Excited-state electron-transfer reactions

Boxer S G, Goldstein R A, Lockhart D J, Middendorf T R and Takiff L 1989 Excited states, electron-transfer reactions, and intermediates in bacterial photosynthetic reaction centers J. Rhys. Chem. 93 8280-94... 

PBE dendrons bearing a focal bipyridine moiety have been demonstrated to coordinate to Ru + cations, exhibiting luminescence from the metal cation core by the excitation of the dendron subunits [28-30]. The terminal peripheral unit was examined (e.g., phenyl, naphthyl, 4-f-butylphenyl) to control the luminescence. The Ru +-cored dendrimer complexes are thought to be photo/redox-active, and photophysical properties, electrochemical behavior, and excited-state electron-transfer reactions are reported. 

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. 

An electronic excited state of a metal complex is both a stronger reductant and oxidant than the ground state. Therefore, complexes with relatively long-lived excited states can participate in inter-molecular electron transfer reactions that are uphill for the corresponding ground state species. Such excited state electron transfer reactions often play key roles in multistep schemes for the conversion of light to chemical energy ( 1). 

These excited-state electron transfer reactions were mainly investigated using time-resolved spectroscopic techniques such as flash photolysis and flash fluorescence. The extensive work on the photochemistry of MLCT excited states is motivated by both the interest in basic science and the potential applications to many areas of chemistry, for example, biochemistry, solar energy, and conducting polymers.130 135... 

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. 

Thermodynamic Aspects of Excited State Electron Transfer Reactions... 

Fig. 12. Schematic representation of the relation between the spectroscopic Stokes shift and the difference in the reorganization barriers for ground and excited state electron transfer reactions. AB = absorption CD = emission B C + D A = Ahv (Stokes shift) 2Ea = reorganization barrier for reaction (35) 2 Ea = reorganization barrier for reaction (36)...

Another important difference between ground-state and excited state electron-transfer reactions is that the orbitals involved are different in the two cases, which may cause a different interaction energy. Detailed examples will be given in Section 9.6. 

Fig. 13. Kinetic scheme for an excited state electron transfer reaction. For details, see text...

As we have seen in Section 8, excited-state electron-transfer reactions can be used for the conversion of light energy into chemical energy. Most of the polypyridine complexes show intense absorption bands in the visible region (see, for examples, Fig. 17) and thus they are particularly suitable for solar energy conversion. 

During the last 6 years, we have studied solvent stabilization of the excited state electron transfer reaction for dimethylaminobenzonitrile (DMABN) (Grassian et al. 1989, 1990 Shang and Bernstein 1992 Warren et al. 1988). Investigation of the cluster behavior of this reaction has led to an important finding that pertains to both condensed phase behavior and the mechanism for the electron transfer,... 

The electronically excited state of a molecule is a new species with chemical properties that can differ from those of the corresponding ground state. Many of the properties of excited states can be predicted from those of ground-state species with comparable electronic configurations, especially the electron-transfer properties of excited states of metal complexes. In this section the relations between ground- and excited-state electron-transfer reactions of transition-metal complexes are discussed. 

The luminescence and excited state electron transfer reactions of (dppe)Pt S2C2(2-pyridine(ium))(H) and (dppe)Pt S2C2(4-pyridine(ium))(H) are dependent on the protonation state of the pyridine [30-35]. The switching on of the luminescence in these compounds results from a change in the ordering of the electronic transitions in the pyridine and pyridinium substituted complexes. Unlike the quinoxaline-substituted complexes, the neutral pyridine complexes have a lowest lying d-to-d transition, which leads to rapid nonradiative decay of the ILCT excited states. However, upon protonation the ILCT becomes the low-lying transition. The pyridinium complexes are room temperature lumiphores with emission from ILCT and ILCT excited states (see Table Ic). 

The rate constant of these bimolecular processes is controlled by several factors. To elucidate these factors, a detailed reaction mechanism must be considered. Since both electron transfer and exchange energy transfer are collisional processes, the same kinetic formalism may be used in both cases [10]. Using an oxidative excited-state electron-transfer reaction as an example (equation (21)), the reaction rate can be discussed on the basis... 

For a reductive excited state electron transfer reaction... 

The nature of the excited states, electron transfer reactions, and the intermediates in bacterial photosynthetic reaction centers has been reviewed in relation to the X-ray crystallographic structural data. This review provides a critical evaluation of the relationships between the structural features of the bacteriochlorophylls and quinone centers and their photophysical and kinetic features. ... 

A/D] ). In sufficiently polar solvents, the ion pairs dissociate to free ions. In non-polar solvents one can frequently observe a new emission, broad and structureless, and red-shifted from that of A or D. The mechanism of exciplex formation is obviously closely tied to that of excited state electron transfer reactions. There is an important... 

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