Cooperative supramolecular polymerization of a perylene diimide derivative and its impact on electron-transporting properties.

H-bonding-promoted supramolecular polymerization of a perylene diimide (PDI) building block and its impact on charge carrier mobility were studied. PDI-1, containing a carboxylic acid group, exhibits H-aggregation in a non-polar solvent decalin while in THF or chloroform it remains in the monomeric form. In contrast, the control molecule PDI-2, containing an ester group does not show aggregation even in decalin, indicating that H-bonding among the carboxylic acid is primarily responsible for H-aggregation, which is further verified by the FT-IR study. A variable temperature UV/Vis study establishes a cooperative pathway for the supramolecular polymerization of PDI-1. Microscopy images show a short fibrillar morphology. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements reveal significantly higher electrical conductivity for the PDI-1 film prepared from decalin compared with that prepared from THF/MeOH or the film of non-aggregated PDI-2 prepared from decalin. By combining the transient absorption spectroscopy data (that estimate the charge carrier generation efficiency) and the TRMC evaluated conductivity, the 1D charge carrier mobility of PDI-1 (µe,1D) is estimated to be 0.24 cm2 V-1 s-1, which is among the top values reported for any PDI derivative measured using the same technique.

Stabilization of Charge Carriers in Picket-Fence Polythiophenes Using Dielectric Side Chains.

Insulated molecular wires (IMWs) are π-conjugated polymers that are molecularly sheathed with an insulating layer and are structurally analogous to electric power cords at the nanoscale. Such unique architectures are expected in molecular electronics and organic devices. Herein, we propose a new molecular design concept of IMWs, in which the sheaths can be customized, thereby enabling the modulation of the electronic properties of the interior π-conjugated systems. To this end, we focused our attention on the dielectric constant of the sheaths, as it governs the electrostatic interaction between charges. Upon doping, charge carriers, such as polaron and bipolaron, were generated regardless of the dielectric properties of the sheaths. Flash-photolysis time-resolved microwave conductivity measurements revealed that intrawire charge carrier mobility was independent of the sheaths. However, we found that the charge carriers could be stabilized by the sheaths with a high dielectric constant owing to the charge screening effect. We expect that IMWs designed in this way will be useful in a variety of applications, where the nature of charge carriers plays an important role, and particularly when redox switching is required (e.g., electrochromic, magnetic, and memory applications).

Repeat protein scaffolds: ordering photo- and electroactive molecules in solution and solid state.

The precise control over the organization of photoactive components at the nanoscale is one of the main challenges for the generation of new and sophisticated macroscopically ordered materials with enhanced properties. In this work we present a novel bioinspired approach using protein-based building blocks for the arrangement of photo- and electroactive porphyrin derivatives. We used a designed repeat protein scaffold with demonstrated unique features that allow for the control of their structure, functionality, and assembly. Our designed domains act as exact biomolecular templates to organize porphyrin molecules at the required distance. The hybrid conjugates retain the structure and assembly properties of the protein scaffold and display the spectroscopic features of orderly aggregated porphyrins along the protein structure. Finally, we achieved a solid ordered bio-organic hybrid thin film with anisotropic photoconductivity.

Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?

For inorganic semiconductors crystalline order leads to a band structure which gives rise to drastic differences to the disordered material. An example is the presence of an indirect band gap. For organic semiconductors such effects are typically not considered, since the bands are normally flat, and the band-gap therefore is direct. Herein we show results from electronic structure calculations demonstrating that ordered arrays of porphyrins reveal a small dispersion of occupied and unoccupied bands leading to the formation of a small indirect band gap. We demonstrate herein that such ordered structures can be fabricated by liquid-phase epitaxy and that the corresponding crystalline organic semiconductors exhibit superior photophysical properties, including large charge-carrier mobility and an unusually large charge-carrier generation efficiency. We have fabricated a prototype organic photovoltaic device based on this novel material exhibiting a remarkable efficiency.

Characterization of a new cold-adapted lipase from Pseudomonas sp. TK-3.

A psychrotrophic Pseudomonas sp. TK-3 was isolated from dirty and cool stream water in Toyama, Japan from which we cloned and characterized the bacterial lipase LipTK-3. The sequenced DNA fragment contains an open reading frame of 1,428 bp that encoded a protein of 476 amino acids with an estimated molecular mass of 50,132 Da. The lipase showed high sequence similarity to those of subfamily Ι.3 lipase and had a conserved GXSXG motif around the catalytic Ser residue. Its optimal temperature was 20-25 °C, lower than in most other subfamily Ι.3 lipases. The lipase exhibited about 30 % of maximal activity at 5 °C. The optimal pH value was 8.0. The activity was strongly inhibited by EDTA and was highly dependent on Ca(2+). Tricaprylin and p-nitrophenyl caprylate were the most favorable substrates among the triglycerides and p-nitrophenyl esters, respectively. LipTK-3 also showed high activity towards natural substrates including edible vegetable oils and animal fats. Furthermore, LipTK-3 was very active and stable in the presence of several detergents, metal ions, and organic solvents. This cold-adapted lipase may prove useful for future applications.