Stokes shift salicylic acid

From a study of the absorption and emission spectra of salicylic acid and 2-methoxybenzoic acid, Weller (1956) concluded that the greatly increased Stokes shift of salicylic acid was due to intramolecular proton transfer (45) in the excited state ... 

On excitation the phenolic group becomes more strongly acidic while the carboxylic acid group becomes a stronger base and proton exchange then occurs between the two. In confirmation of this explanation, methyl salicylate was found to behave similarly, but methyl 2-methoxybenzoate, with no transferable proton, showed a normal Stokes shift. Quenching experiments demonstrated that at room temperature the proton transfer reaction reached equilibrium within the lifetime of the excited state. 

The vast majority of papers devoted to tautomerization dynamics deal with ESIPT reactions. Since Weller s suggestion that the large Stokes shift he measured for salicyhc acid fluorescence was caused by rapid proton transfer in the excited state [62], and the development of techniques to study this on a femtosecond timescale, the field has blossomed. Most of the 2000 papers on tautomerization dynamics is on ESIPT, from both an experimental and a theoretical point of view. The number of compounds exhibiting ESIPT is far too large to discuss here. It ranges from molecules as simple as malonaldehyde to systems as complicated as 3-hydroxyflavone or 2-(2 -hydroxyphenyl)benzothiazole. In particular, substituted salicylic acids and ortho-hydroxybenzaldehydes have attracted much attention from both experimentalists and theoreticians. Weller s idea is depicted in Figure 1.10. 

Comparing these observations to the requirements of TST, we can immediately see a number of problems. Even apart from the fact that the mass of the proton requires it to be treated as a quantum mechanical particle, so that even if there were a well-defined barrier, we would still need to take the possibility of tunneling into account. Transfer of the proton is directly coupled, or may even be driven by a redistribution of electron density in the molecule. In excited-state intramolecular proton transfer (ESIPT) reactions, the redistributed charge almost certainly provides the driving force. The generic picture for such is reaction is due to WeUer [7, 8], who was the first to realize that the enormous Stokes shift of about 10 000 cm he observed in the fluorescence of salicylic acid (X = OH) could be a consequence of a rapid proton transfer in the excited state. 

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