Excitation, electronic exciton states

Weak coupling (U AE, Aw U As) The interaction energy is much lower than the absorption bandwidth but larger than the width of an isolated vibronic level. The electronic excitation in this case is more localized than under strong coupling. Nevertheless, the vibronic excitation is still to be considered as delocalized so that the system can be described in terms of stationary vibronic exciton states. 

FIGURE 15. Typical bisignate exciton coupling CD spectra (upper) and bell-shaped UV (lower) spectra for chromophores with electronic transitions near 300 nm. The shape of the observed CD curve is due to overlapping, oppositely-signed positive and negative CD transitions from electronic excitation into (two) exciton states... 

As is well known, in crystalline solids there may be formed collective electron-excitation states called excitons.8182 Such states are excited only in media with periodic structure and are delocalized over a large volume of atoms (or molecules), their excitation energy being 0.1-0.5 eV lower than the energy of the electron states of isolated molecules that produced them. The nature and spectroscopy of exciton states have been thoroughly studied both experimentally and theoretically. In this section we will... 

Because of periodic symmetry, the electronic excitation states in a molecular crystal are also of collective nature. These are the well-studied exciton states.81 82 Their energy is close to that of discrete electronic states of isolated molecules (4-8 eV), but the excitation envelops a large group of molecules, migrating efficiently up to 100 nm along the crystal.82 In the same manner, because of efficient migration, the excitation of a fragment of a polymer chain rapidly spreads over the whole molecule.37... 

The mixed quantum classical description of EET can be achieved in using Eq. (49) together with the electronic ground-state classical path version of Eq. (50). As already indicated this approach is valid for any ratio between the excitonic coupling and the exciton vibrational interaction. If an ensemble average has been taken appropriately we may also expect the manifestation of electronic excitation energy dissipation and coherence decay, however, always in the limit of an infinite temperature approach. 

According to the size of the system the MD runs have to be carried out in a way not to notice the actual CC excited state (electronic ground state classical path approximation). However, it seems rather reasonable that any back reaction of the actual excited electronic state should be of minor importance since even in the largest studies complexes only singly excited states (single exciton states) are incorporated. 

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