Shape‐Shifting Azo Dye Polymers: Towards Sunlight‐Driven Molecular Devices

Bushuyev, Oleksandr S.; Aizawa, Miho; Shishido, Atsushi; Barrett, Christopher J.

doi:10.1002/marc.201700253

Cited by 78 publications

(54 citation statements)

References 121 publications

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“…DFT calculations have been performed to gain insight into the association tendency of F‐azobenzenes, revealing significant deviation from planarity for the E ‐isomers, which is detrimental to the formation of noncovalent crosslinks. Optimizing the design of F‐azobenzenes regarding their planarity, for example, employing only one ortho ‐F per phenyl ring, could increase the amplitude of G′ phototuning and possibly induce macroscopic motion of the materials for the preparation of synthetic macromolecular machines …”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Reversible Modulation of Elasticity in Fluoroazobenzene‐Containing Hydrogels Using Green and Blue Light

Zhao

Bonasera

Nöchel

et al. 2017

Macromol. Rapid Commun.

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Gels are especially sensitive to their environment and can be designed to readily respond to variations in, typically, pH or temperature. [5] Light is another trigger of choice offering the advantage of precise photopatterning [4] and allows to alter hydrogels' mechanical properties either permanently by implementing photo cleavable groups [6] or reversibly using photochromic molecules, such as spiro pyrans [7,8] or azobenzenes. [9] Following the latter strategy, noncovalent physical gels typically lead to materials with light induced sol-gel transitions, [10][11][12][13] whereas covalent chemical gels can exhibit more finely tuneable phenomena such as pho toinduced motion, [14][15][16][17][18] or softening/ hardening. Photoswitchable hydrogels exhibiting modulation of their mechanical properties have been described; [16,19] how ever, their activation requires UV light, which displays several drawbacks related to its damaging character for the materials and its surrounding, as well as its limited light penetration. [20] Here, we present a polyethylene glycol (PEG) hydrogel incorporating ortho fluoroazobenzenes [21] as cross linkers (see F4-azo-PEG; Figure 1) and exhibiting reversible photomodula tion of elasticity using blue and green light. Ortho fluoroazoben zenes (abbreviated to F azos in the following) were selected as photochromic moieties due to their full addressability with visible light and very high thermal stabilities of the thermo dynamically less stable Z isomers. F azos were functionalized in para positions with amide linkers, since such electron with drawing groups maximize the separation of E and Z isomers' n→π* bands in the visible region [22] (see Figure 2a) and hence promote higher photoisomerization yields using blue (Z→E) or green (E→Z) light. Results and DiscussionF4-azo-PEG gel samples were prepared via strain promoted click cycloaddition between a tetra armed PEG macromon omer (M n = 10 kg mol −1 ) terminated with azide groups (tetra-N3-PEG) and the F azo derivative functionalized with aza dibenzocyclooctynes [23,24] (F4-azo-bis-DBCO). This reac tion typically exhibits fast kinetics at room temperature and does not necessitate the presence of a Cu catalyst, which might be beneficial for the formation of the gel, since once the gelation starts, diffusion of the reactants is reduced. The two starting materials were dissolved in stoichiometric amount in Synthetic Macromolecular Machines Hydrogels are soft materials that have found multiple applications in biomedicine and represent a good platform for the introduction of molecular switches and synthetic machines into macromolecular networks. Tuning their mechanical properties reversibly with light is appealing for a variety of advanced applications and has been demonstrated in the past; however, their activation typically requires the use of UV light, which displays several drawbacks related to its damaging character and limited penetration in tissues and materials. This study circumvents this limitation by introducing all-visible ortho-fluor...

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Section: Discussionmentioning

confidence: 99%

“…Optimizing the design of Fazobenzenes regarding their planarity, for example, employing only one orthoF per phenyl ring, could increase the amplitude of G′ phototuning and possibly induce macro scopic motion of the materials for the preparation of syn thetic macromolecular machines. [26]…”

Section: Discussionmentioning

confidence: 99%

Reversible Modulation of Elasticity in Fluoroazobenzene‐Containing Hydrogels Using Green and Blue Light

Zhao

Bonasera

Nöchel

et al. 2017

Macromol. Rapid Commun.

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“…Overall, combining ortho ‐fluorination with axial chirality allows designing photoswitches that promote light‐driven and long‐lived helical states in liquid crystals. We envision that these versatile and light‐responsive helices can be incorporated into complex molecular systems, to mediate the development of soft machines that are controlled by light entirely …”

Section: Resultsmentioning

confidence: 99%

Long‐Lived Supramolecular Helices Promoted by Fluorinated Photoswitches

Huang

Orlova

Matt

et al. 2017

Macromol. Rapid Commun.

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light into work and power by soft polymer machines has been demonstrated also. [17] Light-responsive cholesteric liquid crystals can be prepared by dissolving a small percentage of chiral photoswitches in a (achiral) nematic liquid crystal host [4,18] (Figure 1). The propensity of these photoswitches, used as dopants, to induce a twist in a given host is characterized by their helical twisting power (HTP), defined as β = × × − ( ) 1 p c ee , where p is the pitch and corresponds to a full 360° rotation of the molecules along the helical axis, c is the concentration of the dopant in wt%, and ee is the enantiomeric excess of the dopant. Chiral switches are typically characterized by large twisting powers for their chirality and can be amplified across length scales effectively. [6] Azobenzenes are typically used as switchable dopants because their trans-cis photoisomerization is associated with large changes in both the geometry and the dipole moment of the molecule. While rod-like trans-azobenzenes are compatible with the nematic order, the bent-like cis-molecules disturb this molecular order significantly. Consequently, the HTP of the trans and the cis isomers are remarkably different. Moreover, coupling azobenzenes to elements of axial chirality yields both a large HTP in the ground state and a large variation of HTP under irradiation with light, compared to chiral azobenzenes that display point chirality only. [11,[19][20][21][22] Light-driven helix inversion has also been reported, when chiral azobenzene dopants with axial chirality were used as dopants. [23][24][25] However, the lack of thermal stability of the cisisomer remains a limitation to reaching the full potential of complex dynamic behavior in these supramolecular systems.Fluorinated azobenzenes display larger thermal stability than their classical counterparts, primarily because the repulsion between the nitrogen lone pairs that destabilizes the cis-isomer is lifted by electron-withdrawing effect of the fluorine atoms in ortho-position. [26][27][28][29] Here, we combine the axial chirality of a binaphthyl moiety with ortho-fluorination of an azobenzene moiety in molecular photoswitches that induce supramolecular helices in liquid crystals in both forms, and thus we demonstrate the photoengineering of long-lived supramolecular helices in soft matter. Results and DiscussionMolecules 1a and 1b were synthesized from commercially available starting materials. Their photochemistry and performance as dopants were investigated by comparing them to the reference azobenzenes 2a [19] and 2b [20,30] in the same Molecular PhotoswitchesChiral azobenzenes can be used as photoswitchable dopants to control supramolecular helices in liquid crystals. However, the lack of thermal stability of the cis-isomer precludes envisioning the generation of long-lived supramolecular helices with light. Here, this study demonstrates thermally stable and axially chiral azobenzene switches that can be used as chiral dopants to create supramolecular helices from (achiral) nematic liq...

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“…59 Very recently, in 2017, a review of light-triggered topographies in azopolymers including a brief description of the supramolecular work in the field was written by Broer and coworkers. 60 Also in 2017, the use of azobenzene as a building block in molecular machine architectures 61 and in light-powered molecular devices 62 were reviewed. Additionally, in early 2018 light control of nano-objects especially focusing on using noncovalent interactions to couple azobenzene surfactants to these objects was reviewed, thus indicating the timeliness of employing supramolecular interactions in the design of photoresponsive materials.…”

Section: Scope Of This Review and Complementary Readingmentioning

confidence: 99%

Supramolecular design principles for efficient photoresponsive polymer–azobenzene complexes

2018

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a Noncovalent binding of azobenzenes to polymers allows harnessing light-induced molecular-level motions (photoisomerization) for inducing macroscopic effects, including photocontrol over molecular alignment and self-assembly of block copolymer nanostructures, and photoinduced surface patterning of polymeric thin films. In the last 10 years, a growing body of literature has proven the utility of supramolecular materials design for establishing structure-property-function guidelines for photoresponsive azobenzene-based polymeric materials. In general, the bond type and strength, engineered by the choice of the polymer and the azobenzene, influence the photophysical properties and the optical response of the material system. Herein, we review this progress, and critically assess the advantages and disadvantages of the three most commonly used supramolecular design strategies:hydrogen, halogen and ionic bonding. The ease and versatility of the design of these photoresponsive materials makes a compelling case for a paradigm shift from covalently-functionalized side-chain polymers to supramolecular polymer-azobenzene complexes.

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Shape‐Shifting Azo Dye Polymers: Towards Sunlight‐Driven Molecular Devices

Cited by 78 publications

References 121 publications

Reversible Modulation of Elasticity in Fluoroazobenzene‐Containing Hydrogels Using Green and Blue Light

Reversible Modulation of Elasticity in Fluoroazobenzene‐Containing Hydrogels Using Green and Blue Light

Long‐Lived Supramolecular Helices Promoted by Fluorinated Photoswitches

Supramolecular design principles for efficient photoresponsive polymer–azobenzene complexes

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