Stern—Volmer relation

Macromolecules display continuous motions. These motions can be of two main natures the molecule can rotate on itself following precise axis of rotation and it can have a local flexibility. The latter can differ from an area of the macromolecule to another. This local flexibility, called also internal motions, allows to different small molecules such as the solvent molecules to diffuse along the macromolecule. This diffusion is in general dependent on the importance of the local internal dynamics. Also, the fact that the solvent molecules can reach the interior hydrophobic core of macromolecules such as proteins means clearly that the term hydrophobicity should be considered as relative and not as absolute. Although the core of a protein is composed 

We have defined the fluorescence lifetime as the time spent by the fluorophore at the excited state. Collisional quenching is a process that is going to depopulate the excited state in parallel to the other processes we have already described in the Jablonski diagram. Therefore, the fluorescence lifetime of the excited state will be lower in presence of collisional quencher than in its absence. 

In the presence of a quencher, the quantum yield will be equal to  

Since the fluorescence intensity is proportional to the quantum yield, the Stern-Volmer equation can be written as  

The Stem-Volmer plot can be obtained also from quenching of fluorescence lifetime. In fact, fluorescence lifetime in the absence of quencher is equal to  

---

Equation (4.18) can be compared to the Stern-Volmer relation (4.10). The multiplying factor Y-1 accounts for the transient term and leads to a slight upward curvature of the Stern-Volmer plot. 

Dynamic quenching of fluorescence is described in Section 4.2.2. This translational diffusion process is viscosity-dependent and is thus expected to provide information on the fluidity of a microenvironment, but it must occur in a time-scale comparable to the excited-state lifetime of the fluorophore (experimental time window). When transient effects are negligible, the rate constant kq for quenching can be easily determined by measuring the fluorescence intensity or lifetime as a function of the quencher concentration the results can be analyzed using the Stern-Volmer relation ... 

A time dependence of IF IF(t)) can be converted to that of [FeCp-X]0 ([FeCp( )]0) on the basis of the Stern-Volmer relation ... 

Equation 24 can be inverted to give the Stern-Volmer relation... 

Finally, it should be mentioned that acetone itself quenches both its excited singlet and triplet states. A Stern-Volmer relation is obeyed by the self-quenching of phosphorence and probably of fluorescence as well . ... 

At 3650 A, the efficiency of luminescence increases with increasing biacetyl pressure . This is just the opposite of what would be expected on the basis of the Stern-Volmer relation. Moreover, it was established that the primary decomposition quantum yield decreases with increasing pressure (the plot of Ijcf) versus biacetyl concentration gives a straight line). The results were explained by the assumption that the molecules absorbing radiation of 3650 A are excited to high vibrational levels (of the upper singlet state) from which dissociation can occur, but luminescence cannot. Luminescence can only occur if vibrational excitation is removed by collision. 

Because the direct measurement of lifetimes is easily performed nowadays, it is also useful to transform the Stern-Volmer relation into the form... 

The generalized Stern-Volmer relation for the type photopolymerization described in Scheme IV is given by eq. 2. 

In the dynamic quenching mechanism (Scheme 23A), the Stern-Volmer relation is represented by Eq. (19), where t and tq mean the lifetime in the presence and the absence of quencher, respectively, and [Co ] represents the concentration of the cobalt(III) complex. 

The light-induced Fmax level of room-temperature chlorophyll fluorescence was recorded with a Perkin-Elmer LS-5 spectrometer using 620 nm excitation and analyzed using conventional and modified Stern-Volmer techniques as previously employed [7-10]. The Stern-Volmer relation, Jq/I = 1 + Ksv [Q]> describes the ratio of the chlorophyll fluorescence intensity in the absence of quencher (Iq) to the level in the presence of added quinone (I) as a function of the quinone concentration ([Q]). The y-intercept of 1 is indicative of complete accessibility of the added quinone to the site of action, while the Stern-Volmer quenching constant (Ksv) measures the effectiveness of the quinone as a fluorescence quencher once it has been transported to its site of action. A modified Stern-Volmer relation, I MI = 1/fa l/ fa-K sv [Q]), describes the fluorescence quenching (AI) when the added inhibitor has limited accessibility to the binding site. The fa parameter is the fraction of chlorophyll fluorescence that may be quenched by the added inhibitor. 

Here O is the quantum yield for a solution of concentration [M], Oq is the quantum extrapolated to infinite dilution, and [M]i is the concentration at which the quantum yield has fallen to half its limiting value Oq. (Note that the quantum yield refers to the monomer fluorescence, not to that of the excimer.) The Stern-Volmer relation holds only when the fluorescence of an excited molecule M is quenched by reaction of M with another molecule. If in this case the dimer were formed in the ground state and were undergoing fluorescence of the light it had absorbed as a dimer, the quantum yield of the monomer fluorescence would not follow equation (6.23). 

Fluorescence-intensity measurements allow us to pursue this clue. The intensity (/) of the shorter-wavelength band, due to excited monomeric pyrene molecules (A ), varies linearly with the concentration [A] in a way indicating quenching by pyrene itself, according to the Stern-Volmer relation, expressed as in Equation (6.22) and Figure 6.4(b) with [Q] = [A] ... 

The limited data from photolyses at 334 nm (open circles in figure IX-C-8) show a somewhat stronger dependence on pressure of added air than the results at 313nm (triangles in figure VII-C-8), but they are too limited to derive an accurate relationship between [M] and f. However, if one assumes that a Stern-Volmer relation is followed... 

---