Photoorientation Process Research Articles

The role of lateral substituent size in azobenzene chromophores on photo- and chiro-optical properties of photoactive polymethacrylates

Properties of functional liquid crystalline (LC) macromolecular materials, especially those showing self-organizing behaviour and targeted for specific purposes, strongly rely on the molecular structure of the mesogens used as side-chains. In order to get inside this relationship, new series of chiral LC polymethacrylates (PMA6Azo-R) with azobenzene groups having different lateral substituents (R), namely the methyl (-Me) and ethyl (-Et) groups on the central benzene ring, were synthesized and their comparative study was carried out. The synthesized polymers consist of photochromic azobenzene fragments that differ in the size of the lateral substituent (R = -H, -Me, -Et). All polymers exhibit the cholesteric phase, whereas the polymer without lateral alkyl group in addition forms the low-temperature SmĈ* phase. Investigations of the photoorientation process under linearly polarised visible light irradiation reveal that an increase in the size of side substituent considerably reduces the values of maximum achievable photoinduced dichroism (from 0.89 for PMA6Azo-H to 0.33 for PMA6Azo-Et that is associated with a decrease in the degree of mesogen anisometry). The effect of the lateral substituent size in azobenzene photochrome on the chiro-optical properties and mesomorphic behaviour of studied polymers was established. The presence of the lateral substituent in the photochrome/mesogen fragment leads to a significant decrease in isotropization temperature. Chiro-optical properties of the polymers and their photoswitching were studied, and it was found, that the introduction of the lateral substituent in the azobenzene fragment prevents the formation of exciton couples, that leads to a decrease in the circular dichroism (CD) values. UV irradiation and E-Z isomerisation of the azobenzene chromophores results in complete disappearance of CD signal; subsequent visible light irradiation does not lead to its full recovery. On the other hand, annealing of the films at temperatures higher than the glass transition almost fully recovers the initial CD values. The designed polymethacrylates can be considered as promising materials for various photonic applications.

Photo-Orientation of Liquid Crystals on Azo Dye-Containing Polymers.

In this communication, we summarise our results related to light-induced orientational phenomena at liquid crystal–polymer interfaces. We investigated photoalignment for various nematics at the interface with the photosensitive polymer layer polymethyl methacrilate functionalised with azo dye Disperse Red 1. It was found that the efficiency of photoalignment exhibits marked differences depending on the structure of the rigid core of the liquid crystal molecules. It was demonstrated that the photo-orientation process is also significantly affected by the type of mesophase in which irradiation is carried out. The observations highlight the importance of the mutual influence of the polymer and the liquid crystal in light-induced processes.

Photooptical Properties of Polymethacrylates Having Cyanoazobenzene‐Containing Side Groups with Lateral Methyl Substituents and Different Spacer Length

ABSTRACTNew photochromic polymethacrylates with different spacer length having azobenzene side groups and lateral methyl substituents were synthesized. The phase behavior of polymethacrylates and their photooptical properties were studied and compared with unsubstituted analogues. It is shown that an introduction of lateral methyl substituents results in almost complete suppression of liquid crystalline (LC) phase formation and strong decrease of photoinduced dichroism values. It is found that rates of the photoinduced E‐Z isomerization and back thermal Z‐E isomerization are almost independent on spacer lengths. Due to the presence of lateral substituents, the photoinduced azobenzene Z‐form shows remarkable long lifetime, and back thermal conversion at room temperature takes more than 10 days. Specific peculiarities of the photoorientation process in polymer films under the polarized UV and visible light action were studied and their mechanism is suggested. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1337–1342

The effect of spacer and alkyl tail lengths on the photoorientation processes in amorphousized films of azobenzene-containing liquid crystalline polymethacrylates

ABSTRACTIn our days, photochromic polymers possessing the liquid crystalline behaviour attract considerable attention of the scientific community due to their functionality and highlighted effects which can be used for opto-electronics and photonics. Herein, the photoorientation processes under the action of the polarised UV (355 nm) and visible (457 nm) light are studied for the amorphousized films of new azobenzene-containing polymethacrylates with different lengths of alkyl spacers and tails (5, 6 and 10 methylene units). The prominent effects of the chemical structure, namely length of the alkyl spacers and tails, as well as wavelength of the excitation light, on the peculiarities of the photoorientation process are demonstrated. Those effects can lead to quite important conclusions concerning possible influence of the microphase separation in polymer films.

No effect of the hydrogen bonds on the physicochemical properties of the guest-host poly(amide imide) azosystems and efficiency of chromophore orientation

In this paper, we show a series of the new noncovalent azosystems based on poly(amide imide) matrices. Azobenzene derivatives were attached to the polymer matrix via hydrogen bonds or formed doped systems. Additionally, glass transition temperatures of azosystems were established by the calculation procedure based on Fox equation. We proved that presence H-bonds between polymer matrix and chromophore does not effect on increase Tg and T5, in comparison to the doped polymers. It was observed that poly(amide imide) matrices are able to the thermally induced intermolecular cross-linking via hydrogen bonds. Functional density theory (DFT) was used to observed both changes in geometry the polymer matrix before and after incorporation of the azo-dyes and H-bonds formation possibility. The presence of H-bonds both intermolecular and between polymer and chromophore was confirmed experimentally by FTIR and 1H NMR spectroscopy. The supramolecular system based on the matrix with pyridine rings in the side chains and 4-[(4-methyl phenyl)diazenyl]phenol was found to show a relatively efficient photo-orientation process. We showed the hydrogen bonds between chromophore and polymer matrix did not provide higher and more stable the photoinduced birefringence in comparison with doped one. It seems that the critical parameter which impacts on values and stability of Δn is azo-dye content.

Photo-Orientation Phenomena in Photochromic Liquid Crystalline Azobenzene-Containing Polymethacrylates with Different Spacer Length

Synthesis and investigation of the phase behavior and photo-optical properties of two chiral liquid-crystalline polymers with azobenzene-containing side groups having different spacer lengths, 6 and 10 methylene groups (PMAzo-6 and PMAzo-10, respectively) are described. Formation of different smectic phases and high-temperature cholesteric phase is studied by polarizing optical microscope, differential scanning calorimetry, and X-ray investigations. It is shown that UV-irradiation induces effective reversible E–Z photoisomerization in polymer solutions and films. Atomic force microscopy (AFM) study reveals substantial changes in the surface topography of the polymer PMAzo-6 film after UV-irradiation, whereas PMAzo-10 surface remains the same. Irradiation by polarized light (457 nm) results in photo-orientation process in polymer films consequential in significant alignment of the chromophores in direction perpendicular to the polarization plane of the incident light. A significant difference is found in thermal stability of the photoinduced alignment; an annealing of PMAzo-10 irradiated samples results in a slight decrease of dichroism values (down to 0.57); whereas the dichroism increases up to very high values (0.91) for PMAzo-6.

ESR and Optical Study of Photo-Orientation in Azobenzene-Containing Liquid-Crystalline Polymer

We report a comprehensive experimental study of photoinduced processes in azobenzene-containing liquid-crystalline polymer. The kinetic and optical characteristics of the photoisomerization reaction and photo-orientation process have been determined. The spin probe technique has been used to elucidate the details of orientation order and rotational mobility of molecular fragments within the liquid-crystalline polymer. The evolution of orientation order parameters <P2>, <P4>, and <P6> in the course of photoinduced ordering of the material as well as the change of liquid-crystalline domain sizes have been studied quantitatively. The obtained experimental results are compared with the predictions of the existing models of photo-orientation.

Photochromic azobenzene functionalised banana–calamitic dimers and trimers: mesophase behaviour and photo-orientational phenomena

Azobenzene functionalised banana–calamitic dimers and a novel type of banana–calamitic–banana trimers have been synthesised. Their thermotropic behaviour and the structure of the mesophases have been characterised by differential scanning calorimetry, polarising optical microscopy and X-ray diffractions measurements. Electro-optical studies evidence the polar properties of the smectic phases. A rare anticlinic–synclinic smectic CP (SmCP) phase transition could be proven for two of the twins upon cooling without an electric field although the molecules are not optically active. Remarkably, the phase sequence SmCa–SmCs has been observed upon cooling, the inverse sequence to that reported for non-chiral compounds up to now. A comparative study of the photochemical properties of dilute solutions and of thin films of two compounds has been performed. Irradiation with polarised ultra-violet (365 nm) and visible light (473 nm) induces a photo-orientation process of the chromophores in a direction perpendicular to the polarisation plane of the exciting light. For a trimeric compound bearing an azobenzene bridge between both bent-core mesogenic units this process is associated with cycles of the E–Z–E isomerisation of the azobenzene groups followed by rotational diffusion of molecules in the films. For a bis(azobenzene) containing dimer the E–Z isomerisation in films is strongly suppressed and the photo-induced orientation has a rotational diffusion mechanism.

Photochromic LC copolymers containing azobenzene and crown‐ether groups

AbstractAn approach for the creation of a novel family of multifunctional crown‐ether‐containing comb‐shaped copolyacrylates consisting of chromophoric (azobenzene), ionophoric (crown‐ether), and mesogenic groups in the same macromolecule was developed. Phase behavior of the copolymers was studied, and correlation between their molecular structure and thermal properties was established. The increase of crown‐ether‐containing groups' concentration up to 26 mol % leads to disruption of nematic order and formation of amorphous phase. Influence of copolymers complexation with potassium perchlorate on mesomorphic properties of such systems was investigated. It was shown that complexation leads to decrease in mesophase thermostability due to the significant reducing of the side group anisometry by perchlorate counter ion. The comparative investigations of photooptical properties and photoorientation processes of copolymers and their complexes were also performed. An essential difference in kinetics of photooptical behavior was revealed; the bulky crown‐ether substituents decrease rotational mobility and prevent photoorientation process of azobenzene fragments diminishing photoinduced orientation and order parameter. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6532–6541, 2008

Photoorientation of azobenzene side groups in a liquid-crystalline polybutadiene-based polymer

The light-induced orientation of azobenzene side groups of a polybutadiene based liquid crystalline polymer was studied. The polymer is based on HO-terminated telechelic polybutadiene Krasol LBH 3000 grafted with 5-(4-{[4-(octyloxy)phenyl]azo}phenoxy)pentane-1-thiol units. Linearly polarized light of wavelengths 325, 365 and 488 nm influenced the photoorientation process. While irradiation with the 325 and 488 nm light generated optical anisotropy, UV light of 365 nm resulted in isotropic films. Pre-irradiation with the 365 nm light and subsequent irradiation with 325 nm led to dichroic ratio of about R D = 11. Further irradiation with 365 nm erased the anisotropy again.

Photoinduced Self-Organization and Photoorientation of a LC Main-Chain Polyester Containing Azobenzene Moieties

Light-induced ordering processes were investigated in spin-coated films of a smectic main-chain polyester with mesogenic 4,4‘-dioxyazobenzene groups in the backbone. Spin-coating results in aggregation due to π−π stacking and out-of-plane orientation. Both are destroyed by UV exposure. However, they are rebuilt by subsequent thermal Z/E isomerization of the azobenzene groups, forming a lamellar structure parallel to the substrate. The irradiation with linearly polarized visible light results in the orientation of the azobenzene moieties perpendicular to the electric field vector, preserving the lamellar structure with values of dichroism up to 0.43. However, the efficiencies of the E/Z photoisomerization and that of the photoorientation process strongly depend on the history of the sample. The scanning force microscopy (SFM) histogram indicates two types of domains in the top layer, one oriented in the normal and the second one formed by tilted azobenzene groups with an average tilt angle of about 50° to the normal. The last spacing is erased by annealing, but is rebuilt due to linearly polarized exposure. The photoinduced anisotropy of a “wet” dewetted ultrathin film of 5 nm is 1 order of magnitude smaller. It demonstrates that the photoreaction and the self-organization influence each other mutually.

Waveguide Spectroscopic Characterization of 3D Anisotropies in Conventionally Photooriented and Annealed Films of Liquid Crystalline and Amorphous Azobenzene Polymers

Films of azobenzene-containing polymers were photooriented in the glassy state in such a way that the azobenzene side groups were oriented preferably perpendicular to the electric field vector. In the case of liquid crystalline polymers ,the photoinduced anisotropies generated in the glassy state were modified by thermotropic self-organization due to annealing above T(g). The conventional photoorientation process results in an oblate order. The changes in photoinduced anisotropies brought about by annealing in the liquid crystalline phase were investigated quantitatively for the first time by us for different polymer compositions and experimental conditions. Different biaxial and homeotropic orders result for liquid crystalline polymers, depending on the experimental conditions. Different polymer structures are compared and the influence of the interfaces is investigated. Orientational gradients can be induced by irradiation or annealing and are for the first time determined by the WKB (Wentzel-Kramers-Brillouin) method.

Light-Induced Orientation of Liquid Crystalline Terpolymers Containing Azobenzene and Dye Moieties

Films of liquid crystalline copoly(methacrylate)s with photochromic azobenzene and benzanilide side groups and related terpolymers with additional dye side groups were oriented by the irradiation with linearly polarized visible or, alternatively, UV light. The orientational behaviors of the terpolymers with benzoxazole, stilbene, and anthracene moieties were compared. The irradiation conditions for the photoorientation process were optimized concerning wavelength and dose with respect to the absorption of the dyes and their limited photostability. Caused by the cooperativity of the photoorientation process, the light-induced ordering of the azobenzene group is connected with the alignment of the nonphotochromic co-mesogenic and the dye side groups even below the glass transition temperature. The light-induced orientational order generated in the glassy state was significantly amplified by the subsequent annealing of the irradiated films at temperatures within the mesophase of the co- and terpolymers. Amplification factors up to 30 were found in this series of polymers. The photoinduction process and the subsequent amplification by thermotropic self-organization were investigated in dependence of the polymer composition, the irradiation dose, and the wavelength of the incident light. The required dose or the irradiation time was significantly reduced by the optimization of light-induced and thermal processing, respectively. In this way, dichroic polymer films containing different types of dyes were created.

Photoorientation of a Liquid-Crystalline Polyester with Azobenzene Side Groups: Effects of Irradiation with Linearly Polarized Red Light after Photochemical Pretreatment

In contrast to the conventional photoorientation process with blue light, an orientation of 4-cyano-4‘-alkoxyazobenzene side groups parallel to the electric field vector of the incident light is generated upon irradiating films of a liquid-crystalline side-chain polymer with linearly polarized red light. The polyester is characterized by smectic and nematic phases g24SX26SA34N46i and a strong tendency to form J-aggregates. The process requires a photochemical pretreatment by irradiation with UV light or an exposure to visible light of high power density to produce a certain concentration of the Z-isomer, which destroys any initial orientational order and J-aggregates. The orientation process is cooperative, whereas the light-induced orientation of the photochromic moiety causes an ordering of the alkylene spacers and even of the main-chain segments into the same direction. The most probable mechanism of this two-step process is the angular-selective transformation of the bulky Z-isomers to the rodlike E-isomeric formed by the red light. The aligned E-azobenzene side groups become strongly J-aggregated. Very high values of dichroism of about 0.8 and birefringence of about 0.3 were generated as a result of this combination of the photoinduced orientation process and the thermotropic self-organization, which take place simultaneously under the irradiation conditions. The process results in a uniaxial prolate order of the film, whereas conventional photoorientation leads to a biaxial oblate order. These two different three-dimensional orders have been characterized by FTIR polarization spectroscopy and exhibit also varying intermediate thermal stabilities.

Photo-orientation at liquid crystal—polymer interfaces

It is suggested that liquid crystal—polymer interfaces are coupled systems, in which the components mutually influence the orientational state of each other. The photo-orientation process at liquid crystal-polymer interfaces provides a striking example of such a coupling. Experiments show that the anisotropic structure generated by polarised light at a polymer surface is strongly affected by the phase of the liquid crystal covering the polymer. Photo-orientation is significantly more efficient when the liquid crystal is in the isotropic phase than when it exhibits orientational order. The observations are interpreted by assuming that in the smectic and nematic phases the liquid crystal stabilises to a large extent polymer chain-segments aligned parallel to the director, while it blocks the photo-induced formation of chain-segments in the perpendicular direction. Other situations, in which the coupling between the liquid crystal and the polymer can be important, are also discussed briefly.

Photoorientation of a Liquid Crystalline Polyester with Azobenzene Side Groups. 1. Effects of Irradiation with Linearly Polarized Blue Light†

The photoorientation process in a polyester with 4-cyano-4‘-alkoxyazobenzene side group and long methylene spacers in the side and the main-chain was studied as a function of irradiation with linearly polarized light of 488 nm under systematic variation of the power density and temperature. This model polymer is characterized by liquid crystallinity (g 24 SX 26 SA 34 n 47 i) and a strong aggregation tendency. The photoorientation is cooperative, i.e., the orientation of the photochromic side group induces the alignment of the ester unit (which is a part of the main-chain) and both methylene segments in the side- and main-chain. The very high values of the normalized linear dichroism up to 0.8 and the birefringence (above 0.3) are due to the interaction of photoorientation and thermotropic self-organization. The induction of anisotropy shows a pronounced dependence on the power density and the working temperature. This is related to the thermal properties of the LC polymer and the resulting ordering behavior. Surprisingly, the photoorientation process is not restricted by J-aggregation of the azobenzene groups even at low power densities. The highest anisotropy at 27 °C (slightly above Tg) is achieved by a low power density (1 mW/cm2 and dose of 0.5 mW/cm2s). Higher power densities result in a decrease of the maximum dichroism and, finally, the initially induced small anisotropy is erased during continuing light exposure. At a constant high power density of about 700 mW/cm2, the saturation value of the dichroism first increases with the irradiation temperature, then passes through a maximum of about 0.8 at 18 °C and finally the film is transferred to the isotropic state at 27 °C. Both effects indicate a significant thermal effect of the laser beam on the polymer film. Taking into account the thermal properties of the investigated polymer it is demonstrated that the photoinduced anisotropy depends strongly on the irradiation conditions.

Photoorientation of azobenzene moieties in self-assembled polyelectrolyte multilayers

Abstract Using the alternating layer-by-layer (LBL) assembly technique, ultrathin polyelectrolyte multilayers with azobenzene-containing bolaamphiphiles or ionenes were prepared. In the initial films the orientational distribution of the azobenzene units is azimuthally isotropic, but a small out-of-plane orientation was found. Except for the photo-inert sample of polyethyleneimine/4,4′-disulfatohexyloxyazobenzene 1 , a Z -rich steady state was always established upon UV irradiation. The photoorientation process of the photochromic units was studied irradiating with linearly polarized visible light. This procedure lead to the induction of a significant in-plane dichroism due to the photoorientation of the photochromic unit, where the long axes become oriented perpendicular to the electric field vector of the incident light. For a sample of polydiallyldimethylammonium with 1 , for example, an in-plane dichroism of about 0.34 was induced. Films of azobenzene-containing ionenes and polystyrenesulfonate showed a dichroism of up to 0.17, which varied with the irradiation conditions. The results demonstrate a restricted mobility of the azobenzene units in the LBL assemblies caused by aggregation, orientational order and/or electrostatic interactions of the charged groups with the polyelectrolyte matrix so that only a rather weak anisotropy can be induced in such systems.

Influence of Liquid Crystallinity and Aggregation on Photo-orientation

The dependence of the Photo-orientation process on the liquid crystallinity and the molecular aggregation is studied. In the case of ordered films the photo-orientation is restricted, but this restriction can be overcome by a photochemical pre-treatment using UV light or by higher power density of the incident Vis light. The optical anisotropy increases with spacer length or the ordering tendency of the polymers.

Planar and Homeotropic Alignment of LC Polymers by the Combination of Photoorientation and Self-Organization

Upon irradiation with linearly polarized light a photoorientation process occurs in spin-coated films of polymethacrylates with 4-hexyloxyazobenzene side groups containing para methoxy and trifluoromethoxy tail groups. It results in the induction of an oblate orientational distribution perpendicular to the electric field vector causing an optical in-plane anisotropy up to an order parameter of about 0.52. The annealing of the photoreoriented films above Tg results in a prolate homeotropic alignment in the center of the irradiated spot up to a degree of order of 0.78, whereas an amplification of the photoinduced in-plane anisotropy is observed in the interim region to the nonirradiated film area. The development of the in-plane and the out-of-plane components is compared for the photoorientation and the subsequent photoreorientation process in this series of polymers. In both cases, the photogenerated order in the glassy state acts as an initializing force for the thermotropic self-organization resulting i...

Interdependence of Photoorientation and Thermotropic Self-Organization in Photochromic Liquid Crystalline Polymers

By irradiation with linearly polarized light optical anisotropy is generated in isotropic films of photochromic LCP's due to a photoorientation process. Generally, the efficiency is much higher in films of polymers with a high content of azobenzene moieties. On the other hand, the photochemically induced orientation of the photochromic side groups affects also the distribution of non-photochromic but mesogenic moieties. Furthermore, in initially isotropic films it acts as initializing force, governing the direction of thermotropic self-organization. The combination of both ordering principles causes a significant amplification of the light-induced optical anisotropy. Investigating several polymers of the same general structure but with a different content of azobenzene side groups an interdependent behaviour was found for the photoorientation of the photochromic and the co-operative orientation of non-photochromic side groups.