Absorption Spectra Of Poly Research Articles

Investigation of nature of excitons in PPDT2FBT and effect of optical interference

Electroabsorption (EA) measurements can be used to identify the type of excitons contributing to the absorption spectra of semiconductors. However, the shape of the EA spectrum may vary depending on the mode of measurement due to the optical interference effects. Analysis without considering these effects may lead to erroneous conclusions. In this work, we present EA measurements and analysis for reflection mode measurements considering optical interference effects. We compared the inferences with transmission mode measurements and discuss the limitations. We identified the nature of excitons associated with each transition in the absorption spectrum of poly[(2,5-bis(2-hexyldecyloxy)-phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]-thiadiazole)] thin film from EA measurements. The bands at 1.89, 2.05, and 2.27 eV had a mixed nature consisting of charge transfer and Frenkel characteristics. Of these, the band at 2.05 eV showed the strongest charge transfer characteristic. From thickness dependent measurements, we showed that the interference effects increase with the thickness of the semiconductor layer. The nature of excitons, however, could still be deduced qualitatively from reflection mode EA measurements.

Vibrational Spectra and Ion–Molecule Interactions in Poly(Vinyl Alcohol) + Lithium Salt (LiNO3, LiClO4, LiBF4)

Raman scattering and infrared absorption spectra of poly(vinyl alcohol) films with added lithium nitrate, perchlorate, and tetrafluoroborate salts were studied. The microstructure of the films and dynamic interactions of particles in them depended largely on the nature, form, and charge state of the molecular anion.

Excitons in Organics Using Time-Dependent Density Functional Theory: PPV, Pentacene, and Picene.

We apply the bootstrap kernel within time-dependent density functional theory to study the one-dimensional chain of polymer polyphenylenevinylene and molecular crystals of picene and pentacene. The absorption spectra of poly(p-phenylenevinylene) has a bound excitonic peak that is well-reproduced. Pentacene and picene, electronically similar materials, have remarkably different excitonic physics, and this difference is also well captured. We show that the inclusion of local-field effects dramatically changes the spectra of both picene and pentacene but not for poly(p-phenylenevinylene).

Separation of overlapping vibrational peaks in terahertz spectra using two-dimensional correlation spectroscopy

In this study, the terahertz (THz) absorption spectra of poly(3-hydroxybutyrate) (PHB) were measured during isothermal crystallization at 90–120°C. The temporal changes in the absorption spectra were analyzed using two-dimensional correlation spectroscopy (2DCOS). In the asynchronous plot, cross peaks were observed around 2.4THz, suggesting that two vibrational modes overlap in the raw spectrum. By comparing this to the peak at 2.9THz corresponding to the stretching mode of the helical structure of PHB and the assignment obtained using polarization spectroscopy, we concluded that the high-frequency band could be attributed to the vibration of the helical structure and the low-frequency band to the vibration between the helical structures. The exact frequencies of the overlapping vibrational bands and their assignments provide a new means to inspect the thermal behavior of the intermolecular vibrational modes. The large red-shift of the interhelix vibrational mode suggests a large anharmonicity in the vibrational potential.

Polarization and temperature dependent spectra of poly(3-hydroxyalkanoate)s measured at terahertz frequencies

Temperature-dependent terahertz (THz) absorption spectra of poly(3-hydroxyalkanoate)s (PHAs) were measured by using a Fourier transform far-infrared (FT-FIR) spectrometer and a THz time-domain spectrometer over a temperature range of 10 K to 465 K with a liquid helium cryostat and a heating cell. Clear differences were observed between the spectra of crystalline and amorphous polyhydroxybutyrate (PHB), indicating that the absorption peaks observed in the THz spectra originated in the higher-order conformation of PHB. The polarization spectra of a stretched PHB sample were measured, and the direction of the vibrational transition moment was determined. The temperature dependences of the spectra reveal frequency shifts and broadening of the absorption peaks with temperature, suggesting large anharmonicity of the vibrational potential. The temperature shift behaviour is quite different in each transition. Some of the transitions show a blue shift, which cannot be explained by a simple anharmonic potential model. Frequency shifts of the peaks were mainly observed below 10 THz, which suggests a large anharmonicity of the vibrational potential at lower frequencies.

Nucleation and Growth Mechanism of Electropolymerization of Methylene Blue: The Effect of Preparation Potential on Poly(methylene blue) Structure

AbstractNucleation and growth mechanism of electropolymerization of methylene blue (MB) in a basic medium and the effect of preparation potential on poly(MB) film structure were investigated by using cyclic voltammetry, potentiostatic current‐time transient, scanning tunneling microscopy (STM), atomic force microscopy (AFM), and UV‐vis. absorption spectroscopy techniques. Electropolymerization of MB has been achieved by potentiodynamic (cyclic voltammetry) and potentiostatic (constant potential) techniques. The potentiostatic current‐time transients fitted with a theoretical model and morphological studies indicate that nucleation and growth mechanism of poly(MB) starts with a progressive layer‐by‐layer nucleation and growth besides random adsorption. Nucleation and growth of poly(MB) follows a process between progressive layer‐by‐layer and 3‐D instantaneous mechanism resulting in highly‐oriented poly(MB) nanofibers with increasing poly(MB) film thickness. Cyclic voltammetry and morphological studies exhibit that poly(MB) film structure changes depending on the preparation potential. Poly(MB) films prepared at the potential values of 900 and 950 mV show a well‐ordered, smooth surface but at the potential values higher than 1000 mV, rough polymer surface arises as overoxidation takes place. UV‐vis. absorption spectra of poly(MB) film and MB monomer show three peaks. The peak at 410 nm for poly(MB) shows 100 nm blue shift when compared to the MB monomer and is attributed to poly(MB) formation on the electrode.

Recombination luminescence of poly(arylene phthalide) films: 1. Electronic and optical features of poly(arylene phthalide)s

Features of the luminescence and absorption spectra of poly(biphenylene phthalide), poly(fluorenylene phthalide), and poly(terphenylene phthalide) films were analyzed. Experimental results obtained for these films by optical methods and thermally stimulated current spectroscopy were compared. Long-lasting afterglow emission from the films after photoexcitation was observed. Possible explanations for the observed optical and thermally stimulated processes in the poly(arylene phthalide)s were suggested in terms of reversible electronic phase transitions induced by external factors in unconjugated polymers.

Sensitization of the electrooptical effect by a merocyanine dye in polymers of azobenzene derivatives

Absorption spectra of poly[4-metacryloyloxy-(4′-carboxy-3′-oxy)azobenzene] films and their complexes with cobalt, without additives or doped with a merocyanine dye whose absorption spectrum lay in the near IR region, were studied before and after switching on an external electric field. An external electric field effect was found on the light transmittance of films within the dye absorption region, which indicated sensitization of the electrooptical effect. The electrooptical properties of the films resulted from spatial reorientation of dipole moments of azobenzene groups in an external electric field, which initiated turning of dye molecules. A phenomenological model was proposed to explain the sensitization of the electrooptical effect.

Synthesis and Absorption Spectra of Poly(3-(phenylenevinyl)thiophene)s with Conjugated Side Chains

Six soluble polythiophene derivatives with phenylene−vinylene conjugated side chains were synthesized by GRIM or Stille methods. The absorption spectra of the polymers show two peaks located in the UV and visible region, where the visible absorption peak is attributed to the π−π* transition of the conjugated main chains and the UV absorption peak results from the conjugated side chains. By doubling the conjugation length of the phenylene−vinylene side chains and reducing the concentration of the conjugated side chains, we synthesized PT4, which possesses a strong and broad absorption in the visible region from 380 to 650 nm and is promising for the application in polymer solar cells. For the polymers of PEHPVT and PMEHPVT where every thiophene unit is attached with a conjugated side chain, the absorption spectra of their films red-shifted by 30 and 50 nm, respectively, in comparison with their solution. After thermal annealing at 130 °C for 10 min, the absorption of the films red-shifted further, and the ...

Electric Field Effects on Photoconductivity and Electronic Absorption Spectra of Photogeneration Sites in Amorphous Molecular Semiconductors

Field dependences of photogeneration quantum yields of charge carriers in films of amorphous molecular semiconductors at high external electric field strengths cannot be always explained in terms of the Onsager or the Poole–Frenkel model, which do not consider the effect of an electric field on basic parameters of these materials. It is assumed that an external electric field shifts the electron density in molecules from its equilibrium distribution, thus altering the probability of electronic transitions. The assumption has been verified in studying the effect of an external electric field on the absorption spectra of poly(N-epoxypropylcarbazole) films doped with a merocyanine dye. Correlation between the theoretical and experimental results has been obtained.

Redox reaction of PEO modified cytochrome c adsorbed on the electrode in ion conductive PEO matrix analyzed with non‐contact optical waveguide spectroscopy

AbstractOptical waveguide spectroscopy was used under non‐contact conditions to analyze the visible absorption spectrum of poly(ethylene oxide) (M w = 150) modified cytochrome c (PEO150‐cyt.c) adsorbed on an indium tin oxide (ITO) glass electrode. The redox reactions of the adsorbed PEO150‐cyt.c were dynamically measured in situ in PEO oligomers (M w = 200). It was confirmed that PEO modification of cyt.c is effective in maintaining the redox activity of the cyt.c after adsorption on the ITO glass electrode. The PEO150‐cyt.c adsorbed on the electrode re‐dissolved gradually in PEO oligomer. Against this, PEO1000‐cyt.c having longer PEO chains (M w = 1000) was found to be adsorbed stably on the electrode. Copyright © 2003 John Wiley & Sons, Ltd.

The electronic behavior of poly(3-octylthiophene) electrochemically synthesized onto Au substrate

We investigate the electronic characteristics and the absorption spectra of poly(3-octylthiophene), POT, films grown by electrochemical methods onto Au substrates. We discuss the results considering the morphological factor. POT films thickness can be controlled by current density in the electropolymerization process. The film roughness depends on the thickness, being about 12% of film thickness. The samples for electrical measurements were made in sandwich structure, Au/POT/metal (metal: Ni, Al). Analyzing current-voltage data we were able to estimate the positive charge carrier mobility (5 x 10-4 cm² V-1 s-1) and the potential barrier height in the metal/polymer interfaces (0.1 eV for Au/POT and Ni/POT and 0.85 eV for Al/POT).

Intermediate neglect of differential overlap excited state calculations in periodic boundary conditions: The absorption spectrum of poly(para-phenylene vinylene)

Much of our current theoretical understanding of the electronic structure of conjugated polymers is based on two-band systems, such as the π-electron model of polyacetylene. But poly(para-phenylene vinylene) (PPV) and many other systems exhibit a number of bands, and this raises new and interesting questions that are beginning to be addressed. Such studies are complicated by the large number of model parameters, and the sensitivity of the predictions to the values used. While the intermediate neglect of differential overlap (INDO) method provides a systematic and tested approach for deriving these parameters from the chemical structure, previous tools could be applied only to oligomers, making it difficult to put the results in the context of other well-studied models. Here, we report INDO calculations on oligomers of PPV as well as long chains with periodic boundary conditions. The long-chain calculations are used to assign the spectral features to transitions between bands, and these assignments are transferred to oligomers by examining how the calculated oligomer spectra evolve with chain length. The effects of various electron–hole symmetry breaking mechanisms on the intensity of peak II (at 3.7 eV) relative to peak III (at 4.7 eV) in the optical absorption spectrum are examined. INDO theory on planar unsubstituted PPV breaks electron–hole symmetry due to inclusion of next-nearest neighbor transfer integrals, giving peak II about 4% of the intensity observed for peak III in the long-chain limit. Alkoxy substitution at the 2 and 5 positions of the phenyl ring is modeled using hydroxy groups (OH-PPV). (This approach is confirmed by replacing the hydroxy substituents with methoxy substituents.) For a planar OH-PPV structure, peak II has 8% of the intensity of peak III. AM1 calculations indicate that in OH-PPV, the vinylene rotates out of the plane of the phenyl rings by 20°, and this raises the intensity of peak II to 15% that of peak III. Finally, finite-chain effects cause oligomers of OH-PPV with between 4 and 8 phenylene units to have a peak II intensity that is 25% that of peak III. Oligomers exhibit additional transitions in the vicinity of peak II, but these have a strong dependence on the length of the oligomer and merge with the lowest-energy peak in the long-chain limit.

Effect of electric field on the optical absorption of amorphous molecular semiconductors doped with polymethine dyes

The effect of a dc electric field on the evolution of the electronic absorption spectra of poly(N-epoxypropylcarbazole) (PEPC), poly(styrene) (PS), and poly(vinyl butyral) (PVB) films doped with methine dyes is investigated. It is revealed that these spectra in the absorption range of the dye undergo transformations depending on the dipole moment and the symmetry of the π-electron structure of dye molecules. The inference is made that the electron density in the dye molecule is redistributed in an external electric field. This redistribution gives rise to the electric field-induced anisotropy in the spatial distribution function of photogenerated electron-hole pairs in amorphous molecular semiconductors.

Absorption changes under UV illumination in doped PMMA

The absorption spectra of poly(methyl methacrylate) (PMMA) doped with the photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA) change under illumination with UV light. Time-resolved measurements of the kinetics of the absorption changes at 352 nm prove the existence of a second light-induced reaction besides the expected light-induced decay of the photoinitiator. We propose a model for this second process which is in good agreement with the observed experimental facts.

Effect of specific energy of heavy ions for 1,2,4,5-tetracyanobenzene radical anion formation

The absorption spectrum of poly(vinyl alcohol) (PVA) film doped with 1,2,4,5-tetracyanobenzene (TCNB) was measured after the irradiation with several hundred MeV of C, Ne and Ar ions. The absorbance due to the TCNB radical anion increased and then decreased with increasing the fluence for all ions. The fluence dependence of the absorbance was analyzed by use of the “chemical” track model with a cylindrical structure. As a result, the estimated radius of the chemical track decreases for C and Ne ions with increasing specific energy (ion energy per atomic mass unit). Furthermore, at the same specific energy, the radius increases with increasing ion mass. The formation yield (G-value) of the TCNB radical anion estimated by optical absorbance and ESR measurements was smaller than that by γ-ray irradiation.

Calculated optical properties of ppp-oligomers

Abstract We have calculated the frequency dependent complex dielectric tensor for bi-and terphenyl within the framework of density functional theory. The good agreement with experimental data confirms that the anisotropy in the optical absorption can be well understood within the band picture. We predict the absorption behavior of ordered films with various chain orientations with respect to the substrate surface. We compare the optical properties of the molecular and the crystal phase. Furthermore, we show differences between the absorption spectra of poly( p -phenylen) (PPP) and its oligomers, respectively.

Synthesis and properties of widely conjugated polyacetylenes having anthryl and phenanthryl pendant groups

The metathesis polymerization of 1- and 2-ethynylanthracenes (1-EA and 2-EA) and 2- and 3-ethynylphenanthrenes (2-EP and 3-EP) in the presence of various WCl 6 -based catalysts produced widely conjugated soluble polymers with relatively high molecular weights. The highest weight-average molecular weights of poly(1-EA) and poly(2-EA) reached 61,000 and 26,000, respectively, when Ph 4 Sn was used as cocatalyst, while those of poly(2-EP) and poly(3-EP) reached 23,000 and 65,000, respectively, with Ph 3 Bi as cocatalyst. In contrast, MoCl 5 -based catalysts were hardly or not effective for these monomers. A large red-shifted peak was observed centering at 570 nm (the cutoff at 750 nm) in the absorption spectrum of poly(1-EA), while the red-shifted peaks were seen around 500 nm (the cutoffs near 700 nm) in the spectra of other polymers, indicating wide conjugations of the polymer chains. The configurational structures of all the polymers confirmed by DSC and 1 H-NMR were trans structures. However, poly(1-EA) and poly(3-EP) appeared to consist partly of cis structures in their main chains. All of the present polymers showed relatively high thermal stability in air compared with poly(phenylacetylene).

Optical absorption in the substituted phenylene-based conjugated polymers: Theory and experiment

We investigate theoretically and experimentally the effects of (2,5) chemical substitution on the optical absorption in the phenylene-based conjugated polymers. Theoretically, substitution destroys both the charge-conjugation symmetry and spatial symmetry that characterize the unsubstituted materials. Within Coulomb-correlated theoretical models, the effect of broken charge-conjugation symmetry alone on the underlying electronic structure and on the absorption spectrum is rather weak. When both broken spatial symmetry and broken charge-conjugation symmetry are taken into account, a strong effect on the electronic structure of polyphenylene derivatives is found. In spite of the strong effect of the broken symmetries on the electronic structure, the effect on the optical-absorption spectrum is weak. This surprising result is a consequence of the subtle nature of the configuration interaction in the substituted polyphenylenes within Coulomb-correlated models. We demonstrate numerically an approximate sum rule that governs the strength of an absorption band at 3.7 eV in the absorption spectra of poly(para-phenylene vinylene) (PPV) derivatives. Although substitution can make a previously forbidden transition weakly allowed, the latter acquires strength from a ``finite-size band'' at about the same energy, and not from a higher-energy band at 4.7 eV, as has been previously claimed. It is further predicted that the 3.7-eV band is polarized predominantly along the polymer-chain axis. We have measured the polarization dependence of the optical absorption in an oriented-substituted PPV film. We found that the two lowest-energy absorption bands are polarized predominantly parallel to the chain axis, while the band at 4.7 eV is polarized predominantly perpendicular to the chain axis. These results are in excellent agreement with the theory.

Gel chromism and anomalous luminescence in poly(3-alkylthiophene)

The absorption spectrum of poly(3-alkylthiophene) gel changes drastically in association with a volume instability upon changing the solvent composition and also the temperature. In the expanded state in chloroform and at high temperature, the photoluminescence is much enhanced compared with the shrunken state in ethanol and at low temperature. This gel chromism and anomalous luminescence are discussed in terms of the change of effective conjugation length induced by the steric hindrance and the dynamics of the photoexcited species.