A fluorescent supramolecular polymer with aggregation induced emission (AIE) properties formed by crown ether-based host–guest interactions

Abstract

A fluorescent supramolecular polymer with aggregation induced emission properties formed by crown ether-based host–guest interactions was prepared. It can be used as a fluorescent sensor for Pd²⁺ ions.

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Supramolecular polymers, obtained as a result of the integration of polymer science and supramolecular chemistry, have received considerable attention during the last decade.¹ Compared with the traditional covalent bonds linked polymers, supramolecular polymers whose repeating units are joined together by non-covalent bonds often exhibit some interesting properties such as stimuli-responsiveness,² self-healing,³ and shape memory⁴ due to their dynamic nature. Different kinds of supramolecular polymers were prepared based on different non-covalent interactions such as hydrogen bonding,⁵ metal–ligand,⁶ π–π stacking,⁷ charge transfer,⁸ and host–guest interactions.⁹ Nowadays, the studies of supramolecular polymers have gradually turned from the preparation and characterization of supramolecular polymers¹⁰ to the functionality of supramolecular polymers, which is of significant importance for their future applications.

Tetraphenylethylene (TPE) is a fluorescent chromophore which shows bright fluorescence emissions with an increase in the concentration or in the solid state.¹¹ Compared with the conventional organic chromophores that display strong fluorescence in dilute solutions and are non-emissive at high concentrations or in the solid state, the aggregation induced emission (AIE) properties of TPE make it an excellent fluorescent molecule to dock with supramolecular polymers, because supramolecular polymers only form at relative high concentrations. Herein, we report a supramolecular polymer formed by crown ether-based host–guest interactions with tetraphenylethylene moieties as the chromophore. It exhibits much higher fluorescence emissions than its monomer due to the AIE properties of TPE. Interestingly, the fluorescence intensity of the supramolecular polymer decreased dramatically by the addition of Pd²⁺ ions, hence it can be used in the detection of Pd²⁺ ions.

As shown in Scheme S1,† monomer 1 was synthesized via a click reaction from one equivalent of TPE unit with two azide groups and two equivalents of dibenzo-24-crown-8 (DB24C8) moieties with one acetylene group. A supramolecular polymer was obtained by mixing equal molar ratios of monomer 1 and linker 2, a bisammonium salt with two dibenzylammonium (DBA) units in solution based on the DB24C8/DBA recognition motif, which has been widely used in the construction of molecular machines and supramolecular gels due to its responsiveness.⁹ At the same time, the aggregated TPE molecules endowed the supramolecular polymer with strong fluorescence at high concentrations (Scheme 1).

Scheme 1 Chemical structures of monomer 1 and linker 2 and cartoon representations of the formation of the fluorescent supramolecular polymer.

Concentration-dependent ¹H NMR measurements (Fig. 1) were first carried out to study the formation of the supramolecular polymers. As the concentrations of monomers 1 and 2 increased, noticeable chemical shift changes were observed for both DB24C8 and DBA. The NMR spectra became complicated and each of the protons of DB24C8 and DBA split into two signals, because this complexation is a slow exchange system. The aromatic protons H₂₂, H₂₅, H₂₆ and benzyloxymethylene protons H₂₇ of 2 shifted downfield, while upfield chemical shifts were found for the benzyl protons H₂₃ and H₂₄. Moreover, the proton signals became more and more broad and a conversion from cyclic oligomers to linear polymers was observed as the concentration increased, indicating a ring-chain mechanism for the formation of this kind of supramolecular polymers.^5a

Fig. 1 Partial ¹H NMR spectra (1 : 1 CD₃CN–CDCl₃, 293 K, 500 MHz) of (a) linker 2; equal molar mixtures of 1 and 2 at different concentrations: (b) 200 mM; (c) 110 mM; (d) 80.0 mM; (e) 50.0 mM; (f) 10.0 mM; (g) 2.0 mM; (h) 0.50 mM; and (i) monomer 1. Peaks of linear polymers and cyclic oligomers are denoted by l and c, respectively.

To further confirm the formation of linear polymeric aggregates, viscosity measurements, a reliable method to reflect the property of polymers, was performed in CH₃CN–CHCl₃ (1 : 1, v/v) using a semi-micro dilution viscometer. From Fig. 2a, it can be seen that the slope of the curve was 1.41 in the low concentration range, while a sudden rise in the viscosity (slope of 2.25) was observed when the concentration exceeded the critical polymerization concentration (CPC, 70 mM), indicating the transition from cyclic oligomers to linear supramolecular polymers. Two-dimensional diffusion-ordered NMR (DOSY) experiments were also carried out to test the mobility of the supramolecular polymer in solution. As the concentration of the equimolar solutions of 1 and 2 increased from 5.00 to 200 mM, the measured weight average diffusion coefficient D decreased from 4.42 × 10⁻¹⁰ to 2.90 × 10⁻¹¹ m² s⁻¹ (Fig. 2b). This result suggested that the intermolecular interactions were weak in the low concentration range, while they became strong in the high concentration range, providing another evidence for the formation of supramolecular polymers.

Fig. 2 (a) Specific viscosity of the linear supramolecular polymer (1 : 1 CH₃CN–CHCl₃, 298 K); (b) concentration dependence of the diffusion coefficient D (1 : 1 CD₃CN–CDCl₃, 293 K, 500 MHz).

Due to the pH-responsive complexation of DB24C8 and DBA, the conversion between the supramolecular polymers and monomers 1 and 2 can be reversibly realized by using NEt₃ and CF₃COOH to adjust the pH of the solution, and this process was confirmed by ¹H NMR (Fig. S9, ESI†).^6g

Electrospinning is a convenient technique to obtain nanofibers, and it requires molecular chain entanglements in the charged fluid to avoid the formation of droplets. Therefore, whether low molecular weight molecules can generate nanofibers by electrospinning is dependent on the existence of their intermolecular interactions. Overlapped nanofibers were obtained by electrospinning as shown in the SEM images (Fig. 3a), and their average diameter was about 1.0 μm (Fig. 3b), indicating the formation of supramolecular polymers with a high degree of chain entanglements in solution. It can be clearly seen that the fibers show strong blue fluorescence when they were irradiated at 350 nm (Fig. 3c and d), which is consistent with the AIE properties of the TPE molecules, because in the fibers the concentration of TPE molecules is very high. Consequently, a fluorescent supramolecular polymer was obtained by docking fluorescent tetraphenylethylene molecules with crown ether-based host–guest interactions.

Fig. 3 (a) SEM image of the electrospun supramolecular polymer nanofibers; (b) enlarged image of a; (c) 350 nm fluorescence emission-irradiated photos of the electrospun supramolecular polymer nanofibers; (d) enlarged image of c.

Then, we investigated the concentration dependent fluorescent intensity of the formed linear supramolecular polymer. Fig. 4 shows the fluorescent response of monomer 1 and the linear supramolecular polymer at different concentrations. Monomer 1 shows a very weak fluorescence peak only at 560 nm even at a high concentration (200 mM). Interestingly, the fluorescent intensity of the corresponding linear supramolecular polymer at the same concentration was enhanced by 6-fold, which is due to the greater restriction of the intramolecular torsional/rotational motions of TPE group in the linear supramolecular polymers.¹¹ As the concentration of the linear supramolecular polymers increased from 75 mM to 200 mM (above CPC of the supramolecular polymer), the fluorescent intensity did not decrease, but increased along with a 13 nm red-shift, which is consistent with other reported AIE systems.¹¹ The fluorescence intensity was further enhanced with a 79 nm blue-shift when the liner supramolecular polymer was in the solid state (Fig. S10, ESI†). The blue-shift could be explained by the morphological change of the aggregates from the amorphous to crystalline state.¹² These results indicate that the linear supramolecular polymer exhibit AIE properties both in solution and in the solid state, making it possible to use this linear supramolecular polymer as a solid supramolecular material.

Fig. 4 Fluorescence spectra of the linear supramolecular polymer (CH₃CN–CHCl₃ = 1 : 1) at different concentrations: red, 200 mM; blue, 150 mM; magenta, 120 mM; olive, 100 mM; navy, 90.0 mM; violet, 85.0 mM; purple, 80.0 mM; wine, 75.0 mM; and black, monomer 1 (200 mM).

Since 1,2,3-triazole units can form metal–ligand complexes with some metal ions, we further studied whether the supramolecular polymer can act as a supramolecular sensor. Fig. 5 shows the fluorescence response of the fluorescent supramolecular polymer to various metal cations. Upon excitation at λ_ex = 350 nm, the supramolecular polymer in the solid state displayed a strong fluorescence peak at 477 nm. No obvious fluorescence intensity changes were observed in the presence of the following metal ions: Na⁺, Mg²⁺, Ca²⁺, Co²⁺, Ba²⁺, Zn²⁺, Cd²⁺, Fe²⁺, Hg²⁺, Pb²⁺, and Ag⁺. However, a dramatic decrease in the fluorescence intensity along with an 8 nm blue-shift was found when 9 μM Pd²⁺ ions were added, which was caused by the energy transfer from the supramolecular polymer to Pd²⁺ by the coordination of Pd²⁺ with the 1,2,3-triazole group.^4f,13 The fluorescence spectral changes of the supramolecular polymer as a function of the Pd²⁺ concentration in the solid state are shown in Fig. 6. As the concentration of Pd²⁺ increased, the fluorescence intensity gradually decreased. A plot of the fluorescence intensity changes versus the concentration of Pd²⁺ was obtained and was found to fit linearly. Thus, the resulting supramolecular polymer can act as a fluorescent sensor for Pd²⁺ ions.

Fig. 5 Fluorescence spectra of the linear supramolecular polymer with different metal ions in the solid state.

Fig. 6 Fluorescence emission spectra of the linear supramolecular polymer in the presence of different concentrations of Pd²⁺ (0–10 μM) in the solid state irradiated at 350 nm. Inset: plot of fluorescence intensity changes of the linear supramolecular polymer versus varied concentrations of Pd²⁺ at 577 nm.

Conclusions

In summary, a linear fluorescent supramolecular polymer formed by crown ether-based host–guest interactions was prepared using TPE as the chromophore. The supramolecular polymer exhibits AIE properties resulting from the TPE group not only in solution, but also in the solid state, and both of them are higher than that of its monomer 1. Moreover, the fluorescence intensity decreased dramatically on the addition of Pd²⁺ due to the coordination interactions of Pd²⁺ with the 1,2,3-triazole group, which allows the fluorescent supramolecular polymer to be applied as a fluorescent sensor for Pd²⁺. This study on supramolecular sensors based on fluorescent supramolecular polymers will enrich the applications of supramolecular polymers as solid supramolecular materials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (91127032, 21174035, 21274034), Program for Changjiang Scholars and Innovative Research Team in Chinese University (IRT 1231), Excellent Young Teachers in Zhejiang Province and Hangzhou Normal University (HNUEYT 2011-01-019), the Opening Foundation of Zhejiang Provincial Top Key Discipline (no. 20121109), National Training Programs of Innovation and Entrepreneurship for Undergraduates (201310346014) and Zhejiang Province's Xinmiao Talent Plan (2014R421064).

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and full characterization of monomer 1, the conversion and fluorescence spectra of monomer 1 and the linear supramolecular polymer. See DOI: 10.1039/c4py01206b

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