Electronic excited state proton transfer:

One may consider the relaxation process to proceed in a similar manner to other reactions in electronic excited states (proton transfer, formation of exciplexes), and it may be described as a reaction between two discrete species initial and relaxed.1-7 90 1 In this case two processes proceeding simultaneously should be considered fluorescence emission with the rate constant kF= l/xF, and transition into the relaxed state with the rate constant kR=l/xR (Figure 2.5). The spectrum of the unrelaxed form can be recorded from solid solutions using steady-state methods, but it may be also observed in the presence of the relaxed form if time-resolved spectra are recorded at very short times. The spectrum of the relaxed form can be recorded using steady-state methods in liquid media (where the relaxation is complete) or using time-resolved methods at very long observation times, even as the relaxation proceeds. 

Marzocchi M P, Mantini A R, Casu M and Smulevich G 1997 Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis J. Chem. Phys. 108 1-16... 

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. 

Excited-state proton transfer relates to a class of molecules with one or more ionizable proton, whose proton-transfer efficiency is different in the ground and excited states. The works of Forster [2-4] and Weller [5-7] laid the foundation for this area on which much of the subsequent work was based. Forster s work led to the understanding of the thermodynamics of ESPT. He constructed a thermodynamic cycle (Forster cycle) which, under certain acceptable approximations, provides the excited-state proton-transfer equilibrium constant (pK f,) from the corresponding ground-state value (pKa) and electronic transition energies of the acid (protonated) and base (deprotonated) forms of the ESPT molecule ... 

An intramolecular excited state proton transfer occurs on irradiation of hypericin 12gi36-i39 Excitation of hypericin in lipid vesicles results in excited state regioselective transfer of a proton to the substrate from one of the peri-hydroxyl groups . Hypericin in its triplet state reacts with reducing agents to afford a long-lived transient presumed to be the resultant radical anion ". Both electron donors and acceptors can quench the fluorescence of hypericin . A detailed review of the reactions of the photosensitizer hypericin has been published. Some of the work described dealt with its photochemical deprotonation in the excited state. ... 

For many molecules, due to extensive redistribution of electron densities, acid-base property in the excited state differs considerably from that in the ground state [33 For instance, aromatic amines are weakly basic in the ground state. But many of them become acidic in the excited state and readily donate a proton to a proton acceptor to produce the anion in the excited state. Such a molecule, which behaves as an acid in the excited state, is called a photoacid similarly, photobases are those that display basic properties in the excited state. In many cases, excited state proton transfer (ESPT) results in dual emission bands. One of these emission bands arises om the neutral excited state and bears mirror image relation with the absorption spectrum. The other emission band is due to the excited deprotonated (anion) or protonated species and exhibits a large Stokes shift. 

Solubility enhancement by use of cyclodextrins is achieved for a number of drug substances, an approach of interest in formulation of drugs for topical, parenteral, and oral use (Stella and Rajewski, 1997 Loftsson and Masson, 2001 Qi and Sikorski, 2001). The solubilizing effect can be extensive even in low concentrations of cyclodextrin. The use of 0.1 M sulfobutyl-ether-P-cyclodextrin increases the solubility of prednisolone acetate and testosterone by a factor of 426 and 2020, respectively (Myrdal and Yalkowsky, 2002). Cyclodextrin encapsulation of a molecule will affect many of its physicochemical properties (Loftsson, 1995). As a result of complexation, solubility, pKa value, spectral properties, and the chemical reactivity of the included substance will change. The cyclodextrins are known to affect molecular orientation and to have an influence on rates and efficiency of electron transfer, excited state proton transfer, and rate of decomposition (Chattopadhyay, 1991 Fox, 1991 Sur et al., 2000). Cyclodextrins can also be used in combination with liposomes a cyclodextrin-liposome entity represents an even more complex environment to the drug molecule (Loukas et al., 1995). 

For the studies of the excited-state proton transfer reactions of aromatic compounds, kinetic analyses by means of fluorimetry, single-photon counting, and laser photolysis methods are very important to obtain the exact data. Their acid-base properties in the excited states can be understood on the bases of thermodynamic analyses and electronic structures. Large changes in the acidity constant of organic compounds upon electronic excitation may be applicable to various fields, especially to biochemistry. 

Ultrafast studies on tautomerism concentrate on compounds that can exhibit hydrogen transfer in the electronically excited state. Hydrogen transfer is a very typical reaction for the interconversion between different tautomeric forms. It converts enol to keto, amino to imino, imino to enamino, and lactim to lactam forms, to name some examples. For time-resolved experiments, excited-state intramolecular proton transfer (ESIPT) is particularly well suited since a short laser pulse in the visible or ultraviolet (UV) spectral region can trigger this process by promoting the molecule into the electronically excited state and initiating the transfer in this way [3]. The vast majority of experiments on tautomerism with ultrafast time resolution are therefore done on compounds exhibiting ESIPT. 

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 ... 

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