Switch of the magnetic field effect on photon upconversion based on sensitized triplet-triplet annihilation.

Magnetic field effects (MFEs) on photon upconversion based on sensitized triplet-triplet annihilation (TTA-UC) are examined in platinum(ii) octaethylporphyrin/9,10-diphenylanthracene (DPA) systems at different DPA concentrations and laser power densities. Positive MFEs on TTA-UC are observed for the first time and are most likely attributable to aggregation of DPA.

Electron Spin Exchange in Linked Phenothiazine-Viologen Charge Transfer Complexes Incorporated in "Through-Ring" (Rotaxane) α-Cyclodextrins.

A series of covalently bound phenothiazine (PHZ) donor and methylviologen (V) acceptor compounds with polymethylene chain spacers (C8 , C10 , C12 ) were incorporated in a "through-ring" (rotaxane) fashion to α-cyclodextrin (α-CD) hosts such that the alkyl chains were fully extended, with the donor and acceptor on opposite sides of the α-CD cylinder. Photoexcitation of the PHZ unit induces electron transfer from the PHZ first excited triplet state to the V moiety, forming a biradicaloid charge-separated state. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy at the X-band and Q-band microwave frequencies was used to investigate the spin exchange interaction, J, in these biradicaloids. Simulation of the spectra using a "static" model for spin-correlated radical pairs allows extraction of the J values, which are negative in sign and have absolute values range from 2 to 1000 Gauss. Comparison of the PHZn V (n = 8, 10, 12) spectra to those obtained using phenyl ether spacers indicates that π-bonds may assist the electronic coupling. The results are discussed in terms of through-bond vs through-space electronic coupling mechanisms.

Effects of silver nanoparticles with different sizes on photochemical responses of polythiophene-fullerene thin films.

Effects of size and coverage density of silver nanoparticles (AgPs) on the fluorescence emission and fluorescence lifetime of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films were investigated. AgPs of 64 nm diameter showed greater effects on the fluorescence decay process of P3HT films as compared with 7 nm AgPs. The fluorescence lifetime (FL) of P3HT decreased from 0.61 to 0.22 ns in the presence of 64 nm AgPs, while no appreciable change (0.60 ns) was seen in the case of 7 nm AgPs. The results suggest that the 64 nm AgPs showed a greater effect on the enhancement of the decay rate of excited P3HT. The photoelectric conversion of thin films consisting of P3HT and phenyl-C61-butyric acid methyl ester (PCBM) was also investigated. AgPs of 7 or 64 nm diameters were first deposited on indium-tin-oxide substrates with controlled surface coverage densities from ~1 to 40%. When the coverage densities of deposited AgPs were ~20% for both 7 and 64 nm, the enhancement of photoelectric conversion efficiency reached maximum. The degree of enhancement in the case of 64 nm AgPs was larger than in the case of 7 nm AgPs.

Split Spy0128 as a potent scaffold for protein cross-linking and immobilization.

Site-specific cross-linking techniques between proteins and additional functional groups have become increasingly important for expanding the utility of proteins in biochemistry and biotechnology. In order to explore powerful techniques for practical bioconjugation applications, we have validated a technique mediated by a unique property of Streptcoccus pyogenes pilin subunit Spy0128, an autocatalytic intramolecular isopeptide formation in Spy0128. Recently, it has been revealed that Spy0128 can be split into two fragments (split-Spy0128 (residues 18-299 of Spy0128) and isopeptag (residues 293-308 of Spy0128)) that were capable of forming an intermolecular covalent complex. We focused on this unique reconstitution property and first studied the bioconjugation of blue and green fluorescent proteins, enabling the direct monitoring of cross-linking reactions by Förster resonance energy transfer (FRET). A fluorescence lifetime study shows that spatial control of two proteins on the Spy0128 scaffold is possible when one protein is fused to the N-terminus of split-Spy0128 and another one is tethered at the N- or C-terminus of the isopeptag. Furthermore, we demonstrated site-specific protein immobilization mediated by the reconstitution of split-Spy0128 and isopeptag. In this case, a split-Spy0128 mutant with a free N-terminal Cys residue was first immobilized onto beads chemically modified with a maleimide group through a Michael addition process. Then, an isopeptagged protein was successfully immobilized onto the split-Spy0128-immobilized beads. These results suggest that Spy0128 is a potent proteinaceous scaffold available for bioconjugation both in solution and at a solid surface.

Magnetic orientation of single-walled carbon nanotubes or their composites using polymer wrapping.

The magnetic orientation of single-walled carbon nanotubes (SWNTs) or the SWNT composites wrapped with polymer using poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEHPPV) as the conducting polymer were examined. The formation of SWNT/MEHPPV composites was confirmed by examining absorption and fluorescence spectra. The N,N-dimethylformamide solution of SWNT/MEHPPV composites or the aqueous solution of the shortened SWNTs was introduced dropwise onto a mica or glass plate. The magnetic processing of the composites or the SWNTs was carried out using a superconducting magnet with a horizontal direction (8 T). The AFM images indicated that the SWNT/MEHPPV composites or the SWNTs were oriented randomly without magnetic processing, while with magnetic processing (8 T), they were oriented with the tube axis of the composites or the SWNTs parallel to the magnetic field. In polarized absorption spectra of SWNT/MEHPPV composites on glass plates without magnetic processing, the absorbance due to semiconducting SWNT in the near-IR region in horizontal polarized light was almost the same as that in vertical polarized light. In contrast, with magnetic processing (8 T), the absorbance due to semiconducting SWNT in the horizontal polarization direction against the direction of magnetic field was stronger than that in the vertical polarization direction. Similar results were obtained from the polarized absorption spectra for the shortened SWNTs. These results of polarized absorption spectra also support the magnetic orientation of the SWNT/MEHPPV composites or the SWNTs. On the basis of a comparison of the composites and the SWNTs alone, the magnetic orientation of SWNT/MEHPPV composites is most likely ascribable to the anisotropy in susceptibilities of SWNTs.