Intersystem crossing, in aromatic

Intersystem crossing in aromatic ketones is very efficient, so that their photochemistry is dominated by the triplet-state processes. The efficient formation of triplet states, the small singlet-triplet energy and the accessible long-wavelength absorption make the aryl ketones excellent triplet sensitisers. 

An alternate explanation for the rapid rate of intersystem crossing in aromatic ketones is that 17,17 triplet levels lie between the singlets and the ,77 triplets,369 and Kearns has interpreted some T S absorption spectra as indicating two such low-lying triplets.105 However, even if two triplets do lie beneath the lowest singlet, the chemical reactivity of the lowest triplet would not be expected to change unless the two triplets were of mixed character. 

Figure 5.5. Relationship between the energy gap AE(T, - Sg) and the logarithm of the rate constant kj of intersystem crossing in aromatic hydrocarbons (data from Birks. 1970).

The theory of intersystem crossing in aromatic molecules has been developed in a series of investigations [90,205,220-227]. The mechanism of intersystem crossing is here more complicated than in the case considered previously for aromatic molecules that contain heteroatoms with n electrons [221]. One of the reasons for this is that spin-orbit coupling in aromatic hydrocarbons is generally small compared to that in... 

Siebrand, W. (1970) Mechanisms of intersystem crossing in aromatic hydrocarbons. Chem. Phys. Let, 6, 192. 

Langan, J.G., Sitzmann, E.V., and Eisenthal, K.B., Picosecond Laser Studies on the Effect of Structure and Environment on Intersystem Crossing in Aromatic Carbenes, Chem. Phys. Lett., 110,521, 1984. 

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. 

Intersystem crossing from Si to and higher triplet states has also been reported in some aromatic molecules Li, R. Lim, E.C. Chem. Phys., 1972, 57, 605 Sharf, B. Silbey, R. 

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