Kinetics of light-induced ordering and deformation in LC azobenzene-containing materials

Optical deformations of azobenzene polymers: orientation approach vs. other concepts

It has been 30 years since the discovery of surface restructuring in thin azopolymer films by two independent research groups. A wide variety of topographical structures have been created by the application of two-/four-beam interference patterns, space light modulators and even helical beams. There are a number of comprehensive reviews which describe in detail the advances in superficial photopatterning of azopolymer films and macroscopic deformations of azonetworks. The theoretical approaches are only briefly touched on in these reviews and often are accompanied by the remark that the phenomenon is far from being understood. In this review, we would like to present the polymer theoretist's point of view on this intriguing problem. We begin by describing a multitude of theoretical approaches and commenting on the pluses and drawbacks of each. Importantly, we show that in most cases the presence of an azopolymer matrix is either ignored or limited to a specific class of azopolymers (liquid-crystalline or elastomeric). We then move to early orientation approaches based on the hypothesis that reorientation of azo-chromophores by modulated polarized light is the sole cause of superficial patterning. At the end of the review a modern orientation approach, as proposed by our own group, is presented. This approach has high predictive power because it can explain a large pool of experimental data for different classes of azopolymers including glassy and liquid-crystalline materials. This is made possible by taking into account both the light-induced orientation process and the change of anisotropic interactions between the chromophores upon their isomerization. Last but not least, this is the only approach that provides an estimate of the light-induced stress large enough to cause plastic deformations of glassy azopolymers. Recent finite element modeling results show remarkable similarity to real patterns and even time-dependent data are well explained. With this, we claim that the puzzle is finally understood and the orientation approach is ready for its implementation for major azopolymer classes.

Unrestricted Chiral Patterning by Laser Writing in Liquid Crystalline and Plasmonic Nanocomposite Thin Films

Obtaining hierarchical structures with arbitrarily controlled chirality remains a challenge. Here, thin films featuring chiroptically bipolar patterns are produced by a device utilizing microscale photothermal re-melting of materials exhibiting chirality synchronization. This device operates autonomously, guided by an algorithm that facilitates the homochiral growth of supramolecular organic helices through controlling their re-melting. The chirality synchronization phenomena of constitutionally achiral molecules grants availability of both handednesses of the helices, enabling unrestricted chiral writing in the film. The collective chiroptical response of assembled molecules is utilised to guide the patterning process, creating a foundation for optically secured information. The established methodology enables achieving dissymmetry factor values for circular dichroism (CD) a magnitude higher than previously reported, as confirmed with state-of-the-art, synchrotron-based Mueller matrix polarimetry (MMP). Moreover, the developed method is extended to nanocomposites comprising gold nanoparticles, providing the opportunity to tune the CD toward the plasmonic region. This strategic application of photothermal processing, specifically laser-directed melting, uncovers the potential to broaden the selection of nanostructured materials with precisely designed functionalities for photonic applications.

Photo-Ordering and Deformation in Azobenzene-Containing Polymer Networks under Irradiation with Elliptically Polarized Light

Azobenzene-containing polymers (azo-polymers) have been a subject of extensive investigations during the last two and half decades, due to their remarkable ability to undergo pronounced alignment and deformation under irradiation with light. The molecular ordering and deformation in azo-polymers of various structures under irradiation with linearly polarized light was described in a series of theoretical works, based on the effect of the reorientation of azobenzene moieties due to the anisotropic character of the photoisomerization processes. In the present study, we generalize the previous orientation approach to describe the photo-alignment and deformation of azo-polymer networks under irradiation with elliptically polarized light. We demonstrate that, in general, the light-induced ordering and deformation have a biaxial symmetry defined by the polarization ellipse. Azobenzene chromophores have a tendency to align along the direction of light propagation, the orientation in the other two directions being dependent of the aspect ratio of the polarization ellipse. This causes deformation of azo-polymer networks along the direction of light propagation, the sign of which (expansion/contraction) is defined by a chemical structure of network strands. Theoretical results are in agreement with experiments and have a practical importance to predict the photo-mechanical response of azo-polymers depending on their structure and on the polarization of light.

Reprocessable Photodeformable Azobenzene Polymers

Photodeformable azobenzene (azo) polymers are a class of smart polymers that can efficiently convert light energy into mechanical power, holding great promise in various photoactuating applications. They are typically of crosslinked polymer networks with highly oriented azo mesogens embedded inside. Upon exposure to the light of appropriate wavelength, they experience dramatic order parameter change following the configuration change of the azo units. This could result in the generation and accumulation of the gradient microscopic photomechanical force in the crosslinked polymer networks, thus leading to their macroscopic deformation. So far, a great number of photodeformable azo polymers have been developed, including some unoriented ones showing photodeformation based on different mechanisms. Among them, photodeformable azo polymers with dynamic crosslinking networks (and some uncrosslinked ones) have aroused particular interest recently because of their obvious advantages over those with stable chemical crosslinking structures such as high recyclability and reprocessability. In this paper, I provide a detailed overview of the recent progress in such reprocessable photodeformable polymers. In addition, some challenges and perspectives are also presented.

Directing Polymorphism in the Helical Nanofilament Phase

Herein, it is reported that the polymorphism in the helical nanofilament (HNF, B₄ ) liquid-crystalline phase depends on the fabrication methods, that is, UV-driven formation and template-assisted self-assembly in the nanoconfined geometry. As a result, uniaxially oriented HNFs with different helical structures were obtained, in which generation of the twisted-ribbon and cylindrical-ribbon polymorphs showed that even the molecular lattice has a different orientation. The detailed structures were directly observed by SEM and grazing-incidence X-ray diffraction with synchrotron radiation. The resultant polymorphs could be used in chiro-optical applications due to the capability for fine control of the helical structures.

Modeling of Nonlinear Optical Phenomena in Host-Guest Systems Using Bond Fluctuation Monte Carlo Model: A Review

We review the results of Monte Carlo studies of chosen nonlinear optical effects in host-guest systems, using methods based on the bond-fluctuation model (BFM) for a polymer matrix. In particular, we simulate the inscription of various types of diffraction gratings in degenerate two wave mixing (DTWM) experiments (surface relief gratings (SRG), gratings in polymers doped with azo-dye molecules and gratings in biopolymers), poling effects (electric field poling of dipolar molecules and all-optical poling) and photomechanical effect. All these processes are characterized in terms of parameters measured in experiments, such as diffraction efficiency, nonlinear susceptibilities, density profiles or loading parameters. Local free volume in the BFM matrix, characterized by probabilistic distributions and correlation functions, displays a complex mosaic-like structure of scale-free clusters, which are thought to be responsible for heterogeneous dynamics of nonlinear optical processes. The photoinduced dynamics of single azopolymer chains, studied in two and three dimensions, displays complex sub-diffusive, diffusive and super-diffusive dynamical regimes. A directly related mathematical model of SRG inscription, based on the continuous time random walk (CTRW) formalism, is formulated and studied. Theoretical part of the review is devoted to the justification of the a priori assumptions made in the BFM modeling of photoinduced motion of the azo-polymer chains.

Security use of the chiral photonic film made of helical liquid crystal structures

The color change of photonic crystals (PCs) has been widely studied due to their beauty and anti-counterfeiting applications. Herein, we demonstrated security codes based on chiral PCs that cannot be easily mimicked and are quite different from the conventional technology used currently. The chiral PCs can be made by self-assembly and the structural colors change based on the polarization of the light in the transmission mode. These color changes are easily detected in real-time and are useful in the fabrication of anti-counterfeiting patterns that show beautiful and diverse color changes with rotating polarizers. We believe this can provide a new platform in various security and color-based applications.

Orientation Control of Helical Nanofilament Phase and Its Chiroptical Applications

Chiral liquid crystal phases show fascinating structural and optical properties due to their inherent helical characteristics. Among the various chiral liquid crystal phases, the helical nanofilament phase, made of achiral bent-shaped molecules, has been of keen research interest due to its unusual polar and chiral properties. This review is intended to introduce the recent progress in orientation control and its application to the helical nanofilament phase, which includes topographic confinement, photoalignment, and chiroptical applications such as photonic crystal and chirality sensor.

Modeling of the photo-induced stress in azobenzene polymers by combining theory and computer simulations

It has been shown recently that the photo-induced deformations in azobenzene-containing polymers of a side-chain architecture can be explained by means of the so-called orientational approach. The explanation is based on the following sequence of steps: (i) reorientation of azobenzenes under illumination, (ii) reorientation of the polymer backbones coupled mechanically to azobenzenes, and (iii) development of large stress in a material. Step (i) is based on the angle selective absorption of the azobenzene chromophore, which is a well established fact. Step (iii) has been validated in a series of recent theoretic studies in an infinite coupling limit. Concerning step (ii), in a real material, the backbone-azobenzene coupling will be always finite, resulting in a decrease of the effective torque sensed by the backbones and in a time delay in their reorientation. To study the relevance of these effects in detail, we perform coarse-grained molecular dynamics simulations of side-chain azobenzene-containing oligomers in bulk at conditions close to the glassy state. The focus is on the dynamical properties of such a system and on its response to the illumination, with the latter modeled either as an orientation potential applied to the azobenzenes or via their stochastic photo-isomerization. By matching the amount of light-induced stress evaluated in both cases, we obtained the equivalent orientation potential as a function of the illumination intensity and the system density.

Coatable Compensator for Flexible Display: Single-Layered Negative Dispersion Retarder Fabricated by Coating, Self-Assembling, and Polymerizing Host-Guest Reactive Mesogens

Antireflective films for flexible display devices should be made by a coating process rather than a stretching process. Additionally, the compensator, which is the core of the antireflective film, must exhibit a wavelength-independent phase retardation in the visible-light region to act as an ideal retarder. To satisfy all of these requirements, we intend to make a single-layered negative dispersion retarder (SNDR) with a single coating process followed by polymer stabilization. To achieve this goal, X-shaped reactive mesogen (X2RM) is newly synthesized as a guest RM and mixed into a host RM that exhibits a smectic A mesophase. Based on the thermal, spectroscopic, microscopic, and scattering analyses combined with computer simulation, the content of X2RM in the HCM026 molecule is optimized to be 40 wt %. The SNDR thin film is fabricated by coating the optimized H-G mixture on the rubbed alignment substrate and subsequent heat treatment. The trans-to-cis photoisomerization of imine bond can help X2RM to be located in the smectic interlayer of the HCM026. The molecular long axis of HCM026 is parallel to the rubbing direction of the alignment layer and the conjugated benzene rings of X2RM aligned perpendicular to the molecular long axis of smectic RM, which is the ideal molecular arrangement of negative dispersion retarder. Additionally, polarized UV polymerization improves the mechanical and chemical stability as well as the molecular orientation of SNDR.

Complex Dynamics of Photoinduced Mass Transport and Surface Relief Gratings Formation

The microscopic and semi-macroscopic mechanisms responsible for photoinduced mass transport in functionalized azo-polymers are far from deeply understood. To get some insight into those mechanisms on "microscopic" scale, we studied the directed photoinduced motion of single functionalized polymer chains under various types of polarized light illumination using Monte Carlo bond fluctuation model and our kinetic Monte Carlo model for photoinduced mass transport. We found sub-diffusive, diffusive and super-diffusive regimes of the dynamics of single chains at constant illumination and mostly super-diffusive regime for directed motion in the presence of the gradient of light intensity. This regime is more enhanced for long than for short chains and it approaches the ballistic limit for very long chains. We propose a physical picture of light-driven inscription of Surface Relief Gratings (SRG) as corresponding to a dynamical coexistence of normal and anomalous diffusion in various parts of the system. A simple continuous time random walk model of SRG inscription based on this physical picture reproduced the light-driven mass transport found in experiments as well as the fine structure of SRG.

Kinetics of Ordering and Deformation in Photosensitive Azobenzene LC Networks

Azobenzene-containing polymer networks are unique compounds that are able to change their shape in response to light, which makes them prospective materials for photocontrollable nano-templates, sensors, microrobots, artificial muscles, etc. In present work, we study the kinetics of light-induced ordering and deformation in two-component polymer networks containing optically inert liquid crystalline (LC) mesogens and azobenzene chromophores. By this, we generalize our previous theory [J. Phys. Chem. Lett. 2017, 8, 1094⁻1098] devoted to the kinetics of photoizomerization in one-component azo-polymers without mesogenic inclusions. The kinetic equations of photoisomerization are used, taking into account the angular selectivity of the photoisomerization with respect to the polarization direction of the light E. After multiple trans-cis-trans photoisomerization cycles, the azobenzenes are reoriented preferably perpendicular to the vector E. This changes the ordering of the mesogens due to the orientational LC interactions between the components. The light-induced reordering is accompanied by network deformation. Time evolution of ordering and deformation is found as a function of the intensity of light and structural parameters of the LC azo-networks, which define the viscosity, the strength of the LC interactions between the components, the volume fraction of the azobenzene moieties, and the angular distribution of azobenzenes in polymer chains. Established structure-property relationships are in agreement with a number of experimental data.

Self-Assembly of Azobenzene Derivatives into Organogels and Photoresponsive Liquid Crystals

A new class of coil-rod-coil molecules with an azobenzene core was synthesized. They were found to form robust organogels in several organic solvents. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), FTIR spectroscopy, UV/Vis absorption spectroscopy, ¹ H NMR spectroscopy, and X-ray diffraction (XRD) revealed that in these organogels, the molecules self-assembled into a nanofiber network with an H-type aggregation mode under the joint effect of π-π stacking, intermolecular hydrogen bonding, and van der Waals forces. Interestingly, the incorporation of the azobenzene mesogene into the rigid core led to photoisomerizable liquid crystal materials, which exhibited quick responsiveness to light and temperature, along with the trans-cis transition stimulated by UV light and heating.