Biological macromolecules possess a specific helical structure with a controlled handedness, which directs their sophisticated functions in living systems. Inspired by such elaborate biological helices, chemists have been challenged to develop artificial helices to mimic the biological helices and their exquisite functions. Dr. Eiji Yashima has been studying for more than two decades the design and synthesis of helical molecules, supramolecules, and polymers with novel structures and functions based on his unique approach and strategy, and has made seminal contributions to our understanding of the importance of the helical chirality of polymers and oligomers for their functions including the sensing of chirality, molecular and chiral recognitions, resolution of enantiomers, and asymmetric catalysis. Furthermore, he has made a key contribution to develop a versatile methodology to directly visualize the helical structures of polymers and foldamers by atomic force microscopy (AFM). His major achievements are briefly summarized below.

1. Discovery of Helicity Induction and Memory Effects in Synthetic Macromolecules

In 1995, Yashima discovered a unique helicity induction with a controlled helix-sense in optically inactive polymers, such as poly(phenylacetylene)s bearing functional pendant groups, through noncovalent bonding interactions upon complexation with nonracemic molecules, resulting in a characteristic induced circular dichroism (ICD) in the p-conjugated polymer backbones. Based on this finding, he developed novel chirality sensing systems for a variety of chiral molecules by CD spectroscopy based on the preferred-handed helicity induction of the functional poly(phenylacetylene)s. Thereafter, he also discovered the unprecedented memory of helical chirality induced in poly(phenylacetylene)s when the chiral molecule is replaced by various achiral ones. This helicity induction and memory effect has been proved a unique and valuable method to constructing helical polymers, such as polyisocyanides with a helicity memory that showed an efficient enantioselectivity during asymmetric reactions as well as a chiral stationary phase for separating enantiomers during HPLC. This method also has a significant advantage from a practical viewpoint such that a preferred-handed macromolecular helicity can be induced in commodity polymers, such as syndiotactic poly(methyl methacrylate) (st-PMMA), which further forms a practically useful stereocomplex with an optical activity upon the inclusion complexation of isotactic PMMA into the helical cavity of st-PMMA. These studies represent major milestones in the field of polymer chemistry and supramolecular chemistry. A number of helical molecules and polymers were then prepared based on the helicity induction and memory concept.

2. Direct Observations of Helical Structures of Polymers, Foldamers, and Small Molecule-based Helical Assemblies

Although a number of studies has reported the synthesis of helical polymers, a preferred-handed helix formation is usually presumed by the CD or optical rotation, which is not straightforward, and does not give unambiguous information about the helical structures, particularly the helix-sense. The direct observation and elucidation of the helical structures of helical polymers including the helical pitch and handedness by microscopy, a long-standing problem in polymer science, was for the first time achieved by Yashima in 2006 using AFM coupled with organic solvent vapor exposure. He found that rod-like helical polymers hierarchically self-assembled on graphite to form regularly arranged two-dimensional crystals of helix-bundles, which allowed one to directly visualize the helical structures that also include the helical pitch, handedness (right or left), helix-sense excess, and helical reversal. This method also made it possible to observe the helical structures of the foldamers and helical assemblies of small molecules and merits further research into helical polymers and oligomers with unique structures and specific functions.

3. Double-Stranded Helical Polymers and Oligomers with Specific Functions

In spite of the numerous variations of single-stranded helical polymers and oligomers, the molecular design strategy for constructing double-stranded helical polymers and oligomers is still limited, despite the natural model, the DNA double-helix. Yashima has developed a modular strategy to construct complementary double helices stabilized by salt bridges with crescent-shaped m-terphenyl backbones, providing the first one-handed complementary double helices. A series of complementary double helices composed of different sequences and chain-lengths and water soluble double helices that exhibit specific functions, such as the molecular recognition toward carbohydrates in water, have also been synthesized. A complementary double-helix dimer composed of achiral strands bridged by achiral diphosphines was also enantioselectively prepared by taking full advantage of the helicity induction and memory effect, which accommodated metal ions, and the Cu(I) complex catalyzed the asymmetric cyclopropanation reaction. The chiral space generated by the complementary double-helix is essential for this high enantioselectivity, thus providing a promising and conceptually new strategy in the broad fields of supramolecular catalysis with a unique double-stranded helical structure. Thereafter, he developed an enantiopure molecular spring consisting of two tetraphenol strands bridged by spiroborates that sandwiches a sodium ion in the middle. The spiroborate helicate exhibited a reversible anisotropic spring-like extension-contraction motion triggered by the successive addition and removal of sodium ions while maintaining its one-handedness, thus leading to the development of molecular machines with unique chiral and mechanical functions originating from their unidirectional spring motions.

The pioneering role and enormous contributions of Dr. Eiji Yashima to the field of chirality and stereochemistry in helical systems including polymers and supramolecules and their application to chiral materials are unquestionable and his significant achievements are highly valued worldwide, therefore, he deserves The Chemical Society of Japan Award.