Electronic properties intersystem crossing processes

In this chapter we have described the photophysics and photochemistry of C6o/C70 and of fullerene derivatives. On the one hand, C6o and C70 show quite similar photophysical properties. On the other hand, fullerene derivatives show partly different photophysical properties compared to pristine C6o and C70 caused by pertuba-tion of the fullerene s TT-electron system. These properties are influenced by (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in case of multiple adducts by the addition pattern. As shown in the last part of this chapter, photochemical reactions of C60/C70 are very useful to obtain fullerene derivatives. In general, the photoinduced functionalization methods of C60/C70 are based on electron transfer activation leading to radical ions or energy transfer processes either by direct excitation of the fullerenes or the reaction partner. In the latter case, both singlet and triplet species are involved whereas most of the reactions of electronically excited fullerenes proceed via the triplet states due to their efficient intersystem crossing. 

Chou and Chi ° studied the Os + complexes with dbm, quinolinate, diazene and C-linked pyridyl azolates and reported the spectroscopic and dynamic measurements, in combination with theoretical analyses. This work has provided the electronically excited state properties of these complexes, such as the energy gap and nature of the lower lying states, the rate of intersystem crossing, and the efficiency of corresponding radiative decay and non-radiative deactivation processes. 

The DFT/MRCI approach reproduces excitation energies and other spin-independent properties of experimentally known electronic states of pyranthione and dithio-succinimide excellently. As far as phosphorescence lifetimes of dithiosuccinimide are concerned, calculations have not yet been completed. For the T] state of pyranthione we find that phosphorescence and nonradiative decay via intersystem-crossing to the So state are concurrent processes occurring at approximately equal rates in the range of 104 s-1, in good accord with experimental data. The Ti - So radiative transition borrows its intensity from two sources ... 

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