Single-Molecule Electronics Based on Porphyrin Molecules for Functionality Emergence

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, organic materials are used as mass materials and most of their characteristic properties arise from the cooperative effects of the molecules. However, if each single molecule had its own specific properties and could interact in a rational way with the other molecules, more sophisticated properties would emerge. With this aim, we studied the electronic and magnetic properties of a single or small number of molecules based on a porphyrin moiety and investigated the integration of these molecular electronic parts.

Synthesis of a Porphyrin-Imide Single Molecular Diode Connected to Single-Wall Carbon Nanotubes (SWNT):

Our ultimate objective is the fabrication of single molecular integrated circuits (ICs). The ability to wire more than three electrodes to a single molecule is essential to achieve this goal. Considering the size of the molecules, SWNTs are the only possible candidates as electrodes. We synthesized a series of porphyrin-imide molecules connected to SWNTs. The presence of the molecules was confirmed by AFM using gold nanoparticles as markers. The electronic properties were measured using PCI-AFM, and their dependency on the molecular structure was studied.

Single-Molecule Magnetic Properties of Porphyrin-based Tb^III Double-decker Complexes:

Single-molecule magnets (SMMs) have attracted wide scientific attention owing to their unique energy barrier to magnetic relaxation and their application in molecular spintronics. 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, many mononuclear complexes with slow magnetic relaxation have been reported. In particular, phthalocyanine (Pc)-based Tb^III 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-based Tb^III 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.

Synthesis of Porphyrin Arrays with Programmed Orders by Sequential Coupling Methods:

              Once single-molecule electronic parts are prepared, they must be connected to form molecular integrated circuits. This can be achieved either through organic synthesis or self-organization.  As a programmable synthetic method for the molecular parts, we developed sequential coupling reactions that are suitable for the automated synthesis of complex molecules. A series of porphyrin arrays were synthesized using the Suzuki and Sonogashira coupling reactions, starting from common key compounds. Spectroscopic analyses of the compounds revealed that the order of the central metals in the arrays significantly affected the electronic properties of the arrays in a way that cannot be predicted from a simple summation of the components.