ESIPT Intramolecular proton-transfer

Lukeman, M. Veale, D. Wan, R Munasinghe, V. R. N. Corrie, J. E. T. Photogeneration of 1,5-naphthoquinone methides via excited-state (formal) intramolecular proton transfer (ESIPT) and photodehydration of 1-naphthol derivatives in aqueous solution. Can. J. Chem. 2004, 82, 240-253. 

Le Gourrierec, D. Ormson, S. M. Brown, R. G. Excited-state intramolecular proton transfer. Part 2 ESIPT to oxygen. Prog. React. Kinet. 1994, 19, 211-275. 

Brousmiche, D. W. Wan, P. Excited state (formal) intramolecular proton transfer (ESIPT) in p-hydroxyphenyl ketones mediated hy water. J. Photochem. Photobiol. A Chem. 2000, 130, 113-118. 

Lukeman, M. Wan, P. Excited state intramolecular proton transfer (ESIPT) in 2-phenylphenol an example of proton transfer to a carbon of an aromatic ring. J. Chem. Soc., Chem. Commun. 2001, 1004-1005. 

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... 

The fundamental approach to a proton transfer process, which is crucial to mimic many chemical and biological reactions, has relied deeply on studies of excited-state intramolecular proton transfer (ESIPT) reactions in the condensed phase. 

Lim SJ, Seo J, Park SY (2006) Photochromic switching of excited-state intramolecular proton-transfer (ESIPT) fluorescence a unique route to high-contrast memory switching and nondestructive readout. J Am Chem Soc 128 14542-14547... 

Ameer-Beg S, Ormson SM, Brown RG et al (2001) Ultrafast measurements of excited state intramolecular proton transfer (ESIPT) in room temperature solutions of 3-hydroxyflavone and derivatives. J Phys Chem A 105 3709-3718... 

Fig. 4 Excited state intramolecular proton-transfer (ESIPT) mechanism of 3-hydroxychromone...

To conclude our description of techniques, the use of nanosecond and picosecond spectroscopy which has been applied to excited state intramolecular proton transfer (ESIPT) will be mentioned briefly (Beens et al., 1965 Huppert et al., 1981 Hilinski and Rentzepis, 1983). A large number of inter-and intramolecular proton transfers have been studied using these methods (Ireland and Wyatt, 1976) but in the case of processes which are thought to involve simple proton transfer along an intramolecular hydrogen bond it is usually only possible to estimate a lower limit for the rate coefficient. 

If the molecule carries its own base, excited-state proton transfer can occur intramolecularly (ESIPT = excited-state intramolecular proton transfer(37)) and becomes more or less independent of the surrounding solvent. The ESIPT reaction is extremely fast (subpicosecond kinetics(38)) and occurs also in rigid glasses and at very low temperatures.06 39) Very often, only the ESIPTproduct P fluoresces, and this is the source of extremely large Stokes shifts which are fairly independent of medium... 

Figure 5.4. Some ESIPT structures. In the lop port of the figure, the intramolecular proton transfer is sketched schematically, exemplifying the small spatial need necessary for this reaction.

The optical properties of the 8-o-PhOH-purine adducts have provided insight into their ground-state structures at the nucleoside level. These adducts have the ability to phototautomerize, through an excited-state intramolecular proton transfer (ESIPT) process, to generate the keto form. This tautomerization depends on the presence of a intramolecular hydrogen (H)-bond between the phenolic OH and the imine nitrogen (N-7). Figure 14 shows normalized absorption and emission spectra for 8-o-PhOH-dG and 8-o-PhOH-dA in aqueous buffered water and hexane. In water, 8-o-PhOH-dG shows only enol emission at 395 nm, while 8-o-PhOH-dA shows enol emission at 374 nm and phenolate emission at 447 nm. In hexane, both adducts show keto emission at 475 nm 8-o-PhOH-dA also shows a small amount of enol emission and no phenolate emission. These results show that in water, the intramolecular H-bond... 

ESIPT = Excited-state intramolecular proton transfer... 

The 1977 review of Martynov et al. [12] discusses existing mechanisms of ESPT, excited-state intramolecular proton transfer (ESIPT) and excited-state double-proton transfer (ESDPT). Various models that have been proposed to account for the kinetics of proton-transfer reactions in general. They include that of association-proton-transfer-dissociation model of Eigen [13], Marcus adaptation of electron-transfer theory [14], and the intersecting state model by Varandas and Formosinho [15,16], Gutman and Nachliel s [17] review in 1990 offers a framework of general conclusions about the mechanism and dynamics of proton-transfer processes. 

The ESIPT of 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) (27) has been explored by Douhal and co-workers [166] for its probe characteristics in a variety of organized media which include cyclodextrin, calixarene, micelle, and HSA. The incorporation of HPMO into hydrophobic cavities in an aqueous medium involves the rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon excitation (280-330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer (28), the fluorescence of which (Xm = 450 170 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2—C bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cav-... 

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

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 application of UV absorbers, i.e. compounds absorbing the harmful solar radiation, represents an effective solution of the problem (Rabek, 1990). The absorbed radiation is deactivated by intramolecular radiative and radiationless processes. The ideal UV absorber is expected to absorb all terrestrial UV-A and UV-B radiation but no radiation having wavelengths higher than 400 nm. Different classes of commercialized UV absorbers fulfil requirements on effective plastics protection. A group of UV absorbers acting by excited state intramolecular proton transfer (ESIPT) mechanism (Pospfsil and Nespurek, 1997) includes phenolic derivatives of benzophenone (37), various benzotriazoles, such as 38 or 39, and 1,3,5-triazine 40. Non-phenolic UV absorbers are represented by oxamide 41 and a-cyanoacrylate 42. 

ESIPT exited state intramolecular proton transfer 3.2.2... 

Different photochemical channels leading either to TICT or to other products can be combined in one and the same molecule. An example involving competition between Excited State Intramolecular Proton Transfer (ESIPT) and TICT formation (Scheme 2) is the molecule Kal [111]. In this case, three fluorescence bands can be expected in principle (the precursor state E and the two product species ESIPT and TICT). The product channels can be selectively blocked in the model compounds Ka2 and Ka3. 

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