Radical ion pairs, recombination

Since the values of i/ depend on several factors noted above, in the absence of additional data such as the temperature dependence of the electron transfer rate constants for i-2 it is difficult to analyze the apparent difference between i/ for the charge separation reaction and that of the radical ion pair recombination reaction. However, the difference between these two values of u is not unreasonable given that the charge separation involves oxidation of an excited state of the donor, while radical ion pair recombination involves two ground state radicals. Small changes in the nuclear coordinates of the donor and acceptor for these two reactions should be sufficient to produce the observed difference in i/. The electronic coupling factor between ZnTPP and AQ should be different than that between ZnTPP " and AQ". 

Figure 5. Plot of log for clgree separation,, and log for radical ion pair recombination, j [, vs edge-to-edge distance between the 7T systems of the ZnTPP donor and anthraquinone acceptor in compounds 1-3 in butyronitrile at 21 .

In all covalent electron donor-acceptor systems produced earlier, triplet states observed by EPR were formed via a spin-orbit intersystem crossing (SO-ISC) mechanism. Another possible mechanism of triplet formation is RP-ISC, mentioned above, which results from radical ion pair recombination, and which had been observed previously by time-resolved electron paramagnetic resonance spectroscopy (TREPR) only in bacterial reaction centers and in the green plant Photosystem I and II reaction centers. These two mechanisms can be differentiated by the polarization pattern of the six EPR transitions at the canonical orientations. In SO-ISC,... 

When donor-substituted and acceptor-substituted alkenes complex and suffer a one-electron transfer, the cation radical of the radical cation-radical anion pair may suffer rearrangement at rates competitive with back electron transfer or with radical ion pair recombination. Some vinylcyclopropanes show this phenomenon (127 —> 128 —> 129 —> 130) 2 TCNE and the diphenylbenzocyclobutene (131) react similarly to form (134) by way of (132) and (133).2 ... 

Solute fluorescence is also induced by charge scavenging followed by radical ion pair recombination... 

In a related study. Levin et have examined the relaxation of triplet exciplexes through electron transfer pathways. The magnetic field dependence of the radical ion pair recombination rate constants indicated that these reactions were predominantly of triplet radical ion pairs recombining through electron... 

The most optimal conditions for monitoring quantum beats are in non-polar solvents for the following reasons [7] (1) the separation of the radial pair is usually much less than the Onsager radius and the radical ion pairs recombine geminately (2) a strong luminescence signal can be observed due to the small solvation energy ... 

These secondary radical ion pairs recombine according to the reaction... 

Figure B 1.16.9 shows background-free, pseudo-steady-state CIDNP spectra of the photoreaction of triethylamine with (a) anthroquinone as sensitizer and (b) and (c) xanthone as sensitizer. Details of the pseudo-steady-state CIDNP method are given elsewhere [22]. In trace (a), no signals from the p protons of products 1 (recombination) or 2 (escape) are observed, indicating that the products observed result from the radical ion pair. Traces (b) and (c) illustrate a usefiil feature of pulsed CIDNP net and multiplet effects may be separated on the basis of their radiofrequency (RF) pulse tip angle dependence [21]. Net effects are shown in trace (b) while multiplet effects can...

Whereas other experimental methods have been used to obtain values of kti no other method provides values of k-t or equilibrium data. There are, however, several important limitations of our method. First, the method is restricted to relatively fast hole transport processes that can compete with charge recombination of the Sa -G+ radical ion pair (Fig. 6). This precludes the use of strong acceptors which can oxidize A as well as G (Fig. 2a). We find that hole transport cannot compete with charge recombination in such systems, even when a charge gradient is constructed which should favor hole transport [35]. Second, the method is unable to resolve the dynamics of systems in which return hole transport, k t, is very slow (<104 s-1) or systems in which multiple hole transport processes occur. Third, since the guanine cation radical cannot be detected by transient spectroscopy, the method is dependent upon the analysis of the behavior of Sa-. In section 3.4 we de-... 

The potential of mean force due to the solvent structure around the reactants and equilibrium electrolyte screening can also be included (Chap. 2). Chapter 9, Sect. 4 details the theory of (dynamic) hydro-dynamic repulsion and its application to dilute electrolyte solutions. Not only can coulomb interactions be considered, but also the multipolar interactions, charge-dipole and charge-induced dipole, but these are reserved until Chap. 6—8, and in Chaps. 6 and 7 the problems of germinate radical or ion pair recombination (of species formed by photolysis or high-energy radiolysis) are considered. 

In Chap. 6, Sect. 4, the effect of a magnetic field on the probability and the rate of (neutral) radical radical-pair recombination was discussed. Much of this can be extended to the case of ion-pair recombination. There are three means by which spin correlation may be lost [403]. 

Recombination fluorescence has been used to study the decay of radical ion pairs generated photolytically.288 Simulation of quantum beats caused by hyperftne interaction in the R and R+ enable the values of hfc to be determined for very short-lived species. In the case of one R excellent agreement with the value of hfc as determined by ESR is reported. The primary reaction in the photolysis of 1-arylalkyl radicals (128) is the heterolytic cleavage of the jS-halogen (X), generating the radical cation (129).289... 

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