Excited state proton transfer reactions

Formosinho, S. J. Amaut, L. G. Excited-state proton transfer reactions. II. Intramolecular reactions. J. Photochem. Photobiol. A Chem. 1993, 75, 21—48. 

Amaut LG, Formosinho SJ (1993) Excited-state proton-transfer reactions. 1. Fundamentals and intermolecular reactions. J Photochem Photobiol A Chem 75 1-20... 

Cheng YM, Pu SC, Yu YC et al (2005) Spectroscopy and femtosecond dynamics of 7-N, N-diethylamino-3-hydroxyflavone. The correlation of dipole moments among various states to rationalize the excited-state proton transfer reaction. J Phys Chem A 109 11696-11706... 

H. Shizuka, Excited state proton-transfer reactions and proton-induced quenching of aromatic compounds, Acc. Chem. Res. 18, 141-147 (1985). 

Takeuchi, S., and Tahara, T. 1998. Femtosecond Ultraviolet-Visible Fluorescence Study of the Excited State Proton Transfer Reaction of 7-Azaindole Dimer. J. Phys. Chem. A 102 7740. 

Fig. 2. Excited-state proton transfer reaction schemes in alkaline (left) and acid (right) solutions.

Figure 10-16. Reactant (Watson-Crick) and product GC structures of the excited state proton transfer reaction. The atom numbering scheme is illustrated for the initial Watson-Crick configuration...

There are also many well-dociunented cases of excited state proton transfer reactions. It has been known for a long time that the acid-base properties of organic molecules such as phenols are drastically modified upon light absorption. About 60 years ago, Forster suggested a simple method for estimating the excited state pif values of photoactivated species from thermodynamic and spectroscopic data (117), which became very popular (Fig. 12). 

Detailed analyses of intramolecular structures are possible. Comparison of NMR and fluorescence data shows meso- and racemic diastereoisomers are found from 2,4-di(2-pyrenyl)pentane 24 jhe polarization of monomer and excimer of 4,9, disubstituted pyrenes have been measured in nematic liquid crystals 25 Quenching of pyrene fluorescence by alcohols in cyclodextrin inclusion complexes has also been studied in detail 26 Solvent effects on the photophysical properties of pyrene-3-carboxylic acid has been used to measure the pJJ, in different solvents 27 Geminate recombination in excited state proton transfer reactions has been studied with... 

The discovery of photoacidity was made by Forster more then 50 years ago . Forster correctly explained the unusual large Stokes shift found in the fluorescence of several classes of aromatic dyes, including 1- and 2-naphthol derivatives as an indication of excited state proton-transfer reaction which results in the formation of the molecular anion still in the excited state. Thus, it become clear that excited-state proton transfer may compete with other radiative and non-radiative decay routes of the photoacid. The main modern-day importance of photoacids lies in their ability to initiate and then to follow acid-base reactions so they may be regarded as optical probes for the study of general proton-transfer reactions. 

Over the years the field of photoacids (and photobases) has been reviewed many times . The most extensive list of photoacids appeared, so far, in a 1976 review by Ireland and Wyatt The hydroxyarenes are the most widely used photoacids. In polar solutions they may undergo an excited-state proton-transfer reaction according to the general reaction scheme of equations 2-5. 

The general validity of the Forster cycle approach is undoubtedly linked first of all with the reality of the assumed microscopic reversibility of exdted-state proton transfer reactions. Secondly, the reliability of the K scale should be checked, when possible, against directly determined K values of well behaved photoacids. Furthermore, for the general validity of the IC scale to hold as defined by the Forster cycle its validity should not depend on the photoacid actually reaching equilibrium conditions or even on observing at all an excited-state proton transfer reaction within the finite lifetime of the excited state. These assertions should be carefully tested and checked before establishing the general applicability of the Forster cycle. 

More direct evidence for the inherent microscopic reversibility of an excited-state proton transfer reaction was found in ps-time-resolved measurements of a strongly reactive photoacid, namely HPTS (Fig. 12.2). With its conjugated-base, fourfold charged, the observation of the back (geminate) recombination of the pro-... 

It is rather similar in both ground-state and excited-state proton transfer reactions of cationic acids. 

Several methods are available for the determination of the excited singlet state pA (pA ). The one that is most popular due to its straightforward and simple application is the Forster cycle.A schematic energy-level diagram for the Forster cycle is shown in Fig. 1. If one assumes that AS is the same for the ground and the excited-state proton-transfer reactions, then it follows that... 

L. Stryer, Excited-state proton transfer reactions A deuterium isotope effect onfluorescence, J. Am. Chem. Soc. 88 (1966), pp. 5708-5712. 

An interesting recent example is the use of Shpol skii matrices to study ultrafast phenomena. The excited state proton transfer reaction in 3-hy-droxyflavone could be calculated from the (relatively) broad bands in the Shpol skii excitation spectrum, using the Heisenberg uncertainty principle. 

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