Institut für Physikalische Chemie und Theoretische Chemie
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The paper is focused on the formation and redispersion of monodisperse BaSO4 nanoparticles in polyelectrolyte- modified microemulsions. It is shown that a cationic polyelectrolyte of low molar mass, e.g. poly(dially1dimethylammonium chloride) (PDADMAC), can be incorporated into the individual inverse microemulsion droplets (L2 phase) consisting of heptanol, water, and an amphoteric surfactant with a sulfobetaine head group. These PDADMAC- filled microemulsion droplets can be successfully used as a template phase for the nanoparticle formation. The monodisperse BaSO4 nanoparticles are produced by a simple mixing procedure and can be redispersed after solvent evaporation without a change in particle dimensions. Dynamic and electrophoretical light scattering in combination with sedimentation experiments in the analytical Ultracentrifuge of the redispersed powder show polyelectrolyte-stabilized nanoparticles with diameters of about 6 nm. The polyelectrolyte shows a "size control effect", which can be explained by the polyelectrolyte-surfactant interactions in relation to the polyelectrolyte-nanoparticle interactions during the particle growth, solvent evaporation and redispersion process. However, the approach used here opens away to produce different types of polyelectrolyte-stabilized nanoparticles (including rare metals, semiconductors, carbonates or oxides) of very small dimensions. (C) 2004 Elsevier B.V. All rights reserved
Deuteration effects on the vibronic structure of the emission and excitation spectra of triangular [ 4] phenylene (D-3h [4]phenylene) were studied using laser-excited Shpol'skii spectroscopy (LESS) in an octane matrix at 4.2 K. For correct assignment of the vibrational modes, the experimental results were compared with calculated frequencies (B3LYP/6-31G*). CH vibrations were identified by their characteristic isotopic shifts in the spectra of deuterated triangular [4]phenylenes. Two CC stretching modes, at 100 cm(-1) and 1176 cm(-1), suitable as probes for bond strength changes in the excited state, were identified. The isotope effect on the internal conversion rates of triangular [4] phenylene was evaluated from measurements of temperature dependent lifetime. Isotope dependency and the magnitude of the internal conversion rates indicate that internal conversion in triangular [4] phenylene is most likely induced by CH vibrations. The results obtained by LESS and lifetime measurements were compared with PM3 PECI calculations of the excited state structure. The theoretical results and the relation between ground and excited state vibration energies of the 1176 cm(-1) probe vibration indicate a reduction of bond alternation of the central cyclohexatriene ring in the excited state