Electron acceptors 8 pulses

Morishima et al. [75, 76] have shown a remarkable effect of the polyelectrolyte surface potential on photoinduced ET in the laser photolysis of APh-x (8) and QPh-x (12) with viologens as electron acceptors. Decay profiles for the SPV (14) radical anion (SPV- ) generated by the photoinduced ET following a 347.1-nm laser excitation were monitored at 602 nm (Fig. 13) [75], For APh-9, the SPV- transient absorption persisted for several hundred microseconds after the laser pulse. The second-order rate constant (kb) for the back ET from SPV- to the oxidized Phen residue (Phen+) was estimated to be 8.7 x 107 M 1 s-1 for the APh-9-SPV system. For the monomer model system (AM(15)-SPV), on the other hand, kb was 2.8 x 109 M-1 s-1. This marked retardation of the back ET in the APh-9-SPV system is attributed to the electrostatic repulsion of SPV- by the electric field on the molecular surface of APh-9. The addition of NaCl decreases the electrostatic interaction. In fact, it increased the back ET rate. For example, at NaCl concentrations of 0.025 and 0.2 M, the value of kb increased to 2.5 x 108 and... 

Karpfen published a study of trends in halogen bonding between a series of amines and halogens and interhalogens [171]. Iodine-containing electron acceptors were not included. This study involved the use of RHF, MP2, and various DFT methods using extended, polarized basis sets and made extensive use of pulsed-nozzle, FT-microwave spectroscopic data (similar to that... 

Carotenoid radical formation and stabilization on silica-alumina occurs as a result of the electron transfer between carotenoid molecule and the Al3+ electron acceptor site. Both the three-pulse ESEEM spectrum (Figure 9.3a) and the HYSCORE spectrum (Figure 9.3b) of the canthaxanthin/ A1C13 sample contain a peak at the 27A1 Larmor frequency (3.75 MHz). The existence of electron transfer interactions between Al3+ ions and carotenoids in A1C13 solution can serve as a good model for similar interactions between adsorbed carotenoids and Al3+ Lewis acid sites on silica-alumina. 

Carotenoid radical intermediates generated electrochemically, chemically, and photochemically in solutions, on oxide surfaces, and in mesoporous materials have been studied by a variety of advanced EPR techniques such as pulsed EPR, ESEEM, ENDOR, HYSCORE, and a multifrequency high-held EPR combined with EPR spin trapping and DFT calculations. EPR spectroscopy is a powerful tool to characterize carotenoid radicals to resolve -anisotropy (HF-EPR), anisotropic coupling constants due to a-protons (CW, pulsed ENDOR, HYSCORE), to determine distances between carotenoid radical and electron acceptor site (ESEEM, relaxation enhancement). 

The photophysical properties of magnesium(II) tetra-(i-butyl)phthalocyanine (27) have been studied in solution, in micelles and in liposomes cation radical formation (CBr4 as electron acceptor) has been detected with UV excitation, or by a two-photon excitation using a pulsed laser in the therapeutic window at 670 nm.118 The Mg11 complex of octa(tri-z -propylsilylethy-nyl)tetra[6,7]quinoxalinoporphyrazine (28) has been prepared as a potential PDT sensitizer. The synthesis is shown in Figure 8. Compound (28) has Amax 770 nm (e = 512,000 M-1 cm-1), d>f = 0.46 and d>A = 0.19 (all in THF, under air).119... 

A number of rate constants for reactions of transients derived from the reduction of metal ions and metal complexes were determined by pulse radiolysis [58]. Because of the shortlived character of atoms and oligomers, the determination of their redox potential is possible only by kinetic methods using pulse radiolysis. In the couple Mj/M , the reducing properties of M as electron donor as well as oxidizing properties of as electron acceptor are deduced from the occurrence of an electron transfer reaction with a reference reactant of known potential. These reactions obviously occur in competition with the cascade of coalescence processes. The unknown potential °(M /M ) is derived by comparing the action of several reference systems of different potentials. 

Interestingly, non-metallic silver clusters, depending on their sizes, may act either as electron donors or as electron acceptors. Using sulfonatopropyl-viologen, SPV (E° for SPV/SPV- = — 0.41 V/NHE), pulse radiolysis established that small silver clusters (n < 4) acted as electron donors (Le. E° for (Ag /Agn < E° for SPV/SPV - ) while, conversely, large silver clusters (n 2 4) were electron acceptors (i.e. E° for Ag/ /Agn > E° for SPV/SPV- ) [511]. Size-dependent electrochemical potentials of silver aggregates have been elucidated (Fig. 82) [506]. 

The analysis of the data of PS I gave quite accurate information on the distance of the spin centres (25.4 0.3 A)301 that compared well with the crystal structure data.68 A problem is the extended it-spin density distribution in the donor and acceptor. For a solid comparison a centre of gravity for the spin must be calculated from experimental or theoretical spin density distributions of the two radicals. Similar data with almost unaltered distances were obtained for PS I with other quinones substituted into the Ai site.147-302This work has been extended to other electron acceptors,303 which show a larger heterogeneity in distances. It has been shown that the lifetime of the RP can also be measured and can even be controlled in the experiments by an additional mw pulse prior to the 2-pulse echo sequence.302 This pulse transfers population to triplet levels which cannot directly recombine to the singlet ground state. This has earlier been shown for the bRC.304,305 The OOP-ESEEM technique has also been applied to various mutants of PS I to characterize them by the measured distances between fixed donor and variable acceptors.254 256-263-264... 

Pulse radiolysis studies on unsymmetrical, chemically linked organic systems have shown the expected fall off in rate constant as AG becomes more favorable.81 In these experiments, advantage is taken of the fact that capture of electrons produced by pulse radiolysis is relatively indiscriminate and in some of the pulse events an electron is captured by the component in the dimeric systems which is the weaker oxidant. Following such an event, the experimental observation made is of the system relaxing by intramolecular electron transfer to the stable redox configuration, as shown for (I)->(2) where A is one of a series of polyaromatic or quinone electron acceptors.81... 

Rate studies of the reaction between cesium and water in ethylenediamine, using the stopped-flow technique, have been extended to all alkali metals. The earlier rate constant (k — 20 NT1 sec.-1) and, in some cases, a slower second-order process (k — 7 Af"1 sec.-1) have been observed. This is consistent with optical absorption data and agrees with recent results obtained in aqueous pulsed-radiolysis systems. Preliminary studies of the reaction rate of the solvated electron in ethylenediamine with other electron acceptors have been made. The rate constant for the reaction with ethylene-diammonium ions is about 105 NCl sec.-1 Reactions with methanol and with ethanol show rates similar to those with water. In addition, however, the presence of a strongly absorbing intermediate is indicated, which warrants more detailed examination. 

Further work used a similar system to inhibit the formation of a second ion pair completely, using the electric field of an initial ion pair. In compound 14, Zn3PN and 9-(N-pyrrolidinyl)perylene-3,4-dicarboximide (pyr-PMI) are the electron donors, while NI and PI are once again electron acceptors.11701 Photoinduced electron transfer from Zn3PN to PI with 416 nm laser pulses occurs with t = 27 ps however, if a 645 nm laser pulse is used to excite pyr-PMI first, this event is completely inhibited. 

If the electron acceptors are in great excess, one can express the relaxation of the excited state population N (t) during and after arbitrary light excitation, through the survival probability of the excited donors after 8 pulse, R(t) as was done in Eq. (3.5). By substituting Eq. (3.415) into Eq. (3.5), we obtain the following for C pulse ... 

The electron transfer from aromatic radical anions to various electron acceptors takes place efficiently in solution. Likewise, when a second solute, pyrene, is added to the MTHF solution of PVB, the electrons transfer from polymer anions to pyrene occurs [50]. The rate constant determined by pulse radiolysis is approximately a third of that of the electron transfer from biphenyl anion to pyrene. 

The Re(CO)s (3) radical, which may also be generated by several other methods, for example, deposition of Re atoms on a CO matrix at 10 K or Pulse Radiolysis of ReBr(CO)s in ethanol, has the theoretically predicted square-pyramidal structure that is maintained in solution at room temperature. Reactions where (3) may play a crucial role after photochemical generation from (1) include halogen abstraction, disproportionation, and electron transfer to an electron acceptor (EA) (Scheme 2). 

A recent study of Cgo in pulse radiolysis reported a spectral feature at 650 nm, assigned to the radical cation [82], This study apparently did not use a near-IR sensitive detector, so that the strong 980 nm absorption was not observed, and it is possible that this 650 nm absorption is caused by products of radical addition to Ceo- Reaction of triplet Ceo with strong electron acceptors produces an exciplex and the free C o radical cation in benzonitrile [24]. 

One particular example of the use of pulse radiolysis to general chemistry was the work of Miller and co-workers on the rates of electron-transfer reactions. These studies, which were begun using reactants captured in glasses, were able to show the distance dependence of the reaction of the electron with electron acceptors. Further work, where molecular frameworks were able to fix the distance between electron donors and acceptors, showed the dependence of electron-transfer rate on the energetics of the reaction. These studies were the first experimental confirmation of the electron transfer theory of Marcus. 

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