Cobaltocenium-functionalized poly(propylene imine) dendrimers: Redox and electromicrogravimetric studies and AFM imaging

The first four generations of cobaltocenium-functionalized, diamino-butane- based poly(propylene imine) dendrimers DAB-dend-Cb-x(PF6)(x) (x = 4, 8, 16, and 32; Cb = {Co(eta (5)-C5H4CON-H)(eta (5)-C5H5)} (1-4) have been synthes ized and characterized. The redox activity of the cobaltocenium centers in 1-4 has been characterized by using cyclic voltammetry and the electrochemi cal quartz-crystal microbalance (EQCM). All of the dendrimers exhibit rever sible redox chemistry associated with the cobaltacenium/cobaltocene redox c ouple. Upon reduction, the dendrimers exhibit a tendency to electrodeposit onto the electrode surface, which is more pronounced for the higher generat ions. Pt and glassy carbon electrodes could be modified with films derived from 1-4, exhibiting a well-defined and persistent electrochemical response . EQCM measurements show that the dendrimers adsorb, at open circuit, onto platinum surfaces at monolayer or submonolayer coverage. Cathodic potential scanning past -0.75 V at which the cobaltocenium sites are reduced, gave r ise to the electrodeposition of multi-layer equivalents of the dendrimers. The additional material gradually desorbs upon re-oxidation so that only a monolayer equivalent remains on the electrode surface. Changes in film morp hology as a function of dendrimer generation and surface coverage were stud ied by using admittance measurements of the quartz-crystal resonator on the basis of its electrical equivalent circuit, especially in terms of its res istance parameter. In general, we find that films of the lower dendrimer ge neration 1 behave rigidly, whereas those of the higher generation 4 exhibit viscoelastic behavior with an intermediate behavior being exhibited by 2 a nd 3. Using tapping-mode atomic force microscopy (AFM), we have been able t o obtain molecularly resolved images of dendrimer 4 adsorbed on a Pt(111) e lectrode.