Self-assembly is fundamental to generating structures on all scales from molecules to galaxies. It is defined as a reversible process in which disordered pre-existing parts (building blocks) of a pre-existing system form ordered structures and patterns. Intelligent materials can be tailored from specifically designed molecular components through self-assembly that are programmed under the control of directional noncovalent physical interactions.

1.1) Helical Supramolecular Self-Assembly. 
A helix is a screw-like twist that can be either right- or left-handed, (Figure 1). Helical structures have been studied for several decades in material, electrical, optical, and life sciences not only out of scientific interest, but also for their practical applications. In biological chiral materials such as DNA and RNA, chiral structures on different length scales are essential. Helices are often constructed by introducing molecular/atomic chirality with the lowest possible symmetry (C1). On the nanometer scale, non-covalent interactions such as hydrogen-bonding cause the handedness of helical conformations. These helical molecules then pack to form the phase structure and thus, exhibit phase chirality. These phase structures further aggregate to form object chirality on the macroscopic scale. It is known that the formation and construction of chiral structures as well as the chirality transfer between length scales is critical to the design, synthesis and construction of molecular and supramolecular chiral structures to achieve the macroscopic properties desired for specific bio-mimetic and electro-optical applications (Figure 2). The purpose of this project was first to build helical structures on different length scales through supramolecular self-assembly processes (“bottom-up” approach), then second to find the origin of helix formation from achiral/chiral building blocks. This will provide insight into new ways to construct molecules that can form chiral structures which will finally enable these helical concepts to be used in practical applications. This project required extensive structural (TEM, WAXD, SAXS, ED) and phase stability analysis (DSC, OM). 

1.2) Biosensors though Molecular Self-Assembly. 

A biosensor is a device for the detection of a biological analyte with a physico-chemical detector. The most widespread example of a commercial biosensor is the blood glucose biosensor, which uses an enzyme to break blood glucose down. The high market demand for such sensors has fueled development of additional sensor technologies. In this research project, we developed simple biosensors which can detect the handedness of both chemical and biological materials using the so-called “sergeant and soldier effect” which is well-known in chiral liquid crystal self-assembly. Recently, we constructed both right-handed and left-handed propeller-patterned chiral nematic droplets together with achiral droplets from achiral biphenyl carboxylic acid compounds (Figure 3). When the two propeller-patterned chiral nematic droplets with the identical handedness merge, a bigger chiral propeller-patterned nematic droplet with the same handedness is formed. On the other hand, if two propeller-patterned chiral nematic droplets with opposite handednesses merge, an achiral nematic droplet is formed. Therefore, the achiral nematic droplets are attributed to being a racemic mixture of both the right-handed and left-handed twisted chiral conformers, and the chiral propeller-patterned nematic droplets are originated from the twisted chiral conformers. Based on the concept of the “sergeant and soldier effect”, if an unknown chiral material invades the twisted chiral conformers, the handedness of all the nematic propeller-patterned droplets will be either right-handed or left-handed depending on the handedness of unknown material. The advantage of this biosensor is its high sensitivity, simple device construction and easy morphological detection.

1.3) Photovoltaic though Molecular Self-Assembly.

Photovoltaic arrays use solar energy to provide electricity for human activities. Solar cells produce direct current from the sun’s rays, which can be used to power equipment or to recharge a battery. Energy conversion efficiencies achieved to date using conductive polymers have been low (4-5% efficiency) for the best cells to date. However, these cells could be beneficial for some applications where mechanical flexibility and disposability are important. In this research project, we would like to improve the efficiency of polymeric solar cells using molecular self-assembly techniques. Since columnar discotic liquid crystals show many extraordinary properties such as one-dimensional electrical conductivity, fast photoconductivity and ferroelectricity, the structures and dynamics of discotic liquid crystals have been studied extensively. Recently we constructed robust discotic columns from the symmetric tapered bisamides. It was found that a discotic column was generated through inter-molecular hydrogen-bonding of four tapered bisamides, and these supramolecular discs further assembled into oblique columnar phases. By controlling the direction of columnar long axis, we can significantly increase the one-dimensional electric conductivity and photoconductivity which are the main drawback in the polymer solar cells (Figure 4). Furthermore, we achieved biaxial molecular orientation from rod-disc molecules, synthesized via the chemical attachment of the six cyanobiphenyl rod mesogens to the periphery of a triphenyl disc-like mesogen group with six alkyl chain linkages. This biaxial molecular arrangement can provide numerous potential electro-optical applications including photovoltaics. Another approach to improve the efficiency of organic/polymeric solar cell is to incorporate inorganic materials inside of the columnar assembly. Recently, we achieved ion-promoted, automorphogenic self-assembly of nanoscale composite fibers using a structurally rigid hexameric macrocycle possessing photonic and electron storage potential and a dodecacarboxylate-terminated dendrimer (Figure 5). Utilizing selected area electron diffraction, we realized that the molecular wires are constructed by an alternating stack of rigid polycationic macrocycles and spherical polyanionic dendrimers. These are expected to be able to improve electron conduction thus increase the efficiency of solar cells.

The self-assembly of block copolymers has received much attention due to the ordering of hierarchical structures on different length scales in one system. The use of the physical properties of block copolymers in such areas as transport, mechanical, electrical, and optics will provide substantial benefits in the future.

2.1) Crystal Orientations and Crystallization Behaviors in Nano-confined Environments. 

In recent years, there has been an interest in the crystallization behaviors and crystal orientations of materials in nano-confined environments (Figure 6). To generate a nano-confined environment, one usually uses crystalline-amorphous diblock copolymers or copolymer blends. These systems are chosen because when the two components strongly phase separate, the phase structure can be lamellar, double gyroid, hexagonal cylinders, or face-centered cubic spheres based on the diblock copolymer or blend volume fractions. If the glass transition temperature (Tg) of amorphous block (or blend) is higher than the melting temperature (Tm) of crystalline block (or blend), a hard nano-confined environment can be created. In this research project, utilizing simultaneous two-dimensional small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) techniques, we can investigate the crystal orientation changes inside or outside of the one-, two-, and three-dimensional hard/soft confined environments, which can open new potential nano-technological applications. The remaining question in this project is why we did not observe the c-axis orientation that is parallel to in the hexagonal columnar phase, even though we can see this in the inversed hexagonal columnar phase confinement. In order to answer this question, we must investigate the effect of increasing the diameter size of the PEO cylinders in the PS matrix of the hexagonal columnar phase. 

2.2) Construction of Honeycomb Structure and Super-Hydrophobic Surface from Block Copolymers. 

Ordered mesoporous polymeric materials are of significant interest in areas such as photonics, separations, and catalysis, which require organized microporous structures on the order of sub-micrometer to micrometer length-scales (Figure 7). When a thin film of block copolymer is cast using an appropriate solvent on a solid substrate, a highly ordered and microporous honeycomb structures with a characteristic length scale is formed. In addition, super-hydrophobic materials with water contact angles higher than 150° have attracted great interest because of their practical application in water repellency, self-cleaning, and antifouling coating. The water contact angle is affected by the water-repellent behavior of the fractal micro-nanoscale binary structure of the materials. Preparation of the rough surface and subsequent coating of the surface with low surface energy material such as fluorine compounds is an essential process in fabricating super-hydrophobic surfaces. However, the use of the fluorine compound comes with certain disadvantages such as low oil repellency, high toxicity and high cost. In this research project, we attempt to form hierarchical structures from a honeycomb structure in common/selective mixed solvents to a super-hydrophobic surface simply by changing the selective solvent content of the non-fluorinated polymer solution without the need for low-surface energy modification. 

Smart polymers have one or more properties that can be significantly altered in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields. Suitably designed structures made from these smart polymers can therefore be made that bend, expand or contract when an external stimulus is applied.

3.1) High Performance Polymers with Excellent Elastic Recovery and Strength. 

Due to their anisotropic properties and the self-assembling capabilities, main-chain liquid crystalline polymers have been used in a range of mechanical, electrical, optical and biological applications. In order to tune these properties, main-chain liquid crystalline polymers, comprised of flexible alkylene spacers between rigid and controlled aromatic units, with various chemical structures and molecular architectures for both mesogenic groups and flexible spacers have been designed. It is common to tailor liquid crystalline stabilities by selecting different mesogenic groups, varying the location of the mesogens, and altering the lengths and types of flexible spacers in the liquid crystalline polymers. Forming their liquid crystalline phases is dependent of the configuration of the meta- and para-linkages in the mesogenic groups (Figure 8). The spacer length can cause periodic changes in the thermodynamic properties of a material based on whether there are an even or odd number of unit in the liquid crystalline polymer spacers. Similar to the bent-core liquid crystalline small molecules, even numbered polyesters exhibited a limited crystal twisting on the micrometer length scale because of configurational bent. In this research project, we can develop high performance smart polymers with excellent elastic recovery and strength by introducing a conformational bend and/or a configurational bend in the mesogenic groups and/or flexible spacers.

3.2) Quantum Dots via a Dendritic Functionalized Template. 

A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes, or excitons in all three spatial directions. Quantum dots can provide a functional platform for the creation of novel materials and devices that benefit from the unique physical and electrical properties arising from the confined nature of there quantized electronic states, which are intermediate between those of the molecular and bulk scales. They also form the basis for new photovoltaic cells, light-emitting diodes, biosensors, and other hybrid materials prepared by directed- and self-assembly techniques. In this research project, we fabricated well-ordered CdS quantum dot composite assemblies on dendronized single wall carbon nanotubes (Figure 9) or dendrimerized chiral cellulose. The size of these dots puts them in the quantum-confined regime and they exhibit novel luminescence properties. Templated CdS quantum dots are useful for fabricating molecular electronic devices based on their unique nanoscale electronic properties as well as for building devices and components for molecular biology, biotechnology, and biomedicine.