Picosecond spectroscopic measurement of very fast intersystem crossing for 9,10-dioxa-anti-bimanes

Picosecond spectroscopy, following the buildup of T₁ → T_n absorption (maximum at 420 nm), shows that the T₁ state of 1,5-diazabicyclo[3,3,0]octa-3,7-diene-2,6-diones(9,10-dioxa-anti-bimanes) is formed within about 10 ps. The nature of the T₁ state was confirmed by decay rates of T₁ → T_n absorption in acetonitrile (n = 0.375 cP, k_nr = 4.5 × 10⁵ s^?1), 1,2-ethanediol (n = 26 cP, k_nr = 1.5 × 10⁴ s^?1 and glycerol (n = 1400 cP, k_nr = 1.3 × 10³ s^?1). The very fast intersystem crossing is ascribed to the proximity of a ³nπ* state to the ππ* (S₁ state produced by light absorption (El-Sayed rule).     

Excited-state proton transfer in chiral environments. 1. Chiral solvents

Excited-state proton transfer from the "super" photoacid 5,8-dicyano-2-naphthol to 2-butanol is faster in the enantiopure solvent than in its racemic form. The difference observed is discussed in terms of long-range order in homo- and heterodimers of 2-butanol.     

颗粒柱塌落中的尺寸效应和瞬态流变性研究

颗粒材料是自然界和工程中广泛存在和普遍应用的材料,泥石流、滑坡和混凝土等均可视为颗粒材料。颗粒材料研究有助于更好地控制相关自然灾害或利用其某些特性。对颗粒材料柱体的塌落动力学研究不仅可以方便理解颗粒材料在瞬态流动时的流变性,还可以引申到泥石流等岩土材料的运动与堆积形态。本文利用扩展多面体离散单元法对颗粒堆积柱的塌落进行了细致的研究,探索高宽比、摩擦系数以及颗粒柱相对尺寸等对柱体塌落的影响。对颗粒集合进行分网并分析每个网格内的应力与应变率之间的关系,讨论其瞬态本构关系。相关研究对于深入理解颗粒材料重力流的动力学性质以及颗粒集合体的堆积形态具有重要意义。     

Zur quantitativen Bestimmung des Acetons im Harn

Ohne Zusammenfassung     

Primitive,auflösbare Permutationsgruppen

Ohne Zusammenfassung     

Comparative study of the photoprotolytic reactions of D-luciferin and oxyluciferin

Optical steady-state and time-resolved spectroscopic methods were used to study the photoprotolytic reaction of oxyluciferin, the active bioluminescence chromophore of the firefly's luciferase-catalyzed reaction. We found that like D-luciferin, the substrate of the firefly bioluminescence reaction, oxyluciferin is a photoacid with pK(a)* value of ～0.5, whereas the excited-state proton transfer (ESPT) rate coefficient is 2.2 × 10(10) s(-1), which is somewhat slower than that of D-luciferin. The kinetic isotope effect (KIE) on the fluorescence decay of oxyluciferin is 2.5 ± 0.1, the same value as that of D-luciferin. Both chromophores undergo fluorescence quenching in solutions with a pH value below 3.     

Effect of electrolytes on the excited-state proton transfer and geminate recombination

Time-resolved emission and steady-state fluorescence techniques are used to study the excited-state intermolecular proton transfer from 8-hydroxypyrene-1,3,6-trisulfonate (HPTS or pyranine) to water in the presence of inert salts, NaCl and MgCl(2). At low salt concentrations, up to about 0.5 M MgCl(2) or about 0.8 M NaCl, the time-resolved emission of both the photoacid and conjugate base can be quantitatively fitted by our diffusion-assisted geminate recombination model. In this concentration range, the proton transfer and geminate recombination rate constants are almost independent of the salt concentrations whereas the proton diffusion constant decreases as the salt concentration increases. At higher salt concentrations, the proton-transfer rate constant decreases while the recombination rate constant increases slightly. For the saturated solution of MgCl(2) (about 5 M at room temperature), the steady-state emission consists of only a single band of the protonated photoacid. Careful examination of the time-resolved emission of HPTS in the presence of a large concentration of MgCl(2) shows that the quality of the fit to the geminate recombination model is rather poor and we fail to find adjustable parameters for a good quality fitting. For this large concentration range of MgCl(2) we were able to get a good fit of the experimental data with a model based on a distribution of proton-transfer rates. The model is consistent with an inhomogeneous water environment next to the excited HPTS molecule in such concentrated solutions.     

Understanding and controlling organic-inorganic interfaces in mesostructured hybrid photovoltaic materials

The chemical compositions and structures of organic-inorganic interfaces in mesostructurally ordered conjugated polymer-titania nanocomposites are shown to have a predominant influence on their photovoltaic properties. Such interfaces can be controlled by using surfactant structure-directing agents (SDAs) with different architectures and molecular weights to promote contact between the highly hydrophobic electron-donating conjugated polymer species and hydrophilic electron-accepting titania frameworks. A combination of small-angle X-ray scattering (SAXS), scanning and transmission electron microscopy (SEM, TEM), and solid-state NMR spectroscopy yields insights on the compositions, structures, and distributions of inorganic and organic species within the materials over multiple length scales. Two-dimensional NMR analyses establish the molecular-level interactions between the different SDA blocks, the conjugated polymer, and the titania framework, which are correlated with steady-state and time-resolved photoluminescence measurements of the photoexcitation dynamics of the conjugated polymer and macroscopic photocurrent generation in photovoltaic devices. Molecular understanding of the compositions and chemical interactions at organic-inorganic interfaces are shown to enable the design, synthesis, and control of the photovoltaic properties of hybrid functional materials.     

Analysis of the decay of picosecond fluorescence in semiconductors: Criteria for the presumption of electroneutrality during the decay of an exponential electron-hole profile

When an exponential profile of electron-hole pairs is photogenerated (in a semiconductor) with a delta-function light pulse, unequal diffusion coefficients of holes and electrons (i.e. D_e≠ D_h) effect deviations from electroneutrality as electrons and holes diffuse into the bulk semiconductor. These deviations will in turn effect errors in the analysis of data (e.g. time resolved fluorescence) when using theory based on the presumption of electroneutrality. We deduce here the experimental conditions required for an effective electroneutrality to be maintained during the course of an experiment. Analyses were carried out using computer simulations without the presumption of electroneutrality and the analytic solution with the presumption of electroneutrality. The differences in the measured fluorescences predicted by the two computations are characterized as a function of a variety of experimental parameters and physical properties: intensity (of the excitation pulse), the absorption of the exciting and emitted light, the the ratio D_h/D_e, bulk dielectric constant of the semiconductor, bulk and surface recombination kinetics. It is shown that a conditon of adequate electroneutrality can be effectively attained when a well defined a minimum number of electron-hole pairs is generated; an upper limit of the number of e^?–k⁺ pairs is also established in order to avoid an intolerable temperature pulse.