Photolysis electron acceptor solvents

We prepared three bifunctional redox protein maquettes based on 12 16-, and 20-mer three-helix bundles. In each case, the helix was capped with a Co(III) tris-bipyridyl electron acceptor and also functionalized with a C-terminal viologen (l-ethyl-V-ethyl-4,4 -bipyridinium) donor. Electron transfer (ET) was initiated by pulse radiolysis and flash photolysis and followed spectrometrically to determine average, concentration-independent, first-order rates for the 16-mer and 20-mer maquettes. For the 16-mer bundle, the a-helical content was adjusted by the addition of urea or trifluoroethanol to solutions containing the metal-loprotein. This conformational flexibility under different solvent conditions was exploited to probe the effects of helical secondary structure on ET rates. In addition to describing experimental results from these helical systems, this chapter discusses several additional metalloprotein models from the recent literature. 

Charge-transfer-to-solvent reactions are a special class of MLCT processes. Coimnonly a halogenated solvent serves as the acceptor with formation of a halide ion and a radical. Snbseqnent reaction chemistry between the oxidized metal species and either the halide ion or radical may take place. For example, photolysis of Fe(et2dtc)2(CO)2 in CHCI3 resnlts in formation of Fe(et2dtc)2Cl via electron transfer to solvent from a MLCT excited state. Similarly, photolysis of (Cp-CH2-Cp)Fe2(/r-CO)2(/(r-dppm) in CHCI3... 

Thus, electron transfers from a series of unhindered, partially hindered, and heavily hindered aromatic electron donors (with matched oxidation potentials) to photoactivated quinone acceptors are kinetically examined by laser flash photolysis, and the free-energy correlations of the ET rate constants are scrutinized [31]. The second-order rate constants of electron transfers from hindered donors such as hexaethylbenzene or tri-icrt-butylbenzene strongly depend on the temperature, the solvent polarity and salt effects, and they follow the free-energy correlation predicted by Marcus theory (see Figure 20A). Moreover, no spectroscopic or kinetic evidence for the formation of encounter complexes (exciplexes) with the photo-activated quinones prior to electron transfer is observed. 

A paper by Suppan draws attention to electrostatic interaction effects on condensed phase photoinduced electron transfer and the need to take account of the fact that solvent is not in reality a uniform dielectric material. Pressure effects on exciplex formation has been exemplified in the pyrene-p-cyanobenzene system. Ternary electron donor acceptor complexes are formed and in the case of anthracene-tetracyanoethylene gives rise to (DO ) dimer radical cations. Laser flash photolysis shows that perylene in acetonitrile undergoes three distinct electron transfer processes, (i) gives pt + MeCNT, (ii) gives... 

Light excitation in the CT absorption bands formally leads to the transfer of an electron from the donor to the acceptor component (optical electron transfer). As a consequence, particularly when this process leads to formation of charges of the same sign in the two components (Fig. 8), one can expect destabilization of pseudorotaxane structures, followed by dethreading. In practice, however, this simple approach does not work because the back electron-transfer process is much faster than the separation of the molecular components, a process which requires extended nuclear motions and solvent rearrangement. In a particular case [24], laser flash photolysis experiments have suggested that a small fraction of the irradiated pseudorotaxane may undergo dissociation. 

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