A close-packed monolayer of TiO 2 nanocrystals was deposited on a conducting glass support using Langmuir Β± Blodgett (LB) techniques and fired. A close-packed mixed monolayer of eicosyl phosphonic acid (I) and the viologen, 1,1'-dieicosyl-4,4'-bipyridinium dichloride (II) was then deposited on the TiO 2 substrate, also using LB techniques. At sufficiently high dilutions of II in I, a single viologen molecule is adsorbed with a known orientation at the surface of each nanocrystal. The resulting assembly was incorporated as the working electrode in an electrochemical cell. Under open circuit conditions, bandgap excitation of a TiO 2 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 potential, 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 nanometer scale.