Search

The synthesis of a novel series of copolymers containing push−pull bisazobenzene (BAz) and monoazobenzene (MAz) groups with various N-alkyl donor groups and with spacers of two and four methylene u...

A diffraction grating with a large diffraction efficiency (η = 25% on the first order) was inscribed on an azobenzene-containing polymer thin film, electrically poled, and characterized using spatially resolved Raman confocal microscopy and near-field scanning optical microscopy (NSOM) coupled with second-order nonlinear optical measurements. Linear (Raman) and nonlinear (second harmonic generation or SHG) polarized microscopic measurements were performed on a grating with ∼1.4 μm periodicity, revealing the molecular orientations in various regions of the sinusoidal relief surface. The most probable distribution functions f(θ) of the chromophore orientations were derived using the two first even parity Legendre polynomials 〈P2〉 and 〈P4〉 together with the two odd order parameters 〈P1〉 and 〈P3〉, as determined by Raman and SHG, respectively. These distribution functions show that the poling treatment is quite efficient and also emphasize the importance of determining both couples of the order parameters in s...

Circular dichroism and circular birefringence were induced in thin films of achiral polymer liquid crystals containing 7 or 15 mol % azobenzene chromophores by irradiation with circularly polarized light (CPL). Circular dichroism was observed not only for the azobenzene moieties but also for the nonphotoactive mesogens, cyanobiphenyl groups, due to a cooperative motion. CPL with opposite handedness produces enantiomeric structures, and a chiroptical switch can be achieved by alternating irradiation with left and right CPL. The level of photoinduced chirality depends on the incorporation methods (doped or chemically bound) of the azobenzene chromophores, the amount of azobenzene units, and the sample temperature. The photoinduced circular dichroism can be erased by heating the films above the clearing temperatures or by annealing the films in the liquid-crystalline phase.

A series of polyurethane elastomers containing aromatic azobenzene chromophore in the hard segment was synthesized by a one-step method. AFM surface study showed that particles on the order of 100 nm to several micrometers appeared after annealing the polymer films. Birefringence was induced in these polymer elastomers. The stability of the photoinduced birefringence could be improved significantly by annealing the polymer films. This also induced microphase separation. The effects of azo contents and molecular weight of soft segments were investigated. Dynamic study shows that photoinducing rate decreased while relaxation rate increased after annealing. Surface relief gratings could be induced only on the pure polyurethane and the elastomer containing 75% bis(azo)-diol. Only a reversible volume hologram was observed for the other elastomers.

A series of poly{4‘-{(X-methacryloyloxyalkylene)methylamino}-4-nitroazobenzene} (pXMAN, where X is the number of methylene units and varies from 2 to 12) polymers with liquid crystalline (LC) prope...

In this study, an achiral liquid-crystalline azo polymer is irradiated with circularly polarized light, and the transmitted polarization data show evidence of photoinduced chirality. When the polarization of the laser is switched from left- to right-handed, the chirality of the polymer film is switched as well. In a more revealing experiment, the handedness of the laser is fixed and the evolution of the ordering is monitored. Two models are described to interpret this result, one based on a uniaxial arrangement with the observed circular anisotropy included on a phenomenological level; the other based on a helical arrangement of chromophores. These models were used to fit the polarization data in order to determine the anisotropic optical constants that characterize the structural arrangement. It was noticed that during the irradiation, the sense of the circular anisotropy switches. This is also discussed in relation to both models presented.

An azo-containing liquid crystalline polymer was irradiated with linearly polarized blue Ar+ light and the resulting linear birefringence and dichroism were monitored. In its amorphous phase, results obtained are typical for azo polymers of similar structure, with the optical constants largest along the direction perpendicular to the laser polarization. In its nematic phase, surprising results are obtained, including two changes in the sign of the dichroism measured at 632.8 nm and, at one point, a quadrupolar transmission profile. Dichroic visible spectra show evidence for two bands of opposite sign in the region of the nπ* transition, which are attributed to the trans and cis isomers. In addition to the conventional photoreorientation of azo groups along an axis perpendicular to the polarization direction of the pump, we propose that, under some conditions, a tendency toward parallel alignment occurs as a result of the optical Freedericksz transition.

Structural order often manifests itself in the anisotropic optical phenomena birefringence and dichroism. Such properties of a material cause changes in the polarization of a probe beam. Characterization of the optical anisotropy, and subsequent inference on the structural arrangement of molecules responsible, may be accomplished in two stages. First, Stokes polarimetry is used to determine the polarization change in the transmitted light. Next, a model of the polarization transfer is used to fit the data in order to obtain values of the birefringence and dichroism which may serve as quantitative order parameters. The technique may be applied to linear as well as circular anisotropy, but is perhaps most valuable for cases in which both types coexist and therefore need to be separated. Our polarimeter is simple in construction and may be assembled from common optical elements.

Two copolymers, poly{4‘-[[[2-(methacryloyloxy)ethyl]ethyl]amino]-4-nitroazobenzene}-co-[(methacryloyloxy)-4-nitrophenyl-(S)-prolinol] (poly(DR1M-co-(S)-NPPMMA)) and poly{4-[2-methacryloyloxy)ethyl]azobenzene-co-[(methacryloyloxy)-4-nitrophenyl-(S)-prolinol] (poly(MEA-co-(S)-NPPMMA)), were synthesized and characterized. They contain side chain azobenzene chromophores (DR1 and MEA, respectively) and a prolinol-based chiral side chain group (NPP). Photoinduced birefringence, polarized absorption spectroscopy, and chiroptical properties (optical rotation and circular dichroism) in films were measured, and the results are discussed in relation to the structure of these copolymers. Cooperative motion of the chiral structural unit is observed for the photoinduced birefringence. Chirality is, however, not transmitted from the chiral structural unit to the azobenzene structural unit at the 1:1 compositions investigated here.

The design of polymeric materials with optimized photoconductivity and nonlinear optical properties is of major interest for emerging optical applications using photorefractive properties. This paper reports the synthesis and preliminary optical characterization of a series of novel, fully functionalized, photorefractive polymers. They were synthesized by a post azo coupling reaction of a polyether carbonate polymer based on the charge transport N,N,N‘,N‘-tetraphenyl-diaminobiphenyl group with 4-nitrobenzene-diazonium tetrafluoroborate by phase transfer catalysis. The amount of the inserted 4-nitro-phenyl-azo group was between 0.1 and 1.7 per structural unit. These polymers absorb in the UV−vis range, because of the presence of donor−acceptor substituted azobenzene chromophores. Light induced change of the refractive index was studied by birefringence measurements (λprobe = 632.8 nm) as a function of azo content in thin films with thickness ranging from 200 to 500 nm. Surface relief grating inscription wa...

Three novel polyurethanes with V-shaped bisazo groups (PU1 and PU2) or rodlike monoazo groups (PU3) in the main chain were synthesized. On irradiation with a linearly polarized laser beam, birefringence was induced in the three films to the level of 0.035, 0.037, and 0.044 for PU1, PU2, and PU3, respectively. The stable birefringence obtained at Tg − T = 33 °C was 71%, 46%, and 19% of that achieved at room temperature for PU1, PU2, and PU3, respectively. The relative stability of the photoinduced birefringence of PU3 decreased linearly with the increase of temperature, while that of PU1 and PU2 was constant even close to Tg. Surface relief gratings with high diffraction efficiency were inscribed on these main-chain polymers. The photoinduced gratings are only partially erasable by heating above the Tg of the polymers.

Irradiation with circularly polarized light of a film of an achiral azobenzene liquid crystalline polymer induces chirality. Circularly polarized light with opposite handedness produces enantiomeric structures. A chirooptical switch based on this phenomenon is reported.

A series of azobenzene-containing side chain liquid crystalline polymers with various spacer lengths (pXMAN, where X, the number of methylene units, varies from 4 to 12) have been synthesized and characterized. Phases and aggregation have been studied by UV-visible spectroscopy as a function of temperature. Polarized spectra were measured before and after irradiation with a resonant pump light at 488 nm. The annealing process on polymers with short spacers (p4MAN-p8MAN) induces a hypsochromic shift while, for longer spacers (p9MAN-p12MAN), it induces a large bathochromic shift. These effects may be due to the strong dipolar interaction and interdigitation of the side chain chromophores in an antiparallel orientation. The irradiation of such organized films with either circularly or linearly polarized light induces a bathochromic shift by breaking the antiparallel organization. This is a consequence of the angular reorientation of the azobenzene moieties as a result of trans-cis photoisomerization. To estimate the orientation of the side chain azobenzene photoactive group, of the aliphatic spacer and of the main chain, linear dichroism in the infrared spectral range was measured using the polarization-modulation technique with in situ irradiation. From these experiments, a high and stable value of photoinduced linear dichroism reveals a strong interaction and self-organization of azobenzene side chain molecules during the relaxation process. The anisotropic reorientation of the polymer main chain and of the spacer is small when compared to the reorientation of the azobenzene side chains, which become perpendicular to the linearly polarized pump beam. Additional birefringence measurements were carried out with in situ irradiation followed by a relaxation cycle for the whole series of polymers in various phases. The stability and the rate of the induced birefringence depend on the initial degree of organization within the thin film and on its thermal history. It is noteworthy that annealing can lead to a significant improvement of the birefringence level (+44%). The results obtained on photoinduced orientation and phase organization are correlated with the polymer properties.

An azocarbazole-based polyimide was synthesized from an azocarbazole diamine monomer and 4,4‘-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) by a two-step polycondensation reaction. Birefringence experiments have been performed on the pure polyimide and on the polyimide mixed with small azo chromophores. Because of its high Tg, the polyimide has a very stable induced birefringence (only 14% loss during the relaxation process). The stability of the mixtures is also very high. Surface relief diffraction gratings were inscribed on these systems. The measured diffraction efficiency was about 0.25% for a diamine-doped polyimide exposed for 1 h to an irradiance of 200 mW/cm2. The surface profile studied by atomic force microscopy showed an amplitude of 30 nm and a regular spacing of 700 nm on a 255 nm thick film. The photoinduced gratings had a high stability without further surface deformation after baking at 240 °C for 1 h in air.

Amorphous copolymers with rigid azobenzene and ester side groups form films in which birefringence can be induced using linearly polarized light. When both the azobenzene and the ester groups are p...

A series of copolymers with two structural units1-(4-nitrophenyl)-2-(4-{[2-(methacryloyloxy)ethyl]ethylamino}phenyl)diazene (DR1M), which contains donor−acceptor substituents in the azobenzene group, and 4-[2-(methacryloyloxy)ethyl]azobenzene (MEA), which has no donor−acceptor substituentswas prepared. The maximum obtainable photoinduced birefringence and the fraction of birefringence conserved after relaxation increase with increasing DR1M concentration in the copolymer. The rate of inducing birefringence is independent on the copolymer composition. In terms of the relaxation of the photoinduced birefringence, copolymers with higher DR1M content relax faster. The β-parameter of the stretched exponential functions describing the growth and relaxation of the photoinduced birefringence also appears to be independent of copolymer composition. The electronic spectra, as well as the birefringence studies, reveal that there is no significant interaction between DR1M and MEA structural units in the copolymers.

Nitrophenyl 4-((2-((2-methyl-1-oxo-2-propenyl)oxy)ethyl)oxy)benzoate (BEM) was synthe- sized and copolymerized with 4'-(((2-((2-methyl-1-oxo-2-propenyl)oxy)ethyl)ethyl)amino)-4-nitroazobenzene (DR1M) in dioxane at 60 °C. BEM and DR1M copolymerize in a random fashion with reactivity ratios rDR1M ) 0.82 and rBEM ) 1.03. The copolymers are amorphous. Interactions between the azo and the BEM side groups produce a shift of absorption maxima in the electronic spectra. Birefringence can be photoinduced and photoerased on films of all copolymers. The photoinduced birefringence per azo chromophore is very high in copolymers with low azo contents, about 4 times that of poly(DR1M). A cooperative motion of BEM groups with the azo groups is postulated as the reason for the additional birefringence. To our knowledge this is the first example of cooperative motion in amorphous polymers. The growth and relaxation of the birefringence are fitted by biexponential equations and "writing" rate constants are calculated for two processes, one is assigned to the fast motion of side groups and the other to the slow motion of the main chain. The fast "writing" rate increases with the DR1M content while the slow "writing" rate is almost constant. From the relaxation of the birefringence, about 85% of it is maintained in copolymers with high BEM contents. The maximum "writing" efficiency obtained for poly- (DR1M-co-BEM) is 5 10-6 cm3/J (at about 50 mol % DR1M) which is higher than that of poly(DR1M).

Poly{(4-nitrophenyl)-3-[N-[2-(methacryloyloxy)ethyl]carbazolyl]]diazene} was prepared and its photoinduced birefringence, diffraction grating, and photorefractive asymmetric two-beam gain coupling were studied. The monomer was obtained by performing azo coupling in a two-phase water−dichloromethane system in the presence of a phase transfer catalyst. Photoinduced birefringence of up to 0.09 was observed and diffraction efficiencies of up to 25% were obtained, with atomic force microscopy studies revealing that the grating profile exhibited a sinusoidal shape. Asymmetric two-beam coupling is observed, indicating photorefractive properties, but the energy transfer phenomena are more complex, involving also diffraction by the photoinduced gratings. The combination of these three optical propertiesphotoinduced birefringence, surface gratings, and two-beam couplingis believed to be unique and may eventually produce an all-optical device built of a single polymer film.

Amorphous copolymers of 4'-[(2-(methacryloyloxy)ethyl)ethylamino]-4-nitroazobenzene (DR1M) and 4-nitrophenyl 4-[2-(methacryloyloxy)ethyl]benzoate (BEM) have been studied by time dependent polarization modulation infrared spectroscopy. Bands due to several chemical groups of the copolymers have been analyzed in order to follow simultaneously the time dependence of the photoactive (DR1M) and inert (BEM) groups during the orientation (linearly polarized laser on) and relaxation (laser off) periods. The results show clearly the cooperative orientation of the BEM groups with the azobenzene side chains. The level of orientation and the rate constants of the two types of structural units have been determined for different copolymer compositions by fitting the orientation and relaxation curves with biexponential functions. The fast writing rate constant of DR1M increases linearly with the DR1M content while the slow writing rate constant is almost constant. The orientation kinetic of BEM groups is rather described by a monoexponential function and is mainly governed by that of photoactive side chains at high DR1M content. During the relaxation process, the fast and slow rate constants of DR1M groups are fairly constant over the whole range of copolymer composition while the degree of orientation of BEM groups is almost completely maintained.

A series of amorphous azobenzene-containing polymers were cast as thin films and shown to produce both reversible volume diffraction gratings and high-efficiency surface gratings by laser irradiation at an absorbing wavelength. The latter process involves localized mass transport of the polymer chains to a high degree, as atomic force microscopy reveals surface profile depths near that of the original film thickness. A mechanism for this phenomenon is proposed which involves pressure gradients as a driving force, present due to different photochemical behaviors of the azo chromophores at different regions of the interference pattern. The phase addition of the two beams in the interference pattern leads to regions of high trans-cis-trans isomerization by the absorbing azo groups, bordered by regions of low isomerization. As the geometrical isomerization requires free volume in excess of that available in the cast films, the photochemical reaction in these areas produces a laser-induced internal pressure ab...

Poly[(4-nitronaphthyl)[4-[[2-(methacryloyloxy)ethyl]ethylamino]phenyl]diazene] (pNDR1M) was prepared. The effect of the size of the azo moiety in the optical storage properties was studied. Both writing and relaxation processes can be described by biexponential functions. The bulkier pNDR1M exhibits a slower writing rate. However, there is no significant difference in the maximum birefringence achieved and in the relaxation rate when compared with the less bulky poly[(4-nitrophenyl)[4-[[2-(methacryloyoxy)ethyl]ethylamino]phenyl]diazene] (pDR1M).

Two amorphous copolymer series and one blend series based on Disperse Red I and methyl methacrylate are compared using absorption spectra and optically induced birefringence on spin-coated films. Observed blue shifts in the absorption spectra with increasing azo concentration suggest that, locally, azo dipoles are paired antiparallel both intramolecularly and intermolecularly. Induced birefringence studies indicate that the intramolecular dipolar interaction affects the orientability of the azo dipoles. The intermolecular dipolar interaction does not appear to have a large effect on orientability. The dependence of induced birefringence on azo content in the sample is different in the blend series (linear) and in the two copolymer series (nonlinear). A triad sequence distribution model is used to explain this difference. Observed writing efficiencies as a function of composition are also analyzed using a sequence distribution model. Dynamics of the inducing and birefringence relaxation processes are compared using biexponential fitting equations. jg p.2

Poly[4-[2-(methacryloyloxy)ethyl]azobenzene] (pMEA) has been synthesized and its reversible optical storage properties tested in comparison with previously reported electron-donor-electron-acceptor substituted azobenzene-containing polymers. The cis-trans thermal isomerization rate is fairly slow in pMEA, and while it is possible to induce birefringence using UV-polarized light, the levels and rates are not reproducible, due to the persistent cis population. Birefringence can, however, be induced with polarized visible light (514.5 nm), but at lower levels than in the previous polymers, due to the lower absorbance levels at this wavelength. The efficiency of the inducing birefringence process is about 4.3 x 10 -6 cm 3 /J and is comparable to the efficiency of the same processes in the previous polymers only if the differences in absorbances are considered. pMEA is also a good substrate for high efficiency diffraction gratings inscription on the film surface. A 90-nm grating (4% efficiency) can be obtained in only 35 min of irradiation with 22 mW/cm 2 at 514.4 nm.

A series of high-Tg side-chain azo aromatic polymers and small molecular weight azobenzenes dissolved in polymers were cast as thin films and subjected to a pumphelax procedure to analyze the thermal cis-trans isomerization behavior of the azo groups. All samples exhibit a fast isomerization process due to strained cis isomers trapped below the glass transition temperature of the film, followed by a slower isomerization process which can be analyzed by a monoexponential decay. Thermal isomerization rate constants are found to depend on the spectral type of the chromophore and type of attachment to the polymer backbone and, less importantly, on the glass transition temperature and the crystalline order of the polymer.

Copolymers of 4'4 ( 2-(acryloyloxy)ethyl)ethylaminol-4-nitroazobenzene (DRlA) with methyl methacrylate (MMA) and of the respective methacrylate monomer (DRlM) with MMA were cast as films and subjected to a pump/relax procedure to analyze the thermal cis-trans isomerization behavior of the azo groups. Blends of polyDRlA with polyMMA were also used in a similar manner. All samples show a fast isomerization process due to strained cis isomers, trapped below the glass transition temperature of the film, followed by a slower isomerization process. The homopolymers, all blends, and the copolymers with very low azo concentrations can be analyzed by a monoexponential decay for the slow process. The isomerization of all other copolymers is analyzed as a sum of three monoexponential processes, in which each process is the isomerization of an azo-centered triad. The rate constants found for the individual processes indicate that isomerization is faster when more azo groups are neighboring the central azo group in the triad. This is proof for the neighboring effect on motion of the azo groups, even in these completely amorphous samples.

Polarized light induces a preferred orientation of azobenzene groups through photochemical trans-cis-trans processes. Stronger orientation tendencies exist if the azo-containing polymers are crystalline or liquid crystalline and the cooperative reorientation of nonactive mesogens of similar shape has been demonstrated. This paper shows that cooperative reorientation is a more general phenomenon and can take place even for a nonactive rigid group of a different shape which is bonded in the main chain while the azobenzene groups are in the side-chains. Comparison is made between a disperse red containing amorphous azo polymer (pDR1A) and a disperse red 19 - containing semi-crystalline azo polymer (pDR19T) with phenylene diacrylate (PD) groups within the main chain. Dichroic ratios are obtaining from infrared and electronic spectra and are correlated with a writing/erasing/rewriting sequence performed with an argon laser. A maximum birefringence of 0.08 can be obtained on pDR1A, while on the birefringence is 0.27 in similar conditions. This is the result of a combination of the semicrystallinity of pDR19T and of the cooperative orientation of PD groups along with the azobenzene groups. The drawback in terms of optical storage is the longer time required to write or erase. Azo polymer, Cooperative motion, IR Dichroism.

Azo polymers have many special properties. In the search for new materials for optical applications, such as reversible optical storage and nonlinear optical devices, azo polymers have attracted much attention. Polymers with low azo concentrations are particulary useful. The writing and erasing efficiency depends on the energy input from the irradiation light during the process. In principle, lower energies would be required to induce local variations of optical properties in polymers with low azo concentrations

Two series of copolymers containing disperse red 1 and methyl methacrylate have been synthesized: poly(4'-(((2-(acryloyloxy)ethyl)ethy l) amino)-4-nitroazobenzene-co-methylmethacrylate)(poly-(DRlA-co-MMA)) and poly(4'- (((2-(methacryloyloxy)ethyl)ethyl)amino)-4-nitroazobenzene-co- methylmethacrylate)(poly(DR1M-co-MMA)). The sequence distribution of the two series of copolymers is calculated on the basis of the estimated reactivity ratio values and analyzed by 13C-NMR spectroscopy. Methylmethacrylate centered triad MMD, DMD, and MMM and disperse red 1 centered triad DDD fractions are derived from the alpha-methyl carbon of MMA and carbonyl carbon in poly (DR1A- co-MMA). Good agreement is found between the spectral deconvolution results and the calculated results. Polymer tacticity is analyzed with the main chain quaternary carbon and carbonyl carbon in poly(DR1M-co-MMA). No spectral sequence distribution information is available for this series. Azo copolymers, Configuration, Sequence distribution, Solution NMR.

The photochemical and thermal cis to trans isomerization of aromatic azo groups covalently bonded within polymers and its practical consequences as well as some other recent applications are reviewed. The kinetics and mechanism of the isomerization of azo polymers are briefly presented and then several recent developments in azo polymers research are discussed. They include liquid crystallinity, nonlinear optical properties, monolayer assemblies and the light induced birefringence and dichroism. Some of the phase transitions of liquid crystalline azo polymers can be induced photochemically by the azo groups isomerization. Second order optical nonlinearity can be obtained in polymers with donor and acceptor substituted azo groups. Using the Langmuir-Blodgett technique, azo polymer monolayers can be built and used as 'command surfaces' of liquid crystal films. The light induced birefringence is a reorientation phenonmenon which is a consequence of the trans-cis-trans isomerization. Future possible applications for a variety of practical devices, such as display devices, optical modulators, optical waveguides, holography, and reversible optical storage are mentioned. Azo polymers, Review, Optoelectronic and photonic applications

Writing, reading, erasing and rewriting processes are performed using laser beams on a poly[4'-[[2-(acryloyloxy)ethyl]ethylamino]-4-nitroazobenzene] (pDRlA) film. The writing and erasing are statistical processes and the level of optical anisotropy induced depends on the photon flux directed at the sample. The time evolution of the writing process is investigated. A computer simulation of these processes is performed taking into account a relaxation process in which some of the reoriented trans molecules are randomized by thermal motion.