Proton transfer from excited electronic

The future is surely for direct dynamical examination of the individual elementary steps in such reactions. For proton transfer from excited electronic states this is already a reality, and we look forward to additional work along the same lines of inquiry for other archetypal reactions. 

Fig. 46. Scheme of optical transitions, explaining the dual fluorescence resulting from proton transfer in excited electronic state. 

Various molecular and quasi-molecular ions can be formed under MALDI conditions. The formation of protonated analyte (A) molecules, [A -F H]+, is generally most important at least for samples containing slightly basic centres, such as the peptides and proteins, MALDI mass spectrometry of which is known to be most facile and reproducible. Therefore, proton transfer from the electronically excited, neutral or ionized, or protonated matrix species is considered to be crucial in the overall MALDI process . Notably, proton transfer can occur already in the condensed phase, followed by desorption of the preformed ions . However, the generation of the [A -F H]+ ions is believed to take place preferably in the so-called plume , that is, in the energized, short-hved and relatively dense vapour phase generated above the solid matrix upon excitation by the laser pulse. The actual proton donor species (be it one or several) in a given case is still a matter of... 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

CT) complex with absorption maxima at 470 and 550nm, was produced. These species were formed only in polar solvents with relatively high proton affinity. The data suggested an intermolecular proton transfer, from electronically excited TNB to the solvent forming the anion... 

The photochemistry of imides, especially of the N-substituted phthalimides, has been studied intensively by several research groups during the last two decades [233-235]. It has been shown that the determining step in inter- and intramolecular photoreactions of phthalimides with various electron donors is the electron transfer process. In terms of a rapid proton transfer from the intermediate radical cation to the phthalimide moieties the photocyclization can also be rationalized via a charge transfer complex in the excited state. 

The dynamic behavior of the chromoprotein is much more clearly multiexponential. Two pronounced decay components were identified with 5-ps (25 % wt.) and 60-ps (25% wt.) lifetimes. The strong weight of these exponentials is not in favor of a comparison to the weak 13-ps decay of the free pigment. It is more likely that they actually reveal a new and specific deactivation channel in the protein complex. It has, in fact, been speculated that light irradiation of the living organism induces an acidification of the intracellular medium [11], so proton transfer from the chromophore to the associated protein was first proposed as the initial phototransduction step. More recent experiments on the isolated pigment in the presence of electron acceptors [12] proved that the optical excitation can induce an electron transfer from the chromophore to an acceptor site possibly situated inside the associated... 

Previously, Ohashi and his co-workers reported the photosubstitution of 1,2,4,5-tetracyanobenzene (TCNB) with toluene via the excitation of the charge-transfer complex between TCNB and toluene [409], The formation of substitution product is explained by the proton transfer from the radical cation of toluene to the radical anion of TCNB followed by the radical coupling and the dehydrocyanation. This type of photosubstitution has been well investigated and a variety of examples are reported. Arnold reported the photoreaction of p-dicyanobenzene (p-DCB) with 2,3-dimethyl-2-butene in the presence of phenanthrene in acetonitrile to give l-(4-cyanophenyl)-2,3-dimethyl-2-butene and 3-(4-cyanophenyl)-2,3-dimethyl-l-butene [410,411], The addition of methanol into this reaction system affords a methanol-incorporated product. This photoreaction was named the photo-NO-CAS reaction (photochemical nucleophile-olefin combination, aromatic substitution) by Arnold. However, a large number of nucleophile-incorporated photoreactions have been reported as three-component addition reactions via photoinduced electron transfer [19,40,113,114,201,410-425], Some examples are shown in Scheme 120. 

The spinless theory that we are using here is approximate for electron transfer but absolutely correct for the proton reactions. The widely studied example is the reversible proton transfer from an excited photoacid to the... 

Photoreduction by amines differs from photoreduction by alcohols in two respects quantum yields are always lower than maximal and rate constants for amine quenching of triplet ketones are very large. These two facts led Cohen 153> and Davidson 154> to suggest that amines react with excited carbonyl compounds by electron transfer followed by proton transfer from the amine radical cation. 

Formation of cycloadducts can be completely quenched by conducting the experiment in a nucleophilic solvent. This intercepts radical cations so rapidly that they cannot react with the olefins to yield adducts. In Scheme 54 the regiochemistry of solvent addition to I-phenylcyclohexene is seen to depend on the oxidizability or reducibiiity of the electron-transfer sensitizer. With ]-cyanonaphthalene the radical cation of the olefin is generated, and nucleophilic capture then occurs at position 2 to afford the more stable radical. Electron transfer from excited 1,4-dimethoxynaphthalene, however, generates a radical anion. Its protonation in position 2 gives a radical that is oxidized by back electron transfer to the sensitizer radical before being attacked by the nucleophilic solvent in position 1. Thus, by judicious choice of the electron-transfer sensitizer, it is possible to direct the photochemical addition in either a Markovnikov (157) or anti-Markovnikov (158) fashion (Maroulis and Arnold, 1979). 

Several groups have studied photoreactions of dicyano-aromatic compounds with alkylbenzenes as the electron donors [8]. Efficient proton transfer from the benzylic position of the alkylbenzene radical-cation, formed by electron transfer to excited DCN, to the counter anion (DCN -) is reported to produce a benzylic radical and... 

Irradiation of aromatic hydrocarbons such as phenanthrene, anthracene, naphthalene, and certain substituted naphthalenes in the presence of NaBH4 and m- or p-(NC)2C6H4 promotes a Birch-type photoreduction.The reaction seems to occur by electron transfer from the excited singlet state of the arene to the electron acceptor giving the arene radical cations, which are then reduced by the borohydride. Other reducing agents such as NaBH, NaBHjCN, and NaBH(OMe)3 have been found to be effective and all lead to different isomer ratios. In a mechanistically related reaction, both fluoren-9-ol and the corresponding acetate are reported to be photoreduced to the parent hydrocarbon in the presence of aliphatic amines. The products arise by photoinduced electron transfer followed by proton transfer from the amine. The yield depends on the structure of the amine and increases in the order primary < secondary < tertiary amine. In... 

Aromatic aldehydes can be reduced to their corresponding alcohols over irradiated TiOj [22], The reaction involves the formation of an a-hydroxyl radical via a single electron transfer from excited TiOj to the aldehyde followed by protonation of the radical anion. The hydroxyl radical then is reduced by a... 

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