Electron transfer processes excited states

Typical values of Ro between chlorophylls and bacteri-ochlorophylls are 60-100 A (assuming = 1). Excitation transfer is therefore a longer range process than is electron transfer (see below). In fact, the structures of anteima complexes have undoubtedly been fine tuned by evolution to minimize excited-state electron transfer processes, while at the same time efficiently delivering energy to the reaction center. 

Topics which have formed the subjects of reviews this year include photosubstitution reactions of transition-metal complexes, redox photochemistry of mononuclear and polynuclear" complexes in solution, excited-state electron transfer processes, transition-metal complexes as mediators in photochemical and chemiluminescence reactions, lanthanide ion luminescence in coordination chemistry, inorganic photosensitive materials," and photocatalytic systems using light-sensitive co-ordination compounds. Reviews have also appeared on the photoreduction of water.Finally, various aspects of inorganic photochemistry have been reviewed in a single issue of the Journal of Chemical Education. 

For a supramolecular system, A-L-B, the driving force of an excited-state electron transfer process can be easily calculated (Eq. 3) on the basis of electrochemical and spectroscopic data on the isolated molecular components (with, in case, small corrections for electrostatic work terms) [6], F or the case of oxidative PET (Figure lb Eq. 2), the relevant expression is given in Eq. 3. 

The occurrence of the excited-state electron transfer process (Eq, 24) competes with nonradiative deactivation and luminescence (Eq. 23), which is therefore a useful handle to evaluate whether Eq. 24 takes place. A necessary condition for the occurrence of Eq. 24 is, of course, that it takes place with negative free energy change. The reduction potential of the Cu.31 / Cu.31 couple can be evaluated... 

Although this was not the main purpose in the design of the dyad, interesting results can also be obtained by exciting the Rh(NN)3 component (eq 21) using near ultraviolet radiation. Upon laser excitation at 298 nm, transient formation and decay of the electron transfer product is observed. This experiment shows that another excited-state electron transfer process (eq 22) can take place in this system (rate constant, 3x10 s" ). Because of the fast generation of the electron transfer product, the... 

Gan, H., KeUett, M.A., and Whitten, D.G., Novel intermolecular and intramolecular photochemical reactions initiated by excited state electron transfer processes, /. Photochem. Photobiol, A Chem., 82, 211,1994. 

Excited state electron transfer also needs electronic interaction between the two partners and obeys the same rules as electron transfer between ground state molecules (Marcus equation and related quantum mechanical elaborations [ 14]), taking into account that the excited state energy can be used, to a first approximation, as an extra free energy contribution for the occurrence of both oxidation and reduction processes [8]. 

We now turn to an example of nonadiabatic chemistry where the nonadiabatic process starts on the ground state, and is followed by an excursion upward onto the excited state electron transfer (see references 2-5). 

Excited-state electron transfer represents one of the most fundamental pathways in chemical and biological processes. This, together with its prospects for application, elaborated in later sections, has been attracting considerable attention. As a result, many review papers and books have been published to address this issue [1-6]. 

Chen KY, Hsieh CC, Cheng YM et al (2006) Tuning excited state electron transfer from an adiabatic to nonadiabatic type in donor-bridge-acceptor systems and the associated energy-transfer process. J Phys Chem A 110 12136-12144... 

Bimolecular excited state electron transfer reactions have been investigated extensively during the last decade (1-3). Electron transfer is favored thermodynamically when the excitation energy E of an initially excited molecule A exceeds the potential difference of the redox couples involved in the electron transfer process. 

Studies of such systems provided a better understanding of the mechanism of electron transfer processes in general. This reaction type is also the basis of almost any type of natural or artificial photosynthesis. Hence it is not surprising that many investigations have been devoted to excited state electron transfer reactions. On the contrary, the reversal of excited state electron transfer has found much less attention although it is certainly not less interesting. 

According to the Franck-Condon principle, the photoexcitation triggers a vertical transition to the excited state, which is followed by a rapid nuclear equilibration. Without donor excitation, the electron transfer process would be highly endothermic. However, after exciting the donor, electron transfer occurs at the crossing of the equilibrated excited state surface and the product state. 

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

But in metal complexes it appears that other processes such as solvent attack on the excited state, electron transfer, and ligand dissociation can lead to excited state deactivation before bond rupture can occur. As seen in the summary in Table III, dissociative cleavage of a multiple metal-metal bond remains to be accomplished. Also, upper excited state reaction is the rule in the multiple bonded systems. 

Intramolecular excited state electron transfer in bi- and polynuclear metal polypyridine complex is a much studied process, especially in relation with the development of molecular wires [38, 51, 52, 83, 87, 88, 318]. The rates are dependent on the nature of the bridging groups linking the metal-polypyridine units. These process will be discussed in detail elsewhere in this book. 

Excited-state electron transfer (ET) is a fundamental complex phenomenon playing a crucial role in a variety of photophysical, photochemical and biochemical reactions (for reviews see, for example. Refs. [ I - 3 ). Owing to the essential role of ET in many processes and in photochemical applications (e.g., solar energy conversion and storage [4-6], photocatalysis [7], photopolymerization [8], information processing and storage [9] and photomedicine [10]) the understanding of the factors which determine the thermodynamics, kinetics and dynamics of the ET processes is very important. 

FIGURE 12.10 Mechanisms of excited-state electron transfer. In the upper reaction, the excited state acts as a reductant, a process commonly termed oxidative quenching. In the lower reaction, the excited state is an oxidant, termed reductive quenching. 

Vinylogous urethane (248) was employed in a radical-mediated synthesis of epilupinine (Scheme 48), where its cyclization was induced by treatment with tributyltin hydride-AIBN <89T5269>. In another radical process, a very efficient synthesis of berbine was achieved by photolysis of compound (249), which induced a diradical cyclization process promoted by an excited-state electron-transfer desilylation (Scheme 49) <85TL5867>. 

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