Intersystem-crossing processes

Time-resolved fluorescence measurements on unsubstituted thiophene oligomers in solution indicate a sharp increase of the fluorescence quantum yield when the number of thiophene units is increased from two to seven [56, 70] in such experiments, we expect the migration of the excitons towards trapping centers to be minimized due to the finite size of the systems and the absence of interchain effects. The evolution of ( f with chain size has been related to a decrease in nonradiative decay rate /cnr, since the radiative decay constant is observed to be almost unaffected 

We have tried to provide a coherent picture of the ISC processes in oligothiophenes in order to rationalize the trends observed experimentally [43]. As first suggested by Rossi et al. in the case of terthiophene, can be expressed as a sum of two contributions, ki and 2- 

6 Geometric and Electronic Structure and Optical Response of Oligo- and Polythiophenes 

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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... 

It is not unreasonable that the substitution of chlorines onto the acenaphthylene ring should have such a significant influence on the intersystem crossing processes, kd and lsc. It is somewhat surprising, however, that the other processes involving spin-inversion were affected to such a small extent. 

If the intersystem crossing process is efficient at this excitation, then the Norrish type II rearrangement must occur from the triplet state. This is further substantiated by a reduction in loss of tenacity with increasing concentration of triplet state quencher. The reduction in loss of tenacity may be equated with interruptions of the chain scission process(es). 

Table II. Rate Constants for Intersystem-Crossing Processes in Co(II), Fe(II) and Fe(III) Complexes.

The intersystem crossing process has opposite effects on the yields of fluorescence and phosphorescence since it depletes the singlet state and populates the triplet state. It is commonly known that heavy ions, such as iodide and bromide, increase intersystem crossing by spin-orbit coupling.(1617) For proteins, fluorescence can be quenched as phosphorescence yield is enhanced. 8,19) However, although the phosphorescence yield is increased, the lifetime is decreased. This effect arises because spin-orbit coupling, which increases the intersystem crossing rate from 5, to Tt, also increases the conversion rate from T, to S0. 

The ko. A, and Fa parameters obtained for a few alkanes are collected in Table 3. kg is around 10 sec A 10 to 10 sec and Fa 10 to 20 kJ mol h In principle, the decay of excited states may involve Si- Sx-type internal conversion transitions [IC, where Sx is some singlet state that gives the product(s) of chemical decomposition] and Si T -type intersystem crossing processes (ISC). The temperature-independent decay was attributed, on the basis of the size of the rate parameter (ko 10 sec ), to Si T -type intersystem crossing. At the same time the temperature-activated decay with a frequency factor of 10 to 10 sec was attributed to an internal conversion process that takes place by overcoming a barrier of Fa 10-20 kJ mol and leads finally to some... 

Note that in this particular intersystem crossing process the nuclear coordinate Q is less than Q0 in both initial and final states. We call eq. (7-12) nonadiabatic intersystem crossover, since the process initiates on the E(Y) curve and ends up on the (11) curve. We call the process (7-9) an adiabatic intersystem crossover, since it follows curve E ) only. 

This reaction has been observed194 in scattering experiments and is thought to be due to a crossing or close approach of the diatomic potential curves between the reactants N+(3P) + He(1S) and the products N+(5S) + He(1S). Further examples of intersystem crossing processes will be discussed in greater detail in Sections IX-XII. 

Intersystem crossover processes between singlet and triplet states are well known in photochemistry. Intersystem crossing processes have been of importance in interpreting benzene photochemistry. Anticipating that... 

Now, in aromatic hydrocarbons intramolecular skeletal vibrations, rather than C—H vibrations, dominate the vibronic coupling contribution to the term J m = — . Furthermore, intermolecular vibrations will have negligible effect on the coupling of the electronic states of interest. Thus, in the case of internal conversion, where the (relatively large) matrix elements are solely determined by intramolecular vibronic coupling, no appreciable medium effect on the nonradiative lifetime is to be expected. On the other hand, intersystem crossing processes are enhanced by the external heavy atom effect, which leads to a contribution to the electronic coupling term. 

As an example of the effect of level shifts in the crystalline state, as just described, consider the observed rates of radiationless transitions in anthracene.45 The first excited 1BSu of the isolated anthracene molecule is located about 600 cm-1 above the second triplet state. Hence, 8 < vv and the intersystem crossing process is quite rapid at room temperature. The fluorescence quantum yield is about 0.3 for this molecule in the gas phase and in solution. In the crystal the first excited singlet state is red shifted (from the gas level) by about 1880 cm- while the second triplet state is hardly affected, so that in this case the energy gap between those two states increases in the crystal. Then the coupling term, v, is smaller in the crystalline state than in solution, thereby leading to a decrease in the rate of the intersystem crossing. The result is that the fluorescence yield in the crystal is close to unity.40... 

To conclude this section, many systems are complicated by the presence of two (or more) low lying states (as in Figure 6). If these states can intercommunicate, one must add to the kinetic scheme various intersystem crossing processes. Thus for the Crin case, one may need to consider the processes in equation (22), where and bisc are the rate constants for intersystem crossing and for back intersystem crossing, respectively. The kinetic analysis at this point can become rather complex.27-28... 

This article examines the dynamics of spin-equilibrium processes, principally from studies in solutions. The properties of the complexes which are relevant to the dynamics studies are first reviewed. Then the techniques used to observe these rapid processes are described. Some aspects of solid-state dynamics are mentioned. Finally, some implications for the description of intersystem crossing processes in excited states and for spin equilibria in heme proteins are described. 

Spin equilibria are thermal intersystem crossing processes. The ground state and the excited state lie within a few hundred wavenumbers of each other and both are thermally populated. There are two photophysical processes in excited states related to the dynamics of thermal spin equilibria. One is the radiationless deactivation of an excited state to a ground state of different spin multiplicity. The other is intersystem crossing between excited states. 

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