Intersystem crossing rates

Since Ag is positive and is negative, Q is larger for the p state than for the a state. Radical pairs in the p nuclear spin state will experience a faster intersystem crossing rate than those in the a state with the result that more RPs in the p nuclear spin state will become triplets. The end result is that the scavenging product, which is fonned primarily from triplet RPs, will have an excess of spins in the p state while the recombination product, which is fonned from singlet RPs, will have an excess of a nuclear spin states. 

Because the spin-orbit interaction is anisotropic (there is a directional dependence of the view each electron has of the relevant orbitals), the intersystem crossing rates from. S to each triplet level are different. 

Figure 6.3. Experimentally determined ratio for the intersystem crossing rate constants, (a) Pyrazine,(19) KJKz > 20. (b) Quinoxaline, 25 KJK2 > 10. (c) 2,3-Dichloroquinoxaline,<25) KX/K2 >15.

Since (/iv//av)/(/i//2) can be measured and kx and k2 are just the observed reciprocal decay constants of levels tx and r2, it is possible to determine the ratio of the intersystem crossing rate constants K KX. This information is difficult if not impossible to obtain in other ways. The results for several molecules are shown in Figure 6.3. 

Scheme 8 gives species with extended 109 or starbust -type structures, which are strongly luminescent even with the large ligand L = PCy3.85 The diphenylfluorene derivative shows a remarkable heavy atom effect on the intersystem crossing rate.78... 

H. B. Gray Multiconfiguration SCF calculations by P. J. Hay indicate that the 166 -366 energy separation is over 1 eV, and there is no evidence for intervening states that could provide a facile intersystem pathway. Thus a relatively small singlet triplet intersystem crossing rate constant is not all that peculiar. 

Table 5.1 Effect of deuteration on intersystem crossing rates (k ) and triplet-state lifetimes...

The intersystem crossing process has opposite effects on the yields of fluorescence and phosphorescence since it depletes the singlet state and populates the triplet state. It is commonly known that heavy ions, such as iodide and bromide, increase intersystem crossing by spin-orbit coupling.(1617) For proteins, fluorescence can be quenched as phosphorescence yield is enhanced. 8,19) However, although the phosphorescence yield is increased, the lifetime is decreased. This effect arises because spin-orbit coupling, which increases the intersystem crossing rate from 5, to Tt, also increases the conversion rate from T, to S0. 

If the intersystem crossing rate with diamagnetic iron porphyrins is slow then the observed correspondence in relaxation times between S=0 and S=2 iron(II) porphyrins may not necessarily indicate that the relaxation pathways are the same. 

The rate of intersystem crossing is just as important as its efficiency. Obviously, if the rate of intersystem crossing is faster than that of diffusion in solution (usually on the order of 1010 sec"1), bimolecular reactions of the excited singlet are precluded. Unfortunately, the intersystem crossing rates are available for only a few carbonyl compounds.11,12 It is known that the rate of intersystem crossing for aliphatic carbonyl compounds (e.g., acetone) is slow (4-20 x 107 sec-1)30 in comparison to that for aromatic carbonyl compounds. Thus, aliphatic (and perhaps some aromatic) carbonyl compounds have an opportunity to react in the excited singlet state. 

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

Intersystem crossing rate constants of ortho- and meta-substituted singlet phenylni-trenes are presented in Table 1Mono- and di-o-fluorine substituents have no influence on ISC rate constants.No effect with meta, meta-difluoro substitution is observed either. Pentafluoro substitution has no effect on fcisc in pentane although a modest acceleration is observed in the more polar solvent methylene chloride.i° - i... 

TABLE 11.3. Intersystem Crossing Rate Constants of Ortho and Meta Substituted Phenylnitrenes"... 

Table 3 Intersystem crossing rate constants of ortho- and meM-substituted phenyl-nitrenes.

Unfortunately there are no experimental results which could be compared with these theoretical curves. All experiments reported in the literature are either made with cyanine dyes showing some excited state absorption or with phthalocyanine dyes which have an extremely high intersystem crossing rate so that triplet populations cannot be neglected. 

The situation is quite different with dyes like the phthalocyanines which have very high intersystem crossing rates ksr (e.g- vanadiumphthalocyanine, kSr >... 

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