Semiconductive and photoconductive properties of the single-molecule magnets ${\mathrm{Mn}}_{12}$-acetate and ${\mathrm{Fe}}_8{\mathrm{Br}}_8$

Resistivity measurements are reported for single crystals of ${\mathrm{Mn}}_{12}$-acetate and ${\mathrm{Fe}}_8{\mathrm{Br}}_8.$ Both materials exhibit a semiconductor-like, thermally activated behavior over the 200–300 K range. The activation energy $E_a$ obtained for ${\mathrm{Mn}}_{12}$-acetate was $0.37\pm 0.05\mathrm{eV},$ 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 ${2E}_a{=E}_g,$ the optical band gap. For ${\mathrm{Fe}}_8{\mathrm{Br}}_8,$ $E_a$ was measured as $0.73\pm 0.1\mathrm{eV},$ 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 ${\mathrm{Mn}}_{12}$-acetate, we also measured photoconductivity in the visible range; the conductivity increased by a factor of about 8 on increasing the photon energy from 632.8 nm (red) to 488 nm (blue). X-ray irradiation increased the resistivity, but $E_a$ was insensitive to exposure.