Synthesis and electrochemical studies of nanoscopic dendritic dumbbells

Several dendritic dumbbells were synthesized using poly(benzyl ether) and poly(benzyl ester) dendrimers as the bell on either side of a rigid rod consisting of 3,3′-dimethoxybenzidine (B) or oligomers of B and 1,4,5,8-naphthalene dianhydride ( A). The poly(benzyl ether) dendrons were synthesized via Frechet's method while the penpheral surfaces of the dendrons were either unmodified berxzyl groups or para-substituted methyl ester benzyl groups. Similarly, the poly(benzyl ester) dendrons were synthesized with tert-butylcarbamate-protected amines in the para position so as to modify the surface functionality of the dumbbell. The reactive terminal group at the core was converted to the acid in order to link the dendrons to the rigid rod so as to form the dumbbell. The normally highly insoluble oligoimides were readily dissolved in a variety of solvents once modified with even lower generation dendrons; i.e. D₂-BABAB-D₂ is readily soluble in acetone, tetrahydrofuran, chloroform, methylene dichloride, dimethyl acetamide and dimethyl sulfoxide in spite of the oligoimide being a 5.4 nm long rigid rod. Electrochemical studies of the dendritic dumbbells containing the electroactive naphthyl diimide unit (A) in the core shows that Bach unit undergoes two single electron reductions per A unit and as oligoimides of this type unmodified by dendrimers, there is no interaction between A groups as the electrons are localized on the A moieties. The studies also show the diffusion coefficients decrease with respect to increasing dendrimer generation and hence, in size as measured by the molecular weight and hydrodynamic radius. The experimental values for the diffusion coefficients are in excellent agreement with the theoretically calculated values for prolate ellipsoids. Also decreasing in a similar manner is the heterogeneous electron transfer coefficient with respect to dendrimer generation. The electron transfer rate constants begin to show inhibition in generation three. We suggest this inhibition arises from the dendrons preventing A from readily reaching the electrode by holding the rigid rods containing A at a distance from and parallel to the electrode surface.