Charge time-resolved kinetics

A consistent picture for dynamics of heterogeneous ET has been emerging in the last 5 years with the development of new experimental approaches. Techniques such as AC impedance, modulated and time-resolved spectroscopy, SECM, and photoelectrochemical methods have extended our knowledge of charge-transfer kinetics to a wide range of time scales. This can be exemplified by comparing impedance analysis, which is limited to k of... 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

Most importantly, the careful kinetic analysis of the rise and decay of the transient species in equation (69) shows that the decarboxylation of Ph2C(OH)CO occurs within a few picoseconds (kc c = (2-8) x 1011 s-1). The observation of such ultrafast (decarboxylation) rate constants, which nearly approach those of barrier-free unimolecular reactions, suggests that the advances in time-resolved spectroscopy can be exploited to probe the transition state for C—C bond cleavages via charge-transfer photolysis. 

Clusters, as possible catalytic reactors, are perfectly dispersed in solutions. They are thus suitable systems for observing, under quasi-homogeneous conditions by time-resolved techniques, the kinetics of catalyzed electron transfer, which would be inaccessible on a solid catalyst. It was demonstrated that the reaction of radiation-induced free radicals COT and (CH3)2COH catalyzed by metal clusters started by the storage of electrons on clusters as charge pools and that electrons were then transferred pairwise to water-producing molecular hydrogen [22,75]. 

Electron transfer from I- into the oxidized Ru photosensitizer (cation), or regeneration of the Ru photosensitizer, is one of the primary processes needed to achieve effective charge separation. The kinetics of this reaction has also been investigated by time-resolved laser spectroscopy [48,51]. The electron-transfer rate from I into the Ru(III) cation of the N3 dye was estimated to be 100 nsec... 

Detailed kinetic exploration of the relaxation of the initially formed radical ion pair would probably be of most utility in addressing the question of the degree of charge separation in exciplex or in radical ion pairs. Rentzepis and coworkers have recently found that in strongly interacting donor-acceptor pairs, for example, indene-chloranil, the initially formed ion pair relaxes within a few picoseconds to an equilibrated solvated complex (67). Time-resolved resonance Raman spectroscopy has also been used recently as a kinetic monitor for radical ion reactivity (68). 

In recent years, direct, time-resolved methods have been extensively employed to obtain absolute kinetic data for a wide variety of alkyl radical reactions in the liquid phase, and there is presently a considerable body of data available for alkene addition reactions of a wide variety of radical types [104]. For example, rates of alkene addition reactions of the nucleophilic ferf-butyl radical (with its high-lying SOMO) have been found to correlate with alkene electron affinities (EAs), which provide a measure of the alkene s LUMO energies [105,106]. The data indicate that the reactivity of such nucleophilic radicals is best understood as deriving from a dominant SOMO-LUMO interaction, leading to charge transfer interactions which stabilize the early transition state and lower both the enthalpic and entropic barriers to reaction, with consequent rate increase. A similar recent study of the methyl radical indicated that it also had nucleophilic character, but its nucleophilic behavior is weaker than that expressed by other alkyl radicals [107]. 

If excitation is weak or partner concentration is small, then the free ions are produced in low concentration and their bimolecular recombination is too slow to be seen in the timescale of the geminate reaction. Therefore the kinetics of the latter is often studied separately with a fast time-resolved technique. Alternatively, the free-ion quantum yield found from the initial concentration of ions participating in the slow bimolecular recombination can be used to calculate

charge separation quantum yield tp is the usual subject of numerous investigations. Here we will concentrate only on two of them, where this quantity was studied as a function of not only the recombination free energy but of the solvent viscosity as well. These investigations were carried out on the following systems ... 

Kinetic and Mechanism of the Charge Separation in Ti02 Colloid Nanoparticles as Studied by Time-Resolved EPR Technique. 

The mechanism of the charge separation in the colloidal Ti02 particles was studied at low temperature by direct time-resolved and light-modulated EPR techniques [91]. The recombination kinetics in the nanocrystalline semiconductor particles usually is very fast, on... 

In order to learn about the true quantum efficiency of photogeneration one therefore has to study the photoinduced charge generation mechanism at faster time scales. Pump probe spectroscopy utilising a few optical-cycle laser pulses (5-6 fs) in the visible spectral range with broadband frequency conversion techniques [89] now makes it possible to study extremely fast optically-initiated events with unprecedented time resolution. Such a setup was used to time-resolve the kinetics of the charge transfer process from a polymer chain to a fullerene moiety in thin films of poly[2-methoxy, 5-(3, 7 -dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) and [6,6]-phenyl C6i butyric acid methyl ester (PCBM). Solutions prepared from 1 wt% solutions of toluene on thin quartz substrates were studied. 

Warman JM, Gelinck GH, de Haas MP. (2002) The mobility and relaxation kinetics of charge carriers in molecular materials studied by means of pulse-radiolysis time-resolved microwave conductivity Dialkoxy-substituted phenylene-vinylene polymers. J Phys Condensed Matter 14 9935-9954. 

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