Electron transfer in the excited state

There is another compensating effect at a metal electrode as a consequence of such an electron transfer in the excited state. The species D+ and D which are generated have acceptor or donor levels below or above the Fermi level of the metal where a reverse electron transfer is now possible. In the steady state reached within the lifetime of the products D+ and D at the electrode the photocurrents are compensated by the reverse electron transition from the products. Only if a feist consecutive reaction brings one of the products to an energy state where no reverse electron transfer is possible a net photocurrent can be observed at a metal electrode. It is not clear whether up to now any reaction has been found which... 

Fig. 18. Electron energy correlations for regeneration of dyes after electron transfer in the excited state by redox reactions with electron donors or acceptors...

The electron transfer in the excited state is reversed when the molecule returns to the ground state, the leuco dye being oxidized back by ferric ion. A similar system includes mixed inorganic solutions such as (I2/I ) + (Fes+/Fe2+). In such electron transfer reactions, the acceptor has a much lower electron affinity in the ground state than the donor. Thus, the... 

Excited state redox potentials may be different from those of the ground state values. Redox reactions can be initiated on electronic excitation against the electrochemical gradients. Electron transfer in the excited state may be reversed in the ground state. An important example is photosynthesis in plants. 

The argument on the mechanism of photosensitized charge transfer, as to whether the charge transfer excitation is responsible, or the excitation of an isolated molecule carries out electron transfer in the excited state is not always clear. Even if no CT complex is detected by spectroscopy, this is not conclusive evidence that CT interaction at the ground state does not exist. There are possibilities of overlapping absorptions of the CT band with specific absorptions of donor or acceptor. 

Similar results were obtained [139] with the three dimethoxybenzenes and acrylonitrile, methacrylonitrile, and crotonitrile. The amounts of substitution products decrease in the order acrylonitrile (49%) > methacrylonitrile (45%) > crotonitrile (6%), which agrees with the electron affinities of these compounds. Simultaneously, the amount of addition product increases acrylonitrile, 0% methacrylonitrile, 38% crotonitrile, 67%. In the series of anisole and the dimethoxybenzenes with crotonitrile, the amount of substitution products decrease in the order ortho- and para-dim ethoxy benzene > meta-dimethoxyben-zene > anisole, which is just the reverse of the order of their oxidation potentials. Ohashi et al. [139] have attempted to relate the photochemical behavior of these systems to the free enthalpy of electron transfer in the excited state as calculated with the Rehm-Weller equation, AG = E(D/D+) - E(A /A) - el/eR - AE00. 

Numerous such reactions can be found in the exhaustive book of Balzani and Carassiti on the photochemistry of coordination compounds, published in 1970 [1]. This monograph was of extreme importance in the development of this research area. It is noteworthy that it contains relatively little photo-redox chemistry it is only about ten years ago that electron transfer in the excited state was considered to be important. Interestingly, a special issue of the Journal of Chemical Education was recently devoted to inorganic photochemistry [2] electron transfer reactions are clearly among the most studied and applied photochemical reactions of today. 

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... 

Pandey and co-workers have generated arene radical cations by PET from electron-rich aromatic rings [119]. The photoreaction is apparently initiated by single-electron transfer from the excited state of the arene to ground state 1,4-dicyanonaphthalene (DCN) in an aerated aqueous solution of acetonitrile. Intramolecular reaction with nucleophiles leads to anellated products regio-specifically. The author explains the regiospecifidty of the cyclization step from... 

Substituent effects on the electron-transfer processes between pyrrolidinofullerenes and tetrakis(dimethylamino)ethylene (TDAE) were studied in both the ground state and excited triplet state. ° Equilibrium constants and rate constants for forward and backward electron-transfer processes in the ground state, in addition to rate constants of the forward electron transfer in the excited triplet state were measured. 

Photochemical decomposition of cationic photoinitiators can be sensitized by energy or electron transfer from the excited state of a sensitizer. Certain sensitizers, such as isopropylthioxantone, anthracene or even certain dyes absorbing in the... 

Electron transfer from the excited states of Fe(II) to the H30 f cation in aqueous solutions of H2S04 which results in the formation of Fe(III) and of H atoms has been studied by Korolev and Bazhin [36, 37]. The quantum yield of the formation of Fe(III) in 5.5 M H2S04 at 77 K has been found to be only two times smaller than at room temperature. Photo-oxidation of Fe(II) is also observed at 4.2 K. The actual very weak dependence of the efficiency of Fe(II) photo-oxidation on temperature points to the tunneling mechanism of this process [36, 37]. Bazhin and Korolev [38], have made a detailed theoretical analysis in terms of the theory of radiationless transitions of the mechanism of electron transfer from the excited ions Fe(II) to H30 1 in solutions. In this work a simple way is suggested for an a priori estimation of the maximum possible distance, RmSiX, of tunneling between a donor and an acceptor in solid matrices. This method is based on taking into account the dependence... 

In the excited singlet state the dimer of bacteriochlorophyll possesses a redox potential of 930 mV, which is sufficient to reduce the intermediate primary acceptor J. The rate of electron transfer from the excited state of bacteriochlorophyll dimer P to J is quite high (k 1011 s-1). The high rate of electron transfer ensures a high quantum yield (0 1) of the charge separation process... 

Whilst [ Ru(bipy)3]2t itself is incapable of splitting water, its electron-transfer properties have been utilized for hydrogen production in a series of reactions involving cocatalysts (see equations 21 to 26). The first step involves electron transfer from the excited state complex to an electron relay (R), which in its reduced form is capable (in the presence of a suitable heterogeneous redox catalyst) of reducing protons to hydrogen. The [Ru(bipy)3]3+ which is formed is then capable of... 

From the point of view of light stability and range of absorptivity, inorganic redox systems might be more interesting. Photoinduced electron transfer in an aqueous solution of tris-(2, 2 -bipyridine) ruthenium (II) has been found to decompose water in to a mixture of H2 and 02. The Complex can serve both as an electron donor and electron acceptor in the excited state. The efficiency is low because of barrier to electron transfer. SVhen spread as a monolayer on glass slides after attaching to a surfactant... 

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