Reorientation Of Chromophores Research Articles

Unraveling the surface properties of PMMA/azobenzene blends as coating films with photoreversible surface polarity.

Various reports demonstrated that azobenzene derivatives are the chromophore of choice in photoresponsive surfaces showing reversible surface polarity. Hitherto the surface study of coating films based on polymer/azobenzene blends using contact angle measurements remained unexplored. To provide insight into the surface polarity of polymer/dye blend films, poly(methyl methacrylate) (PMMA) blends containing photoresponsive 4-hydroxy-4′-methylazobenzene (AZO1) and 4,4′-dimethylazobenzene (AZO2) as coating films on clear glass substrates are investigated in this work. Contact angle measurements were carried out to unravel the role of substituents in the surface polarity and the orientation of chromophores in the coating matrices before and after UV light (λmax = 365 nm) irradiation. Changes in water contact angles measured on the PMMA/azobenzene coating films indicated that the surface polarity is reversible as the chromophores underwent reversible trans–cis isomerisation. It has been revealed that the repeated trans–cis isomerisation led to the random reorientation and arrangement of chromophores in PMMA/AZO1 coating films. Then, to indicate the possibility of the disruption of interfacial interactions due to the repeated trans–cis isomerisation processes, as a proof of concept experiment, it is shown that the commercial acrylic-based pressure-sensitive sticker which adhered strongly to the PMMA/AZO1(13) coating film is peeled off from the coating surface after being subjected to a cycle of UV light irradiation for 12 hours, followed by dark conditions for another 12 hours within 14 days. The proof of concept study will lead to more development of smart photoresponsive coating films using simple polymer/dye blends.

Theory of Light‐Induced Deformation of Azobenzene Elastomers

AbstractSummary: A microscopic theory is proposed to describe light‐induced deformation of photo‐sensitive elastomers bearing azobenzene chromophores in their strands. We use an orientation approach in which it is assumed that the light‐induced deformation is caused by reorientation of azobenzene chromophores with respect to the electric vector of the linearly polarized light, E, due to the trans‐cis‐trans photoisomerizaion process whose efficiency depends on the orientation of the chromophores with respect to the vector E. In the framework of the Gaussian approximation for elasticity of network strands it is shown that the value of the light‐induced deformation depends on the chemical structure of network strands, namely, on the orientation distribution of chromophores around the main chains which is related to the length and elasticity of spacers. Depending on the chemical structure, azobenzene elastomers can demonstrate expansion or uniaxial contraction along the vector E, as well as non‐monotonic deformation with increasing light intensity (expansion at small light intensities and contraction at high ones).

Light-Induced Deformation of Azobenzene Elastomers: A Regular Cubic Network Model

We propose a microscopic theory of light-induced deformation of azobenzene elastomers bearing photosensitive azo-moieties in their strands. The theory is based on the orientation approach (Toshchevikov et al. J. Phys. Chem. B 2009, 113, 5032), in which the light-induced mechanical stress originates from reorientation of chromophores with respect to the electric vector of the light. A regular cubic network model built from freely jointed polymer chains bearing azobenzene chromophores is used to calculate the light-induced deformation of azobenzene elastomers under homogeneous light illumination. We show that the photomechanical behavior of azobenzene elastomers is very sensitive to their chemical structure: it depends on the orientation distribution of chromophores around the main chains which is defined by the chemical structure of spacers. Depending on the chemical structure, azobenzene elastomers demonstrate either expansion or uniaxial contraction along the electric vector of the light. The magnitude of the light-induced deformation depends on the degree of cross-linking: the larger is the degree of cross-linking, the smaller is the magnitude of deformation. Additionally, we discuss possible bending motions of azobenzene elastomers under inhomogeneous light illumination, when the light intensity changes inside the polymer due to the absorption of the light energy by the material.

Photoassisted Corona Poled YLD-124/DR1-co-PMMA Electrooptic Device Using Photoisomerization

A photoassisted corona poling technique (PACP) was applied to a binary chromophore system that was composed of YLD-124 and DR1-co-PMMA to enhance the poling efficiency of an electrooptic (EO) polymer device. The DR1 chromophores as a photoaddressable host material were resonantly irradiated by a polarized pumping green laser beam at a wavelength of 532 nm which is within its molecular absorption band. This process caused the DR1 chromophore's reorientation via a cis-trans photoisomerization process. The coupled YLD-124 system's dipoles were then poled with the assistance of a static electrical field applied to the EO core layer. In this letter, we report the experimental demonstration of the photoassisted corona poling scheme in a binary chromophore system that improved Mach-Zehnder optical modulator performance by lowering its <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> . It was shown that the device's <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> was reduced by 21%-23% compared to a regular device that was only corona poled without the green light assistance.

Nonlinear refraction in photo-induced anisotropic states of azo-dye containing materials

The optical anisotropic states in the azo-dye Disperse Red 13 doped poly(methyl methacrylate) polymeric film are induced by polarized light and the nonlinear refractive index in these states are studied by the Z-scan technique. It is found that the n 2 of the sample can be controlled by changing both the polarization direction and the intensity of the 514 nm light. Photo-induced isomerization and reorientation of azo chromophores in polymer matrices are used to explain the results.

Exciton migration and quenching in poly(propylene imine) dendrimers

Exciton migration between chromophore groups of the poly(propylene imine) dendrimer in chloroform solution and in solid state has been investigated by means of the time-resolved fluorescence measurements. Fluorescence decay kinetics, dynamic band shift and the depolarization rate have been analyzed. Exciton migration in a single dendrimer was found to be slow in comparison with temperature-dependent chromophore reorientation time of 150–600 ps. In a solid state chromophore groups form collective excitonic states responsible for the dendrimer film fluorescence. Exciton migration and localization to the lowest energy sites within the distributed density of states take place on a subnanosecond–nanosecond time scale.

Photoregulation of Ion Permeation through a Polyelectrolyte Multilayer Membrane by Manipulating the Chromophore Orientation

Toward the realization of nanoscale device control, we report a novel method for photoregulation of ion flux through a polyelectrolyte multilayer membrane by chromophore orientation that is adjusted either by illumination at normal incidence or by slantwise irradiation at an angle of 10 degrees with respect to the surface. Our results indicate that the chromophore reorientation caused by the slantwise irradiation controls the effective pore size and, consequently, the transport behavior on the nanoscale. The slantwise illumination, which includes six EZE photoisomerization cycles generated by alternately irradiating with ultraviolet (lambda = 360 nm) and visible (lambda = 450 nm) light, reversibly switches the orientation of E-azobenzene in the membrane between 53 +/- 2 degrees (high tilt) and 17 +/- 5 degrees (low tilt) with respect to the surface. The novel feature of this light-gated valve system is its extremely long-lived open-switch state; this behavior stands in contrast to that of other systems based on labile photoisomers, which tend to instantly return to the thermodynamically stable state.

Relaxation effect of stilbene azobenzene derivatives on their holographic properties

The material relaxation effect on holographic properties of stilbene azobenzene derivatives in the form of glassy films has been experimentally studied. Holographic grating recording with the period of 2 μm was made by a He-Ne laser at 633 nm in the self-diffraction mode. The readout was made simultaneously in order to follow the fast self-diffraction efficiency changes. The existence of the optimal material storage time (6-51 day) is established enabling the most efficient recording. Material relaxation amplitude and the holographic recording efficiency increased when the chromophore concentration was increased, especially above the threshold of about 70 mass %. It is also found that 633 nm recording due to the modulation of refraction and absorption indices is accompanied by the formation of surface relief grating. The conclusion is made that holographic recording in stilbene azobenzene derivatives at 633 nm is due to the chromophore reorientation by linearly polarized light possibly including trans-cis-trans transformations.

Preparation of side‐on bisazobenzene‐containing homopolymers and block copolymers via ATRP and studies on their photoisomerization and photoalignment behaviors

AbstractThe preparation of a series of novel homopolymers and copolymers containing bisazobenzene chromophores with side‐on structure in the side chains via atom transfer radical polymerization (ATRP) were presented. UV–vis spectra of the thin films of these polymers under irradiation of 488 nm Ar+ laser suggested that the photoisomerization of the bisazobenzene chromophores happened mainly on one of the two azo groups in the bisazobenzene chromophores with similar probability due to their side‐on structure. Good photoalignment behaviors of these polymers were proved by photoinduced birefringence measurements because side‐on structure permitted the two azo groups in the bisazobenzene chromophores both participated in the trans–cis–trans photoisomerization cycles equally to induce the whole chromophore reorientation. Furthermore, the reorientation axis located at the middle of chromophore decreased the sweep volume during photoalignment. The impetus for this study was to evaluate the photoisomerization and photoalignment process of side‐on bisazobenzene‐containing polymers and to find possible applications in the photosensitive devices. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3460–3472, 2007

Thermodynamic Theory of Light-Induced Material Transport in Amorphous Azobenzene Polymer Films

It was discovered 10 years ago that the exposure of an initially flat layer of an azobenzene-containing polymer to an inhomogeneous light pattern leads to the formation of surface relief structures, accompanied by a mass transport over several micrometers. However, the driving force of this process is still unclear. We propose a new thermodynamic approach that explains a number of experimental findings including the light-induced deformation of free-standing films and the formation of surface relief gratings for main inscription geometries. Our basic assumption is that under homogeneous illumination, an initially isotropic sample should stretch itself along the polarization direction to compensate the entropy decrease produced by the photoinduced reorientation of azobenzene chromophores. The magnitude of the elastic stress, estimated by taking the derivative of the free energy over the sample deformation, is shown to be sufficient to induce plastic deformation of the polymer film. Orientational distributions of chromophores predicted by our model are compared with those deduced from Raman intensity measurements.

Peculiarities of electric field alignment of nonlinear optical chromophores incorporated into thin film polymer matrix

The electric field poling process of free-standing films of poly(methyl methacrylate) (PMMA) matrix doped with the nonlinear optical compound 4-(dimethylamino)-4'-nitrostilbene (DMANS) was investigated by molecular simulation methods. The influence of the vacuum/bulk interfacial regions on static and dynamic properties, including the glass transition temperature Tg and the field-induced chromophore reorientation, was studied by employing films of three different thicknesses and by comparison with previous work on the bulk system. The interfacial region, defined as the region, where the local density increases from zero to the bulk density, is about 2 nm wide, independent of the film thickness. Tg decreases with decreasing film thickness, in accord with previous experimental work and theoretical predictions. The resistance against field-induced chromophore reorientation in the liquid state is found to increase strongly with the film thickness.

Self-Assembly of a Hydrophobically Modified Naphthalene-Labeled Poly(acrylic acid) Polyelectrolyte in Water:Organic Solvent Mixtures Followed by Steady-State and Time-Resolved Fluorescence

The solution properties of two water-soluble polymers, poly(acrylic acid) (PAA), covalently labeled with the fluorescent hydrophobic dye naphthalene (Np), have been investigated in water:organic solvent mixtures. The naphthalene chromophores have been randomly attached, onto the polymer, with two different degrees of labeling. Fluorescence measurements (steady-state and time-resolved) have been used to follow the photophysical behavior of the polymers and consequently report on the self-association of the polymers in the mixed organic (methanol or dioxane):aqueous solutions. The emission spectra of the high-labeled Np PAA reveal the presence of monomer and excimer bands whereas with the low-labeled polymer only monomer emission is observed. The excitation spectra collected at the monomer and excimer emission bands show significant differences, depending on the water content of the mixture, which indicate the simultaneous presence of preformed and dynamic dimers as routes to excimer formation. The time-resolved data decay profiles of the high-labeled polymer in the mixtures were always triple exponential whereas in pure methanol and dioxane they follow biexponential laws. The data in the mixtures are consistent with two types of monomers and one excimer. Both monomers are able to give rise to excimer in the excited state, one type involving the movement of long distant Np chromophores and the other involving a local reorientation of adjacent Np chromophores. These correspond to different decay times: (1) a long which corresponds to the long distant approach of non-neighboring Np chromophores forming an excimer and (2) a short corresponding to the fast adjustment of two neighboring Np chromophores in order to have the adequate parallel geometry. An additional decay time corresponding to the excimer decay was found to be present at all wavelengths. All the decay times were dependent on the water content of the mixture. An estimation of the two excimer forming rate constants values is made for the mixed media considered in this work. On the whole, using both steady-state and time-resolved fluorescence parameters, and by comparing data for a polymer with a small number of hydrophobes with a more highly modified one, it is possible in great detail to demonstrate how association is controlled by solvent quality for the hydrophobes and by the distance between hydrophobes.

Surface relief gratings on polymer dispersed liquid crystalsby polarization holography

We report the observation of surface relief gratings (SRGs) on polymer dispersed liquid crystal films after polarization holographic recording, demonstrating the formation of SRGs in systems without azo compounds, where photoisomerization and chromophore reorientation processes do not occur. Permanent SRGs, several hundred nanometers deep, are recorded on the surface of a polymeric material containing oriented liquid crystal droplets. The results suggest that SRG growth under uniform intensity irradiation is not exclusively related to the photoisomerization, but is a more general phenomenon which can involve different photoinduced chemical and physical mechanisms sensitive to the light polarization state. These effects contribute to the formation of anisotropic structures during the recording process.

Syntheses of bisazo‐containing polymethacrylates using atom transfer radical polymerization and the photoalignment behavior

AbstractThree kinds of photoresponsive polymethacrylates containing different bisazo chromophores were prepared with atom transfer radical polymerization and characterized with proton nuclear magnetic resonance, gel permeation chromatography, and ultraviolet–visible spectra. These polymers had similar molecular weights, molecular weight distributions, glass‐transition temperatures, and absorption coefficients. The irradiation of these polymer films with a linearly polarized laser could induce birefringence because of the reorientation of the bisazo chromophores through trans–cis–trans isomerization cycles of double azo bonds, and the corresponding mechanism was also examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4237–4247, 2004

Photorefractivity of hyperbranched polyisophthalate endcapped with pendant carbazolyl and azobenzene groups

A novel hyperbranched polyisophthalate containing pendant carbazolyl and azobenzene groups at the terminals was synthesized, and its photorefractivity has also been evaluated by the effective electro-optic (EO) coefficient, two-beam couple (TBC) coefficient, diffraction efficiency and photorefractive (PR) response time. This PR polymer has a low glass transition temperature of 21 °C without any extra plasticizer added, which provides a necessary free volume for EO chromophore reorientation during the recording process at room temperature. The obtained PR film exhibited an effective EO coefficient of 30 pm V −1, a TBC coefficient of 59 cm −1, a diffraction efficiency of 17%, and a modulation index of 1.67 × 10 −3 at the wavelength of 632.8 nm with an applied electric field of 80 V μm −1 at room temperature. The TBC coefficient was found to be beam-intensity dependent, compared with what is found in inorganic crystals. Moreover, a fast PR response time of 10 ms was deduced at an erasing beam intensity of 1.0 W cm −2.

Electro-optic response of chromophores in a viscoelastic polymer matrix to a combined dc and ac poling field

We present a generalized model for the electro-optic response of chromophores in a viscoelastic polymer matrix to a combined dc and ac applied poling field. The model includes a local molecular field of random orientation at each chromophore site to take into account the influence of the polymer matrix on the chromophore reorientation. Hence the model relies on a physically intuitive picture of chromophore dynamics in a viscoelastic polymer matrix. The dynamics is described by the rotational diffusion equation, where the local molecular field is inferred and a solution is presented with a variational approach. We obtain an analytical expression for the electro-optic response both at the modulating frequency and at two times the modulating frequency, having explicit frequency and molecular-field dependence. The model is successfully compared with frequency-resolved ellipsometric measurements in an azo-dye containing polymer guest–host system that is poled in a combined dc and ac electric field. The experimental setup is a modified Teng–Man ellipsometer in a balanced detection scheme, and, as a model system for investigating the electro-optic response, we use the chromophore Disperse Red 1 in a polymer matrix of poly(methyl methacrylate). Finally, the model is compared with independent experiments on a 2,5-dimethyl-4-(p-nitrophenylazo)-anisole containing photorefractive polymer composite, and good qualitative agreement is found.

Reversible Polarization Gratings on Thin Films of Polyoxetanes Bearing 4-(N,N-Diphenyl)amino-4′-nitroazobenzene Chromophores

Polarization gratings were fabricated by the two-beam coupling method on the surface of a series of polyoxetanes containing 4-(N,N-diphenyl)amino-4′-nitroazobenzene pendants with the azobenzene chromophores either side-on attached or end-on attached through short or long spacers to the main chain. The dynamics of formation of the gratings or writings was studied in relation to the diffraction efficiency. The erasing behavior of the gratings was also examined, irradiating a linearly polarized single beam. The temporal stability of the diffraction efficiency was further studied after removing the coupled excitation beam. Both the mode of attachment of the chromophore to the main chain and the length of the side-chain spacer greatly influenced the dynamic properties of the diffraction gratings. The presence of a long side-chain spacer and side-on attachment of the chromophore resulted in faster writing and erasing behavior. A preliminary study by solid-state 13C NMR of the relaxational behavior of the main-chain CH2 groups and the pendant chromophores provided a reasonable picture to help explain the photo-induced reorientation and relaxation of the azo chromophores at the molecular level.

Comparison of the birefringence in an azobenzene-side-chain copolymer induced by pulsed and continuous-wave irradiation

A comparative study of photoinduced birefringence in azobenzene-side-chain copolymers under continuous-wave (cw) and pulsed irradiation is presented. Absolute refractive index changes were determined by the attenuated total reflection spectroscopy. We clearly demonstrated that pulsed, similarly to cw, irradiation induces a three-dimensional reorientation of azobenzene chromophores. For the same light dose, pulsed irradiation using ns pulses of appropriate pulse energy led to higher values of birefringence, which can be attributed to a cooperative motion of azobenzene side chains and thermal effects induced by the laser pulses.

Electric field-induced second harmonic generation studies of chromophore orientational dynamics in photorefractive polymers

Photorefractive (PR) polymers are promising for use in various applications that require fast response times. The main factors that determine the PR speed in polymers are photoconductivity and chromophore reorientation in an electric field. In this article, we investigate the chromophore reorientational dynamics in various PR composites using the electric field second harmonic generation (EFISHG) technique, and then relate it to the PR dynamics observed in a four-wave mixing holographic experiment. We also report on the enhancement of the EFISHG signal in the presence of HeNe light, which we attribute to a photoinduced internal electric field that formed in the PR polymer.

Unusually Fast Optically Induced Birefringence in Polyoxetanes Bearing 4‐( N , N ‐Diphenyl)amino‐4′‐nitroazobenzene Chromophores

Two new polyoxetane polymers bearing azobenzene-type chromophore pendants were prepared, and their photoinduced molecular orientation and relaxation were studied. The two polymers differ in the mode of attachment of the chromophores, which are either laterally (Pox1) or perpendicularly (Pox2) bound to the polymer backbone. The two polymers reveal unusually fast (∼2 s to saturation at 0.4 W/cm2) photoinduced reorientation of chromophores and decay behavior at room temperature. Side-on attachment of the chromophores (Pox1) results in a significantly faster response to polarized light and higher maximum birefringence than end-on attachment. The kinetics of photoinduced orientation and relaxation decay were quantitatively analyzed and the results are discussed in relation to the structures of the polymers.