Triplet recombination

Fig. 10. Energy level diagram for the electron transfer reactions of chloranil and TMPD, respectively, with cis- and frans-l-phenylpro-pene [157, 158]. The generation of radical ion pairs is shown as solid lines, whereas singlet and triplet recombinations are depicted as dashed and wavy lines, respectively...

The second group of isomerizations, featuring triplet recombination as a key step, owe their rearrangement to a mismatch of the energy surfaces of the ground and triplet state potential surfaces. They typically require the involvement of two consecutive intermediates, either two radical cations or a radical cation and a triplet state (if the latter is generated by triplet recombination). The rearrangement occurs because there is no unique correspondence between minima on the potential... 

In this context it is useful to remember that the concept of the possible recombination of triplet radical ion pairs is not an ad hoc assumption to rationalize certain Z - E isomerizations, although the CIDNP effects observed during an isomerization reaction played a key role in understanding this mechanism. Triplet recombination has been accepted in several donor-acceptor systems as the mechanism for the generation of fast (optically detected) triplets [169-171], and invoked for several other reaction types [172]. The CIDNP technique is a sensitive tool for the identification of this mechanism, for example, in the geometric isomerization of Z- and E-1,2-diphenylcyclopropane and in the valence isomerization of norbornadiene (vide infra). Most of these systems have in common that the triplet state can decay to more than one minimum on the potential surface of the parent molecule. 

Rearranging Eqs. (3.586), we can give the uniform definitions for the singlet and triplet recombination efficiencies... 

Figure 3.75. The experimental quantum yields of singlet ( ) and triplet ( ) recombination products as well as the charge separation yield (A)- Solid lines—their fitting with UT using the exponential rates for ionization, Wi = fVcexp[—2(r — a)// ], and for recombination to either the ground state, Ws = W, exp[—2(r — a) fin], or the triplet excited state, Wr = W, exp[—2(r — cj)//r]. Parameters obtained from the best fit are the following Wc = 29.12ns 1, Ws = 77ns 1 — 1.21V, l] = 0.81 A, lR = 1.24A, k = 0.75 ns 1, a = 7.5 A. (From Ref. 223.)...

At kt = 1011 M 1 s 1, the theoretical line is almost as straight as in the original experimental works (Fig. 3.79). However, at lower rates of triplet recombination 1/ levels off earlier and at a lower level, making the studied dependence curvilinear. 

This expression is represented by a small intermediate step with height 0.25 seen on the thick line. However, with a further increase in AG, the triplet recombination constant is not as high any more, but falls down to zero, giving way to the alternative recombination channel backward to the excited singlet state. Therefore % rises further on with AG, up to x = 1 ... 

Such a complex behavior of x(AG ) in the presence of the triplet channel is smoothed at faster diffusion when ko is closer to the maximal value of the recombination constants. In this case there is a single step border [dashed line in Fig. 3.81(a)] between the irreversible (to the left) and reversible (to the right) reactions. The position of this border essentially depends on the true height of k,. In Figure 3.81 (Vi) one can see how the border marked by this line is shifted to the right with the increasing rate of triplet recombination. 

Figure 3.82. Rehm-Weller plot for a few systems which differ by the rate of triplet recombination (a) the theoretical curves for k l /ko = 20 0.1 10 s 10 8 () (from right to left)—the remaining parameters are the same as in Figure 3.81(b) (b) interpolation through the experimental points from Figure 3.2 of Ref. 234.

Let us consider the pathways of PET in solution shown in Fig. 18. After formation of a geminate ion-pair, ion dissociation may take place in competition with reversible electron transfer, electron return, triplet recombination, and product-forming reactions. If we disregard product-forming steps and triplet recombination, the quantum yield, [Pg.52]

Table 6-2. Solvent effect on the geminate singlet and triplet recombination yields ( (t)s and (t>r) and the yield of the ion radicals ((()i) for Ks = 6.176 A/ns and Kt = 18.53 A/ns. Relative diffusion coefficients defined by D = D(Py) + D(DMA) and dielectric constants (e) for various solvents are employed by the values at 25 °C. (Reproduced from Ref. [27] by permission from The American Institute of Physics)...

In the context of the potential Cope rearrangement of hexa-1,5-diene radical cations (Section 2.4.1), we mentioned the triplet recombination of radical ion pairs generating a biradical [202, 203]. Because of continuing interest in this type of reaction we briefly mention two additional examples involving radical cationic systems discussed in this review, viz., the isomeric 1,2-diphenylcyclopropane radical cations, cis- and trans- 3 , and norbornadiene radical cation, 91 +. 

Another example of triplet recombination involves the formation of quadricyclane upon electron transfer from norbornadiene to 1-cyanonaphthalene (1-CNN) [226]. Once again, the radical ion pair energy lies above 3 eV, so an intermediate on the... 

N photons present in the driving laser mode. When we include the interaction Vl the triplets recombine into new triplets with eigenvectors (dressed states)... 

V. S. Gladkikh, A. I. Burshtein, G. Angulo, and G. Grampp. Quantum yields of singlet and triplet recombination products of singlet radical ion pairs. Phys. Chem. Chem. Phys., 5(12) 2581-2588, 2003. A. I. Burshtein and A. Y. Sivachenko. Photochemical accumulation and recombination of ion pairs undergoing the singlet-triplet conversion. Chem. Phys., 235(1-3) 257-266, 1998. 

B2) Magnetic interactions, which provide information on the properties of the P+BH radical pair, i.e., its singlet-triplet splitting J, and its triplet recombination rate k. ... 

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