Intersystem crossing spin-orbit

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. 

The primary step is the absorption of a quantum, which results in an excitation of the Ti-orbital. This singlet orbital can undergo intersystem crossing with spin change to a triplet stage ((t, (t), which has been found in brominated arenes (ref. 29). The photolytic reaction in methanol is much slower compared to hexane (ref. 27). This is shown for 1,2,3,4-TBDD, for which the decay constants ha/e been determined in both solvents ... 

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

Frosch(84,133) have explained the external heavy-atom effect in intersystem crossing by postulating that the singlet and triplet states of the solute, which cannot interact directly, couple with the solvent singlet and triplet states, which themselves are strongly coupled through spin-orbit interaction. Thus the transition integral becomes<134)... 

In addition, it can be shown that second-order vibronic perturbation will make possible some intersystem crossing to the 3B3u(n, tt ) state. However, this second-order perturbation should be much less important than the first-order spin-orbit perturbation.(19) This will produce the unequal population of the spin states shown in Figure 6.1. In the absence of sir the ratios of population densities n are given by the following equations ... 

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

Intersystem crossing is a spin-forbidden process between states of different multiplicity, so the magnitude of the spin-orbit coupling is important in controlling the rate of intersystem crossing. Transitions between... 

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

Intersystem crossing (i.e. crossing from the first singlet excited state Si to the first triplet state Tj) is possible thanks to spin-orbit coupling. The efficiency of this coupling varies with the fourth power of the atomic number, which explains why intersystem crossing is favored by the presence of a heavy atom. Fluorescence quenching by internal heavy atom effect (see Chapter 3) or external heavy atom effect (see Chapter 4) can be explained in this way. 

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