# Joining Two Switches in One Nano-Object: Photoacidity and Photoisomerization in Electrostatic Self-Assembly

Alexander Zika, Mohit Agarwal, Ralf Schweins, Franziska Gröhn

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Alexander Zika, Mohit Agarwal, Ralf Schweins, Franziska Gröhn. Joining Two Switches in One Nano-Object: Photoacidity and Photoisomerization in Electrostatic Self-Assembly. Chemistry - A European Journal, 2022, 29 (10), pp.e202203373. 10.1002/chem.202203373. hal-05520016

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# Joining Two Switches in One Nano-Object: Photoacidity and Photoisomerization in Electrostatic Self-Assembly

Alexander Zika,[a] Mohit Agarwal,[a, b] Ralf Schweins,[b] and Franziska Gröhn*[a]

**Abstract:** Multi-switchable supramolecular nano-objects that respond to irradiation of different wavelengths with changes in size and shape have been built from two different water-soluble molecular switches, joined by attachment to the same polyelectrolyte. Accordingly, two wavelength-specific reac-

tions, namely the excited-state proton dissociation of a photoacid and the *cis*-trans isomerization of an azo dye, are combined in one supramolecular nano-object that is stable in aqueous solution. The concept has potential in the fields of sensors, molecular motors, and transport.

## Introduction

Functional supramolecular nanoparticles that respond to an external trigger[1-4] denote an emerging field due to their great potential as carrier systems,[5-7] sensors,[8,9] and optoelectronics.[10,11] Recently, supramolecular nano-assemblies that respond to more than one trigger have elicited a growing amount of scholarly attention.[12-20] When it comes to multi-switchable assemblies in solution, the classical usage of amphiphiles is predominant, while only most recently singular examples of using different non-covalent interactions have been described.[21,22] In solid materials, one system responding to different wavelength irradiation has been reported.[23] Yet, no nano-assemblies combining two switches and responding to irradiation at different wavelengths in aqueous solution are known at all, even though this would be highly desirable, for example, for complex, triggerable drug delivery.

It is possible to effectively create supramolecular assemblies through self-assembly, using π-π interactions,[24-27] amphiphilicity,[28-31] or ionic interactions.[32-35] In recent years, we have demonstrated that electrostatic self-assembly of polyelec-trolytes and multivalent oppositely charged aromatic ions can lead to nano-objects of various shapes in solution, where the combination of ionic and π-π interactions (between counter-ions) directs the assembly formation: in particular the total Gibbs free energy ΔG, resulting from the sum of the electro-

static along with the π-π contribution, controls the aggregation number, while the ΔH/ΔS balance encodes the aspect ratio.[36,37]

Supramolecular structures have the advantage that they can be triggered by changes in pH[37-40] and temperature.[41-44] Light-responsive systems are of particular interest due to the noninvasiveness of the trigger.[45-51] Electrostatic self-assembly allowed to address the size of nano-objects in solution.[50] In the recent past, we have utilized the special property of photoacids, an excited-state proton dissociation, to trigger both, nano-object formation and size.[51-53]

Herein, we present a multi-responsive system of electrostatically self-assembled particles with double-light and pH responsiveness. The model system (Scheme 1) comprises a fourth-generation poly(amidoamine) dendrimer (G4), which is a 4 nm-sized flexible branched hydrophilic molecule carrying 62 tertiary and 64 primary protonatable amine groups as a cationic model macroion, together with the divalent anionic photoacid sodium 1-naphthol-3,6-disulfonate (1 N36S), and the photo-isomerizable di-anionic azo dye acid yellow 38 (AY38). Following excitation, the photoacid undergoes an increase in acidity of the hydroxy

<figure><img src="image_3.png" /><figcaption>Scheme 1. Schematic representation of photoacid-dye-dendrimer assemblies in different states.</figcaption></figure>

<footnote>[a] A. Zika, M. Agarwal, Prof. Dr. F. Gröhn
Department of Chemistry and Pharmacy &
Interdisciplinary Center for Molecular Materials
Friedrich-Alexander Universität Erlangen–Nürnberg
Egerlandstraße 3, 91058 Erlangen (Germany)
E-mail: Franziska.groehn@fau.de</footnote>

<footnote>[b] M. Agarwal, Dr. R. Schweins
DS / LSS, Institut Laue–Langevin
71 Avenue des Martyrs, CS 20 156, 38042 Grenoble Cedex 9 (France)</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202203373</footnote>

<footnote>© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_4.png" /></figure>

proton, whereas pH responsiveness originates from the pH-
dependent protonation of the photoacid and the dendrimer.

Results and Discussion

Assemblies are formed by mixing aqueous solutions of the
building blocks set to pH 7.0 where the dendrimer's primary
amine groups are protonated. Hence, the cationic polyelectro-
lyte associates with the anionic photoacid and azo dye. To
investigate these assemblies, we vary the loading ratio *l*,
defined as the total molar concentration of negative charges,
that is, photoacid and azo dye charges, divided by the molar
concentration of the cationic polyelectrolyte charges, and the
molar ratio *k*, defined as the molar concentration ratio of
photoacid to azo dye.

Atomic force microscopy (AFM) was performed to gain a first view on assembly sizes and size-triggering upon irradiation. Figure 1 illustrates that the irradiation with $\lambda$=300 nm leads to an increase in assembly size from $d$=(36 $\pm$ 5) nm (state A$_{1}$) to $d$=(79 $\pm$ 16) nm (state B$_{1}$) and subsequently isomerization of AY38 (state C$_{1}$) causes a size change to $d$=(257 $\pm$ 54) nm. In the last two steps, the size decreases to $d$=(198 $\pm$ 33) nm (state D$_{1}$) after light exclusion, and changing to pH 7 chemically results in particles with $d$=(64 $\pm$ 13) nm (state F). In this manner, electrostatically self-assembled particles of a certain size are formed, which can be addressed by the different triggers. The latter switching also demonstrates a reversibility. At the same time, due to structural changes upon deposition onto the surface and drying, AFM can give only a first impression, and solution characterization methods are needed in conjunction.

Dynamic light scattering (DLS) was performed to character-
ize the assembly sizes in the solution. Figure 2a shows the
results at different irradiation steps (loading ratio *l* = 5.0, molar
ratio *k* = 2.0). Directly after the nano-assemblies are formed
(state A₁), the hydrodynamic radius is *R*<sub>H</sub> = 20 nm (*σ* = 0.08).
After irradiation with *λ* = 300 nm (state B₁), the size slightly
increases to *R*<sub>H</sub> = 24 nm (*σ* = 0.07). This is attributed to both the
more highly charged photoacid and the higher protonation of
the dendrimer as the pH value decreased to 4.3 through
photoacid dissociation. Subsequent irradiation at *λ* = 380 ±
20 nm (affecting the azo dye) to state C₁ an increase to *R*<sub>H</sub> =
134 nm (*σ* = 0.07). This can be understood with the isomer-
ization to the cis state of the azo dye, in which the dye–dye
interaction is much weaker than in the trans state,<sup>[50]</sup> leading to
less stabilization and thus growth of the nano-assemblies.
Keeping the solution in the dark (state D₁), notably, the size of

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Irradiation-dependent AFM of photoacid–dye–dendrimer assemblies at *l* = 5.0 and *k* = 2.0 after deposition onto a mica surface.</figcaption></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 2.** a) Multi-switching of photoacid–dye–dendrimer assemblies: a) DLS, electric field autocorrelation functions $g_1(\tau)$, and distributions of relaxation times $A(\tau)$ at a scattering angle of $\theta$ = 90° for Cycle I. b) SLS-SANS, scattering curves and structural fits for Cycle I. c) DLS, $R_H$ of the Cycle I and II depending on the different states.</figcaption></figure>

the assemblies does not revert completely and decreases only
slightly to R<sub>H</sub> = 124 nm (σ = 0.06) due to a photoprotective
effect (to be discussed below). Changing back to pH 7.0
completes the cycle (state F<sub>i</sub>). Relative size changes correspond
to each other in DLS and AFM, whereas the structures in AFM
are substantially smaller due to the shrinking upon drying
during the AFM sample preparation (Table S1 in the Supporting
<figure><img src="image_9.png" /></figure>

Information). Next to the reversibility, DLS also showed, that this cycle can be repeated at least three times (Figure S1a).

Small-angle neutron-scattering (SANS, using D11@ ILL) and static light scattering (SLS) were combined to understand how the assemblies' shape and structure are influenced by the switching of the two molecules (Figure 2b). In the non-irradiated status, the resulting nanoparticles are spheres with a size of 100 nm (state Aᵢ). Excitation of the photoacid (state Bᵢ) again reveals a spherical geometry with an increased size of 180 nm. Subsequent isomerization of the azo dye (state Cᵢ) changes the nanoparticle shape to an ellipsoid with a length of 800 nm (major axis) and a diameter of 240 nm (minor axis). By keeping the particles in the dark and changing back to pH 7.0 chemically, the shape changes to the core-shell sphere with a total size of 280 nm and a shell thickness of 26 nm (state Fᵢ). The size does not reach 100 nm of the starting point because the azo dye transformation is not completed (see below), due to the photoprotective effect, where the electrostatic interaction with the dendrimer hinders the isomerization.[⁵⁰]

Combining two light switchable molecules allows for another cycle: For Cycle II, the irradiation order of photoacid and azo dye is reversed (Scheme 2). Isomerization of the azo dye leads to particles with $R_H=478$ nm ($\sigma=0.07$) at state Bᵢ. This is nearly twice as large as the corresponding state in Cycle I, that is, the non-irradiated photoacid does not stabilize the particles as well. Subsequent excitation of the photoacid (state Cᵢ) results in a size decline to $R_H=447$ nm ($\sigma=0.07$). The first irradiation changes the shape from the sphere to an ellipsoid with a length of 830 nm (major axis) and diameter of 270 nm (minor axis). After the second irradiation, a significant difference between both cycles can be found: In Cycle II, subsequent irradiation of the photoacid does not lead to a shape change, although the size decreases (to 780 nm (major axis) and 220 nm (minor axis)). Changing back to state Fᵢ leads to the same shape and size as in Cycle I. These shape changes are also confirmed

by considering the radius of gyration determined by SLS: The shape sensitive parameter $R_G/R_H$ shows that the assemblies are more elongated after irradiation of the azo dye in both cycles (Figure S3), while the irradiation of the photoacid does not lead to any differences. The reversibility is also confirmed by SLS.

To understand the size control and inner structure of the assemblies, ζ-potential, UV-Vis spectroscopy and isothermal titration calorimetry (ITC) were performed (Figures 3 and S4). Overall, assemblies are negatively charged and electrostatically stabilized in solution, as evidenced by the negative ζ-potential (Figure 3a). The anionic charge is caused by an overstoichio-metric binding of azo dye molecules. Together with the self-assembly thermodynamics elucidated by ITC of the ternary and, for comparison, the corresponding binary systems, it can be surmised that AY38 binds electrostatically to the dendrimer and undergoes mutual π-π interaction, while the photoacid binds electrostatically and less strongly. The photoacid itself has two binding steps: The first is endothermic and entropically driven, corresponding to the release of the initial counterions of the sulfonate groups. The second binding step is exothermic and in combination with IR spectroscopy reveals that photoacids undergo mutual hydrogen bonding. The last binding step,

<figure><img src="image_10.png" /><figcaption>**Scheme 2.** Scheme of the different shapes depending on the irradiation steps according to SANS.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 3.** a) Effective surface charge density $\sigma_{eff}$ depending on the different switching steps for Cycles I and II. b) UV/Vis spectroscopy of Cycle II.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

again an endothermic one, can be understood by the increased dendrimer concentration leading to inter-cationic dendrimer repulsion, subsequent disassembly and decrease in assembly size. The difference in binding between both anionic building blocks with the cationic dendrimer is due to the different secondary interactions and their strength.

Figure 3a considers the effective charge density derived from the ζ-potential as a function of the switching step. The dependency corresponds to the size dependency (Figure 2b); the more negative the charge density, the more total surface can be stabilized, that is, the smaller the particles, similar to the behavior of the assemblies without photoacid.[50] In the first step of Cycle I, irradiation at λ = 300 nm leads to an increase in the negative surface charge density (a decrease in its absolute value, state Bᵢ): Next to the formation of the more highly charged photoacid molecule, the excitation also leads to a pH decrease to pH 4.3, where the dendrimer is further protonated with a degree of ionization close to 90%, corresponding to 113 positive charges per G4 dendrimer.[54] Subsequent irradiation with λ = 380 ± 20 nm (state Cᵢ) leads to a further increase in surface charge density to a value close to zero, which is indicative of reduced electrostatic stabilization and, as a consequence, aggregation and growth in correspondence with the increased particle size. Upon keeping the samples in the dark and changing to pH 7.0 (state F), the surface charge density decreases back to the starting point, demonstrating the reversibility and thus, the thermodynamic control of the triggered equilibrium self-assembly. Regarding Cycle II, the direct isomerization of the azo dye (state Bᵢ) leads to an increase in the surface charge density to an even higher value (a negative surface charge density with a smaller absolute value) than in Cycle I. This difference is caused by the photoacid which has only two negative charges in Cycle II, providing less electrostatic stabilization than in Cycle I. Next, the excitation of the photoacid (state Cᵢ) yields a slight further decrease in surface charge density. Further, Cycle II corresponds to Cycle I.

UV-Vis absorption spectra given in Figure 3b elucidate the underlying molecular changes of the nano-assemblies. In Cycle II, irradiation at λ = 380 ± 20 nm (state Bᵢ) shows a large influence on the isomerization, as evidenced by the appearance of two bands at λ = 322 and 445 nm. Table 1 summarizes the azo dye's degrees of isomerization in the nano-assemblies. The amount of cis conformation is 91%, while dark storage and subsequent pH change (state Fᵢ) lead to back-isomerization to trans with a degree of 85%. Thus, following the completion of Cycle II, the spectra resemble those prior to irradiation. Comparing Cycle I (state Cᵢ) and Cycle II (state Bᵢ), the isomerization in Cycle I takes place to a higher degree. Due to the lower affinity of the cis dye to the dendrimer in comparison to the trans isomer, there is less repulsion for the photoacid to

interact with the dendrimer. Interestingly, a more expressed change is observed in Cycle I for the isomerization to the trans configuration in the dark, which happens less for Cycle I (state Dᵢ and Dᵢᵢ). This is attributed to the photoprotective effect, where the azo dye is kept in its state due to electrostatic interaction with the dendrimer.

Further, it is necessary to develop a fundamental understanding of the size and shape changes. Table 2 summarizes the enthalpies for the ternary system investigated herein as well as for the binary systems and dye dilution experiments to determine the π-π interaction, obtained by ITC. ITC of the ternary system revealed (Figure S4a) that the azo dye binds exothermically and overstoichiometrically through electrostatic and π-π interactions with the dendrimer, in analogy to the binary dendrimer-Ay38 system (Figure S4c, and measured earlier[50,55]). By contrast, the photoacid binds less strongly and electrostatically only (with some H-bonding contribution, Figure S4a and b and Scheme 3). Compared to the binary systems, the binding enthalpies are less negative for both the anionic building blocks attaching to the dendrimer, which, in turn, is attributed to the electrostatic repulsion with the third building block.

Thus, understanding the size changes qualitatively is quite straight forward: ITC and ζ-potential show that the cis dye binds to the dendrimer less strongly and to a lesser extent than the trans dye (in the ternary system here in analogy to binary systems measured earlier[50,55]), while the photoacid increases its valency upon irradiation and subsequently attaches more strongly. Accordingly, the charge of the assembly changes, which leads to an adjustment in the size of the electrostatically stabilized particles. Put differently, the aggregation number is controlled thermodynamically in analogy to a previous study on

<table><caption>**Table 1.** Degree of dye isomerization from *trans* to *cis* and from *cis* to *trans* as derived from UV-Vis spectroscopy.</caption><thead><tr><th>Head 1[a]</th><th>trans→cis</th><th>cis→trans</th></tr></thead><tbody><tr><td>Cycle I</td><td>92%</td><td>87%</td></tr><tr><td>Cycle II</td><td>85%</td><td>91%</td></tr></tbody></table>

<table><caption>**Table 2.** Binding enthalpies measured by ITC for AY38-1 N36S–G4 assemblies and comparison systems.</caption><thead><tr><th>System</th><th>Process</th><th>ΔH [kJ mol<sup>-1</sup>]</th><th>Interaction</th></tr></thead><tbody><tr><td rowspan="3">G4-AY38-1 N36S dark and irradiated</td><td>1 N36S binding to G4</td><td>-11.8</td><td>total (electrostatic + H bonding)</td></tr><tr><td>trans-AY38 binding to G4</td><td>-37.3</td><td>total (electrostatic + π-π)</td></tr><tr><td>cis-AY38 binding to G4</td><td>-20.0</td><td>total (electrostatic + π-π)</td></tr><tr><td rowspan="2">G4-AY38 dark and irradiated</td><td>trans-AY83 binding to G4</td><td>-42.5</td><td>total (electrostatic + π-π)</td></tr><tr><td>cis-AY38 binding to G4</td><td>-26.2</td><td>total (electrostatic + π-π)</td></tr><tr><td>G4-1 N36S</td><td>1 N36S binding to G4</td><td>-53.9</td><td>total (electrostatic + H bonding)</td></tr><tr><td rowspan="4">AY38 into water (comparison for π-π interaction) dark and irradiated G4-AY38-1 N36S (calcd)</td><td>trans-AY38 self-stacking</td><td>-29.3</td><td>π-π trans-AY38</td></tr><tr><td>cis-AY38 self-stacking</td><td>-2.0</td><td>π-π cis-AY38</td></tr><tr><td>trans-AY38 binding to G4 electrostatic contribution</td><td>-8.0</td><td>trans-AY38-G4 electrostatic</td></tr><tr><td>cis-AY38 binding to G4 electrostatic contribution</td><td>-18.0</td><td>cis-AY38-G4 electrostatic</td></tr></tbody></table>
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Scheme 3.** Schematic overview on assembly formation and internal structure (state A).</figcaption></figure>

non-switchable systems of ours.[36] This is further substantiated by the fact that mixing the uncharged building blocks at pH 10 and changing the pH to 7.0 afterwards, that is charging the building blocks and initiating the assembly, results in the same nano-objects (Figure S6).

In stark contrast, previous structure-directing effects do not hold for the particle shape. The strong alteration from spherical to elongated ellipsoids upon isomerization of the AY38 is especially significant. Against this backdrop, it is remarkable that the $\Delta H$ of the cis-AY38 $\pi$-$\pi$ interaction ($\Delta H = -2.0 \text{ kJ mol}^{-1}$) is much weaker than for the trans-AY38 $\pi$-$\pi$ interaction ($\Delta H = -29.3 \text{ kJ mol}^{-1}$). In fact, the cis isomer shows nearly no $\pi$-$\pi$ interaction, which most likely is due to steric hindrance in the cis form. Thus this is in contrast to the previous finding where stronger $\pi$-$\pi$ interaction (a more negative $\Delta H_{\text{dye-dye}}$) promoted anisotropy on the nanoscale.[36] In this case, the anisotropic shape of the assemblies herein is not controlled by secondary $\pi$-$\pi$ interactions. Instead, it must be ionic effects that encode the shape.

Figure 3a makes it evident that the effective surface charge density (here related to a sphere with the radius $R_H$) assumes significance and smaller particles need a higher charge density to be stabilized. On the one hand, in analogy to classical colloid and nanoparticle science (though we study hydrophilic assemblies from water-soluble building blocks here), the energy needed to create a surface is proportional to this surface, that is, here to the particle surface. On the other hand, the Poisson-Boltzmann equation and Debye-Hückel approximation suggest that for smaller particles, the electrostatic potential can be approximated to scale with the charge. For this reason, Figure 4a plots both the particle surface (obtained from SANS) and the effective particle charge (derived from the $\zeta$-potential) as a function of the aggregation number (from SANS), in conjunction with the shapes of the particles identified. It is also noteworthy that both curves overlay almost exactly, thus ascertaining the importance of the charge-surface relation (rather than with the $\zeta$-potential or the surface charge density).

It is evident that an increase of surface required for the purpose of stabilization with charge occurs steeply; for higher aggregation numbers it is no longer possible to increase the size of a sphere only, but particles must adopt a geometry with

<figure><img src="image_15.png" /><figcaption>**Figure 4.** a) Particle charge $Q_{\text{eff}}$ (measured from the $\zeta$-potential) and particle surface A (measured by SANS) as function of aggregation number (measured by SANS). b) Enthalpy-entropy compensation plot for light-switchable binary and ternary systems; orange circles: AY38-G4 dendrimer, green triangles: AY38-1 N36S-G4, gray squares: various azo dyes from previous studies.[36]</figcaption></figure>

a larger surface and ellipsoids with a higher surface per aggregation number (particle volume) form. Here, the aggregation number is determined thermodynamically. At the same time, the thermodynamically encoded aggregation number must geometrically fit in terms of surface/volume ratio, that is, in terms of particle shape.

From a molecular point of view, the thermodynamic binding equilibria of the connecting ions to the macroion, based on all the molecular effects – ionic binding, $\pi$-$\pi$ interaction, geometric effects – cause a certain aggregation number and a certain net charge. Thus, upon irradiation and subsequent isomerization from *trans* to *cis*, the increase in size and change from a spherical to an ellipsoidal shape is due to the “missing” $\pi$-$\pi$ interactions.

Furthermore, Figure 4b pictures the enthalpy-entropy master plot defining regions of isotropic and anisotropic nano-objects, in analogy to the one that was previously introduced by us (with prior data points shown in gray).[36] It pictures the enthalpic versus the entropic contribution to the free energy change for the binding of one small molecule to the macroion. Evidently, despite a totally different molecular and nanoscale mechanism for particle shape control, this master plot still holds for the system herein. Self-assembly with a large gain in enthalpy but a negative change in entropy leads to spherical particles, while self-assembly with smaller enthalpy gain and larger entropy gain (i.e., smaller entropy loss or entropy gain).
<figure><img src="image_16.png" /></figure>

leads to the formation of elongated, anisotropic particles. While there is some lack of clarity with regard to the details of this scenario, this might point to a very general connection of nanoscale particle shape with interaction thermodynamics, which might be widely applied to understand the shape of self-assembled nanoparticles.

## Conclusions

In conclusion, we have demonstrated in this study that the ternary electrostatic self-assembly of a photoacid, an azo dye, and a cationic dendrimer leads to a novel multi-switchable system in aqueous solution. By joining two different ionic molecular switches through a polyelectrolyte, the resulting nano-objects can respond to changes in pH and different irradiation wavelengths with changes in size and shape. The supramolecular assemblies exhibit a size between R<sub>H</sub>=20 and 134 nm and shapes ranging from spheres via core-shell spheres to ellipsoids. Nanoscale size and shape changes are caused by excitation-dependent photoacid dissociation and azo dye isomerization. Particle sizes can be understood according to the extent of overstoichiometrically π-π bound dye molecules. Importantly, it is the particle surface required to accommodate a certain (particle) charge and volume that determines particle shapes. Thermodynamic data reveal an enthalpy-entropy master plot with spherical and elongated structures falling into different regions that, in turn, may be widely valid for shapes of supramolecular nano-objects. This new type of electrostatically self-assembled multi-switchable nanoparticles could pave the way for new sensors and molecular transport.

## Experimental Section

Essential experimental procedures/data, a comparison of AFM and DLS, reversibility and UV/Vis of cycle I and autocorrelation function and SANS of cycle II, SLS results of both cycles, ITC measurements, and a comparison of different synthesis procedure can all be found in the Supporting Information [z1].

## Acknowledgements

We gratefully acknowledge the German Science Foundation (DFG) for funding, the Institut Laue-Langevin (ILL), France, for the beamtime, and the Partnership for Soft Condensed Matter (PSCM) for providing us with light scattering instrument support. This work benefited from the use of the SasView application (www.sasview.org), originally developed under NSF award DMR-0520547. The DOI for the beamtime on D11 at the ILL is: 10.5291/ILL-DATA.9-11-2077. Open Access funding enabled and organized by Projekt DEAL.

## Conflict of Interest

The authors declare no conflict of interest.

**Keywords:** azo dyes · electrostatic self-assembly · light responsiveness · multi-switchable nano-objects · shape change

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Manuscript received: October 31, 2022

Accepted manuscript online: November 6, 2022

Version of record online: December 27, 2022