Proton transfer reactions in the excited state

The aim of this lecture is to provide a qualitative description of reversible proton transfer reactions in the excited-state, using the extended theory of diffusion influenced reactions. The complete equations and numerical procedures may be found in the literature [10-14]. Major results include (i) the asymptotic power-law decay and the evidence for diffusive kinetics [10] (ii) The salt effect [11] and the Naive Approximation for the screening function [17, 11] and (iii) an extension [18] of the theory for approximating the effect of competing geminate and homogeneous proton recombination expected atdow pH values. 

Proton Transfer Reactions in the Excited State. Proton transfer reactions involving aromatic bases and acids can take place in the excited state as well as the ground state because proton transfer in such systems is an extremely fast process. Since there is no reason why the excited-state and ground-state surfaces should touch, these are X-type reactions. In other words, we are dealing with an equilibrium between an excited acid HX and a base B to form an excited salt BH (X ) . The equilibrium constant for this process, and so the pK of the excited acid (pK ), will normally differ from that of the acid in its ground state. 

Scheme 15.9 Two examples of proton transfer in the excited state. The top (with -naphthol, [73]) is illustrative of an intermolecular proton transfer (to the solvent) jwocess whereas the bottom (with indigo, [22]) is illustrative of an intramolecular proton transfer. In the two cases the kinetic scheme (in the middle) applies with a single ground-state species however in the case of indigo, the back-proton transfer reaction in the excited state is unlikely...

Bartl K, Funk A, Schwing K, Fricke H, Kock G, Martin HD, Gerhards M (2009) IR spectroscopy applied subsequent to a proton transfer reaction in the excited state of isolated 3-hydroxyflavone and 2-(2-naphthyl)-3-hydroxychromone. Phys Chem Chem Phys 11 (8) 1173-1179... 

Recent developments in ultrafast spectroscopy have enabled us to investigate directly the ultrafast proton-transfer reactions in the excited state of aromatic compounds. The effect of electronic sfiucture on proton transfer rate is of great interest not only from fundamental aspects in reaction dynamics, but also from the viewpoint of developing new photoacids. Among a number of photoacids investigated so far, 1- and 2-naphthols (1-NL and 2-NL) are representative compounds for investigating... 

X lO s )-Either substituent at thepara-position shows only slightinfluences on the rate. The results of a kinetic study [94] are summarized in Table 2.5 together with the and p values. The less prominent effect of the para substituent is qualitatively explained by the direction of <- A transition moment, which is perpendicular to the direction of the two substituents. A quantitative explanation for the remarkable substituent effects on proton transfer rate can be made in terms of the free energy change (AG ) for the proton transfer reactions in the excited state. In Figure 2.5, the values of log(fcjj ) are plotted as a function of AG [94]. 

The NaCl effect upon the proton-transfer reactions in the excited state of 2-naphthol has been studied by Harris and Selinger [41]. They have reported that the enhancement of the fluorescence of 2-naphthol is due either to a disruption of the water structure by the high concentration of Na and Cl ions or to an increase in the activity coefficient of the excited 2-naphthol. In previous works [53,106,107], it is shown that NaCl is a very weak quencher there is a weak quenching ability for CT, but no ability for Na. The NaCl effect on the proton dissociation reactions in water at 300 K has been studied by means of nanosecond and picosecond spectroscopy with fluorimetry [108]. The proton dissociation rate constant k decreases with an increase of [NaCl] according to the equation... 

The third type of experiment is photolysis, where the product is one of a tautomer pair [2, 7, 75]. Again, almost aU reactions studied are keto-enol tautomerizations where the proton transfer is not direct but in a number of steps via the solvent Since the first step is often an ionization (proton transfer to solvent molecule), which is thought to be diffusion-controlled [67], it does give some insight into proton transfer reactions, but exact elucidation is hard, since often there are numerous possibiHties for reaction mechanisms and roles of solvent molecules and internal vibrations [76, 77]. In view of the lack of understanding of proton transfer reactions, it would be much better to have a simpler and more direct way to initiate intramolecular proton transfer. This possibility is offered by looking at intramolecular proton transfer reactions in the excited state, which can be initiated much faster and followed on a much shorter timescale than ground-state reactions. 

Research of a more fundamental nature - not directly geared toward finding useful applications - has been reported on two other groups of molecules. 7-Azaindole is another biologically relevant molecule since it is closely related to indole, the core of the amino acid tryptophan. Tryptophan is an important reporter molecule in protein spectroscopy, and replacement of the indole group by an azaindole makes it even more suitable for its simpler decay characteristics and red-shifted spectrum [90]. It was also extensively investigated by Kasha and coworkers [91], and has been the subject of much theoretical work[92].The tendency of 7-azaindole to form dimers in particular solvents has also led to the study of double proton transfer reactions in the excited state [93, 94]. Some of these issues are complicated by the possible presence of anion fluorescence [95] (Figure 1.13). 

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