Azobenzene Units Research Articles

Azobenzene-containing side-chain ionic metathesis polymers: Facile synthesis, self-assembly and photoresponsive behavior

A series of well-defined azobenzene-functionalized ionomers were produced via ring-opening metathesis polymerization of norbornene-dicarboximide tethered to an ionic group further connected with an azobenzene unit in ionic liquid. The corresponding azobenzene-containing non-ionic polymers were also synthesized by the similar method. All the polymerizations were confirmed to be the characteristic of living based on the linear relationship between molecular weight and feed ratio. As expected, the distinct photoresponsive properties from azobenzene units in ionomers were observed under alternating irradiation of UV and visible light, which cannot be achieved by traditional azobenzene-containing non-ionic polymers. The ability and efficiency of trans−cis photoisomerization was depressed to some extent due to the presence of ionic groups nearby azobenzene units. Moreover, the sizes and morphologies of these ionomers can be modulated through changing the number of the methylene spacer to generate regular and uniform spherical nanostructures, ultimately leading to the unique photoisomerization behaviors. All these interesting results will not only provide a facile and efficient method for constructing photosensitive ionic metathesis polymers, but also expand the optical properties and application prospect of azobenzene-containing polymers.

Photoresponsive Palladium and Nickel Catalysts for Ethylene Polymerization and Copolymerization

AbstractIn this contribution, we install an azobenzene functionality in olefin polymerization catalysts and use light to modulate their properties via photoinduced trans–cis isomerization of the azobenzene moiety. The initially targeted azobenzene‐functionalized α‐diimine palladium and nickel catalysts are not photoresponsive. To address this issue, an imine–amine system bearing interrupted conjugation with the metal center, and a sandwich‐type α‐diimine system bearing an azobenzene unit at a position covalently far from the metal center were prepared and studied. We demonstrate that light can be used to tune their properties in ethylene polymerization and copolymerization with polar comonomers, enabling light‐induced control of the polymerization processes, polymer microstructures and polymer properties. More interestingly, the light‐mediated property changes were attributed to ligand electronic effects in one system and ligand steric effects in the other.

Transfer, Amplification, Storage, and Complete Self‐Recovery of Supramolecular Chirality in an Achiral Polymer System

AbstractSupramolecular chirality and its complete self‐recovery ability are highly mystical in nature and biological systems, which remains a major challenge today. Herein, we demonstrate that partially cross‐linked azobenzene (Azo) units can be employed as the potential chiral trigger to fully heal the destroyed helical superstructure in achiral nematic polymer system. Combining the self‐assembly of Azo units and terminal hydroxyl groups in polymer side chains allows the vapor‐induced chiral nematic phase and covalent fixation of the superstructure via acetal reaction. The induced helical structure of Azo units can be stored by inter‐chain cross‐linking, even after removal of the chiral source. Most interestingly, the stored chiral information can trigger perfect chiral self‐recovery (CSR) behavior after being destroyed by UV light, heat, and solvents. The results pave a new way for producing novel chiroptical materials with reversible chirality from achiral sources.

Transfer, Amplification, Storage, and Complete Self-Recovery of Supramolecular Chirality in an Achiral Polymer System.

Supramolecular chirality and its complete self-recovery ability are highly mystical in nature and biological systems, which remains a major challenge today. Herein, we demonstrate that partially cross-linked azobenzene (Azo) units can be employed as the potential chiral trigger to fully heal the destroyed helical superstructure in achiral nematic polymer system. Combining the self-assembly of Azo units and terminal hydroxyl groups in polymer side chains allows the vapor-induced chiral nematic phase and covalent fixation of the superstructure via acetal reaction. The induced helical structure of Azo units can be stored by inter-chain cross-linking, even after removal of the chiral source. Most interestingly, the stored chiral information can trigger perfect chiral self-recovery (CSR) behavior after being destroyed by UV light, heat, and solvents. The results pave a new way for producing novel chiroptical materials with reversible chirality from achiral sources.

Photoresponsive Amide-Based Derivatives of Azobenzene-4,4'-Dicarboxylic Acid-Experimental and Theoretical Studies.

Azobenzene derivatives are one of the most important molecular switches for biological and material science applications. Although these systems represent a well-known group of compounds, there remains a need to identify the factors influencing their photochemical properties in order to design azobenzene-based technologies in a rational way. In this contribution, we describe the synthesis and characterization of two novel amides (L1 and L2) containing photoresponsive azobenzene units. The photochemical properties of the obtained compounds were investigated in DMSO by UV-Vis spectrophotometry, as well as 1H NMR spectroscopy, and the obtained results were rationalized via Density Functional Theory (DFT) methods. After irradiation with UV light, both amides underwent trans to cis isomerization, yielding 40% and 22% of the cis isomer of L1 and L2 amides, respectively. Quantum yields of this process were determined as 6.19% and 2.79% for L1 and L2, respectively. The reverse reaction (i.e., cis to trans isomerization) could be achieved after thermal or visible light activation. The analysis of the theoretically determined equilibrium structure of the transition-state connecting cis and trans isomers on the reaction path indicated that the trans-cis interconversion is pursued via the flipping of the substituent, rather than its rotation around the N=N bond. The kinetics of thermal back-reaction and the effect of the presence of the selected ions on the half-life of the cis form were also investigated and discussed. In the case of L1, the presence of fluoride ions sped the thermal relaxation up, whereas the half-life time of cis-L2 was extended in the presence of tested ions.

On the Low-Lying Electronically Excited States of Azobenzene Dimers: Transition Density Matrix Analysis.

Azobenzene-containing molecules may associate with each other in systems such as self-assembled monolayers or micelles. The interaction between azobenzene units leads to a formation of exciton states in these molecular assemblies. Apart from local excitations of monomers, the electronic transitions to the exciton states may involve charge transfer excitations. Here, we perform quantum chemical calculations and apply transition density matrix analysis to quantify local and charge transfer contributions to the lowest electronic transitions in azobenzene dimers of various arrangements. We find that the transitions to the lowest exciton states of the considered dimers are dominated by local excitations, but charge transfer contributions become sizable for some of the lowest electronic transitions in stacked and slip-stacked dimers at short intermolecular distances. In addition, we assess different ways to partition the transition density matrix between fragments. In particular, we find that the inclusion of the atomic orbital overlap has a pronounced effect on quantifying charge transfer contributions if a large basis set is used.

Unlocking Entropic Elasticity of Nematic Elastomers Through Light and Dynamic Adhesion

AbstractNematic liquid crystal elastomers (LCEs) generally show soft elasticity, masking entropic elasticity inherent in crosslinked networks. They macroscopically deform with little stress increase under straining because of the soft shear mode attained by nematic director rotations. Moreover, the nematic interaction in main‐chain LCEs can arrest the ergodic response to cycling strain. It manifests hysteresis, slow relaxation, and increased viscosity, which critically affect their mechanical applications such as actuation and adhesion. Here, it is shown that entropic elasticity fully recovers after photo‐isomerization of azobenzene units incorporated in a nematic LCE. With the light‐induced bent cis‐isomers at room temperature, the degree of soft elasticity, viscosity, and adhesion are tangibly lowered. Mechanical responses rationalize that cis isomers unlock the frozen configurational degree of freedom by reducing nematic order in the same manner observed upon temperature increase. The present results not only unveil how the nematic interaction suppresses entropic elasticity leading to the soft state, but also offer a guide to tuning viscoelasticity toward dynamic mechanical applications.

A photogated photoswitchable [2]rotaxane based on orthogonal photoreactions

A photoswitchable [2]rotaxane with unique photogated macrocyclic shuttling is constructed by incorporating the photoswitchable azobenzene (AB) and nonphotoactive biphenyl (BP) units into the axle as two terminal recognition sites, bridged by a naphthalene (NAP) unit. The UV (λ = 379 nm)/blue (λ = 440–450 nm) lights triggered Z/E photoisomerization of the AB unit can drive the cyclobis(paraquat-p-phenylene) (CBPQT4+) macrocycle to shuttle between the two terminal recognition sites. Such macrocyclic translational movement can be photogated by a steric barrier generating from the green-light (λ = 520–525 nm)-triggered cycloaddition reaction between the NAP unit on the thread and a triazolinedione (TAD) additive. Upon standing in dark, the photocycloadduct undergoes disassociation which restores the phototriggered macrocyclic shuttling ability. The construction of such photogated photoresponsive molecular shuttle can provide new approach to improve the switching controllability of rotaxane, which is beneficial to fabricate rotaxane-based nanosystems and materials with high performance.

Synthesis of Discrete Conjugated Fluorene‐Azo Oligomers for the Investigation of Azobenzene Position‐Dependent Physical Properties and Photoresponsive Behavior

AbstractIt is well known that macromolecular structure, including composition, topology and sequence, significantly determines the properties of conjugated polymers/oligomers. To accurately understand the structure‐induced properties, discrete oligomers/polymers with molecularly defined structures are the ideal model because they provide distinct nonstatistical results. Herein, a series of discrete conjugated fluorene‐azo oligomers with different positions of the azobenzene units along the main chain (i.e., sequence) are efficiently prepared based on CuAAC “click” chemistry via a stepwise chain‐growth strategy. Position‐dependent behavior has an evident impact on the properties of oligomers, such as thermal properties, optical properties and photoresponsive behavior in both solution and thin film states. In addition, the fluorene‐azo oligomers show less photodegradation of 9,9‐dioctyl‐fluorene in the solid‐state than oligofluorene during UV light irradiation; however, position‐dependent photodegradation is found in solution. Optimized configurations and UV–vis absorption spectra of fluorene‐azo oligomers are calculated by density functional theory. Experimental results and theoretical analysis indicate that the location of azobenzene can play a dominant role in determining the properties of fluorene‐azo oligomers resulting from sequence‐mandated conformational changes. Undoubtedly, this work can provide deeper insight into the structure–property relationships based on precisely sequence‐defined polymer structures.

Study of Aggregation Behavior of Predesigned Azobenzene-Cholesteryl Derivatives in Deep Eutectic Solvents

Structurally isomeric cholesteryl-appended azobenzene derivatives (azo-1 to azo-5) with various substituents, such as H/unsubstituted, ether, ester, and nitro at the terminal position of azobenzene units were designed, and synthesized. The gelation ability and aggregation behavior of the above synthesized azobenzene-cholesteryl derivatives in deep eutectic solvents (DES) such as Zinc Chloride: Ethylene Glycol (Zn:EG), Choline Chloride: EG (Ch:EG), Choline Chloride: Urea (Ch: Urea), and Choline Chloride: Glycine (Ch: Gly) were studied. The results revealed that all the azo derivatives formed semi-transparent and strong/hard eutectic gels in at least one DES except azo-4 which formed gel in two DES. The morphological analyses by scanning electron microscopy (SEM) exhibited entangled dense fibrous, flowers, and sheet-like textures, depending on the nature of DES as well as azo derivatives. Like all azobenzene-based organo-gelators, UV-triggered gel-to-sol transition was expected for these eutectic gels. However, these eutectic gels did not undergo the gel-to-sol transition under UV irradiation. This could be due to the hardness of the gel, which arrests the structural transformation from trans-to-cis during photolysis. It was further confirmed by absorption profiles of before and after irradiation of eutectic gels. Regarding application, an attempt has been made to use eutectic gels as a template for the synthesis of nanomaterials and the results revealed that the azo-4 gel can be used to prepare aggregated highly dense nanorods of copper chloride.

Comparative study of the vibrational spectra of carboxylate azocalix[4]arenes and azothiacalix[4]arenes

The vibrational spectra of calix[4]arene (p-CAC) and thiacalix[4]arenes with azobenzene units having carboxylate groups in the para (p-CATC) and meta position (m-CATC) were recorded and analyzed.The calculated structures of molecules p-CAC, p-CATC, and m-CATC are consistent with experimental X-ray data. The most stable and polar is the cone conformation with four intramolecular cooperative hydrogen bonds. In classical calixarenes, the hydrogen bonds are shorter and, therefore, more durable than in thiacalixarenes. The cone conformation retains for all the studied calixarene molecules. The energy differences between E- and Z-forms of azobenzene groups in p-CAC, p-CATC, and m-CATC are 254.4, 260.2, and 249.4 kJ/mol, respectively. The shape of the molecules changes noticeably upon isomerization of azobenzene groups.Upon passing from thiacalixarenes to classical calixarenes, the dipole moment of the molecule increases. The polarity of calixarene molecules changes upon isomerization of azobenzene units. Ionization energy electron affinity and electrophilicity are higher in classical calixarenes compared to thiacalixarenes. There are active sites of reaction and interaction in the narrow and wide rims of the calixarene molecules. Carboxylate groups are most active in the formation of complexes with metals.

Azo-Enhanced Raman Scattering for Enhancing the Sensitivity and Tuning the Frequency of Molecular Vibrations.

Raman scattering provides stable narrow-banded signals that potentially allow for multicolor microscopic imaging. The major obstacle for the applications of Raman spectroscopy and microscopy is the small cross section of Raman scattering that results in low sensitivity. Here, we report a new concept of azo-enhanced Raman scattering (AERS) by designing the intrinsic molecular structures using resonance Raman and concomitant fluorescence quenching strategies. Based on the selection of vibrational modes and the enhancing unit of azobenzenes, we obtained a library of AERS molecules with specific Raman signals in the fingerprint and silent frequency regions. The spectral characterization and molecular simulation revealed that the azobenzene unit conjugated to the vibrational modes significantly enhanced Raman signals due to the mechanism of extending the conjugation system, coupling the electronic–vibrational transitions, and improving the symmetry of vibrational modes. The nonradiative decay of azobenzene from the excited state quenched the commitment fluorescence, thus providing a clean background for identifying Raman scattering. The most sensitive AERS molecules produced Raman signals of more than 4 orders of magnitude compared to 5-ethynyl-2′-deoxyuridine (EdU). In addition, a frequency tunability of 10 distinct Raman bands was achieved by selecting different types of vibrational modes. This methodology of AERS allows for designing small-molecule Raman probes to visualize various entities in complex systems by multicolor spontaneous Raman imaging. It will open new prospects to explore innovative applications of AERS in interdisciplinary research fields.

Atomically Precise Engineering of Single‐Molecule Stereoelectronic Effect

AbstractCharge transport in a single‐molecule junction is extraordinarily sensitive to both the internal electronic structure of a molecule and its microscopic environment. Two distinct conductance states of a prototype terphenyl molecule are observed, which correspond to the bistability of outer phenyl rings at each end. An azobenzene unit is intentionally introduced through atomically precise side‐functionalization at the central ring of the terphenyl, which is reversibly isomerized between trans and cis forms by either electric or optical stimuli. Both experiment and theory demonstrate that the azobenzene side‐group delicately modulates charge transport in the backbone via a single‐molecule stereoelectronic effect. We reveal that the dihedral angle between the central and outer phenyl ring, as well as the corresponding rotation barrier, is subtly controlled by isomerization, while the behaviors of the phenyl ring away from the azobenzene are hardly affected. This tunability offers a new route to precisely engineer multiconfigurational single‐molecule memories, switches, and sensors.

Atomically Precise Engineering of Single-Molecule Stereoelectronic Effect.

Charge transport in a single-molecule junction is extraordinarily sensitive to both the internal electronic structure of a molecule and its microscopic environment. Two distinct conductance states of a prototype terphenyl molecule are observed, which correspond to the bistability of outer phenyl rings at each end. An azobenzene unit is intentionally introduced through atomically precise side-functionalization at the central ring of the terphenyl, which is reversibly isomerized between trans and cis forms by either electric or optical stimuli. Both experiment and theory demonstrate that the azobenzene side-group delicately modulates charge transport in the backbone via a single-molecule stereoelectronic effect. We reveal that the dihedral angle between the central and outer phenyl ring, as well as the corresponding rotation barrier, is subtly controlled by isomerization, while the behaviors of the phenyl ring away from the azobenzene are hardly affected. This tunability offers a new route to precisely engineer multiconfigurational single-molecule memories, switches, and sensors.

Photo-induced changes in azobenzene-containing soft materials

Photocontrol of molecular alignment is an exceptionally-intelligent and useful strategy. It enables us to control optical coefficients, peripheral molecular alignments, surface relief structure, and actuation of substances by means of photoirradiation. Epoxy linear polymers with azobenzene and alkyl side groups were synthesized and studied. They showed interesting and variable photo-response upon visible-light irradiation due to trans–cis–trans photoisomerization of the azobenzene units. The influence of physical crosslinks between alkyl chains on optical and mechanical properties was also investigated. Mechanical characterization of the polymers was done by nanoindentation. Light induced reversible or irreversible changes in the polymer after being irradiated with visible light, depend on the polarization states of light used. When using circularly polarized light, reversible mechanical response is observed. Permanent structural changes in films are related to anisotropic alignment of azobenzene molecules (after being illuminated with linear polarized light) and/or physical crosslinks of alkyl chains. This study demonstrated the importance of the polymer architecture and the presence of domains of alkyl chains on the potential photo responsiveness.

Role of Alicyclic Conformation-Isomerization in the Photomechanical Performance of Azobenzene-Functionalized Cross-Linked Polyimides Containing Tetra-Substituted Cyclohexane Moieties.

The classical "chair-twist boat-boat" conformational dynamics (CD) of cyclohexane is thermally activated. Here we report on the photoinduced/azobenzene-assisted CD of bilaterally fused cyclohexane moieties contributing to large photomechanical response of cross-linked azobenzene-functionalized polyimides (X-azoPI), based on 1,2,4,5-cyclohexane-tetracarboxylic-dianhydride (CHDA), exhibiting a photobending angle and photogenerated stress, up to ∼90° and 370 kPa, respectively. In contrast, X-azoPI containing planar pyromellitimide (PMDI) or cage-like bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic-diimide (BCDI) show smaller photomechanical responses. The superior photomechanical performance of X-azoPI with constrained cyclohexane-diimide (CHDI) units is attributed to an increased mobility of segments comprising "hinged" p-phenylene rings, azobenzene, and CHDI units in the cross-link sites. Blue light irradiation initiates the motions driven by photoisomerization/reorientation of azobenzenes connected to CHDI units, whose CD is then amplified, leading to longer-range segmental mobility, more local free volume, and culminating in large photoinduced bending. The trapping of redistributed CHDI's stereoisomers in X-azoPI backbone at Troom is implicated for the observed photothermal memory.

Preparation of new enforcement polyamide nanocomposite filled by ternary layer double hydroxide and investigation of electrochemical activity, optical and thermal properties

Herein, we report synthesis and manipulation of new reinforced polyamide nanocomposites filled by modified ternary (Mg, Zn, Al) layered double hydroxide by a solution intercalation technique. First, a new optically and electrochemically active polyamide 5 containing a new combination of azobenzene unit, imine function, triphenylamine segment and pyridine moiety in the main chain was prepared by reaction of a newly dicarboxylic acid 3 and 2,6-diaminopyridine 4. Then, two resulting nanocomposites containing 3 and 6% of DLDH were prepared and structures, morphology, electrochemical activity, thermal and optically properties of them were investigated. Also, dye-modified ternary layered double hydroxide was obtained by a co-precipitation method. The π–π* transition related to trans azobenzene chromosphere showed a single absorption between 290 and 350 nm. The electrochemically active behavior of resulting films onto glassy carbon exhibited cyclic voltammetry peak at 0.5–0.9 V, which was enhanced by filling polymer matrix with modified DLDH. On the other hand, significant improvements at T5, T10 and char yields of resulting nanocomposites were observed as compared to the polymer.

A photoresponsive palladium complex of an azopyridyl-triazole ligand: light-controlled solubility drives catalytic activity in the Suzuki coupling reaction.

Herein, the design and synthesis of a click-derived Pd-complex merged with a photoswitchable azobenzene unit is presented. While in the trans-form of the switch the complex showed limited solubility, the photogenerated cis-form rendered the molecule soluble in polar solvents. This light-controllable solubility was exploited to affect the catalytic activity in the Suzuki coupling reaction. The effect of the substrate and catalyst concentration and light intensity on the proceeding and outcome of the reaction was studied. Dehalogenation of the aryl iodide starting material was found to be a major side reaction; however, its occurrence was dependent on the applied light intensity.

Norbornenyl-based amphiphilic ABA-triblock azobenzene copolymers: Synthesis, photoresponsive and self-assembly properties

A series of well-defined ABA-triblock copolymers P(NB-PtBA)x-b-P(NB-PFP)y-b-P(NB-PtBA)x were synthesized by living ring-opening metathesis polymerization (ROMP) of norbornenyl-poly(tert-butyl acrylate) (NB-PtBA), norbornenyl-pentafluorophenyl (NB-PFP) and NB-PtBA in a sequential manner by the use of Grubbs’ third generation catalyst (G3). The corresponding amphiphilic ABA-triblock azobenzene copolymers P(NB-PAA)x-b-P(NB-Azo)y-b-P(NB-PAA)x were obtained in high yields after subsequent post-polymerization modifications, including the reaction of active esters with amine-functionalized azobenzene to introduce the azobenzene unit, and the hydrolysis reaction to remove the tert-butyl groups. Under alternating UV and visible light illumination, the light response behaviors of the polymers were demonstrated by UV–Vis spectroscopy, and the reversible changes were observed as a result of the reversible trans-cis photoisomerization of azobenzene mesogens. These amphiphilic copolymers can also spontaneously self-assemble into spherical nanoparticles in an aqueous medium, and the hydrodynamic diameter increases as the hydrophobic NB-Azo content increases.

Consequences of Chirality in Directing the Pathway of Cholesteric Helix Inversion of π-Conjugated Polymers by Light.

Control over main-chain motion of chiral π-conjugated polymers can lead to unexpected new functionalities.Here, it is shown that by combining photoswitchable azobenzene units in conjugation with chiral fluorene comonomers and appropriate plasticizers, the polymer organization and chiroptical properties of these alternating copolymers steered by light and its state of polarization can be dynamically controlled. The configuration of the stereogenic centers in the side chains of the fluorene units determines the handedness of the cholesteric organization in thermally annealed films, indicating cooperative behavior. The polymer alignment and helicity of the supramolecular arrangement can be switched by irradiating with linearly and circularly polarized light, respectively.Intriguingly, when switching the handedness of thermally induced cholesteric organizations by illuminating with circularly polarized light that is opposite to the handedness of the cholesteric phases, a nematic-like intermediate state is observed during helix interconversion. By the sequence of irradiation with left and right circularly polarized light followed by thermal annealing, an asymmetric motion, reminiscent of that seen in molecular motors is observed. These findings suggest that functional conjugated polymers can exhibit emergent properties at mesoscopic scale.