Photoexcited singlet

Table 5 Reduction potentials of photoexcited (singlet and triplet) electron acceptors .

Quite recently, the reaction of photoexcited (singlet or triplet) silylene centers with dihydrogen has also been studied346. These excited states abstract hydrogen atoms from H2 directly. For the excited singlet state, the process takes place with no discernable barrier. 

An earlier procedure reported from this laboratory for the isolation of sulfides was based on the selective oxidation of the sulfides to the sulfoxides with photoexcited singlet oxygen followed by silica gel chromatographic separation of the sulfoxides (9). Reduction of the isolated sulfoxides back to the sulfides and a subsequent chromatographic purification resulted in the isolation of the sulfides as a pale yellow oil. However, when the sulfur is in a five-membered ring, the photooxidation may lead to side reactions (22). Therefore, other oxidation methods had to be explored. 

CIDNP and CIDEP Studies.—Two mechanisms have been proposed to account for the observation of electron spin polarization in radical reactions (CIDEP), the first being termed the radical pair mechanism, in which the polarization results from the mixing of the singlet and triplet states of the radical pair by the magnetic interactions within the radicals, and the second the triplet mechanism, in which the polarization originates in the triplet as a result of spin-selective intersystem crossing from the photoexcited singlet state, as is known to occur in several systems from ODMR measurements (see above). It has recently been pointed out 480 that, for the latter mechanism, unequal population of the triplet sub-levels will depend upon zero-field D and E terms and Zeeman terms, but also... 

That there is magnetic resonance of only spin 1/2 excitations eliminates most of the theoretical models for the HE band in trans-iCH),. These include singlet excitons [53,541 and breather modes [55,56] for which 5 = 0, as well as triplet excitons [57] for which 5 = 1. Of all the models for the HE photoabsorption band in the literature, only 5 has spin 1/2. Thus, the only viable explanation for the HE band in the photomodulation spectrum is photogenerated 5". However, 5 cannot be directly photogenerated because the photoexcited singlet state cannot directly decay into an pair. There... 

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