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

Single-crystal Ni~dmit!2 salts, ~Ph4P!@Ni~dmit!2#3 , ~Bu4N!2@Ni~dmit!2#7•2CH3CN, and ~Me3S!@Ni~dmit!2#2 , have been synthesized. All show semiconducting behavior in their temperature-dependent dc conductivities. Room-temperature polarized reflectance measurements have been made over the range between 100 and 32 000 cm ~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 dc transport measurements. The stacking-axis conductivity shows the effect of electron-molecular vibration interaction; analysis for ~Me3S!@Ni~dmit!2#2 yields a dimensionless electron-phonon coupling constant l;0.27. For light polarized perpendicular to the stacking axis, only weak vibrational features are observed.