Proton Recombination and Acid-Base Neutralization

Acids are in equilibrium with their conjugate bases in protic solvents, where the relative concentrations depend on the pK value. The observed dynamics of an electronically excited photoacid, typically interpreted as the proton transfer rate to the (protic) solvent [77, 78], is thus governed by the equilibration dynamics to the new configuration - as long as the photoacid and conjugate photobase remain in the electronically excited state - as dictated by the new excited state pJ a value. Depending on the pFI of the solvent one can observe the reversible time-depen-dent geminate recombination of the photobase with the released proton [79-83], or even the reaction of the photobase with other protons present in solution. 

Proton transfer dynamics of photoacids to the solvent have thus, being reversible in nature, been modelled using the Debye-von Smoluchowski equation for diffusion-assisted reaction dynamics in a large body of experimental work on HPTS [84—87] and naphthols [88-92], with additional studies on the temperature dependence [93-98], and the pressure dependence [99-101], as well as the effects of special media such as reverse micelles [102] or chiral environments [103]. Moreover, results modelled with the Debye-von Smoluchowski approach have also been reported for proton acceptors triggered by optical excitation (photobases) [104, 105], and for molecular compounds with both photoacid and photobase functionalities, such as lO-hydroxycamptothecin [106] and coumarin 4 [107]. It can be expected that proton diffusion also plays a role in hydroxyquinoline compounds [108-112]. Finally, proton diffusion has been suggested in the long time dynamics of green fluorescent protein [113], where the chromophore functions as a photoacid [23,114], with an initial proton release on a 3-20 ps time scale [115,116]. 

The diffusive kinetics of geminate pairs have been predicted to show a time-dependent decaying behavior [117-122]. Early experiments showed, in contrast, a decay, with a being dependent on the proton concentration [123]. Experiments on longer time ranges with improved sensitivity are prerequisites for an accurate determination of the asymptotic behavior [124]. In fact, recent measurements on HPTS have demonstrated the validity of the theoretically predicted decay law (see Fig. 14.4) [125]. For 5-(methanesulfonyl)-l-naphthol a kinetic transition from power law to exponential has been reported due to a short photobase lifetime [126]. 

Neutralization of the photoacids as a result of direct proton transfer to (scavenging) bases has also been explored in time-resolved studies. Whereas initial work 

---