Azobenzene

The thermal cis-to-trans isomerisation process has been studied for a series of para-, ortho- and polyhydroxy-substituted azobenzenes in different solvents. The kinetics of the thermal backreaction for the p-hydroxy-substituted azobenzenes depend strongly on the nature of the solvent used, with relaxation times ranging from 200–300 milliseconds in ethanol to half an hour in toluene. Otherwise, the process rate is mainly independent of the solvent nature for the ortho-substituted analogs. Polyhydroxy-substituted azobenzenes show very much faster kinetics than the para- and ortho- monohydroxyazoderivatives. With relaxation times of 6–12 milliseconds in ethanol, they are optimal molecules for designing fast optical switching devices. All the hydroxyazoderivatives thermally isomerise from the metastable cis form to the thermodynamically stable trans isomer through a rotational mechanism.

Nano-layered hybrid compounds composed of a polyfluoroalkyl azobenzene surfactant (abbreviated as C3F-Azo-C6H) and layered inorganic nanosheets undergo three-dimensional morphological changes such as reversible shrinkage and expansion of interlayer spaces, and nanosheet sliding by photo-irradiation. Previously, we have investigated the photoreactivity of C3F-Azo-C6H/clay nano-layered hybrids in various microenvironments and found a remarkable enhancement in the photoreactivity for the cis-trans photo-isomerization reaction (F cis-trans = 1.9). In this paper, nanosecond and microsecond dynamics of trans-C3F-Azo-C6H and its assembly in various microenvironments have been studied by laser flash photolysis to get deeper insight into the extraordinary reactivity of the molecular assembly in the nano-layered microenvironment. In solution, the molecular trans-C3F-Azo-C6H exhibited only a depletion of the transform of azobenzene upon the laser pulse excitation. On the other hand, in the case of the C3F-Azo-C6H/clay hybrid film, the depletion of the transform was drastically recovered in three steps on nano-and microsecond timescales. This indicates that the once reacted C3F-Azo-C6H molecule (cis-C3F-Azo-C6H) was reverted back to the transform after the laser pulse. It is considered that the excess energy provided by the photo-excitation, which is immediately dissipated to the surrounding media through the intermolecular vibrational modes in solution, is trapped in the nano-layered microenvironment to thermally revert the cis-form back to the transform. Conversely, in the case of cis-trans isomerization of the C3F-Azo-C6H/clay hybrid film upon photo-irradiation, the reactivity would be much enhanced by the additional contribution of the thermal excess energy efficiently trapped in the nano-layered microenvironment. As compared with the hydrocarbon analogue (C3H-Azo-C6H), the subsequent recovery was very much enhanced in the C3F-Azo-C6H/clay film. The polyfluoroalkyl part of the surfactant layer plays a key role in the retarded dissipation of the excess energy by photo-excitation, which might be coupled with the three-dimensional morphological motion with efficient isomerization reactions.

An oxidative dimerization reaction of aromatic amines utilizing tert-butyl hypoiodite (tBuOI) under mild reaction conditions leads to aromatic azo compounds (see scheme). The method allows access to unsymmetric aromatic azo compounds, which are difficult to prepare by conventional synthetic methods, in a selective manner.

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A photoaddressable azobenzene-side-chain polymer (see Figure) optimized to exhibit a large optical birefringence of Δn = 0.5 at 650 nm is described. The birefringence extends well into the telecommunication wavelength region, which, together with the high optical transparency above 650 nm, promises applications for integrated photonic device applications. The writing dynamics of the polymer are examined, and it is shown that the rewriteability is limited by photobleaching.

Liquid-Crystalline Elastomers (LCEs) are materials that combine the entropic properties of a crosslinked polymer melt with the enthalpic properties of a liquid-crystalline state of order. LCEs show unique characteristics: visco-elasticity and order at the same time in one system. The elastic and the viscous properties come from the crosslinking and friction of the polymer chains, respectively, while the orientation comes from the mesophase which keeps the polymer backbone aligned. LCEs behave as normal polymer networks or rubbers when no energy-storing mesophase is present. This state of disorder can be induced by means of temperature or light.  Thermally, the change in shape of LCEs can easily reach 300% when all the enthalpy stored by the mesophase is released and the crosslinked polymer chains are free to move and adopt a random coil conformation. The light-induced local disorder can be achieved when shape-changing molecules are incorporated in the LCE matrix. These compounds are able to absorb light, rearrange themselves in a new shape and subsequently disturbing the mesophase. This results in the molecules that are keeping the order no longer being able to sustain the retractive force of the polymer backbone,  and the material contracts, exerting an actuating force. But how does a light sensitive side-chain LCE elastomer behave? And a main-chain LCE? What about nematics or smectics? Is a different  kind of actuation, besides the common retractive force,  possible? To answer these questions, new chemistry needs to be developed, together with new physics to understand the systems, and new applications need to be created.

Molecular switches that can undergo reversible switching
between two or more different states in response to chemical,
electrochemical or photochemical stimuli, have attracted much
attention in recent decades due to their promising applications
in chemical sensing, photocontrollable biological process and
molecular machines [1,2]. Most prevalent switches, including
interlocked molecules, azobenzene, spiropyrans, diarylethenes
and fulgides, have been widely investigated and employed for the
construction of stimuli responsive systems and materials

A comparative study of UV-irradiation-induced refractive index modulation in two analogous monodomain nematic side-chain liquid-crystalline elastomer materials is conveyed. In one of them, mesogenic azobenzene derivatives are incorporated as pendant co-monomers, and in the other as crosslinking units. The dependence of the optical diffraction properties on the polarization state of the probe beam reveals that diffraction is predominantly of a bulk origin for both materials. For prolonged UV exposures, the material with pendant azobenzene derivatives exhibits practically constant diffraction properties, while the material with cross-linking azobenzene units shows a profound decrease of diffraction efficiency with increasing exposure time. The difference is attributed to the photoinduced alignment of the azobenzene molecules, which is much stronger in the material with the crosslinking azobenzene units.

In this work we show the feasibility of using an azopolymer as an actuator to induce nano-and microscale movements controlled with light from the far field. We study azopolymers and their interaction with multi-walled carbon nanotubes (MWCNTs) by inducing surface relief gratings (SRG) through optical illumination. Upon different optical treatments, the MWCNTs are organized in the troughs or the crests of a surface relief grating. Large scale organization of MWCNTs has potential in applications such as transparent electronics.

A comparative study of UV-irradiation-induced refractive index modulation in two analogous monodomain nematic side-chain liquid-crystalline elastomer materials is conveyed. In one of them, mesogenic azobenzene derivatives are incorporated as pendant co-monomers, and in the other as crosslinking units. The dependence of the optical diffraction properties on the polarization state of the probe beam reveals that diffraction is predominantly of a bulk origin for both materials. For prolonged  UV  exposures, the material with pendant azobenzene derivatives exhibits practically constant diffraction properties, while the material with cross-linking azobenzene units shows a profound decrease of diffraction efficiency with increasing exposure time. The difference is attributed to the photoinduced alignment of the azobenzene molecules,  which is much stronger in the material with the crosslinking azobenzene units.

We describe the complex time dependence of the buildup of force exerted by a clamped photoelastomer under illumination. Nonlinear (non-Beer) absorption leads to a bleaching wave of a significant cis isomer dye concentration deeply penetrating the solid with a highly characteristic dynamics. We fit our experimental response at one temperature to obtain material parameters. Force-time data can be matched at all other temperatures with no fitting required; our model provides a universal description of this unusual dynamics. The description is unambiguous since these are clamped systems where gross polymer motion is suppressed as a possible source of anomalous dynamics. Future experiments are suggested.

Monodomains of nematic main-chain liquid-crystalline elastomers containing azo derivatives in the polymer backbone have been studied by means of opto-mechanical experiments at different temperatures for the evaluation of the mechanical response, kinetic rates, activation energies, and the isomerization mechanism. The increase in the crosslinking density of the network enhances the light-induced stress in the sample, leading to better performance and more tunability of the systems whenompared to nematic side-chain liquid-crystalline elastomers.

In-vivo, real-time study of the local and collective cellular biomechanical responses requires the fine and selective manipulation of the cellular environment. One innovative pathway is the use of photoactive bio-substrates such as azobenzene-containing materials (azopolymers), with optically tunable properties. In this work we show an innovative simple method to optically stimulate cells locally, by light-excitation of an azo-polymer derivative bio-substrate. Excited cells exhibit spectacular motility and reversible area shrinkage, which is dependent on the illumination. The photomechanical mechanisms taking place at the substrate and the cell/environment mechanical phenomena require further investigation.

Organic-inorganic hybrids composed of polyfluoroalkyl azobenzene surfactant (abbreviated as C3F-Azo-C6H) and inorganic layered compounds are able to undergo reversible three-dimensional morphology changes such as interlayer space changes and nanosheet sliding in a giant scale due to reversible trans-cis isomerization of the azobenzene moiety upon photo-irradiation. In this paper, we have systematically studied the relationship between the layered hybrid microstructures of C3F-Azo-C6H-clay and their photoreactivity for understanding the mechanism of the photo-induced morphology change. The photoreactivity was found to be very much affected by the surrounding microenvironments. As compared with it in solution, the cis-trans photo-isomerization in C3F-Azo-C6H-clay nano-layered film was substantially enhanced with the quantum yield exceeding unity (F = 1.9), while the trans-cis isomerization was rather retarded. The corresponding hydrocarbon analogue of the azobenzene surfactant (C3H-Azo-C6H) did not show such an enhancement. The enhancement was discussed in terms of a cooperative effect among adjacent azobenzene moieties along with polyfluoroalkyl chains and the inorganic clay nanosheet to prevent a dissipation of the excess energy being liberated during the photo-isomerization within the nano-layered microenvironment.

Amide linkage was introduced into azo compound at para position and its optical storage properties were investigated. Synthesized compounds showed liquid crystalline behavior when electron withdrawing group was inserted in the chemical structure. UV/Vis study showed that the photoisomerization effect in solution occurred at 18-24 sec (E-Z) and 5-11 hours (Z-E), whereas in solids it occurred at 30 sec (E-Z) and 5 hours (Z-E). Photoisomerization effect of amide based azodyes in the presence of hydrogen bonding is discussed for the first time. Effect of terminal electronic withdrawing groups on hydrogen bonding is speculated to be the reason behind the surprising behavior. Strong evidence for the structure property relations reported here is useful for applications such as optical storage device in which one can tune the structure according to one’s requirement.

The mechanical behaviour of monodomain nematic side-chain liquid-crystalline elastomerscontaining azoderivatives as pendant groups or crosslinkers has been studied under UVirradiation and in the darkness at different temperatures. From the evaluation of the opto-mechanical experiments, the mechanical efficiency,kinetic rates, activation energies and the isomerizationmechanism of the azocompounds in the liquid-crystal-line matrix could be determined, as well as the effect ofthe chemical constitution of the azobenzene deriva-tives and their role in the elastomeric network.