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 NiPcI_x 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 NiPcI_1.0. These crystallize in the space group D_4h²-P4/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 F_o² > 3σ(F_o²). 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) Å. 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 Mössbauer spectroscopic measurements indicate that iodine is present predominantly if not exclusively as I₃^- for all NiPcI_x where x ≤ 3. Optical spectroscopic and X-ray powder diffraction studies of the x ≠ 1.0 phases suggest that mixtures of several discrete phases are present. Single-crystal electron spin resonance studies (ESR) of NiPcI_1.0 reveal that the iodine oxidation is ligand centered yielding π radical cations. The charge distribution thus can best be represented as [Ni^IIPc]^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 NiPcI_1.0 crystals has been investigated by four-probe techniques. Room-temperature conductivities along the crystallographic stacking direction are in the range 260-750 Ω^-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 (ρ ∼ T^1.9±0.2) 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.