Excitons conjugated chains

The appearance of one or more CT-excitons below the conduction band of the conjugated chain may not appear to be of major importance for the properties of the material. In fact the consequences of the occurrence of excitons are significant. Slater and Shockley (1936) demonstrated that the descriptions of the system by Bloch functions, i.e. the band model, and by localised excitations, i.e. excitons, were related to one another by a unitary transformation. They were also the first to consider the impact of the... 

The initial application of quantum mechanics to the electronic states of a perfect linear conjugated chain, as in the Htickel model discussed in Section 4.2.5 and above, led to a model of a one-dimensional semiconductor with well-defined valence and conduction bands. This labelling of the electronic states is widespread in the literature. On the other hand, when electron correlation is included, the electronic states are more localised and an exciton description is more appropriate. The disorder present in all but a few exceptional cases inevitably leads to the conclusion that the electronic states must be localised by chain defects. The extent to which the electronic states of conjugated polymers are localised, i.e. deviate from the band model, has been a matter of debate. There is a growing body of experimental and theoretical evidence, discussed in Sections 9.4.2 and 9.4.3 below, that suggests that the exciton description is closer to the truth. 

According to a large number of experimental studies, the most stable photogenerated species in the lowest excited states of conjugated chains are electron-hole pairs bound by Coulomb attraction and associated to a local deformation of the backbone, i.e., polaron-excitons [18]. A good insight into the properties of these species can be provided by quantum-chemical calculations our recent theo-... 

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