Molecular structure-property relationships for electron-transfer rate attenuation in redox-active core dendrimers

Two series of redox-active, iron-sulfur core dendrimers of the general structure (nBu(4)N)(2)[Fe4S4(S-Dend)(4)] (Dend = dendrons of generations 1 through 4) were prepared. Heterogeneous electron-transfer rate constants indicated that the rigid series of dendrimers were more effective at attenuating the rate of electron transfer than were the flexible series of dendrimers. These results were rationalized using computationally derived models which indicated an offset and mobile iron-sulfur core in the flexible series of molecules and a more central and relatively immobile iron-sulfur core in the rigid series of molecules. Further consideration of these data indicated that, while the dendrimers containing rigid ligands had better encapsulated redox cores for a given molecular weight, these molecules had higher electron-transfer rates for a given molecular radius.