Azobenzene Molecules Research Articles

Two-Dimensional Nanoarchitectonics with Fluorination and Alkylation on Photoresponsive Azobenzene Molecules Self-Assembled at the Solid Surface

Two-Dimensional Nanoarchitectonics with Fluorination and Alkylation on Photoresponsive Azobenzene Molecules Self-Assembled at the Solid Surface

Thermoplasmonic Effect Enables Indirect ON-OFF Control over the Z-E Isomerization of Azobenzene-Based Photoswitch.

Proper formulation of systems containing plasmonic and photochromic units, such as gold nanoparticles and azobenzene derivatives, yields materials and interfaces with synergic functionalities. Moreover, gold nanoparticles are known to accelerate the Z-E isomerization of azobenzene molecules in the dark. However, very little is known about the light-driven, plasmon-assisted Z-E isomerization of azobenzene compounds. Additionally, most of the azobenzene-gold hybrids are prepared with nanoparticles of small, isotropic shapes and azobenzene ligands covalently linked to the surface of nanostructures. Herein, a formulation of an innovative system combining azobenzene derivative, gold nanorods, and cellulose nanofibers is proposed. The system's structural integrity relies on electrostatic interactions among components instead of covalent linkage. Cellulose, a robust scaffold, maintains the material's functionality in water and enables monitoring of the material's plasmonic-photochromic properties upon irradiation and at elevated temperatures without gold nanorods aggregation. Experimental evidence supported by statistical analysis suggests that the optical properties of plasmonic nanometal enable indirect control over the Z-E isomerization of the photochromic component with near-infrared irradiation by triggering the thermoplasmonic effect. The proposed hybrid material's dual plasmonic-photochromic functionality, versatility, and ease of processing render a convenient starting point for further advanced azobenzene-related research and 3D printing of macroscopic light-responsive structures.

Quantifying Hydrogen-Bonding Interactions in the Self-Assembly of Photoresponsive Azobenzene Amphiphiles at the Air-Water Interface.

Amphiphilic azobenzene molecules offer ample scope to design functional supramolecular systems in an aqueous medium that can be controlled by light. Despite their widespread applications in photopharmacology and optoelectronics, the self-assembly pathways and energy landscapes of these systems are not well understood. Here, we report combined molecular dynamics (MD) simulation and surface manometry studies on a specially designed alkylated, meta-substituted azobenzene derivative to quantify the hydrogen-bonding interactions in the self-assembled monolayers of its photoisomers. The z-density profile, radial distribution function, order parameters, and hydrogen bond analyzed using MD simulations corroborated the experimental observations of changes in surface pressure, dipole moment, and thickness of the monolayers. Even a small change in the number of hydrogen bonds in the molecule-molecule and molecule-water interactions causes significant changes in the monolayer properties. These results are fundamentally important for engineering photoresponsive molecules with tailored properties for applications in targeted drug delivery and other industrial applications.

Experimental and numerical investigation of the photo-mechanical response of azobenzene filled soft elastomers, Part I: Experimental investigations

Photosensitive elastomers have the ability to undergo significant deformations upon the illumination by light of a specific wave length. Compared to other non-mechanical stimuli used in materials such as electro- or magneto-active polymers, light offers interesting advantages such as ultra high application speed and spatial precision. Moreover, as light powers and controls the movement of the material, the need for stiff wiring or external energy sources is eliminated. This becomes especially important in the design of miniature sized applications such as nano-scale robots. In the scope of the present work the conversion of photonic energy into a mechanical response originates from switching molecules embedded into a soft polymer matrix. Upon stimulation by UV-light, these molecules can switch from their stable trans - into a meta-stable cis-isomer, which induces a mechanical response on the macro scale. Depending on the experiment this can be visible as a deformation of the material or an increase in stiffness. This contribution presents the results of various mechanical and photo-mechanical experiments performed with the silicone Elastosil P 7670™ mixed with azobenzene molecules. These experiments are conducted in such a fashion that the obtained results are well suited for the identification of material parameters that arise from a photo-viscoelastic continuum modeling approach.

Spatially resolved work-function manipulation of azobenzene-functionalized self-assembled monolayers by optical stimulation

Strongly differing static dipole moments of the trans and cis isomers of photochromic azobenzene allow for optical switching of the work function of azobenzene-functionalized self-assembled monolayers (SAMs). We apply these properties in a fundamental experiment to manipulate the area size of the switched SAM. Azobenzene molecules were excited by ultraviolet laser illumination, and the transient isomerization profile of the SAM was spatially resolved recording photoemission electron microscopy images. Thereby, we demonstrate the spatial tuning of the SAM's work function and discuss the role of the laser spot profile in generating sharp edges or gradual changes of the work function.

Photo-responsive doxorubicin delivery: Nanogel systems based on azobenzene and host-guest interactions enhanced by squeezing action

Conventional chemotherapy methods impact both normal and cancerous cells; therefore, it is essential to design drug delivery systems to reduce undesired drug effects. Nano-scaled drug delivery systems have the advantage of high retention time in blood as well as the capability of conventional penetration into tissue barriers. In this study, light-sensitive biocompatible nanogels with a core-shell structure have been synthesized and characterized as drug delivery system loaded with doxorubicin (DOX). These smart nanogels possess a hydrophobic core, coated with hydrophilic starch polymeric chains, which are modified with β-cyclodextrin (βCD). The core-shell formation is based on host-guest interaction between azobenzene rings as photochromic agents and βCD hydrophobic cavities. The mechanism of drug release is based on the cis-trans isomerization of azobenzene molecules upon light irradiation and dissociation of crore-shell entities. Due to this isomerization, shrinkage of hydrophobic core and removal of shell occurred simultaneously, which assist the drug release. In conclusion, a nanoscale carrier was designed with sustainable drug release under light irradiation as an external and non-contact stimulus. The synthesized nanogels were characterized by FT-IR, 1H NMR, SEM, DLS, and TEM. In addition, the MTT assay proposed that the viability of the A-431 cells has plunged in the presence irradiated nanogels, also histological studies were performed to evaluate the efficacy of the prepared nanogels in cancerous tissues.

Light-induced modulation of viscoelastic properties in azobenzene polymers.

Photo-induced isomerization of azobenzene molecules drives mass migrations in azopolymer samples. The resulting macroscopic directional photo-deformation of the material morphology has found many applications in literature, although the fundamental mechanisms behind this mass transfer are still under debate. Hence, it is of paramount importance to find quantitative observables that could drive the community toward a better understanding of this phenomenon. In this regard, azopolymer mechanical properties have been intensively studied, but the lack of a nanoscale technique capable of quantitative viscoelastic measurements has delayed the progress in the field. Here, we use bimodal atomic force microscopy (AFM) as a powerful technique for nanomechanical characterizations of azopolymers. With this multifrequency AFM approach, we map the azopolymer local elasticity and viscosity, with high resolution, after irradiation. We find that, while in the (previously) illuminated region, a general photo-softening is measured; locally, the Young modulus and the viscosity depend upon the inner structuring of the illuminating light spot. We then propose a possible interpretation based on a light-induced expansion plus a local alignment of the polymer chains (directional hole-burning effect), which explains the experimental observations. The possibility to access, in a reliable and quantitative way, both Young modulus and viscosity could trigger new theoretical-numerical investigations on the azopolymer mass migration dynamics since, as we show, both parameters can be considered measurable. Furthermore, our results provide a route for engineering the nanomechanical properties of azopolymers, which could find interesting applications in cell mechanobiology research.

Mechanical modulation effects of gold electrodes on geometries and electronic transport properties of azobenzene molecular junctions

The azobenzene molecular junction has aroused much interest of scientists due to its switching property arising from its photoinduced isomerism. Owing to the existence of the cis and trans isomers, the electronic transport properties of the azobenzene molecules are promised to show significant differences. The experimental investigations indicate that the cis azobenzene molecule commonly shows high conductance, while the trans azobenzene molecule shows low conductance. However, the computations give the opposite results. To reveal this significant difference, the effects of electrode mechanical modulation on the geometries and electronic transports of the azobenzene molecules are studied. The effects of the electrode geometries on the electronic transports of the azobenzene molecular junctions are also investigated. The electrode compressing process and the electrode stretching process of the azobenzene molecular junctions are simulated based on the first principles calculations. The electronic transport properties are further calculated by using non-equilibrium Green’s function (NEGF) method. The numerical results show that the structures of the cis and trans azobenzenes with sulfur anchors are stable in the stretching process and compressing process of electrode. For the cis azobenzene molecular junction, the cis geometry remains unchanged until the electrode distance is stretched to about 0.1 nm longer than the stable electrode distance of the trans azobenzene molecular junction. Though the trans azobenzene molecule is bent when squeezed by the electrodes, the C—N—N—C dihedral still maintains its trans structure even when the electrode distance is compressed to about 0.2 nm shorter than the stable electrode distance of the cis azobenzene molecular junctions. It is intriguing that the conductance values of cis and trans azobenzene molecular junctions vary inversely with the electrode distance. The conductance value of the cis azobenzene molecular junction increases with the elongating of the electrode distance, while the conductance value of the trans azobenzene molecular junction increases with the compression of the electrode. The conductance is very sensitive to the electrode distance for both the cis azobenzene molecular junction and the trans azobenzene molecular junctions, which can change more than 10 times with the change of the electrode distance. In the lower bias regime, the conductance of the trans azobenzene is higher than that of the cis one if the two electrodes are planar. However, when the molecule is sandwiched between two pyramid-shaped electrodes, the condutance of the cis azobenzene is higher. Thus, the higher conductance of cis azobenzene may be caused either by the pyramid-shaped electrodes or by the large electrode distance.

Surface-mediated dynamic cooperative motions in azobenzene polymer films

Abstract Azobenzene (1,2-diphenyldiazene in IUPAC name) is a widely studied, simple, rodlike photochromic molecule. The trans- (E) to cis- (Z) photoisomerization of this molecule was discovered a long time ago (nearly 90 yr). Nevertheless, the allure and utility of this molecule never faded and it has not surrendered a leading position among many other organic photochromic molecules. New applications for photoswitching molecular and macromolecular systems are still being proposed extensively. By linking the azobenzene molecule to liquid crystals (LCs) and polymer systems, a number of fascinating motility functions have been newly realized. This article overviews our research activities using azobenzene-containing monolayers and LC polymer films. The topics range across surface photoalignment of LCs, both from solid surface and free surface; photoalignment of hierarchical structures such as mesohybrids and block copolymers; photomechanical motions and morphological switching in block copolymer monolayers; high-density brushes of azobenzene side chain LC polymers (SCLCPs); photo-triggered mass migrations in azobenzene SCLCP films; and polymer motions via Marangoni flow, etc. Demonstrations of these dynamic photofunctions are expected to provide great opportunities for practical smart applications.

Photo-modulated regeneration of hypercrosslinked polymer adsorbents for water treatment

Hypercrosslinked polymers (HCPs) are highly effective adsorbents for removing dyes from water. However, the regeneration of such adsorbents is typically achieved with energy-intensive thermal methods or with solvents that generate chemical waste. Here in this proof of concept study, we used HCPs loaded with azobenzene molecules (HCPs@Azo) that were developed in our previous work, and four types of dyes – Methyl Orange (MO), Methylene Blue (MB), Rhodamine B (RB), Uniblue A (UA), at concentrations ranging from 10 to 500 mg L−1, to demonstrate the feasibility of photo-modulated regeneration of HCP-based adsorbents. Azobenzene in HCP pores reduced dye adsorption by 4–31 %, but upon photo-isomerisation from trans to cis states, led to the desorption of 1.7–20 % dyes that were previously adsorbed. These desorption capacities were 71–209 % higher than using water for regeneration. Compared to the traditional method using organic solvents such as methanol to regenerate HCP-based adsorbents, our approach of photo-modulated regeneration led to the desorption of 50 % more MO and MB dyes. HCPs@Azo also demonstrated good reusability, showing comparable photo-modulated adsorption and desorption capacities after four cycles of UV–visible light irradiation. Results from this work can potentially pave the way for solvent-less regeneration of adsorbents, creating a waste-less adsorbent regeneration process.

Design and Characterization of Azobenzene Electrolytes for Aqueous Redox Flow Batteries

Aqueous redox-flow batteries (RFBs) containing organic redox electrolytes are a promising technology for grid-scale renewable energy storage. Most electrolytes currently under consideration for these RFBs contain charge-storing molecules that are drawn from a narrow collection of molecular scaffolds, and undergo rapid decomposition, which results in high rates of capacity fade. To inform efforts at developing more stable aqueous RFB electrolytes, it is critical that the kinetics of charge carrier decomposition and its relationship to capacity fade are well understood.In this work, we explore a relatively new class of redox-active organic molecules known as azobenzenes as potential charge carriers in aqueous RFBs. The electrochemical properties and cycling performance of a range of substituted azobenzenes are characterized using cyclic voltammetry, chronoamperometry, and constant current-constant voltage cycling. Decomposition products are identified using UV-vis and proton nuclear magnetic spectroscopy, as well as mass spectrometry. We discern the influence of decomposition during flow cell cycling by combining in-line UV-vis spectroscopy with multivariate curve resolution, which allows us to quantify decomposition kinetics and estimate UV-vis spectra of intermediate species/decomposition products. These results are being used to drive de novo computational design of new azobenzene molecules that can eliminate or substantially mitigate the identified decomposition pathways.

The Synergistic Effects Between Liquid Crystal and Crystalline Phase on Photo-Responsive Elastomers Toward Quick Photo-Responsive Performance.

Adopting only a small amount of azobenzene molecular to design liquid crystal photo-responsive materials capable of quick response and flexible adjustability is in high demand but is challenging. Herein, we introduced azobenzene molecules into polyurethane elastomer containing crystalline structure for preparing azobenzene liquid-crystal elastomers (ALCEs) and discovered this phenomenon of the synergistic effects between liquid crystal and crystalline phase. The key point of the work is that the synthetic ALCEs can utilize the reversible isomerism capability of azobenzene molecules under light irradiation, which can pry the motion of the macromolecular crystalline region in system to realize the large macroscopic deformation of the photo-responsive behavior. Obviously, the ALCEs sample containing azobenzene molecule and polyethylene glycol (PEG) crystallization can quickly bend illuminated by UV light and rapidly straighten under green light. Under the same UV irradiation, the bending speed, final bending angle, recovery rate and recovery ratio of ALCEs are larger than that of ALCEs without any crystalline structure. This devised ALCEs based on the synergistic effects between liquid crystal and crystalline phase can break through the current dilemma that the application of traditional azobenzene photo-responsive materials is limited by their concentration, greatly expanding the design thought and their scope of application. This article is protected by copyright. All rights reserved.

In silico investigation of the photoisomerization mechanism of push-push azoderivatives

The design of novel chromophores showing specific photophysical traits is nowadays crucial for the development of new dyes and optical devices. Azobenzene derivatives comprise the largest fraction of industrial dyes and remain one of the main functions to be employed in the design of general-purpose photoactivated switches. In this context, we have investigated the optical and photoisomerization properties of two push-push azo derivatives, from a purely theoretical perspective, to outline a mechanistic landscape that can contribute to setting the grounds for the future goal of formulating dyes with specific photophysical properties. Both derivatives show downhill excited-state potential energy surfaces, hinting at fast photoactivated isomerization. In addition, both the trans and cis forms show nearly complementary absorption spectra leading to discriminatory color and differential excitation possibilities. Additionally, the reverse cis to trans ground state thermal isomerization reactions show higher energy barriers than the parent azobenzene molecule, supporting their potential use in different technological applications.

Complex Monte Carlo Light-Driven Dynamics of Monomers in Functionalized Bond Fluctuation Model Polymer Chains.

We study Monte Carlo dynamics of the monomers and center of mass of a model polymer chain functionalized with azobenzene molecules in the presence of an inhomogeneous linearly polarized laser light. The simulations use a generalized Bond Fluctuation Model. The mean squared displacements of the monomers and the center of mass are analyzed in a period of Monte Carlo time typical for a build-up of Surface Relief Grating. Approximate scaling laws for mean squared displacements are found and interpreted in terms of sub- and superdiffusive dynamics for the monomers and center of mass. A counterintuitive effect is observed, where the monomers perform subdiffusive motion but the resulting motion of the center of mass is superdiffusive. This result disparages theoretical approaches based on an assumption that the dynamics of single monomers in a chain can be characterized in terms of independent identically distributed random variables.

On-chip mixing of cancer cells and drug using LED enabled 2D opto-wetting droplet platforms

Droplets of microliter size serve as miniaturized reaction chambers for practical lab on a chip (LoC) applications. The transportation and coalescence of droplets are indispensable for realizing microfluidic mixing. Light can be used as an effective tool for droplet manipulation. We report a novel platform for LED-based transport and mixing of cell-encapsulated microdroplets for evaluating dose response of cancer drugs. Microcontroller enabled LEDs (Light-emitting diodes) were used to actuate droplet movement on Azobenzene coated planar silicon substrates. Droplet transport was initiated by the spatial gradient in solid-liquid interfacial tension developed through LED triggered photoisomerization of Azobenzene substrate. Detailed UV-Visible characterization of Azobenzene molecule was performed for different LED light intensities and wavelengths. A complete standalone opto-wetting toolbox was developed by integrating various components such as a microcontroller, UV LED (385 nm), blue LED (465 nm), and Azobenzene coated photoresponsive substrate. 2D transport of DI water droplets (10–30 μl) along simple trajectories was demonstrated using this device. Subsequently, the proposed opto-wetting platform was used for performing drug evaluation through on-chip mixing of droplets containing cancer cells (A549—Lung cancer cells) and cancer drug (paclitaxel). Separate cell viability analysis was performed using MTT assays, where the cytocompatibility of Azobenzene and UV light (385 nm) on A549 cells were studied. The dosage response of paclitaxel drug was studied using both MTT (3-(4,5-Dimethylthiazolyl-2)−2,5-diphenyltetrazolium bromide) and live-dead cell assays. The results obtained indicate the potential use of our device as a cost-effective, reliable opto-wetting microfluidic platform for drug screening experiments.

Machine learning modeling of the absorption properties of azobenzene molecules

We present a machine learning framework for modeling the absorption properties of azobenzene molecules – an important class of organic compounds with many potential photochemical applications. The framework utilizes predictors based on the chemical composition and structure of each molecule and consists of separate regression models trained to predict the absorption at distinct wavelengths, covering the UV and visible light ranges. Despite the relatively small size of the dataset (330 molecule-absorption spectrum pairs), the models were able to learn to accurately predict the absorption at fixed wavelengths, as well as the position and intensity of the maximum absorption. These predictions can be used to rapidly screen thousands of candidate molecules for a variety of potential applications, reducing the need for time-consuming and expensive experiments or first-principles computations.

Reversible Photo-switching of G‐Quadruplex by Backbone-Inserted Azobenzene

Introduction: Taking the typical human telomeric G-quadruplex (G4) sequence as a reference, we designed four photoresponsive DNA sequences by inserting azobenzene moieties into a planar interlayer and the end surfaces of the G4 structure. Method: The photo-responsive G4 molecules were investigated by melting curve, FRET, CD, and gel electrophoresis. Results and Discussion: The measurements showed that the photo-responsive G4 molecules took the stable four-strand structure under visible light, but after UV light irradiation the G4 structures tended to be disentangled. When azobenzene molecules were inserted at the end surfaces of the G4 structure, the Tm difference of the photo-responsive G4 between visible light and UV light reached more than 30 oC. Conclusion: In a temperature range from 20 to 53 oC, the reversible transformation of the G4 structure can be realized solely by light irradiation.

Collective transport and reconfigurable assembly of nematic colloids by light-driven cooperative molecular reorientations.

Nanomotors in nature have inspired scientists to design synthetic molecular motors to drive the motion of microscale objects by cooperative action. Light-driven molecular motors have been synthesized, but using their cooperative reorganization to control the collective transport of colloids and to realize the reconfiguration of colloidal assembly remains a challenge. In this work, topological vortices are imprinted in the monolayers of azobenzene molecules which further interface with nematic liquid crystals (LCs). The light-driven cooperative reorientations of the azobenzene molecules induce the collective motion of LC molecules and thus the spatiotemporal evolutions of the nematic disclination networks which are defined by the controlled patterns of vortices. Continuum simulations provide physical insight into the morphology change of the disclination networks. When microcolloids are dispersed in the LC medium, the colloidal assembly is not only transported and reconfigured by the collective change of the disclination lines but also controlled by the elastic energy landscape defined by the predesigned orientational patterns. The collective transport and reconfiguration of colloidal assemblies can also be programmed by manipulating the irradiated polarization. This work opens opportunities to design programmable colloidal machines and smart composite materials.

Environment polarity effects on the microscopic nonlinear optical properties of some asymmetrical azobenzene molecules

Solvent as a complex environment can surround solute molecules and modify their function. In this work, different solute–solvent interaction effects on the nonlinear optical behavior of some azo dyes were studied at the molecular level. In this case, spectroscopic technique and density functional theory were used. According to the results, molecular dipole transition, changes between ground and excited state dipole moments, substituent, and environment polarity play considerable effects on the molecular nonlinear optical responses. In addition to solvent effects on the molecular first- and second-order hyperpolarizability, the two-photon absorption cross-section is also modified due to solvent-induced interactions. Moreover, the determination of the contribution of dominant solvent effects on the molecular nonlinear optical behavior can facilitate the studies on the application of optical samples in designing various optical systems.

Co‐Harvest Phase‐Change Enthalpy and Isomerization Energy for High‐Energy Heat Output by Controlling Crystallization of Alkyl‐Grafted Azobenzene Molecules

Photoisomerization‐induced phase change are important for co‐harvesting the latent heat and isomerization energy of azobenzene molecules. Chemically optimizing heat output and energy delivery at alternating temperatures are challenging because of the differences in crystallizability and isomerization. This article reports two series of asymmetrically alkyl‐grafted azobenzene (Azo‐g), with and without a methyl group, that have an optically triggered phase change. Three exothermic modes were designed to utilize crystallization enthalpy (∆Hc) and photothermal (isomerization) energy (∆Hp) at different temperatures determined by the crystallization. Azo‐g has high heat output (275–303 J g−1) by synchronously releasing ∆Hc and ∆Hp over a wide temperature range (−79 °C to 25 °C). We fabricated a new distributed energy utilization and delivery system to realize a temperature increase of 6.6 °C at a temperature of −8 °C. The findings offer insight into selective utilization of latent heat and isomerization energy by molecular optimization of crystallization and isomerization processes.