Phototautomeric forms

Figure 3.14 Double proton transfer in pyrroloquinolines in alcohol solutions, (a) Structure of 1 H-pyrrolo[3,2-h]quinoline (7) and a scheme of the photoinduced tau-tomerization leading from a normal form to a phototautomeric form [52, 61, 62]. (b) Structure of 7,8,9,10-tetrahydropyrido[2,3-o] carbazole (30) [52] and pyrido[2,3-o] car-bazole (31) [52]. (c) Fluorescence of 7, 30, and 31 (solid lines) and of the corresponding tautomeric model structures (dotted lines), shown as 7m, 30m, and 31 m. The...

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... 

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],... 

The phototautomerism of the uracil (113) has been studied in a quantum mechanical investigation. The study suggests that enols will be formed only in the excited state. Irradiation of (114) in frozen benzene with added tri-fluoroacetic acid affords the cyclized product (115) as well as other related compounds. 

Pyrex-filtered irradiation in de-aerated acetonitrile solution. The resultant enols can be trapped by dimethyl maleate to yield the adducts (27) and (28) in the ratios and yields shown. Trapping of the enols can also be acomplished using dimethyl acetylenedicarboxylate or ethyl propiolate as the dienophile. A study of the phototautomerism of methyl salicylate (29) into (30) at 77 K has been carried out and this again involves a 1,5-hydrogen transfer. Interestingly, the authors report that triplet state emission occurs from the transient keto form (30). The influence of aryl substituents (p-Me and -MeO) upon the photochemically induced intramolecular proton transfer in salicylic acid has also been studied in detail. 

Most azobenzene derivatives can be isomerized reversibly between the E and Z isomeric forms by light or heat. Some derivatives may show phototautomerism, photodimerization, photoreduction, and fading. The effects discussed in this chapter are based on E-Z photoisomerization of aromatic azobenzenes (Rau, 1990) also known in the literature as transits photoisomerization. 

The lactim-lactam phototautomerization was studied by means of 2-(6 -hydroxy-2 -pyridyl)benzimidazolium in water [61]. It was found that two pathways exist, namely, a water-assisted proton translocation by probably a double proton transfer, and a two-step process during which the molecule dissodates and forms a zwitteri-onic species which is protonated at the pyridine nitrogen. The disappearance of the lactim tautomer after optical excitation takes less than 1 ns, while the zwitterionic form and the lactam tautomer have an exdted-state Hfetime of a few nanoseconds. Studies on 5-(4-fluorophenyl)-2-hydroxypyridine revealed that, after optical excitation of the lactim form, a tautomeric equiHbrium is established by proton transfer processes, again on a subnanosecond timescale [62]. 

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