Transient Absorption of Radical Cations

Another way of obtaining information on the time-averaged mobility of charge carriers on their way across a sample of arbitrary thickness is, to simultaneously measure the current and the stationary charge carriers density via transient absorption and to infer die mobility from j = enpF. Such experiments have been performed on blends of conjugated oligomers derived from tm-stilbene amine and a 

Finally the questions will be addressed as to whether or not a transient electroluminescence experiment can yield complementary information on charge carrier mobilities in organic LEDs. Starting from a simplified picture that neglects any delayed injection processes, holes and electrons are injected into the LED from opposite sides and move toward each other. If injection starts at t=0, the fronts of the hole and electron clouds begin to interpenetrate at a time td = d/E(p++pCi. For approaches the hole transit time across the sample, implying that 

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Hole Transfer in DNA by Monitoring the Transient Absorption of Radical Cations of Organic Molecules Conjugated to DNA... 

Kawai K, Majima T (2004) Hole Transfer in DNA by Monitoring the Transient Absorption of Radical Cations of Organic Molecules Conjugated to DNA. 236 117-137 Kee TP, Nixon TD (2003) The Asymmetric Phospho-Aldol Reaction. Past, Present, and Future. 223 45-65... 

Kiyohiko Kawai and Tetsuro Majima, Hole transfer in DNA by monitoring the transient absorption of radical cations of organic molecules conjugated to DNA, Topics in Current Chemistry, 236, 117-137 (2004). 

This is shown by the formation of transient absorptions of both the donor radical cations and the C6o or C70 radical anion [120,125,127,133,139,141,... 

However, the initial step of the electron transfer reaction strongly depends on the solvent polarity. By changing the solvent to less polar or nonpolar solvents like benzene or nonaromatic hydrocarbons the transient absorptions of 3C 0, G)0 and donor radical cation appear immediately after the laser pulse. The decay of all the absorptions is also completed at the same time. The fast appearance and the fast decay of the Go and donor radical cation absorption suggest that there is an interaction between fullerene and donor in less polar and nonpolar solvents before laser irradiation [120,125,133-139],... 

Another spectroscopic technique that should be able to quantify (Po is transient absorption of the photosensitizer radical cation. This was attempted in combination with a peculiar approach to lower the value of the oxidation potential of the photosensitizer, and hence to increase the energy of the HOMO and make HOMO-LUMO gap smaller than the triplet state energy. As shown in Table I, this can be achieved with Zn rather than Pd metalloporphyrins derivatives. In fact, the zinc complex of the bacteriopurpurinimide derivative identified in Fig. 8, ZnBPur, has F t = 1-4 eV, F i/2° = 0.45 V, and F i/2 =-0.81V versus SCE (81), and the following sequence of reactions becomes exothermic in polar solvents... 

The decay of the transient absorption of the carotenoid radical cation at 960 nm was fitted as the sum of two exponentials it is likely that the long-hved component involves bimolecular processes and is only approximated as an exponential. 

As noted above, most kinetic studies of alkene radical cations in solution have focused on aryl-substituted systems since they have convenient optical propenies and have been extensively studied by other techniques. Radical cations are frequently identified on the basis of their characteristic UV-visible absorptions and the comparison of their spectra to those obtained in matrices at low temperature." However, a number of other diagnostic tests are also commonly employed to identify these intermediates. For example, their kinetic behavior as a function of solvent nucleophilicity or added nucleophiles is analogous to that of other electrophilic species. Thus, reaction with nucleophiles such as azide and halide ions provides support for the assignment of a transient to a radical cation, although it will not serve to eliminate a carbocation intermediate. More useful in the latter respect is the method of generation of the transient since PET does not in general lead to the formation of carbocations. Quenching of the observed transient with a more easily oxidized... 

Lemmetyinen and coworkers have reported the mechanistic analysis for photoamination of la with ethylenediamine in the presence of p-DCB in MeCN-HjO (9 1) by laser flash photolysis [57]. During the irradiation, the short-lived transient appeared at 420 nm, followed by the formation of long-lived transient (420 nm) due to the cation radical of la. The short-lived transient absorption is due to the exciplex between la andp-DCB and/or ion pair between the cation radical of la and the anion radical of p-DCB. By the addition of ethylenediamine, the absorption of exciplex was affected but the absorption of the cation radical of la was not affected. Therefore, the nucleophilic attack of the amine occurred on the exciplex between la and p-DCB. They have concluded that nucleophilic attack of the amine to the cation radical of la seems to be impossible. 

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