light‐switchable azobenzene‐containing macromolecules

FEATURE ARTICLE

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Light-Switchable Azobenzene-Containing Macromolecules: From UV to Near Infrared

Philipp Weis and Si Wu*

P. Weis, Dr. S. WuMax Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz, GermanyE-mail: [email protected]

DOI: 10.1002/marc.201700220

the large amount of soldiers.[1] In addi-tion, switching only the end group of a polymer can change the conformation and properties of the whole polymer.[2] Furthermore, switching a polymer side chain can stimulate the motions of the polymer backbone.[3]

Different external stimuli such as light,[4] heat,[5] and pH,[6] have been used to switch polymers. Among these stimuli, light is a clean and non-contact stimulus, provides high spatiotemporal resolution, and can be switched on and off fast. Macromolecules, which can be switched using light, are photoswitch-able macromolecules. A photoswitch-able macromolecule usually exhibits two forms: “A” and “B”. While A is respon-

sive to a wavelength range, B is responsive to another wave-length range. A and B can be interconverted into each other by light irradiation with distinct wavelengths. One of the most prominent photoswitchable units incorporated into macromol-ecules is azobenzene (Figure 2). Azobenzene has a stable trans isomer and a metastable cis isomer. Trans-to-cis isomerization is induced by UV light irradiation and cis-to-trans isomeriza-tion can be induced by visible light irradiation or heating. Photoswitching of azobenzene in solution is usually very effi-cient. However, small molecule azobenzene compounds are usually crystalline, which are difficult to be switched with light in the solid state. Introducing azobenzene into macromolecules can facilitate photoisomerization in the solid state. In addition, polymer matrices make azobenzene processable and macro-molecular architectures endow azobenzene with new functions and applications.[1,3,7]

We use the term “azo-macromolecules” to represent azopoly-mers and azobenzene-functionalized (bio)macromolecules such as peptides and proteins. Conventional azo-macromolecules are UV-light responsive. However, UV light cannot penetrate into tissue deeply and may damage biological components. In addition, sunlight contains only ≈5% UV light. Furthermore, UV light may damage organic materials or polymers due to photooxidation or other UV-induced side reactions. Therefore, conventional UV light-responsive azo-macromolecules are problematic for the above-mentioned applications. Recently, visible and near-infrared (NIR) light-responsive azo-macromol-ecules have been developed to solve the problems of UV light-responsive azopolymers.

We will not comprehensively review azopolymers here because there are comprehensive and very good reviews during the development of azopolymers.[1,3,7] In this Feature Article,

Stimuli-Responsive Polymers

Azobenzene-containing macromolecules (azo-macromolecules) such as azobenzene-containing polymers (azopolymers) and azobenzene-functional-ized biomacromolecules are photoswitchable macromolecules. Trans-to-cis photoisomerization in conventional azo-macromolecules is induced by ultra-violet (UV) light. However, UV light cannot penetrate deeply into issue and has a very small fraction in sunlight. Therefore, conventional azo-macromol-ecules are problematic for biomedical and solar-energy-related applications. In this Feature Article, the strategies for constructing visible and near-infrared (NIR) light-responsive azo-macromolecules are reviewed, and the potential applications of visible- and NIR-light-responsive azo-macromolecules in biomedicine and solar energy conversion are highlighted. The remaining chal-lenges in the field of photoswitchable azo-macromolecules are discussed.

1. Introduction

The success of molecular machines has motivated polymer chemists to synthesize macromolecular machines. Molecular machines contain switchable units for the control of their motions and functions, and this concept can be applied to macromolecular machines. Macromolecular machines also need switchable units to drive them to work. The develop-ment of switchable macromolecules by introducing switchable units into polymers or biomacromolecules is a topic of intense investigation. Switchable macromolecules have a variety of architectures (Figure 1) such as homopolymers, copolymers, crosslinked polymers, telechelic polymers, hyperbranched polymers, dendrimers, and supramolecular polymers. Com-pared with small molecules, macromolecules have advanced and hierarchical structures, which provide many possibilities to induce switchable units into macromolecules. Different from switchable small molecules, switchable macromolecules may have several segments that have cooperative, collective, or syn-ergistic motions. These motions may endow macromolecules with some interesting properties and functions that do not exist in small molecules. For example, a copolymer contains no more than 10% switchable units as “commanders” and more than 90% non-switchable units as “soldiers”.[1] The switching of the small amount of commanders can guide the motions of

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we focus on visible- and NIR-light-responsive azo-macromol-ecules. We present the current state of visible and NIR-light-responsive azo-macromolecules, their potential applications, and the remaining challenges.

2. Azo-Macromolecules

2.1. UV-Light-Responsive Azo-Macromolecules

Before we discuss newly developed visible- and NIR-light-responsive azo-macromolecules, we use UV-light-responsive azo-macromolecules as examples to introduce fundamentals and general properties of azo-macromolecules.

A typical UV-light-responsive azopolymer in the trans form has a strong π−π* absorption band in the UV range and a weak n–π* band in the visible range (Figure 3a). UV irradiation can induce trans-to-cis photoisomerization. The cis isomer has a stronger n–π* band in the visible range, making the cis-to-trans back isomerization feasible using visible light. The reversible photoisomerization not only changes the color of the azopoly mer but may also influence properties.

In a pioneer work, Ikeda et al. demonstrated that a trans azopolymer exhibited liquid crystalline (LC) phases, while the cis form of the same polymer was amorphous.[8] Light can induce photoisomerization and anisotropic-to-isotropic phase transition in the azopolymer. Many LC azopolymers show photoinduced phase transitions, which show applications in photopatterning and information storage (Figure 3b).

Recently, we demonstrated that the glass transition tem-perature (Tg) of the azopolymer 1 can be switched by light (Figure 3c).[9] Trans 1 was a solid with Tg above room tempera-ture, whereas cis 1 was a liquid with Tg below room tempera-ture. UV light irradiation of trans 1 could transform it into cis 1 and therefore induce a solid-to-liquid transition at room tem-perature (Figure 3c). Photoswitchable Tg is useful for applica-tions such as light-healable coatings, reducing of surface rough-ness, and transfer printing.

Azopolymers also show applications as photoactuators. Irradiating azopolymer elastomers with UV light may induce contraction,[10] bending,[11] twisting,[12] and oscillation.[13] Photoactuators are promising for solar energy conversion and mechanical devices.

Philipp Weis studied chem-istry at Johannes Gutenberg University of Mainz and spent an exchange semester at the Department of Materials Science and Engineering, Cornell University. He finished his diploma thesis on photoresponsive polymers in the group of Dr. Si Wu in the department of Prof. Hans-Jürgen Butt at the

Max Planck Institute for Polymer Research. He is currently working as a PhD student on stimuli-responsive polymers in the same group, under the supervision of Dr. Si Wu.

Si Wu is a group leader at the Max Planck Institute for Polymer Research (MPIP), Mainz, Germany. He obtained his bachelor’s degree in polymer science in 2005 at the University of Science and Technology of China (USTC), Hefei, China. He was supported by the joint doctoral promotion program working on photoresponsive

polymers at MPIP and USTC. After he received his PhD in 2010, he worked as a postdoctoral researcher at MPIP. He was promoted to a group leader at MPIP in 2012, and is now heading an experimental group focusing on the syn-thesis of photoresponsive polymers and photoresponsive nanomaterials.


Figure 1. Different architectures of switchable macromolecules: a) switchable side-chain homopolymer; b) switchable side-chain copolymer; c) switchable crosslinked polymer; d) switchable supramo-lecular polymer; e) switchable main-chain homopolymer; f) switchable main-chain copolymer; g) switchable end-capped polymer; h) switchable dendrimer; and i) switchable hyperbranched polymer.

Figure 2. Photoisomerization of azobenzene.



Azobenzene was proposed to be used as a solar thermal fuel.[14] The trans isomer can absorb light and be converted into the cis isomer, while the cis isomer stores the photo energy. The cis isomer can release the stored energy as heat. Recently, Grossman and co-workers demonstrated the use of an azopoly mer as solar energy storage.[15] The use of azopolymers provides the opportunity to modify the structures of the monomers and polymer backbone to enhance the stored energy density, improve the optical chargeability and photostationary state, and collect photons across a greater portion of the solar spectrum.

Azopolymers have been used to construct nano-carriers such as micelles and vesicles.[16] These nano-carriers have poten-tial applications in light-controlled drug delivery. In an early work, Zhao and co-workers synthesized amphiphilic diblock

copolymers that contain a hydrophobic azobenzene-containing block and a hydrophilic poly(acrylic acid) block (Polymer 2 in Figure 4).[16a] Polymer 2 formed micelles in water/dioxane mix-tures. UV light induced trans-to-cis isomerization and micelle-to-solution transition, while visible light induced cis-to-trans isomerization and solution-to-micelle transition (Figure 4b). The reversible morphology change makes azobenzene-containing block copolymers interesting for light-induced drug release.

The functions of biomacromolecules such as proteins and peptides are strongly dependent on their conformation. Azoben-zene has been introduced into biomacromolecules for photo-controlled structural changes.[17] Trans–cis photoisomerization of azobenzene bound to biomacromolecules induced conforma-tion change. Azobenzene-functionalized biomacromolecules provide a platform to control biological systems with light.[18]

Azopolymers are also building blocks for supramolecular materials. UV-light-responsive hydrogels, colloids, self-healing materials, and surfaces have been prepared using azopolymer based supramolecular assemblies.[19]

UV-light-responsive azo-macromolecules are good model systems for fundamental studies. They are also used for some applications in which UV light is suitable. However, they are problematic for biomedical or solar-energy-related applications. In contrast, visible- and NIR-light-responsive azo-macromole-cules are better suited for these applications.

2.2. Visible and NIR-Light-Responsive Azo-Macromolecules

Introducing substituents to azo groups in macromolecules can change the absorption bands. How substituents influence absorp-tion of small molecule azobenzene compounds have been well documented.[20] Therefore, visible and NIR-light-responsive azo-macromolecules can be prepared by introducing corresponding small molecule azobenzene compounds to macro molecules. In this section, we introduce azo-macromolecules which can be switched using visible or NIR light via one-photon process.

2.2.1. Amino-Type Azopolymers

The amino-type azopolymer is a kind of visible light-respon-sive azopolymer, which contains an amino group as an elec-tron-donating substituent.[3] Polymer 3 is a typical amino-type azopolymer (Figure 5).[21] Compared to conventional UV-light-responsive azopolymers, its π−π* band redshifts to the visible range (absorption maximum: 406 nm). Due to this redshift, the π−π* band and the n–π* band partially overlap, making it dif-ficult to separately trigger trans-to-cis or reverse isomerization. The spectral overlap is good for inducing trans-cis-trans cycling using a wavelength at the overlapped range. Orientation of the azo groups in a film of 3 can be induced using linearly polar-ized blue light.[21]

2.2.2. Push–Pull-Type Azopolymers

Another type of visible- or NIR-light responsive azopolymers is push–pull-type azopolymers. The azo groups of push–pull


Figure 3. a) Absorption spectra of a typical azopolymer in trans and cis forms. b) Polarized optical microscope images of an anisotropic azo-polymer film before irradiation, after masked UV irradiation, after subse-quent visible light irradiation, and after thermal annealing. c) Chemical structures and optical microscope images of the azopolymer 1 in trans and cis forms. Adapted with permission.[9] Copyright 2016, Nature Pub-lishing Group.



azopolymers have electron donating and withdrawing substituents.[3] The π−π* bands of push–pull-type azopolymers are at visible or NIR range.[22] Polymer 4 is a typical push–pull-type azopolymer (Figure 6). The π−π* band of the trans isomer is at ≈500 nm, which completely overlaps with the n–π* band of the cis isomer.[23] Therefore, irradiating the push–pull-type azopolymer using visible light induces both trans-to-cis and cis-to-trans isomerization simultaneously. This property is good for photoinduced orientation and photoinduced surface relief grat-ings, which are based on trans-cis-trans cycling. It is difficult to obtain a cis-rich state for push–pull-type azopolymers because of the spectral overlapping and the short half-life of the cis isomer.

In some early studies, push–pull-type azo chromo-phores were covalently bound to polymers.[22,24] Later, push–pull azo dyes were also induced to polymers via doping or supramolecular interactions.[25] The supramolecular strategy can prepare homogenous polymeric materials and avoid com-plicated synthesis.[26] For example, Priimägi et al. prepared

polymer films that contain azo dyes through hydrogen bonding (Figure 7).[27] Hydrogen bonding suppressed dye aggregation with the dye concentration as high as 30 wt%. Linearly polar-ized light induced birefringence in the polymer film. Hydrogen bonding reduced mobility of the molecules in the anisotropic film, and therefore the photoinduced birefringence had long-term stability. Another advantage of supramolecular azopoly-mers is that the content of azo dyes is tunable. It is easy to adjust the dye concentration for applications which require either a high or low content of azo dyes.


Figure 4. a) Chemical structure of the amphiphilic azobenzene block copolymer 2. b) Scheme of photoinduced reversible solution-to-micelle/vesicle transition. Adapted with permission.[16a] Copyright 2004, Amer-ican Chemical Society.

Figure 5. UV-vis absorption spectrum of a typical amino-type azopolymer 3. The inset shows the chemical structure of 3. Adapted with permis-sion.[21] Copyright 2008, The Royal Society of Chemistry.

Figure 6. UV-vis absorption spectrum of a typical push–pull type azo-polymer 4. The inset shows the chemical structure of 4. Adapted with permission.[23] Copyright 2011, The Royal Society of Chemistry.

Figure 7. Visible light-responsive supramolecular azopolymer. a) Chem-ical structure of 5; b) absorption spectra of supramolecular azopolymers with different azo contents. Adapted with permission.[27] Copyright 2007, AIP Publishing LLC.



2.2.3. Tetra-Ortho-Substituted Azo-Macromolecules

Both amino-type and push–pull-type azopolymers have short half-lives in the cis form; they are not suitable for applications that require long half-lives in the cis form. To solve this problem, a new type of visible light-responsive azobenzene, tetra-ortho-substituted azobenzene, has been invented recently.[28] The substituents are methoxy, F, Cl, or Br. In tetra-ortho-substituted

azobenzenes, the n–π* band of the trans isomer redshifts and separates from the n–π* band of the cis isomer. There-fore, green or red light can induce trans-to-cis isomerization by exciting the n–π* band of the trans isomer; blue light can induce the reverse isomerization by exciting the n–π* band of the cis isomer. Importantly, the cis isomer of a tetra-ortho-sub-stituted azobenzene has a long half-life (up to 2 years at room temperature).[28b,29] Tetra-ortho-substituted azobenzenes have


Figure 8. Ortho-substituted azobenzene incorporated into macromolecules/polymers for biomedical applications. a) Visible light switching a peptide crosslinked by a tetra-chloro-substituted azobenzene (Compound 6) for bioimaging in vivo. b) Protein release from a supramolecular hydrogel con-structed by a tetra-ortho-methoxy-substituted azopolymer (7) and a β-cyclodextrin-functionalized polymer (8) controlled by red light. The photograph shows the hydrogel in water before and after red light irradiation, which releases the protein. Panel (a) adapted with permission.[28c] Copyright 2013, American Chemical Society. Panel (b) adapted with permission.[31] Copyright 2015, The Royal Society of Chemistry.



been introduced into polymers or biomacromolecules such as proteins or peptides. Their responsiveness to red light opens an avenue toward deep-tissue biomedical applications.

Woolley and co-workers crosslinked a peptide with a tetra-ortho-substituted azobenzene (Figure 8a).[28c] Fluorescein was also covalently bound to the peptide. Fluorescein was chosen since its emission spectrum overlaps to different extents with the n–π* bands of the trans and cis isomers of the tetra-ortho-substituted azobenzene; thus, fluorescence should be quenched to different extents by the trans and cis isomers via fluorescence resonance energy transfer. Switching the fluorescence with red light for in vivo applications was demonstrated (Figure 8a).

Han and coworkers grafted tetra-ortho-methoxy-substi-tuted azobenzene to a polyacrylate backbone.[30] This random copolymer consisting of hydrophobic azobenzene units and hydrophilic acrylic acid units can self-assemble into nanoparticles in water. Trans-to-cis and cis-to-trans reversible photoisomerization in this polymer was induced with green and blue light, respectively.

Our group prepared a red-light-responsive supramolec-ular hydrogel consisting of a tetra-ortho-methoxy-substituted azopolymer (Polymer 7) and a β-cyclodextrin functionalized polymer (Polymer 8) (Figure 8b).[31] A trans azo group in 7 formed a host-guest complex with a β-cyclodextrin in 8, while the cis isomer had very weak interaction with β-cyclodextrin. Red light irradiation on the supramolecular hydrogel induced photoisomerization and disassembly of the supramolecular complex, which resulted in a gel-to-sol transition. The hydrogel was used as a protein carrier. Proteins loaded in the hydrogel were released after red light irradiation (Figure 8b). This type of red light-responsive supramolecular complex was also used as valves for red light-controlled drug release from mesoporous silica nanoparticles.[32]

Tetra-ortho-methoxy-substituted azopolymers also show potential applications in reversible photopatterning. We synthesized a tetra-ortho-methoxy-substituted azobenzene homopoly mer (Polymer 9 in Figure 9a).[33] Red and green light can switch 9 from trans to cis and blue light can induce reverse isomerization. Thus, switching 9 with visible light prevented photodamage caused by UV light. In addition, the azo chromophores in 9 do not π−π stack, which make photo-switching in solid state more efficient. Therefore, fully revers-ible photo patterning was achieved in a film of 9 (Figure 9b,c). In the pristine film no pattern is observable. After irradi-ating the film through a mask with red light, a pattern was written in the film that can be erased by blue light irradia-tion. This irradiation cycle was repeated for several times to demonstrate reversible photo patterning with visible light (Figure 9c).

Another application of polymer 9 is solar thermal fuel. Solar thermal fuels are materials that absorb solar light and store the energy chemically. For example, trans azobenzene can be excited by light and transition to cis azobenzene, a higher energy isomer. The cis isomer can store the energy difference between the trans and cis states and release it as heat. In our recent work, 9 was combined with the UV-light-responsive azopolymer 10, a dye and a UV-pass-filter to create a solid state solar thermal cell (Figure 10a).[34] Solar irradiation triggered trans-to-cis photoisomerization of both azopolymers. The dye and UV-pass-filter blocked wavelengths which can induce cis-to-trans back isomerization. In the solar thermal cell, 10 stores the photo energy of UV light, a small portion of sun light; 9 stores the photo energy of visible light, a major part of sun light. The use of both 9 and 10 for simultaneous UV and visible light storage enhances the efficiency of the solar thermal cell.


Figure 9. Visible light-responsive azopolymer for reversible photopatterning. a) Chemical structure of 9; b) Absorption spectrum of trans and cis 9. c) Optical microscopy images showing photopatterning cycles of a film of 9. Irradiating the film with red light through a mask writes a pattern into the film which is erased by blue light irradiation. Adapted with permission.[33] Copyright 2016, American Chemical Society.



Another sunlight-driven azopolymer system was prepared using the azo cross-linker 11 and LC monomers (Figure 10b).[35] Compound 11 showed trans-to-cis and cis-to-trans isomeriza-tion under green and blue light irradiation, respectively. The LC polymer film incorporated with 11 showed chaotic oscilla-tion under sunlight irradiation (Figure 10b). This work dem-onstrated that it is feasible to directly convert solar energy to mechanical energy.

Some visible light-responsive tetra-ortho-substituted azoben-zenes such as tetra-thiol,[36] protonated tetra-methoxy,[37] and tetra-isopropoxy[38] have not been introduced into macromolecules yet. By changing the tetra-ortho-substituted

groups, the absorption bands are tunable and the resistance of azo-macromolecules in biological environment can be improved.

2.2.4. Biomacromolecules with Bridged Azobenzenes

Bridged azobenzene is another type of visible light-responsive azobenzene.[39] Opposite to conventional azobenzene, the cis isomer of the bridged azobenzene is the thermodynamically more stable form. Separated absorbance bands of cis and trans isomers in bridged azobenzene enabled almost complete photo-switching in both directions. Bridged azobenzene was incor-porated into a peptide (Figure 11).[40] Visible light was used to control the conformation of the peptide, as demonstrated by circular dichroism (CD) spectra of the trans azo peptide, which forms a helix compared to the cis azo peptide which forms a coil. In addition, the cis isomer had high resistance against reduction by glutathione, making it useful for biomedical applications.

2.3. Red- or NIR-Light-Responsive Azopolymers Based On Nonlinear Optical Processes

Section 2.2 shows switching azo-macromolecules with visible or NIR light based on a one-photon process. Visible or NIR light-switching of azopolymers can be also achieved using indi-rect ways based on nonlinear optical processes.

2.3.1. NIR Photoswitching of Azopolymers Based On Two-Photon Absorption

NIR photoswitching of azobenzene can be achieved via simul-taneous two-photon absorption.[41] UV-light-responsive azoben-zene (e.g., absorption at 365 nm) can simultaneously absorb two NIR photons (e.g., λ = 730 nm), which induce photo-isomerization. The advantage of two-photon absorption is that it has high spatial resolution because it only occurs at the laser focus. For example, Ambrosio et al. fabricated gratings on a film of azopolymer 18 using a pulsed laser at 800 nm via two-photon absorption (Figure 12).[42] Using this method they produced structures down to 250 nm, which is far below the half-wavelength diffraction limit of the laser beam, and diffrac-tion gratings with periods between 500 nm and 2 µm. How-ever, two-photon absorption is not efficient even when pulsed lasers are used. Furthermore, two-photon absorption is a spot-by-spot process that is slow and not suitable for photoswitching of macro scopic samples.

2.3.2. NIR Photoswitching of Azopolymers Based On Upconverting Nanoparticles

Photon upconversion is more efficient than simultaneous two-photon absorption.[43] Recently, NIR photoisomerization assisted by lanthanide-doped upconverting nanoparticles (UCNPs) have been developed.[44] UCNPs are inorganic NPs that can convert


Figure 10. Visible light-responsive azopolymers for solar-energy-related applications. a) A solar thermal cell fabricated using a dye layer, a layer of the visible light-responsive azopolymer (9), a UV-pass filter, and a layer of the UV-light-responsive azopolymer (10). The two filters block unwanted wavelengths, while the azopolymers store light as chemical energy. b) LC polymer film crosslinked by a tetra-ortho-fluoro-substituted azobenzene (11). Near a window the self-standing film acts as a chaotic sunlight driven actuator. Panel (a) adapted with permission.[34] Copyright 2016, Wiley-VCH. Panel (b) adapted with permission.[35] Copyright 2016, Nature Publishing Group.



NIR light to UV or visible light. The upconverted UV or vis-ible light can induce isomerization of azobenzene.[45] Yu, Li, and their co-workers incorporated UCNPs into an azotolane-containing cross-linked LC polymer film (Figure 13).[46] UCNPs converted NIR light to blue light, which induced fast bending of the film. After turning off the NIR light, the film returned to the initial state. This work showed that photoactuation can be triggered using NIR light, which is better suited than UV light for biomedical applications.

2.3.3. Photoswitching of Azopolymers Based On Triplet-Triplet Annihilation Upconversion

Triplet-triplet annihilation (TTA) upconversion is another type of upconversion.[47] A typical TTA upconversion system contains a photosensitizer and an annihilator. The photosensitizer is excited to a singlet state after absorbing NIR or visible light. The excited photosensitizer relaxes to a triplet state, and transfers its energy to the annihilator, which is then excited to a triplet state. Two excited annihilators at the triplet state can then annihilate: one of them is transiting to the ground state and the other to a singlet state. The annihilator at the singlet state transits back to the ground state and emits a high-energy photon.[47]

Based on TTA upconversion, red light was used to actuate a crosslinked LC azopolymer film (Figure 14).[48] Platinum(II) tetraphenyltetrabenzoporphyrin (22) and 9,10-bis(diphenylphosphoryl)anthracene (23) were the sensi-tizer and the annihilator, respectively. The absolute quantum yield of red-to-blue upconversion for this system was as high as 9.3 ± 0.5%. The TTA upconversion system was assembled


Figure 11. Bridged azobenzene incorporated into a peptide for visible light-controlled peptide folding. Circular dichroism (CD) spectra indicate an increase in the helix content after violet light irradiation. Multiple photoswitching cycles were performed in 5 mM reduced glutathione solution and the absorption at 495 nm was plotted against time. Adapted with permission.[40] Copyright 2012, Wiley-VCH.

Figure 12. NIR photoswitching of an azopolymer based on two-photon absorption. a) Absorption spectrum of a thin film of Polymer 18 and its chemical structure. b) AFM images of grating periodicity between 1 and 2 µm on a film of 18 fabricated by two-photon absorption. The inset shows gratings down to 500 nm. Adapted with permission.[42] Copyright 2009, AIP Publishing LLC.




with a LC azopolymer film, which was responsive to blue light. Red light was converted to blue light via TTA upconversion. The upconverted blue light then induced photoisomerization in the polymer film, resulting in film bending. Yu and co-workers further demonstrated that red light passed through a piece of tissue and induced bending. Thus, TTA upconversion enables photoactuation in deep tissue.

3. Summary and Outlook

Although UV-light-responsive azo-macromolecules have been comprehensively investigated, there are only few examples of intrinsic visible or NIR-light-responsive azo-macromolecules.

Implementing new small mole cule azobenzene compounds into macromolecules is a straightforward way to prepare new visible or NIR-light-responsive azo-macromolecules. Sev-eral visible- or NIR-light-responsive azobenzenes have been reported and not introduced into macromolecule systems yet: (1) For example, heterodiazocines (Compounds 24 and 25) are visible light-responsive azo compounds (Figure 15).[49] They have high switching efficiencies and quantum yields. (2) Another type of visible- or NIR-light-responsive azo com-pounds are BF2 complexed azo compounds (Compound 26–33) (Figure 16).[50] These compounds have well-separated trans and cis absorption bands, which made switching efficient. In addition, thermal relaxation of the cis state is slow (t1/2 = 12.5 h for 26). Their photoconversion and photoisomerization

Figure 13. UCNP-assisted photoisomerization and bending. a) Chemical structures of the monomer 19 and the cross-linker 20. b) Bending of a film containing UCNPs and azotolane induced by NIR light irradiation via the NIR-to-blue upconversion process. Adapted with permission.[46] Copyright 2011, American Chemical Society.

Figure 14. Red light-induced photoisomerization and bending of an azopolymer film based on TTA upconversion. a) Chemical structures of the photo-sensitizer 22, annihilator 23, monomer 19, and cross-linker 21. b) Film bending induced by red light via the TTA upconversion process. Adapted with permission.[48] Copyright 2013, American Chemical Society.




quantum yields are higher than those of azobenzene. Fur-thermore, these compounds are stable against reduction by glutathione, making them promising for use in photo-pharmacolgical and optogenetical applications. (3) An azopyri-dine functionalized Ni-porphyrin complex is also visible light switchable.[51] (4) A ferrocene containing azobenzene (FcAB) has been used as a visible-light-responsive system.[52] In the Fe(II) state the trans FcAB can be switched to cis FcAB using green light. After oxidation of Fe(II) to Fe(III) the cis FcAB can be switched to trans FcAB using green light with the same wavelength. The FcAB system uses the existence and absence of a metal-to-ligand charge-transfer (MLCT) of FcAB to trigger the photoisomerisation. Therefore, a variety of small molecule azobenzenes can be used as building blocks to construct new visible- or NIR-light-responsive azo-macromolecules.

Photoswitching of azopolymers based on upconversion still have some open questions to tackle. Although upconversion is more efficient than two-photon absorption, it still requires high excitation intensity. However, high-intensity light may damage

skin, tissue, and other biological components due to the photo-thermal effect.[44a,53] This problem limits biomedical applica-tions of photoswitchable azopolymers based on upconversion. Enhancing upconversion efficiency and the energy transfer efficiency between upconverters and azopolymers are possible ways to solve this problem.

Another interesting topic is preparing visible- and NIR-light-responsive azopolymers with different architectures (Figure 17). Visible-light-responsive azobenzene-containing homopolymers,[33,34] random copolymer,[30,31] hyperbranched polymers,[54] and crosslinked polymers,[35] have been prepared. In the future, advanced polymer synthesis techniques should be applied to prepare visible-light-responsive azopolymers with other architectures[1,55] such as dendrimers, block copolymers, and polymer brushes, some of which are under investigation by our group. Azopolymers with advanced architectures might be interesting to construct light-responsive nanomaterials and interfaces.

Combining visible- or NIR-light-responsive azobenzenes with 2D polymers such as graphene and carbon nanotubes still

Figure 15. Potential building blocks for future visible-light-responsive azopolymers. a) Chemical structures and b) absorption spectra of bridged heteroatom azobenzenes. Adapted with permission.[49] Copyright 2016, American Chemical Society.

Figure 16. Potential building blocks for future visible- and NIR-light-responsive azopolymers. a) Chemical structures of BF2 bridged azobenzene and b) absorption spectrum of Compound 26. Adapted with permission.[50a] Copyright 2012, American Chemical Society.

Figure 17. Potential visible or NIR-light-responsive azo-macromolecules with advanced polymeric structures.




awaits exploration (Figure 17). UV-light-responsive azobenzene has been grafted to carbon nanotubes or graphene to construct solar thermal cells.[56] Due to the template effect of the carbon materials, the energy storage capacity is greatly enhanced. We expect the use of graphene or carbon nanotubes modified with visible or NIR-light-responsive azobenzene to further enhance the efficiency of solar thermal cells.

Light-directed self-assembly of nanoparticles decorated with small molecule azobenzene has been extensively inves-tigated.[57,58] The combination of nanoparticles with azo-macromolecules may result in new systems for light-directed self-assembly. Such a research topic is interdisciplinary and is anticipated to broaden the knowledge of azo-macromolecules and nanoparticles.

AcknowledgementsWe acknowledge the Deutsche Forschungsgemeinschaft (DFG, WU 787/2-1) and the Fonds der Chemischen Industrie (FCI, No. 661548) for their financial support.

Keywordsazobenzene, macromolecular machines, photoisomerization, photo-switches, stimuli-responsive polymers

Received: April 8, 2017Revised: May 15, 2017

Published online:

[1] K. Ichimura, Chem. Rev. 2000, 100, 1847.[2] a) P. Theato, Angew. Chem. Int. Ed. 2011, 50, 5804; b) H. Akiyama,

N. Tamaoki, Macromolecules 2007, 40, 5129.[3] A. Natansohn, P. Rochon, Chem. Rev. 2002, 102, 4139.[4] a) D. Bleger, Z. Yu, S. Hecht, Chem. Commun. 2011, 47, 12260;

b) J.-F. Gohy, Y. Zhao, Chem. Soc. Rev. 2013, 42, 7117.[5] a) M. I. Gibson, R. K. O’Reilly, Chem. Soc. Rev. 2013, 42, 7204;

b) D. Roy, W. L. A. Brooks, B. S. Sumerlin, Chem. Soc. Rev. 2013, 42, 7214; c) F. D. Jochum, P. Theato, Chem. Soc. Rev. 2013, 42, 7468.

[6] G. Kocak, C. Tuncer, V. Butun, Polym. Chem. 2017, 8, 144.[7] a) Y. Yu, T. Ikeda, J. Photochem. Photobiol., C 2004, 5, 247;

b) S. Lee, H. S. Kang, J.-K. Park, Adv. Mater. 2012, 24, 2069; c) D. Wang, X. Wang, Prog. Polym. Sci. 2013, 38, 271; d) D. Habault, H. Zhang, Y. Zhao, Chem. Soc. Rev. 2013, 42, 7244; e) A. Priimagi, A. Shevchenko, J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 163.

[8] T. Ikeda, O. Tsutsumi, Science 1995, 268, 1873.[9] H. Zhou, C. Xue, P. Weis, Y. Suzuki, S. Huang, K. Koynov,

G. K. Auernhammer, R. Berger, H.-J. Butt, S. Wu, Nat. Chem. 2017, 9, 145.

[10] a) H. Finkelmann, E. Nishikawa, G. G. Pereira, M. Warner, Phys. Rev. Lett. 2001, 87, 015501; b) M. H. Li, P. Keller, B. Li, X. Wang, M. Brunet, Adv. Mater. 2003, 15, 569.

[11] a) Y. Yu, M. Nakano, T. Ikeda, Nature 2003, 425, 145; b) C. L. van Oosten, D. Corbett, D. Davies, M. Warner, C. W. M. Bastiaansen, D. J. Broer, Macromolecules 2008, 41, 8592; c) L. Zhang, H. Liang, J. Jacob, P. Naumov, Nat. Commun. 2015, 6, 7429.

[12] S. Iamsaard, S. J. Aßhoff, B. Matt, T. Kudernac, J. J. L. M. Cornelissen, S. P. Fletcher, N. Katsonis, Nat. Chem. 2014, 6, 229.

[13] K. M. Lee, M. L. Smith, H. Koerner, N. Tabiryan, R. A. Vaia, T. J. Bunning, T. J. White, Adv. Funct. Mater. 2011, 21, 2913.

[14] a) J. Olmsted Iii, J. Lawrence, G. G. Yee, Sol. Energy 1983, 30, 271; b) H. Taoda, K. Hayakawa, K. Kawase, H. Yamakita, J. Chem. Eng. Jpn. 1987, 20, 265.

[15] D. Zhitomirsky, E. Cho, J. C. Grossman, Adv. Energy Mater. 2016, 6, 1502006.

[16] a) G. Wang, X. Tong, Y. Zhao, Macromolecules 2004, 37, 8911; b) W. Su, Y. Luo, Q. Yan, S. Wu, K. Han, Q. Zhang, Y. Gu, Y. Li, Macromol. Rapid Commun. 2007, 28, 1251; c) Y. Li, Y. He, X. Tong, X. Wang, J. Am. Chem. Soc. 2005, 127, 2402; d) S. Wu, Q. Zhang, C. Bubeck, Macromolecules 2010, 43, 6142.

[17] a) A. A. Beharry, G. A. Woolley, Chem. Soc. Rev. 2011, 40, 4422; b) F. Ercole, T. P. Davis, R. A. Evans, Polym. Chem. 2010, 1, 37.

[18] P. Gorostiza, E. Y. Isacoff, Physiology 2008, 23, 238.[19] a) I. Tomatsu, A. Hashidzume, A. Harada, Angew. Chem. Int.

Ed. 2006, 45, 4605; b) S. Tamesue, Y. Takashima, H. Yamaguchi, S. Shinkai, A. Harada, Angew. Chem. Int. Ed. 2010, 49, 7461; c) Y. Zhou, D. Wang, S. Huang, G. Auernhammer, Y. He, H.-J. Butt, S. Wu, Chem. Commun. 2015, 51, 2725; d) P. Wan, Y. Wang, Y. Jiang, H. Xu, X. Zhang, Adv. Mater. 2009, 21, 4362; e) Y. Xiong, Z. Chen, H. Wang, L.-M. Ackermann, M. Klapper, H.-J. Butt, S. Wu, Chem. Commun. 2016, 52, 14157; f) X. Liu, M. Jiang, Angew. Chem. Int. Ed. 2006, 45, 3846.

[20] a) H. Rau, in Photochemistry and Photophysics, Vol. 2 (Ed: J. Rabeck), Boca Raton, FL, 1990, pp. 119–141; b) H. M. D. Bandara, S. C. Burdette, Chem. Soc. Rev. 2012, 41, 1809.

[21] S. Wu, X. Yu, J. Huang, J. Shen, Q. Yan, X. Wang, W. Wu, Y. Luo, K. Wang, Q. Zhang, Journal of Materials Chemistry 2008, 18, 3223.

[22] a) X. Wang, S. Balasubramanian, J. Kumar, S. K. Tripathy, L. Li, Chem. Mater. 1998, 10, 1546; b) X. Wang, J. Kumar, S. K. Tripathy, L. Li, J.-I. Chen, S. Marturunkakul, Macromolecules 1997, 30, 219.

[23] S. Wu, L. Wang, A. Kroeger, Y. Wu, Q. Zhang, C. Bubeck, Soft Matter 2011, 7, 11535.

[24] a) D. Y. Kim, S. K. Tripathy, L. Li, J. Kumar, Appl. Phys. Lett. 1995, 66, 1166; b) P. Rochon, E. Batalla, A. Natansohn, Appl. Phys. Lett. 1995, 66, 136.

[25] a) T. Srikhirin, A. Laschitsch, D. Neher, D. Johannsmann, Appl. Phys. Lett. 2000, 77, 963; b) N. Tirelli, U. W. Suter, A. Altomare, R. Solaro, F. Ciardelli, S. Follonier, C. Bosshard, P. Günter, Macromolecules 1998, 31, 2152; c) T. Seki, Macromol. Rapid Commun. 2014, 35, 271.

[26] a) J. Gao, Y. He, F. Liu, X. Zhang, Z. Wang, X. Wang, Chem. Mater. 2007, 19, 3877; b) X. Sallenave, C. G. Bazuin, Macromolecules 2007, 40, 5326; c) S. Wu, S. Duan, Z. Lei, W. Su, Z. Zhang, K. Wang, Q. Zhang, J. Mater. Chem. 2010, 20, 5202; d) M. Poutanen, O. Ikkala, A. Priimagi, Macromolecules 2016, 49, 4095; e) S. Wu, C. Bubeck, Macromolecules 2013, 46, 3512; f) S. Wu, J. Huang, S. Beckemper, A. Gillner, K. Wang, C. Bubeck, J. Mater. Chem. 2012, 22, 4989; g) S. Iamsaard, E. Villemin, F. Lancia, S.-J. Ahoff, S. P. Fletcher, N. Katsonis, Nat. Protocols 2016, 11, 1788; h) N. Zettsu, T. Ogasawara, N. Mizoshita, S. Nagano, T. Seki, Adv. Mater. 2008, 20, 516.

[27] A. Priimagi, M. Kaivola, F. J. Rodriguez, M. Kauranen, Appl. Phys. Lett. 2007, 90, 121103.

[28] a) A. A. Beharry, O. Sadovski, G. A. Woolley, J. Am. Chem. Soc. 2011, 133, 19684; b) D. Bléger, J. Schwarz, A. M. Brouwer, S. Hecht, J. Am. Chem. Soc. 2012, 134, 20597; c) S. Samanta, A. A. Beharry, O. Sadovski, T. M. McCormick, A. Babalhavaeji, V. Tropepe, G. A. Woolley, J. Am. Chem. Soc. 2013, 135, 9777.

[29] C. Knie, M. Utecht, F. Zhao, H. Kulla, S. Kovalenko, A. M. Brouwer, P. Saalfrank, S. Hecht, D. Bléger, Chem. – Eur. J. 2014, 20, 16492.

[30] G. Wang, D. Yuan, T. Yuan, J. Dong, N. Feng, G. Han, J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2768.




[31] D. Wang, M. Wagner, H.-J. Butt, S. Wu, Soft Matter 2015, 11, 7656.[32] D. Wang, S. Wu, Langmuir 2016, 32, 632.[33] P. Weis, D. Wang, S. Wu, Macromolecules 2016, 49, 6368.[34] A. K. Saydjari, P. Weis, S. Wu, Adv. Energy Mater. 2017, 7, 1601622.[35] K. Kumar, C. Knie, D. Bléger, M. A. Peletier, H. Friedrich, S. Hecht,

D. J. Broer, M. G. Debije, A. P. H. J. Schenning, Nat. Commun. 2016, 7, 11975.

[36] S. Samanta, T. M. McCormick, S. K. Schmidt, D. S. Seferos, G. A. Woolley, Chem. Commun. 2013, 49, 10314.

[37] M. Dong, A. Babalhavaeji, M. J. Hansen, L. Kalman, G. A. Woolley, Chem. Commun. 2015, 51, 12981.

[38] D. Wang, M. Wagner, A. K. Saydjari, J. Mueller, S. Winzen, H.-J. Butt, S. Wu, Chem. – Eur. J. 2017, 23, 2628.

[39] R. Siewertsen, H. Neumann, B. Buchheim-Stehn, R. Herges, C. Näther, F. Renth, F. Temps, J. Am. Chem. Soc. 2009, 131, 15594.

[40] S. Samanta, C. Qin, A. J. Lough, G. A. Woolley, Angew. Chem. Int. Ed. 2012, 51, 6452.

[41] S. Kawata, Y. Kawata, Chem. Rev. 2000, 100, 1777.[42] A. Ambrosio, E. Orabona, P. Maddalena, A. Camposeo, M. Polo,

A. A. R. Neves, D. Pisignano, A. Carella, F. Borbone, A. Roviello, Appl. Phys. Lett. 2009, 94, 011115.

[43] a) F. Auzel, Chem. Rev. 2004, 104, 139; b) F. Wang, X. Liu, Chem. Soc. Rev. 2009, 38, 976.

[44] a) S. Wu, H.-J. Butt, Adv. Mater. 2016, 28, 1208; b) C.-J. Carling, J.-C. Boyer, N. R. Branda, J. Am. Chem. Soc. 2009, 131, 10838.

[45] L. Wang, H. Dong, Y. Li, C. Xue, L.-D. Sun, C.-H. Yan, Q. Li, J. Am. Chem. Soc. 2014, 136, 4480.

[46] W. Wu, L. Yao, T. Yang, R. Yin, F. Li, Y. Yu, J. Am. Chem. Soc. 2011, 133, 15810.

[47] J. Zhou, Q. Liu, W. Feng, Y. Sun, F. Li, Chem. Rev. 2015, 115, 395.[48] Z. Jiang, M. Xu, F. Li, Y. Yu, J. Am. Chem. Soc. 2013, 135, 16446.[49] M. Hammerich, C. Schütt, C. Stähler, P. Lentes, F. Röhricht,

R. Höppner, R. Herges, J. Am. Chem. Soc. 2016, 138, 13111.[50] a) Y. Yang, R. P. Hughes, I. Aprahamian, J. Am. Chem. Soc. 2012,

134, 15221; b) Y. Yang, R. P. Hughes, I. Aprahamian, J. Am. Chem. Soc. 2014, 136, 13190.

[51] S. Venkataramani, U. Jana, M. Dommaschk, F. D. Sönnichsen, F. Tuczek, R. Herges, Science 2011, 331, 445.

[52] M. Kurihara, A. Hirooka, S. Kume, M. Sugimoto, H. Nishihara, J. Am. Chem. Soc. 2002, 124, 8800.

[53] a) Z. Chen, W. Sun, H.-J. Butt, S. Wu, Chem. – Eur. J. 2015, 21, 9165; b) S. He, K. Krippes, S. Ritz, Z. Chen, A. Best, H.-J. Butt, V. Mailänder, S. Wu, Chem. Commun. 2015, 51, 431.

[54] L. Wang, Y. Chen, L. Yin, S. Zhang, N. Zhou, W. Zhang, X. Zhu, Polym. Chem. 2016, 7, 5407.

[55] D. L. Jiang, T. Aida, Nature 1997, 388, 454.[56] a) T. J. Kucharski, N. Ferralis, A. M. Kolpak, J. O. Zheng,

D. G. Nocera, J. C. Grossman, Nat. Chem. 2014, 6, 441; b) W. Luo, Y. Feng, C. Cao, M. Li, E. Liu, S. Li, C. Qin, W. Hu, W. Feng, J. Mater Chem. A 2015, 3, 11787.

[57] R. Klajn, J. F. Stoddart, B. A. Grzybowski, Chem. Soc. Rev. 2010, 39, 2203.

[58] D. Manna, T. Udayabhaskararao, H. Zhao, R. Klajn, Angew. Chem. Int. Ed. 2015, 54, 12394.

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