  Resistivity measurements are reported for single crystals of Mn12-Acetate and
Fe8Br8. Both materials exhibit a semiconductor-like, thermally activated
behavior over the 200-300 K range. The activation energy, Ea, obtained for
Mn12-Acetate was 0.37 +/- 0.05 eV, which is to be contrasted with the value of
0.55 eV deduced from the earlier reported absorption edge measurements and the
range of 0.3-1 eV from intramolecular density of states calculations, assuming
2Ea = Eg, the optical band gap. For Fe8Br8, Ea was measured as 0.73 +/- 0.1 eV,
and is discussed in light of the available approximate band structure
calculations. Some plausible pathways are indicated based on the crystal
structures of both lattices. For Mn12-Acetate, we also measured
photoconductivity in the visible range; the conductivity increased by a factor
of about eight on increasing the photon energy from 632.8 nm (red) to 488 nm
(blue). X-ray irradiation increased the resistivity, but Ea was insensitive to
exposure.
