Formation of a ground-state non-fluorescent complex

Let us consider the formation of a non-fluorescent 1 1 complex according to the equilibrium 

The excited-state lifetime of the uncomplexed fluorophore M is unaffected, in contrast to dynamic quenching. The fluorescence intensity of the solution decreases upon addition of Q, but the fluorescence decay after pulse excitation is unaffected. Quinones, hydroquinones, purines and pyrimidines are well-known examples of molecules responsible for static quenching. 

Considering that the fluorescence intensities are proportional to the concentrations (which is valid only in dilute solutions), this relationship can be rewritten as 

A linear relationship is thus obtained, as in the case of the Stern-Volmer plot (Eq. 4.10), but there is no change in excited-state lifetime for static quenching, whereas in the case of dynamic quenching the ratio I0/I is proportional to the ratio to/t of the lifetimes. 

In some cases, evidence for the formation of a complex can be obtained (e.g. changes in the absorption spectrum upon complexation), but in the absence of such evidence, the interaction is likely to be non-specific and the model of an effective sphere of quenching is more appropriate. A nonlinear variation of Io/I is predicted in the latter case, but at low quencher concentration, exp(VqNa[Q]) 1 + VqNa[Q]. 

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The term static quenching implies either the existence of a sphere of effective quenching or the formation of a ground-state non-fluorescent complex (Figure 4.1) (Case A of Section 4.2.1). 

Following an external perturbation, the fluorescence quantum yield can remain proportional to the lifetime of the excited state (e.g. in the case of dynamic quenching (see Chapter 4), variation in temperature, etc.). However, such a proportionality may not be valid if de-excitation pathways - different from those described above - result from interactions with other molecules. A typical case where the fluorescence quantum yield is affected without any change in excited-state lifetime is the formation of a ground-state complex that is non-fluorescent (static quenching see Chapter 4). 

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