Bimolecular reaction fluorescence quenching

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

Bimolecular reactions such as quenching, either by molecules of the same kind, self-quenching, or by added substances, impurity quenching, inhibit emission because frequency of bimolecular collisions in gases as well as in solution, k 1010. v can compete with fluorescence emission. Solvent quenching may involve other physical parameters as well such as solute-solvent interactions. Since the solvent acts as the medium in which the solute molecules are bathed, solvent quenching may be classified under unimolecular processes and a clear distinction between it and internal conversion St - S0 is difficult. 

Fig. 49. Estimates for the rate coefficient of a second-order bimolecular reaction (such as fluorescence quenching) in solution. The quencher species, for instance, is present in large quantities, occupying a fraction c of the total volume. If this species has a spherical radius of R, the volume fraction is c = 4irI Jp/3, where p is the density of...

If one studies only the fluorescence quenching by irreversible bimolecular ionization (3.52), there is seldom any need to trace the fate of the charged products. On the contrary, those who are interested in photoinduced geminate recombination (3.188) rarely care about the kinetics of ionization, its quenching radius, and all the rest studied in Section III. All that they need to obtain the charge separation yield is the initial ion distribution mo(r), prepared by photoionization. However, the latter is scarcely so simple as in Eq. (3.201), which is usually favored. Even so, the initial separation ro is not a fitting parameter but the characteristic interion distance, which is dependent on the precursor reaction of photoionization. 

Scheme 1. Bimolecular fluorescence quenching Scheme 2. Fluorescence quenching by reaction. a geminate recombination.

Bimolecular quenching reactions of excited states are typically represented by the pseudo-first-order reaction kinetics, Equation (30.6a). When a distribution of probe environments exists, the difference between the macroscopic fluorescence decay profile observed and microscopic fluorescence decay rates present must be recognized. The composite distribution of both unimolecular decay rates and the respective bimolecular reaction rates are combined in the distribution of observable decay rates. Hence, the parameters in Equation (30.6a) have to be redefined in the Gaussian distribution terminology to yield Equation (30.6b). 

In order to develop catalytic effects of cyclodextrins for bimolecular reactions, it needs to include two guest molecules simultaneously in a cyclodextrin (CD) cavity. Several examples of cyclodextrin-catalyzed bimolecular reactions have been reported. Rideout and Breslow have found that Diels-Alder reactions of cyclo-pentadiene with butenone, cyclopentadiene with acrylonitrile, and anthracene-9-carbinol with N-ethylmaleimide in water are markedly accelerated by 3-cyclodextrin (3-CD) (1). Komiyama and Hirai have reported site-selective Reimer-Tiemann reactions of phenols in cyclodextrin solutions (2). In most of these reactions, however, each substrate molecule is relatively small so that a 3-CD cavity may include simultaneously an additional reactant molecules. We found previously that the fluorescence quenching of pyrene and naphthalene by trimethylamine (TMA) or dimethylamine (DMA) in water is catalyzed by g-CD (3) Since the pyrene molecule is too large to be incorporated completely in the 3-CD cavity, it has been assumed that pyrene binds to a rim of the CD cavity to form a pyrene-capped CD complex and a remain-... 

The fluorescence quenching can be regarded as a probe reaction for investigating the effects of CD on bimolecular reactions. Especially, the structural requirements for guest molecules to form three-component complex can be studied by this method. Formation of three-component complexes is essentially important to realize CD-catalyzed bimolecular reactions. In the present work, we studied the fluorescence quenching of naphthalene, 1-methylnaphthalene, and acenaphthene by TMA in aqueous 3-CD solutions to clarify the steric factors affecting the formation of the three-component complexes. 

The effect of excimer kinetics on fluorescence decays of monomers and excimers upon excitation with a short pulse was studied first by Birks et al. [119]. They took into account all the relevant processes that proceed after the excitation of a low fraction of monomers by an ultrashort pulse and derived the rate equations describing the monomer and excimer decays. Most processes involved in the Birks scheme are monomolecular and depend only on the concentration of the excited species and on the first-order rate constant one of them is a bimolecular process and depends on the concentrations of both the excited and ground-state molecules. They include (1) monomer fluorescence, (rate constant fM), (2) internal monomer quenching, M —>M, ( iM). (3) excimer formation, M - -M D (bimolecular reaction, i.e., the rate depends on the product of the rate constant and concentration of the ground-state... 

CE Bunker, Y-P Sun. Evidence for enhanced bimolecular reactions in supercritical CO2 at near-critical densities from a time-resolved study of fluorescence quenching of 9,10-bis(phenylethynyl)anthracene by carbon tetrabromide. J Am Chem Soc 117 10865, 1995. 

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

The fluorescence Intensity of substituted stilbenes and stilbene analogues provides a useful indicator of photochemical reactivity. Virtually all of the reported bimolecular photochemical reactions of electronically excited stilbenes involve stilbenes which are fluorescent at room temperature in solution. The absence of fluorescence is indicative of a singlet lifetime too short (< 100 ps) to allow for efficient bimolecular quenching. 

Finally let us consider the very unusual bimolecular addition of dienes to cyclobutanones (34), a flagrant violation of Rule 7. The reactions which were run at -78 °C can be understood if we assume that the rate of cleavage of to a biradical is slowed down because of a decrease in the available thermal activation. However, a moderate yield of oxetane is formed from cyclobutanone and butadiene (19) at 10°C. From work on the quenching of fluorescence by alkanones (53), we anticipate the rate of... 

Bimolecular quenching reactions of excited states are typically represented by the pseudo-first-order reaction kinetics, equation 6a. When a distribution of probe environments exists, the difference between the macroscopic fluorescence decay profile observed and microscopic fluores-... 

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