ROLE OF SOLVATION IN THE ELECTROGENERATED CHEMILUMINESCENCE OF HEXANUCLEAR MOLYBDENUM CLUSTER ION

The solvent dependence of the electrogenerated chemiluminescence (ecl) of the hexanuclear cluster complex Mo6Cl14(2-) has been investigated. Overall ecl quantum yields and relative production yields of electronically excited Mo6Cl14(2-) ion have been measured for the annihilation of Mo6Cl14- by 4-cyano-N-methylpyridine (CMP) and Mo6Cl14(3-) in acetone (Ac), acetonitrile (AcN), propionitrile (PrN), butyronitrile (BuN), dichloromethane (DCM), and 1,2-dichloroethane (DCE). Partitioning of the electrochemical excitation energy between excited- and ground-state reaction channels can be described by electron-transfer models. A striking result to emerge is that the formation yield of Mo6Cl14(2-*) does not correlate explicitly with the solvent's dielectric constant but rather parallels the kinetic diameter of the solvent. This result suggests that electron transfer occurs to Mo6Cl14- encapsulated in a solvent shell for polar solvents such as Ac, AcN, PrN, and BuN. Conversely, despite a significant difference in the kinetic diameters of the more weakly coordinating solvents DCM and DCE, similar excited-state formation yields imply that the solvent shell about the Mo6Cl14- ion is not retained during electron-transfer annihilation. Our observations, which are in accordance with simple ion-dipole models, establish the utility of chemiluminescence as a sensitive measure of specific solvation of reactants participating in bimolecular electron-transfer events.