Excitation quencher

Identification of the Chlorophyll Excitation Quencher in Aggregated LHCII.124... 

The excited quencher can then lose its enetgy by a variety of processes. 

Both the decay channels are realized independently. The decay in which the excited quencher Q would be formed is usually largely endoergonic and, therefore, such a pathway of degrading the electronic energy is implausible. 

First, we have prepared a series of co-polymers of styrene with an excitation quencher, 2-(2 -hydroxy-5 -vinylphenyl)-2H-benzotriazole, abbreviated 2H5V. Secondly, we have looked at the effect of temperature on the photophysical properties of these polymers. 

Scheme 9.12 Schematic illustration of the deactivation of excited quencher molecules.

Here, the excited analyte molecule, F, transfers excitation energy to a quencher molecule, Q, causing deactivation of F and forms an excited quencher molecule, Q. For collisional quenching, the decrease in intensity often follows the well-known Stem-Vol-mer equation ... 

The observation of any unimolecular process of the excited quencher following light absorption by the donor is called energy transfer sensitization, and is the only direct proof of the involvement of energy transfer in the quenching process. The quantum yield of a sensitized process is given by... 

RTIL + P-carotene —> RTIL + P-carotene where an excited molecule (RTIL ) transfers excitation energy to a quencher molecule P-carotene, causing deexdtation of RTTL and forming an excited quencher molecule, P-carotene. If P-carotene is a fluorescent sjjedes, its fluorescence, called sensitized fluorescence, can be detected. This is the phenomenon allowing observation of fluorescence from a molecule like P-carotene that maybe difficult to excite directly because of the optically forbidden (for symmetry reason) singlet electronic states. The all-frans P-carotene in standard solvent, for example hexane, fluorescence quantum yield of electronic state S2 is 2 x 10- (Sherve et al., 1991 Andersson, 1992) and of Si state is 4 X lO- (Wasielewski, 1986,1989). 

These reactions are modelled in terms of a diffusional step or kd[ff), resulting in the formation of a bimolecular encounter complex. This is followed by competing pathways for dissociation of the complex or or energy transfer k or k ). Deactivation of the excited quencher is described by p. The observed CPL is directly proportional to the difference in excited state concentrations of the two enantiomers, and this may be related to the specific rate constants introduced above. If, for example, we make the reasonable assumption that the diffusion and deactivation are independent of chirality, one can derive the following expression for AN t)... 

Chou J Z and Fiynn G W 1990 Energy dependence of the reiaxation of highiy excited NO2 donors under singie coiiision conditions vibrationai and rotationai state dependence and transiationai recoii of CO2 quencher moiecuies J. Chem. Rhys. 93 6099-101... 

Excited states can also be quenched. Quenching is the same physical process as sensitization, but the word quenched is used when a photoexcited state of the reactant is deactivated by transferring its energy to another molecule in solution. This substance is called a quencher. 

Another useful technique for measuring the rates of certain reactions involves measuring the quantum yield as a function of quencher concentration. A plot of the inverse of the quantum yield versus quencher concentration is then made Stern-Volmer plot). Because the quantum yield indicates the fraction of excited molecules that go on to product, it is a function of the rates of the processes that result in other fates for the excited molecule. These processes are described by the rate constants (quenching) and k (other nonproductive decay to ground state). 

A plot of 1 versus quencher concentrations, [Q], then gives a line with the slope k /k. It is usually possible to assume that quenching is diffusion-controlled, permitting assignment of a value to k. The rate of photoreaction, k, for the excited intermediate can then be calculated. 

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

It is possible that Q is an excited state of Q if so, we will assume that its emission spectrum does not contribute to the fluorescence intensity at Vcnr Q is called a quencher, because in its presence the fluorescence intensity of solute A is reduced. 

Kaneko et al. [80, 81] prepared copolymers of AA (93.9-95.9 mol%) and vinylbipyridine (1.6-3.7 mol%) with pendant Ru(bpy)2+ (2.4-2.5 mol%) (25). The quenching of the excited state of the pendant Ru(II) complex by MV2+ was accelerated in alkaline aqueous solution owing to the electrostatic attraction of the cationic quencher. Interestingly, the quenching efficiency was dependent on the molecular weight of 25. The quenching of the polymer with MW 2100 occurred... 

In an aqueous solution containing 26 and 27 the excited state of the Ru(II) complex in 26 essentially has no chance to be directly quenched by the donor quencher in 27, because a strong electrostatic repulsion acts between 26 and 27. Sassoon and Rabani added methoxydimethylaniline (MDMA, 28) to this system... 

It has been shown in Chapter 5, the fluorescence quenching of the DPA moiety by MV2 + is very efficient in an alkaline solution [60]. On the other hand, Delaire et al. [124] showed that the quenching in an acidic solution (pH 1.5-3.0) was less effective (kq = 2.5 x 109 M 1 s 1) i.e., it was slower than the diffusion-controlled limit. They interpreted this finding as due to the reduced accessibility of the quencher to the DPA group located in the hydrophobic domain of protonated PMA at acidic pH. An important observation is that, in a basic medium, laser excitation of the PMAvDPA-MV2 + system yielded no transient absorption. This implies that a rapid back ET occurs after very efficient fluorescence quenching. 

In order to clear up the mechanism of inactivation of excited states, we examined the processes of quenching of fluorescence and phosphorescence in PCSs by the additives of the donor and acceptor type253,2S5,2S6 Within the concentration range of 1 x 1CT4 — 1 x 10"3 mol/1, a linear relationship between the efficiency of fluorescence quenching [(/0//) — 1] and the quencher concentration was found. For the determination of quenching constants, the Stem-Volmer equation was used, viz. 

Iq/I — t — KgI0 [Q], in which Kg is the bimolecular rate constant of interaction of quencher Q with the excited states of the PCS, t is the lifetime of excited molecules with no quencher, I0 is the quantum yield of fluorescence in the absence of the quencher, and I is the quantum yield of fluorescence in the presence of the quencher. 

The luminescence of an excited state generally decays spontaneously along one or more separate pathways light emission (fluorescence or phosphorescence) and non-radiative decay. The collective rate constant is designated k° (lifetime r°). The excited state may also react with another entity in the solution. Such a species is called a quencher, Q. Each quencher has a characteristic bimolecular rate constant kq. The scheme and rate law are... 

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