Quenching diffusion-controlled

The dynamics of both static and dynamic quenching of the fluorescent singlet states of diazapyrenium salts by nucleotides has been investigated by Brun and Harriman using sub-nanosecond time-resolved transient absorption spectroscopy [88]. Observation of the reduced acceptor DAP+ (Table 5) supports an electron transfer mechanism for fluorescence quenching. Diffusion-controlled rate constants were observed for quenching of DAP + by all four deoxynucleotides. Excitation of... 

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. 

Note. Both the rearrangement In t-ButanoI) and the double bond isomerization of (114) (In Benzene) are quenched in a diffusion-controlled process by suitable triplet acceptors e.g., naphthalene or 2,5-dimethylhexa-2,4-diene). The rearrangement (114) (118) -I- (120) is also observed on irradiation in... 

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. 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

Since these values are of the same order of magnitude, Hammond assumed that the quenching was diffusion controlled with kq = 2 x 109 liters/mole-sec in (3.21). With this value for kv, the rates of the other processes can be determined and are shown in Table 3.1. As we shall see, these values agree well with those obtained by Linschitz using the technique of flash photolysis. 

Backstrom and Sandros<54-55) found that the phosphorescence of biacetyl in benzene solution at room temperature was quenched at a diffusion-controlled rate by aromatic hydrocarbons when the triplet energy of the hydrocarbon was sufficiently below that of biacetyl. 

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 )... 

If quenching is a diffusion-controlled process (k 3xl09L mol 1 s ), the lifetime t 3x 10-7 s coincides with the lifetime of triplet acetophenone (product of peroxyl radical disproportionation in oxidized ethylbenzene). 

Studies on the quenching of photoexcited 9-phenan-thrylmethyl pivalate by MEK, a model forathe-PMMA-Phe/MEK system, provide a value of k 7.3 X 108 M s in cyclohexane (11). This value is nearly one order of magnitude lower than the diffusion-controlled rate. For reactions in which a diffusion step precedes a chemical step, the relationship between k and kd ff is given by ... 

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). 

By comparing time-resolved and steady-state fluorescence parameters, Ross et alm> have shown that in oxytocin, a lactation and uterine contraction hormone in mammals, the internal disulfide bridge quenches the fluorescence of the single tyrosine by a static mechanism. The quenching complex was attributed to an interaction between one C — tyrosine rotamer and the disulfide bond. Swadesh et al.(()<>> have studied the dithiothreitol quenching of the six tyrosine residues in ribonuclease A. They carefully examined the steady-state criteria that are useful for distinguishing pure static from pure dynamic quenching by consideration of the Smoluchowski equation(70) for the diffusion-controlled bimolecular rate constant k0,... 

From Ah = 0.8x10 1 mole i s i and straightforward kinetics the rate constant for radiationless decay of triplet nitrobenzene could be derived Aat = 0.6 X 10 s i. Thus it is easily unterstood why oxygen quenching (assumed to be diffusion controlled does not affect photoreduction in 2-propanol to a large extent upon irradiation in air-saturated ([O2] 10 M) solutions, the quantum... 

Diffusion-controlled quenching of the transients is observed with oxygen, azulene and ferrocene,... 

Quantitative data on rates of reaction have been obtained for some of the triplet reactions. Assuming triplet quenching to be approximately diffusion controlled, the rate constants for the reactions between excited species and nucleophile are 10 -10 1 mole s . The data show that in comparing and interpreting quantum yields—even in the case of related systems—one should proceed to determine separately rate constants as well as intersystem crossing efficiencies and lifetimes of the reacting excited species. 

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