Intramolecular proton transfer, laser studies

Excited-state intramolecular proton transfer (ESIPT) processes are important for both practical and fundamental reasons. o-Hydroxybenzaldehyde (OHBA) is the simplest aromatic molecule displaying ESIPT and serves as a model system for comparison with theory. TRPES was used to study ESIPT in OHBA, monodeuterated ODBA and an analogous two-ring system hydroxyacetonaph-tone (HAN) as a function of pump laser wavelength, tuning over the entire enol... 

The dependence of intramolecular proton transfer on solvent friction has been established for 2-(2 -hydroxy-5-methylphenyl) benzotriazole in alcohol and other solvents. Excited-state proton transfer in 2-(2 -hydroxyphenyl) benzothiazole has also been studied 2 Photophysical properties and laser performance of w, w -bis ( oxazol-2-yl)- -oligophenylenes in dioxane have been measured at room temperature. +p increases with the number of phenyl rings between terminally positioned oxazoyl groups. 

The methods that generate quinone methides were reviewed, along with a detailed analysis of the mechanisms of the reactions of these electrophiles with nucleophiles. " Quinone methide (10), the para isomer and the zwitterionic meta isomer, were obtained by photolysis of 2-phenylphenol derivatives substituted with a hydroxyadamantane. The mechanisms of decomposition of these intermediates were studied by a combination of product analysis and laser flash photolysis. Irradiation of 1-hydroxypyrene results in intramolecular proton transfer from OH to carbon atoms at the 3, 6, and 8 positions resulting in quinone methide intermediates (e.g. the zwitterion (11)). These revert to starting material by proton loss, a process that is monitored by deuterium labelling. 

Ultrafast studies on tautomerism concentrate on compounds that can exhibit hydrogen transfer in the electronically excited state. Hydrogen transfer is a very typical reaction for the interconversion between different tautomeric forms. It converts enol to keto, amino to imino, imino to enamino, and lactim to lactam forms, to name some examples. For time-resolved experiments, excited-state intramolecular proton transfer (ESIPT) is particularly well suited since a short laser pulse in the visible or ultraviolet (UV) spectral region can trigger this process by promoting the molecule into the electronically excited state and initiating the transfer in this way [3]. The vast majority of experiments on tautomerism with ultrafast time resolution are therefore done on compounds exhibiting ESIPT. 

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. ... 

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