(Invited) Switching of Single Molecule Magnetism and Conductance in Porphyrin-Based Terbium Double-Decker Complexes
Authors: Ogawa Takuji 
Published: February 2020

Organic functional materials are utilized in commonly used electronic products such as organic light emitting diodes and liquid crystals in cellular phones and televisions. Currently, materials are used in mass and most of their characteristic properties arise from the cooperative interaction of a number of molecules or atoms. As the result, many physical properties disappear when the size of the material becomes smaller than a certain size.
For example, ferromagnetism is observed in larger samples than ca. 15 nm in diameter because collaboration of many spins in the sample is required for the property. As the result, the density of memory size in hard disk is limited to ca. 3 TB/in2 with ordinal magnetic materials. If the single molecules can work as magnets, the density can increase to 600 TB/in2 theoretically. Molecules which can work as magnets in single molecule are called single molecule magnet, and first found as a Mn12 cluster by Sessoli et al., in 1993.1 During the last two decades, polymetallic transition-metal complexes with strong intramolecular exchange coupling have been typical research targets because their high-spin ground state and large anisotropy may impart a higher energy barrier than that of single–metal-ion complexes. Since Ishikawa reported SMMs based on single lanthanide ions in 2003,2 many mononuclear complexes with slow magnetic relaxation have been reported. In particular, phthalocyanine (Pc) Tb double-decker compounds display a higher blocking temperature, below which magnetic relaxation is slow. Moreover, the magnetic relaxation behavior of double-decker complexes is sensitive to their coordination mode, which can be easily tuned through their flexible structure. This variation in the coordination modes makes it possible to control their magnetic properties via external stimuli such as a redox reaction or pulse current from an STM chip, resulting in unique switching properties.
A tetraphenylporphyrin (TPP) Tb double-decker complex was synthesized, and the crystal structures of both the protonated and deprotonated forms have been determined. The structure analysis demonstrated for the first time in tetrapyrrole-based double-decker complexes that a proton is localized on the pyrrole ring nitrogen for charge balance. The AC magnetic susceptibility measurements revealed that the protonated form does not show SMM behavior although the anionic form does act as an SMM. The SMM behavior of the double-decker complex can be reversibly switched through only a single proton.3,4 We studied the synthesis and study of octaethylporphyrin (OEP) Tb double-decker complexes with different electronic structures comprising protonated, anionic, and radical forms. Magnetic susceptibility measurements revealed that only the anionic and radical forms of the OEP Tb double-decker complexes exhibited SMM properties. Scanning tunneling microscopy (STM) investigations revealed that these OEP Tb complexes form well-ordered monolayers upon simple dropcasting from dilute dichloromethane solutions. All three complexes form an isomorphic pseudo-hexagonal 2D pattern, regardless of the differences in the electronic structures of their porphyrin-Tb cores. This finding is of interest for SMM technology since chemical transformations in ultrathin films of these materials will not require any detrimental reorganization.5 Further, we effect sub-molecular scale patterning to create an ordered assembly of SMMs by using (OEP)-Tb double-decker complex, Tb(OEPH)(OEP)), which exhibits non-SMM properties but changes into the SMM molecule Tb(OEP)2 by discharging a hydrogen atom. We locally formed an SMM molecule Tb(OEP)2 by injecting tunneling electrons using scanning tunneling spectroscopy (STS) on the ordered film of Tb(OEPH)(OEP), where the spin of the converted Tb (OEP)2 molecule was detected by a Fano dip of Kondo resonance in scanning tunneling spectroscopy.
Single molecular conductance of Tb(TPP)2 and Tb(TPP)(Phthalocyanine) complexes were also studied using break-junction methods to show that in the later complex the conductance changed greatly by deprotonation.
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