Susumu Tsuda

Synthesis of Organic-Soluble Conjugated Polyrotaxanes by Polymerization of Linked Rotaxanes

Extensive research on the use of cyclodextrin for insulating pi-conjugated polymer chains has been carried out. However, the resulting polyrotaxanes do not exhibit high and constant covering ratios and are generally insoluble in organic solvents. Here we demonstrate a new method of synthesizing permethylated cyclodextrin-based polyrotaxanes involving the polymerization of linked rotaxane monomers. The insulated molecular wires obtained by this method are highly soluble in organic solvents and have a high covering ratio, rigidity, and photoluminescence efficiency. A cholesteric liquid-crystal phase was observed for these highly rigid polyrotaxanes, in which threading of a pi-conjugated polymer chain through chiral macrocycles occurs.

Insulated Molecular Wire with Highly Conductive π-Conjugated Polymer Core

We have recently developed a new method for synthesizing polyrotaxanes with a high covering ratio, rigidity, photoluminescence efficiency, and solubility in a variety of organic solvents through the polymerization of structurally defined rotaxane monomers. The rigid rodlike structure of the pi-conjugated core polymers in these polyrotaxanes is thought to facilitate the effective transport of charge carriers. Here we applied this method to the synthesis of a polyrotaxane having a poly(phenylene ethynylene) backbone by the Sonogashira copolymerization of a structurally defined rotaxane with a linker molecule. According to time-resolved microwave conductivity and transient absorption spectroscopy measurements, the hole mobility along the pi-conjugated polymer chain of the polyrotaxane thus formed was extremely high and comparable to that in amorphous silicon.

Single-Molecule Conductance of π-Conjugated Rotaxane: New Method for Measuring Stipulated Electric Conductance of π-Conjugated Molecular Wire Using STM Break Junction

An electronic conductance with small fluctuations, which is stipulated in single-molecule junctions, is necessary for the precise control of single-molecule devices. However, the suppression of conductance fluctuations in conventional molecular junctions is intrinsically difficult because the fluctuations are related to the contact fluctuations and molecular motion. In the present study involving experimental and theoretical investigations, it is found that covering a single π-conjugated wire with an α-cyclodextrin molecule is a promising technique for suppressing conductance fluctuations. The conductance histogram of the covered molecular junction measured with the scanning tunneling microscope break-junction technique shows that the conductance peak for the covered junction is sharper than that of the uncovered junction. The covering technique thus has two prominent effects: the suppression of intramolecular motion, and the elimination of intermolecular interactions. Theoretical calculations of electronic conductance clearly support these experimental observations.

Preparation and Characteristic Control of Conducting Polymer/Metal Oxide Nano‐Hybrid Films for Solar Energy Conversion

We review our recent investigation of photoinduced polymerization of thiophene inside TiO2 nanopores to form nanostructured polythiophene/TiO2 heterojunction films and the related kinetic studies. The nanohybrid film possesses dense heterojunction and electronic connection within the TiO2 nanoporous domain. Photo-polymerization proceeded in 3 stages, (i) photoexcitation of bithiophene covalently attached to the TiO2 surface, (ii) an electron injection reaction from the surface attached thiophene to the TiO2 and (iii) an electron transfer from a thiophene reactant in an electrolyte to the surface attached oxidized bithiophene. The nanohybrid film was applied to a sensitized-type photoelectrochemical solar cell, substantiating direct application of the nanohybrid film to electronic devices. Despite increase in light harvesting efficiency, wavelength dependent incident photon-to-current conversion efficiency decreased with the light irradiated polymerization time. In order to identify a factor controlling photocurrent efficiency, kinetic studies at TiO2/bithiophene/electrolyte interfaces were conducted, and their parameters to the solar cell functions were related. Comparison of emission studies between the bithiophene adsorbed TiO2 and Al 2O3 revealed the electron injection from the excited bithiophene into the TiO2 with the efficiency of nearly 100 %. The charge recombination between the bithiophene cation and the electron in the TiO2 appeared to be fast with a half decay time of 70 ?s in comparison to the ruthenium dye sensitized TiO2 film (?1 ms). The bithiophene regeneration kinetics was slightly faster, clarifying the inferior photocurrent performance.