摘要:

AbstractPolyimides such as 6F‐6F and 6F‐ODA and model N‐arylphthalimides are stabilized against photooxidative degradation by their electron donor (D) – acceptor (A) character. We have investigated the precise origin(s) of this effect using D and A substituents on the N‐aryl groups of these compounds. The lowest excited singlet state (S1) of N‐arylphthalimides is an intramolecular charge transfer (ICT) state. A nominally twisted compound, N‐(2‐t‐butylphenyl)phthalimide, shows greatly diminished CT absorption and blue‐shifted fluorescence with reduced quantum yield when compared to the 4‐t‐butyl isomer with an identical N‐aryl donor group. It therefore seems unnecessary to claim that the ICT state of phthalimides is a so‐called TICT state. Quantum yield and fluorescence lifetime measurements lead to the conclusion that enhanced internal conversion from the ICT state (S1) to the ground state makes a significant contribution to photostabilization of these compounds by suppressing formation of the reactive triplet state. Further stabilization of polymer films may be afforded by triplet state self‐quenching which is enhanced for 6F‐ODA in increasingly poor solvents. N‐alkylarylphthalimides in which the aryl and phthalimide groups are not formally conjugated but, rather, joined by flexible methylene ‘spacers’, exhibit a different kind of fluorescent intramolecular CT singlet state whose formation can also stabilize these compounds b

ISSN:1022-1360    

DOI:10.1002/masy.19971160103

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA new technique that combines nonphotochemical hole burning with multichannel detected fluorescence line narrowing has been used to obtain vibrationally resolved fluorescence spectra of squaraine chromophores in polymer matrices at 1.4 K. At a fixed excitation frequency, the intensities of the zero‐phonon lines decay with time due to nonphotochemical hole burning, leaving behind a broader background attributed to emission from molecules excited into phonon sidebands. Subtracting the spectrum of the hole‐burned sample from the initial one leaves predominantly a zero‐phonon line excited spectrum exhibiting enhanced vibrational structure. Spectra of the same squaraine in polystyrene and polyethylene matrices show differences in the frequencies and intensities of the phonon sidebands, indicating differences in the frequencies and strengths of the matrix modes coupled to the electronic transition of the chromophore. The phonon densities of states inferred through different measurement techniques are compared and related to electronic dephasing

ISSN:1022-1360    

DOI:10.1002/masy.19971160104

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractWe study the photophysical properties of the pyridine‐based polymers: poly(p‐pyridyl vinylene) (PPyV) and poly(p‐pyridine) (PPy). The primary photoexcitations in the pyridine‐based polymers are singlet excitons. We observe direct intersystem crossing (ISC) on picosecond time scales with the volume density of triplet excitons varying with the sample morphology (film or powder). These effects are demonstrated clearly by examining the millisecond photoinduced absorption characteristics of powder and film forms of PPyV. The pyridine‐based polymers have been shown to be promising candidates for polymer light‐emitting devices, both “conventional” diode device and symmetrically configured ac light‐emitting (SCALE) device. Here we examine the role of “insulating” layers and their interfaces with the emitting layer and electrodes in the SCALE device operation, with emphasis on the central role of the polym

ISSN:1022-1360    

DOI:10.1002/masy.19971160105

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractPhotoinduced absorption measurements on poly(phenylenevinylene), PPV, or its derivatives, illuminated with photons of energy past the absorption edge revealed that some percentage of the photons create a long‐lived excitation rather than the expected excitons. We present the evidence that these excitations are polaron pairs, which are essentially excimers. The reasons why this percentage varies from sample to sample are discussed. Also discussed are the reasons why polaron pairs in some derivatives of PPV emit light while in others they do not. Calculations with a relatively simple Hamiltonian can account well for the peaks in the photoinduced absorption produced by these pair

ISSN:1022-1360    

DOI:10.1002/masy.19971160106

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractHole mobilities have been measured in a series of triarylamine molecules doped into a poly(styrene) host. The results are described by a formalism based on disorder, due to Bässler and coworkers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameter of the formalism is the energy width of the hopping site manifold. The width is described by a model based on dipolar disorder. According to the model, the width is comprised of a dipolar component and a van der Waals component. The dipolar component is in good agreement with the recent analysis of Young. The results show that the van der Waals component increases with decreasing dopant concentration. This effect, which is not predicted from first order treatment arguments, is attributed to increasing structural randomness with dilution

ISSN:1022-1360    

DOI:10.1002/masy.19971160107

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractFemtosecond pulses were used to study photoexcitation dynamics in polydiacetylene 4BCMU film. Ultrafast photoinduced dichroism and optical Kerr gate responses were observed. These measurements allowed estimation of the diffusion constant (D ∼ 0.1 cm2/s), the nonlinear index of refraction (n2∼ 10−8esu), and third‐order susceptibility (δχ(3)∼ 6 × 10−10esu), in the film. We also found that the sign of the photoinduced transmission signal was dependent on the pump energy at the low energy absorption edge of the film. Possible explanation

ISSN:1022-1360    

DOI:10.1002/masy.19971160108

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe effect of blending of alkylated polymers, which have different backbone structures, was investigated in order to improve the electronic properties of conducting polymers. Comb‐shaped flexible polymer, poly(octadecyl acrylate) (PODA), was blended with rigid alkylated conducting polymers, poly(3‐dodecyl‐thiophene)(PDDT), and polyaniline emeraldine base (PANI)/p‐dodecyl‐benzenesulfonic acid (DBSA) complex, respectively, to investigate the effect of long alkyl chain of flexible polymer on the conformational mode change of rigid backbones and the effect of intermolecular interaction between these alkylated polymers. Optical microscopy was applied to observe the morphology change and obtain the phase diagrams of these blends. The intermolecular interactions that occurred in these blends were explained for each different characteristic peak obtained with FT‐IR spectra. Solvatochromism (red‐shift) of PDDT/PODA binary blends in solid state due to the planarity change of rigid backbone in the presence of PODA and electrochromism of PANI(DBSA)4/PODA ternary blends due to the hydrogen bonding between the nitrogen cation of PANI complex and carbonyl group of PODA are observed in UV‐Vis‐NIR spectra. Interestingly, the increase of conductivity was observed in the presence of 5 wt% of PODA in PDDT/PODA binary blends and a homogeneous smectic liquid crystalline structure was clearly confirmed by cross polarized optical microscopy in PANI(DBSA)4/PO

ISSN:1022-1360    

DOI:10.1002/masy.19971160109

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractRigid‐rod aromatic LC polyester with long alkyl side chains and two thermotropic LC polyesters (PET40/OBA60 and PB‐10) were studied by fluorescence spectroscopy to investigate their charge transfer interactions corresponding to LC configuration and changes during phase transit

ISSN:1022-1360    

DOI:10.1002/masy.19971160110

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractPhotochemical phase transition of push‐pull type azobenzene liquid crystal (LC), which contains both an electron donor and an acceptor in both ends of the azobenzene moiety, has been explored. Polymer azobenzene LCs were prepared, which show nematic (N) LC behavior in thetransform while no LC phase in thecisisomer. Photoirradiation of a very thin film of the azobenzene LCs (∼ 200 nm) in thetransform resulted in disappearance of the N phase due totranscisphotoisomerization of each mesogen, and the N phase recovered quickly when the irradiated sample was kept in the dark because ofcis‐transthermal isomerization and reorientation oftrans‐azobenzenes. Time‐resolved measurements by the use of a laser pulse (355 nm; 10 ns FWHM) revealed that the N to isotropic (I) phase transition took place in 200 μs. The thermal I‐N phase transition of the push‐pull type azobenzene LC occurred in 800 ms at 135 °C. This response is faster by one order of magnitude than the response of azobenzene LCs without charge‐tra

ISSN:1022-1360    

DOI:10.1002/masy.19971160111

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractSurface relief gratings were optically produced on a number of azobenzene‐based polymer films. The surface grating formation was investigated by monitoring the diffraction efficiency and using atomic force microscopy. The effect of the structure of the chromophores on surface modulation was investigated. The surface deformation process depended on the polarization state of the writing beams. The localized variations of the light intensity and alteration of the resulting electric field polarization were essential writing conditions to the formation of the surface relief gratings. The surface pattern from straight edge diffraction established that the surface profile of the recorded gratings is proportional to negative gradient of the intensity pattern incident on the polymer fil

ISSN:1022-1360    

DOI:10.1002/masy.19971160112

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY