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Eingeladener Vortrag
- nein (80)
Photo- and thermo-oxidative stability of aromatic spiro-linked bichromophoric cross-shaped molecules
(2000)
An extensive investigation of the photostability and the thermo-oxidative stability is presented for 2,2,7,7-tetrakis(biphenyl-4-yl)-9,9-spirobifluorene and 2,2,4,4,7,7-hexakis(biphenyl-4-yl)-9,9-spirobifluorene. Both compounds are conjugated fully aromatic systems that are being discussed as active functional materials for a variety of advanced applications. The effect of atmosphere, sample thickness and preparation procedure on photo-oxidative degradation are investigated in detail by absorption and fluorescence spectroscopy. Distinct mechanisms are described in terms of relevant parameters such as the quantum yields of the photo-oxidation and the fluorescence. No oxidative degradation could be detected under nitrogen. In ambient air a strong decrease of the fluorescence performance is found due to effective quenching by defective chromophores. Chemiluminescence investigations were performed to characterise the thermo-oxidative behaviour in the temperature region between 300 and 450 K. It becomes clear that even a stable chemical structure such as the investigated aromatic system does not guarantee sufficient photostability with regard to light emitting properties.
About this Journal
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.
On the thermal behaviour and thermo-oxidative stability of liquid crystalline triphenylene compounds
(1999)
Columnar discotic materials are considered for applications in the area of photoconductivity and light-emitting diodes. A major requirement is their stability at elevated temperatures and in the presence of oxygen. The thermal and thermo-oxidative behaviour of discotic triphenylene derivatives was investigated by us using various methods, in particular by chemiluminescence (CL), UV-vis absorption spectroscopy and in situ thermogravimetry-mass spectroscopy (TG-MS). Various degradation processes are described for increasing temperature, and their influences on functional properties are discussed.
Polarized fluorescence and orientational order parameters of a liquid-crystalline conjugated polymer
(1999)
We report a study of the orientational order of aligned thin films of the liquid crystalline conjugated polymer poly(9,9-dioctylfluorene). Steady state polarized fluorescence measurements were used to determine the orientational order parameter <P2> and <P4>. The influence of intermolecular and intramolecular excitation energy transfer on the degree of polarization is discussed. The role of film morphology is also examined by comparison of the results for glassy and crystalline films.
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.
Zwischen Chemie, Wirtschaft und Politik - Aus den Aufgaben einer Chemisch-Technischen Bundesanstalt
(1995)
Innovation; Was ist das?
(2005)
Forschen - Prüfen - Beraten; Ressortforschung für Sicherheit und Regelsetzung am Beispiel der BAM
(2005)
Conjugated polymers based on 1,4-phenylenevinylene units are very promising materials for applications as light-emitting diodes. A major requirement is their stability at elevated temperatures of operation. The thermo-oxidative behaviour of a soluble poly(1,4-phenylenevinylene) derivative, poly[2,5-bis(2-ethylhexyloxy)-1,4-phenylene vinylene], was investigated by using chemiluminescence and UVVis absorption spectroscopy. Extremely sensitive chemiluminescence is successfully applied since even minor chemical changes could lead to a considerable loss of photo- and electro-optical properties. Various degradation processes are described as a function of time and temperature and their influences on functional properties are discussed. The investigated material does not show sufficient thermo-oxidative stability within the temperature range of intended use in contact with air. For industrial application, direct contact with oxygen during processing and operation has to be avoided.
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.
A liquid crystalline mainchain conjugated polymer for polarized electroluminescence applications
(1997)
Polarized fluorescence and orientational order parameters of a liquid crystalline conjugated polymer
(1999)
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 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.