Electronic Structure Mechanism of Axial Ligands on Itinerant Electrons and Negative Magnetoresistance in Axially-Ligated Iron ( III ) Phthalocyanine Molecular Conductors

Partially-oxidized Iron(III) phthalocyanine (FeIIIPc) with axial CN or Br ligands are molecular conductors with giant negative magnetoresistance. Electron conduction occurs via intermolecular overlapping of Pc p-orbitals, while negative magnetoresistance is brought about by intramolecular interaction between Fe-d and Pc-p orbitals. Aside from permitting slip-stacked solid-state arrangement, axial ligands can further enhance the p-d interaction of FeIII(Pc) depending on the strength of ligand field energies that proportionally leads to larger negative magnetoresistance. However, the strong ligand field of CN results in conductivity reduction due to the p-accepting nature of the ligand which enhances electron gradient in the oxidized Fe3+, thereby localizing itinerant electrons in Pc, as evidenced by charge transfers between Fe-d and CN-p orbitals. In contrast, the p-donating nature of Br ligands complements the electron deficiency of Fe3+, resulting in the delocalization of itinerant electrons in the Pc system, thus creating a highly conducting molecular system with giant negative magnetoresistance.


INTRODUCTION
Phthalocyanine (Pc) is a planar molecule composed of four circular N-linked isoindole rings forming a fully-conjugated 18 p-electron system -ideal structural and chemical characteristics as building blocks of functional materials.In recent years, molecular engineering has been employed to the Pc moiety to utilize its potential solid-state applications, particularly as organic conductors.Octahedral-coordinating central metal such as Co 3+ (d 6 ) and Fe 3+ (d 5 ) are incorporated to Pc to enable the attachment of linear axial ligands such as CN, thereby forming slip-stacked solid-state arrangement between adjacent Co III /Fe III (Pc)(CN) 2 units, resulting in the intermolecular overlapping

Brief communication
of p-orbitals between Pc rings (HOMO), and thus generating electron conduction path/band upon partial-oxidization of the Pc system (electron hole transport) 1 .
Fe III (Pc)L 2 system.At magnetic field of 15 Tesla, Fe III (Pc)(CN) 2 and Fe III (Pc)Br 2 have exhibited 93% and 67% reduction of electrical resistivity (GNMR), respectively.However, the intensity of GNMR in Fe III (Pc)L 2 (L = CN > Br) have an inverse effect on the normal electrical conductivity (at ambient temperature and magnetic field) of the Fe III (Pc)L 2 series which resulted in the order: L = CN < Br, wherein a 2-order electrical conductivity decrease in Fe III (Pc)(CN) 2 attributed to p-d interaction, and no decrease in the electron transport of Fe III (Pc)Br 2 were observed, even as the Br-ligated species also has a relatively strong p-d interaction as demonstrated by its GNMR profile 3 .
For years, there was a prevailing concept that the existence of p-d interaction in the Fe III (Pc)L 2 system will always consequently equate to electron localization (decrease in conductivity).However, an absence of electron localization was observed in the Fe III (Pc)Br 2 system, where its electrical conductivity is in the same order as its Co homologue, Co III (Pc) Br 2 , in which there is no p-d interaction.In terms of molecular structure, the electronic conduction band width of Fe III (Pc)L 2 stood unaffected by the p-d and GNMR interplay, as it remained to be based on the effectiveness of the intermolecular Pc p-p orbital overlap brought about by the steric effect of the axial ligands 7 .Thus, the origin of the p-d interaction-electron localization -GNMR interplay phenomenon in the Fe III (Pc)L 2 system may be traced to its electronic structure.And the elucidation of the electronic structure mechanism of Fe III (Pc)L 2 may provide new prospects in the design of functional highly conducting molecular systems with GNMR.Axial ligands can fur ther modify the properties of the metal phthalocyanine system.For Co III (Pc)L 2 system, electrical conductivity are correlated to the effectiveness of intermolecular Pc p-p orbital overlap emanating from the bulkiness/ steric effect of the axial ligands which are in the order: L = CN > Cl > Br 2 .
The GNMR is brought about by the interaction between the d-orbital of Fe 3+ and the p-orbital of the Pc moiety.Interestingly, it was observed that the intensity of p-d interaction is affected and can be modulated by the varying magnitudes of axial ligand field energy 3 .
Axial ligands affect the electronic structure of the Fe III (Pc) system resulting in the modulation of orbital interactions.Ab initio calculations derived the differences (proximity) between the p and d orbital energies which correspond to the intensity of p-d interaction as 8.5450 eV and 7.8655 eV for L = Br and CN, respectively 4 .It was established that the axial ligand correspondingly lifts Fe-d orbitals nearer to Pc-p orbitals depending on its field energy, thereby intensifying p-d interaction.Moreover, the extent of the p-orbitals of the axial ligands results into stronger d-d interactions 5,6 .Thus, higher axial ligand field energy equates to stronger p-d orbital interaction.
The intensity of the p-d interaction has a direct correlation effect on the GNMR of the were suspended in 40 mL 1:3 dimethylformamide (DMF) : acetone solvent system and subsequently mixed via a sonicator.A current of 5 μA was then applied to the system.Electrocrystallization of the partially-oxidized TPP[Fe(Pc)Br 2 ] 2 takes 2-3 weeks to accomplish.The product conversion is between 60-70%.

RESULTS AND DISCUSSION
The electronic absorption spectra is an effective method to experimentally describe the electronic structure and corresponding orbital interactions of the Fe III (Pc)L 2 , which is a unique single molecule p-d system.
The charge transfer (CT) between Fe 3+ and CN ligands in Fe III (Pc)(CN) 2 and the absence of CT between Fe 3+ and Br ligands in Fe III (Pc)Br 2 , despite having p-d interactions, points to the axial ligands as the origin of the electron localization/delocalization mechanism in the Fe III (Pc)L 2 system.That is, the oxidized state of Fe 3+ (vacancy in e g (dp) enables the attraction of electron density from electron-rich Pc (Fe-d yz to Pc-p z ), while axial CN which is a strong p-acceptor further intensifies the withdrawal of electron density from Fe 3+ (to CN p x -orbital), thereby resulting to the localization of itinerant electrons in Pc.In contrast, the strong p-donor character of Br appears to compensate for the electron deficiency of Fe 3+ , thereby favoring electron delocalization that results in the easing of electron transport on the part of the Pc moiety.Moreover, the absence of the MLCT peaks in Co III (Pc)(CN) 2 system (Fig. 1) further ascertains that the nature of MLCT1 and MLCT2 are induced by the p-d electron interplay in the Fe III (Pc) (CN) 2 species.

CONCLUSION
The strong p-d interaction in Fe III (Pc) L 2 system always result in GNMR.Axial ligands enhance the p-d interaction depending on their field energy.It is established that the p-d interaction in Fe III (Pc)L 2 does not always result in electron localization.Electron localization is not dependent on the p-d but on the electronic character of the axial ligands.
Employing axial ligands that are p-donor (weak field energies) by nature will still significantly enhance the p-d interaction of Fe III (Pc) to produce GNMR with the absence electron localization.p-donor ligand such as Br compensates for the electron deficiency of the oxidized Fe 3+ , thereby easing itinerant electrons in Pc to favor electron delocalization.Thus, it is possible to create a highly conducting magnetic molecular system with GNMR such as Fe III (Pc)Br 2 which has strong p-d interaction and unhampered electron transport.The characteristic intra-Pc ring p → p* transitions: Q (1a 1u (p) → 1e g (p*)) at around 15000 cm -1 (665 nm), and B/Soret: (1a 2u (p) → 1e g (p*) at around 27500 cm -1 (365 nm) were observed for all species.Also, for the Fe III (Pc)(CN) 2 species, two metal to axial ligand charge transfers (MLCT) were detected: MLCT1 = (e g (dp) → 1b 1u (p*) at around 18500 cm -1 (540 nm), and MLCT 2 = (e g (dp) → 1b 2u (p*) at around 25000 cm -1 (400 nm) 8 .