Ground-state intramolecular proton transfer

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 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... 

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>. 

Fig. 10b), where an ultrafast intramolecular excited-state charge-induced proton transfer occurs in tens of femtoseconds after the vertical excitation of the ground state. The proton transfer alters the energy landscape and IC to the ground-state surface takes place with no cis-trans isomerisation, unlike other chromophores, where the intramolecular proton transfer is not possible. 

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

Frey W, Elsaesser T (1992) Femtosecond intramolecular proton transfer of vibrationally hot molecules in the electronic ground state. Chem Phys Lett 189 565-570... 

The absorption spectra of the hydroxyphenylbenzo-triazole derivatives in various solvents and polymer films indicate that two ground-state forms of the molecules exist. These species are proposed to be a planar and non-planar form of the stabilizers. The position of the equilibrium between these two forms is affected by both the polarity and the hydrogen-bonding strength of the medium. The blue fluorescence (A.max = 400 nm) observed for these stabilizers originates from an excited-state species in which intramolecular proton transfer is disrupted. 

Figure 6.22 Intramolecular proton transfer in the charge-transfer state of 2-hydroxybenzophenone. The zwitterionic form reverts to the ground state in a time of ps...

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. 

In summary, anils show fluorescence from a state (most probably TT-TT ) whfch Is similar to the cls-gulnold tautomer observed In appropriate solvents even in the ground state. This state can only be produced by Intramolecular proton transfer In the excited state. Since the energy difference between the phenolic and qulnold ground state isomers is rather small In the anils (see Section IV-B), the Increase In (-0H) acidity and/or (=N-)baslclty needs not be as large as in the case of salicylic esters. 

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