Magnetic Field Effect in Highly Pure, Highly Fluorescent Conjugated Polymers
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ABSTRACT We report on the magnetic field effect (MFE) on the photoluminescence in films of a highly fluorescent conjugated laddertype poly(paraphenylene) (LPPP). We observe no MFE for the pure material, however, the introduction of a very small content of chemical defects via photo-oxidation leads to a MFE proportional to the amount of defects. The chemically induced increase in MFE is correlated with a change in other properties of the LPPP films, such as photoluminescence emission and excitation spectra, transient photoluminescence and infrared spectra. INTRODUCTION Due to their low content of chemical defects [1] the class of conjugated laddertype poly(paraphenylenes) (LPPP) is highly interesting as a model system for basic physical research [2] and attractive for applications. Using LPPP blue and white light emitting devices (LEDs) [3] were demonstrated as well as highly directional, linearly polarized, narrowline (; 2 nm) emission from optically pumped polymer waveguides [4]. The effect of weak static magnetic fields on the photoluminescence (PL) quantum yield (QY) [5] and the photoconductivity (PC) [6] of conjugated polymers was attributed to the PL emission being partly due to geminate recombination of charge carriers. In [7] a significant enhancement of this MFE is found for the PC in the presence of oxygen. The aim of our investigation is to study how strong the MFE is in LPPP which is characterized by high intrachain order [8] and a very low defect content [1]. EXPERIMENTAL The preparation of the LPPP powder is described in [9]. For our investigations we have used the m-LPPP which is distinguished by its methyl substituent (see Fig. 1), increasing the chemical stability as compared to the hydrogen substituted form. The polymer films were prepared by dropcasting from toluene, chloroform and tetrahydrofuran solutions of m-LPPP on glass, sapphire and silicon substrates. All subsequent spectroscopic investigations were either performed on the same or on identically treated samples. Photo-oxidation of the polymer films and solutions is performed by the infrared filtered ultraviolet/visible radiation of a 1000 W high pessure Xe lamp. Photoluminescence excitation (PLEx) and emission (PLEm) spectra were recorded with a Shimadzu spectrofluorophotometer RF-5301PC. The time resolved PL data were obtained with a photomodulation spectrometer I.S.S. GREG200 300 Xe. The infrared spectra were recorded with a Bomem MB 102 Fourier transform infrared spectrometer. The MFE was investigated with a self-built MARY spectrometer [10], where MARY refers to "MAgnetic field effects on Reaction Yield". A 150 W high pressure Xe lamp (Osram XBO 150/1 OFR) together with appropriate filters was used as excitation source. The sample was placed in a Bruker research magnet B-E 10 and exposed to very homogeneous small magnetic fields B up to 10 mT. In order to detect the small MFE we used a modulation technique. A pair of additional coils, connected in series, was placed onto the pole shoes of the research magnet. This pair of coils was sim
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