S - T Intersystem Crossing

If excited states underwent only reactions which produce readily measurable effects, determination of intersystem crossing yields would be straightforward. For example, the earliest attempts to estimate 

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of 

Wilkinson has recently described a novel approach.113 It has long been known that solvents containing heavy atoms markedly quench the fluorescence of aromatic hydrocarbons, and it has been shown that this effect arises from an enhancement of the rate of intersystem crossing. Thus the ratio of phosphorescence to fluorescence for naphthalene irradiated at 77°K can be increased more than a hundredfold upon addition of some isopropyl iodide to the solvent.114 The same effect has been noted upon changing from hydrocarbon glasses to frozen krypton and xenon matrices.115 Wilkinson found that the decrease in fluorescence intensity from irradiated solutions of anthracene and some of its derivatives upon addition of bromobenzene is attended by an increase in T-T absorption intensity.116 The fluorescence quenching follows the Stern-Volmer law  

Straightforward kinetic analysis reveals that the exact relation between the relative fluorescence decrease and the relative T-T absorption increase depends on d lsc, which can then be extracted from data obtained at various quencher concentrations.113 

Such intermolecular heavy atom effects on S - T transitions are 

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The failure to observe photosubstitution in the presence of a sensitizer in which the latter is the principal absorber, the invariance of product quantum yield with wavelengths shorter than 350 nm (onset of n -> -n absorption), and the observation that chloride and bromide ions (known to catalyze S-+T intersystem crossing) strongly diminish the quantum yields of these reactions, strongly points to the lowest excited ir- n singlet state as the reactive species in these transformations. Excitation into the n->ir absorption band results in little product formation. A triplet state may, however, be involved in the photoamination of nitrobenzene.a41)... 

The dilution technique makes use of the different concentration dependence of a) S-T-intersystem crossing, and b) [1 -f-2]-cyclo-addition of a carbene to an olefin. The decay of the metastable singlet state is monomolecular, while the stereospecific addition is of the first order with respect to the concentration At high dilution with an inert solvent such as hexafluorobenzene or octafluoro-cyclobutane etc., the same cis-/trans-cyclopropane ratio should be obtained with cis- or trans-olefin as the starting compound. 

S,- T, intersystem crossing and therefore enhances the formation of diazacy-clooctatetraene on direct irradiation. 

Figure 16. Excitation energy dependence of nonradiative decay rates in vapor-phase pyridine, pyrazine, and pyrimidine. The dashed lines represent S, - T intersystem crossing, whereas the solid lines represent the second nonradiative process attributed to photochemical reaction. (From ref. [11] with permission.)...

In contrast, the direct 185-nm excitation of 48 (Table 7 entry 5) preferably produces NBD, which indicates an ineffective S- T intersystem crossing, as is the case in the 185-nm photolysis of 49 (Table 7 entry 10). The thermolysis of both azoalkanes 48 and 49 was reported to yield NBD as the main product (Table 7 entries 1,6). Therefore, the 185-nm photodenitrogenation of 48 and 49 results from higher excited singlet states (Scheme 11). Furthermore, fast internal conversion (ca 10" s) may lead according to Kasha s rule to the population of vibrationally excited states which exhibit enough energy to overcome the small activation barrier (12-30 kJ mol ) of the N2 elimination from Si. 

The high spin-orbit coupling constant of the platinum nucleus (x = 4,481 cm ) should facilitate both the S T intersystem crossing process and the T S radiative decay. However, the extent to which it does so in a complex will depend upon the contribution of metal atomic orbitals to the excited state. In many simple Pt(II) complexes with relatively small hgands, the metal s involvement is such that triplet state formation is very fast, of the order of 10 s [7]. Since this greatly exceeds typical singlet radiative rate constants of aromatic ligands, emissimi is then... 

Substituent effects on Reaction (178) are quite informative. It was found, for example, that the reaction cannot be detected for X = CH3CO, NO2, or (CH3)2N. It appears that for these substituents the S state of the m-stilbene isomer does not have an electronic distribution which is suitable for cyclization. The change in electronic distribution is most easily visualized in the case of the acetyl substituent. The acetylstUbenes would be expected to have S states which are of the n- -nr rather than of the tt —> tt type. Since bromostilbenes undergo photocyclization despite the enhanced rates of S — T intersystem crossing in these molecules, it may be concluded that the rate of cyclization is extremely fast provided the process is favored by the electron density distribution of the lowest excited singlet state of the cis isomer. 

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