Enhancement, singlet-triplet benzene

A well-known example46,47,141 of such singlet triplet enhancement is provided when the molecule A is benzene and the paramagnetic molecule B is oxygen... 

Striking evidence for the role of the hyperfine interaction on the time dependence of ion-pair recombination was found by Brocklehurst [56, 409]. He ionized a solution of p-terphenyl in decalin (benzene, squalane etc.) with 90Sr (3 particles and observed the time dependence of the fluorescence intensity. When the experiment was repeated, but with a magnetic field (< 0.16 T) applied, the fluorescence intensity was enhanced by approximately 40% after about 10—20 ns. The magnetic field splits the triplet levels so that only T0 can mix with S0 and the initial singlet... 

The radiation induced cis-trans isomerization of 2-butene in benzene, benzene-dr, toluene, and pyridine has been studied. The triplet yield in benzene is 4.7 molecules per 100 e.v. benzene-dr, is similar. The yield in toluene is 6 molecules per 100 e.v. Nitrous oxide reduces the isomerization yield by about 40%. This indicates a G value of about 2 for excited states produced by the recombination of benzene ions with electrons. Xenon and krypton (up to 1M) increase the isomerization yield from 2.4 to 3.1 and 2.9 respectively, whereas argon has no effect. From the effect of xenon, which is attributed in part to enhanced intersystem crossing, it is estimated that the yield of singlet states which do not cross to the triplet in unperturbed benzene solutions is 1.3 molecules per 100 e.v. 

Selective formation of the azirane 11 from the triplet-excited azoalkanes 3 in polar media is rationahzed in terms of solvent stabilization of the more polar transition state for the (i-CC-bond cleavage vs.that for the a-CN-bond scission. This solvent polarity effect, observed for the liquid-phase photolysis of azoalkanes 3, is similar to the photochemical behavior of the azoalkane 3a in the interior of the zeoHtes. The polar zeohte environment enhances the formation of azirane 1 la compared to the solution photochemistry in benzene. In contrast to zeolites, the formation of azirane 11 a, a triplet-state product, is completely suppressed in the crystaUine state and the housane 10a, a singlet-state product, is formed selectively. ... 

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