Electron-transfer mechanism, excited state

The remaining sections of this chapter will be mostly concerned with reactions following mechanism I. In this case, the photocurrent magnitude is directly connected to the ratio between the rates of heterogeneous electron transfer and excited state decay. In addition, the effect of the Galvani potential difference on the photocurrent magnitude provides valuable information on the reactivity of the metalloporphyrin at the liquid/liquid boundary. 

It turns out that the found mechanism is not as schematically simple as the SDDJ model, and it is more appropriate to be referred to as coupled proton-electron transfer in excited state , rather than hydrogen-atom transfer . This mechanism further requires the study of recombination dynamics between the separated charges in clusters and solvents. 

However, the mechanism of the transfer is not the excited state hydrogen atom transfer (ESHAT) but should be more appropriately characterized as a coupled proton-electron transfer in excited state. (See Ref. [159, 192] for reviews of proton-coupled electron transfer.)... 

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]. 

A general theory of the aromatic hydrocarbon radical cation and anion annihilation reactions has been forwarded by G. J. Hoytink 210> which in particular deals with a resonance or a non-resonance electron transfer mechanism leading to excited singlet or triplet states. The radical ion chemiluminescence reactions of naphthalene, anthracene, and tetracene are used as examples. 

Fig. 9. Proposed electron-transfer mechanism for the sensitization of YbIn luminescence by the excited state of tryptophan (Trp). Figures are energies of the states in eV. Redrawn from (deW. Florrocks et al., 1997).

The intramolecular mechanism, illustrated on the left-hand side of Figure 6.8, is based on four separate operations [52]. (a) Destabilization of the stable translational isomer light excitation of the photoactive unit P (step 1) is followed by the transfer of an electron from the excited state to the Al station, which is encircled by the macrocycle (step 2) with the consequent deactivation of this station such a photoinduced electron-transfer process has to compete with the intrinsic decay of P (step 3). (b) Ring displacement the ring moves from the reduced station Ah to A2 (step 4), a step that has to compete with the back electron-transfer process from Ah (still encircled by the macrocycle) to the oxidized photoactive unit P+ (step 5). This is the most difficult requirement to meet in the intramolecular mechanism, (c) Electronic reset a back electron-transfer process from the free reduced station Ah to P+ (step 6) restores the electron-acceptor power to the Al station, (d) Nuclear reset as a consequence of the electronic reset, back movement of the ring from A2 to Al takes place (step 7). 

The results for the monomer 9b and the dimer 9c reveal equal solvent dependence of the discussed states. However, in these two compounds no BCT is present. The heat of formation, AHf, of the CT states is increasing almost linearly from monomer to trimer. These trends prove the observations resulting from the photophysical measurements and confirm the suggested charge-transfer behavior of exTTF-oPPE -C 0- Thus, the calculations support the hypothesis that in all solvents an excitation of the triad results in the CT state, which substantiates our interpretation of the electron-transfer mechanism. 

A state-of-the-art description of broadband ultrafast infrared pulse generation and multichannel CCD and IR focal plane detection methods has been given in this chapter. A few poignant examples of how these techniques can be used to extract molecular vibrational energy transfer rates, photochemical reaction and electron transfer mechanisms, and to control vibrational excitation in complex systems were also described. The author hopes that more advanced measurements of chemical, material, and biochemical systems will be made with higher time and spectral resolution using multichannel infrared detectors as they become available to the scientific research community. 

The mechanism of electron transfer reactions in metal complexes has been elucidated by -> Taube who received the Nobel Prize in Chemistry for these studies in 1983 [xiv]. Charge transfer reactions play an important role in living organisms [xv-xvii]. For instance, the initial chemical step in -> photosynthesis, as carried out by the purple bacterium R. sphaeroides, is the transfer of electrons from the excited state of a pair of chlorophyll molecules to a pheophytin molecule located 1.7 mm away. This electron transfer occurs very rapidly (2.8 ps) and with essentially 100% efficiency. Redox systems such as ubiquinone/dihydroubiquinone, - cytochrome (Fe3+/Fe2+), ferredoxin (Fe3+/Fe2+), - nicotine-adenine-dinucleotide (NAD+/NADH2) etc. have been widely studied also by electrochemical techniques, and their redox potentials have been determined [xviii-xix]. 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

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