Intersystem crossing , substituent effect

Finally, in many of the perturbation calculations of the effect of substituents and other structural changes, an important tacit assumption is made and it is far from obvious that it is always fulfilled. As already discussed, the physical argument on which the calculation is based is that the value of the initial slope, or the height of a small barrier along the way, determine the rate at which the photochemical reaction occurs. However, the experimental value with which comparison is made usually is not the reaction rate but the quantum yield, which of course also depends on rates of other competing processes and these may be affected by substitution as well. For instance, the rate at which fluorescence occurs is related to the absorption intensity of the first transition, the rate of intersystem crossing may be affected by introduction of heavy atoms... 

Generation of 78 by thermolysis or photolysis of a diazoalkane or diazirine precursor, however, affords the singlet carbene, whose 1,2-H shift to ethene is opposed by a barrier of only 0.678 to 1.298 kcal/mol. Consequently, even in cryogenic matrices, singlet 78 rearranges more rapidly than it intersystem crosses to the triplet, which has therefore not been detected by UV or ESR in either an Ar matrix at 8 K or a Xe matrix at 15 K." The lifetime of singlet 78 at ambient temperature has been estimated at <0.5 ns.89,98b (Note the enormous spectator substituent effect of Cl the lifetime of MeCCl is 740 ns,60 at least 1500 times longer than that of MeCH.)... 

In general, the presence of heavy atoms as substituents of aromatic molecules (e.g. Br, I) results in fluorescence quenching (internal heavy atom effect) because of the increased probability of intersystem crossing. In fact, intersystem crossing is favored by spin-orbit coupling whose efficiency has a Z4 dependence (Z is the atomic number). Table 3.3 exemplifies this effect. 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

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... 

Substituent effects on intersystem crossing 269 Cyclization to azirines 273... 

We could not study the effect of the strong rr-donor para-methoxy and dimethylamino substituents, or the iodine substituent on this reaction, because of rapid intersystem crossing at all temperatures. ... 

Adams and Cherry (78) have investigated the effects of stilbene substitution on the behavior of their excited complexes with fumaronitrile and find that the rate constants for fluorescence and nonradiative decay are insensitive to substitution, but that the rate constant for intersystem crossing is increased by electron-donating substituents (lower stilbene oxidation potential). This trend is attributed to a decrease in the energy gap between the excited complex and locally excited 3t (Fig. 4). The observed energy gap dependence of the exciplex lifetime could also account for the absence of fluorescence (or cycloadduct formation, see Section IV-B) from the excited charge-transfer complexes of t-1 with stronger electron acceptors such as maleic anhydride (76) or tetracyanoethylene (85). 

As can be seen from Table 11 all carbena-cyclopentadienes (2) add stereospecifically as singlets (S). The only exceptions are tetrachloro-cyclopentadienylidene (2g) and the fluorenyhdenes 2c, 2d). The nonstereospecific addition — which is of the same order as for diphenyl-carbene — in 2g is most probably due to a heavy-atom effect of chlorine favouring intersystem crossing to the triplet (T). An aromatic ring as in 2c and 2d (as well as Br substituents) also facilitates the S—T inter con version step. 

The singlet excited state photochemistry of cyclopropenes is dominated by ring cleavage reactions (Section IV.B.2), but in the triplet manifold [2 -I- 2] cycloaddition is the major pathway. Because of inefficient intersystem crossing the first excited triplet state of a cyclopropene is best populated by sensitization. Despite the added energy involved with these species there is no effective pathway available for ring cleavage in the absence of added olefin stereospecific dimerization to a cis-transoid-cis-tricyclohexane, e.g. 145, occurs. The smallest of the C(3) substituents is endo in the product ". A stepwise... 

However, flash photolysis and substituent-effect studies indicate that these complexes are orders of magnitude more photoactive when the lowest-energy ES of the complex is LF in character Complexes with low-energy MLCT states show little photolability. The photochemistry for these complexes can be explained by an ES model where initial MLCT excitation is followed by rapid internal conversion-intersystem crossing to the lowest energy ES, and the nature of the ES determines the overall photoactivity . ... 

Oxygen was also found to complicate these studies. Under freeze-pump-thaw (FFT) conditions there were fewer side reactions and a consistent (albeit minor) o behavior for the meta substituents. Simple degassing was not sufficient to avoid oxidized products that clouded the substituent effects. This observation may imply that intersystem crossing has a significant impact on the product distribution of benzyl photocleavages. 

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