Molecular Addressing? Selective Photoinduced Cooperative Motion of Polar Ester Groups in Copolymers Containing Azobenzene Groups

Photoinduced Reorientation and Photofunctional Control of Liquid Crystalline Copolymers with in Situ-Formed N-Benzylideneaniline Derivative Side Groups

The influence of the annealing conditions on the thermally stimulated photoinduced molecular reorientation of a photoinactive liquid crystalline polymethacrylate with phenyl aldehyde and benzoic acid side groups (P1) doped with 4-methoxyaniline, which forms photoalignable 4-methoxy-N-benzylideneaniline (MNBA) side groups in situ, was investigated. Light exposure and subsequent thermal stimulation under a N₂ atmosphere realized sufficient cooperative molecular reorientation (D > 0.7), but the simultaneous thermal hydrolysis of the MNBA groups under humid air lowered the molecular reorientation performance. By contrast, subsequent thermal hydrolysis of MNBA after molecular reorientation introduced different aromatic amines into the reoriented P1 film, which regulated the birefringence and photofunctionality of the oriented film.

Enhanced Rates of Photoinduced Molecular Orientation in a Series of Molecular Glassy Thin Films

Photoinduced orientation in a series of molecular glasses made of small push-pull azo derivatives is dynamically investigated for the first time. Birefringence measurements at 632.8 nm are conducted with a temporal resolution of 100 ms to probe the fast rate of the azo orientation induced under polarized light and its temporal stability over several consecutive cycles. To better evaluate the influence of the azo chemical substituents and their electronic properties on the orientation of the whole molecule, a series of push-pull azo derivatives involving a triphenylaminoazo core substituted with distinct electron-withdrawing moieties is studied. All resulting thin films are probed using polarization modulation infrared spectroscopy that yields dynamical linear dichroism measurements during a cycle of orientation followed by relaxation. We show here in particular that the orientation rates of small molecule-based azo materials are systematically increased up to 7-fold compared to those of a reference polymer counterpart. For specific compounds, the percentage of remnant orientation is also higher, which makes these materials of great interest and promising alternatives to azobenzene-containing polymers for a variety of applications requiring a fast response and absolute control over the molecular weight.

Liquid crystalline side-chain triblock copolymers consisting of a nematic central subblock edged by photochromic azobenzene-containing fragments: their synthesis, structure and photooptical behaviour

Synthesis, phase and photooptical properties of new symmetrical fully liquid crystalline triblock copolymers.

Formation of surface relief grating in polymers with pendant azobenzene chromophores as studied by AFM/UFM

We studied peculiarities of the structural reconstruction within holographically recorded gratings on the surface of several different amorphous azobenzene-containing polymers. Under illumination with a light interference pattern, two processes take place in this type of polymer. The first process is the light-induced orientation of azobenzene units perpendicular to the polarization plane of the incident light. The second one is a transfer of macromolecules along the grating vector (i.e. perpendicular to the grating lines). These two processes result in the creation of a volume orientation grating (alternating regions of different direction or degree of molecular orientation) and a surface relief grating (SRG)-i.e. modulation of film thickness. One can assume that both orientation of molecules and their movement might change the local mechanical properties of the material. Therefore, formation of the SRG is expected to result also in modulation of the local stiffness of the polymer film. To reveal and investigate these stiffness changes within the grating, spin-coated polymer films were prepared and the gratings were recorded on them in two different ways: with an orthogonal circular or orthogonal linear polarization of two recording light beams. A combination of atomic force microscopy (AFM) and ultrasonic force microscopy (UFM) techniques was applied for SRG development monitoring. We demonstrate that formation of the phase gratings depends on the chemical structure of polymers being used, polymer film thickness, and recording parameters, with the height of grating structures (depth of modulation) increasing with both the exposure time and the film thickness. UFM images suggest that the slopes of the topographic peaks in the phase gratings exhibit an increased stiffness with respect to the grating depressions.

Photoinduced reordering in thin azo-dye films and light-induced reorientation dynamics of the nematic liquid-crystal easy axis

We theoretically study the kinetics of photoinduced reordering triggered by linearly polarized (LP) reorienting light in thin azo-dye films that were initially illuminated with LP ultraviolet pumping beam. The process of reordering is treated as a rotational diffusion of molecules in the light intensity-dependent mean-field potential. The two-dimensional diffusion model which is based on the free energy rotational Fokker-Planck equation and describes the regime of in-plane reorientation is generalized to analyze the dynamics of the azo-dye order parameter tensor at varying polarization azimuth of the reorienting light. It is found that, in the photosteady state, the intensity of LP reorienting light determines the scalar order parameter (the largest eigenvalue of the order parameter tensor), whereas the steady state orientation of the corresponding eigenvector (the in-plane principal axis) depends solely on the polarization azimuth. We show that, under certain conditions, reorientation takes place only if the reorienting light intensity exceeds its critical value. Such threshold behavior is predicted to occur in the bistability region provided that the initial principal axis lies in the polarization plane of reorienting light. The model is used to interpret the experimental data on the light-induced azimuthal gliding of the liquid-crystal easy axis on photoaligned azo-dye substrates.

Kinetics of photoinduced ordering in azo-dye films: two-state and diffusion models

We theoretically study the kinetics of photoinduced ordering in azo-dye photoaligning layers and present the results of modeling performed using two different phenomenological approaches. A phenomenological two-state model is deduced from the master equation for the one-particle distribution functions of an ensemble of two-level molecular systems by specifying the angular redistribution probabilities and by expressing the order parameter correlation functions in terms of the order parameter tensor. Using an alternative approach that describes light-induced reorientation of azo-dye molecules in terms of a rotational Brownian motion, we formulate the two-dimensional diffusion model as the free energy Fokker-Planck equation simplified for the limiting regime of purely in-plane reorientation. The models are employed to interpret the irradiation time dependence of the absorption order parameters defined in terms of the principal extinction (absorption) coefficients. Using the exact solution to the light transmission problem for a biaxially anisotropic absorbing layer, these coefficients are extracted from the absorbance-vs-incidence angle curves measured at different irradiation doses for the probe light linearly polarized parallel and perpendicular to the plane of incidence. It is found that, in the azo-dye films, the transient photoinduced structures are biaxially anisotropic whereas the photosteady and the initial states are uniaxial.

Photoinduced chirality in azobenzene-containing polymer systems

In this article, we will review the phenomena on circularly-polarized-light (CPL)-induced chirality in polymeric systems with photochromophores in their side-chain or main-chain. In the side-chain polymeric systems, the photoinduced chirality arises from superstructural chirality, i.e., helix structure, via aggregating of achiral azobenzene molecules in side chains. On the other hand, in the main-chain polymeric system, the macroscopically induced chirality can originate from individual chirality of chromophores in their main-chain backbone. The CPL irradiation on polymeric films with photochromophores to produce macroscopic chirality has been realized as potentially useful in the development of optical-switching, optical-storage, and light-driven devices.

Ab initio calculation of the vibrational and electronic spectra of trans- and cis-azobenzene

We report accurate geometries and harmonic force fields for trans- and cis-azobenzene determined by second-order Møller-Plesset perturbation theory. For the trans isomer, the planar structure with C(2h) symmetry, found in a recent gas electron diffraction experiment, is verified. The calculated vibrational spectra are compared with experimental data and density functional calculations. Important vibrational frequencies are localized and discussed. For both isomers, we report UV spectra calculated using the second-order approximate coupled-cluster singles-and-doubles model CC2 with accurate basis sets. Vertical excitation energies and oscillator strengths have been determined for the lowest singlet n(pi)* and (pi)(pi)* transitions. The results are compared with the available experimental data and second-order polarization propagator (SOPPA) and density functional (DFT) calculations. For both isomers, the CC2 results for the excitation energies into the S(1) and S(2) states agree within 0.1 eV with experimental gas-phase measurements.

Photoinduced three-dimensional orientational order in side chain liquid crystalline azopolymers

We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced three-dimensional (3D) orientational order in the films of liquid crystalline (LC) azopolymers. The technique is used to identify 3D orientational configurations of trans azobenzene chromophores and to characterize the degree of ordering in terms of order parameters. We study two types of LC azopolymers which form structures with preferred in-plane and out-of-plane alignment of azochromophores, respectively. Using irradiation with the polarized light of two different wavelengths, we find that the kinetics of photoinduced anisotropy can be dominated by either photoreorientation (angular redistribution of trans chromophores) or photoselection (angular selective trans-cis isomerization) mechanisms depending on the wavelength. At the early stages of irradiation, the films of both azopolymers are biaxial. This biaxiality disappears on reaching a state of photosaturation. In the regime of photoselection, the photosaturated state of the film is optically isotropic. But, in the case of the photoreorientation mechanism, anisotropy of this state is uniaxial with the optical axis dependent on the preferential alignment of azochromophores. We formulate the phenomenological model describing the kinetics of photoinduced anisotropy in terms of the isomer concentrations and the order parameter tensor. We present the numerical results for absorption coefficients that are found to be in good agreement with the experimental data. The model is also used to interpret the effect of changing the mechanism with the wavelength of the pumping light.

2000 Macromolecular Science and Engineering Award LectureThe versatility of azobenzene polymers

The well-known trans–cis–trans photoisomerization of azobenzenes produces at least three different kinds of motion in the polymer materials to which the azobenzenes are bound. The first is a photoinduced motion of the azobenzene groups only, and they can align in a selected position with respect to the light polarization. The second is a macroscopic motion of huge amounts of polymeric material, producing surface deformation, and the third is a reorganization of smectic domains in liquid crystalline polymers. These motions and their consequences are briefly discussed in relation to the polymer structure and some possible photonic applications are mentioned.Key words: photoinduced orientation, azobenzene polymers, surface gratings, photonics, thermochromism, photochromism, photorefractivity, photoinduced chirality and switching.

Photoinduced birefringence in copolymers containing Disperse Red 1 and styrene

A series of copolymers of 4'-[[[2-(methacryloyloxy)ethyl]ethyl]amino]-4-nitroazobenzene (DR1M) and an optically "inert" styrene comonomer were tested for photoinduced birefringence. The level of saturated birefringence increased linearly with the content of DR1M in the copolymer. The birefringence per mole of azo chromophore was constant along the copolymer series.Key words: azobenzene, photoinduced birefringence, copolymers, trans-cis isomerization.