Stimuli-Responsive Supramolecular Polymers from Amphiphilic Phosphodiester-Linked Azobenzene Trimers.

An amphiphilic phosphodiester-linked azobenzene trimer has been exploited in the development of stimuli-responsive, water-soluble supramolecular polymers. The trimer can reversibly undergo thermal and photoisomerization between Z- and E-isomers. Its self-assembly properties in aqueous medium have been investigated by spectroscopic and microscopic techniques, demonstrating that E- and Z-azobenzene trimers form supramolecular nanosheets and toroidal nanostructures, respectively. By virtue of the E/Z photoisomerization of the azobenzene units, the two different supramolecular morphologies can be switched by photoirradiation. The findings pave a way towards stimuli-responsive, water-soluble supramolecular polymers which hold great promise in the development of smart functional materials.

Introduction

The field of supramolecular polymers (SPs) has attracted tremendous interest although it did not emerge as a new branch in polymer science until the late 20th century. SPs are assemblies of repeating building blocks held together by non‐covalent interactions. In particular, SPs composed of aromatic amphiphilic building blocks hold great promise in biological applications due to their dynamic nature, unique physical and chemical properties and nanostructures that resemble those of living systems. Following the principles of molecular self‐assembly, a large diversity of intriguing SPs with unprecedented topologies including 2D nanosheets, 1D nanotubes, vesicles, fibers, toroids and helicoids have been reported.[ , ] Among them, stimuli‐responsive SPs are of prime importance in the development of smart and adaptive functional materials, ultimately leading to biological functions. Spurred by exciting results obtained via light‐induced unfolding–refolding of helical coils by mimicking the functions of biopolymers, we set ourselves the task of preparing an amphiphilic, phosphodiester‐linked azobenzene‐based trimer to gain external control over the nanostructures of the resultant SPs.

Our interest in azobenzene (AZB) derivatives stems from their intriguing structural features and the ability to undergo reversible isomerization between E‐ and Z‐configuration, triggered by irradiation, temperature, electric field or mechanical stress. E‐AZB is planar with a dipole moment of zero while Z‐AZB is not planar but slightly tilted with a dipole moment of 3.1 D, due to the steric repulsion by the phenyl rings. As a result, the overall length of an AZB core changes from 0.9 nm to 0.55 nm for E‐ and Z‐isomers, respectively. In addition, the Z‐isomer is more hydrophilic than the E‐isomer. Taken together, E‐ and Z‐isomers are expected to exhibit unlike self‐assembly properties in aqueous environment. AZB has frequently been used to impart responsivity into building blocks that self‐assembled into SPs with different properties and diverse nanostructures by changing dimensions and cooperativity of the self‐assembly in response to a specific stimulus.[ , , ] In close relation with the actual work, electrostatically assembled nanoparticles can, under UV‐light irradiation, undergo a significant change in size or shape. As a continuation of our ongoing work, we have prepared an amphiphilic, phosphodiester‐linked azobenzene trimer as depicted in Figure 1, which can thermally and photochemically undergo a reversible E–Z isomerization. The negative charge of the phosphate group ensures water solubility, while the hydrophobic nature of AZB units facilitates the intramolecular folding and the intermolecular assembly of the trimers in an aqueous environment. By a combination of UV/Vis spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM) techniques, it has been borne out that trimers self‐assemble into μm‐sized 2D nanosheets while trimers form toroidal nanostructures, which undergo a light‐triggered reversible interconversion, in stark contrast to the aforementioned nanoassemblies[ , , ] in terms of reversibility.

Figure 1

Reversible isomerization of to trimer upon UV‐light irradiation and, vice versa, upon irradiation with visible light or heating, and light‐triggered reversible interconversion of SPs with different morphologies.

Results and Discussion

The preparation of trimer was accomplished by a solution‐phase synthesis[ , ] using an AZB‐derived phosphoramidite followed by HPLC purification (Supporting Information). was kept as a stock solution in ethanol. As it is composed of three azobenzene segments, several isomeric states are expected: EEE, EEZ/EZE, EZZ/ZEZ, and ZZZ. At room temperature, the stock solution consists of a mixture of these isomers in a ratio of 47:39:11:3, indicative of a strong predominance of the E‐isomer (77 % overall). The photoisomerization mechanism can proceed either through out‐of‐plane rotation of the N=N double bond upon excitation to the bright S 2 (ππ*) state or in‐plane inversion at one of the nitrogen atoms upon excitation to the dark S 1 (nπ*) state. The photoisomerization from the E‐ to Z‐isomer occurs upon UV‐light irradiation at 366 nm and then the back conversion to the E‐isomer takes place either by heating above 50 °C or, in a slower process, by exposure to ambient light. The isomerization process can be readily monitored by UV/Vis spectroscopy. As depicted in Figure 2, at r.t. predominant E‐configuration (henceforth ) which strongly absorbs at 355 nm (black curve), transforms to predominant Z‐configuration ( ) upon irradiation at 366 nm (blue curve). The intense absorption bands at 355 nm and 240 nm correspond to the long‐ and short‐axis polarized π→π* transitions, respectively. A very weak band at 450 nm is attributed to the n→π* transition. Clearly, UV‐light irradiation of the stock solution for 10 min leads to an increase in the absorbance at 450 nm and a significant decrease in the absorbance at 355 nm, accompanied by a concomitant emergence of a new absorption at 320 nm. These spectral changes provide unambiguous evidence for efficient E–Z photoisomerization with a photostationary state of 90 % Z‐isomer and 10 % E‐isomer.

Figure 2

UV/Vis absorption spectra of recorded in ethanol at ambient conditions (black line), after UV‐light irradiation (blue line), and after heating to 80 °C (red line); [ ]: 5 μM. Top: Illustration of the orientation of electric transition dipole moments (short and long axis).

The heating of the solution of (10 min at 80 °C) leads to the back conversion to the thermodynamically more stable (red curve). The increase in the intensity of the π→π* band at 80 °C (red) and 20 °C (black) indicates a near quantitative thermal isomerization of Z‐isomer to E‐isomer with a photostationary state of 96 % E‐isomer and 4 % Z‐isomer. These results show that the configuration of the trimer in ethanol can be controlled by external factors: at elevated temperatures, it predominantly exists as E‐isomer, whereas the Z‐configuration is successfully induced by irradiation at 366 nm.

The self‐assembly properties of the described amphiphilic trimer were studied in a mixture of aqueous buffer (10 mM sodium phosphate buffer at pH 7.2 and 10 mM sodium chloride containing 10 % ethanol by volume). As illustrated in Figure 3, the absorption spectrum of at 80 °C (red line) resembles the one obtained in ethanol at 20 °C (black line), indicating that exists as a molecularly dissolved trimer that adopts a random coil structure. Upon cooling down to 20 °C, significant spectral changes (blue line) are observed. Firstly, the long‐axis polarized π→π* band (355 nm) is blue‐shifted by 50 nm and experiences a dramatic decrease in intensity, which serves as strong evidence for the formation of H‐aggregates.[ , ] Concomitantly, the short‐axis polarized π→π* band is red‐shifted by 10 nm, indicative of the formation of J‐aggregates. The complete disappearance of the n→π* band after the cooling of the solution in the dark unambiguously manifests the successful conversion of ‐ to .

Figure 3

Comparison of UV/Vis absorption spectra of in ethanol at 20 °C (black line), and aqueous medium (10 mM sodium phosphate buffer, pH 7.2, 10 mM sodium chloride, ethanol 10 vol %) at 80 °C (red line) and 20 °C (blue line); [ ]: 5 μM.

Figure 4 A shows the sharp emergence of an H‐band at 305 nm at the expense of the π→π* absorption band (355 nm) upon slow cooling of the solution from 70 °C to 20 °C, providing clear evidence that the supramolecular polymerization occurs via a nucleation–elongation process starting at 65 °C and reaching a plateau at 40 °C. The morphologies of the formed SPs were investigated by AFM and TEM. As shown by AFM in Figure 4 B, the self‐assembly leads to the formation of two‐dimensional (2D) objects (nanosheets), which exhibit a highly regular structure. They extend over several micrometers in length and possess a height of 2.5±0.1 nm (Figure 4 C). The shape and the micrometer size range of SPs is confirmed by TEM imaging (Figure 4 D), showing the sheet‐like objects deposited on a carbon‐coated holey copper substrate. The formed objects resemble the previously reported nanosheets obtained via supramolecular polymerization of amphiphilic pyrene trimers in terms of shape, extension, and height. It can therefore be assumed that trimers are arranged in a stair‐like fashion in SPs (illustrated in Figure 4 E), similar to said pyrene trimers. Thus, as demonstrated by UV/Vis spectroscopy and microscopic imaging, planar E‐AZB cores stack in a J‐aggregation mode along one direction and in an H‐aggregation mode along a second direction, leading to a 2D supramolecular polymerization by a cooperative nucleation–growth process.

Figure 4

A) Supramolecular polymer formation curve upon cooling of the solution from 70 °C to 20 °C (0.1 °C per minute). B) AFM image of the resultant SPs of deposited on APTES‐modified mica. C) Height profile of the cross‐section line on the AFM image. D) TEM image of SPs of . E) Graphical illustration of a stair‐like arrangement of ‐AZB chromophores in supramolecular polymers.

As expected, the cooling gradient has a significant impact on the size of the formed SPs. Fast cooling (10 °C per minute, Figure S1 ) and slow cooling (0.1 °C per minute, Figures S2 and S3) of the solution from 70 °C to 20 °C results in small (<1 μm) and large (>1 μm) nanosheets, respectively. Moreover, the AFM image of the obtained SPs after aging for several days (Figure S1C) reveals imperfections in the nanosheets due to the SPs’ dynamic nature.

Next, the spectroscopic and self‐assembly properties of were addressed. Two possible ways to obtain self‐assembled structures of are demonstrated.

The first strategy involves the direct photoisomerization of previously prepared supramolecular polymers by irradiation with UV‐light (366 nm). As depicted in Figure 5 A, two distinct changes in the absorption spectrum of SPs (black line) are observed: a gradual decrease in the intensity of the π→π* bands with a substantial red‐shift of the long‐axis polarized band from 305 nm to 320 nm, and the progressive appearance of the n→π* transition band over prolonged irradiation time. All these intrinsic spectral signatures indicate the photoisomerization from to .

Figure 5

A) Evolution of the UV/Vis absorption spectra of SPs (black line) in aqueous medium (10 mM sodium phosphate buffer, pH 7.2, 10 mM sodium chloride, ethanol 15 vol %) monitored after irradiation with UV‐light (366 nm) for 5 min (green line), 30 min (blue line) and 60 min (red line); [ ]: 5 μM. B, C) AFM images alongside the height profile of the nanostructures.

It is worth noting that E–Z photoisomerization only takes place in aqueous solutions containing 15 % or more ethanol. Figure S4 displays the influence of ethanol content in the medium on the completion of the E–Z photoisomerization process. In solutions with less than 15 % ethanol, UV‐light irradiation leads to negligible changes in the position and intensity of π→π* bands, suggesting that azobenzene chromophores essentially retain the sheet‐like molecular assembly, as evidenced by AFM images (Figure S4). Obviously, 15 % ethanol is a prerequisite to the successful isomerization via irradiation.

The resulting is relatively stable in the absence of visible light or heat sources. As illustrated in Figure 5 B, the self‐assembly of forms toroidal nanostructures with a diameter of approximately 100 nm and a height of 3 nm. In contrast to planar and nonpolar E‐AZB units, nonplanar and polar Z‐AZB units show a lower tendency for strong π–π stacking. As such, trimers are likely to form curved stacks leading to the observed toroidal nanostructures, as discussed in the literature for various types of building blocks.[ , ] Additional AFM images of the resulting toroids are depicted in Figure S5 (SI). The presence of nanoobjects in the 50–100 nm range in solution at room temperature was further corroborated by dynamic light scattering (DLS) experiments (SI, Figure S6).

An alternative approach to obtaining SPs consists in the addition of an aliquot of obtained by photoisomerization after irradiation at 366 nm in ethanol solution, to an aqueous medium followed by gentle mixing. Self‐assembly of affords nanostructures (Figures S7 and S8) with identical diameters and heights to those prepared by the aforementioned E–Z photoisomerization process. We therefore infer that the addition of an ethanol solution of leads to the spontaneous formation of SPs at room temperature. Interestingly, in this procedure the self‐assembly occurs without annealing, possibly via an isodesmic polymerization process.

To understand the thermodynamical stability of SPs, temperature‐dependent UV/Vis absorption spectra were measured by heating a solution of from 20 °C to 80 °C while keeping it in the dark (Figure 6). Up to a temperature of 40 °C, the spectrum remains essentially unchanged. Starting from 50 °C, Z‐to‐E isomerization occurs as evidenced by the occurrence of clearly defined isosbestic points at 260 nm, 325 nm and 435 nm. Both absorption bands around 320 nm and 450 nm (n→π*), characteristics of , decrease at the benefit of a strong π→π* absorption band at 355 nm, characteristics of . Thus, under the exclusion of light SPs are preserved below 40 °C and undergo disassembly and thermal isomerization to the molecularly dissolved with further increase of the temperature.

Figure 6

Temperature‐dependent UV/Vis absorption spectra of in an aqueous medium (10 mM sodium phosphate buffer, pH 7.2, 10 mM sodium chloride, ethanol 15 vol %) measured after the equilibration for 5 min at each temperature (solutions kept in the dark); [ ]: 5 μM.

The photostability and reversibility properties of SPs have been investigated by monitoring the absorption changes as a function of exposure time with appropriate light irradiation. UV/Vis absorption measurements have been performed with great caution to avoid undesired thermal conversion after different exposure times (Figure 7). Upon exposure of freshly prepared (red line) to ambient light for 30 min, significant changes are observed (blue line). A profound decrease in the n→π* absorption band accompanied with the appearance of J‐band (250 nm) and H‐band (305 nm) demonstrates the direct conversion of SPs into SPs. Upon photo‐irradiation for 120 min, no further noticeable spectral changes are observed. The final spectrum resembles that of SPs (blue line in Figure 3). Irradiation of SPs at 427 nm using an LED lamp further shows that Z–E isomerization promotes the disassembly of the toroidal nanostructures and the temporary appearance of ill‐defined aggregates. Further progression finally leads to the reassembly of the dissolved ‐AZB3 into the nanosheets (see SI, Figure S9).

Figure 7

Evolution of the UV/Vis absorption spectra of (red line) in an aqueous medium (10 mM sodium phosphate buffer, pH 7.2, 10 mM sodium chloride, ethanol 15 vol %) monitored after exposure to ambient light for 30 min (blue line), 120 min (green line) and 150 min (black line); [ ]: 5 μM.

Interestingly, multi‐layered nanosheets are formed after exposure of solution to ambient light at room temperature for 15 min, as shown in Figure 8 and Figure S10. At this point, the two individual SP morphologies (toroids on the surface of the nanosheets) are co‐existing, indicating the ongoing Z–E photoisomerization process. The height profile of the resulting nanostructures confirms that layers with a uniform thickness of ca. 2.5 nm are arranged on top of each other. The size of each layer is smaller than the one below, suggesting that individual nanosheets are formed on top of an already existing one. As demonstrated above, the hydrophobic intermolecular interactions in the forms of J‐ and H‐aggregation act cooperatively in two dimensions, leading to the formation of nanosheets upon self‐assembly of . In contrast, non‐planar and polar Z‐AZB units cannot stack in an H‐aggregation fashion due to their steric hindrance and dipole–dipole interactions forming curved stacks via J‐aggregations. Isomerization to E‐AZB enables nucleated H‐aggregation, which changes the internal order and reduces the spontaneous curvature. We hypothesize that the primary nucleation process takes place simultaneously with the growth process. During the photoisomerization process, toroids that are already adsorbed on existing nanosheets undergo transformation into small nanosheets that then serve as templates for the subsequent transformation of additional toroids.

Figure 8

A) AFM image of multi‐layered nanosheets formed after the photoisomerization from to at ambient light conditions. B) Height profile of the cross‐section on the AFM image.

Conclusion

The present paper describes the preparation and self‐assembly properties of a novel stimuli‐responsive amphiphilic AZB‐based trimer. The initial mixture of configurational isomers of in ethanol is converted into at temperatures above 50 °C or, alternatively, into by irradiation with UV‐light at temperatures below 50 °C. In aqueous medium, trimers self‐assemble into μm‐sized 2D nanosheets while trimers form toroidal nanostructures. Both nanosheets and toroids are thermally disassembled at temperatures above 50 °C. Photoisomerization of the AZB units by exposure to UV or visible light provides a means to reversibly destroy or recover 2D nanosheets. The obtained μm‐sized 2D nanosheets of ‐ can undergo UV‐light‐induced photoisomerization in an aqueous medium to form ‐ toroidal structures. During the photoisomerization from ‐AZB3 to ‐AZB3, multi‐layered nanosheets with a uniform height are observed. The findings pave a way towards new, stimuli‐responsive water‐soluble SPs which are important for the development of smart functional materials.

Conflict of interest

The authors declare no conflict of interest.

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Supporting Information

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