Proton Transfer The ESIPT Mechanism

Acid/base equilibria can be very different in ground and excited states. A well-known example is (I-naphthol, which becomes highly acidic in the excited state and transfers a proton to the surrounding water solvent acting as base within the excited 

If the molecule carries its own base, excited-state proton transfer can occur intramolecularly (ESIPT = excited-state intramolecular proton transfer(37)) and becomes more or less independent of the surrounding solvent. The ESIPT reaction is extremely fast (subpicosecond kinetics(38)) and occurs also in rigid glasses and at very low temperatures.06 39) Very often, only the ESIPTproduct P fluoresces, and this is the source of extremely large Stokes shifts which are fairly independent of medium 

As a rule, however, most of the ESIPT dyes exhibit weak to very weak fluorescence quantum yields, i.e., the quantum yield of the nonradiative processes is near unity. Such dyes, if they show little permanent photodestruction, can be used as ultraviolet (UV)-stabilizers of polymers, such as Tinuvin (a hydroxy-benzotria-zole),(45) because they efficiently convert UV radiation, harmful for the polymer, into harmless heat. The mechanisms of these nonradiative decay paths are often linked to 

Intramolecular Folding The Excimer/Exciplex Mechanism and Dewar Isomerization (Butterfly Mechanism) 

Butterfly-type folding has also been discussed in the context of rhodamine dyes adsorbed on surfaces/53 In this case, too, nonexponential fluorescence decays are observed. A comparison of the consequences of the different reaction mechanisms with respect to free volume sensing is given in Section 5.4. 

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