Critical reviews in solid state and materials sciences
Paging:
Volume 3 (1973) nr. 3 pages 243-271
Year:
1973-04-01
Contents:
The electronic states generally characterize solids as insulators, semiconductors, or metals,1 but in some respects organic molecular crystals are an exception to ths scheme. To describe the lower energy states of these solids, the concepts of a valence band and a conduction band are not very useful because the electrons are hghly localized. These states are described better by the Frenkel tight-binding model in which molecular states are the progenitors of crystal states.2 Generally at higher energies, crystal states not present in the free molecule are expected.3 These states, the ionic states, consist of an electron and an ionized molecule. Such states may be similar to the exciton and conducting states of other solids or may be somewhat unique to the molecular solid, e.g., ion pair states in which an electron and hole are localized on neighboring lattice sites. In spite of the apparent importance of the ionic states in understanding electron and energy transfer phenomena in molecular aggregates4, little direct information on these crystal states has been obtained.
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