Singlet-state participation

Guaiacoxyacetoveratrone. Unusual Effects of Solvent, Oxygen and Singlet State Participation", Can. J. Chem., 1991, 69, 104. 

Berinstain, A.B. In Singlet State Participation and Solvent Effects on the Photodegradation of a-guaiacoxyacetoveratrone, a Lignin Model Compound Concordia University, Institute for Co-operative Education, Department of Chemistry and Biochemistry Work Term Report 1990. 

Table I shows that in either dioxane or acetonitrile the quantum yield for degradation of I, is unaffected by the presence of 0.1 M of triplet quencher, either sorbic acid, naphthalene or cyclohexadiene. In ethanol, triplet quenchers reduce < >d from 0.34 to 0.14. Quantum yields for intersystem crossing, as determined by a laser opto-acoustic technique ( ), were 0.36 in ethanol and 0.59 in dioxane. These results agree with our earlier report (3), and indicate that significant reactivity occurs from St of I in protic solvents, and that reaction occurs exclusively from Sx in aprotic solvents. While triplet quenching experiments cannot rigorously exclude participation by short-lived higher triplet states, Palm et al (9) have obtained conclusive evidence from CIDNP experiments for singlet-state participation in a series of aryloxy-acetophenones. Note that the triplet state of I is formed in aprotic solvents, and that in deaerated solutions at room temperature it decays by first-order kinetics with a lifetime of 200 ns (3). Remarkably, despite having lifetimes about 100 times longer than other, differently-substituted, aryloxyacetophenones (the longer lifetimes may...

JA Schmidt, AB Berinstain, E de Rege, C Heitner, LJ Johnston, and JC Scaiano. Photodegradation of the Lignin Model a-guaiacoxyacetoveratrone. Unusual Effects of Solvent, Oxygen, and Singlet State Participation. Cart J. Chem. 69 104—107, 1991. 

Chlorination of aromatic compounds under irradiation has been studied extensively (Wagner, 1969). With benzene, the product is a mixture of stereoisomeric hexachlorocyclohexanes with yields 104pmol.J 1. This certainly points to chain reaction with the initiation either from a dissociation, Cl2 2C1, or from the participation of the first excited singlet state of benzene 0B2u) giving... 

Electron-transfer catalytic cycles with oxygen were also discovered in photochemical reactions with participation of an excited sensibilizer (9,10-dicyanoanthracene [DCNA]) and stilbene. The sensitizer assists an electron transfer from the substrate to oxygen. Oxygen transforms into the superoxide ion. Stilbene turns into benzaldehyde. In the absence of the sensitizer, this reaction does not take place even on photoirradiation (when oxygen exists in the first singlet state). In the singlet state,... 

The participation of the singlet states of dyes such as rose bengal (RB, tetraiodotetrachlorofluorescein, sodium salt) and MB during the sensitized oxygenation of amines was demonstrated by Davidson (40). Fluorescence quenching Stem-Volmer constants were reported for several amines, halide anions, and for 3-carotene and were consistent with charge transfer stabilization of an exciplex quenching intermediate. 

Intramolecular cycloaddition for 5-phenyltricyclo[5.2.1.02i6]deca-4,8-dien-3-one, 102, producing the cage product, 103 has been reported to proceed through an upper singlet state [82] (Scheme 8). The participation of an upper singlet was determined by a comparison of yields obtained using different excitation wave-... 

C S, So-H heat (10 M0 s) ISC inter system crossing T, ->So -t heat (10" -10 s) ISC S, T, -H heat (10-"-10" s). P, Reaction product from the singlet state (intramolecular) P2 (intermolecular) reaction product from the triplet state. The reactions from Sj and T] may also include electron or energy transfer reactions. The arrows in the boxes represent the spin orientation of the electrons in the participating MOs. 

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