Detailed studies of the structure, conductivitity, magnetoresistance, optical spectra, and magnetic properties (susceptibility, EPR) for the new molecular metal tetrabenzporphyrin iodide (H2(tbp)I) and the electrical, spectral, and magnetic properties of Ni(tbp)I are reported. Paramagnetic transition-ion impurities were carefully excluded during the synthesis of H2(tbp)I and Ni(tbp)I, and both materials show much higher, metal-like conductivites than previously seen for less-pure Ni(tbp)I. Comparison of the specular reflectance data for Ni(tbp)I and H2(tbp)I allows a distinction between purely ring π-transitions and metal-involved charge-transfer transitions, and the spectra fix the energy levels of the π orbitals involved in conduction. Transport, magnetic, and optical properties show that both H2(tbp)I and Ni(tbp)I are ring-based conductors that have metal-like conductivities, varying as ∼1/T. down to ca. 30-40 K. However, the remaining level of defects is higher in the tbp conductors than in H2(pc)I, and whereas the latter is metallic down to the mK temperature range, the defects in the (tbp) compounds localize the conduction electrons at ∼10 K (Ni(tbp)I) and ∼30 K (H2(tbp)I), leading to transport through one-dimensional variable-range hopping. EPR g-values for H2(tbp)I and Ni(tbp)I are close to that for the free electron and are nearly temperature-independent. The line widths for both samples are extremely narrow and also are nearly temperature-independent. These results show that Ni(tbp)I does not display doubly-mixed valence, as thought earlier: Paramagnetic impurities significantly altered the EPR signals of the prior samples. H2(tbp)I crystallizes in the space group P4/mcc with cell constants of a = 14.173(10) Å and c = 6.463(4) Å. Full-matrix least-squares refinement of 63 variables gave an R index of 0.061 on F02.
|Original language||English (US)|
|Number of pages||7|
|State||Published - Dec 1 1997|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry