Chemiluminescence: A mechanistic probe of electron transfer reactions

The chemiluminescent reactivity of M{dollar}sb6{dollar}X{dollar}sb8{dollar}Y{dollar}sb6sp{lcub}2-{rcub}{dollar} (M = Mo, W; X, Y = Cl, Br, I) clusters in nonaqueous solution has been used to investigate the mechanism of electron transfer reactions. The partitioning of the electrochemical excitation energy upon annihilation of electrogenerated Mo{dollar}sb6{dollar}Cl{dollar}sb{lcub}14{rcub}sp{lcub}3-{rcub}{dollar} with a series of W{dollar}sb6{dollar}X{dollar}sb8{dollar}Y{dollar}sb6sp-{dollar} ions has been determined from overall electrogenerated chemiluminescence (ecl) quantum yields and chemiluminescence spectra. The electrochemical excitation energy is partitioned to produce Mo{dollar}sb6{dollar}Cl{dollar}sb{lcub}14{rcub}sp{lcub}2-*{rcub}{dollar} and W{dollar}sb6{dollar}X{dollar}sb8{dollar}Y{dollar}sb6sp{lcub}2-*{rcub}{dollar} with essentially equal probability. Analysis of the equal distribution with current electron-transfer theories suggests that the electronic coupling and reorganizational energy for the conversion of M{dollar}sb6{dollar}X{dollar}sb8{dollar}Y{dollar}sb6sp-{dollar} {dollar}to{dollar} M{dollar}sb6{dollar}X{dollar}sb8{dollar}Y{dollar}sb6sp{lcub}2-*{rcub}{dollar} and M{dollar}sb6{dollar}X{dollar}sb8{dollar}Y{dollar}sb6sp{lcub}3-{rcub}{dollar} {dollar}to{dollar} M{dollar}sb6{dollar}X{dollar}sb8{dollar}Y{dollar}sb6sp{lcub}2-*{rcub}{dollar} by simple electron exchange are equal. The free-energy dependence of the M{dollar}sb6{dollar}X{dollar}sb8{dollar}Y{dollar}sb6sp{lcub}2-{rcub}{dollar} ecl in acetonitrile and dichloromethane was investigated with four series of structurally and electronically related electroactive organic compounds. The yields for the formation of electronically excited Mo{dollar}sb6{dollar}Cl{dollar}sb{lcub}14{rcub}sp{lcub}2-{rcub}{dollar} ion produced by the electron-transfer reaction of Mo{dollar}sb6{dollar}Cl{dollar}sb{lcub}14{rcub}sp{lcub}3-{rcub}{dollar} with electroactive organic acceptors and the reaction of Mo{dollar}sb6{dollar}Cl{dollar}sb{lcub}14{rcub}sp-{dollar} with electroactive organic donors have been measured over a wide potential range by simply varying the reduction potential of the electroactive organic reagents. The dependence of the formation yield of Mo{dollar}sb6{dollar}Cl{dollar}sb{lcub}14{rcub}sp{lcub}2-*{rcub}{dollar}, {dollar}phisb{lcub}rm es{rcub}{dollar}, on the driving force of the annihilation reaction is similar for the four series in both solvents. {dollar}phisb{lcub}rm es{rcub}{dollar} is immeasurable ({dollar}<{dollar}10{dollar}sp{lcub}-5{rcub}{dollar}) for reactions with free energies positive of a threshold value. Over a narrow free energy range just negative of threshold, {dollar}phisb{lcub}rm es{rcub}{dollar} rapidly increases. And with increasing exergonicity of the electron-transfer reaction, {dollar}phisb{lcub}rm es{rcub}{dollar} asymptotically approaches a limiting value less than unity. Analysis of these excited-state production yields using Marcus theory reveals that unit efficiencies for excited-state production are circumvented by long-distance electron transfer. The distance this electron transfer occurs can be mediated by solvent and solute interactions, and calculations establish that the electron-transfer distance is equal to the radii of the reactants plus the diameter of two solvent molecules. Ecl efficiencies of the hexanuclear cluster ions are not only perturbed by intermolecular factors but also are dramatically effected by ligand coordination sphere. Additionally, the effects of temperature and potential step sequence on the ecl efficiencies of the hexanuclear cluster ions have also been investigated.