Short-and long-range order effects on resonance energy transfer in crystals and glasses

Resonance energy transfer by the Förster-Dexter mechanism in a cubic crystal and in a rigid homogeneous medium is studied. The homogeneous medium is modeled using a hard-sphere fluid (HSF) radial distribution function. This distribution is more realistic than the commonly used uniform distribution with excluded volume (UDEV) function. For the dipole-dipole mechanism, both models yield essentially the same donor luminescence decay, except for small critical radii. For the exchange mechanism, however, the two models differ significantly. In particular, to fit a given experimental decay, the UDEV model requires both a larger effective Bohr radius and a larger rate constant at collisional distance than the HSF model.     

Enzyme kinetics with a twist

A different approach to enzyme kinetics stressing the cyclic nature of the catalytic process is presented. The time-dependence of the substrate concentration is derived in a simple way not invoking the quasi-steady-state approximation. According to this approach the turnover rate can be written as the ratio of two parameters with a direct meaning: enzyme efficiency and average cycle duration. Real kinetic data for two enzyme-substrate pairs is used to show that the enzyme kinetic efficiency is best measured by the turnover rate.     

Size effects in triplet-triplet annihilation: I. Standard and statistical approaches

The dynamics of triplet-triplet annihilation is theoretically studied in linear chains and nanoparticles as a function of size M, the rate of excitation migration W, and the rate of excitation annihilation V. It is shown that a sum of two exponentials is usually sufficient for fitting experimental phosphorescence and triplet-triplet absorption decays. The first term describes the decay of domains containing initially one triplet, while the second term reflects the disappearance of domains containing initially two triplets.     

Constantin Carathéodory and the axiomatic thermodynamics

The birth, raise, development and fortunes of a fundamental theory in thermodynamics, the axiomatic thermodynamics, a creation of Constantin Carathéodory, is thoroughly presented together with a summary of Carathéodory's biography. Axiomatic thermodynamics is centered around some interesting properties of Pfaffian differential equations, which are here introduced and used for some well-known cases in thermodynamics.     

THERMODYNAMIC AND PARTICLE-DYNAMIC STUDIES ON SYNTHESIS OF SILICA NANOPARTICLES USING MICROWAVE-INDUCED PLASMA CVD

1. Introduction1.1 Silica nanoparticles and synthesis methods Silica (SiO2) nanoparticles are widely used in industry asan active filler for polymer reinforcement, a rheologicaladditive in fluids, a free flow agent in powders, and anagent for chemical mechanical polishing during IC (inte-grated circuit) fabrication (Sniegowski & de Boer, 2000).Silica powder is also used for producing silicon carbide(Koc & Cattamanchi, 1998) or opaque silica aerosols (Leeet al., 1995). Many methods can …     

Photophysical study of bis(naphthalimide)-amine conjugates: toward molecular design of excimer emission switching

The fluorescence properties of two bis(1,8-naphthalimides) with amino-containing spacers are investigated, giving special emphasis to the observation of excimer emission. It is found that a minor elongation of the spacer by two methylene units gives rise to a quantitative shut-down of the broad and red-shifted excimer emission. Furthermore, a switching of this emission is established through manipulation of a photoinduced electron transfer process, which involves the amino spacer. Protons as well as protic solvents lead to substantial excimer emission with lifetimes of 12 to 27 ns. The excimer quantum yield takes a maximum value of Φ(f) = 0.07 (acetonitrile with 1 equiv trifluoroacetic acid). The increased virtual Stokes shifts (ca. 150 nm) as compared to the fluorescence of monomeric 1,8-naphthalimides are an alternative approach to obtain colored, significant, and long-lived fluorescence from these chromophores. As an additional excited state pathway, the occurrence of homo-Fo?rster resonance energy transfer (homo-FRET) is established by fluorescence polarization measurements and calculation of the corresponding critical Fo?rster radius (R(0) ca. 13 ?). The average interchromophore distance between the naphthalimides is estimated as 7.5 ? and 9.5 ? for the dyad with the shorter and the longer spacer, respectively. These observations and the absence of a rise time component for excimer emission are in agreement with the formation of a "loose" ground state dimer, which upon excitation undergoes a fast geometrical adjustment to the excimer structure where the chromophores are at contact distance.     

Intra- and intermolecular heavy-atom effects on the fluorescence properties of brominated C60 polyads

A series of brominated mono-methano[60]fullerene malonate derivatives (two diads and one triad) are investigated for intramolecular and external heavy-atom effects on their fluorescence. Significant internal and external heavy-atom effects are observed in the three cases. It is shown that the internal effect doubles when going from the diads to the triad. In bromobenzene and in iodobenzene, the external effect is predominant, and diads and triad behave identically.     

A Fluorescent and Phosphorescent Nanoporous Solid: Crystalline Calix[4]arene

Calix[4]arene forms elongated nanoporous microcrystals. The pores are linear nano-channels (1.7 nm diameter) arranged in a honeycomb network. The crystals luminesce at room temperature according to a variety of processes that include monomer fluorescence (lifetime of ca. 1.1 ns), dimer fluorescence (lifetime of ca. 5.4 ns), and monomer phosphorescence (lifetime of ca. 2 s). The dimers result from pi-orbital overlap of adjacent phenol groups from neighboring nano-channels, with C-C distances of ca. 4 A.