Photoinduced charge separation. Dipoles, exciplexes and ion pairs

Depending on the system, the extent of photoinduced charge separation varies from intramolecular to intermolecular and further to ion-radical pairs of arbitrary separation. Transient dc conductivity techniques allow study of all these stages in photoinduced charge separation with subnanosecond time resolution. In the simplest case, photoexcitation of a molecule can result in intramolecular charge transfer, i. e. the creation of an excited state dipole moment, which exists only until the molecule relaxes to the ground state. In the case of an intermolecular charge transfer, there is a nonzero probability for the charges to dissociate and create free ion radicals. The larger the initial charge separation the greater is their probability of dissociation. The probability increases even more in the case of photoionization where the photoelectron thermalizes at a distance from the parent cation, creating the so called geminate ion radical pair.