Electron Transfer in Self-Assembled Monolayers of N-Methyl-N′-carboxyalkyl-4,4′-bipyridinium Linked to Gold Electrodes

Electron transfer processes in monolayer assemblies formed by covalent linkage of N-methyl-N′-carboxyalkyl-4,4′-bipyridinium (1) to cystamine, chemisorbed onto gold electrodes, are examined. The resulting monolayer exhibits nonordered structure being reflected by a similar rate constant to the bipyridinium redox sites, that is independent of the bridging alkyl chain length, k_et = 550 s⁻¹. The nonordered structure is also confirmed by the low electron transfer coefficients α_a and α_c, corresponding to 0.25 ± 0.05 and 0.3 ± 0.05, respectively. By treatment of the nonordered monolayer assembly with 1-hexadecanethiol, C₁₆SH, a more densely packed organized monolyer is formed, where the alkyl-substituted bipyridinium sites are stretched in the monolayer configuration. In these assemblies the electron transfer rate constants to the bipyridinium sites depend on the alkyl chain length bridging the redox site to the electrode. The electron transfer rate constants follow Marcus theory and the electron tunneling coefficient corresponds to β = 0.006 A⁻¹.