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

Abstract

The synthesis of two charge trapping dendrimers (T-0 and T-1) 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 (S-0, S-1, S-2, and S-3) built exclusively of benzyl viologens (3, 9, 21, and 45 subunits). The diffusion coefficients obtained from RDE experiments and the corresponding hydrodynamic radii (r(h)) reflect the linear growth of r(h) with the dendrimer generation number. Experimental r(h) values are well supported by MM+ force field calculations. Pimerization of peripheral benzyl viologen radical cations is observed in T-0 and T-1 similar to that described earlier for the electron sponge series. Pimerization increases with increasing dendrimer generation. Electrochemical evidence for charge trapping in the dendrimers T-0 and T-1 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 T-0 and T-1. 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. (C) 2001 Elsevier Science B.V. All rights reserved.