Intersystem process

The improved order in films cast from toluene may also be induced by prolonged illumination at low temperatures, as recently discovered at the Technion [155]. Delocalized photoexcitations among adjacent chains may lead to several characteristic properties that are not common in isolated chains. These are [71] (i) reduced PL quantum efficiency, (ii) PL redshift, (iii) relatively large generation of PP excitations, rather than intrachain excitons, (iv) more substantial delayed PL due to PP recombination, (v) reduced formation efficiency of triplet excitons since the intersystem process is hampered by the interchain delocalization [156], and (vi) increased exciton dissociation efficiency in Ceo" doped polymer films. It is thus important to review the ultrafast excitation dynamics in MEH-PPV in comparison with those of DOO-PPV. 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

The natural processes of intersystem crossing and internal conversion will quickly (e.g. 50 ns) carry the molecule from this excited electronic surface to the ground electronic surface without a collision,... 

FIGURE 7.4 Modified Jablonski diagram showing transitions between excited states and the ground state. Radiative processes are shown by straight lines, radiationless processes by wavy lines. IC = internal conversion ISC = intersystem crossing, vc = vibrational cascade hvf = fluorescence hVp = phosphorescence. 

Thus if one starts with one pure isomer of a substance, this isomer can undergo first-order transitions to other forms, and in turn these other forms can undergo transitions among themselves, and eventually an equilibrium mixture of different isomers will be generated. The transitions between atomic and molecular excited states and their ground states are also mostly first-order processes. This holds both for radiative decays, such as fluorescence and phosphorescence, and for nonradiative processes, such as internal conversions and intersystem crossings. We shall look at an example of this later in Chapter 9. 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

Equation (49) applies to both the forward and reverse rate constant, /clh and Ichl- Consequently, the thermodynamic parameters for the intersystem crossing process are related according to ... 

An alternative mechanism that has been suggested [93, 118] for the intersystem crossing process is based on a twist movement of the octahedral ligand arrangement. Two modes designated by M 3 and Ml and illustrated in Fig. 13... 

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