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 cystamin e, chemisorbed onto gold electrodes, are examined. The resulting monol ayer exhibits nonordered structure being reflected by a similar rate c onstant to the bipyridinium redox sites, that is independent of the br idging alkyl chain length, k(et) = 550 s-1. The nonordered structure i s also confirmed by the low electron transfer coefficients alpha(a) an d alpha(c), corresponding to 0.25 +/- 0.05 and 0.3 +/- 0.05, respectiv ely. By treatment of the nonordered monolayer assembly with 1-hexadeca nethiol, C16SH, a more densely packed organized monolyer is formed, wh ere the alkyl-substituted bipyridinium sites are stretched in the mono layer configuration. In these assemblies the electron transfer rate co nstants to the bipyridinium sites depend on the alkyl chain length bri dging the redox site to the electrode. The electron transfer rate cons tants follow Marcus theory and the electron tunneling coefficient corr esponds to beta = 0.006 A-1.