Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (18)
- Beitrag zu einem Tagungsband (4)
- Beitrag zu einem Sammelband (3)
- Vortrag (2)
- Sonstiges (1)
Sprache
- Englisch (28) (entfernen)
Schlagworte
- Chemiluminescence (3)
- Oxidation (2)
- Bromination (1)
- Chemilumineszenz (1)
- Chlorination (1)
- Conjugated polymer (1)
- Dendrimers (1)
- Diphenylethylenderivate (1)
- Liquid crystal (1)
- Liquid-cristalline conjugated Polymer (1)
Eingeladener Vortrag
- nein (2)
A liquid crystalline mainchain conjugated polymer for polarized electroluminescence applications
(1997)
Measurements with linearly polarized exciting and fluorescent light show that the excimerlike fluorescence of pyrenebutyric acid in alkaline solution is polarized. The degree of polarization r depends on temperature and, very pronounced, on the wavelength of excitation. This occurs in the spectral region where the excitation spectrum of the excimer differs from the corresponding monomer spectrum. The value of r, after correction of rotational depolarization, is 0.36 and indicates that single transition moments of absorption and emission coincide nearly in their direction. These results, together with the fluorescence time decay laws, strongly support the conclusion that, at least in an hydrophilic surrounding, not excimers but ground-state stable dimers of pyrene are formed.
The thermo-oxidative degradation of a polymeric optical cable is investigated by chemiluminescence, The results are reliable and reproducible. Two distinct processes are reported marked by a peak and a plateau behavior versus the time, respectively. Both processes are ruled by thermally activated processes. Beside the dependencies of temperature and time, the influence of absorbed water is discussed. Chemiluminescence is proposed as a promising candidate for a suitable testing method assessing the thermo-oxidative stability of plastic optical fibers and cables. it requires not more than a simple one-day testing procedure and has the advantage that it can be carried out even within the lo cv temperature ranges of the cables' intended use.
On the basis of Wittig-Horner reactions and protection group techniques compound 7 for the core and the components 9a-c and 11a-c for the dendrons were prepared and linked in the final step. The convergent synthesis yielded constitutionally and configurationally pure dendrimers (2a-c, 2a'-c') which consist of distyrylbenzene units. Their thermo-oxidative stability in the presence of air was studied by chemiluminescence and compared to the dendrimers 1 consisting of stilbene units.
Polarized fluorescence spectroscopy is used to investigate the photophysical behavior of poly(p-phenylphenylenevinylene) (PPPV) in polystyrene matrix in comparison with oligomeric model compounds. For this purpose PPPV is modeled by a chain consisting of a distribution of independent oligomeric segments. Excitation energy transfer (EET) between the segments depends on the wavelength of excitation, not only for transfer along an isolated polymer chain, but also for intermolecular transfer at high concentration of PPPV. The spatial range of EET, as indicated by fluorescence depolarization, is reduced for excitation at the long wavelength edge of the absorption spectrum ("red-edge-effect").
The excitation energy transfer (EET) of a bichromophoric cross-shaped molecule was investigated by stationary polarized fluorescence spectroscopy in the solid state. For this purpose 2,2[prime],7,7[prime]-tetrakis(biphenyl-4-yl)-9,9[prime]-spirobifluorene was embedded in a polymeric bisphenol-A-polycarbonate (PC) matrix. The dependence of the fluorescence on concentration and wavelength was determined. The role of the intermolecular and intramolecular EET is dealt with separately and discussed by means of the degree of polarization. The intermolecular excitation energy transfer is described in terms of a Förster transfer mechanism. The intramolecular transfer is prevented for the zero-point vibrational levels by the molecular cross-shaped structure, but is found for a wide range of wavelength, presumably based on vibrationally excited states.
The photophysical behavior of poly(p-phenylphenylenevinylene) (PPPV) and several oligomeric
model compounds in isotropic and anisotropic samples is investigated by means of polarized fluorescence
spectroscopy. As long as excitation energy transfer and mutual alignment are considered, the behavior of
PPPV can be modeled by a chain consisting of a distribution of independent oligomeric segments. The
results of fluorescence polarization, e.g., the nonzero value of r in isotropic PPPV, cast doubt on the application
of the semiconductor band model but favor an interpretation of spectroscopic properties in terms of molecular
transitions. The orientation coefficients of the oligomers and of PPPV in stretched polyethylene are determined.
The change of fluorescence spectra of oriented PPPV in stretched polyethylene demonstrates directly that
conformational changes of the macromolecule influence the pathway of energy transfer.