Giant Photoconductivity in NMQ[Ni(dmit)2]

The simple molecular salt NMQ[Ni(dmit)(2)] (NMQ = N-methylquinolinium, dmit = 1,3-dithiol-2-thione-4,5-dithiolate) functions as a diamagnetic insulator with an activation energy E-a(dark) of 0.20 eV. However, at 300 K, it exhibits ca. 40 times higher conductivity (sigma(UV)) under UV irradiation [(375 +/- 5) nm, 15.7 mW cm(-2)] than it does under dark conditions (sigma(dark)). The ratio sigma(UV)/sigma(dark) rapidly increases with decreasing temperature and reaches ca. 880 at 200 K. From the temperature dependence of sigma(UV), the activation energy under irradiation E-a(UV) is 0.12 eV. These observations cannot be explained as the result of sample heating during UV irradiation. Rather, the X-ray photoelectron spectra of the sulfur and nickel atoms, the calculated band structure, and the UV/Vis spectra of the salt can all be explained consistently as follows: charge transfer between the Ni(dmit)(2) moieties upon exposure to 375 nm UV light induces melting of the charge-ordered state and produces the unusually large photoconductivity of NMQ[Ni(dmit)(2)].