A controversy has arisen regarding the photophysical properties of 9,10-diphenylanthracene (DPA), a popular emission quantum yield standard. In order to elucidate the causes of the disagreement in the literature, fluorescence quantum yields, lifetimes, and oscillator strength's were measured for DPA in ethanol, 3-methylpentane, cyclohexane, and benzene solutions. The effects of high concentrations and self-absorption were dramatized by determining the measured lifetime for a series of concentrations of DPA in cyclohexane, demonstrating that the optical density in the 0-0 band must be kept below 0.05/cm in DPA to avoid reabsorption. It is proposed that these effects constitute a partial explanation for the discrepancies in the literature. In addition, the universal application in quantum yield measurements of the correction factor ns2/nr2 for differences between the index of refraction of the sample ns and the reference nr is questioned. By means of a variable slit arrangement, we have shown that the proper correction is a strong function of the geometry of the sample compartment. This effect suggests that wide utilization of the standard correction term may also provide a source of error. Taking these deficiencies into consideration the following fluorescence quantum yields and lifetimes of DPA at 293 K were determined: 0.95, 8.19 ns in ethanol; 0.93, 7.88 ns in 3-methylpentane; 0.86, 7.58 ns in cyclohexane; 0.82, 7.34 ns in benzene.

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Highly fluorescent conjugated polymer nanoparticles were prepared directly by polymerization in aqueous miniemulsion, employing Glaser coupling polymerization as a suitable step-growth reaction. A 4,4'-dinonyl-2,2'-bipyridine-modified catalyst was found to be suited for the polymerization in the aqueous heterophase system. Nanoparticles of poly(arylene diethynylenes) (arylene = 2,5-dialkyoxy phenylenes and 9,9'-dihexyl fluorene) with molecular weights in the range of M(n) 10(4) to 10(5) g mol(-1) and with sizes of < or = 30 nm, as observed by TEM, result. N,N'-di(4-ethynylphenyl)-1,7-di[4-(1,1,3,3-tetramethylbutyl)phenoxy]perylene-3,4:9,10-tetracarboxdiimide or 2,7-diethynylfluorenone was converted completely during the heterophase polymerization to afford colloidally stable nanoparticles of poly(arylene diethynylenes) with 0.1-2 mol % covalently incorporated perylene dye and 2-9 mol % of covalently incorporated fluorenone dye, respectively. Fluorescence spectroscopy of the aqueous dispersions reveals an efficient energy transfer to the dye in the nanoparticles, which enables a variation of the luminescence emission color between red (lambda(em) (max.) ca. 650 nm) and the green emission of the nanoparticles without dye.

The p h o t d i t i o n of C l s O (C, symmetry) at 248 and 193 nm has been studied by photofragment translational spectroscopy under various scattering angles. At 248 nm the decay proceeds along the molecular channel to SO + C12 but mainly along the fast (<10-l2 s) radical channel yielding C1+ SOCl. The anisotropy parameters show that the molecular and the radical decays arise from initial preparation of two states with A' and A" symmetry, respectively. At 193 nm the molecular and the radical processes are strongly dominated by a threebody dissociation (>8Wo) producing SO + C1+ CI. The photofragment anisotropies are consistent with the three competing, subpicosecond dissociation reactions starting on a single potential energy surface accessed via an A" -A' singlet transition. Furthermore, the fragment kinetic energy distribution of the radical channel at 248 and 193 nm indicates that SOCl has an excited electronic state at -9000 cm-I.

Single-molecule fluorescence microscopy was used to investigate the dynamics of perylene diimide (PDI) molecules in thin supported polystyrene (PS) films at temperatures up to 135 °C. Such high temperatures, so far unreached in single-molecule spectroscopy studies, were achieved using a custom-built setup which allows for restricting the heated mass to a minimum. This enables temperature-dependent single-molecule fluorescence studies of structural dynamics in the temperature range most relevant to the processing and to applications of thermoplastic materials. In order to ensure that polymer chains were relaxed, a molecular weight of 3000 g/mol, clearly below the entanglement length of PS, was chosen. We found significant heterogeneities in the motion of single PDI probe molecules near T(g). An analysis of the track radius of the recorded single-probe molecule tracks allowed for a distinction between mobile and immobile molecules. Up to the glass transition temperature in bulk, T(g,bulk), probe molecules were immobile; at temperatures higher than T(g,bulk) + 40 K, all probe molecules were mobile. In the range between 0 and 40 K above T(g,bulk) the fraction of mobile probe molecules strongly depends on film thickness. In 30-nm thin films mobility is observed at lower temperatures than in thick films. The fractions of mobile probe molecules were compared and rationalized using Monte Carlo random walk simulations. Results of these simulations indicate that the observed heterogeneities can be explained by a model which assumes a T(g) profile and an increased probability of probe molecules remaining at the surface, both effects caused by a density profile with decreasing polymer density at the polymer-air interface.