Photoinduced Electron Transfer in Donor-Acceptor Systems

5 Photoinduced Electron Transfer in Donor—Acceptor Systems 

Interdisciplinary research on charge transfer processes has been going on for a long time. The general outline of an intra- or intermolecular photoinduced electron transfer in a donor-acceptor composite can be divided into steps for a clearer understanding [72], Here the letters D and A denote charge donor and acceptor, respectively, and 1 and 3 indicate whether the excited state is a singlet or triplet. 

At each intermediate step, the process can relax back to the ground state by releasing energy in the form of emitted radiation or heat. In step 3, the symbol 5 is introduced. It denotes the fraction of charge transferred, continuously in the range between 0 6 1, where 6 — 1 is the state where the whole electron has been transferred. For the formation of the ion radical pair in step 4, certain conditions must be fulfilled  

After a photon has excited the conjugated polymer to form an exciton, the C60 accepts one electron due to its high electron affinity and establishes the anion Cg0. What is left on the polymer chain is a cation radical, i.e., a positive polaron, as depicted in Fig. 1.14, which is a mobile charge carrier that can move along the polymer backbone. This transfer is an exothermal reaction, where energy from the system is released. 

In many cases it will be seen, that the simple five-step scheme given above is not sufficient to describe more complex systems, i.e., donor-acceptor couples in solution or covalently linked donor acceptor couples. More sophisti- 

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