Assembly of Molecular Building Blocks: Molecular Assembly Chemistry and Functional Expression

Taku Hasobe (Associate Professor, Department of Chemistry)

What is molecular stacking via the bottom-up method?

In conjunction with the recent rapid development of nanotechnology, terms like "top-down" and "bottom-up" are frequently used, and you have probably heard them at least once. In recent years, bottom-up methods have been attracting particular attention. To put it simply, the bottom-up method in nanotechnology involves stacking individual atomic and molecular units, each smaller than one nanometer (10⁻⁹ m), one by one to create a minute "assembly" of a certain size. In other words, it might be easier to visualize the "molecular stacking via the bottom-up method" discussed in this article if you think of it as LEGO blocks or a puzzle, where molecules are treated as individual blocks or pieces.

Strategies for molecular integration using the bottom-up method in chemistry

So, what strategy should we adopt to stack these extremely small molecules as we desire? Of course, we cannot assemble them directly with our own hands like LEGO blocks or a puzzle. One strategy we chemists choose is to skillfully use chemical bonds, such as covalent bonds and hydrogen bonds, for assembly. Generally, among chemical bonds, covalent bonds are very strong, while other chemical bonds (or interactions) like coordination bonds, hydrogen bonds, and intermolecular forces have relatively different bond strengths. Therefore, by skillfully utilizing these differences (a supramolecular chemistry approach), we can arrange and stack molecules in desired locations.

What is the design of molecular assemblies for eliciting functions?

Next, will the molecular stacks we synthesize using the above methods truly exhibit remarkable functions? Our group is actively developing photoelectric conversion systems, typified by solar power generation, using supramolecular chemistry methods. As for how we design them, the key is to carefully examine the mechanism to elicit the desired function. For example, photoelectric conversion, which transforms light energy into electrical energy, is a function that only manifests when three distinct processes occur sequentially: ① light absorption (and exciton diffusion), ② carrier charge generation (charge separation: a state where holes and electrons are separated), and ③ carrier transport. Therefore, we design and synthesize molecular assemblies based on physicochemical knowledge to ensure these photoelectric conversion functions operate systematically, and then proceed to actual device applications.

[Specific Example: Application to Solar Cells]

Part 1: Construction of P/N junction interfaces at the molecular level

The organic molecules used here include porphyrin (an electron-donating molecule: P), which has a structure similar to chlorophyll, a pigment in natural photosynthesis, and the spherical molecule fullerene C60 (an electron-accepting molecule: N). Both have a molecular size of about 1 nanometer. Figure 1 shows an example of the process of higher-order self-organization. First, porphyrin is organized with gold nanoparticles via covalent bonds through an alkanethiol chain. Meanwhile, when fullerene is mixed in a good solvent in which it dissolves easily (toluene), supramolecular formation of porphyrin-fullerene occurs, enabling the construction of a P/N junction interface (charge separation interface) at the single-molecule level. Next, by mixing them in the presence of a poor solvent in which they do not dissolve well (acetonitrile), particulate assemblies of about 100 nm in size are formed, which can impart both light-harvesting and carrier transport functions within the assembly. In particular, lengthening the alkyl chain spacer (n=5, 11, 15) subtly changes the distance between the nearest two porphyrin molecules, making it easier for fullerene to be incorporated. In other words, this means that the P/N junction interface has been precisely controlled at the molecular level. In fact, when a solar cell is fabricated by forming a thin film of these particulate assemblies on an electrode substrate, the photocurrent response extends not only over the entire visible light region but also into the near-infrared region (around 950 nm) derived from the charge-transfer absorption band. The photoelectric conversion characteristics show an incident photon-to-current conversion efficiency (IPCE value) of about 60% and a photoelectric conversion efficiency of about 2%, achieving an approximately 50-fold improvement in photoelectric conversion efficiency through higher-order self-organization. A similar effect was obtained using this assembly design strategy with dendrimers, which are dendritic polymers, and chain-like polymers.

Part 2: Control of P/N junction interfaces within rod-shaped molecular assemblies

If anisotropic rod-shaped assemblies can be synthesized instead of isotropic particulate ones, new functions and developments can be expected, such as improved optical and electronic properties due to enhanced crystallinity and directional control of carrier transport. Using different bonding modes (coordination bonds and intermolecular forces) as driving forces, we also succeeded in creating fullerene-encapsulated porphyrin nanorods, as shown in Figure 2, with a fullerene (N) layer on the inside and a porphyrin (P) layer on the outside. In this rod-shaped structure with two separate inner and outer layers, systematic operation of photoelectric conversion is expected—light absorption in the outer P layer, charge separation at the P/N junction interface, and carrier transport in the respective P and N layers. Indeed, good carrier charge generation and photoresponse across the entire visible light spectrum were observed.

Part 3: Construction of P/N junction interfaces on carbon nanotube surfaces

Unlike the spherical molecule fullerene C60, carbon nanotubes (CNTs), which have a characteristic cylindrical structure, are expected to exhibit good carrier transport. Furthermore, if porphyrin molecules, which have a two-dimensional π-planar structure, can be systematically coated onto the CNT surface (supramolecular organization), it would be possible to improve not only solubility but also light-harvesting and charge separation properties, leading to applications in photoelectric conversion devices. We investigated the supramolecular organization of porphyrin and single-walled carbon nanotubes (SWCNTs) and succeeded in constructing rod-shaped assemblies as shown in Figure 3. When a photoelectric conversion cell was similarly fabricated, the coating of the SWCNT surface with porphyrin (forming a P/N interface) resulted in a significant, two-orders-of-magnitude improvement in photoelectric conversion characteristics compared to a system with SWCNTs alone.

Future prospects: Applications to catalytic systems and electronics in general are also possible

The molecular assemblies presented in this article can be applied not only to photoelectric conversion, such as in solar cells, but also to a wide range of other fields, including general electronics and catalytic systems, by pre-programming them with the required functions. For example, by utilizing the carrier charges generated during photoelectric conversion, they can also be used for chemical energy conversion (such as hydrogen production) and material conversion. Recently, we also designed and synthesized a porphyrin hexamer with triphenylene, a type of polycyclic aromatic hydrocarbon, as the central stacking unit, as shown in Figure 4. Simply by dissolving this hexamer in an organic solvent, casting it onto a substrate, and drying it, a unique and clean linear monolayer pattern structure appears. This patterning is expected to be developed as "molecular wiring" for future ultra-fine electronics technology. Thus, molecular assembly chemistry, which utilizes synthetic chemistry and supramolecular chemistry, is not limited to mere manufacturing in the field of chemistry but is expected to play an important role in materials science in the future.