Excited state intramolecular proton transfer ESIPT process

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... 

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 tautomerization at room temperature is caused by photoinduced proton transfer and due to the large distance between O and N atoms it cannot proceed intramoleculary. The process is solvent assisted as seen from the measured emission spectra in different solvents [53] in apolar or nonprotic polar solvents only the emission coming from 2 IE is detected, while in hydrogen-bonded solvents (alcohols or water for instance) a new strongly Stokes-shifted band appears in addition. This new band has been attributed to emission by a phototautomer 21K, formed by an adiabatic excited state intramolecular proton transfer (ESIPT) process in a complex between the excited enol form and solvent molecules [54, 55] (Figure 12.10). 

The excited-state intramolecular proton-transfer (ESIPT) process (Scheme 2) in 4-methyl-2,6-dicarbomethoxyphenol (11) was shown to be perturbed by CD complexation [38]. This molecule absorbs at 322nm as an intramolecularly... 

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. 

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 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... 

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. 

Proton transfer is one of the simplest of all elementary chemical processes, the kinetics of which play an important role in many biological processes. Many examples of tautomerism (the equilibrium between two different isomers) involve proton transfer. Of the many systems studied the photon-stimulated Excited State Intramolecular Proton Transfer (ESIPT) in 3-hydroxyfiavone (3-HF) (C15 Hio O3), which is an important plant compound, has many desirable features, making it an ideal model system. 

Another fluorescent probe 26, based on 1-amidoanthraquinone and with calix[4] arene in the cone conformation, was developed (Fig. 28.6) [54]. Probe 26 exhibits very weak fluorescence because of the excited state intramolecular proton transfer (ESIPT) between the amide NH and the quinone oxygen. The addition of F significantly enhances the fluorescence emission due to inhibition of the ESIPT process. The F ions interact with the amide NH groups on the two arms, thereby inhibiting the ESIPT process and thus increasing the fluorescence emissirMi. 

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. 

ESIPT process (Excited-State Intramolecular Proton Transfer) [96-98]. Upon irradiation, E forms exhibit an intramolecular proton transfer process from the hydroxy to the imine nitrogen atom of the C=N group, which is characterized by intense fluorescence and large Stokes shift. Fluorescence intensity quenching is attributed to the complete or partial substitution of hydroxyl proton, which blocks the excited state processes. 

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. 

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