Quenching sphere

The results of this study show a definite quenching of the 418 nm phosphorescence emission of DMT. One would expect that the quenching effect, in a rigid glass, would fit the Perrin model (73). A plot in In 4>0/4> versus concentration of 4,4 -BPDC yielded a straight line, the slope of which was identified with NV. The radius, R, of the active volume of quenching sphere was calculated by the following equation ... 

Based on luminescence studies, we postulated triplet-triplet energy transfer by electron exchange as the mechanism of photostabilization and we calculated an active quenching sphere with a radius, R0, of 19.7 A for 2,6-ND. Because the value of R0 is larger than 15 A, we postulated that energy migration was occurring. 

When M and Q cannot change their positions in space relative to one another during the excited-state lifetime of M (i.e. in viscous media or rigid matrices), Perrin proposed a model in which quenching of a fluorophore is assumed to be complete if a quencher molecule Q is located inside a sphere (called the sphere of effective quenching, active sphere or quenching sphere) of volume Vq surrounding the fluorophore M. If a quencher is outside the active sphere, it has no effect at all on M. Therefore, the fluorescence intensity of the solution is decreased by addition of Q, but the fluorescence decay after pulse excitation is unaffected. 

A plot of ln(Jo//j versus [Q] yields Vq. The values of VqNa are often found to be in the range of 1-3 L mol-1. This corresponds to a quenching sphere radius of about 10 A, which is somewhat larger than the van der Waals contact distance between M and Q. 

In this mechanism the probe (sensitizer) containing Q in the quenching sphere around the probe is quenched entirely, but the probe containing no Q is not quenched at all (see Fig. 19.5 (b)). When the process involves both dynamic and static mechanisms, the loll and to/t plots are depicted as shown in Fig. 19.5 (c). 

N,jV-diethylaniline MTHF, methyl-tetrahydrofuran a The R values in Angstrom units refer to the radius of the Perrin quenching sphere of action ... 

The 0 value depends on the quenching sphere radii R and can reach an extremely high value (>10 ) with appropriate molecules and conditions. 

The incorporation equilibrium of Q into the quenching sphere around a probe is it a one-step equilibrium or a multi-step one ... 

For a multi-step equilibrium between Q and probe, is the static quenching rate is proportional to the number of Q in the quenching sphere, or independent of the number of Q ... 

On the other hand, quenching of the Sj state of ZnTTPS proceeds via electron transfer between the fluorophore and 1 within the quenching sphere of action, consistent with the AG° of the charge-separation process. An important outcome of this work is the demonstration that either the Sj state only or both the S, and S2 states can be quenched by judiciously selecting the quenchers used. This has pertinent implications for the design of efQcient molecular logic devices. 

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