Excited-state acidity intramolecular proton transfer

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... 

From a study of the absorption and emission spectra of salicylic acid and 2-methoxybenzoic acid, Weller (1956) concluded that the greatly increased Stokes shift of salicylic acid was due to intramolecular proton transfer (45) in the excited state ... 

Salicylic acid and its methylated derivatives have been reexamined recently (Schulman and Gerson, 1968 Kovi et al., 1972b) and the pH-dependence of the intramolecular proton transfer reaction has been investigated. In both chloroform and aqueous media, salicylic acid both in the uncharged and carboxylate forms undergoes intramolecular proton transfer in the excited state. In the case of methyl salicylate, the proton transfer occurs intramolecularly in chloroform but intermolecularly in aqueous media. By comparison with the behaviour of 2-methoxybenzoic acid and methyl salicylate,... 

Following brief discussion of the acidity of excited states (Section II), the spectral properties of the three groups are discussed in Sections III to V. Not all substances possibly showing this effect will be covered. The discussion is rather restricted to compounds on which experiments have been performed which contribute to the elucidation of the mechanism of intramolecular proton transfer. In the last section the three groups will be discussed together. 

Intramolecular proton transfer in salicylic derivatives can be understood qualitatively considering the increased acidity of phenolic OH groups and the Increased basicity of carboxyl and carbonyl groups in the first excited singlet state (1,2). 

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. 

Porco et al. reported the synthesis of ( ) methyl rocaglate using [3 + 2] dipolar photocycloaddition reaction of an oxidopyrylium betaine derived from excited state intramolecular proton transfer reaction of 3-hydroxyflavin and methyl cinnamate [144]. Methyl rocaglate was obtained by a base-mediated a-ketol rearrangement followed by hydroxy-directed reduction sequence. They subsequently succeeded in the asymmetric synthesis of methyl rocaglate using functionalized TADDOL derivative (34) (Figure 2.30) as a chiral Bronsted acid (Scheme 2.77) [145]. 

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). 

Comparing these observations to the requirements of TST, we can immediately see a number of problems. Even apart from the fact that the mass of the proton requires it to be treated as a quantum mechanical particle, so that even if there were a well-defined barrier, we would still need to take the possibility of tunneling into account. Transfer of the proton is directly coupled, or may even be driven by a redistribution of electron density in the molecule. In excited-state intramolecular proton transfer (ESIPT) reactions, the redistributed charge almost certainly provides the driving force. The generic picture for such is reaction is due to WeUer [7, 8], who was the first to realize that the enormous Stokes shift of about 10 000 cm he observed in the fluorescence of salicylic acid (X = OH) could be a consequence of a rapid proton transfer in the excited state. 

The excited state properties of hydroxyaromatic compounds (phenols, naphthols, etc) are of interest to a wide audience in chemistry, including those interested in the environmental decomposition of phenols, chemical physicists interested in the very fast dynamics of excited-state proton transfer (ESPT) and excited-state intramolecular proton transfer (ESIPT), physical chemists interested in photoionization and the photochemical pathways for phenoxyl radical formation, and organic photochemists interested in the mechanisms of phenol and hydroxyarene photochemistry. Due to space limitations, this review is restricted to molecular photochemistry of hydroxyaromatic compounds reported during the last three decades that are of primary interest to organic photochemists. It also includes a brief section on the phenomenon of enhanced acidity of phenols and other hydroxyaromatics because this is central to hydroxyarene photochemistry and forms the basis of much of the mechanistic photochemistry to be discussed later on. Several reviews that offer related coverage to this work have also appeared recently. This review does not cover aspects of electron photoejection from phenols or phenolate ions (and related compounds such as tyrosine) or phenol OH homolysis induced photochemically, as shown in Eq. (39.1), as these are adequately covered elsewhere ... 

Fluorescence Spectroscopic Evidences of Excited-State Intramolecular Proton Transfer of Salicylic Acid and Methyl Salicylate... 

Kim, Y, Yoon, M., and Kim, D., Excited-state intramolecular proton transfer coupled-charge transfer of N,N-dimethylaminosahcylic acid in aqueous P-cyclodextrin solutions, /. Photochem. 

Catalan, J., J.C. del Valle, J. Palomar, C. Diaz, and J.L.G. de Paz (1999), The six-membered intramolecular hydrogen bond position as a switch for inducing an excited state intramolecular proton transfer (ESIPT) in esters of o-hydroxynaphthoic acids, J. Phys. Chem. A, 103, 10921-10934. 

The proton transfer may occur rapidly after the excitation and form a tautomer, when either acidic or basic moieties of the same molecule become stronger acids or bases in the excited state. The majority of reactions of this type involve the proton transfer from an oxygen donor to an oxygen or nitrogen acceptor, although a few other cases are known, where a nitrogen atom can function as a donor and a carbon atom as the acceptor. Usually an intramolecular hydrogen bond between the two moieties of a molecule facilitates the proton transfer. 

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. 

That true acid-base equilibrium is achieved in the excited state. In shortlived excited states, where proton transfer is slow, e.g. diffusion controlled or intramolecular, this is unlikely to be the case. 

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