Azobenzene Polymer Research Articles

Structure Control for Fine Tuning Fluorescence Emission from Side-Chain Azobenzene Polymers

New fluorescent azobenzene dyes and side-chain polymers have been synthesized and characterized and their photophysical properties studied. A series of azobenzene dyes having different fluorophores such as phenol (S1), phenylphenol (S2) and naphthol (S3) incorporated in them were synthesized. S2 had unusually high fluorescence with a quantum yield of phi f = 0.2 recorded in dichloromethane (DCM), whereas S1 and S3 were found to be weakly fluorescent. The azobenzene dyes were converted into methacrylate monomers having short ethyleneoxy spacers and then free radically polymerized. Phenylphenol-based azobenzene polymer (P2) continued to show fluorescence, whereas fluorescence was completely quenched in the case of phenol (P1)- and naphthol (P3)-based polymers. Phenylphenol, though twisted in the ground state is known to have a more planar geometry in the excited state--a factor that enables it to retain its fluorescence behavior even when it is incorporated as part of an azobenzene unit. In contrast, naphthol, which is a better fluorophore compared to phenylphenol, loses much of its emissive behavior upon coupling to the azobenzene unit. The extent of trans to cis photoisomerization in solution was very low (approximately 17%) for P2 after 30 min of continuous irradiation using 365 nm light, in contrast to approximately 40% for P1 under identical conditions. This is attributed to the steric repulsion brought about by the bulky phenylphenol units that restrict rotation. A 2-fold enhancement in fluorescence emission was observed for P2 upon irradiation by UV light at 360 nm, which relaxed to the original intensity in about 7 day's time. The higher emission of the cis azobenzenes is generally attributed to an inhibition of photoinduced electron transfer (PET) mechanism. The emission of P2 showed a concentration dependence which increased initially and then decreased in intensity with the formation of a new red-shifted peak at higher concentration due to aggregation. Irradiation of the fluorescence quenched highly concentrated (1 x 10(-3) M) sample of P2 showed an enhancement in emission from aggregates at 532 nm.

Surface Organization, Light-Driven Surface Changes, and Stability of Semifluorinated Azobenzene Polymers

A series of polymers with 4-perfluoroalkyl-modified azobenzene side groups was investigated for its light-induced changes in surface properties. The ultraviolet (UV) light activated trans to cis isomerization of the azobenzene group, and the influence of molecular order and orientation on this process were studied using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and water contact angle measurements. Light-induced molecular reorganization in the near-surface region was studied by NEXAFS using in situ UV irradiation of polymer thin films. Differential scanning calorimetry and wide-angle X-ray scattering studies showed that sufficiently long fluoroalkyl groups formed well-ordered smectic mesophases in the bulk, as well as on the surface, which was evidenced by NEXAFS. The disruption of mesogen packing by photoisomerization was found to be influenced by the fluoroalkyl segment length. Surfaces with perfluorohexyl and perfluorooctyl groups that showed high orientational order were also highly resistant to light-induced changes. In such cases, the trans-cis isomerization resulted in greater lowering of the azobenzene phenyl ring order parameters than the perfluoroalkyl order parameters. UV exposure caused reorientation of the phenyl rings of the azobenzene group, but the terminal perfluoroalkyl segments remained more or less ordered.

Elastic Properties of Photoswitchable Azobenzene Polymers from Molecular Dynamics Simulations

Nanomechanical devices or molecular machines will, for a broad range of applications, most likely be powered by light or other kinds of electromagnetic radiation. The major reasons are ease of addressability, picosecond reaction times to external stimuli, and compatibility with a broad range of ambient substances, such as solvents, electrolytes, or gases. Azobenzene is a well-studied photoactive system, which can be photoswitched selectively from an extended trans to a more compact cis conformation by using light of wavelength 365 nm, whereas the reverse cis-to-trans isomerization is induced by light of wavelength 420 nm (Figure 1a). 6] Many processes, such as light-driven ion transport through biological membranes, can be steered by conformational switching of azobenzene chromophores. Azobenzene has been used frequently in synthetic photoresponsive systems for the regulation of the geometry and function of biomolecules. The isomerization of individual azobenzene molecules has also been studied by scanning tunneling microscopy. 21] Recently, a bistable polyazobenzene peptide was synthesized as a model system for a light-powered molecular machine, and its mechanical properties were characterized by means of single-molecule atomic force microscopy (AFM) experiments (Figure 1b, c). 23] Optical switching of the azobenzene polymers between their extended trans and compact cis conformations was demonstrated, and the corresponding change in the contour length of the polymer was detected. Thereby, in analogy to an Otto cycle, Gaub and co-workers established an optomechanical operating cycle, in which “optical” contraction against an external force delivered net mechanical work. Thus, they demonstrated that azobenzene polymers indeed hold great promise for future applications in nanotechnology, for example, as light-triggered molecular switches or cargo lifters. In a related experiment, Vancso and co-workers characterized a redox– mechanical cycle by using electrochemical AFM-based single-molecule force spectroscopy.

Confinement of surface patterning in azo-polymer thin films

Azobenzene polymer thin films are known to spontaneously generate surface patterns in response to incident light gradients. This peculiar process is investigated in terms of the dynamics of the various azobenzene photomotions, which occur on different length scales. In particular, the formation and thermal erasure of surface relief gratings are measured as a function of film thickness and by using combinatorial samples with thickness gradients. The thermal erasure of gratings in this system provides a direct measure of the glass-transition temperature, which is found to deviate substantially from the bulk value. Thin azo films exhibit a glass transition up to 50 K higher than the bulk. These dynamical measurements allow the authors to probe the length scale of mass transport, which is found to be approximately 150 nm. Furthermore, surface mass transport is completely arrested in thin films<40 nm. According to these results, mass transport involves the coordinated motion of many polymer chains in the depth of the sample, rather than surface diffusion of individual chains.

Photoinduced chirality in achiral liquid crystalline azobenzene polymers containing different polar side groups

Circular dichroism (CD) was induced in the films of two achiral liquid crystalline polymers, poly[(4′-(6-(methacryloyloxy)hexyl)oxy)-(4-X)azobenzene] (PM6X, X = C and N for cyano and nitro, respectively), by irradiation with one handed circularly polarized light (CPL) at 442 nm. The CD and UV spectra of the polymeric films suggest that left-CPL induces a left helical structure and right-CPL reverses the resulting structure into a right helix. For PM6C, the photoinduced CD values of the nematic film are much higher than in the casting and isotropic film. The photoinduced chirality of PM6C films is higher than PM6N due to a higher polarity of nitro groups in PM6N. The PM6N’s CD intensity around 450 nm is increased with the irradiation time of linearly polarized light (LPL) followed by left-CPL irradiation. The correlation between azobenzene chromophores and circularly polarized light will be discussed in the paper.

Hologram replication technique in glass plates using corona charging

The authors propose a technique of recording in glass plates using corona charging. The recording material used in this study is conventional soda-lime glass. A surface-relief hologram on an azobenzene polymer film coated on a glass plate can be recorded as an electric charge distribution in the glass plate using corona charging. The hologram recorded in the glass plate can be reconstructed as a surface-relief structure on a fresh azobenzene polymer film coated on the glass plate, again using corona charging.

Self-assembled spontaneous structures induced by a pulsed laser on a surface of azobenzene polymer film

The paper presents the self-assembled regularly patterned unique structures induced by the irradiation of pulsed laser beam on a surface of azobenzene polymer film. Self-assembled patterns depend on the polarization of laser beam. Linearly polarized beam gave rise to the ripple structures whose grating vector is perpendicular to the polarization of laser beam. Pitch of the ripple structures (Λ) was related to the wavelength (λ) of irradiated laser beam; Λ=λ∕n. Λ=λ∕1.05 for λ=266 and 355nm and Λ=λ∕1.3 for λ=420–540nm were obtained. These results implied that the ripple structures were produced by the interference of the incident beam with the optical wave scattered in a surface (waveguide mode light). The difference of n value is explained by the wavelength dispersion of refractive indices in the azobenzene polymeric matrix. Circularly polarized beam gave rise to the concentric wavelike structure. Pitch of wavelike structure was analyzed using two dimensional Fourier transform imaging techniques. Pitch of wavelike structure increases with increasing the number of laser shots. Furthermore, with increasing irradiation time, well-defined honeycomblike structure was also developed.

Temperature Dependence of Photoinduced Birefringence in anAzobenzene Polymer

The photoinduced birefringence in an azobenzene polymer is investigatedat different temperatures between −20°C to 50°C. It isfound that there is a peak value of photoinduced birefringence in thetemperature dependence of the photoinduced birefringence under a certainpumping intensity. With the pump light in 90 mW/cm2, the peak value ofthe photoinduced birefringence appeared at about 0°C. The effect oftemperature on the photoinduced birefringence is discussed using thecompetition mechanism between the photoinduced reorientation and thethermal random motion.

Molecular tracer diffusion in thin azobenzene polymer layers

Translational diffusion of fluorescent tracer molecules in azobenzene polymer layers is studied at different temperatures and under illumination using the method of fluorescence recovery after photobleaching. Diffusion is clearly observed in the dark above the glass transition temperature, while homogeneous illumination at 488nm and 100mW∕cm2 does not cause any detectable diffusion of the dye molecules within azobenzene layers. This implies that the viscosity of azobenzene layers remains nearly unchanged under illumination with visible light in the absence of internal or external forces.

Photomechanical Effects in Azo-Polymers Studied by Neutron Reflectometry

Neutron reflectometry is used to study photomechanical effects in thin films of azobenzene polymer cast onto silicon substrates. A significant photoexpansion effect, up to 17%, is observed at 25 °C, due to the free volume requirement of the azobenzene chromophore photoisomerization. Above a distinct crossover temperature of ∼50 °C, the material response is inverted and instead photocontraction effects, of more than −15%, are observed. In this case the combined photomotion and thermal mobility enables aggregation and crystallization of the azobenzene dipoles. The photomechanical effects, which can be reversed, occur readily using a variety of irradiation powers, incident polarizations, and film thickness values. This photomechanical behavior, which appears to be general to all azo materials, is likely the origin for a wide variety of curious photomotions observed in these systems, including macroscopic bending of samples and micron-scale surface mass transport.

Nanoscale Heterogeneities in Nematic Azobenzene Polymethacrylates for Optical Nanowriting

AbstractESR studies of the rotational dynamics of the cholestane spin probe dissolved in nematic azobenzene polymethacrylates from above TNI to below Tg are presented. Different dynamic regimes were recognized, in which the molecular sites or distributions of molecular sites were populated depending on the structure of the polymers and their thermal histories. We highlight the significance of such spatial‐temporal nanoscale heterogeneities in view of possible application of the azobenzene polymers in erasable optical nanowriting.

Low Driving Voltage and Analysis of Azobenzene Polymer DopedLiquid Crystal Grating

We mix azobenzene polymer and liquid crystal incertain ratio. Then the mixture is injected into cells. Nonlinearly photoinducedbirefringence takes place when linearly polarized ultraviolet is applied with thepattern photomask covering on the cells, which results in the formation ofazobenzene polymer doped liquid crystal grating. The obtained grating ischaracterized by an optical microscope and a He–Ne laser. The results indicate thatthe samples have clear grating structure, and the diffraction efficiencies can bemodulated by electric field. The sample driving voltage is 0.6 V/μm. It islower than the driving voltage of holographic polymer dispersed liquid crystaltransmission grating and could be matched with the driving integrated circuit.

Surface Relief Grating Couplers Using Azobenzene Liquid-Crystalline Polymer Films

Input and output grating couplers with a grating period of 1.3 µm were fabricated in an azobenzene liquid-crystalline polymer film by holography. When a He-Ne laser beam at 633 nm was incident to the input grating, the beam was coupled, propagated in the polymer film, and then was taken out from the output grating. The coupling efficiency of the input and output coupler was estimated as about 1.1% in TE mode. Upon irradiation of the film with UV light to cause trans-cis photoisomerization of the azobenzene moiety, the intensity of the output beam was repeatedly changed.

Numerical Analysis of Photoinduced Surface Relief Formed on Azobenzene Polymer Film by Optical Near-Field Exposure

The physical model of photoinduced mass transport, which was previously proposed, was improved. A novel numerical method was developed to analyze electromagnetic field distribution and energy transformation in more detail. The electromagnetic field was calculated using a modified cubic interpolated propagation/constrained interpolation profile (CIP) method. The mass transport was calculated using a particle method proposed previously. The photoinduced surface relief of an azobenzene polymer film formed by optical near-field exposure was simulated using a proposed physical model and a numerical method. The numerical results were compared with experimental results. The surface profile and energy dependence of thne photoinduced surface relief formation obtained numerically were coincided with the experimetal results.

Control and Suppression of Surface Relief Gratings in Liquid‐Crystalline Perfluoroalkyl–Azobenzene Polymers

AbstractSemifluorinated azobenzene liquid‐crystalline side‐chain polymers with different perfluoroalkyl lengths are used to develop surface relief gratings (SRGs). With sufficient fluorinated side‐chain lengths, surface relief gratings could be completely suppressed due to the self‐assembled liquid‐crystalline order at the surface. This indicates that there is a surface‐dependent nature to SRG formation. The SRG formation from the semifluorinated azobenzene liquid‐crystalline side‐chain polymers is compared to methoxy‐terminated azobenzene side‐chain polymers, and results show that the perfluoroalkyl–azobenzene polymers require a longer formation time than the methoxy‐terminated polymers.

Characterization of saturable absorbers using an open-aperture Gaussian-beamZscan

We present a theoretical investigation on the open-aperture Gaussian-beam $Z$-scan technique for two dominant saturable absorption models, by using the Adomian decomposition method. Analytic expressions of the $Z$-scan traces are deduced for a cw laser and a pulsed laser, respectively; in particular, for performable simulations, the optimal sum upper limit is found. We give an experimental demonstration, in which a side-chain azobenzene polymer poly [6-[1-(4-(4-nitrophenylazo)phenyl) piperazine] hexyl methacrylate] (Pda) film behaves as a test sample. The results manifest that our theory is reasonable and the Pda film is a good saturable absorber at the wavelength of $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

Holographic grating recording in azobenzene functionalized polymers

Abstract A group of 22 polymers have been synthesized to test their suitability for recording holographic gratings. Polyamides, polyimides, polyesters and their combinations were functionalized with pendant azobenzene groups containing single or double N=N. The polymers were studied using a standard degenerate two-wave mixing technique, which enables measurement of light-induced periodic modification of polymer refractive index and absorption coefficient by analysis of the diffracted light. Two qualitatively different configurations of the holographic polarization recording were used, s-s and s-p. The relationship between structural properties of polymer matrix and azobenzene groups and the holographic grating recording kinetics and light diffraction efficiency was investigated.

Image contrast reversal based on photoinduced optical rotation in azobenzene polymers

Image contrast reversal is demonstrated by using photoinduced rotation of the azimuth of the light polarization ellipse in an azobenzene polymer. The elliptically polarized plane wave of monochromatic blue light first passes an image transparency. Then the beam falls onto the polymer sample. The polarization azimuth of the light transmitted through the brighter areas of the image undergoes greater rotation after passing through the polymer film. Ellipticity remains unchanged. Finally, an analyser converts zones of the beam with different azimuths to different intensities. Thus, the contrast of the image can be processed, and even reversed by using a single beam for recording and readout.

Simultaneous X-ray and Visible Light Diffraction for the Investigation of Surface Relief and Density Grating Formation in Azobenzene Containing Polymer Films

ABSTRACT The development of surface relief and density patterns in azobenzene polymer films was studied by diffraction at two different wavelengths. We used x-ray diffraction of synchrotron radiation at 0.124 nm in combination with visible light diffraction at a wavelength of 633 nm. In contrast to visible light scattering x-ray diffraction allows the separation of a surface relief and a density grating contribution due to the different functional dependence of the scattering power. Additionally, the x-ray probe is most sensitive for the onset of the surface grating formation.

Temperature-controlled neutron reflectometry sample cell suitable for study of photoactive thin films

We describe a novel cell design intended for the study of photoactive materials using neutron reflectometry. The cell can maintain sample temperature and control of ambient atmospheric environment. Critically, the cell is built with an optical port, enabling light irradiation or light probing of the sample, simultaneous with neutron reflectivity measurements. The ability to measure neutron reflectivity with simultaneous temperature ramping and/or light illumination presents unique opportunities for measuring photoactive materials. To validate the cell design, we present preliminary results measuring the photoexpansion of thin films of azobenzene polymer.