Triplet excitation migrating

In alicyclic hydrocarbon solvents with aromatic solutes, energy transfer (vide infra) is unimportant and probably all excited solute states are formed on neutralization of solute cations with solute anions, which are formed in the first place by charge migration and scavenging in competition with electron solvent-cation recombination. The yields of naphthalene singlet and triplet excited states at 10 mM concentration solution are comparable and increase in the order cyclopentane, cyclohexane, cyclooctane, and decalin as solvents. Further, the yields of these... 

The corresponding photocyclization of S-arylvinyl sulfides to dihydro-thiophens has been shown to proceed via short-lived thiocarbonyl ylide intermediates, formed in turn by cyclization of triplet excited states. 1-Phenylthio-3,4-dihydronaphthalene (49), for example, affords the colored thiocarbonyl ylide (50)44 the isolated products are derived by processes involving hydrogen abstraction and migration. The greater efficiency of the photocyclization of 2-thioaryloxyenones (51) to dihydrothiophens (52) has been attributed to the additional stabilization afforded to the ylide (53) by the carbonyl group.45 Other similar photocyclizations have been reported.46... 

The spin state of the nitrene in the photoi earrangement of the tri-phenylmethyl azides had not been identified with certainty. The authors were in favour of a triplet intermediate, because in the sensitized photoreaction, where the azide was directly promoted to the triplet excited state, the reaction products and the migration aptitudes were the same as in the direct (unsensitized) photolysis. This, however, may only mean that the spin state of the nitrene has no effect on the migratory aptitudes of the substituents. 

H. E. Zimmerman. Thus triplet excited 4,4-diphenylcyclohex-2-en-l-one gives the phenyl migration product 5,6-bicyclo[3.1.0]hexan-2-one in high trans (or endo) diastereoselectivity (> 250 1). A different pathway (named Type A rearrangement) was found for cyclohexadienones (obviously directly connected with the di-7t-methane rearrangement) ... 

Clearly, this is a simplistic view, since there is a considerable variation in 0g among the PS homopolymers. Yet the excimer fluorescence to fluorescence ratio in DMM solutions is very similar from polymer to polymer. A possible alternative, or additional, route to chain scission may involve intramolecular triplet energy migration and transfer processes. It has been shown (J ) that intramolecular triplet migration is facile in both PS and P(S-alt-MMA), but that sequences of two or more MMA-units interrupt the migration process. If an excited triplet trapping site can release its excess energy without bond scission,... 

---