Singlet production

Postulate (ii) derives from the proposition that in any elementary process a singlet product will be more readily formed from a singlet reactant than from a triplet reactant, since multiplicity changes are of low probability. A triplet reactant could of course be converted to a triplet product but in most cases this is unlikely from purely energetic considerations. Consequently the components of triplet radical pairs tend to separate. 

In the absence of a long-lived intermediate, the time constants for product formation must be the same for the two branches of the parallel reactions giving triplet products or singlet products each appearance time is given by the reciprocal of the sum of the rates for the two parallel channels. As discussed above the appearance time for the production of the singlet... 

For a more in-depth interpretation of the inertness of dioxygen, the fact that 02 is a triplet state bi-radical, i.e. it has two unpaired electrons in the 2jig orbitals, needs to be considered. It follows that the oxidation of singlet state substrates by the triplet 02 to form singlet products is spin-forbidden and, as a consequence, relatively slow. 

Direct irradiation of 130a-c and 130f led to modest yields of 136a-c and 136f. The yields of the amine could be reduced by the addition of the triplet quencher TMDD. Triplet sensitized photolysis led to increased yields of the amine, even in cases in which the reduction product was not found in direct irradiation experiments. " Triplet sensitized photolysis always led to some of the singlet products 132,133, and/or 134. ... 

The rate of singlet product formation from any precursors is... 

These formulas account for the singlet products in either the excited or ground states. Making the same substitution, kf. for kc, we obtain from Eq. (3.595) the contact value of psr ... 

Figure 3.78. The inverse quantum yield of geminate singlet production as a function of z = kD/kt = 4.2/r] at different spin-conversion rates measured by y — KO- k fkt (from Ref. 231). The upper straight line corresponds to y = 105, the intermediate dashed line to y = 10, and the lower dotted line to y = 0. The experimental points are borrowed from Ref. 227.

The geminate contribution to singlet production in this limit is not linear because the inequalities (3.634) do not hold. Regarding the bulk efficiency vv, it decreases with the spin conversion and in the limit x > oo turns to zero (L 1, w —> 0). 

Thermolysis of peroxide [29c] in benzene solution generates a chemiluminescent emission whose spectrum is identical to the fluorescence spectrum of photoexcited p-dimethylaminobenzoic acid under similar conditions. Thus the direct chemiluminescence is attributed to the formation of the singlet excited acid. The yield of directly generated excited acid is reported to be 0.24% (Dixon and Schuster, 1981). Since none of the other peroxybenzoates generate detectable direct chemiluminescence it was not possible to compare this yield to the other peroxides. However, by extrapolation it was concluded that the dimethylamino-substituted peroxide generates excited singlet products at least one thousand times more efficiently than does the peroxyacetate or any of the other peroxybenzoates examined. 

Because ps(0 is a real quantity, we can choose the initial condition as shown by Eq. (15-15). In order to explore limiting cases to discover how large the low field effect could be under the right conditions, Timmel et al, [15] adopted an approximation that the singlet and triplet radical pairs disappear with first-order kinetics with a rate constant of A In this approximation, the singlet product yield (d ) is given by... 

Intersystem crossing to the So surface is believed to occur at the 1,4-biradical stage, and to yield one of the three possible types of singlet product (reactant, olefin, cycloalkanol) depending on the geometry at which it occurs, as discussed in Section 4.3.4. 

Cholesta-3,5-diene and similar transoid dienes undergo a variety of complex photochemically induced reactions, which differ for singlet- and triplet-excited states. Singlet products are derived in part from the very strained 3,5 4,6-bis-cyclo ( bicyclobutane ) intermediates of the type (315), which open by solvent attack to give such products as (316). The singlet-excited diene may alternatively react via... 

---