Intersystem Crossing, Phosphorescence, and Delayed Fluorescence

In the related photon counting histogram technique, a histogram of the intensity of fluorescence from individual molecules is collected as molecules diffuse through the focal volume of a confocal microscope [278-280]. If the sample contains a mixture of molecules with different fluorescence properties, the histogram can reveal the relative amplitude of the fluorescence from a single molecule of each class, as well as the number of molecules in each class. 

As we discussed in Chap. 2, the angular momentum associated with an individual electron s spin must have a projection of either hjl (spin state a) or —hjl (spin state P) on the axis of a magnetic field. A molecule in an excited singlet state (spin 

The magnitude of spin-orbit coupling can be evaluated by considering the interaction of the magnetic dipole associated with an electron s spin with the intramolecular electric and magnetic fields, including the magnetic field created by the electron s own orbital motion [26, 281-286]. In some molecules 

One reason that intersystem crossing from an excited singlet state ( Pi) to an excited triplet state C P ) usually occurs more rapidly than phosphorescence or radiationless decay of to the ground state is that the energy gap between the zero-point vibrational levels of f l and usually is much smaller than the gap between and the ground state. Within a related series of molecules, the rates of radiationless intramolecular transitions decrease approximately exponentially with the 0-0 energy difference between the initial and final states (AEoo)- 

Triplet states also can form by back reactions of radical-pair states that are created by photochemical electron transfer. We discuss this process briefly in Box 10.2. 

---