Future research directions

　Synthetic chemistry stands out as the most creative discipline, offering boundless opportunities to create value. The imaginative process of molecular design, coupled with the creativity of molecular assembly and editing technology, ensures an endless array of targets. A molecular model, combined with powerful chemical reactions, becomes the gateway to unlocking infinite possibilities in terms of physical and biological properties and functions. Notably, the study of fullerenes serves as a representative example, showcasing the unforeseen properties and functions hidden within. New forms of carbon often possess undiscovered or unprecedented properties, unknown at the time of their discovery.

　In the realm of molecules, history attests that aesthetically pleasing structures often coincide with remarkable functionality. Molecules possess the transformative power to change the world. Reflecting on the history of science, breakthroughs related to the creation and discovery of new molecules, as well as the exploration of unprecedented physical and biological properties, have led to nonlinear advancements. Moreover, the innovative connection of molecules and fields seemingly unrelated in common sense can lead to the creation of new value, echoing Steve Jobs’ famous quote about “connecting the dots.”

　Our approach involves the simultaneous creation of molecules that embody these innovation factors. We embark on an exploration of uncharted territories, aspiring to craft unique molecules that blend structural beauty with unparalleled functionality, thereby creating new materials. Additionally, we aim to activate our molecules in the unexplored boundaries between fields that have never coexisted before, delving into new frontiers. Our commitment to molecule creation chemistry is grounded in three key pillars: nanocarbon click chemistry for “molecular synthesis and editing,” molecular nanocarbon materials science focusing on “extraordinary structure and function,” and molecular nanocarbon biology dedicated to “exploring uncharted territories.” Through this multidimensional approach, we aim to drive innovation and shape the future landscape of science and technology.

Nanocarbon click chemistry

Fused aromatic compounds, like nanographene, constitute a vital class of compounds extensively utilized in electronic materials, including organic thin-film transistors, solar cells, and organic electroluminescence devices. These compounds are increasingly recognized as “molecular nanocarbons,” representing a crucial avenue for harnessing nanocarbons as “molecules” in the future. However, the synthesis of molecular nanocarbons encounters challenges such as “multi-step-process problems” and “unsynthesized problems.” We have pioneered novel synthetic methodologies, such as the APEX reaction, revolutionizing the synthesis of molecular nanocarbons and fused-ring aromatic compounds.

In the pursuit of “molecular nanocarbon biology,” a key focus of our future research, synthesis difficulties pose a bottleneck in the biological applications of these compelling compounds. Hence, developing a more robust synthetic method is essential. We are set to develop the “nanocarbon click reaction” as a new concept, expected to significantly impact not only molecular nanocarbon biology research but also future research in molecular nanocarbon materials science and a wide array of fields.

Molecular nanocarbons and “functional” molecules, encompassing organic electronic materials, plastics, medicines, agrochemicals, proteins, and amino acids, predominantly feature benzene rings. These rings, serving as a ubiquitous skeletal structure, play diverse functional roles. However, benzene rings, known for their stability and low reactivity, are typically used as the backbone of functional molecules. The transformative potential lies in the ability to functionalize benzene rings under “selective” and “mild” conditions, akin to click reactions. Our endeavors have resulted in advancements like the direct conversion of benzene rings into molecular nanocarbons and bioactive molecules (catalytic C–H activation) and the APEX reactions. However, their reactivity and selectivity remain insufficient, necessitating high temperatures and catalysts. Our ultimate goal is to apply these reactions to complex systems, aiming for direct intracellular conversion.

To address these challenges, we are committed to establishing nanocarbon click chemistry, targeting nanocarbon structures and benzene rings. We aim to elevate the reaction power to a level comparable to the click reaction that earned the Nobel Prize in Chemistry in 2022. Nanocarbon click chemistry is pivotal, unlocking possibilities for the development of both molecular nanocarbon materials science and molecular nanocarbon biology.

Molecular nanocarbon materials science

　While we have successfully synthesized a variety of molecular nanocarbons and investigated their functionalities, our current objective is to design unique molecules that outperform conventional ones in both structure and function. Our ultimate ambition is to craft a molecule surpassing the capabilities of even fullerenes.

Synthesis of ultimate molecular nanocarbons

　We have successfully synthesized nanocarbons with diverse curved and flat structures. At RIKEN, our current emphasis is on synthesizing three molecules that can be regarded as ultimate structures. Looking ahead, we plan to introduce specific molecular structures in our ongoing research.

All-carbon electronic materials

　In the field of organic electronics, materials that exhibit fundamental physical properties such as hole transport, electron transport, semiconductor conductivity, and luminescence are indispensable. The field has witnessed remarkable progress through the discovery and advancement of superior organic materials. Triarylamines with nitrogen atoms dominate hole-transport materials, while electron-transport materials are led by electron-deficient heterocycles and fullerene derivatives. Polythiophenes and oligothiophenes are prevalent in the domain of organic semiconductors. The hallmark of these molecular groups lies in the incorporation of heteroatoms to attain desired physical properties.

　However, we aim to challenge this conventional wisdom. Our goal is to disrupt this “common sense” by introducing all-carbon electronic materials composed solely of carbon and hydrogen atoms. We aspire to establish these materials as the new standard, ushering in a nonlinear breakthrough in the field of organic electronics.

Structure-property mapping of molecular nanocarbons

　While we possess strengths in synthetic chemistry, the development of the aforementioned materials poses a significant challenge, requiring extensive time and manpower through the conventional “molecular design → synthesis → physical property evaluation” approach. This challenge is attributed to the extremely large diversity of nanographene structures. Even when restricting our focus to nanographene, a polycyclic aromatic hydrocarbon with 10 or fewer six-membered rings, the theoretically feasible structures amount to an overwhelming 20,620. Compounding the complexity, many of these structurally simple molecules have never been synthesized.

　To address this complexity, we employ a multidisciplinary approach, integrating quantum chemical calculations, machine learning, AI, and robotic synthesis to construct structure-property maps of molecular nanocarbons. We aim to expedite the achievement of our goals using cutting-edge nanocarbon synthesis technology by narrowing down the group of candidate molecules that exhibit the desired physical properties.

　We envision that the structure-property maps generated for nanographenes and molecular nanocarbons will serve as a kind of “periodic table,” offering researchers worldwide a valuable resource for their endeavors in chemistry. These maps are poised to become a significant asset, contributing to the advancement of research and innovation in the field.

Molecular nanocarbon biology

　Molecular nanocarbons possess distinctive attributes not present in traditional bio-functional molecules, including a broad 2D π-plane, flexible construction of 2D and 3D structures, as well as photo/electronic responsiveness and bioorthogonality. By leveraging these unique features, we are confident in our ability to create molecular nanocarbons that drive bio-innovation through a purposeful molecular design.

　We believe that the introduction of the concept of “molecular nanocarbon biology” will enhance the value and creativity of nanocarbons. This concept is poised to contribute to the ongoing social impact of nanocarbons within the field of materials science.

Bioactive molecular nanocarbons

Traditionally, drug discovery has relied on empirical rules like Lipinski’s rule of five to define drug-likeness. However, the chemical space of nanocarbons significantly deviates from established medicinal criteria, leading to skepticism about their biological activity. Despite this, there has been a lack of thorough verification. Our recent discovery of several bioactive nanocarbon molecules challenges these perceptions and is poised to usher in guiding principles for the development of bioactive compounds.

Pharmaceutical companies, which have extensively explored drug candidates within the chemical space defined by Lipinski’s rule of five, face challenges in discovering new drugs globally. Molecular nanocarbons, serving as outliers, present the potential for groundbreaking discoveries. The success of our recent research on bioactive nanocarbon molecules represents a significant expansion of the conventional chemical space. This breakthrough is anticipated to establish a new paradigm in drug discovery research, introducing novel exploration areas and revitalizing the pursuit of new therapeutic agents.

Controlling protein-protein interactions by molecular nanocarbons

　We are inclined to explore the control of protein-protein interactions (PPIs), as a distinctive avenue for molecular nanocarbons, given their central role in the development of costly antibody drugs. PPIs play crucial roles in various biological processes, including cell proliferation, differentiation, and signal transduction, making them noteworthy targets for novel drug discovery. Although achieving this with conventional small organic molecules is challenging, we believe that the wide π-plane and unique physical properties of molecular nanocarbons offer precise control over PPIs.

　Through preliminary screening utilizing our nanocarbon library, we have recently identified a molecular nanocarbon with the capability to regulate PPI. Moving forward, in tandem with optimizing molecular structures and delving into structural biology research, we plan to undertake drug discovery verification, laying a solid foundation for therapeutic drug development. We believe that molecular nanocarbons possess the potential to establish a new standard for PPI-controlling molecules. This is attributed to their high hydrophobicity, abundant aromatic rings, and susceptibility to π–π interactions.

Delivery of proteins and nucleic acids by molecular nanocarbons

　Biopolymers like proteins and nucleic acids play crucial roles in basic research and are increasingly gaining prominence as pharmaceuticals (such as antibody drugs and nucleic acid drugs) and tools for genome editing. The direct introduction of proteins is particularly advantageous, as it circumvents the need for transcription, translation, or post-translational modifications associated with protein expression using nucleic acids. This approach is especially ideal for producing induced pluripotent stem (iPS) cells, where precise timing of functional expression is crucial for inducing cell differentiation.

　However, biopolymers generally face challenges due to their limited membrane permeability caused by their size and surface charge. Recently, we have designed and synthesized a molecular nanocarbon that demonstrates the ability to transport nucleic acids within mammalian cells. Furthermore, we have identified a nanocarbon molecule capable of transporting proteins, which poses a more significant challenge than nucleic acids. Looking ahead, our goal is to pioneer a new class of standard materials for the transport of proteins and nucleic acids, addressing both fundamental and practical needs in this evolving field.

Nanocarbon peptides

We plan to innovate by integrating nanocarbons with appealing physical properties into widely used peptides in biology and materials science, giving rise to a novel class of materials termed “nanocarbon peptides.” Our exploration will span the realms of chemistry, biology, and materials science. By combining the high bioorthogonality of nanocarbons with the excellent biocompatibility of peptides, these materials can serve as biocompatible substances unaffected by the in vivo environment, particularly valuable in biological applications.

Moreover, incorporating the distinctive π-plane characteristic of nanocarbons into peptides that contribute to aggregate structures opens up possibilities for their use as supramolecular materials with unique assembly modes in the materials field. Nanocarbon peptides represent a pioneering category of “connecting-the-dots” hybrid molecules, enabling molecular nanocarbons to function in a dimension beyond their traditional scope, presenting infinite possibilities.

In our ongoing research, we also plan to explore additional themes, and details about these themes will be introduced later.