Excited state intramolecular proton transfer ESIPT

Foster, K. L. Baker, S. Brousmiche, D. W. Wan, P. o-Quinone methide formation from excited state intramolecular proton transfer (ESIPT) in an o-hydroxystyrene. J. Photochem. Photobiol. A Chem. 1999, 129, 157-163. 

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. 

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... 

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. 

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. 

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. 

Dihydrobenzoxanthenes 27 result from the photolysis of 2-(l-naphthyl)phenols a quinone methide intermediate is postulated. The twisted nature of the o-hydroxybiaryl system facilitates excited-state intramolecular proton transfer (ESIPT) at both the 2 - and 7 -positions <03JA1164>,... 

Semi-empirical AMI-SCI calculations have been performed to rationalize the photophysical behavior of two series of compounds one comprising of 2-(2 -hydroxyphenyl)benzoxazole, 2-(2 -hydroxyphenyl)benzimidazole (HBI), and 2-(2 -hydroxyphenyl)benzothiazole, and the other of 2-(2 -hydroxyphenyl)oxazole (HPO), 2-(2 -hydroxyphenyl)imi-dazole (HPI), and 2-(2 -hydroxyphenyl)thiazole (HPT). These compounds exhibit intramolecular rotation as well as excited state intramolecular proton transfer (ESIPT). The results suggested that for the first series of compounds two rotational isomers are present in the ground state of HBO and HBI while HBT has a single conformer under similar circumstances. For the other series, existence of rotamers depends very much on the polarity of the environment <2003IMS335, 2002JST(604)87>. 

For some molecules, in the excited state, a hydrogen atom is transferred to a group within the same molecule. This is known as excited state intramolecular proton transfer (ESIPT) [33a,49-68]. The product of ESIPT is a phototautomer,... 

Two papers deserve to be reported here. The absence of excited-state intramolecular proton-transfer (ESIPT) in 3-hydroxy-2-methyl-phenyl-4-pyridinone (129) was explained by the twist (48°) of the dihedral angle (AMI calculations) (94CPL(220)229). The tiimethyl derivative of N-(l-naphthyl)-pyridinium 130 shows temperature dependent multiple fluorescence. B3LYP/6-31G(d) calculations indicate that several minima on the SI-hypersurface are responsible for this behavior (01MI127). 

The development of picosecond laser techniques has led to a renewed interest in the spectroscopy and kinetics of aromatic molecules which may undergo excited state intramolecular proton transfer (ESIPT, fig. 1). The ESIPT reaction is evidenced by a large Stokes shift for fluorescence from the proton-transferred molecule. ... 

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