Ultrafast electron transfer processes

Experiments carried out on various blends with MDMO-PPV PCBM weight ratios ranging from 1 3 to 1 0.5 all displayed the same ultrafast electron transfer process, with a dynamics which was found to be almost independent of concentration. For much lower PCBM concentrations (weight ratios lower than 1 0.05), the formation time of the PA band increases to a few ps and the formation rate becomes a linear function of PCBM concentration. This indicates that, as previously observed [94], at low acceptor concentrations we enter a new regime in which the charge transfer process is mediated by disorder-induced diffusion of the excitations, which migrate until they reach a site favourable for charge transfer. 

Figure 8. Energy-level diagram of ultrafast electron-transfer processes in aqueous sodium chloride solution. Transitions (eV) correspond to experimental spectroscopic data obtained for different test wavelengths. The abscissa represents the appearance and relaxation dynamics of nonequilibrium electronic populations (CTTS ", CTTS, (e hyd) fCl e pairs). The two channels involved in the formation of an s-like ground hydrated electron state (e hyd, c hyd ) (dso reported in the figure. From these data, it is clear that the high excited CTTS state (CTTS ) corresponds to an ultrashort-lived excited state of aqueous chloride ions preceding an electron photodetachment process.

Aqueous ionic solutions represent a paradigm for the study of early branching between ultrafast nonadiabatic and adiabatic electron transfers. The very recent experimental observations of specific counterion effects on electronic dynamics provide direct evidence of complex influences of inhomogeneous ion-ion distributions on ultrafast electron-transfer processes. These microscopic effects are particularly evident in IR electronic traiectories in sodium chloride solution (86, 92). 

Many spectroscopic methods have been employed for the investigation of such systems For example, wide-band, time-resolved, pulsed photoacoustic spectroscopy was employed to study the electron transfer reaction between a triplet magnesium porphyrin and various quinones in polar and nonpolar solvents. Likewise, ultrafast time-resolved anisotropy experiments with [5-(l,4-benzoquinonyl)-10,15,20-triphenylpor-phyrinato]magnesium 16 showed that the photoinduced electron transfer process involving the locally-excited MgP Q state is solvent-independent, while the thermal charge recombination reaction is solvent-dependent . Recently, several examples of quinone-phtha-locyanine systems have also been reported . 

The ultrafast photoreactions in PNS of these proteins take place immediately after conversion from the FC state to vibrationally unrelaxed or only partially relaxed FI state [1-3]. For PYP [1] and Rh [3], the primary process is twisting of the chromophore, which causes the ultrafast fluorescence quenching, in the course of the isomerization, while the primary process for FP [2] is the ultrafast electron transfer leading to the fluorescence quenching reaction in PNS. Thus, in spite of the different molecular structures of PYP, Rh and FP chromophores and different kind of photoinduced reactions, these photoresponsive proteins show ultrafast and highly efficient photoreactions from FI state of similar nature (vibrationally unrelaxed or only partially relaxed), suggesting the supremely important role of the PNS controlling the reactions. 

A microscopic theory for describing ultrafast radiationless transitions in particular for, photo-induced ultrafast radiationless transitions is presented. For this purpose, one example system that well represents the ultrafast radiationless transaction problem is considered. More specifically, bacterial photosynthetic reaction centers (RCs) are investigated for their ultrafast electronic-excitation energy transfer (EET) processes and ultrafast electron transfer (ET) processes. Several applications of the density matrix method are presented for emphasizing that the density matrix method can not only treat the dynamics due to the radiationless transitions but also deal with the population and coherence dynamics. Several rate constants of the radiationless transitions and the analytic estimation methods of those rate... 

The 7i-stacked bases of ds DNA might be expected to provide a better medium for bridge-mediated electron transfer than the sigma bonds of proteins or hydrocarbons. It has in fact been proposed by Turro and Barton [18d] that ultrafast photoinduced electron transfer processes involving intercalated donors and acceptors can occur with little or no distance dependence. According to this paradigm, duplex DNA can function as a molecular wire or r-way . 

Aminocoumarins 101-103 present ultrafast fluorescence quenching by the fluorescence up conversion method when the electron donating partners (aniline or N,N-dimethylaniline) are present as solvents. An intermolecular electron-transfer process between the coumarins and the anilines is indicated to be active in depressing the quenching rate171. 

In addition, mixed-valence intermediates feature prominently in mechanistic approaches aimed at electron-transfer processes, including ultrafast valence exchange and multielectron catalysis (where mixed-valency during stepwise processes is inevitable) The modelling and the classification derived have... 

There now exists a large body of experimental data supporting ultrafast electron transfer from MLCTexcited states to the acceptor states of anatase Ti02.112 Most, but not all, of these studies have focused on the famous N3 dye first prepared by Nazeeruddin, cw-Ru(dcb)2(NCS)2.39 In a recent study, an excitation wavelength dependence of the injection process was time resolved.113 Femtosecond injection was attributed to the singlet state and a slower picosecond process from the thermally equilibrated triplet state. 

A class of futuristic solar cells, often called hot carrier solar cells, seeks to harvest the full energy of solar photons. Such cells would utilize the additional energy content of a blue photon relative to ared one.126 In present-day solar cells, equilibrated carriers are collected and hence all absorbed photons with energy greater than the bandgap contribute equally to the measured efficiency. The realization of such hot carrier solar cells therefore requires electron transfer processes that are competitive with nonradiative decay of molecules or phonon relaxation in solids.126 Literature data indicate that such relaxation occurs on a femtosecond timescale. The ultrafast... 

With the intensive development of ultrafast spectroscopic methods, reaction dynamics can be investigated at the subpicosecond time scale. Femtosecond spectroscopy of liquids and solutions allows the study of sol-vent-cage effects on elementary charge-transfer processes. Recent work on ultrafast electron-transfer channels in aqueous ionic solutions is presented (electron-atom or electron-ion radical pairs, early geminate recombination, and concerted electron-proton transfer) and discussed in the framework of quantum theories on nonequilibrium electronic states. These advances permit us to understand how the statistical density fluctuations of a molecular solvent can assist or impede elementary electron-transfer processes in liquids and solutions. 

Reaction Centers from Photosystem II of Green Plants. As a final example we mention the ultrafast research on the photosynthetic reaction center from photosystem II (PSII) of green plants. The reaction center contains six chlorophylls (Chi) and two pheophytins (Pheo). The absorption bands of all these pigments overlap extensively. Therefore, it is even more difficult to assign the different steps in the electron transfer process. Large uncertainty exists about the transfer time from the primary donor to the pheophytin. For instance, the transfer time was reported to be 3 psec. However, a time constant of 21 psec was reported for the same electron transfer event.A difference in the experimental condi-... 

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