Rate constants bimolecular quenching

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.)...

Pulsed-laser photolysis of the azirines 158a-c in the presence of electron-dehcient alkenes (44,66) allowed the determination of the bimolecular quenching rate constants (feq) for reactions with acrylonitrile (1.0-5.4 x 10 and... 

The luminescence quantum efficiency when quencher is present is termed q and the bimolecular quenching rate constant is termed kq. 

If the steady-state approximation for the concentrations of S Q and Q" in Scheme I in 13.4.3 is made, then the observed bimolecular-quenching rate constant is ... 

Whereas the observed decay profile no longer is characterized by a single decay rate, the steady-state fluorescence intensity becomes dependent on both 7obs and fc>bs. The typical Stern-Volmer plot is no longer represented by equation 7a, but rather by equation 7b, where fcobs is defined by equation 6b, fc q is the bimolecular quenching rate constant, fco is the probe s mean excited-state unimolecular decay rate constant, fcobs is the mean observed decay rate constant, 70 is the distribution parameter of the Gaussian for the unimolecular decay, and 7obs is the distribution parameter for the observed unimolecular decay rate. 

SOM HaS Reactions.—The photolysis of SOa in Ha at 147 nm and 116—123.6 nm has been studied,388 and emission from OH(/f - X), S ( -> X), and SO(B -+ X) was observed in addition to SOa fluorescence. The quantum yield of S03 formation was found to be <0.4 at 123.6 nm. A fuller paper has appeared on single vibronic level fluorescence from the state of SOa vapour. This confirms that there are two different groups of rovibronic states, one of which is very long-lived (80—600 (is), the other being short ( 50 (jis) and with a bimolecular quenching rate-constant that is an order of magnitude greater than the gas-kinetic rate.388 The extensive implications of this important observation were discussed in Volume 5,887 and will not be repeated here. 

The accessibility of quenchers to probes in supramolecular structures can be inferred from bimolecular quenching rate constants. The necessary assumptions have been discussed in Section II.B. Quenching can be studied by the decrease of the fluorescence intensity with the addition of a quencher and the data are treated by employing the Stem-Volmer equation ... 

The triplet excited state characteristics of the crown-ether squaraines, determined by energy transfer sensitization, are also summarized in Table 1. The triplet states of the crown ether squaraine derivatives underwent a selfquenching process with bimolecular quenching rate constants varying from 0.5 X 10 to 1.9 X 10 M s with the ground state of the dyes to yield the radical cation and anion (Reactions 1 and 2). 

The bimolecular quenching rate constants, of pyrene fluorescence in various systems are presented in Table II. The values of with the quenchers such as oxygen, nitromethane, and sodium iodide in PA-I8K2 polymer-micellar solutions are much smaller than in water or in homogeneous nonpolar solvents (viz. heptane) and even smaller than in SDS micellar solutions. The data suggest that the penetration of the selected quenchers to the pyrene hosted in the polymer micelle is inhibited by the main chain of the host polymer micelle. This restrictive effect is larger than in simple micellar systems, and it results from the more rigid environment of pyrene in polymer micellar systems. 

Figure 30.11 shows that the observed decay rate constants of both DPA and pyrene vary linearly with surface oxygen concentration when surface oxygen concentration is used as the independent variable. The bimolecular quenching rate constants obtained from the slopes of these plots decrease with a decrease in temperature, as... 

The photochemistry of this class of complexes has also been studied extensively. For example, the high-energy XLCT emission of 4a in benzene has been found to be quenched by pyridine [47], The bimolecular quenching rate constant was estimated to be 5.9 0.5 X 10 dm- mol" s". The mechanism for the quenching has been ascribed to the exciplex formation between the excited complex and the quencher. 

Table 8 Bimolecular Quenching Rate Constants for the Oxidadve Quenching Reactions of 25b, 30, and 31 with Pyridinium Acceptors...

The phosphorescent states of the polynuclear gold(I) complexes 50a, b and 51a, b have also been found to react with MV [132,133]. The quenching mechanism has been found to be electron transfer in nature. The excited state reduction potentials of these polynuclear gold(I) phosphine complexes have also been estimated by oxidative quenching experiments with a series of structurally related pyridinium acceptors. For example, the bimolecular quenching rate constants for the photoreactions between 50b and the pyridinium acceptors are listed in Table 14. A plot of iRT/F) n fc, vs. Eu2 values of the pyridinium ions is displayed in Fig. 10. The excited state reduction potentials, RT In Xkv values. 

Bimolecular quenching rate constant Radiative rate constant Stem-Volmer constant Rate constant for vibrational relaxation... 

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