Chapter 6
Charge and Energy Transfer Processes
Abstract In this chapter we shall present the peculiar features of charge and excitation energy transfer processes (CT and ET) that are of basic importance in photosynthesis, photovoltaics, and other areas of biochemistry and technology. The migration
of charge or excitation energy between distinct chromophores implies a dramatic
change in the electronic wavefunction, so the general nonadiabatic theory we have
already discussed also applies to these processes. However, some peculiar features
distinguish charge and energy transfer from other nonadiabatic processes. If the two
chromophores are placed in two molecules free to move in gas or liquid phase,
the transition can only take place during a collision or encounter, so the kinetics of
bimolecular processes plays an essential role. However, just because their interaction
is a basic requirement for the process to occur, in structured biological or artificial
photosystems the single units are fixed at suitable relative positions and orientations.
In typical situations, such arrangements also determine easily discernable spectral
features. Whenever the interaction between the involved chromophores is not too
large, the initial and final electronic states of the CT or ET process constitute a physically sound diabatic representation, which allows to analyze theoretically the main
features of the dynamics.
Keywords Charge transfer · Energy transfer · Quenching · Sensitization
Exciton coupling
6.1 Gas-Phase Collisions
The most relevant bimolecular processes in photochemistry are reactions, excitation
energy transfers (ET), and charge transfers (CT). For a bimolecular event to occur,
the two partners must get close enough; i.e., we need what is called a collision in gas
phase or an encounter in liquid phase.
In gas phase, we can take as reference a system made of rigid spheres. The kinetic
theory of gases provides the number of collisions per unit time and unit volume
between identical spheres:
N coll = 8
π K B T
M X
1/2
R
2
X C
2
X
(6.1)
© Springer International Publishing AG, part of Springer Nature 2018
M. Persico and G. Granucci, Photochemistry, Theoretical Chemistry
and Computational Modelling, https://doi.org/10.1007/978-3-319-89972-5_6
179
Charge and Energy Transfer Processes
Abstract In this chapter we shall present the peculiar features of charge and excitation energy transfer processes (CT and ET) that are of basic importance in photosynthesis, photovoltaics, and other areas of biochemistry and technology. The migration
of charge or excitation energy between distinct chromophores implies a dramatic
change in the electronic wavefunction, so the general nonadiabatic theory we have
already discussed also applies to these processes. However, some peculiar features
distinguish charge and energy transfer from other nonadiabatic processes. If the two
chromophores are placed in two molecules free to move in gas or liquid phase,
the transition can only take place during a collision or encounter, so the kinetics of
bimolecular processes plays an essential role. However, just because their interaction
is a basic requirement for the process to occur, in structured biological or artificial
photosystems the single units are fixed at suitable relative positions and orientations.
In typical situations, such arrangements also determine easily discernable spectral
features. Whenever the interaction between the involved chromophores is not too
large, the initial and final electronic states of the CT or ET process constitute a physically sound diabatic representation, which allows to analyze theoretically the main
features of the dynamics.
Keywords Charge transfer · Energy transfer · Quenching · Sensitization
Exciton coupling
6.1 Gas-Phase Collisions
The most relevant bimolecular processes in photochemistry are reactions, excitation
energy transfers (ET), and charge transfers (CT). For a bimolecular event to occur,
the two partners must get close enough; i.e., we need what is called a collision in gas
phase or an encounter in liquid phase.
In gas phase, we can take as reference a system made of rigid spheres. The kinetic
theory of gases provides the number of collisions per unit time and unit volume
between identical spheres:
N coll = 8
π K B T
M X
1/2
R
2
X C
2
X
(6.1)
© Springer International Publishing AG, part of Springer Nature 2018
M. Persico and G. Granucci, Photochemistry, Theoretical Chemistry
and Computational Modelling, https://doi.org/10.1007/978-3-319-89972-5_6
179