Structure-Property Relationships for Electron Transfer Kinetics in Metal Tris(bipyridine) Core Dendrimers

Abstract

Structure property relationship in the redox-active core dendrimers were systematically studied by probing the rate and driving force for electron transfer. An isostructural series of redox-active, metal tris(bipyridine) core dendrimers were synthesized for this purpose. Various synthetic routes were attempted to introduce the bulky dendritic moieties to the bipyridine units with high yields. Heterogeneous electron transfer kinetics was studied by electrochemical methods. In the second generation of these dendrimers, a large attenuation of electron transfer rate was observed qualitatively. A newly designed thin layer electrode was constructed and utilized to study heterogeneous electron transfer kinetics in the second generation dendrimers. In the finite condition, the slow heterogeneous electron transfer kinetics in second generation dendrimers could be studied by computer simulation. Homogeneous electron self-exchange kinetics was studied by nuclear magnetic resonance spectroscopy. The rate attenuation of electron transfer with dendrimer generation was not the same as the behavior found in heterogeneous, electrochemical electron transfer rate determinations. While a large attenuation was observed between the zeroth and first generation, the attenuation of electron transfer between the first and second generation was insignificant. This was rationalized by the concept of core mobility. The redox core in slow exchange limit can move in a non-rate limiting fashion toward a neighboring redox core with the result that the structural effect of the dendrimer is reduced and electron transfer is facilitated in larger dendrimers. For further studies, thermodynamic activation parameters were also obtained by variable temperature nuclear magnetic resonance studies.