Photoinduced Birefringence Measurements Research Articles

Photoresponsive Behavior of Heterocyclic Azo Polymers with Various Functional Groups

Here, we report on efficiency of light-induced processes in a series of methacrylic polymers containing heterocyclic azo moieties covalently attached to the polymer backbone. The optical response of the polymers was investigated in experiments on photoinduced birefringence measurements and surface relief gratings inscription upon irradiation with polarized blue light. The experimental results indicate that the presence of acceptor-type (Br) substituent in the side benzene ring linked to the heterocyclic ring has a beneficial effect on efficiency of birefringence generation, while incorporation of donor-type (CH3, OCH3) substituents lowers the level of photoinduced birefringence. Moreover, the process of surface relief structure formation was successfully observed for all studied polymers. The obtained surface-modulation depth reached up to 100 nm for two out of four investigated materials. No straightforward relation between the induced azo chromophore alignment level and the efficiency of mass-transport process is observed.

Vapour deposited nanocomposite films of perylene bis azo-imides with improved photoresponsiveness by visible light

The paper presents preparation of nanocomposite thin films of three newly synthesized perylene bis azo-imides in polyimide matrix by vapour deposition, results of DFT quantum chemical calculations on the optimized molecular geometry of trans (E) and cis (Z) isomers of the studied dyes as well as photoinduced birefringence measurements to explore potential optical application of these novel materials. Vapour deposited single films of the dyes were studied by reflectance UV-VIS and fluorescence spectroscopy in order to investigate the nature of electron transitions and donor-acceptor interactions between phenylazo fragments and perylene core. The nanocomposite films were prepared through vapour deposition of ODA (4,4′-oxydianiline) and PMDA (pyromellitic dianhydride) as the host polymer matrix and of corresponding perylene bis azo-imide dyes as “guest” at two various concentrations 5 % and 10 % w/w. It was found that nanocomposite films have improved photoresponsivness by applying visible light at λ = 532 nm compared to the single deposited films of dyes. This study shows the applicability of the synthesized dyes embedded in the polyimide matrix as visible-light activated molecular switches.

On stress – strain responses and photoinduced properties of some azo polymers

Mechanical properties of two amorphous azo polymers were characterized in tensile measurements and correlated with photoinduced behaviour of the materials observed upon irradiation with blue light of moderate intensity. Side-chain azobenzene poly(amide imide)s capable of forming high-quality free-standing layers of a large area and thickness of a few micrometers were chosen for the investigations. High Young's modulus of ca. 2 GPa and yield stress of the order of 30 MPa were found for the materials. Photoinduced birefringence measurements evidenced efficient chromophore orientation in both polymers, while polymer mass transport upon illumination with interference pattern was hardly observed. Nevertheless, subtle periodic changes in the surface topography of the films after irradiation with right- and left-handed circularly polarized beams suggested that opto-mechanical stress reached the value of a few tens of MPa, i.e., the minimum value needed for inducing plastic deformations in the studied azo polymers.

Influence of supramolecular interactions on photoresponsive behavior of azobenzene poly(amide imide)s

Abstract This study presents the comparison between the photoresponsive behavior of a series of six poly(amide imide)s bearing azobenzene-derivative chromophores as side groups. The solubility, molecular weight, glass transition temperature and thermal stability of the polymers are reported. The chosen azopolymers contain one or two azochromophores in the repeating units attached to the main chain at different locations, i.e., one of the aromatic cores of azobenzene is a part of backbone connected in the 2 and 4 position with either the amide or/and imide groups. UV–vis spectroscopy shows that the chromophore position affects the polymer absorption band. The photoinduced birefringence measurements reveal large differences in the photoresponsive behavior of the materials. The birefringence of order of 0.02 and 0.05 is achieved for the polymers of moderate molecular weight containing one azo-dye per structural unit attached between the amide groups or for the polymers with two chromophores per structural unit placed between both the amide and imide groups, respectively. The lack of optical anisotropy is observed for the polymers of a low molecular weight containing the azochromophores located between the imide groups, indicating a very low efficiency of the trans–cis isomerization process in these materials. The explanation of the observed relationship based on H-bond formation is confirmed by FTIR and 1H NMR analysis.

Thermal, optical and photoinduced properties of a series of homo and co-polyimides with two kinds of covalently bonded azo-dyes and their supramolecular counterparts

The paper describes the synthesis and characterization of new aromatic polyimides with one or two different moieties of the azo-dyes covalently attached to the polymer backbone and their supramolecular analogues. Azo-functionalized polyimides were prepared using post-polymerization method including the introduction of Disperse Red 13 and/or 4-[4-(6-hydroxyhexyloxy)phenylazo]pyridine to homo and co-polyimides containing hydroxyl groups via Mitsunobu reaction. The degree of functionalization of polymers with chromophores was estimated by UV–Vis spectroscopy. Polyimides containing hydroxyl groups were applied as matrixes to create supramolecular systems based on hydrogen bonds. Hydrogen-bond interactions in azosystems were studied by FTIR spectroscopy. The polymers were characterized by 1H NMR, FTIR, X-ray, UV–Vis, DSC and TGA methods. The photoisomerization process was investigated in supramolecular systems. The light-induced anisotropy was studied in a holographic gratings recording experiment and by photoinduced birefringence measurements. The polymer films were investigated by atomic force microscopy (AFM) after the diffraction grating recording to confirm formation of surface relief gratings (SRGs). To the best of our knowledge, that the first time photoinduced anisotropy has been studied by birefringence measurements in polyimides containing two different azo-dyes.

Large and highly stable photoinduced birefringence in poly(amideimide)s with two azochromophores per structural unit

We present a series of novel poly(amideimide)s containing two covalently bonded azobenzene chromophores in the polymer repeating unit. The chromophores possess different substituent in para position such as fluorine, hydroxyl, methyl or do not contain any of them. The synthesized polymers were characterized by 1H NMR, FTIR spectroscopies and elemental analysis. The polymers showed high glass transition temperatures (250–280°C), which were significantly separated from the temperatures of thermal decomposition (345–590°C). The photoinduced birefringence measurements were performed for continuous wave irradiation at 405nm. A very large, as for polyimides, birefringence of 0.06 was observed with the growth dynamics dependent on the chromophore substituent. The generated birefringence exhibited an exceptionally low relaxation. Apart from the poly(amideimide) with the methyl substituent it decreased by only 1.5% just after switching off the excitation light, and maintained its level during the next 500s.

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We present a series of novel poly(amideimide)s containing two covalently bonded azobenzene chromophores in the polymer repeating unit. The chromophores possess different substituent in para position such as fluorine, hydroxyl, methyl or do not contain any of them. The synthesized polymers were characterized by 1H NMR, FTIR spectroscopies and elemental analysis. The polymers showed high glass transition temperatures (250–280 °C), which were significantly separated from the temperatures of thermal decomposition (345–590 °C). The photoinduced birefringence measurements were performed for continuous wave irradiation at 405 nm. A very large, as for polyimides, birefringence of 0.06 was observed with the growth dynamics dependent on the chromophore substituent. The generated birefringence exhibited an exceptionally low relaxation. Apart from the poly(amideimide) with the methyl substituent it decreased by only 1.5% just after switching off the excitation light, and maintained its level during the next 500 s.

Poly(etherimide)s and poly(esterimide)s containing azobenzene units: Characterization and study of photoinduced optical anisotropy

In this paper novel processable, photoresponsible aromatic polymers with imide rings and covalently boned azobenzene units are presented. Prepolymerization strategy based on polycondensation of two diamines with azobenzene group, that is, 2,4-diamino-4′-cyanoazobenzne and 2,4-diamino-4′-nitroazobenzene and two dianhydrides, was utilized for preparation of the new photochromic polyimides. The obtained polymers differ in the chemical structure of polymer backbone and the kind of substituents on azobenzene moieties. Polymers were characterized and evaluated by FT-IR, 1H NMR, X-ray, UV-vis, DSC, and TGA methods. The synthesized azopolymers exhibited glass transition temperatures in the range of 140–170 °C, thermal stability with initial decomposition temperatures ( T d) in the range of 266–290 °C, and excellent solubilities in common organic solvents, providing optical-quality films. The light-induced optical anisotropy was studied in obtained polymers by photoinduced birefringence measurements. The birefringence was generated by the femtosecond pulses at 400 nm via a linear absorption of the trans-azobenzene. A large light-induced birefringence (Δ n = 0.02) was achieved in poly(esterimide)s contrary to poly(etherimide)s. The birefringence exhibited a very stable behavior after switching off the writing light indicating a permanent storage capacity.

Laser irradiation effects on the optical properties of layer-by-layer PPV/Congo Red thin films

In this paper we investigate the optical properties of thin films produced through the layer-by-layer (LbL) technique with a poly (p-phenylene vinylene) (PPV) precursor and the Congo Red (CR) azodye. Some significant changes are observed when the PPV/CR film is irradiated with polarized laser. An increase of about 10 times in the PPV emission intensity was observed, with an irradiation time of 100min. Photoinduced birefringence measurements revealed that a high birefringence was produced in the region of the film submitted to photo-irradiation, while a photobleaching phenomenon was observed in the same region. However, such phenomena were not detected in measurements performed at low temperatures. The results indicate that a degradation process of the CR molecules is occurring with photo-irradiation, and the changes observed in the photoluminescence spectra are correlated with the combined effects of degradation and energy transfer between the PPV light-emitting conjugated segments and CR azochromophores.

Post and prepolymerization strategies to develop novel photochromic poly(esterimide)s

AbstractThis paper reports novel photoresponsive, processable poly(esterimide)s functionalized with the azobenzene and tricyanovinylene groups. Post and prepolymerization strategies were utilized for preparation of the new photochromic polymers with imide rings. The postpolymerization method of azopolymer synthesis included a functionalization of precursor poly(esterimide) containing dialkylaminophenyl groups with diazonium salt of aniline. The precursor poly(esterimide) was obtained from synthesized 2,2′[N‐phenylethyloaniline‐di(4‐estro‐1,2‐dicarboxylic)]anhydride and 4,4′‐methylene bis(2,6‐dimethylaniline). The prepolymerization strategy based on polycondesation of 2,4‐diaminoazobenzene with two dianhydrides, that is, with or without the azobenzene group. The new dianhydride with azobenzene unit was obtained. The reaction of post‐tricyanovinylation was carried out for substitution of prepared polymers with tricyanovinylene groups. The synthesized polymers were characterized and evaluated by FTIR, 1H NMR, X‐ray, UV‐Vis spectroscopies, and DSC. The polymers exhibited glass transition temperatures in the range of 119–208°C and good solubilities in common organic solvents, providing optical‐quality films. Thermal stability of the polymers films was investigated by UV‐Vis spectroscopy. Preliminary investigations of the optical anisotropy in chosen poly(esterimide)s were carried out by photoinduced birefringence measurements. Relatively high photoinduced birefringence (Δn = 0.01) was measured for polyimide with covalently bonded azobenzene substituted with strong acceptor group. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Synthesis, Characterization, and Study of Photoinduced Optical Anisotropy in Polyimides Containing Side Azobenzene Units

In this paper, novel processable aromatic polymers with imide rings and attached as side-chain azobenzene units are presented. Polymers differ in the chemical structures of chromophores and polymer backbones. Azopolymers were obtained by a two-step synthetic approach. This includes the preparation of a precursor poly(esterimide) and poly(etherimide) with pendant phenolic hydroxyl groups, followed by the covalent bonding of NLO chromophores onto the polyimide backbone by the Mitsunobu reaction. The degree of functionalization of polymers was estimated by UV-vis spectroscopy. Polymers were characterized and evaluated by FT-IR, (1)H NMR, X-ray, UV-vis, DSC, and TGA methods. The synthesized polymers exhibited glass transition temperatures in the range of 167-228 degrees C, thermal stability with decomposition temperatures in the range of 275-446 degrees C, and excellent solubilities in common organic solvents. The light-induced optical anisotropy was studied in obtained azopolymers with the help of a holographic grating recording technique. Two polarization geometries were applied for the grating inscription s-s and p-p. The influence of the polarization geometry on the diffraction efficiency dynamics and on the depth of the surface modulation was not observed, which is different from results reported in the literature. Surface relief gratings, which appeared after the light exposure, were observed by atomic force microscopy. Additionally, the optical anisotropy in poly(esterimide)s was investigated by photoinduced birefringence measurements. For the first time, in polyimide with covalently bonded azobenzene derivatives, the high photoinduced birefringence (Delta n = 0.01) was measured.

Photoinduced Birefringence in Nanostructured SiO2:DR1 Sol–Gel Films

Pump-probe photoinduced birefringence measurements were carried out in amorphous and in nanostructured sol-gel films with Disperse Red 1 (DR1) azochromophores embedded in a SiO2 matrix. X-ray diffraction (XRD) patterns determine the long-range nanostructure order of the films, exhibiting a lamellar nanostructure, with two different d-spacings, due to the presence during the sol-gel process of the Sodium Dodecyl Sulfate (SDS) or of the Cetyltrimethylammonium Bromide (CTAB) ionic surfactants. The photoinduced birefringence measurements were performed on fresh and on heat treated films as a function of the pumping time. The measurements give us information about the effect of the nanostructures on the azochromophores orientation dynamics. As a result, for the same azochromophores concentration, annealed nanostructured films exhibited the largest azochromophore mobilities but by the other side, amorphous films had the largest signal intensities. Besides, we established a phenomenological model for the analysis of the azochromophores orientation in the films as a function of the pumping time.

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

Wide-field polarimetric analysis of photoinduced birefringence in azo-dye-doped thin films: irradiance and time dependence

We have investigated by wide field polariscopy the behaviour of photoinduced birefringence (PIB) in sol–gel hybrid disperse red 1 (DR1) doped films. This technique allows the mapping of the local values of a bell-shaped pump beam irradiance and those of the corresponding photoinduced birefringence. By means of a point-to-point correlation of these maps, we have tracked the dependence of PIB, simultaneously and continuously, on exposure times and pump irradiances. We have then applied some of the available theoretical models, representing birefringence as an analytical function of irradiance and time, to interpolate the experimental PIB data. As compared to the usual PIB measurements, which neglect the spatial distribution of the pump irradiance, our results provide a more detailed and reliable empirical basis on which models of the orientational mechanisms of the photoisomerizable molecules can be developed.