Phototautomerization

The ESIPT mechanism of phototautomerization allows the employment of types 346 and 347 compounds and their analogs as UV stabilizers useful for synthetic polymers. Such a stabilizer -rapidly converts the energy ab-... 

Shukla MK, Leszczynski J (2002) Phototautomerism in uracil a quantum chemical investigation. J Phys Chem A 106 8642... 

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... 

Recent studies on the phototautomerization mechanism of the nitroenamine functionality in 2-(nitromethylene)-tetrahydro-l,3-thiazine were performed using complete active space self-consistent field reaction path computations... 

An investigation into the phototautomerism of 7-hydroxy-4-methylcoumarin has been undertaken (74JA4699). 

Leszczynski et al. have shown interest in comparing experimental (matrix isolation) and calculated (B3LYP) IR data (frequency and intensity) to discuss the tautomerism of benzo-annelated pyridonone, pyrazinone, and pyrimidinone (144-148). These equilibria were well reproduced by theoretical calculations carried out at the QCISD and QCISD(T) levels. The combined experimental and theoretical results reveal links between aromaticity and tautomerism. Moreover, a UV-induced phototautomeric reaction transforming the oxo forms into the hydroxy tautomers was observed for all (except 3-hydroxyisoquinoline) studied compounds [144], The interest of Leszczynski in problems related to tautomerism, aromaticity, and proton transfer is also apparent in a study of (lH-aza-hetero-2-ylidene)-acetaldehyde and 2-azahetero-2-yl-ethanol tautomeric pairs [145],... 

Very similar behaviour was found for salicylamide and salicyl-anilide (Schulman et al., 1973c) but 3- and 4-hydroxybenzoic acids showed no intra- or intermolecular phototautomerism, possibly because of the very short lifetimes of the Sj states involved. Also, the ionization sequences in the ground and lowest excited singlet states appear to be the same for these molecules and their methyl esters (Paul and Schulman, 1974). 

A temperature dependence study of the dimer phototautomerism indicated that at 77°K proton tunnelling is the dominant mechanism for proton transfer in the excited state. Results obtained on a deuterium substituted compound were consistent with this mechanism. 

A similar type of phototautomerism has been observed in lumi-chromes and alloxazines (Song et al., 1974). In the singlet state the N(l) proton is shifted to the N(10) position and the catalytic effect of pyridine and acetic acid is explained in terms of 1 1 complexes like that postulated in the 7-azaindole case. 

A kinetic study of the photohydroxylation of the quinone (371) in potassium hydroxide and perchloric acid solution, has been reported. The phototautomerism of the quinone (372) has been studied and the effect of pH and acidity evaluated. - ... 

In the first part of this article, we present a kinetic study on the isomerization of l-phenylazo-2-hydrozynaphthalene (1PA2N) measured by picosecond spectroscopy. The selection of this compound was based on its known photochemical properties of cis-trans isomerization and tautomerization by proton transfer. The phototautomerization of 1PA2N is evidenced by the fact that in solution this compound is a mixture of the azo tautomer A and the hydrazo tautomer as shown in Figure 2.1. It is thought that the inter-... 

Other Processes - The phototautomerism of A-salicylideneanilines has been studied at 77 A study of the fluorescent behaviour of the... 

The phototautomerism within derivatives of [2,2 -bipyridyl]-3,3 -diol has been described and the influence of substituents close to the reactive hydrogens has been assessed. Intramolecular photochemical proton transfer in the imidazoles (160) and (161) has been studied,Other workers have reported on the photochemical behaviour of the imidazole (162) and the influence exerted by change of solvent. Proton transfer occurs within the excited state of the benzazole derivatives (163), and details of excited state proton transfer within the flavylium salt (164) have been published. ... 

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