Selection rules for intersystem crossing

According to the selection rules for intersystem crossing known as El Sayed s rules (El Sayed, 1963) transitions... 

The selection rules for intersystem crossing were treated theoretically by van der Waals et al [26] for selected molecules in detail. Figure 111 shows the particularly simple example of the energetically lowest electronic states of quinoxaline. The spin-orbit coupling mixes two pairs of singlet and triplet states in such a way that both contributions select the triplet component Tz) as the radiative (phosphorescing) component. 

The mechanism of ONP is based on the hyperfine coupling and on the selection rules for intersystem crossing. In detail, it is complicated. We refer the reader to the relevant literature, which is cited in the article by Hausser and Wolf [29]. 

The selection rules for intersystem crossing (ISC) were given in Section 9.2. If a higher triplet is produced, it too will obey Kasha s Rule and relax to Ti. Since its two modes of relaxation to So, phosporescence and ISC, both involve spin-inversion, Ti will be comparatively long-lived. Except when trapped at very low temperatures, there is ample time for the three components of the triplet T , and to reach equilibrium before it phosphoresces, relaxes by ISC or reacts chemically. 

The magnitude of 1kisc is governed by El-Sayed s selection rules that is, the rate of intersystem crossing from the lowest singlet state to the triplet manifold is relatively large when the transition involves a change of orbital type. For example ... 

In low-temperature solids, the spin-polarization of the lowest triplet state has often been observed at a zero field for many molecules [13]. This phenomena occurs through the selection rule in the intersystem crossing (ISC) from the lowest excited singlet state (S ) to T , as shown in Fig. 5-3. Here, the rate constant of the ISC (A isc) can be given by the spin-orbit interaction (Hso) as follows ... 

Path 2 Nonradiative crossing to Ti followed by rapid vibrational equilibration to Ti. This is followed by a radiative transition Ti Sq. The radiation emitted is called phosphorescence. The nonradiative intersystem crossing (ISC) is much slower than the vibrational equilibrations, but competes with the fluorescent emission in the molecules that exhibit phosphorescence. The radiative transition, Ti So, is usually very slow since the triplet-singlet transition is spin-forbidden by the selection rules. Consequently, the phosphorescence persists for some time after the exciting radiation is turned off. 

The factor that most strongly influences allowedness of a transition is spin. Transitions between states with different spins, such as singlet and triplet, are very inefficient. Direct So to T1 absorption is rarely important, and the reverse emissive process, phosphorescence, occurs quite slowly. The heavy atom effect can relax this selection rule through a spin-orbit effect. As a result, S-T interconversions are much more facile in molecules that contain atoms such as Br or 1. The second major factor that influences the efficiency of transitions is the general spatial overlap of the wavefunctions for the two states. This term favors it,it transitions over ,ir transitions, for reasons discussed above. Another important factor is a general energy gap law. For processes such as intersystem crossing, the smaller the gap between the... 

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