Proton transfer switch

Aviram also proposed an intramolecular hydrogen atom transfer switch, based on H bonding in ortho-quinone-cathecol systems [15,16]. It has been confirmed that there is interamolecular H atom transfer in such a system [ ]. Mitani is working toward a proton transfer switch [17]. 

During the course of the work, we also conducted further studies of the reactions of proton hydrates with CH3COCH3 and CH3COOCH3. The reaction mechanisms were found to change from proton transfer to ligand switching and ultimately to an association process, which would be equivalent to adsorption in the case of bulk systems. 

In this section, we switch gears slightly to address another contemporary topic, solvation dynamics coupled into the ESPT reaction. One relevant, important issue of current interest is the ESPT coupled excited-state charge transfer (ESCT) reaction. Seminal theoretical approaches applied by Hynes and coworkers revealed the key features, with descriptions of dynamics and electronic structures of non-adiabatic [119, 120] and adiabatic [121-123] proton transfer reactions. The most recent theoretical advancement has incorporated both solvent reorganization and proton tunneling and made the framework similar to electron transfer reaction, [119-126] such that the proton transfer rate kpt can be categorized into two regimes (a) For nonadiabatic limit [120] ... 

Lim SJ, Seo J, Park SY (2006) Photochromic switching of excited-state intramolecular proton-transfer (ESIPT) fluorescence a unique route to high-contrast memory switching and nondestructive readout. J Am Chem Soc 128 14542-14547... 

We next focus on the use of fixed-site cofactors and coenzymes. We note that much of this coenzyme chemistry is now linked to very local two-electron chemistry (H, CH3", CH3CO-, -NH2,0 transfer) in enzymes. Additionally, one-electron changes of coenzymes, quinones, flavins and metal ions especially in membranes are used very much in very fast intermediates of twice the one-electron switches over considerable electron transfer distances. At certain points, the chains of catalysis revert to a two-electron reaction (see Figure 5.2), and the whole complex linkage of diffusion and carriers is part of energy transduction (see also proton transfer and Williams in Further Reading). There is a variety of additional coenzymes which are fixed and which we believe came later in evolution, and there are the very important metal ion cofactors which are separately considered below. 

Fig. 6 An intrinsic molecular switch positioned in an STM tunnel junction. Here, the mechanism of intramolecular switching is independent of the surface. It is the intramolecular transfer of two protons which is supposed to shift the molecular orbitals of molecule since one phenyl is bonded to a sulfur atom. The protons transfer is triggered by the tip to surface electric field...

Figure 5.12 depicts the corresponding adiabatic one-dimensional potential for the covalent rHF proton-transfer coordinate, showing the barrierless switch-over at equilibrium between FH F and F HF bond patterns. The potential well is seen to be extremely flat in the neighborhood of equilibrium, corresponding to the extremely low IR frequency of the proton-transfer mode (1299 cm-1, red-shifted... 

Numerous processes may be linked to proton transfer and protonation/deprotonation reactions (for general descriptions of proton transfer see, for instance, [8.214-8.217]). Proton-triggered yes/no or +/- switching is contained in the ability of polyamine receptors and carriers to bind and transport cations when unproton-ated and anions when protonated also, zwitterions such as amino acids may change from bound to unbound or vice versa, when they undergo charge inversion as a function of pH. 

Thermally or photochemically induced proton transfers represent bistable switching processes and are of interest for information storage. A lateral transfer of information on the surface of biological membranes is thought to occur by fast proton conduction through protonic networks [8.234]. 

The second type of mechanism offers reversible switching without space-consuming transfer processes. These dyes are electrocyclically reacting dyes, where the transition from one form to another form happens by electron shift or proton transfer, without the rearrangement of bulky molecule segments. Fulgides have been proposed for this purpose. Fulgides are bis (methylene) amber acid anhydrides (130), eg,... 

Push-pull acid-base catalysis has been proposed to account for the proton switch mechanism which occurs in the methoxyaminolysis of phenyl acetate (Scheme 11.14) where a bifunctional catalyst traps the zwitterionic intermediate. A requirement of efficient bi-functional catalysis is that the reaction should proceed through an unstable intermediate which has p values permitting conversion to the stable intermediate or product by two proton transfers after encounter with the bifunctional catalyst the proton transfer with monofunctional catalysts should also be weak. 

Interfacial monolayer, multilayer and polymer species which exhibit interesting examples of light and electrically stimulated functions such as isomerization and proton transfer in ISAs are also presented in this chapter. Such materials may represent the precursors for electrooptic switches and addressable molecular-based machines. 

Photostimulated molecular motion is an important photophysical phenomenon frequently exploited in molecular switches. The molecular electronic rearrangements accompanying optical excitation may stimulate nuclear rearrangement of the excited species. Like electron and energy transfer, such processes compete with radiative events and therefore reduce the measured lifetime and quantum yield of emission. The most important nuclear rearrangements in supramolecular species are proton transfer and photoisomerization. 

Like proton transfer, photoisomerization is a fundamentally important photochemical process. The two most important forms of photoisomerization are valence isomerization and stereoisomerization. The latter is probably the most common photoinduced isomerization in supramolecular chemistry. It may occur in systems in which the photoactive component has unsaturated bonds which can be excited, and this effect may be exploited for optical switching applications. A number of interfacial supramolecular complexes capable of undergoing cis-trans photoisomerization have been studied from this perspective - some examples are outlined in Chapter 5. 

There has been a resurgence of interest in proton-coupled redox reactions because of their importance in catalysis, molecular electronics and biological systems. For example, thin films of materials that undergo coupled electron and proton transfer reactions are attractive model systems for developing catalysts that function by hydrogen atom and hydride transfer mechanisms [4]. In the field of molecular electronics, protonation provides the possibility that electrons may be trapped in a particular redox site, thus giving rise to molecular switches [5]. In biological systems, the kinetics and thermodynamics of redox reactions are often controlled by enzyme-mediated acid-base reactions. 

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