PARTITIONING OF THE ELECTROCHEMICAL EXCITATION-ENERGY IN THE ELECTROGENERATED CHEMILUMINESCENCE OF HEXANUCLEAR MOLYBDENUM AND TUNGSTEN CLUSTERS

The electrogenerated chemiluminescence (ecl) of the hexanuclear cluster complexes M6X8Y62−(M = Mo, W; X, Y = Cl, Br, I) in dichloromethane has been investigated. Overall ecl quantum yields and relative production yields of electronically excited Mo6Cl142−and W6X8Y62−(X, Y = Cl, Br, I; X = Cl, Y = Br; X = I, Y = Br) have been measured for the annihilation of Mo6Cl143−and W6X8Y6−. In all cases, equal or nearly equal partitioning of the electrochemical excitation energy is observed. This result suggests that the rates of electron transfer of the trianion Mo6Cl143−and monoanion W6X8Y6−to produce their respective excited states are equal owing to similar electronic coupling and activation barriers for Mo6Cl142−* ↔ Mo6Cl143−and W6X8Y62−* ↔ W6XgY6−exchange. Consistent with partitioning studies are the electron-transfer reactions of Mo6Cl142−* with aromatic amine donors (D) and W6X8Y62−* with aromatic nitro or quinone acceptors (A), to produce Mo6Cl143−and W6X8Y6−, respectively. Slopes of −0.49 and −0.51 for the plots of RT ln kqvs the free energy driving force for Mo6Cl142−*/D and W6X8Y62−*/A electron transfers, respectively, are in the accordance with the predictions of Marcus’ theory. As originally inferred from equal partitioning ratios, analysis of the quenching rates reveals that Mo6Cl142−* ↔ Mo6Cl143−and W6XgY62−* *↔ W6X8Y6−exchange reactions possess similar electron-transfer activation barriers (λ(W6X8Y62−*/W6X8Y6−) = 1.01 eV and λ(Mo6Cl142−*/Mo6Cl143−) = 1.11 eV). These ecl partitioning and quenching results establish that the M6XgY6−and M6X8Y63−electrogenerated intermediates contribute equally to the excited-state production in the ecl reactions of these cluster ions. © 1990, American Chemical Society. All rights reserved.