Spin conversion triplet recombination

The border between the irreversible and reversible ionization lies at a AG, value where the rate of the backward transfer to the ground state equals the rate of reverse transfer to the excited state kb(AG°) = kc(AG°). As a rule AG° is negative and not small, so that the border (indicated by the dashed line in Fig. 3.47) is far below the resonance reached at AG, = 0. To shift it up another channel of charge, recombination should be opened and its rate must be faster than kc. This ionic reaction may be parallel to that included in scheme (3.90) or recombination through the triplet channel proposed in Refs. 107 and 150. The latter is discussed in Section XI among other reactions affected by the spin conversion. 

The only difference from the single-channel EM outlined above (Section V.A) is the substitution of k et by the sum of the spin-allowed and spin-forbidden transfer rates k et + k c, to the ground and triplet states, respectively. Like k-eh the intersystem crossing rate kKC does not depend on viscosity. Moreover, EM does not separate the two different steps of the forbidden transition spin conversion to the triplet RIP and subsequent allowed electron transfer into the triplet product [212-216]. However, as has been shown in Section XI.A, even in the case of a single channel but spin-forbidden reaction (I), one should discriminate between the spin conversion and subsequent recombination through electron transfer. The qualitative difference between the spin-allowed and... 

A similar result was expected in Ref. 221 for cps(c>) = 1 — cp(cr), but the obtained difference between the recombination rates in the opposite limits was half as much kc for the slowest conversion and kj2 for the fastest one. This is because the isotropic Ag mechanism determining the spin conversion in Ref. 221 mixes the singlet with the 7b) sublevel only. In the rate approximation one can easily get the same, assuming that the spin transitions between the singlet and triplet RIPs occurs with equal rates in the forward and backward directions as in Eq. (3.585b). However, the transition from the slow to the fast conversion limit resulting from the rate approximation differs somehow from that obtained with the Hamiltonian approach in Ref. 221. 

With the constraint (3.601) at any spin conversion (p(r) is exactly the same as in the equation of the spinless theory, (3.211). This is because the transitions between the singlet and triplet RIPs do not modulate their recombination rates, leaving z = kc/4na as well as product yields, 1 [Pg.318]

Figure 3.76. The effect of spin conversion on /E (a) and

Even more straightforward evidence in favor of recombination to the triplet state was obtained by heavy-atom substitution into the fluorescence quencher, which also enhances the rate of spin conversion. By measuring the transient absorption of both ion radicals and the triplet products of their recombination, it was demonstrated that the quantum yield of triplets increases when the charge separation yield 9(0) decreases as a result of heavy-atom substitution [225]. As was shown in Figure 3.76, triplet state is faster than to the singlet state, while the quantum yield of triplets produced from the singlet precursor only increases with ks. Hence these data also indicate that the triplet channel of recombination is the most efficient. 

As a result of spin conversion, the initial singlet state of this pair changes to a triplet one and thus opens the way for the electron transfer to a triplet product as well. This becomes possible in parallel with the allowed RIP recombination through a singlet channel, to either the excited or ground states, M- or M- as in Section XI.C ... 

As has been shown, the fitting of the linear viscosity dependence of chemiluminescence is completely different if the geminate recombination is considered alone or accompanied by the bulk reaction. In the former case the faster the spin conversion, the better, while in the latter case it can be set to zero provided the rate of electron transfer through the triplet channel is high enough. A similar alternative will be presented in the next section. There the combination of geminate and bulk reaction appears more preferable, especially because the spin conversion carried out by the hyperfine interaction is usually weak. 

The first two represent the reversible ionization of the triplet excitations and accumulation of triplet RIPs. In the absence of the spin conversion, since there is no geminate recombination of triplet RIPs to the ground state these kernels are equal. R describes the recombination of triplet RIPs to the triplet excited states. The last kernel represents the recombination of ions to either the triplet or ground state, in proportion to the equilibrium weights of competing channels. 

The conversion of [D+ A-] from the singlet to triplet state allows the geminate pair to recombine not only in the ground state but also into the excited triplet product, 3D (Fig. 3.80). However, the quantum yield of the latter inspected in Section XI.B is never as large as necessary to make the ionization fully irreversible. This can be possible only if one takes into account the free ion recombination in the bulk. Since these ions meet with uncorrelated spins, 75% of the newly born RIPs appear in the triplet state from where they recombine fast and irreversibly into the same triplet product, 3D. This mechanism allows extending the Rehm-Weller diffusional plateau up to the border between exergonic and endergonic electron transfer at AG, = 0. 

In the encounter RP, the ratio of singlet and triplet spin state populations is 1 3 as shown by reaction (9-12c). The MIE arises almost completely from the T-S conversion of tripler RP. Thus, the recombination probability of peroxy radicals with terminal atoms is higher than that of radicals with terminal 0 or 0 atoms. In the case of polypropylene, the a value of 1.060 0.005 was obtained for O, but 1.015 0.010 for 0. This result certified that (MIE) a(CIE) for reaction (9-12). 

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