upon the addition of 2 equiv of tQ[14]. From the change in fluorescence intensity, we inferred that the inclusion complex was mainly formed with 1:1 stoichiometry (inset in Figure 2B). The binding constant (K) for tQ[14]−2-ASP was estimated as 8.37 × 104 mol/L according to the literature.25 Thus, both the absorption and fluorescence spectra were indicative of host− guest interactions. Hence, we also concluded that multibond rotation due to intramolecular charge transfer (ICT) is found to occur and the restriction of intramolecular rotation of 2-ASP in the cavity of tQ[14] plays a major role in determining its photophysical properties.24 Correspondingly, UV−vis and fluorescence spectrometric results revealed that the interaction

Fluorescence emission spectra were recorded on a Varian Cary Eclipse spectrofluorometer (Varian, Inc., Palo Alto, CA). Stock solutions of tQ[14] (2 × 10−3 mol/L), the guests (2ASP/2-ASPH+ and 4-ASP/4-ASPH+, 1 × 10−4 mol/L), and MV2+ (2 × 10−3 mol/L) were prepared in doubly distilled water. The pH of the solutions was adjusted using Tris−HCl. These stock solutions were combined to give solutions containing a fixed guest concentration of 2.0 × 10−5 mol/L in the presence of different concentrations of tQ[14] ((0−4) × 10−5 mol/L) in each solution, and each solution was characterized by absorption spectra and fluorescence spectra. The maximum excitation wavelengths (λex) were 438, 323, 454, and 335 nm for the tQ[14]−2-ASP, tQ[14]−2-ASPH+, tQ[14]−4-ASP, and tQ[14]−4-ASPH+ complexes, respectively. The photomultiplier gain was medium, with 10 nm emission and excitation bandwidths. For each experiment, three replicate measurements were made. Isothermal Titration Calorimetry (ITC) Measurements. Thermodynamic parameters and binding constants (K) for the tQ[14]−guest complexes were determined by titration calorimetry using a Nano ITC instrument (TA). All solutions were prepared in doubly distilled water and degassed prior to the titration experiments. The evolved heat was recorded at 298.15 K. The concentration of tQ[14] in the sample cell (1.3 mL) was 1 × 10−4 mol/L. Typical ITC titration was carried out by titrating the 2-ASP/4-ASP (pH 6) and 2-ASPH+/4-ASPH+ (pH 2) solutions (1 × 10−3 mol/L, 10 μL aliquots, at 250 s intervals) into the tQ[14] solution. The heat of dilution was corrected for by injecting the guest solution (free guest) and subtracting the values from the corresponding values obtained for the host−guest titration. Computer simulations (curve fitting) were performed using Nano ITC analytical software. 1 H NMR Measurements. The 1H NMR spectra were recorded at 25 °C on a WNMR-I 500 MHz NMR spectrometer (Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences). D2O was used as a field-frequency lock, and the observed chemical shifts are reported in parts per million (ppm) relative to that for the internal standard (tetramethylsilane at 0.0 ppm). Analytical Application. To obtain the calibration curves, (20−300) × 10−7 mol/L solutions of MV2+ were added to the corresponding aliquots of the concentrated stock solution in the presence of the tQ[14]−2-ASP (1:1, 2.0 × 10−5 mol/L) complex in aqueous solution. According to the International Union of Pure and Applied Chemistry recommendations, a blank solution was measured (n ≥ 20) to determine the precision and limit of detection of the method. 11120

DOI: 10.1021/acs.jpcb.7b10285 J. Phys. Chem. B 2017, 121, 11119−11123

Article

The Journal of Physical Chemistry B

Figure 2. Absorption spectra (A) and fluorescence spectra (λex = 438 nm) (B) of 2-ASP (20 μM, pH 6) in the presence of different stoichiometries of tQ[14]/μM: 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 24, 28, 32, 36, and 40.

Figure 3. Absorption spectra (A) and fluorescence spectra (λex = 323 nm) (B) of 2-ASPH+ (20 μM, pH 2) in the presence of different stoichiometries of tQ[14]/μM: 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, and 40.

in the tQ[14]−4-ASP system was similar to that in the tQ[14]−2-ASP system (Figure S4 in SI) and the binding constant (K) for tQ[14]−4-ASP was 5.85 × 104 L/mol. The protonated form of 2-ASPH+ shows an absorption maximum at 323 nm and a fluorescence peak at 570 nm in Tris−HCl buffer (pH 2). Compared to those in the tQ[14]−2ASP complex, different spectral changes were observed upon the gradual addition of tQ[14]. As shown in Figure 3A, the absorption band at 323 nm showed a decrease in intensity and gradually red-shifted to 330 nm, demonstrating the interaction of 2-ASPH+ with tQ[14]. The fluorescence intensity at 570 nm increased upon the gradual addition of tQ[14], and an approximately 3-fold fluorescence enhancement was observed with the addition of 2 equiv of tQ[14], as shown in Figure 3B. Correspondingly, UV−vis and fluorescence spectrometric results revealed that the interaction in the tQ[14]−4-ASPH+ system was similar to that in the tQ[14]−2-ASPH+ system (Figure S5 in SI). Isothermal titration calorimetry (ITC) was also employed to investigate the binding behavior of tQ[14] with guests. Equilibrium association constants (Ka) and thermodynamic parameters from the ITC experiments are listed in Table 1, and ITC profiles are shown in Figure S6 (Supporting Information). It can be seen that the protonated forms of the guests showed higher binding affinities toward tQ[14] compared to those of the nonprotonated forms. The observations may be explained in terms of a combination of ion−dipole and electrostatic interactions between the positively charged dimethylamino moiety and the polar carbonyl groups of tQ[14]. Subsequently, the switching behavior under neutral and acidic conditions was studied by 1H NMR spectroscopic titrations. The resonances of 2-ASP were assigned on the basis

Table 1. Stability Constants and Thermodynamic Parameters for the Complex Formation of Guests with Twisted Cucurbit[14]uril at 25 °C K (M−1)

guests 2-ASP 4-ASP 2-ASPH+ 4-ASPH+

(3.15 (3.26 (2.59 (2.90

± ± ± ±

0.57) 0.82) 0.38) 0.18)

ΔH(kJ mol−1) × × × ×

4

10 104 105 105

−52.8 −56.6 −32.3 −35.9

± ± ± ±

4.36 3.29 0.66 0.38

TΔS(kJ mol−1) −28.6 −30.8 −1.42 −4.71

of its rotating-frame Overhauser effect spectroscopy (ROESY) and gradient correlation spectroscopy (g-COSY) two-dimensional (2D) NMR spectra (Figures S7−S10, SI). Upon the addition of 1.0 equiv of 2-ASP to a solution of tQ[14], all of the proton signals of 2-ASP were shifted downfield in the tQ[14]complex, except for those of H4 and H5, and they were broadened. This indicated that 2-ASP and tQ[14] formed a complex in which almost all of the protons of the guest resided in a deshielding microenvironment, suggesting that the guest may remain at the portal of tQ[14], leading to a dynamic balance between the free and complexed states (Figures 4a,b and S11). This binding mode is shown in Scheme 1,I. However, when 1.0 equiv of 2-ASPH+ was complexed with 1 equiv of tQ[14], as shown in Figure 4c,d (Figure S12), the signals of protons H1, H3, H4, and H5 of 2-ASPH+ were shifted upfield. In contrast, the signals of protons H2 and H6−H10 of 2-ASPH+ were shifted downfield. This implies that the benzene ring, the dimethylamino moiety thereon, and the double bond moved deeper into the cavity of tQ[14], as illustrated in Scheme 1,II. Additionally, ROESY and g-COSY 2D NMR experiments 11121

DOI: 10.1021/acs.jpcb.7b10285 J. Phys. Chem. B 2017, 121, 11119−11123

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The Journal of Physical Chemistry B

Figure 4. 1H NMR spectra of tQ[14] upon titration with 2-ASP (D2O, 500 MHz) and 2-ASPH+ (pD 2, 500 MHz): (a) 0 and (b) 1 equiv of tQ[14] were added to 2-ASP; (c) 0 and (d) 1 equiv of tQ[14] were added to 2-ASPH+.

Scheme 1. Possible Binding Modes of tQ[14] with 2-ASP

Figure 5. Reversible switching controlled by successively adding H+ and OH−.

constructed on the basis of deprotonation by acid−base neutralization between tQ[14] and a hemicyanine dye. This study lays down a foundation for constructing molecular INHIBIT logic gates by supramolecular interaction. Analysis Application. In addition, we employed other chemical additives, such as methyl violet (MV2+), in view of varying binding affinities toward tQ[14] as stimuli to elicit the tQ[14]/2-ASP-responsive behavior;8 MV2+ is a nonselective contact herbicide that is used to control weeds and grasses. It is often present as environmental residues because of its water solubility and nonvolatility. According to the fluorescence spectra (Figure S24), the addition of MV2+ to the tQ[14]−2ASP complex resulted in a complete decrease of the emission intensity. It is envisaged that stimuli-responsive supramolecular assemblies might be applicable in turn-off fluorescent sensors to sense MV2+. According to IUPAC recommendations, a linear calibration graph was obtained in the concentration range (20− 300) × 10−7 mol/L (Figures S25 and S26) and the detection limit was 3.2 × 10−7 mol/L; the corresponding parameters are shown in Table S2.

provided further evidence for the formation of these two complexes (Figures S13−S16). Simultaneously, we also examined the interaction of 4-ASP/ 4-ASPH+ with tQ[14] through 1H NMR spectroscopic titrations. The resonances of the guest were assigned on the basis of the previous work by another group.23 According to the titration 1H NMR spectra, the interaction of 4-ASP/4-ASPH+ with tQ[14] was in a similar inclusion mode (Figures S17− S23). ON/OFF Switching. It should be noted that the switching between 2-ASP/tQ[14] and 2-ASPH+/tQ[14] is reversible. The signaling responses could be switched back and forth by successively adding H+ and OH−, converting the fluorescent state between “OFF” and “ON” (Figure 5). In this supramolecular system, the ON state (output 1 = 1) was defined as that corresponded to the strong fluorescence at 563 nm, whereas the OFF state (output 2 = 0) corresponded to the weak emission at 563 nm; 20 μM 2-ASPH+ at pH 2 in the absence of tQ[14] was considered as the initial state. For input, the presence of H+ or OH− was defined as the “1” state and their absence as the “0” state. With no input (0, 0) or with H+ input (1, 0) alone, the output was 0. When subjected to OH− (0, 1) alone, the fluorescence of the solution increased sharply, giving an output signal of 1. When the two inputs appeared together (1, 1), the fluorescence was extremely weak and the output returned to 0. The truth table is listed in Table S1, which indicates that an INHIBIT logic gate was constructed. In other words, a resettable molecular logic gate system was

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CONCLUSIONS In summary, we have presented stimuli-responsive supramolecular assemblies between twisted cucurbit[14]uril and hemicyanine dyes. It is significant that the binding behavior changes could be reversibly adjusted by protonation and deprotonation of the hemicyanine dyes. The “OFF−ON− OFF” behavior of the host−guest complexes with the alternating addition of H+ and OH− has been utilized to develop a model INHIBIT logic gate. Such a strategy might be explored for application in turn-off fluorescent sensors or targeted drug delivery. 11122

DOI: 10.1021/acs.jpcb.7b10285 J. Phys. Chem. B 2017, 121, 11119−11123

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The Journal of Physical Chemistry B

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b10285. NMR spectroscopic data, UV−vis absorption, fluorescence spectroscopies, and ITC (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ying Huang: 0000-0002-1823-9197 Xin Xiao: 0000-0001-6432-2875 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (Grant No. 21562015), Excellent Youth Scientific Talents Foundation of Guizhou Province (Grant No. [2017] 5636), Talents support project of Guizhou Province of Education (QJH-KY-2016-059), the Natural Science Fund of the Science and Technology, Department of Guizhou Province (Grant No. JZ-2014-2005), and the Innovation Program for High-level Talents of Guizhou Province (No. 2016-5657).

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REFERENCES

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DOI: 10.1021/acs.jpcb.7b10285 J. Phys. Chem. B 2017, 121, 11119−11123