Charge trapping in dendrimers with a viologen skeleton and a radial redox gradient

The synthesis of two charge trapping dendrimers (T0 and T1) consisting of one phenyl viologen in the core and 4 or 12 benzyl viologens in the branches and periphery is described. Their electrochemical properties are presented together with those of four closely related dendritic electron sponges (S0, S1, S2, and S3) built exclusively of benzyl viologens (3, 9, 21, and 45 subunits). The diffusion coefficients obtained from RDE experiments and the corresponding hydrodynamic radii (rh) reflect the linear growth of rh with the dendrimer generation number. Experimental rh values are well supported by MM+ force field calculations. Pimerization of peripheral benzyl viologen radical cations is observed in T0 and T1 similar to that described earlier for the electron sponge series. Pimerization increases with increasing dendrimer generation. Electrochemical evidence for charge trapping in the dendrimers T0 and T1 is presented. Results from cyclic voltammetry are difficult to interpret because of dendrimer adsorbate formation but spectroelectrochemical experiments indicate fast reduction and slow oxidation of the phenyl viologen core in T0 and T1. Slow oxidation is explained by the activation barrier of ca. 0.25 eV related to the tailored endergonic electron transfer from the core phenyl viologen to peripheral benzyl viologens. The possibility of electron transfer shunts related to excessive conformational freedom of the benzyl viologen branches is discussed.