trans–cis Photoisomerization of Azobenzene‐Conjugated Dithiolato‐Bipyridine Platinum(II) Complexes: Extension of Photoresponse to Longer Wavelengths and Photocontrollable Tristability

Azobenzene derivatives modified with dithiolato‐bipyridine platinum(II) complexes were synthesized, revealing their highly extended photoresponses to the long wavelength region as well as unique photocontrollable tristability. The absorptions of trans‐ 1 and trans‐ 2 with one azobenzene group on the dithiolene and bipyridine ligands, respectively, cover the range from 300 to 700 nm. These absorptions are ascribed, by means of time‐dependent (TD)DFT calculations, to transitions from dithiolene(π) to bipyridine(π*), namely, interligand charge transfer (CT), π–π*, and n–π* transitions of the azobenzene unit, and π–π* transitions of the bipyridine ligand. In addition, only trans‐ 1 shows distinctive electronic bands, assignable to transitions from the dithiolene(π) to azobenzene(π*), defined as intraligand CT. Complex 1 shows photoisomerization behavior opposite to that of azobenzene: trans‐to‐cis and cis‐to‐trans conversions proceed with 405 and 312 nm irradiation, which correspond to excitation with the intraligand CT, and π–π* bands of the azobenzene and bipyridine units, respectively. In contrast, complex 2 shows photoisomerization similar to that of azobenzene: trans‐to‐cis and cis‐to‐trans transformations occur with 365 and 405 nm irradiation, respectively. Irradiation at 578 nm, corresponding to excitation of the interligand CT transitions, results in cis‐to‐trans conversion of both 1 and 2 , which is the longest wavelength ever reported to effect the photoisomerization of the azobenzene group. The absorption and photochromism of 4 , which has azobenzene groups on both the dithiolato and bipyridine ligands, have characteristics quite similar to those of 1 and 2 , which furnishes 4 with photocontrollable tristability in a single molecule using light at 365, 405, and 578 nm. We also clarified that 1 and 2 have high photoisomerization efficiencies, and good thermal stability of the cis forms. Complexes 3 and 5 have almost the identical photoresponse to those of their positional isomers, complexes 2 and 4 .     

Coordination‐Driven Macrocyclization for Locking of Photo‐ and Thermal cis→trans Isomerization of Azobenzene

Both trans and cis isomers of azobenzene‐linked bis‐terpyridine ligand L1 were incorporated in rigid macrocycles linked by Fe^II(tpy)₂ (tpy: terpyridine) units. The complex of the longer trans‐ L1 is dinuclear [(trans‐ L1 )₂ ? Fe^II₂], whereas the complex of the shorter cis‐ L1 is mononuclear [cis‐ L1? Fe^II]. The complex cis‐ L1? Fe^II was not only thermally stable but also photochemically inactive. These results indicate a perfectly locked state of cis‐azobenzene. The stable macrocyclic structure of cis‐ L1? Fe^II causes locking of the isomerization. To the best of our knowledge, this is first example of dual locking of photo‐ and thermal isomerization of cis‐azobenzene.     

Molecular Design for Reversing the Photoswitching Mode of Turning ON and OFF DNA Hybridization

A new photoswitch for DNA hybridization involving para‐substituted azobenzenes (such as isopropyl‐ or tert‐butyl‐substituted derivatives) with L ‐threoninol as a linker was synthesized. Irradiation of the modified DNA with visible light led to dissociation of the duplex owing to the destabilization effect of the bulky substituent on the trans‐azobenzene. In contrast, trans‐to‐cis isomerization (UV light irradiation) facilitated duplex formation. The direction of this photoswitching mode was entirely reversed relative to the previous system with an unmodified azobenzene on D ‐threoninol whose trans form turned on the hybridization, and cis form turned it off. Such reversed and reversible photoswitching of DNA hybridization was directly demonstrated by using fluorophore‐ and quencher‐attached oligonucleotides. Furthermore, it was revealed that the cis‐to‐trans thermal isomerization was greatly suppressed in the presence of the complementary strand owing to the formation of the more‐stable duplex in the cis form.     

Association and phase behavior of hydrophobically modified photoresponsive poly(acrylamide)s in the presence of ionic surfactant

 The extent of association between the cationic surfactant TTAB and a series of hydrophobically modified polyacrylamides (HPAMs) containing an N-n-alkyl and substituted azobenzene hydrophobic sidegroup has been studied utilizing a cationic surfactant-selective membrane electrode. Binding of TTAB to the polymer hydrophobes is found to increase with increasing hydrophobicity of the hydrophobe. In the presence of electrolyte, aqueous solutions of HPAMs and ionic surfactant exhibit an associative phase separation. The temperature or clearing point (CP) at which the system goes from a one phase to two-phase system are reported. The area of the two-phase region is found to increase with increasing electrolyte concentration, hydrophobicity of the hydrophobe for the high molecular weight HPAMs, and decreasing hydrophobicity for low molecular weight HPAMs. Exposure of HPAMs containing an azobenzene hydrophobe to UV light results in a decrease in interaction between the hydrophobe and surfactant and a corresponding decrease in the CP due to conversion of azobenzene from the more hydrophobic trans form to the less hydrophobic cis isomer. Received: 23 September 1996 Accepted: 11 March 1997     

Photoinduced switching in self-assembled multilayers of azobenzene-containing ionene polycations and anionic polyelectrolytes

Ultrathin molecular assemblies of new ionene polysoaps bearing azobenzene units in the main chain and anionic polyelectrolytes have been prepared upon electrostatic layer-by-layer adsorption on charged substrates. Ionenes could be adsorbed in the trans- and cis-rich state of their azobenzene units. Use of cis-rich polymer was found to be advantageous because up to three times more material could be adsorbed per dipping cycle as from the solution of the trans polymer. Alternate irradiation with UV (<370 nm) and visible (>450 nm) light allowed to switch between the trans isomer and the cis-rich photostationary state. Photoconversion of ionenes in multilayers is lower than in solution, but higher than for multilayers of azobenzene bolaamphiphiles reported recently.     

Photocontrol of the Catalytic Activity of a β‐Cyclodextrin Bearing Azobenzene and Histidine Moieties as a Pendant Group

An azobenzene group was linked to β‐cyclodextrin via a histidine spacer ( 1 ) to produce a photoresponsive catalyst. The ester hydrolysis of p‐nitrophenyl acetate, Boc‐L ‐alanine‐p‐nitrophenyl ester and Boc‐D ‐alanine‐p‐nitrophenyl ester was examined in the presence of trans‐ 1 or cis‐ 1 . In the case of cis‐ 1 , the cyclodextrin cavity was used as the substrate binding site during imidazole‐catalyzed ester hydrolysis. This was not possible in the case of trans‐ 1 due to the inclusion of the trans‐azobenzene moiety in the cyclodextrin cavity. Consequently, the catalytic mechanism switches in an on‐off fashion on UV irradiation, associated with the conversion of the azobenzene moiety of 1 from trans to cis.     

A visible light responsive azobenzene‐functionalized polymer: Synthesis,self‐assembly,and photoresponsive properties

A novel visible light responsive random copolymer consisting of hydrophobic azobenzene‐containing acrylate units and hydrophilic acrylic acid units has been prepared. The azobenzene molecule bearing methoxy groups at all four ortho positions is readily synthesized by one‐step conversion of diazotization. The as‐prepared polymer can self‐assemble into nanoparticles in water due to its amphiphilic nature. The tetra‐o‐methoxy‐substituted azobenzene‐functionalized polymer can exhibit the trans‐to‐cis photoswitching under the irradiation with green light of 520 nm and the cis‐to‐trans photoswitching under the irradiation with blue light of 420 nm in both solution and aggregate state. The morphologies of the self‐assembled nanoparticles are revealed by TEM and DLS. The controlled release of loaded molecules from the nanoparticles can be realized by adjusting pH value since the copolymer possesses pH responsive acrylic acid groups. The fluorescence of loaded Nile Red in the nanoparticles can be tuned upon the visible light irradiation. The reversible photoswitching of the azobenzene‐functionalized polymer under visible light may endow the polymer with wide applications without using ultraviolet light at all. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2768–2775     

A Natural Glycyrrhizic Acid‐Tailored Light‐Responsive Gelator

The construction of stimuli‐responsive materials by using naturally occurring molecules as building blocks has received increasing attention owing to their bioavailability, biocompatibility, and biodegradability. Herein, a symmetrical azobenzene‐functionalized natural glycyrrhizic acid (trans‐ GAG ) was synthesized and could form stable supramolecular gels in DMSO/H₂O and MeOH/H₂O. Owing to trans–cis isomerization, this gel exhibited typical light‐responsive behavior that led to a reversible gel–sol transition accompanied by a variation in morphology and rheology. Additionally, this trans‐ GAG gel displayed a distinct injectable self‐healing property and outstanding biocompatibility. This work provides a simple yet rational strategy to fabricate stimuli‐responsive materials from naturally occurring, eco‐friendly molecules.     

Photoinduced Formation of an Azobenzene‐Based CD‐Active Supramolecular Cyclic Dimer

A series of new photo‐responsive amino acid‐derived azobenzenedicarboxylic acid derivatives (S)‐ 1 a – e were synthesized. Compound (S)‐ 1 a in the trans form exhibited no circular dichroism (CD) signal in DMF under ambient conditions, whereas intense Cotton effects were observed upon UV irradiation, indicating the formation of a chiral supramolecular structure in the cis form. The CD signals disappeared when trifluoroacetic acid (TFA) was added to the solution. The ester counterpart [(S)‐ 1 a′ ] showed no CD signal. Hydrogen bonding between the carboxy groups seemed necessary for constructing the supramolecular structure. The kinetic studies of cis to trans isomerization of (S)‐ 1 a demonstrated that the formation of a chiral supramolecule enhances the stability of the cis‐azobenzene structure. The ESI mass spectrum of stilbenedicarboxylic acid (S)‐ 4 , an analogue of (S)‐ 1 b , confirmed the formation of a dimer. A theoretical CD study revealed that (S)‐ 1 a in the cis form should be present as a cyclic chiral dimer.     

Synthesis of a Stimuli‐Responsive P‐Chiral Polymer with Chiral Phosphorus Atoms and Azobenzene Moieties in the Main Chain

A reaction of the P‐chiral compound (S,S)‐1,2‐bis(boranato(tert‐butyl)methylphosphino)ethane with an azobenzene derivative gave stimuli‐responsive polymers with P‐chiral phosphines in the main chain. This is the first example of a stimuli‐responsive P‐chiral polymer. The polymer isomerized from the trans to the cis form upon UV irradiation and reverted to the trans form reversibly. The polymer was able to coordinate to platinum, and the resulting polymer complex exhibited the Cotton effect owing to the chirality of the phosphorus atoms. The structure of the P‐chiral polymer obtained could be changed reversibly by light and thermal stimuli, and the polymer chain was induced to rotate helically when complexed with transition metals through the chiral phosphorus atoms.     

Light-Responsive Oligothiophenes Incorporating Photochromic Torsional Switches

We present a quaterthiophene and sexithiophene that can reversibly change their effective π-conjugation length through photoexcitation. The reported compounds make use of light-responsive molecular actuators consisting of an azobenzene attached to a bithiophene unit by both direct and linker-assisted bonding. Upon exposure to 350 nm light, the azobenzene undergoes trans-to-cis isomerization, thus mechanically inducing the oligothiophene to assume a planar conformation (extended π-conjugation). Exposure to 254 nm wavelength promotes azobenzene cis-to-trans isomerization, forcing the thiophenic backbones to twist out of planarity (confined π-conjugation). Twisted conformations are also reached by cis-to-trans thermal relaxation at a rate that increases proportionally with the conjugation length of the oligothiophene moiety. The molecular conformations of quaterthiophene and sexithiophene were characterized by using steady-state UV-vis spectroscopy, X-ray crystallography and quantum-chemical modeling. Finally, we tested the proposed light-responsive oligothiophenes in field-effect transistors to probe the photo-induced tuning of their electronic properties.     

Synthesis and photoisomerization of an azobenzene derivative bearing two β-cyclodextrin units at both ends

An azobenzene derivative with two β-cyclodextrin units was synthesized as a novel photoswitchable host. It undergoes trans-to-cis photoisomerization in aqueous solution with 65.8% cis at the photostationary state and returns to the original trans from with half life of 54,8 h at 25°C. It exhibits circular dichroism bands in the π-π* and n-π* absorption regions of azobenzene in spite of the fact that the azobenzene unit cannot be included in the cyclodextrin cavity, and their absolute dichroism intensities of the cis form become much larger than those of trans one.     

Motion Control of Polymeric Nanomotors Based on Host–Guest Interactions

Controlling the motion of artificial self‐propelled micro‐ and nanomotors independent of the fuel concentration is still a great challenge. Here we describe the first report of speed manipulation of supramolecular nanomotors via blue light‐responsive valves, which can regulate the access of hydrogen peroxide fuel into the motors. Light‐sensitive polymeric nanomotors are built up via the self‐assembly of functional block copolymers, followed by bowl‐shaped stomatocyte formation and incorporation of platinum nanoparticles. Subsequent addition of β‐cyclodextrin (β‐CD) leads to the formation of inclusion complexes with the trans‐isomers of the azobenzene derivatives grafted from the surfaces of the stomatocytes. β‐CDs attachment decreases the diffusion rate of hydrogen peroxide into the cavities of the motors because of partly blocking of the openings of the stomatocyte. This results in a lowering of the speed of the nanomotors. Upon blue light irradiation, the trans‐azobenzene moieties isomerize to the cis‐form, which lead to the detachment of the β‐CDs due to their inability to form complexes with the cis‐isomer. As a result, the speed of the nanomotors increases accordingly. Such a conformational change provides us with the unique possibility to control the speed of the supramolecular nanomotor via light‐responsive host–guest complexation. We envision that such artificial responsive nano‐systems with controlled motion could have potential applications in drug delivery.     

Photoinduced liquid crystal blue phase by bent-shaped cis isomer of the azobenzene doped in chiral nematic liquid crystal

A photoresponsive azobenzene molecule DCAZO2 with two cholesteryl groups linked to both sides of the azobenzene group is doped in a mixture of nematic liquid crystal E7 and chiral dopant S811 (61.9 wt% E7, 36.1 wt% S811 and 2.0 wt% DCAZO2). Cooled from isotropic phase to 33.0°C, chiral nematic liquid crystal (N*LC) was formed in the sample and then the temperature was kept unchanged at 33.0°C. UV light irradiation induces the trans–cis photoisomerisation and thus an obvious phase transition. When the azobenzene groups isomerise to a cis-saturated state, the UV light was turned off and the white light was turned on at the same time. The bent-shaped cis isomer then turns back to the planar trans isomer gradually. A blue–green platelet texture representing cubic blue phase (BP) was observed and the size of the platelets was increased along with the cis–trans isomerisation. UV–vis absorption spectra indicate that the photoinduced BP exists when the isomerisation degree is between 79% and 18%, and further cis–trans isomerisation change BP back into N*LC. The large geometric structure of the cholesteryl groups and the large bent angle θ of the cis isomer are supposed to be responsible for the interesting result.     

Observation of Collective Photoswitching in Free-Standing TATA-Based Azobenzenes on Au(111)

Light-induced transitions between the trans and cis isomer of triazatriangulenium-based azobenzene derivatives on Au(111) surfaces were observed directly by scanning tunneling microscopy, allowing atomic-scale studies of the photoisomerization kinetics. Although the azobenzene units in these adlayers are free-standing and spaced at uniform distances of 1.26 nm, their photoswitching depends on the isomeric state of the surrounding molecules and, specifically, is accelerated by neighboring cis isomers. These collective effects are supported by ab initio calculations indicating that the electronic excitation preferably localizes on the n–π* state of trans isomers with neighboring cis azobenzenes.     

Synthesis of methyl‐substituted azobenzene–carbazole conjugated copolymers with photoinduced structural changes

Azobenzene switches its structure instantaneously by reversible trans‐to‐cis and cis‐to‐trans photoisomerization with light irradiations. Dynamic change in polymer structure is expected via introducing an azobenzene unit into the main chain. In this study, a set of methyl‐substituted azobenzene–carbazole conjugated copolymers is synthesized by the Suzuki–Miyaura coupling method. Introduction of methyl substituents to the azobenzene unit of the monomer, and polymerization in a high‐boiling solvent improve the molecular weight of the polymer. Decrease of effective conjugation length due to the twisted structure of the main chain allows progress of photoisomerization. The microstructure of the polymer was determined with grazing incidence X‐ray diffraction (GIXD) measurements using synchrotron radiation. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1756–1764     

Mechanism aspects of th e ring-opening polymerization of the episulfides compared to epoxides

The cis- and trans-2-butene episulfides polymerize with cationic catalyts differently than reported for the corresponding oxides. Where the cis-oxide gave amorphous disyndiotactic polymer, the cis-sulfide gives crystalline racemic diisotactic polymer since this polymer could be asymmetrically synthesized in optically active form. Also the same crystalline polymer was obtained with coordination catalysts. Where the trans-oxide gave only crystalline, meso-diisotactic polymer, the trans-sulfide gives mainly amorphous polymer which, in one case, did slowly crystallize. The difference between the trans forms appears due to the longer C? S bond which lowers steric hindrance and thus isomer selection in the attack of episulfide on the growing sulfonium ion to give less steroregular polymer. The difference in the cis forms may result from the sulfur atom in the last chain unit coordinating with the counterion. The greater hindrance around oxygen in the comparable oxide polymers may prevent the same mechanism from being utilized. The cationic polymerization of isobutylene sulfide gives both crystalline and amorphous polymer. NMR evidence indicates that the amorphous polymer results from substantial head-to-head, tail-to-tail polymerization, along with the expected head-to-tail polymerization. The same phenomenon occurs, but to a lesser extent, in cationic isobutylene oxide polymerizations. The preparation and properties of high molecular weight, head-to-tail isobutylene oxide and sulfide polymers from R₂Mg-NH₃ coordination catalysis are described.     

Observation of Collective Photoswitching in Free‐Standing TATA‐Based Azobenzenes on Au(111)

Light‐induced transitions between the trans and cis isomer of triazatriangulenium‐based azobenzene derivatives on Au(111) surfaces were observed directly by scanning tunneling microscopy, allowing atomic‐scale studies of the photoisomerization kinetics. Although the azobenzene units in these adlayers are free‐standing and spaced at uniform distances of 1.26 nm, their photoswitching depends on the isomeric state of the surrounding molecules and, specifically, is accelerated by neighboring cis isomers. These collective effects are supported by ab initio calculations indicating that the electronic excitation preferably localizes on the n–π* state of trans isomers with neighboring cis azobenzenes.     

Photo-Induced Switchable Binding Behavior of Bridged Bis(β-cyclodextrin) with an Azobenzene Dicarboxylate Linker

A bridged bis(β-cyclodextrin) (1) with an azobenzene dicarboxylate linker was synthesized, and its binding behavior with a fluorescent dye, acridine red (AR), was investigated by means of fluorescence spectroscopy. Due to the photo-induced conversion of the azobenzene dicarboxylate linker from the trans-conformer to the cis-conformer, 1 exhibits a different binding behavior before and after UV light irradiation, giving a stronger binding ability towards AR in the cis form. This switchable binding behavior of 1 may open a new channel to the design of azobenzene-linked dimeric receptors as photo-induced molecular devices.     

Photoswitchable Adhesives Using Azobenzene‐Containing Materials

Photoinduced reversible solid‐to‐liquid transitions of azobenzene‐containing materials can control adhesion. Photoswitchable adhesives based on azobenzene‐containing small molecules and polymers are under intense investigation. The melting points or glass transition temperatures of such azobenzene‐containing materials in trans and cis forms are above and below room temperature, respectively. Photoswitching of these materials results in reversible trans‐cis isomerization and solid‐to‐liquid transitions. The solid trans azobenzene‐containing materials have strong adhesion and the liquid cis azobenzene‐containing materials have weaker adhesion. In this Minireview, we introduce adhesives based on azobenzene‐containing small molecules and polymers. The remaining challenges and perspectives in the field of photoswitchable adhesives using azobenzene‐containing materials are also discussed.