Resonance raman spectroscopy of polydiacetylene langmuir-blodgett films

Resonance Raman excitation profiles of two vibrational modes have been measured for Langmuir-Blodgett films of a polydiacetylene. The polymer films were formed by exposing multilayer samples of the Cd salt of 10,12-pentacosadiynoic acid to ultraviolet radiation. Raman spectra were measured for incident laser photon energies between 1.85 and 2.20 eV (670-564 nm), the region over which the films exhibited optical absorption associated with an electronic transition of the π-electrons on the conjugated backbone. The excitation profiles have been used to determine both the strength of the vibronic coupling and the differences in vibrational frequency between the ground and excited states. The results are in reasonable agreement with those obtained previously for polydiacetylene single crystals.     

Solid-State Polymerization Behavior of 1,3-Bis(3-quinolyl)-1,4-butadiyne

Summary 1,4-Bis(3-quinolyl)-1,3-butadiyne (DQ) is known to be polymerized in solid state to give the corresponding polydiacetylene. However, the polymer yield of DQ bulk crystals is low. Thus, we prepared several types of DQ crystals by different procedures to find the reason for the low polymer yield. We found three modifications of DQ crystals and they were evaluated by spectroscopic measurements and X-ray diffraction. DQ bulk crystals (Crystal I) and thermally grown DQ crystals on Crystal I (Crystal II) have the same structure classified to Type A, which gives regular polydiacetylene structure in low polymer yields. DQ crystals grown on glass plates by sublimation (Crystal III) belong to the second modification of Type B. DQ nanocrystals prepared by the reprecipitation method (Crystal IV) are Type B, and thermally grown DQ crystals on nanocrystals (Crystal V) are Type C. Crystals of Types B and C could be polymerized in low yields without showing excitonic absorption of polydiacetylene indicating irregular polymerization other than 1,4-addition. For crystals of Type A, we found that the 1,4-addition polymerization proceeded only in near-surface portions of the crystals. It can be plausibly explained that mobile monomers in the near-surface portion are only able to take part in their polymerization, resulting in low polymer conversion.     

Micropatterning of polydiacetylene based on a photoinduced chromatic transition and mechanism study

A photoinduced chromatic transition from blue to red for the polydiacetylene 10,12‐pentacosadiynoic acid has been studied. This transition produces an obvious change in the ultraviolet–visible absorption and fluorescence emission spectra. A two‐dimensional micropattern has been realized on the basis of this change and imaged with a confocal fluorescence microscope. Detailed information on the mechanism of the chromatic transition has been obtained by the application of resonance Raman and Fourier transform infrared methods. The results indicate that the conformational change of alkyl side chains constricted by hydrogen‐bonded head groups imposes strain on the polymer backbone and finally leads to a drastic decrease in the π‐electron‐conjugation length. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 942–946, 2007     

Optical excitations in stoichiometric uncapped ZnS nanostructures

We calculate the optical absorption spectra of low-energy uncapped zinc sulfide nanostructures found by global optimisation (basin-hopping/simulated annealing) using time-dependent density functional theory (TD-DFT) and compare the results with experimental spectra. We predict that for all nanostructures studied the lowest excited state found by TD-DFT corresponds to an exciton with an exciton binding energy that is much larger than that of excitons in bulk zinc sulfide. We further show that for the more symmetrical nanostructures some of the excitons are dark and that the absorption on-sets, the energy of the lowest exciton, for the different nanostructures show no clear evidence of quantum confinement. We propose that this apparent lack of quantum confinement finds its origin in the fact that the lowest exciton is not evenly spread over the whole nanostructure but shows large contributions for specific groups of atoms. Finally, we show that the predicted optical absorption spectra fit with those reported experimentally.     

Direct measurement of polymer segment orientation and distortion in shear: semi-dilute solution behavior

The shear induced backbone segment orientation and deformation of the polymeric chromophore diacetylene 4-butoxycarbonylmethylurethane (4BCMU) in semi-dilute solution has been measured with an extended dichroism technique. At low shear rates the random coil, visco-elastic polymer shows orientation in the flow direction. At higher shear rates a reduction in the average conjugation length is observed with an increase in the number of segments orienting perpendicular to the flow direction. Novel behavior, which is not consistent with standard models, is observed for this visco-elastic polymer at high shear rates. The results presented are discussed in view of prior experimental and theoretical work.     

Enhanced Fluorescence and Local Vibrational Mode in Near‐White‐Light‐Emitting ZnO:Mg Nanorods System

We report exciton and phonon properties of ZnO:Mg nanorods of different Mg doping concentration. X‐ray diffraction studies (XRD) confirm the growth of wurtzite phase ZnO nanostructures. XRD reveals doping‐induced shift in peaks and formation of secondary phase related to Mg. Optical properties of the prepared nanorods are investigated by using UV‐Visible absorption and photoluminescence spectroscopic techniques. Optical absorption studies show strong free excitonic absorption of ZnO and extra absorption bands related to the defect centers of the secondary phase (MgO) formed after Mg doping. Photoluminescence studies show sharp band in UV region and defects‐related broad band emission in the visible range. Gaussian‐fitted photoluminescence spectra show that the emission is composed of free exciton recombination and its longitudinal optical (LO) phonon replica. In addition, Mg‐related local vibrational mode observed in Raman and FTIR spectra after Mg doping, indicates the incorporation of Mg into the lattice positions of wurtzite ZnO.     

Optical, electrical and discharge profiles for (PVC + NaIO4) polymer electrolytes

A sodium ion conducting polymer electrolyte based on poly (vinyl chloride) (PVC) complexed with NaIO₄ was prepared using a solution-cast technique. Optical properties such as direct and indirect optical energy gap, and optical absorption edge were investigated in pure and doped PVC films from their optical absorption spectra in the 200–600 nm wavelength region. The direct optical energy gap for pure PVC lies at 3.14 eV while it ranges from 2.60 to 3.45 eV for different composition doped films. Similar behavior was observed for the indirect optical energy gap and absorption edge. It was found that the energy gaps and band edge values shifted to higher energies on doping with NaIO₄ up to a dopant concentration of 10 wt%. Measurements of ionic conductivity and transference number were made to investigate the order of conductivity and charge transport in this polymer electrolyte. Transference number values show that the charge transport in this polymer electrolyte is predominantly due to ions (t _ion = 0.93). The conductivity increases with increase in concentration of the salt and with temperature. Using this electrolyte, cells were fabricated and their discharge profiles were studied under constant load. Miscibility studies were performed using X-ray diffraction (XRD) and Fourier Transform Infrared analysis (FT-IR) measurements.     

Supramolecular Nanofibers Self-Assembled from Foldamers: Structure Control through Preassembly

We used a helical polymer backbone (polyacrylamide) as a scaffold to organize perylene diimide chromophores into well-defined foldamers, which further undergo self-assembly into supramolecular tube-like arrays in aqueous media, as revealed by cryo-TEM imaging. The arrays are supramolecular polymers, whose structure is templated by folded primary building blocks, representing a useful tool for directing self-assembly . Exciton migration in the supramolecular arrays was studied by transient absorption and revealed a moderate exciton diffusion propensity.     

Chain helicity of a poly(phenylacetylene) with chiral centers between backbone and mesogenic groups on side chains

An optically active poly(phenylacetylene), poly{2-(R)-methyl-2-[4′-(tetradecyloxy)biphenyl-4-yloxy]propyl 4-ethynylbenzoate} (P1) is synthesized in a high yield with a high molecular weight (1.3 × 10⁶ g/mol) by [Rh(nbd)Cl]₂ catalyst. The chemical structure of the polymer is characterized by IR and NMR spectroscopies with satisfactory analysis data. The polymer is soluble in many common organic solvents. It shows circular dichroism (CD) band in the absorption region of the polyacetylene backbone, indicating that P1 is chiroptical owing to the formation of helical conformation with an excess screw sense. Combining the results of CD and light scattering experiments, we find that addition of poor solvent can induce the neighboring helical segments within a single polymer strand to become closer, resulting in exciton coupling prior to polymer precipitation. The exciton coupling can be kept in the solid state after solvent evaporation.     

Shape memory properties and unusual optical behaviour of an interpenetrating network of poly(ethylene oxide) and poly(2‐hydroxyethyl methacrylate)

An interpenetrating polymer network (IPN) with shape memory properties was prepared by using poly(2‐hydroxyethyl methacrylate) (PHEMA) and poly(ethylene oxide) (PEO). PHEMA acts as a fixed phase and PEO as a switching phase. The switching action of PEO is due to the reversible process of melting and crystallization. It was observed that the shape recovery of the IPN increases with increasing crosslinker concentration up to an optimum value and decreases thereafter. In addition to the shape memory property, the IPNs show a reversible change in optical properties from translucent to opaque. The change in optical properties is quite different from that observed in a semicrystalline polymer system where the transparency increases as a result of the melting of crystals. This behaviour of the IPN is explained in terms of H‐bonding of PEO with PHEMA. Fourier transform infrared spectroscopy was used to study the H‐bonding between PEO and PHEMA. © 2019 Society of Chemical Industry     

Optical characterisation of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix for third generation photovoltaics

ABSTRACT: Silicon nanocrystals with an average size of approximately 4 nm dispersed in SiO2/Si3N4 hybrid matrix have been synthesised by magnetron sputtering followed by a high-temperature anneal. To gain understanding of the photon absorption and emission mechanisms of this material, several samples are characterised optically via spectroscopy and photoluminescence measurements. The values of optical band gap are extracted from interference-minimised absorption and luminescence spectra. Measurement results suggest that these nanocrystals exhibit transitions of both direct and indirect types. Possible mechanisms of absorption and emission as well as an estimation of exciton binding energy are also discussed.     

Investigation of optical properties of Bi12GeO20 sillenite crystals by spectroscopic ellipsometry and Raman spectroscopy

Bi₁₂GeO₂₀ (BGO) compound is one of the fascinating members of sillenites group due to its outstanding photorefractive and photocatalytic characteristics. The present paper aims at investigating optical properties of BGO crystals by means of spectroscopic ellipsometry and Raman spectroscopy measurements. Bi₁₂GeO₂₀ single crystals grown by Czochralski method were structurally characterized by X-ray diffraction (XRD) experiments and the analyses showed that studied crystals have cubic crystalline structure. Raman spectrum exhibited 15 peaks associated with A, E and F modes. Spectroscopic ellipsometry measurement data achieved in the energy region between 1.2 and 6.2 eV were used in the air/sample optical model to get knowledge about complex pseudodielectric constant, pseudorefractive index, pseudoextinction and absorption coefficients of the crystals. Spectral change of real and imaginary part of complex pseudodielectric constant were discussed in detail. Band gap energy of Bi₁₂GeO₂₀ single crystals was calculated to be 3.18 eV using absorption coefficient dependency on photon energy. Critical point energies at which photons are strongly absorbed were determined by utilizing the second energy derivative spectra of components of complex pseudodielectric function. Fitting of both spectra resulted in the presence of four interband transitions with energies of 3.49, 4.11, 4.67 and 5.51 eV.     

Optical biaxiality of nematic LC-side chain polymers with laterally attached mesogenic groups

Summary Liquid crystalline polymers with mesogenic groups laterally attached to the polymer backbone were investigated by polarizing microscopy with respect to their optical properties. The nematic polymers were macroscopically oriented in a magnetic or electric field. Polarization microscopic investigations with convergent polarized light show optical biaxial behavior of the nematic phase.     

X-ray photochemistry: ω-Chloro olefin sulfones

To be efficacious as an x-ray resist, a material must efficiently undergo a desirable chemical change upon being irradiated with x-rays. The probability of such a change taking place is given by the product of the fractional x-ray absorption and the “inherent sensitivity,” which we define as the yield per unit energy absorbed. Increasing the absorption of the polymer, as, for example, by chlorinating an olefin sulfone thereby increasing the absorption at wavelengths just below the chlorine absorption edge will result in improved usefulness if, and only if, this increased energy absorbed can be used by the polymer to enhance the desired chemical reaction. The “usefulness” of chlorinating is thus dependent on the efficiency of intramolecular energy transfer. We have studied the efficiency of this transfer by monitoring the loss of SO₂ from the main-chain backbone as a function of absorbed x-ray energy, using x-ray wavelengths both abov and below the chlorine absorption edge. The polymers studied were 1-olefin sulfones, with and without the chlorine atom in the ω-position. Through the use of this series of polymers it was hoped to observe an effect due to the changing separation of the chlorine atom from the backbone. The experiments indicate that (a) the inherent sensitivity is independent of irradiating wavelength, for both the chlorine-containing and the non-chlorine-containing polymers; (b) the hexene polymers are considerably more sensitive than the lower members of the series (which exhibit a sensitivity which is approximately independent of the side chain length); and (c) the chlorine-containing polymers are less sensitive than the non-chlorine-containing species. At this time the explanation for these observations is speculative.     

Synthesis of porphyrin-appended poly(fluorene-alt-carbazole): photoluminescent and electrochemical studies

Poly(fluorene-alt-carbazole) appended with meso-tetraphenylporphyrin (MTPPC) linked through oxyethylene spacer and the parent poly(N-hexyl-2,7-carbazole-alt-9,9-di-n-hexyl fluorene) (P1) were synthesized and characterized. Optical absorption properties of the polymers containing 5% of MTPPC (P2) and 10% of MTPPC (P3) in the polymer backbone show extended optical absorption toward long wavelength compared to the parent polymer P1. Optical absorption studies of the polymers showed that polymer P1 has absorption at 384 nm in solution, whereas polymers P2 and P3 have absorption at 384 nm corresponding to polymer backbone and soret band at 421 nm with Q bands at around 518 and 554 nm in solution. P1 shows photoluminescence emission around 417 nm, whereas P2 and P3 fall around 656 nm. Thermal, electrochemical, and quenching studies of the polymers have been studied and the details are reported.     

Influence of side chains on electrochromic properties of green donor–acceptor–donor polymers

Three solution processable cathodically coloring green electrochromic polymers, based on 2,3-diphenyl-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine, have been synthesized by oxidative FeCl₃ polymerization. The polymers were designed with solubilizing alkyl and oligoethylene oxide side chains to achieve solubility and processability. All three polymers have a small electrochemical bandgap (1.8–1.9 eV) and low oxidation potentials. Spectroelectrochemical studies of polymer films on ITO reveal that the alkyl side chains in head-to-head position on the polymer backbone promote a defined high-energy absorption peak and suppress tailing of charge-carrier absorption into the visible region. Kinetic studies, based on transmission measurements applying a square-wave potential between reduced and oxidized states, show that the polymer with exclusively oligoethylene oxide side chains (P3) had the fastest response times, monitored at the low-energy absorption maxima. The best performing polymer (P1) showed a good optical contrast in the visible region with a ΔT of 26% at 700 nm. An initial test of the electrochemical stability showed that the oligoethylene oxide containing polymers had superior stability over 500 full switches.     

Thickness threshold of structural ordering in thin MEH-PPV films

The microstructure of thin poly-[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) films was investigated by photoluminescence measurements and X-Ray diffraction analysis over thickness from several tens up to several hundreds of nanometer. Obtained results were correlated with exciton diffusion length obtained by the photovoltage method. All measurements revealed that there is a threshold thickness for the microstructural order as well as for exciton diffusion length which are primary physical properties for function of any device made up from such films. Development of more ordered phase can be observed above the critical thickness of about 150 nm. Below the threshold, the polymer chains are preferentially in conformations promoting localized intrachain interactions. Above the threshold, two main features can be observed: (i) the conjugation length of polymer chain segments reaches its maximum and (ii) polymer stacking in more ordered and larger crystalline domains influences strongly the material properties, namely enhances the exciton diffusion length.     

An evaluation of thermochromic prints based on microencapsulated liquid crystals using variable temperature colour measurement

A variable temperature colour measurement technique has been developed to characterise the temperature-dependent colour change, known as 'colour play', observed with thermochromic prints based on microencapsulated liquid crystals. The effect is most pronounced over a black background. The liquid crystals exhibiting the thermochromic effect adopt a chiral nematic phase with an underlying smectic A phase. A variety of methods for presenting and interpreting the data obtained, as the colour of the print passes through the spectrum, is explored. The effect of temperature on the wavelength of reflected light, a * b 相似文献   

Efficient Exciton Quenching by Hole Polarons in the Conjugated Polymer MEH-PPV

Recently published near field scanning optical measurements (NSOM) on the conjugated polymer MEH-PPV exhibited a strong dependence of the photoluminescence intensity on the applied electric fields at the NSOM tip. The observed effect is apparently due to exciton quenching by hole polarons. In the present paper, a model “single carrier” electro-modulated-photoluminescence device is used to further explore the exciton quenching effect of hole polarons in MEH-PPV. Hole polarons, created by charge injection from an ITO electrode, are observed to dramatically quench the photoluminescence intensity of MEH-PPV. The Stern-Volmer quenching efficiency of a hole polaron in conjugated polymer thin films was measured to be 390 nm³. This value, and other data presented herein, are consistent with the published NSOM photoluminescence modulation measurements and offer further evidence that hole polarons are efficient photoluminescence quenchers in MEH-PPV.     

Optical Energy Estimation of Irradiated Polypropylene

The optical absorption has been studied under exposure to gamma radiation. Thin films of polypropylene were irradiated with a ⁶⁰Co source and doses ranged up to 6 kGy. Analysis of the optical absorption spectra indicated that both direct and indirect electron transitions take place in the polymer. The observed optical energy gap (E_opt) and energy gap-tail (ΔE) for irradiated films were determined from the measured absorption spectra. The average values of (E_opt) and (ΔE) are 5.85 eV and 0.5 eV, respectively. There is no detectable change in the optical energy gaps and tails under the applied gamma-ray doses.