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Keywords:

  • Photoconductors;
  • Molecular crystals;
  • Charge transfer;
  • S ligands;
  • ­Simple salts;
  • Semiconductors

Abstract

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 Ea(dark) of 0.20 eV. However, at 300 K, it exhibits ca. 40 times higher conductivity (σUV) under UV irradiation [(375 ± 5) nm, 15.7 mW cm–2] than it does under dark conditions (σdark). The ratio σUV/σdark rapidly increases with decreasing temperature and reaches ca. 880 at 200 K. From the temperature dependence of σUV, the activation energy under irradiation Ea(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].