On the Mechanism of Singlet-Triplet interaction

On the Mechanism of Singlet-Triplet Interaction 179 Table 7.6 Radiative and nonradiative decay parameters for phenanthrene [207] and the DAPs. 

A 30 G splitting triplet superimposed on a large singlet is also observed after irradiation of the polymer at 77°K [344]. The contribution of the triplet was greatly enhanced by the presence of electron acceptors [344]. It thus seems clear that the additives markedly affect the mechanism of radiolysis by interaction with the cation radical. Additives with a hi i electron affinity inhibit cation and electron recombination and thus increase the yield of a-pyridyl radicals which are involved in crosslink formation. Additives that have a lower ionization potential than pyridine (9.28 eV) would interact with the polymer radical-cation according to... 

According to this sequence, formation of cis- and tnms-stilbenes is preceded by formation of a magnetosensitive ion radical by a singlet-triplet conversion. This means that spin polarization must be observed in cis- and fram-stilbenes, and the isomerization rate must depend on the intensity of the magnetic field. These predictions were confirmed experimentally (Lyoshina et al. 1980). Hence, the ion radical route for trans/cis conversion is the main one under photoirradiation conditions. Until now, the mechanisms assumed for such processes have involved energy transfer and did not take into account donor-acceptor interaction. This interaction makes the process energetically more favorable. 

The distance between the chromophores Is approximately 21 8 as measured on molecular models and no ground state Interaction could be observed. In dioxane, the singlet energy transfer occurs by a Forster-type mechanism. On the basis of this mechanism, the distance can be calculated from the transfer efficiency. For [16a] a value of 21.211.6 X was found, while for [I6b] one obtained a value of 16.7 1.4 X. This latter discrepancy was explained on the basis of preferential orientation of the chromophores. That this phenomenon Indeed plays an Important role, was proved by Leermakers (59,60) In a series of indole alkaloids. The same author (59) also Illustrated the absence of triplet-triplet transfer at low temperature in compound [16b]. 

---