Ground-state intramolecular proton transfer reactions

Dynamics of Ground- and Excited-State Intramolecular Proton Transfer Reactions... 

Excited-state intramolecular proton transfer (ESIPT) exhibits different regularities [49, 50]. Commonly, this is a very fast and practically irreversible reaction proceeding along the H-bonds preexisting in the ground state. Therefore, only the reaction product band is seen in fluorescence spectra. Such cases are not interesting for designing the fluorescence reporters. The more attractive dual emission is... 

Below a molecular diagram is given of the ground and the transition states for the intramolecular proton transfer reactions in the formamidine molecule XIII [66] ... 

FIGURE 2.10 Intramolecular proton transfer reactions of 2-(2-hydroxy-4-methoxyphenyl)-4,6-diphenyl-l,3,5-triazine in the Sj state (kj ) and ground state and light-induced opening of the intramolecular hydrogen bond. (From Waiblinger, F., Keck, J., Stein, M., Fluegge, A.P., Kramer, H.E.A., Leppard, D., J. Phys. Chem. A, 2000,104,1100. With permission.)... 

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. 

The intramolecular proton transfer (IPT) reaction is unfavorable in the ground state from both the thermodynamic as well as kinetic reasons for both compounds. However, both factors favor the ESIPT process in the lowest excited singlet and triplet states. 

Light-induced intramolecular proton transfer has long been suspected to play an important role in the mechanism for photochromism in both crystals and solutions. One class of compounds that has been extensively studied is the A -salicylidene anilines. The main evidence for invoking an excited-state proton-transfer step in the photochromic mechanism comes from the observation that the colored species exists as the keto amine tautomer (in the ground state) while the bleached form exists as an enol imine (in the ground state). Replacement of the labile hydrogen with a methyl group prevents the photochemical reaction that leads to the colored species. Picosecond transient spectroscopy was used to study iV-salycylidene-o-toluidine (NST) and the related compound 2-(o-hydroxyphenyl)benzothiazole (HBT). ... 

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...

The conclusion we can draw from all this research is that there is still no coherent picture of intramolecular ground and excited-state proton transfer reactions in tautomers. The topic is complicated from an experimental as well as a theoretical point of view, and many questions remain. Intramolecular ground-state proton transfer is hard to study directly, and although femtosecond pulsed lasers allow initiating and following proton transfers in the excited state on a very short time scale, these methods bring their own complications to the interpretation of the results. ... 

---