Formation of radical-ions and their reaction with monomers

1 Formation of radical ions and their reaction with monomers 

With a donor molecule of low ionization energy and an acceptor of high electron affinity, electron transfer may occur even in the ground state of the complex. When the differences in electron affinity are not pronounced, a structure without bond predominates in the ground state. Electron transfer can be considerably boosted by excitation, by a photon for example. A strong polarization of the donor-acceptor complex occurs [292], leading to its dissociation to radical ions in a solvent of suitable polarity. When a convenient monomer (e. g. A-vinylcarbazole [293], styrene [294], a-methylsty-rene [295], etc.) acts as donor, it can be polymerized by the generated radical ions. 

Absorption of a quantum by a DA complex in the ground state is not the only way of generating an excited donor-acceptor complex (called exciplex). According to Weller [296] and Led with [297] in can also be formed by interaction of an excited component of the complex with a non-excited 

A molecule in the excited state is a better electron donor and a better electron acceptor than an unexcited molecule [292, 298]. The occurrence of exciplexes generated by donor-acceptor interaction of a photoexcited component is frequent. The exciplex thus formed is not necessarily identical to the exciplex formed by excitation of DA complex from its ground state [293]. The relations between DA complexes, excited DA complexes, and exciplexes (from the same components) are illustrated in the scheme [297] 

Here (D, A)SK, (D, A )SK and (D, A)SK are used to designate a collision donor-acceptor complex, (D+, A )FC the photoexcited donor-acceptor complex (of the Franck-Condon type), and (D + A )s a radical ion pair where both components are separated by solvent. The frequency v, of the exciting radiation is generally different for the donors, D, complexes, DA, and acceptors, A. Each step in scheme (79) is probably reversible, and the position and rate equilibruim establishment (of which the solvation and desolvation processes are the slowest) determine the rate and effectiveness of product formation. A review of donor-acceptor interactions and of the properties of DA bonds was published, for example, in refs. [299 and 300] and in the original communication [301], 

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