Optical and photoinduced electron transfer in ion pairs of coordination compounds

Abstract

Electrostatic attraction between charged coordination compounds and oppositely charged counter ions in solution leads to ion pairing. Although ion pairs are loosely bound species, their spectroscopic and photochemical properties may markedly differ from those of the individual components. Upon favourable mutual energetic positions of the redox orbitals additional ion pair charge-transfer (IPCT) optical transitions may be observed. The various contributions to the energy of the IPCT band absorption maximum and an increment system for the prediction of the position of these maxima are discussed. Depending on the choice of the components and the irradiation wavelength, ion pairs may undergo energy transfer, photoinduced electron transfer and optical electron transfer respectively. East subsequent thermal reactions have to compete with back electron transfer in order to achieve high yields of permanent photo-redox products. Mechanistic aspects of the product formation are discussed with respect to ion pairs of, for example, cobalt compounds may find use as photoinitiators for microimaging and curing applications, as well as photocatalysts for selective organic synthesis.