Optical and transport studies of Ni(dmit)(2)-based organic conductors

Single-crystal Ni(dmit)(2) salts, (Ph(4)P)[Ni(dmit)(2)](3), (Bu(4)N)(2)[Ni(dmit)(2)](7) . 2CH(3)CN, and (Me(3)S)[Ni(dmit)(2)](2), have been synthesized. All show semiconducting behavior in their temperature-dependent de conductivities. Room-temperature polarized reflectance measurements have been made over the range between 100 and 32 000 cm(-1) (12 meV-4, eV). For Light polarized along the Ni(dmit)(2) stacking axis, all spectra show an energy gap, with superimposed vibrational fine structure at low frequencies and charge-transfer bands at high frequencies. Band gaps determined from the optical conductivities are consistent with thermal activation energies from de transport measurements. The stacking-axis conductivity shows the effect of electron-molecular vibration interaction; analysis for (Me(3)S)[Ni(dmit)(2)](2) yields a dimensionless electron-phonon coupling constant lambda similar to 0.27. For light polarized perpendicular to the stacking axis, only weak vibrational features are observed.