Does thiol-functionalized viologen monolayer memorize anion present when forming the monolayer?

We investigated the effect of the presence of an additive anion while formi ng a self-assembled monolayer of a thiol-functionalized redox active specie s upon the behavior in aqueous solutions. A bromide salt was added in an ac etonitrile solution of a thiol-functionalized viologen (viologen-thiol: N-p entyl-N'-(11-mercapto)undecyl-4,4'-bipyridinium bishexafluorophosphate) in which the self-assembled monolayer was formed on a polycrystalline Au elect rode. We examined the structure and electrochemical behavior of the resulti ng monolayer-modified electrode in three different aqueous electrolyte solu tions. The viologen-thiol monolayer prepared in the presence of Br- exhibit ed obviously different behavior in both KBr and KF electrolyte solutions fr om that prepared in the absence of Br-. On the other hand, the difference w as minor in KPF6 solution. The difference of the electrochemical behavior w as represented by the coverage of the viologen-thiol and the formal potenti al of viologen dication/radical cation redox couple. The memory that the vi ologen-thiol monolayer was formed in the presence of Br- was retained in KF and KBr solutions, though it was erased in the electrolyte solution contai ning PF6-, which is a softer anion than Br-. However, no definitive differe nce of the film structure was deduced from the electroreflectance study in regard to the monomer content and the average orientation of the viologen m oiety. (C) 2001 Elsevier Science B.V. All rights reserved.