Excited-state intramolecular proton transfer products

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... 

Products isolated from the thermal fragmentation of A-arylbenzamide oximes and A-arylbenzamide O-phenylsulfonyl oximes have been accounted for by invoking a free-radical mechanism which is initiated by the preferential homolysis of the N-O bond." Time-resolved IR spectroscopy has revealed that photolysis of A, A -diphenyl-l,5-dihydroxy-9,10-anthraquinone diimine affords acridine-condensed aromatic products via excited-state intramolecular proton transfer." The absolute and relative rates of thermal rearrangements of substituted benzyl isocyanides have been measured,and it has been found that the relative rates are independent of temperature and exhibit excellent Hammett correlations. Thionitrosoarene (25), thought to be generated by desulfurization of the stable A-thiosulfinylaniline (24), has been established" " as an intermediate in the formation of 3,3a-dihydro-2,l-benzisothiazole (26) from o-alkylthionitrosoarene (24). 

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. 

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,... 

Phenols bearing bulky hydrojyallqrl substituents afford quinone methides upon photodeprotonation and dehydration (37) or excited state intramolecular proton transfer (38). Subsequent reaction with nucleophilic solvents gives the corresponding photosolvolysis products. Photodehydration of 3-hydro qrbiphenyl derivatives, i.e. (39), is also initiated by phenol deprotonation, but in this case zwitterionic species with lifetimes in the microsecond range are formed as photosolvolysis intermediates. Quinone methides have also been generated by irradiation of 2-allq nylphenols. ... 

Products of addition to styrene double bonds can arise as a result of light induced electron transfer reactions. Lewis has studied the intramolecular reaction of l-phenyl-w-amino alkenes (422) 289,290 products arise from electron transfer from the amine nitrogen to the excited state of the styryl group followed by intramolecular proton transfer in the radical ion pair produced. The resultant biradical then couples to yield the isolated products (423) and (424). Sensitisation of the intermolecular analogue of this reaction by 1,4-dicyanobenzene has been reported and is proposed to occur by electron transfer from the styrene to the excited state of the sensitiser followed by attack of an amine on the styrene radical cation. This ultimately leads to the product of anti-Markovnikov addition of the amine across the double bond of the styrene. This is similar to the sequence long since established by... 

Electronically excited phthalimides can act as good electron acceptors and carboxylic acids are documented to serve as electron donors in photoinduced SET reactions. It is likely then, that in the excited state, an electron transfer process between the phthalimide system and the carboxylic acid would occur, leading to charge-separated diradical 6. Proton transfer from the carboxylic acid function (iT" is an electrofugal group) would form a carboxy radical 7, which could undergo rapid decarboxylation to azomethyne ylide 8. This reactive species is transformed into the final decarboxylated product 3 by intramolecular proton transfer (Scheme 16.4). 

The third type of experiment is photolysis, where the product is one of a tautomer pair [2, 7, 75]. Again, almost aU reactions studied are keto-enol tautomerizations where the proton transfer is not direct but in a number of steps via the solvent Since the first step is often an ionization (proton transfer to solvent molecule), which is thought to be diffusion-controlled [67], it does give some insight into proton transfer reactions, but exact elucidation is hard, since often there are numerous possibiHties for reaction mechanisms and roles of solvent molecules and internal vibrations [76, 77]. In view of the lack of understanding of proton transfer reactions, it would be much better to have a simpler and more direct way to initiate intramolecular proton transfer. This possibility is offered by looking at intramolecular proton transfer reactions in the excited state, which can be initiated much faster and followed on a much shorter timescale than ground-state reactions. 

Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)g in AOT/heptane-RMs was studied and the Hfe time of the redox products in reverse micellar system was found to increase by about 300-fold compared to conventional photosystem [335]. The authors have presented a kinetic model for overall photochemical process. Kang et al. [336] reported photoinduced electron transfer from (alkoxyphenyl) triphenylporphyrines to water pool in RMs. Sarkar et al. [337] demonstrated the intramolecular excited state proton transfer and dual luminescence behavior of 3-hydroxyflavone in RMs. In combination with chemiluminescence, RMs were employed to determine gold in aqueous solutions of industrial samples containing silver alloy [338, 339]. Xie et al. [340] studied the a-naphthyl acetic acid sensitized room temperature phosphorescence of biacetyl in AOT-RMs. The intensity of phosphorescence was observed to be about 13 times higher than that seen in aqueous SDS micelles. 

Formal intramolecular hydrogen abstraction (proceeding through coupled electron and proton transfer) occurs in the titi triplet excited state of furanone derivatives, upon acetone photosensitization. After hydrogen transfer from the tetrahydropyran to the (I position of the furanone moiety, radical recombination leads to the final products (18). The results of computational studies on model structures are in accordance with experimental observations and reveal that the reactivity and selectivity are mainly determined by the hydrogen-abstraction step. ... 

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