Aggregation-cased quenching

Due to the overlap of the monomer fluorescence emission and absorption from the aggregates, a quenching process may occur by a resonance interaction. However, in other cases [85, 86] where we determine the extinction coefficients for ground-state monomers and dimers, by simply taking into account the fraction of the excitation radiation absorbed by the monomers (which are emissive species) and aggregates (from which we could not detect any fluorescence emission), a good superposition between the calculated and experimental fluorescence intensity curve was obtained. 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

The first iridium complex used in PHOLED devices was fac tris(2-phenylpyridine) iridium Ir(ppy)3 complex [282]. It has a short triplet lifetime ( 1 ps) and high phosphorescent efficiency (p = 40% at room temperature in solution) [283]. However, in the solid state, most iridium complexes showed very low phosphorescent QE due to aggregate quenching. In most cases, the complexes have to be diluted in host materials to avoid reducing the... 

The long lifetime of phosphorescence allows it to be used for processes which are slow—on the millisecond to microsecond time scale. Among these processes are the turnover time of enzymes and diffusion of large aggregates or smaller proteins in a restricted environment, such as, for example, proteins in membranes. Phosphorescence anisotropy is one method to study these processes, giving information on rotational diffusion. Quenching by external molecules is another potentially powerful method in this case it can lead to information on tryptophan location and the structural dynamics of the protein. 

This chapter discusses four methods of gas phase ceramic powder synthesis by flames, fiunaces, lasers, and plasmas. In each case, the reaction thermodynamics and kinetics are similar, but the reactor design is different. To account for the particle size distribution produced in a gas phase synthesis reactor, the population balance must account for nudeation, atomistic growth (also called vapor condensation) and particle—particle segregation. These gas phase reactors are real life examples of idealized plug flow reactors that are modeled by the dispersion model for plve flow. To obtain narrow size distribution ceramic powders by gas phase synthesis, dispersion must be minimized because it leads to a broadening of the particle size distribution. Finally the gas must be quickly quenched or cooled to freeze the ceramic particles, which are often liquid at the reaction temperature, and thus prevent further aggregation. 

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