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Tailoring Nanoscale Functions through Precision Control of Molecular Interactions
Prof. Takuzo Aida in the first 15 years of his research carrier had worked for precision polymer synthesis with metal complexes. However, when he received an independent research position in 1994, he changed his research direction completely as described below.
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Prof. Takuzo Aida
The University of Tokyo
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1. Light-Harvesting Dendrimers
In 1993, Aida and coworkers published an important paper on edendrimer porphyrinf, in which a porphyrin functionality is encapsulated by a large poly(benzyl ether) dendrimer framework (ChemComm 1993, 1260). This is the first dendrimer having a covalently attached dye unit in the interior and allowed for exploring important properties of dendritic macromolecules using the interior porphyrin as a probe. This work also triggered a large number of related works, which demonstrated a wide variety of potential applications of dendritic porphyrins (Nano Lett. 2005, 5, 2426). In the course of this study, Aida happened to notice the occurrence of an energy transfer from the dendronized benzyl ether units to the porphyrin core and found later that the efficiency is particularly high (~80% quantum efficiency) when the surrounding dendrimer can adopt a spherical morphology (JACS 1998, 120, 10895). This is one of the three pioneering papers on light-harvesting functions of dendrimers. He also confirmed an analogous morphology effect on harvesting of visible (ACIE Minireview 2004, 43, 150; ACIE 2003, 42, 4060; JACS 2006, 128, 10527) and lower-energy photons (Nature 1997, 388, 454; JACS 2005, 127, 10020). More recently, Aida developed an efficient photochemical water reduction using a water-soluble dendrimer with an encapsulated conjugated polymer (JACS 2004, 126, 12084) and erewritable security inkf using a liquid crystalline metallo-dendrimer with a phosphorescent core (Nature Mat. 2005, 4, 546).
2. Mesoporous Silica for Nanoscale Processing of Polymers
In 1999, Aida published a seminal paper reporting that a mesoporous silica-supported metallocene, in conjunction with methylalumoxane, allows for eextrusion polymerizationf of ethylene from the nanoscopic silicate channels to produce uniform nanofibers of an ultrahigh molecular weight, linear polyethylene (Science 1999, 285, 2113). Despite a large entropic bias toward a folded conformation, polyethylene molecules in the nanofibers are crystallized with an extended-chain conformation, leading to an enhanced mechanical strength of the material (important for ehigh-valuef added products such ball-proof jackets and canvases). Such reinforced material is industrially manufactured via post processing by gel spinning. However, supported metallocenes on mesoporous silica allow synthesis and processing at the same time. eExtrusionf is a universal biological process for reinforcing mechanical properties of biopolymers.
In 2001, Aida et al. also reported a pioneering work on the synthesis of functional mesoporous silica using rod micelles of functional amphiphiles as templates (ACIE 2001, 40, 3803). This new synthetic methodology has a great advantage over the epost-treatmentf method so far employed, since it guarantees dense decoration of the silicate channels with organic functionalities (ChemComm Feature Article 2000, 2399). In fact, Aida et al. demonstrated that diacetylene and pyrrole groups, incorporated into the silica channels by this method, are post-polymerizable into electrically conductive nano-cables. He also proposed a concept of elizard templatef, which bears a condensable head group and a cleavable lipophilic tail (JACS 2004, 126, 988). Lizard templates, designed for post-removal of the structure-directing alkyl tails, surely provide a universal synthetic strategy toward molecularly engineered inorganic channels (CEJ 2007, 13, 1731).
3. Engineering and Applications of Biological Nano Machines
In biological systems, molecular chaperons, tubular-shaped protein assemblies, assist refolding of denatured proteins through trapping and subsequent release triggered by their opening motion upon binding with ATP. In 2003, Aida et al. reported that certain molecular chaperons not only are capable of trapping artificial substances such as semiconductor nanoclusters but also releasing them in response to ATP (Nature 2003, 423, 628). Using a chaperon originating from a heat-shock virus, the nanocluster entrapped is highly stabilized and survives even at 90 ‹C. However, when ATP is added at 37 ‹C, the nanocluster is released and decomposed immediately. Aida et al. also reported a chemically and genetically engineered chaperon that carries multiple azobenzene groups site-specifically at the entrance parts of the protein (JACS 2006, 128, 3764). This engineered chaperon responses to ATP and light, and serves as an eAND-logic gatef for protein refolding. These achievements suggest a potential utility of molecular chaperons for intelligent DDS.
In 2006, Aida also published a highly interesting paper, featuring a scissor-like fully synthetic molecular machine, which traps guest molecules and can induce their conformational motion by light (Nature 2006, 440, 512). This machine is driven by photochemical isomerization of an azobenzene unit, attached to a chiral ferrocene core for a pivotal motion, and demonstrates that a mechanical motion of a molecule can be transmitted to the other through non-covalent bonds.
4. Soft Composite Materials with Dispersed Carbon Nanotubes
In 2003, Aida reported a special effect of ionic liquids on dispersion of carbon nanotubes (Science 2003, 300, 2072). Commercially available carbon nanotubes are heavily entangled and difficult to process. However, upon being ground in ionic liquids, bundled carbon nanotubes are exfoliated into much finer bundles, resulting in the formation of a physical gel referred to as ebucky gelf. Using polymerizable ionic liquids, one can fabricate bucky plastics, which are highly reinforced mechanically and conductive electrically (Small 2006, 2, 554). Buck gels have the potential for many applications. Through collaborations, Aida et al. developed printable actuators (ACIE 2005, 44, 2410) and stretchable integrated circuits (Science 2008, XXX, XXXX). In addition to these, only in a short period, many interesting applications have already been reported (CEJ Concepts, 2007, 13, 5048).
5. Molecularly Engineered Graphitic Nanotubes (2003 ~)
Aida et al. reported in 2004 the first conductive tubular assembly, which may be referred to as egraphite nanotubef (Science 2004, 304, 1481). The self-assembling component consists of a hexabenzocoronene (HBC) core, which bears hydrophilic tetraethylene glycol and hydrophobic dodecyl side. This engineered molecular graphene self-assembles in a helical manner into long nanotubes with uniform diameter and wall thickness of 20 and 3 nm, respectively. Incorporation of stereogenic centers into this HBC amphiphile allows the formation of nanotubes with a one-handed helical chirality (PNAS 2005, 102, 10801, Adv. Mater. 2006, 18, 1297). ADMET (JACS 2005, 127, 8284) and ROMP (JACS 2006, 128, 14337) of amphiphilic HBCs with reactive TEG termini allow stitching of the assembled HBC units. Reversible photochemical stitching is possible when the TEG termini are appended with a coumarin unit (JACS 2006, 128, 4220). Aida et al. also succeeded in fabricating photoconductive nanotubes that consist of a nanoscale coaxial heterojunction of donor and acceptor layers (Science 2006, 314, 1761, JACS 2007, 129, 9276).
Conclusions
Overall, Takuzo Aida has made, just in the last one decade, a number of seminal achievements in the interdisciplinary fields of molecular science. Considering a general significance of his achievements along with their big impacts to a variety of other research fields, he was awarded the Chemical Society of Japan Award.
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