Quenching,bimolecular

The downward curvature of the Stern-Volmer intensity plots necessitates a model more complex than a single species quenched bimolecularly. We have evaluated... 

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]. 

Agranovich V M, Efremov N A and Kirsanov V V 1980 Computer simulation of kinetics of excitation bimolecular quenching by Monte-Carlo method Fiz. Tverd. Tela 22 2118-27... 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

To minimize quenching of triplets due to bimolecular collisions, phosphorescence studies are generally carried out in rigid glasses at 77°K. Some materials which may be used to produce these glasses are listed in Table... 

A nonlinear plot of loge[ecd vs. T indicates that bimolecular processes such as triplet-triplet annihilation or triplet quenching are contributing to triplet state deactivation. 

Hence the steady-state population of triplets should increase under heavy-atom perturbation. However, this conclusion is valid only if unimolecular decay is the main route leading to triplet state depopulation. If bimolecular triplet quenching as shown below is more important than unimolecular decay by several orders of magnitude, kd could be increased as much or more than klte without decreasing the steady state triplet population<136) ... 

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. 

The constant of bimolecular quenching for oxygen is very high and reaches 10loL mol 1 s it means that at the atmospheric pressure, when oxygen concentration may be as high as Q 10 3 M, km 107 s. Therefore, for... 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

The next group of bimolecular interactions (3) shown in Table 1, includes noncontact interactions, in which fluorescence quenching occurs due to radiative and nonradiative excitation energy transfer [1, 2, 13, 25, 26]. Energy transfer from an excited molecule (donor) to another molecule (acceptor), which is chemically different and is not in contact with the donor, may be presented according to the scheme ... 

Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

The quenching of benzophenone phosphorescence has been used by Mar and Winnik (1981) as a photochemical probe of hydrocarbon chains in solution. The bimolecular reaction for quenching the triplet state of 4-methoxy-carbonylbenzophenone [24] by 1-pentene occurs at rates which are below the diffusion limit by two to three orders of magnitude. Consequently, the intramolecular quenching reactions of to-alkenyl esters of benzophenone-4-carbo-xylic acid [25] occurs under conformational control. In [25] the point of... 

The mechanistic basis of 3 and 4 is a double application of the argument used for the YES operation with 1. Each receptor in 3 is capable of launching a PET process if the lumophore is powered up by ultraviolet excitation. In other words, the fluorescence of the 9-cyanoanthracene unit is efficiently quenched by either the amine or the 1,2-dioxybenzene group within the benzo-15-crown-5 ether unit. These quenching processes are predictable from thermodynamic calculations or from related bimolecular quenching experiments in the literature. The small separation of the amine from the lumophore ensures rapid PET kinetics. On the other hand, the presence of four bonds between the benzo-15-crown-5 ether and the lumophore is probably responsible for the incomplete quenching seen between this pair. No information is yet available regarding possible folded conformations of 3. 

In the presence of earlier generations, the luminescence quenching of aqueous solutions of Ru(L)32+ by methyl viologen (Figure 13.11) was found to follow Stern-Volmer bimolecular kinetics with a quenching constant /cq 5 x 109 M-1s-1, typical for the bimolecular quenching in homogeneous solutions. This... 

Quenching is a bimolecular process that is, it involves the collision of both Si and Q molecules. Thus, in the presence of the quencher, where the rate constant is kQ and the rate of deactivation by quenching is QJ ... 

Case C Q is not in large excess and mutual approach of M and Q is possible during the excited-state lifetime. The bimolecular excited-state process is then diffusion-controlled. This type of quenching is called dynamic quenching (see Section 4.2.2). At high concentrations of Q, static quenching may occur in addition to dynamic quenching (see Section 4.2.4). 

The same experiment can be carried out quantitatively. By taking into account radiationless processes, namely, internal conversion fcIC, intersystem crossing isc, and bimolecular quenching kQ[Q] with a quencher Q, the time-dependent concentrations of the donor D and the acceptor A in the excited singlet state Si, [Ds,] and [As,] can be expressed as follows ... 

Where quenching is present, the bimolecular quenching rate constant, fa, is assumed constant across the distributions, although for different classes of sites the rate constant may vary. For decay curves for a single site, Eq. (4.5) is used but r in the exponential decay part depends on quencher concentration and fa. 

Figure 4.13. Intensity Stem-Vo I met plot for a double Gaussian distribution of lifetimes as a function of distribution width K. The two lifetimes are 5 and 15 with bimolecular quenching rate constants of 0.30 and 0,01, respectively. Theunqucnchcd intensity contribution is 0.5 for each lifetime. The R for each distribution is indicated next to the curve. (Adapted from Ref. 54.)...

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