Photo-reversible supramolecular hyperbranched polymer based on host–guest interactions

Abstract

A novel class of photo-responsive A₂–B₃ type supramolecular hyperbranched polymer with excellent optical properties can be polymerized and depolymerized reversibly by alternating UV/Vis light irradiation.

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As a new class of materials, supramolecular polymers with non-covalent structures have attracted significant interest in the past few decades.¹ To date, various supramolecular polymers with different architectures, such as linear,² branched,³ star-shaped,⁴ and dendronized,⁵ have been prepared based on hydrogen bonding, metal–ligand binding and host–guest interactions, which exhibit unique properties and potential applications.⁶ In contrast to conventional covalent polymers, reversibility is a basic and crucial feature of supramolecular polymers. To design and develop new supramolecular materials, the realization of reversibility is extraordinarily significant. However, up to date, the majority of supramolecular polymers can hardly undergo reversible changes and switch their aggregated nanostructures reversibly in response to certain external stimuli (e.g., light, voltage etc.). Therefore, utilization of the reversibility of non-covalent interactions to construct reversible supramolecular polymers is still a great challenge. Herein, we report a novel class of A₂–B₃ type photo-reversible supramolecular hyperbranched polymer (SHP) through the host–guest complexation of azobenzene dimer (Diazo) and β-cyclodextrin trimer (β-CD₃) (Scheme 1). As expected, the resulting supramolecular polymer possesses branched structure and excellent optical properties. More importantly, the branched structure and optical properties can be switched reversibly by ultraviolet/visible (UV/Vis) light irradiation alternatively.

Scheme 1 Schematic representation of the photo-controlled polymerization and depolymerization of a β-CD₃/Diazo supramolecular hyperbranched polymer based on host–guest interactions.

The synthetic procedure for the preparation of β-CD₃ and Diazo is given in Schemes S1 and S2†, respectively. Considering that click chemistry is a very selective and effective synthetic method with mild and clean conditions,⁷β-CD₃ was synthesized by a click reaction of an azide group in mono-(6-azido-6-deoxy)-β-cyclodextrin and three alkyne groups in tripropargylamine in the presence of copper(I) bromide (CuBr) and 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA). Diazo was prepared via quaternization reaction between 4-bromomethyl-azobenzene (Azo-Br) and 4,4′-dipyridine. The formation of β-CD₃ and Diazo is clearly evidenced by ¹H NMR and Q-TOF-MS. The detailed characterization data are described in the ESI†.

With increasing initial concentrations, the branched supramolecular polymer is expected to form as a result of intermolecular complexation between equimolar Diazo and β-CD₃ in dimethyl-formamide/water (DMF/H₂O, 1/1, v/v). As shown in Fig. 1, the 2D ¹H NMR ROESY spectrum of an equimolar solution of β-CD₃ (15 mM) and trans-Diazo (15 mM) in DMF-d₇/D₂O (1/1, v/v) shows that the signals of trans-Azo protons of Diazo are correlated with the signals of β-CD's inner protons. This result indicates the complexation between β-CD and trans-Azo moiety. After UV irradiation at 365 nm, the trans-Diazo in solution transformed into the cis form due to photoisomerization. In the 2D ¹H NMR ROESY spectrum of the resulting solution (see ESI, Fig. S5†), the correlation peaks between cis-Azo protons of Diazo and β-CD's inner protons of β-CD₃ disappear, indicating the dissociation of cis-Azo moiety from the cavity of β-CD.

Fig. 1 Partial 2D ROESY ¹H NMR spectrum of a 1/1 mixture of trans-Diazo and β-CD₃ in DMF-d₇/D₂O (1/1, v/v) at 15 mM at 30 °C. Asterisks indicate the signals of DMF-d₇ and D₂O.

In order to determine the association intensity of the trans-Azo guest/β-CD host and cis-Azo guest/β-CD host in DMF/H₂O (1/1, v/v), we performed UV-Vis measurements by detecting the UV absorption of trans-Azo-Br at 327 nm and cis-Azo-Br at 428 nm with increasing concentration of β-CD. According to the modified Hildebrand–Benesi equation,⁸ the association constants for the 1/1 inclusion complex of β-CD with trans-Azo-Br and cis-Azo-Br are 5.36 × 10³ M⁻¹ and 3.15 × 10² M⁻¹, respectively (see ESI, Fig. S6 and S7†). The result indicates that the association constant of β-CD with trans-Azo is about 17-fold higher than that of β-CD with cis-Azo, which is in good agreement with the 2D ROESY observation.

Viscometry was performed to provide direct physical evidence for the formation of a non-covalent supramolecular polymer. As shown in Fig. 2a, a double-logarithmic plot of specific viscosity as a function of the initial concentrations of equimolar solution of β-CD₃ and Diazo in DMF/H₂O (1/1, v/v) yields a curve with slope of 1.05 at low concentrations, confirming the existence of small species. At high concentrations, the curve slope increases to 2.02. Considering that the conventional supramolecular linear polymers have slopes of 3–6,⁹ the low value (2.02) suggests the formation of branched supramolecular polymers. The critical polymerization concentration (CPC), above which an SHP can be formed, was found to be 8.6 mM. Moreover, the result of dynamic light scattering (DLS) in Fig. 2b demonstrates that the hydrodynamic radius of a supramolecular polymer gradually increases from 1.3 nm to 4 nm with increasing initial concentrations, further suggesting the formation of supramolecular polymers of increasing size. After UV irradiation of supramolecular polymer solution, the hydrodynamic radius of supramolecular polymers greatly decreases (e.g., from 4 nm to 1.16 nm at 20 mM), which directly confirms the reversible depolymerization of supramolecular polymers. Combining the structures of A₂ and B₃ building blocks with the results of 2D ROESY, viscometry, and DLS analysis, it can be concluded that an A₂–B₃ type SHP is successfully prepared by self-organization of Diazo (A₂) and β-CD₃ (B₃) at relatively high concentrations according to the Flory theory concerning branched polymers.¹⁰

Fig. 2 (a) Double-logarithmic plot of specific viscosity of equimolar solution of Diazo and β-CD₃ in DMF/H₂O (1/1, v/v) versus molar concentration. Data represent mean standard deviation (n = 3). (b) Hydrodynamic radius of a supramolecular polymer before and after UV irradiation as a function of molar concentration. Data represent mean standard deviation (n = 3).

Concerning the morphology of SHP, we firstly performed atomic force microscopy (AFM) to observe their aggregated morphology. Fig. 3a presents branched structures with sizes of several microns, which is formed from the SHP aggregation viahost–guest interactions between β-CD and trans-Azo group after evaporation of supramolecular polymer solution. While polymer solution is being irradiated by UV light at 365 nm, the SHP is dissociated due to much lower association constant between β-CD and cis-Azo group. After evaporation of solution, because of high concentration, the monomers of β-CD₃ and cis-Diazo naturally accumulate together, resulting in the formation of disordered particles (Fig. 3b). Moreover, the morphology conversion of a branched aggregation structure into disordered particles can be reversibly achieved by UV/Vis light irradiation alternatively.

Fig. 3 AFM amplitude images of a photo-responsive SHP (a) before and (b) after UV irradiation.

To get the morphology of SHP in solution, the liquid-phase AFM was performed. In Fig. S10†, it is clear that an SHP in solution forms nanoparticles with a height of 2–3 nm. By comparison with the results of DLS, the reduction in size might be caused by absorption and further collapse of supramolecular polymer nanoparticles onto the mica surface.

To study the photo-controlled polymerization and depolymerization behaviors of the β-CD₃/Diazo (1/1, mol mol⁻¹) system, UV-Vis measurements were carried out. As depicted in Fig. 4a, the absorption band at 323 nm decreases dramatically, and simultaneously the absorption band at around 432 nm increases slightly after UV irradiation at 365 nm. The absorption bands at around 323 and 432 nm can be ascribed to π–π* (H-aggregate) of the trans form and n–π* (J-aggregate) of the cis form of azobenzene group, respectively.¹¹ In comparison with pure Diazo solution, the absorption intensity at around 323 nm of the β-CD₃/Diazo solution is enhanced due to the inclusion complexation between β-CD₃ and trans-Diazo. The variation in the absorption bands induced by UV irradiation is indicative of the photoisomerization of Diazo from the trans form to the cis form. By irradiation with Vis light at 450 nm, the π–π* absorption increases remarkably with a slight decrease in the n–π* absorption, indicating that the photoisomerization of Diazo undergoes a reversible change from the cis form to the trans form. Interestingly, this reversible photoisomerization process can be recycled many times by alternative irradiation of the solution with UV and Vis light (Fig. 4b).

Fig. 4 (a) UV-Vis spectra and (c) fluorescence emission spectra (λ_ex = 360 nm) of β-CD₃/Diazo (1/1, mol mol⁻¹) and Diazo in DMF/H₂O (1/1, v/v) before UV irradiation, after UV irradiation and after Vis irradiation. (b) Changes in the maximum absorbance at around 323 nm and (d) changes in the maximum emission intensity at around 440 nm upon alternating irradiation with UV and Vis light. The concentration of Diazo was kept at 2.5 × 10⁻⁵ M.

In addition, the fluorescence emission spectra of β-CD₃/Diazo (1/1, mol mol⁻¹) solution by excitation at 360 nm are shown in Fig. 4c. For pure Diazo in DMF/H₂O (1/1, v/v), the fluorescence intensity of both the trans form and cis form is very low as a result of luminescence quenching effect by the intermolecular π–π* or n–π* stacking interaction between azobenzene chromophores. There is no significant difference between them, suggesting that the intermolecular π–π* and n–π* stacking interactions of Diazo have an equivalent effect on the fluorescence intensity of azobenzene moieties. On the contrary, the fluorescence intensity of β-CD₃/Diazo solution is greatly enhanced in comparison with pure Diazo solution. β-CD group of β-CD₃ can encapsulate the azobenzene group of Diazo into its internal cavity to act as an insulating layer viahost–guest interaction. Correspondingly, the intermolecular π–π* stacking interaction between azobenzene chromophores in solution is weakened,¹² resulting in the fluorescence enhancement of β-CD₃/Diazo solution. Importantly, the fluorescence intensity is significantly different between β-CD₃/trans-Diazo and β-CD₃/cis-Diazo systems, which can be ascribed to the different association intensities of β-CD₃ with trans-Diazo and cis-Diazo. As a result, the β-CD₃/trans-Diazo system with high association constant exhibits stronger fluorescence than that of the β-CD₃/cis-Diazo system. Also, the periodic variation in fluorescence intensity of β-CD₃/Diazo solution can be achieved by alternating UV/Vis light irradiation, as shown in Fig. 4d.

In summary, a novel class of photo-responsive supramolecular hyperbranched polymer has been successfully prepared by self-assembly of azobenzene dimer and β-CD trimer. The resulting supramolecular polymer exhibits a branched structure and excellent optical properties which can be switched reversibly by alternating UV/Vis light irradiation through the reversible association and disassociation of the non-covalent connection in the backbone. This novel class of photo-reversible SHP is promising fluorescent materials as well as self-healing materials.

Acknowledgements

This work is sponsored by the National Natural Science Foundation of China (20974062, 30700175) and National Basic Research Program 2009CB930400, Shanghai Leading Academic Discipline Project (no. B202), China National Funds for Distinguished Young Scientists (21025417) and State Key Laboratory of Chemical Engineering (No. SKL-ChE-10C02).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and characterization data. See DOI: 10.1039/c1py00426c

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