Charge effects on the photochemistry of the Co(EDTA)^-·I^- system in the presence of polyammonium macrocyclic receptors

The effects of polyammonium macrocycles on the spectroscopic and photochemical properties of the Co(EDTA)^-·I^- ion-pair have been investigated. The addition of a macrocycle to aqueous solutions containing Co(EDTA)^- and I^- causes an increase of the absorbance in the region of the ion-pair charge-transfer band, as well as an increase of the quantum yield for the intramolecular photooxidation reduction of the ion-pair. Both these effects are mainly, if not only, due to an increase of the association constant between Co(EDTA)^- and I^-, caused by the positive charge of the macrocycle bound to the complex. On the contrary no change was observed on the intrinsic photoreactivity of the excited ion-pair. This last result is discussed in comparison with the effects already observed on the ligand photodissociation of MC excited states of Co(III) cyanide complexes.