Chemical, Spectral, Structural, and Charge Transport Properties of the “Molecular Metals” Produced by Iodination of Nickel Phthalocyanine

This paper presents a detailed study of the solid-state chemical, spectral, structural, and charge transport properties of the materials which result from treating nickel phthalocyanine (NiPc) with elemental iodine. A range of NiPcIx stoichiometries is obtained where x = 0 to ca. 3.0; electrical conductivities of compressed polycrystalline samples are comparable with those of other “molecular metals”. Single crystals were obtained for NiPcI10. These crystallize in the space group D4H2-PA/mcc, with two formula units in a unit cell having dimensions a = 13.936 (6), c = 6.488 (3) Å. Full-matrix least-squares refinement of 65 variables gave a final value of the conventional R index (on F) of 0.042 for 375 reflections having Fo2>3σ(F02). The crystal structure consists of stacked, planar NiPc units (staggered by 39.5°) and disordered chains of iodine atoms extending in the c direction. The NiPc units have crystallographically imposed symmetry 4/m. The interplanar Ni-Ni separation is 3.244 (2) Å, and the intramolecular Ni-N distance, 1.887 (6) A. Analysis of the diffuse scattering pattern arising from disordered iodine chains reveals that iodine is present as I3-. An I-I distance of 3.00 Å and a I-I distance of 3.72 Å are derived from the diffuse scattering. Resonance Raman and iodine-129 Mossbauer spectroscopic measurements indicate that iodine is present predominantly if not exclusively as I3- for all NiPcI* where x < 3. Optical spectroscopic and X-ray powder diffraction studies of the x 9<1.0 phases suggest that mixtures of several discrete phases are present. Single-crystal electron spin resonance studies (ESR) of NiPcIli0 reveal that the iodine oxidation is ligand centered yielding w radical cations. The charge distribution thus can best be represented as [NiIIPc]0.33+(I3-)0.33, although there is ca. 0.002 unit of charge back-transferred from each I3 unit to the metallomacrocycle stack. Susceptibility measurements by ESR and static techniques can be interpreted in terms of a narrow bandwidth metal (ca. 0.37 eV) and a significant contribution from van Vleck paramagnetism. The electrical conductivity of NiPcIli0 crystals has been investigated by four-probe techniques. Room-temperature conductivities along the crystallographic stacking direction are in the range 260-750 “1 cm-1 and carrier mean free paths are in the range 3.3-8.2 Å. The temperature dependence of the conductivity is metallic (p-Tl9±02) down to ca. 55 K, at which point there occurs an abrupt reduction in conductivity. Neither the resonance Raman of the I3”, the ESR line width, nor the magnetic susceptibility is sensitive to this transition.