Elimination of cross-talk and modulation of function in an organized heterosupramolecular assembly

A close-packed monolayer of TiO2 nanocrystals was deposited on a conducting glass support using Langmuir-Blodgett (LB) techniques and fired. A close-p acked mixed monolayer of eicosyl phosphonic acid (I) and the viologen, 1,1' -dieicosyl-4,4'-bipyridinium dichloride (II) was then deposited on the TiO2 substrate, also using LB techniques. At sufficiently high dilutions of II in I, a single viologen molecule is adsorbed with a known orientation at th e surface of each nanocrystal. The resulting assembly was incorporated as t he working electrode in an electrochemical cell. Under open circuit conditi ons, bandgap excitation of a TiO2 nanocrystal results in electron transfer to a viologen molecule. No electron transfer between the viologen molecules adsorbed at different nanocrystals is observed. At a positive applied pote ntial, electron transfer following bandgap excitation is largely suppressed . Considered are the implications of these findings for the development of practical devices based on modulatable function addressable on the nanomete r scale.