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Over the past 30 years, molecular recognition, host-guest chemistry, and supramolecular chemistry have received much attention. Crown ethers, cryptands, and cyclophanes have been extensively used as host molecules. In biological systems, macromolecular recognition, which is the recognition of macromolecules by other macromolecules, plays an important role in maintaining life. Based on biological systems, Harada initiated studies on gmacromolecular recognitionh using simple artificial systems.
1. Polyrotaxanes and Tubular Polymers
Harada discovered that cyclodextrins (CDs) form complexes with various polymers with high selectivity. He first prepared polyrotaxanes by capping complexes between polymers and CDs with bulky stoppers (Nature 1992). Additionally, he prepared tubular polymers by binding neighboring CDs with short cross-linking reagents and then removed the bulky stoppers and polymer chain (Nature 1993). He synthesized the smallest molecular tube in the world. Successively he found that γ-CD forms double chain inclusion complexes (Nature 1994). His observations are epoch-making discoveries. Moreover, he demonstrated that an STM tip can manipulate CD rings (Molecular Abacus). Later, he discovered that CDs form complexes with hydrophilic polymers as well as hydrophobic polymers, such as polypropylene, polybutadiene, polyisobutylene, and even poly(dimethyl siloxane) and polysilanes.
2. Dynamic Aspects of Polyrotaxanes
The threading process can be observed in real time via NMR spectroscopy on cationic (viologen) polymers. Using this knowledge, Harada prepared a Molecular Shuttle using polymethylene chains as stations, viologen units as potential surfaces, and α-CD as rings. During these experiments, he discovered that multi-cationic units prevent dethreading of the CD ring from the dumbbell component of the rotaxane by electrostatic repulsion (Electric Trap, JACS (2000) & Science (Editorfs Choice).
Recently, it was discovered that the sense of threading of the α-CD rings onto a polymer chain is completely controlled by the 2-methylpyridyl group at the end of the dumbbell. The α-CD ring passes over the 2-methylpyridyl group from the wider secondary hydroxyl group side. Harada reported the threading dynamics of CD derivatives with PEG chains attached to the primary face in competition with adamantyl carboxylic acid as a guest (JACS, 2006, 2007).
3. Construction of Supramolecular Polymers
Harada demonstrated that when a guest is attached directly to CD ring, a chemically modified CD forms various types of supramolecular architectures, e.g., linear polymers, rings (Daisy Chain Necklace), helices, and alternating supramolecular polymers. In addition, he successfully obtained supramolecular [2]rotaxane polymers. CD dimers form supramolecular polymers with a guest dimer.
4. Supramolecular Catalysis
Harada discovered that CDs initiate polymerization of lactones to give polyester-tethered CDs. Different lactones exhibit different selectivities. CDs play not only a role as an enzyme-like binding pocket, but also serve as chaperone proteins. Recently, he reported that CD dimers can efficiently catalyze ring-opening polymerization. One of the CD rings serves as a clamp part similar to DNA polymerase(Nature Highlight, 2011).
5. Supramolecular Sensors
Harada and his group employed CDs attached to π-conjugated polymers as sensors for various compounds. The addition of guest compounds can alter the fluorescence of CD conjugated poly(phenylene ethynylene).
6. Side-chain Recognition of Polymers by CDs
Harada revealed that the side chains of brush polymers are recognized by CDs in a manner reminiscent of natural polymers, proteins, and nucleic acids. He demonstrated that α-CD polymers and guest polymers containing azobenzene (AB) as a side chain form a gel in water. Irradiation with UV light converts the gel into a sol, but exposure to visible light or heating restores the gel. That is, the conversion is reversible. trans-AB is included in α-CD to become a cross-linker, while cis-AB departs from the α-CD cavity to become a sol.
7. Self-Healing Gels
Mixing β-CD polymers with guest polymers containing ferrocene in water yields a hydrogel. Harada reported that even if a gel is cut in half, once the pieces come into contact with each other, they self-heal into a single gel (Nature Communications 2011). In addition, the self-healing process can be controlled by redox stimuli.
8. Macroscopic Self-Assembly
Harada observed that a host gel containing a β-CD strongly binds to a guest gel containing adamantane moieties. Molecular recognition plays important roles at both the molecular and macroscopic levels. When pieces of α-CD gel, β-CD gel, n-Bu gel, and t-Bu gel are mixed and shaken in water, α-CD gel binds only to n-Bu gel and β-CD gel sticks only to t-Bu gel to give an alternating or checkered structure. These results clearly indicate that a host gel recognizes the corresponding guest gel through molecular recognition to form macroscopic structures. Because host gels can detect small differences in the guests, this system can be applied to differentiate macroscopic materials through molecular recognition of various guests. This is the first instance of a molecular recognition event used to assemble large objects.
When α-CD gel and azobenzene (AB) gel are mixed and shaken in water, they bind to each other and form an assembly. Irradiating the assembly with UV light causes the AB gel to cleave the α-CD gel and bind with the β-CD gel. Harada successfully controlled the association and dissociation of the gel assembly via light irradiation (Nature Communications).
9. Functionalized Monoclonal Antibodies
Monoclonal antibodies (MA) against achiral Rh complex were used as a second-sphere coordinating ligand for Rh complexes. The complex catalyzes hydrogenation of amino acid precursors to give 100% L-amino acid. MA for the porphyrin-acceptor complex functions as an artificial photosynthetic system to produce H2. Harada reported the highest efficiency for an artificial protein-porphyrin photosynthetic system.
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