Intermolecular quenching

Several colloidal systems, that are of practical importance, contain spherically symmetric particles the size of which changes continuously. Polydisperse fluid mixtures can be described by a continuous probability density of one or more particle attributes, such as particle size. Thus, they may be viewed as containing an infinite number of components. It has been several decades since the introduction of polydispersity as a model for molecular mixtures [73], but only recently has it received widespread attention [74-82]. Initially, work was concentrated on nearly monodisperse mixtures and the polydispersity was accounted for by the construction of perturbation expansions with a pure, monodispersive, component as the reference fluid [77,80]. Subsequently, Kofke and Glandt [79] have obtained the equation of state using a theory based on the distinction of particular species in a polydispersive mixture, not by their intermolecular potentials but by a specific form of the distribution of their chemical potentials. Quite recently, Lado [81,82] has generalized the usual OZ equation to the case of a polydispersive mixture. Recently, the latter theory has been also extended to the case of polydisperse quenched-annealed mixtures [83,84]. As this approach has not been reviewed previously, we shall consider it in some detail. 

A porphinatoaluminum alkoxide is reported to be a superior initiator of c-caprolactone polymerization (44,45). A living polymer with a narrow molecular weight distribution (M /Mjj = 1.08) is ob-tmned under conditions of high conversion, in part because steric hindrance at the catalyst site reduces intra- and intermolecular transesterification. Treatment with alcohols does not quench the catalytic activity although methanol serves as a coinitiator in the presence of the aluminum species. The immortal nature of the system has been demonstrated by preparation of an AB block copolymer with ethylene oxide. The order of reactivity is e-lactone > p-lactone. 

Exploration of collective effects in multiple transfers that appear when the donor and acceptor are the same molecules and display the so-called homotransfer. In this case, the presence of only one molecular quencher can quench fluorescence of the whole ensemble of emitters coupled by homotransfer [32]. The other possibility of using homo-FRET is the detection of intermolecular interactions by the decrease of anisotropy [33]. 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

Figure 3 Different processes for losing energy from the excited state (1) direct CL (2) molecular dissociation (3) chemical reaction with other species (4) intramolecular energy transfer (5) intermolecular energy transfer (in case of a fluorophore, indirect CL) (6) isomerization (7) physical quenching. (Adapted from Ref. 1.)...

Encapsulation starting from the readily available triacetonamine derivative 4-oxo-TEMPO and propylaminetrimethoxysilane, in fact, prevents the intermolecular quenching of the radicals bound at the silica surface that has been found to be responsible for the loss of activity of TEMPO tethered at the surface of commercial silica. 

Y Kawamura, J Brooks, JJ Brown, H Sasabe, and C Adachi, Intermolecular interaction and a concentration-quenching mechanism of phosphorescent Ir(III) complexes in a solid film, Phys. Rev. Lett., 96 017404-1-017404-4, 2006. 

Finally, there can be rather large torsional tunneling splittings due to O—H torsion in the monomers (see the previous section), which are likely to be quenched almost completely by the intermolecular hydrogen bond. These tunneling processes in the monomers can themselves be affected by weaker intramolecular hydrogen bond interactions, such as C—H O contacts. 

We now look at the intermolecular deactivation of an excited molecule by another molecule (of the same or different type), a process called quenching. Any substance that increases the rate of deactivation of an electronically-excited state is known as a quencher and is said to quench the excited state. 

Overview of the intermolecular de-excitation processes of excited molecules leading to fluorescence quenching... 

Methods based on intermolecular quenching or intermolecular excimer formation... 

The serious drawback of the methods of evaluation of fluidity based on intermolecular quenching or excimer formation is that the translational diffusion can be perturbed in constrained media. It should be emphasized that, in the case of biological membranes, problems in the estimation of fluidity arise from the presence of proteins and possible additives (e.g. cholesterol). Nevertheless, excimer formation with pyrene or pyrene-labeled phospholipids can provide interesting in-... 

The choice of method depends on the system to be investigated. The methods of intermolecular quenching and intermolecular excimer formation are not recommended for probing fluidity of microheterogeneous media because of possible perturbation of the translational diffusion process. The methods of intramolecular excimer formation and molecular rotors are convenient and rapid, but the time-resolved fluorescence polarization technique provides much more detailed information, including the order of an anisotropic medium. 

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

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