Conversion and Intersystem Crossing

After excitation has occurred, there are several processes which are important in the deactivation of the excited states. Those discussed in this section will be nonradiative, that is, they do not involve the emission of light. 

Internal conversion is a nonradiative transition between states of like multiplicity (e.g., singlet to singlet, triplet to triplet, but not singlet to triplet)  

The conversion from to 5, is an isoenergetic process that is followed by vibrational relaxation of the new vibrationally hot state. 

A nonradiative transition between states of different multiplicity is called intersystem crossing  

Two important factors that influence the rate of these nonradiative processes are  

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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. 

Thus we see that we have three processes which can compete for deactivation of the excited singlet fluorescence, internal conversion, and intersystem crossing. If we increase the rate of the latter by adding a heavy atom, this should result in a decrease or quenching of the fluorescence intensity ... 

The quantum efficiency of fluorescence of a molecule is decided by the relative rates of fluorescence, internal conversion and intersystem crossing to the triplet state. Up to the present time it has proved impossible to predict these relative rates. Thus, whilst it is now possible to calculate theoretically the wavelengths of maximum absorption and of maximum fluorescence of an organic molecule, it remains impossible to predict which molecular structures will be strong fluorescers. Design of new FBAs still relies on semi-empirical knowledge plus the instinct of the research chemist. 

The possible fate of excitation energy residing in molecules is also shown in Figure 2. The relaxation of the electron to the initial ground state and accompanying emission of radiation results in the fluorescence spectrum - S0) or phosphorescence spectrum (Tx - S0). In addition to the radiative processes, non-radiative photophysical and photochemical processes can also occur. Internal conversion and intersystem crossing are the non-radiative photophysical processes between electronic states of the same spin multiplicity and different spin multiplicities respectively. 

Radiationless transitions (internal conversion and intersystem crossing) between electronic states are isoenergetic processes and are drawn as wavy arrows from the v = 0 level of the initial state to a vibrationally-hot (v > 0) level of the final state. 

Describe the general features of internal conversion and intersystem crossing. 

Understand the importance of the overlap of vibrational probability functions and the energy gap law in determining the rate of internal conversion and intersystem crossing. 

In Chapters 2 and 4, the Franck-Condon factor was used to account for the efficiency of electronic transitions resulting in absorption and radiative transitions. The efficiency of the transitions was envisaged as being related to the extent of overlap between the squares of the vibrational wave functions, /2, of the initial and final states. In a horizontal radiationless transition, the extent of overlap of the /2 functions of the initial and final states is the primary factor controlling the rate of internal conversion and intersystem crossing. 

Thus, the efficiency (rate) of internal conversion and intersystem crossing depends on both electronic and vibrational factors ... 

G. L. Cui and W. Thiel. Generahzed trajectory surface-hopping method for internal conversion and intersystem crossing, J. Chem. Phys., 141 (2014). 

A, 3A and A are molecules in first excited singlet state, molecules in triplet state and in the ground state respectively. In radiationless processes such as internal conversion and intersystem crossing the excess energy is lost to the environment as thermal energy. Some of the unimolecular processes are represented by a Jablonski diagram in Figure 5.1. Radiative transitions me denoted... 

RADIATIONLESS TRANSITIONS—INTERNAL CONVERSION AND INTERSYSTEM CROSSING... 

This expression, it may be recalled, is similar to the one obtained for intramolecular radiationless transfer rate for internal conversion and intersystem crossing (Section 5.2.1). For intermolecular cases... 

This occurs very commonly in photochemistry and is indeed an intrinsic part of well-known phenomena such as internal conversion and intersystem crossing. Theoretical treatments of inorganic photochemistry are beyond the scope of this chapter, however.30... 

Generally, the occurrence of unimolecular radiationless transitions such as internal conversion and intersystem crossing may be inferred from quantum yield measurements. The common experimental observation in such cases is the lack of a net reaction after absorption of a photon. The Franck-Condon principle that implies radiative transitions with quantum yields of less than unity also applies to radiationless processes, as it prohibits vertical transitions between surfaces separated by large energy gaps and favors those at Zero Order surface crossings. 

Much more is becoming known about the rates of the physical processes in competition with proton exchange reactions in excited states. (For an excellent review see Henry and Siebrand, 1973.) The factors which determine the rate constants (k) for internal conversion and intersystem crossing are neatly summarized in the Golden Rule of time-dependent perturbation theory ... 

As an example of application of the method we have considered the case of the acrolein molecule in aqueous solution. We have shown how ASEP/MD permits a unified treatment of the absorption, fluorescence, phosphorescence, internal conversion and intersystem crossing processes. Although, in principle, electrostatic, polarization, dispersion and exchange components of the solute-solvent interaction energy are taken into account, only the firsts two terms are included into the molecular Hamiltonian and, hence, affect the solute wavefunction. Dispersion and exchange components are represented through a Lennard-Jones potential that depends only on the nuclear coordinates. The inclusion of the effect of these components on the solute wavefunction is important in order to understand the solvent effect on the red shift of the bands of absorption spectra of non-polar molecules or the disappearance of... 

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