Thermal Cis-Trans Isomerization Rates of Azobenzenes Bound in the Side Chain of Some Copolymers and Blends

Photomodulation of the Mechanical Properties and Photo-Actuation of Chitosan-Based Thin Films Modified with an Azobenzene-Derivative

A sophisticated comprehension of the impacts of photoisomerization and photothermal phenomena on biogenic and responsive materials can provide a guiding framework for future applications. Herein, the procedure to manufacture homogeneous chitosan-based smart thin films are reported by incorporating the light-responsive azobenzene-derivative Sodium-4-[(4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)diazen-yl]-benzenesulfonate (TEGABS) in the biopolymer through electrostatic interactions. When irradiated with UV-light the TEGABS/chitosan films show a biresponse, comprising the E→Z photoisomerization with a half-life of 13 - 20 h and the light-induced evaporation of residual moisture leading to an increase in the reduced indentation modulus (up to 49%) and hardness. Freestanding films of TEGABS/chitosan show actuation up to 13° while irradiated with UV-light. This work shows the potential of biogenic polysaccharides in the design of biresponsive materials with photomodulated mechanical properties and unveils the link between the humidity of the environment, residual moisture, and the photomodulation of the mechanical properties.

Optical and computational study of the trans ↔ cis reversible isomerization of the commercial bis-azo dye Bismarck Brown Y

The trans-cis-trans isomerization behaviour of Bismarck Brown Y (BBY) during and after irradiation with visible light, was characterized in detail for the first time by means of optical pump-probe experiments, to study the geometric inter-conversion of bis-azobenzene both in solution and embedded in multi-layered polymeric thin films. The rate constants observed for the thermal cis-trans back isomerization permit a determination of how the thermal isomerization is influenced by its local environment. In both solution and when incorporated into multi-layered thin films, the thermal relaxation observed for the commercial azo dye BBY showed a highly unusual biexponential decay, which clearly demonstrates two distinct isomerization processes. The cis decay showed an anomalous fast isomerization process on the timescale of milliseconds, followed by a slower isomerization process with a cis lifetime on the order of seconds. It was further observed that the faster isomerization process was influenced more by its local environment than was the slower process. The faster isomerization process also displayed a higher rate constant in aprotic solvents such as THF and DMF compared to that observed in protic solvents such as ethanol and water. Additionally, a higher rate constant was observed in solution compared to the multi-layered thin films where motion of the azo molecules was likely more restricted. Following recrystallization of the BBY azo dye, the more expected monoexponential decay was observed for the cis isomer in solution, with a single cis lifetime calculated on the timescale of seconds. This timescale corresponded well to values predicted by density functional theory calculations.

Controlling resonant surface modes by arbitrary light induced optical anisotropies

In this work the sensitivity of Bloch Surface Waves to laser-induced anisotropy of azo-polymeric thin layers is expe rimentally shown . The nanoscale reshaping of the films via thermal-Scanning Probe Lithography allows to couple light to circular photonic nanocavities, tailoring on-demand resonant BSW confined within the nanocavity.

Topographical transition of submicron pillar array of azo molecular glass induced by circularly polarized light

The well-aligned submicron patterns on surfaces have attracted wide attention from scientific curiosity to practical applications. Understanding their formation and transition is highly desirable for efficient manufacture of the patterns for many usages. Here, we report a unique observation on self-organized topographical transition of submicron pillar array of an azo molecular glass, induced by irradiation with circularly polarized light. During gradual erasure of the patterns upon exposure to the light, which is a property of this material, a new set of pillars unexpectedly emerge with new one in middle of each triangle cell of the original array. The highly regular pillar array with triple area density is formed and finally stabilized in the process, as revealed by thorough investigation reported here. This unusual observation and its rationalization will be of benefit for deep understanding of the light–matter interaction and can be expected to be applied in different areas.

Hydrogen Bond Enhances Photomechanical Swing of Liquid-Crystalline Polymer Bilayer Films

Mechanical swing is common in nature, such as sound waves, wingbeat of birds, and heartbeat, which is important to convert input energy into continuous motion. Here, we report a photodriven swing actuator composed of commercially available polyimide (Kapton) and azobenzene-containing liquid-crystalline polymers. The liquid-crystalline polymers act as the photoactive layer, which were synthesized by copolymerization of one benzenecarboxylic acid-containing monomer (M₆BCOOH) and one azobenzene-containing monomer (M₆ABOC₂) with different molar ratios. The Kapton layer with a high elastic modulus is photoinert and functions as the substrate layer. After thermal annealing, the film displays chaotic swing under continuous irradiation of actinic light. Interestingly, the swing amplitude is greatly enhanced by the existence of supramolecular hydrogen bonding in liquid-crystalline polymer films. It is the introduction of M₆BCOOH to the copolymer that accelerates the trans-cis photoisomerization rate of azobenzenes. Also, it forms a hydrogen bond as physical crosslinking sites, enabling the polymer film to work as a whole. Thus, it enhances the driving force for photomechanical deformation. Moreover, it improves the elastic modulus of the photoactive layer and modulates the swing behavior of the bilayer strip. More importantly, the formation of a hydrogen bond in the form of acidic dimers has a spatial confinement effect, extending the timescale of photodriven swing. The photomechanical self-vibration of the bilayer film can be ascribed to the combination of the photoisomerization process of azobenzenes with the local photosoftening effect of liquid-crystalline polymers.

Spectroscopic study of the pH dependence of the optical properties of a water-soluble molecular photo-switch

In this report we present a UV-Vis spectroscopic study of the pH dependent optical properties of 4,4'-azobenzene dicarboxylic acid in aqueous solution. A combination of chemical (acid-base) and light stimuli is combined to demonstrate that the system undergoes two types of optical switching cycles. At neutral pH the azobenzene undergoes photo-induced cis-trans isomerisation. Upon pH reduction the UV-Vis spectra show changes consistent with aggregation of the azobenzene photo-switch. The photo-responsive behaviour is dependent on the pH and conformation of the azobenzene. The optical properties of the system are dependent on the mode of pH reduction and the isomeric cis/trans composition of the photo-switch, showing hierarchical control of self-assembly.

Modeling of Azobenzene-Based Compounds

Azobenzene is by far the most studied photochromic molecule and its applications range from optical storage to bio-engineering. To exploit the great potential of azobenzene, one must achieve deep understanding of its photochemistry as single molecule in solution AS WELL AS in-chain moiety and pendent group in macromolecular structures. With the advent of computer-aided simulation scientists have been able to match experimental data with computational models. In this chapter, a review on the modeling of azobenzene-containing molecules in different conditions and environments IS provided with a special focus on advanced applications of photo-controllable materials, such as molecular machines and photoactivation of bio-molecules.

Molecularly Engineered Azobenzene Derivatives for High Energy Density Solid-State Solar Thermal Fuels

Solar thermal fuels (STFs) harvest and store solar energy in a closed cycle system through conformational change of molecules and can release the energy in the form of heat on demand. With the aim of developing tunable and optimized STFs for solid-state applications, we designed three azobenzene derivatives functionalized with bulky aromatic groups (phenyl, biphenyl, and tert-butyl phenyl groups). In contrast to pristine azobenzene, which crystallizes and makes nonuniform films, the bulky azobenzene derivatives formed uniform amorphous films that can be charged and discharged with light and heat for many cycles. Thermal stability of the films, a critical metric for thermally triggerable STFs, was greatly increased by the bulky functionalization (up to 180 °C), and we were able to achieve record high energy density of 135 J/g for solid-state STFs, over a 30% improvement compared to previous solid-state reports. Furthermore, the chargeability in the solid state was improved, up to 80% charged from 40% charged in previous solid-state reports. Our results point toward molecular engineering as an effective method to increase energy storage in STFs, improve chargeability, and improve the thermal stability of the thin film.

Synthesis of photoresponsive main-chain oligomers with azobenzene moieties via ADMET oligomerization and their micellization properties

We report the synthesis of azobenzene functionalized linear unsaturated/saturated polyolefin-oligomers.

Submolecular Plasticization Induced by Photons in Azobenzene Materials

We demonstrate experimentally for the first time that the illumination of azobenzene derivatives leads to changes in molecular environment similar to those observed on heating but that are highly heterogeneous at the submolecular scale. This localized photoplasticization, which can be associated with a free volume gradient, helps to understand the puzzling phenomenon of photoinduced macroscopic material flow and photoexpansion upon illumination far below the glass transition temperature (T(g)). The findings stem from the correlation of infrared (IR) spectral band shifts measured upon illumination with those measured at controlled temperatures for two amorphous DR1-functionalized azo derivatives, a polymer, pDR1A, and a molecular glass, gDR1. This new approach reveals that IR spectroscopy can be used as an efficient label-free molecular-scale thermometer that allows the assignment of an effective temperature (T(eff)) to each moiety in these compounds when irradiated. While no band shift is observed upon illumination for the vibrational modes assigned to backbone moieties of pDR1A and gDR1 and a small band shift is found for the spacer moiety, dramatic band shifts are recorded for the azo moiety, corresponding to an increase in T(eff) of up to nearly 200 °C and a molecular environment that is equivalent to thermal heating well above the bulk T(g) of the material. An irradiated azo-containing material thus combines characteristic properties of amorphous materials both below and above its bulk T(g). The direct measurement of T(eff) is a powerful probe of the local environment at the submolecular scale, paving the way toward better rationalization of photoexpansion and the athermal malleability of azo-containing materials upon illumination below their T(g).

Facile assembly of light-driven molecular motors onto a solid surface

In order to improve the rotary motion of surface assembled light-driven molecular motors, tetra-acid-functionalized motors were bound to an amine-coated quartz surface without prior activation of the acid groups. In contrast to earlier bipodal motors, the tetravalent motor showed no significant reduction in the rotation speed when attached to a surface.

Long-term stability at high temperatures for birefringence in PAZO/PAH layer-by-layer films

Optical memories with long-term stability at high temperatures have long been pursued in azopolymers with photoinduced birefringence. In this study, we show that the residual birefringence in layer-by-layer (LbL) films made with poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) alternated with poly(allylamine hydrochloride) (PAH) can be tuned by varying the extent of electrostatic interactions with film fabrication at different pHs for PAH. The dynamics of both writing and relaxation processes could be explained with a two-stage mechanism involving the orientation of the chromophores per se and the chain movement. Upon calculating the activation energies for these processes, we demonstrate semiquantitatively that reduced electrostatic interactions in films prepared at higher pH, for which PAH is less charged, are responsible for the longer stability at high temperatures. This is attributed to orientation of PAZO chromophores via cooperative aggregation, where the presence of counterions hindered relaxation.

Adhesion of photon-driven molecular motors to surfaces via 1,3-dipolar cycloadditions: effect of interfacial interactions on molecular motion

We report the attachment of altitudinal light-driven molecular motors to surfaces using 1,3-dipolar cycloaddition reactions. Molecular motors were designed containing azide or alkyne groups for attachment to alkyne- or azide-modified surfaces. Surface attachment was characterized by UV-vis, IR, XPS, and ellipsometry measurements. Surface-bound motors were found to undergo photochemical and thermal isomerizations consistent with unidirectional rotation in solution. Confinement at a surface was found to reduce the rate of the thermal isomerization process. The rate of thermal isomerization was also dependent on the surface coverage of the motors. In solution, changes in the UV-vis signal that accompany thermal isomerization can be fit with a single monoexponential decay. In contrast, thermal isomerization of the surface-bound motors does not follow a single monoexponential decay and was found to fit a biexponential decay. Both one- and two-legged motors were attached to surfaces. The kinetics of thermal isomerization was not affected by the valency of attachment, indicating that the changes in kinetics from solution to surface systems are related to interactions between the surface-bound motors.

Efficient surface-relief gratings in hydrogen-bonded polymer-azobenzene complexes

We show that efficient photoinduced surface-relief gratings can be inscribed in polymer-azobenzene complexes which are bonded by phenol-pyridine hydrogen bonding. The grating inscription was studied as a function of chromophore concentration and the molecular weight of the host polymer, both of which can be easily tuned without demanding organic synthesis. Stable gratings with modulation depth as high as 440 nm and with diffraction efficiency exceeding 40% were inscribed in the equimolar complexes. Our results demonstrate that phenol-pyridine hydrogen bonding not only allows one to increase the chromophore content until each polymer unit is occupied but is also sufficiently strong to induce mass migration of the polymer chains in a manner comparable to covalently functionalized polymers.

Photo-mechanical effects in azobenzene-containing soft materials

The change in shape inducible in some photo-reversible molecules using light can effect powerful changes to a variety of properties of a host material. The most ubiquitous natural molecule for reversible shape change is the rhodopsin-retinal protein system that enables vision, and this is perhaps the quintessential reversible photo-switch. Perhaps the best artificial mimic of this strong photo-switching effect however, for reversibility, speed, and simplicity of incorporation, is azobenzene. This review focuses on the study and application of reversible changes in shape that can be achieved with various systems incorporating azobenzene. This photo-mechanical effect can be defined as the reversible change in shape inducible in some molecules by the adsorption of light, which results in a significant macroscopic mechanical deformation of the host material. Thus, it does not include simple thermal expansion effects, nor does it include reversible but non-mechanical photo-switching or photo-chemistry, nor any of the wide range of optical and electro-optical switching effects for which good reviews exist elsewhere.

Azo polymers for reversible optical storage. 8. The effect of polarity of the azobenzene groups

Poly[4′-[[(2-methacryloyloxy)ethyl]ethylamino]azobenzene] (pMAEA) was prepared. Its reversible optical storage properties were studied and compared with the properties of previously reported poly[4-(2-methacryloyloxy)ethylazobenzene] (pMEA) and poly[4-nitrophenyl-4′-[[2-(methacryloyloxy)ethyl]ethyl-amino]phenyldiazene] (pDR1M). The effect of the dipole moment of the azobenzene moiety on the optical storage properties was studied by comparing the photochemically induced birefringence "writing" and "relaxation" behaviors. The writing and relaxation rates are both in the order pDR1M > pMAEA > pMEA. In addition, pDR1M, which contains donor/acceptor substituents in the azobenzene side chain, exhibits the highest saturation level of optically induced birefringence. The fraction of birefringence conserved after relaxation is lowest for pMEA, which has no donor/acceptor substituents in the azobenzene side chain, and highest for pDR1M. Keywords: polymers, azobenzene type, birefringence, photochemically induced orientation, reversible optical storage.