Axially Ligated Phthalocyanine Conductors with Magnetic Moments

Abstract

This mini-review describes electrical conductivity, magnetic properties, and magnetotransport properties of one-dimensional partially oxidized salts composed of axially ligated phthalocyanines, TPP[M(Pc)(CN) 2 ] 2 (TPP = tetraphenylphosphonium, Pc = phthalocyaninato), with M of Fe (d 5 , S = 1/2) and Cr (d 3 , S = 3/2). These salts are isomorphous, and π-π interactions in the crystal, that becomes the origin of the charge carriers, are nearly the same. Both the Fe and Cr salts show carrier localization and charge disproportionation which is enhanced by the interaction between local magnetic moments and conduction π-electrons (π-d interaction). However, the magnetic properties are slightly different between them. M = Fe has been found to show unique anisotropic magnetic properties and antiferromagnetic short-range magnetic order between the d-spins. On the other hand, for M = Cr, its magnetic moment is isotropic. Temperature dependence of the magnetic susceptibility shows typical Curie-Weiss behavior with negative Weiss temperature, but the exchange interaction is complicated. Both M = Fe and M = Cr show large negative magnetoresistance, reflecting the difference in the anisotropy. The magnetoresistance ratio (MR) is larger in the Fe system than in the Cr system in the low magnetic field range, but MR in the Cr system exceeds that in the Fe system when the magnetic field becomes higher than 15 T. We discuss the mechanism of the giant negative magnetoresistance with reference to the d-d, π-d, and π-π interactions.

Evaluation of the Giant Ferromagnetic π–d Interaction in Iron-Phthalocyanine Molecule

“…Furthermore, our calculated DDCI values for J πd are in excellent agreement with the estimate of Murakawa et al of J πd / k B > 500 K and confirm the strong intramolecular π–d interaction expected for Pc systems, a central result of this work. Our evaluations significantly improve the previously reported CASPT2 estimates by Yu et al for TPP­[Fe­(Pc)­(L) 2 ] 2 .…”

“…To build this matrix, both the energy and wave function of the six lowest Sz = 0 magnetic states (one quintet, three triplet, and two singlet states) must be determined. Notice that our evaluations do not take into account spin−orbit coupling effects, which are assumed to be responsible for the magnetic anisotropy and in turn for the anisotropy of the negative magnetoresistance in axially ligated iron-phthalocyanine, 51,16,17,32 but they are expected to provide accurate estimates of the dominant isotropic interactions. DDCI calculations 52,53 were performed to obtain the energy and wave function of the implied states in the monomer and dimers.…”

“…Materials based on metal-phthalocyanine molecules are molecular conductors of different dimensionality depending on the coordination of the metal atom. They form one-dimensional face-to-face stacking conductors in absence of axial ligands as in M­(Pc), M = Ni, Cu, ..., and Pc = phthalocyaninato, − while they behave as multidimensional conductors when the central metal is coordinated to axial ligands as in M­(Pc)­(L) 2 , with M = Fe, Co, Cr, Mn, Ru, ..., and L = CN, Br, Cl. – If the metal atom has a non-null magnetic moment, the interaction between the Pc π and metal d electrons, that is, between the itinerant π-electrons and localized magnetic moments on the metal atom, modifies the transport properties of the molecular conductor. This interaction is at the origin of many interesting phenomena with remarkable importance in the field of molecular electronics and spintronics, such as charge ordering, Kondo effect, and giant magnetoresistance.…”

“…In the work by Murakawa et al the single crystals of the neutral radical [Fe­(Pc)­(CN) 2 ]·2CHCl 3 were fabricated in situ by electrochemical oxidation of THA­[Fe 3+ (Pc 2– )­(CN – ) 2 ] (THA = tetraheptylammonium), but no X-ray data were provided. Our calculations are then based on the available X-ray crystallographic data of the isostructural [Co­(Pc)­(CN) 2 ]·2CHCl 3 system. , Since the size of Fe 3+ ion is almost the same as that of Co 3+ (ionic radius r­(Fe 3+ ) = 0.64 Å and r­(Co 3+ ) = 0.63 Å), the geometry of the Pc unit is not affected, and both crystals are isomorphous as occurs with TPP­[Fe­(Pc)­(CN) 2 ] 2 and TPP­[Co­(Pc)­(CN) 2 ] 2 compounds …”

Elucidating the Electronic Structure and Magnetic and Conducting Properties of μ-Oxo Mn-phthalocyanine [MnPc(CN)]2O Complex

“…Metal phthalocyanines (MPc) form one-dimensional face-to-face conductors, − while the presence of axial ligands gives multidimensional conductors as the packing is controlled by the π–π interactions . When the metal has a magnetic moment, the interaction of the d electrons of the metal and the π electrons of the Pc ring generates a modulation in the conducting behavior and singular properties such as the negative magnetoresistance found in TPP­[M­(Pc)­(CN) 2 ] 2 for M = Fe, − Cr, and Mn or in the related TPP­[Fe­(tbp)­(CN) 2 ] 2 with tbp = tetrabenzoporphyrin.…”