Linked viologen-anthraquinone molecules are attached to TiO2 nanocryst
allites supported on a conducting glass substrate. The resulting assem
bly is incorporated as the working electrode in an electrochemical cel
l. Under open-circuit conditions, band-gap excitation of the semicondu
ctor nanocrystallite results in viologen-mediated electron transfer to
anthraquinone. Prior application of a negative potential step, which
results in 2e(-)/2H(+) reduction of anthraquinone, permits electron tr
ansfer only to viologen. At positive applied potentials, electron tran
sfer following band-gap excitation is largely suppressed. Some implica
tions of these findings for modulation of Function in molecular device
s are considered.