Intramolecular fluorescence quenching processes

Fluorescent Probes with an Efficient Intramolecular Fluorescence Quenching Process in the Base Form Possibly Related to the Formation of a Nonemissive TICT State... 

Recent theory suggests that cyclization phenomena (1) are much more sensitive to excluded volume effects than other properties of polymer chains. Intramolecular fluorescence quenching processes in molecules containing appropriate end groups permit one to study both the dynamics and thermodynamics of end-to-end cyclization. As a consequence, the sensitivity of polymer cyclization to excluded volume can be examined. 

Appropriate combinations of boronic acid and fluorophores lead to a remarkable class of fluorescent sensors of saccharides (Shinkai et ah, 1997, 2000, 2001). The concept of PET (photoinduced electron transfer) sensors (see Section 10.2.2.5 and Figure 10.7) has been introduced successfully as follows a boronic acid moiety is combined intramolecularly with an aminomethylfluorophore consequently, PET from the amine to the fluorophore causes fluorescence quenching of the latter. In the presence of a bound saccharide, the interaction between boronic acid and amine is intensified, which inhibits the PET process (Figure 10.42). S-l is an outstanding example of a selective sensor for glucose based on this concept (see Box 10.4). 

Phosphorescence most commonly follows population of Ti via ISC from Si, itself excited by absorption of light. The Ti state is usually of lower energy than Si, and the long-lived (phosphorescent) emission is almost always of longer wavelength than the short-lived (fluorescent) emission. The relative importance of fluorescence and phosphorescence depends on the rates of radiation and ISC from Si the absolute efficiency depends also on intermolecu-lar and intramolecular energy-loss processes, and phosphorescent emission competes not only with collisional quenching of Ti but also with ISC to So-... 

The collisional fluorescence quenching and phosphorescence induction processes have been later observed for a large number of small and medium-size molecules. The interpretation of these results was however rather confusing collision-induced intersystem crossing considered as a transition from the pure singlet to the pure triplet state is in apparent contradiction with the Wigner rule of spin conservation, at least in the case of light collision partners that cannot affect the intramolecular spin-orbit interaction. 

Figure 6 depicts the process of cation enhancement of fluorescence. Bis(crown ether) 4 at 2 x 10 molar concentration in the dichloromethane phase experiences intramolecular quenching. Using model compound 3 we determined that 75 percent of the fluorescence is quenched, presumably by intramolecular EDA quenching. When the metal ion in the water phase can be extracted by bis(crown ether) 4 into the dichloromethane phase, the complexation decreases the intramolecular quenching dramatically. 

When a metal complex like osmium(II) fris(bipyridyl) or a metallocene, as in compound 3, is appended to the ligand, the MLCT excited state is quenched by energy transfer to the adjacent metal center. The rate of this quenching decreases as an anionic substrate binds between the two metal eomplexes. A similar switching mechanism is seen in the calix[4]diquinone-appended complex, 4. In this case, the fluorescence emission is quenched via an intramolecular electron transfer to the quinone. Anion binding between the Ru complex and the quinone blocks the quenching process, and the emission intensity is significantly recovered. 

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