  The interest in monomolecular films as electric conductors arises from the
search for innovative materials. The utility of non-covalently bonded films is
limited because they are mechanically unstable and consist of poorly connected
domains. Consequently, charge transfers in these films are limited to the
distances in the order of a micrometer. Here we show that a recently developed
gas phase assembling method (Burtman, V., Zelichenok, A., Yitzchaik, S. (1999)
Angewandte Chemie Inter. Ed. 38, 2041-2045.), which produces robust dense
monolayers of NTCDI covalently attached to the surface of silicon, allows one
to overcome this scale limitation. These virtually insulating monolayers can be
photo-chemically populated with cation-radicals via ejection of electrons into
the semi-conducting base. The positive charges of cation-radicals can migrate
as far as several millimeters within microseconds in a random walk fashion thus
demonstrating the macroscopic connectivity of the film. Since the charges exist
as cation-radicals, which are potent oxidants, their migration is coupled to
transfer of the free energy of their reduction and is driven by the redox
potential gradient. Reduction of cation-radicals by an anode converts this free
energy into electromotive force. We show how these films can be implemented in
solar energy conversion and basic time-resolved distance-controlled studies of
sequences of ultra-fast electron transfers.
