In the periodic table of the elements, ruthenium is a transition element which is positioned between iron and osmium. Together with platinum, palladium, rhodium, and iridium, ruthenium is a precious metal. In the case of a ruthenium complex [Ru(bpy)2(PBImH)]2+(bpy=2,2’-bipyridine) (structure shown in Figure 1) which has been configured with a ligand known as pyridyl-benzimidazole (PBImH), the ruthenium changes from a +divalent to a +trivalent. This causes an increase in the acidity of the imino N-H proton of the configured benzimidazole and facilitates dissociation. When an iron (II) ion which has been hydrated in water is oxidized and becomes an iron (III) ion, protons are dissociated from the water which had configured to the iron. This facilitates the transformation to iron hydroxide (II) and results in sediment of a rusty iron color. By increasing the oxidation number of central metal ions, this phenomenon is similar to how the acidity of ligands increases when the electric charge of ligands is drawn to central metal for which the positive electric charge has increased. However, when [Ru(bpy)2(PBImH)]2+ is irradiated with visible light, light emissions with extremely large wavelengths are shown around 650 nm. Nevertheless, when the N-H of the ligands for this complex is deprotonated, the intensity of the light emission decreases dramatically. In this way, in the case of this ruthenium complex, electrons, protons and light move in cooperation. As shown in Figure 1, new chemical species are generated by the stimuli of electrons, protons, and light, and the species exert functionality. For example, Figure 2 shows two ruthenium complexes which were formed from bis (pyridyl-benzimidazole) to which pyridyl-benzimidazole had been connected. The interaction between Ru-Ru changes significantly depending on whether or not protons exist. Analysis of the intervalence charge transfer (IVCT) among photo-induced atoms when the oxidation count for the two ruthenium is in a differing Ru2+-Ru3+ mixed atomic state shows that deprotonating the bridging ligand causes the intensity of the IVCT band to increase and the interaction between Ru-Ru to heighten2). The movement of protons and the movement of electrons are associated a cause a change in physical properties. In this way, by introducing a solution system into a solid system, it is possible to create devices from the functions of molecules which involve electrons and light.
Figure 2: Switching of ruthenium interaction due to protonating/deprotonating of the bridging ligand inside of the ruthenium mixed-valence dinuclear complex.3)
Organic phosphonic acid possesses the chemical formula R-P(O)(OH)2 (R=organic group). As shown in Figure 3, it is a dibasic acid capable of dissociating two protons in water. The protons are dissociated and it becomes organic phosphonate [R-P(O)(O)2]2-. When a variety of metal ions are added to a water solution in which organic phosphonate has been dissolved, it is easy to create insoluble sediment. The organic bisphosphonic acid which possesses phosphonic acid groups on both sides of the organic group disassociates, and both sides undergo ionization to become bisphosphonate. This bisphosphonate forms a crystalline solid which has a two-dimensional layered structure which serves as a bridge between metal ions. (Figure 3)
My research group utilized how compounds which possess this phosphonic acid group selectively adhere to the surfaces of a variety of oxidized objects. First, we solidified a primer layer on the surface of complexes which possess side chains of the phosphonic acid group. Then, we created a new functional nano-structure on top of the primer layer through a method which uses the complexation of metal ions to create a regular layered structure. In recent years, various groups throughout the world have begun conducting research on using layered structure construction methods realized through successive complexation as a method for constructing precisely-controlled surface structures. The research conducted by my group is notable for two points: 1) we use redox activation complexes in the molecular unit, and 2) we control the orientation of molecules on the surface by adhering multiple organic groups which are secured to the surface. This nano-structure manufacturing method (also known as Layer-by-Layer or LbL) is easily manufactured by dipping a substrate into solution. Furthermore, through the successive use of different molecules, it is possible to change the sequence and create a variety of combinations. This has enabled us to perform chemistry which uses “molecular building blocks” similar to molecular Lego blocks on the solid surfaces. (Figure 4)
Figure 4: Oxide substrates such as ITO, sapphire and quartz are dipped into a complex solution (3 hours) and then dipped into a zirconium (IV) solution. After washing, they are once again dipped into a complex solution. By repeating these procedures, it is possible to create a multi-layer complex molecular membrane.
Another method for creating a layered membrane is the Langmuir-Blodgett (LB) technique, which uses an air-water interface. By utilizing the property in which amphipathic molecules are oriented on an air-water interface, this technique transcribes the molecules onto a substrate at a certain surface pressure by utilizing surface area-surface pressure isotherms. My research group recognized that titanium oxide nano-sheet colloids (chemically stripped by researchers led by Dr. Takayoshi Sasaki of the National Institute for Materials Science) possess amphiphilicity and transcribe to substrates when surface pressure is applied. We then discovered a method for arranging single-layer nano-sheets which possess an irregular shape onto the surface of solids with almost no gaps (Figure 5).4) This discovery was made by coincidence when Dr. Kezhi Wang (previously stationed at Chuo University as a JSPS Doctoral Researcher; currently a Professor at Beijing Normal University) was washing an LB trough. Today, many researchers use this method to create single-layer membrane on nano-sheets. In addition to molecules which are nano-size building blocks, inorganic nano-sheets and nano-particles are also promising Lego blocks.
Figure 5: At an LB trough which contained nano-sheet colloid aqueous solution, it is possible to gather amphiphilical nano-sheets when moving the divider on top of the surface to cause compression. This makes transcription possible as a membrane on the substrate at specified surface pressure. This figure shows a conceptual diagram for creating LB membrane, as well as AFM images4) of the titanium oxide, niobate oxide, ruthenium oxide, and manganese oxide which were transcribed onto the silicon substrate.
Figure 7: Conceptual diagram (layers grouped and displayed as a single layer) of energy levels for “0” and “1” conditions as caused by pulse electric potential application of the hetero-junction layer membrane ITO (||(Ru-NP)4|Zr|(Ru-CP)4) and occurrence mechanism of photocurrent caused by 573 nm optical irradiation.
Figure 8: Repetitive pattern of optical illumination and electric potential application to hetero-junction molecular membrane, and the photocurrent response to that pattern (demonstrates that repetition is possible as memory)
This makes it possible to use electric potential application to create two conditions of “0” and “1” for the hetero-junction molecular membrane. These conditions can be detected through near infrared light. In addition, as shown in Figure 8, when performing optical irradiation for the two conditions after the application of -0.5 V and +0.7 V and then measuring the photoelectric current, cathode current was observed in the “0” condition and anode current was observed in the “1” condition. In both cases, the action spectrum of the photoelectric current clearly indicates the involvement of Ru-NP excitement. We have confirmed the existence of a molecular memory function which operates stably even when repeatedly writing to and reading from the device. 5)
In the case of silicon devices, a pn junction is created by performing doping for the silicon, manufacturing p-type and n-type semiconductors, and then connecting them. These devices operate as diodes, solar cells, and optical diodes. In the case of molecules, the orbital energy of molecules is manipulated to connect two molecular layers with different energy states, and the device response is determined by the movement of excited electrons caused by light at the junction layer.
Normally, a pH meter is used to measure proton concentration. Specifically, the difference in electric potential between the inside and outside of a thin membrane is measured and then converted into the proton concentration. By skillfully utilizing the collaborative movement of electrons and protons in the complex as introduced above, use as a pH sensor is possible. In the case of complexes where bis (benzimidazole) pyridine has configured to ruthenium, electron movement and proton movement occurs in conjugation, and the RuII/RuIII redux potential shifts toward negative potential as the solution pH rises. When considering the fusion of nano-carbon material and complexes, I believed that this pH dependency of redux potential could be applied to the pH sensor. Next, I focused on how mechanical pencils used in everyday lift are made using graphite which possesses conductivity, and how that graphite could also be used as an electrode for electrochemical measurement. In order to adhere a ruthenium complex which possesses a pyrene base with a bis (benzimidazole) pyridine ligand to a graphite surface, I synthesized a complex which has a large π conjugate system in the side chain (Figure 9). When I adhered this complex to the surface of a mechanical pencil and conducted electrochemical measurement, I found that the RuII/RuIII response changes together with the solution pH, and that the complex serves as a compact pH sensor. In this way, new functionality is imbued in the interface by securing to the surface a complex molecular system with conjugation between electrons and protons. Application to sensors and devices is possible.6)
Figure 9: A simple and ultra-compact pH sensor which was created by modifying a ruthenium complex with proton conjugate electron movement into a carbon nano-tube surface and then securing it around the core of a mechanical pencil. A model diagram and the electric potential pH response are also shown.6)
Among the various substances which exist in this world, I have focused my research on metal complexes which mainly possess metal ions. I have researched the functions of metal complexes which are realized through the mutual involvement of electrons, proton, and light. Some examples include electron migration and oxidation-reduction for complexes, collaborative proton migration, light emission, and optically inducted electron migration in an excited state. The interaction of light and metal complexes creates an arena for the performance of electrons and protons. In order to efficiently use light, electrons, and protons to generate new functionality, it is necessary to create nano-structures for complexes which are composed of molecular building blocks.7) I strongly feel the importance of skillful molecular design for creating more opportunities to utilize light, electrons and protons, and the importance of skillful arrangement and orientation. Molecular nano-material possess nano-structure which is precisely controlled based on metal complexes, which are structural units that possess clear geometry. This molecular nano-material is become increasingly important for the creation of new nano-devices and energy conversion elements.
Finally, during many years of research, my discussions with countless outstanding researchers and my laboratory work with associate professors, postdoctoral students, graduate students, and undergraduate students has encouraged my experiments and enable me to realize new results. Through in-depth review of daily research results, the contributions of these individuals paved the road for sowing the seeds of new research and creating “Something New.” For this I am extremely grateful.
1) G. Sprintschnik, et al., J. Am. Chem. Soc., 98, 2337 (1976) (There have been reports that the results of this thesis could not be replicated in subsequent experiments. However, this experiment was a major trigger in conducting research on the photo-dissociation of water using visible light.)
2) M. Haga, et al, Coord. Chem. Rev., 132, 99 (1994)
3) M. Haga, et al., Inorg. Chem., 30, 3843 (1991)
4) M. Muramatsu, et. al., Langmuir 21, 6590（2005）
5) T. Nagashima, et al., Chem. Eur. J, 22, 1658 (2016)
6) H. Ozawa et al., Chem. Lett., 42, 1059(2013)
7) Masaaki Haga “New Developments in Interface Molecular Science” (edited by The Chemical Society of Japan) (Kagukudojin), Chap. 4, pp. 78-86 (2011).