Quencher, definition

The relative fluorescence quantum yield defined in Eq. (3.8) is the ratio of the stationary singlet excitation concentration in the presence of quenchers to the same concentration in their absence. By substituting into this definition N from Eq. (3.654a), we confirm that the fluorescence quantum yield obeys the Stem-Volmer law (3.363) with the same constant as in Eq. (3.364), but with the contact [Pg.339]

The nonstereospecific (using Zimmerman s definition) [108] nature of this reaction has been demonstrate by the use of stereoisomeric substrates which lead to oxetanes 114 in identical diastereomeric ratios. Another possibility for obtaining stereoisomerically pure oxetanes is the use of alkenes which simultaneously serve as quenchers for the carbonyl triplets, e.g., dienes. The photocycloaddition of acetone and 2-methyl-2,4-hexadiene (115) represents such a process leading to two regioisomeric oxetanes 116 and 117 where the substrate configuration is retained in 117 [75]. 

Primary process" has been used in accordance with this recently suggested definition (2) "Any continuous sequence of one or more primary steps which starts with the light absorption step." In this sense a primary step is "any one of the elementary transformations of an excited state molecule of the species which absorbs light. The absorption step Itself is also a primary step" (2). Important primary processes of OTM compounds which are described here include (1) absorption, (ii) dissociative reactions, (iii) intramolecular "twisting" isomerizations, (iv) intermolecular energy transfer, (v) inter-molecular electron transfer, (vi) luminescence. Reactions involving OTM compounds as quenchers have also been included. 

This mechanism can be compared with a reaction procedure, in which the quencher is not present. A definition of two different quantum yields, one in the presence of quenching molecules B ( ) and one in the absence of the quenching molecules (v>o)> gives the ratio... 

Which compound acts as a quencher of Oo A common definition Is lacking and there Is some confusion In the literature on this point. In a broad sense, a 02 quencher Is a compound that accelerates the deactlvatJ on of I02 In a given system without reacting Itself with I02. Required also Is that the rate of 02 deactivation with quencher Q (eq. 14) Is higher than the solvent-dependent decay of I02 (eq. 15) In a given solvent ... 

Therefore, a short, solvent-dependent (k ,see Table 2) definition of quencher Q Is as follows In a given system, a quencher of IO2 is a compound fulfilling the requirement kq > kd/(Q) and acting mainly as a physical quencher (kq >> k ). 

It is clear from the examples we have given and from the definitions of the two types of quenching, that static quenching occurs when a complex is formed between two molecules. Collisional quenching does not need the formation of a stable complex with a long lifetime. The fluorophore and the quencher enter into collision inducing the decrease of the fluorescence lifetime and intensity of the fluorophore. 

Thus, taking into consideration the above definitions, it will be possible to distinguish between the two types of quenching without measuring the fluorescence lifetime at different quencher concentrations. 

The photolysis of Ru(NHj)g and Ru(en)3 was studied initially by Matsubara and Ford at pH 3 in 0.2 M NaCl. They found that aquation was the dominant process and the quantum yield of 0.25 for Ru(NHj) was constant between 313 and 405 nm. However, the quantum yield for Ru(en)3 decreased from 0.18 to 0.06 between 313 and 366 nm as yet there is no definitive explanation for this effect. More recently, Neumann and co-workers have used sensitizers and quenchers to study the Ru(NH3)g system. They concluded that the photoactive state is 17.3 0.4xl0 cm" above the ground state and that it is the state. Although the spin-forbidden transition to this state has not been observed, it is predicted by theory to occur at 17.7x10 cm", i.e. 565 nm. 

Consider a Stern-Volmer-type analysis of a system such as in Figure 16.8, but with one additional process, the conversion of A to photochemical product B with rate constant Show that a plot of relative quantum yield for product formation vs. [Q] is linear and can give a value for the lifetime of A if we assume a value for C,- The definition of relative quantum yield is the quantum yield in the absence of quencher divided by the quantum yield in the presence of quencher. 

Note that Equations [16] and [21] are identical in form, and from both equations Stern- Volmer plots can be made in order to obtain the constants Ks, and Kd, respectively. This can be carried out simply by measurements of the luminescence intensity in the presence and absence of the quencher. However, such a study does not distinguish between the two mechanisms. The easiest and most definitive way to determine the quenching mechanism is to carry out a study of the lifetimes in the presence and absence of the quencher. The excited state lifetime decreases in the presence of quencher if dynamic quenching is the mechanism involved. Note that according to Equation [17] a Stern-Volmer plot can be obtained by plotting (Xq/x) on the y-axis instead of Iq/I ... 

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