Excitonic Processes in other Systems

Energy conduction processes by exdtons and various excitonic phenomena also play an important role in other molecular systems besides molecular crystals especially in biological systems. Molecular crystals provide an exemplary field from which a fundamental understanding of these processes can be gained. 

A nice example is the primary photosynthesis process. Here, light energy which is absorbed by the antenna chlorophyll molecules is conducted to a chlorophyll dimer in the reaction centre. Only there do the chemical reactions take place which lead to charge separation and finally to photosynthesis. For more details, cf [M6], Sect. 20.7. 

LH II (outer part) and LH I (inner part) are ordered in a ring-shaped and nearly coplanar arrangement. In the middle of LH I is the reaction centre with the two BChl molecules of the special pair . From [53]. See also [M6], Sect. 20.7, and compare the coloured plate in the appendix. 

In Fig. 6.28, the path of the excitation energy is again shown schematically. The spectroscopic and time-resolved analysis of these systems shows that the Si states of the chlorophyll molecules are in fact split excitonically. Their resonance energy lies at 300 cm for chlorophyll molecules with the mutual orientation shown schematically in Fig. 6.28 these are the B 850 molecules. One can also distinguish between coherent excitons and more-or-less incoherent excitons. 

In polymers, also, excitons are frequently the lowest-lying excited states. They can therefore play an important role for photoprocesses in polymers. This is also true of the key substance of genetics, the DNA helix. Here, again, the excitation energy can be conducted to reactive side groups via excitonic processes. 

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