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- Chemiluminescence (1)
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- poly(p-phenylphenylenevinylene) (1)
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 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.
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").
In films of poly(para-phenylphenylenevinylene) a photoreaction has been observed. The significant feature is a decrease of the fluorescence emission intensity during irradiation. It is assumed that a cycloaddition between C=C- double-bonds leads to crosslinks in the material.
Under similar conditions of excitation a photoreaction has not been observed in poly(p-phenylenevinylene) prepared via the precursor-route.
A method is developed to determine orientational order parameters <P2> and <P4> of films by steady state fluorescence polarization in a spectrometer with 90° angle between excitation and emission beam. This is the geometry usually found in commercial fluorescence spectrometers. The method includes the determination of the fundamental molecular anisotropy rM of the fluorescent chromophore (doped or intrinsic) in stretched and unstretched films and the determination of instrumental correction coefficients from an isotropic film. As an example, the method is applied to orientation analysis of polyethylene and polystyrene films doped with fluorescent molecules of repetitive styrene units. It is concluded that in uniaxially stretched polyethylene-films orientation of these molecules is dominated by their molecular shape; in polystyrene films orientation is influenced by additional interactions.
Stationary polarized fluorescence spectra of poly(1,4-phenylene-1-phenylvinylene), poly(1,4-phenylene-1,2-diphenylvinylene) and poly[1,4-phenylene-1,2-bis(4-phenoxyphenyl)vinylene] in tetrahydrofuran solution and in polystyrene films were recorded at room temperature. In these matrices, different behaviour is found concerning the Stokes shift, the degree of polarization and the quantum yield; it is interpreted as a viscosity induced effect. The commonly accepted model of a conjugated polymer chain to consist of independent segments of various length should not be applied to the poly(phenylene-arylvinylene)s studied in this work.