Hiroaki Yonemura

External magnetic field effects on photoinduced electron transfer reactions in phenothiazine-viologen-linked systems complexed with cyclodextrins☆

Abstract External magnetic field effects on photoinduced electron transfer in phenothiazine-viologen-linked compounds were examined in the presence of cyclodextrins in aqueous solutions. Laser excitation (351 nm) afforded a phenothiazine-viologen radical pair, when the linked compound, with twelve methylene groups as a spacer, was complexed with either α- or β-cyclodextrin. The decay rate of the radical pair was reduced by an order of magnitude on going from 0 to 0.3 T, above which the rate remained constant. The results were explained on the basis of electron spin relaxation in the CDx complexes with extended spacer as suggested by the fluorescence and NMR spectra.

Anomalously stable cyclodextrin complexes of phenothiazine-viologen linked compounds with a long spacer chain

Abstract Distinct NMR signals due to stable cyclodextrin (CDx) complexes were detected by the use of phenothiazine-viologen linked compounds in combination with either α- or β-CDx. Encasing of the spacer alkyl chains by CDx was suggested to explain the complexing behavior.

Effect of high magnetic fields on the reverse electron transfer process in an α-cyclodextrin inclusion complex of phenothiazine-viologen chain-linked compound

The effect of high magnetic fields (< 14 T) on the lifetime of a triplet biradical generated by a photoinduced intramolecular electron transfer reaction of the title compound has been studied in water at room temperature by using pulse-magnet laser flash photolysis apparatus. On increasing the magnetic field from zero to ca. 1 T, the lifetime of the biradical increases steeply, and then decreases gradually in higher magnetic fields. The lifetime at ca. 13 T is about 30% of that at 1 T. This drastic reversal of the effect is qualitatively interpreted by a spin-lattice relaxation (SLR) mechanism. The SLR induced by the anisotropic Zeeman interaction is responsible for the decrease in lifetime in the higher magnetic fields. The mechanism is discussed in detail based on model calculations.

Magnetic field effects on photoinduced electron transfer and the succeeding processes in phenothiazine-viologen linked compounds incorporated into cyclodextrins or reversed micelles☆

Abstract Laser-induced electron transfer in phenothiazine-viologen linked compounds afforded highly active radical pairs, when the linked compounds were incorporated into the cavity of either α- or β-cyclodextrin. Remarkable magnetic field effects on the radical decay rate were observed, and it was ascribed to Zeeman splitting of triplet sublevels of the radical pair. Similar magnetic field effects were also observed with the phenothiazine-viologen linked compounds trapped at walls of microscopic water pools in reversed micelles. Spectroscopic evidences were provided to indicate that the spacer between phenothiazine and viologen units is in extended conformations, and the magnetic field effect was explained in terms of the relaxation mechanism. In zero magnetic field, the radical decay rate was appreciably reduced with decrease of the spacer chain length. Increased singlet-triplet energy separation in the linked compounds with shorter spacer was suggested to be responsible for the reduced decay rate.

Photoinduced intramolecular electron-transfer reactions in carbazole-fullerene and phenothiazine-fullerene linked compounds in benzene and benzonitrile as studied by fluorescence, transient absorption, time-resolved EPR, and magnetic field effects

Photoinduced intramolecular electron-transfer reactions in carbazole (Cz)-fullerene (C60) (Cz(8)C60) and phenothiazine (Ph)-C60 (Ph(n)C60 (n=8, 10, 12)) linked compounds have been investigated in benzene and benzonitrile by fluorescence, transient absorption, and time-resolved electron paramagnetic resonance measurements, and by magnetic field effects on the decay rate constants of the photogenerated biradicals. In benzonitrile, photoinduced intramolecular electron transfer from Cz to the singlet excited state of C60 (1C60*) occurred in Cz(8)C60, but not to the triplet excited state (3C60*), while the intramolecular electron-transfer to both1C60* and3C60* occurred in Ph(n)C60 (n=8, 10, 12). In benzene, on the other hand, no electron transfer to both1C60* and3C60* took place in all linked compounds. These results were interpreted in terms of the different Gibbs free energy changes in the two solvents.

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

Abstract 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.

Magnetic field effects on the decay rate of photogenerated biradical from intramolecular electron transfer of triplet excited fullerene in a fullerene-phenothiazine linked compound

Abstract Transient absorption spectra of a fullerene (C 60 )–phenothiazine (PH) linked compound indicated that the intramolecular electron transfer occurred in benzonitrile, while not in benzene. In benzonitrile, the lifetime (118 ns) of photogenerated biradical was very long, in spite of being around the top region in Marcus theory. The lifetime of the biradical was enhanced by 1.8 times in the presence of magnetic fields (>0.2 T). The magnetic field effects verified that the triplet biradical was generated from the intramolecular electron transfer from PH to the triplet C 60 . The long lifetime is most likely ascribed to spin multiplicities of the biradical.

Photoinduced intramolecular electron transfer reactions in fullerene—phenothiazine linked compounds: effects of magnetic field and spacer chain length

Spectroscopic and electrochemical properties of two fullerene(C60)-phenothiazine(PH) linked compounds with different spacer chain length have been compared in benzonitrile (polar solvent) and in benzene (non-polar solvent). Transient absorption and fluorescence spectra indicated that photoinduced intramolecular electron transfer occurred in benzonitrile, but not in benzene. The results are due to solvent effect on energy levels of the photogenerated biradical. The driving forces for the electron transfer were determined by measuring the redox potentials of the C60 and PH moieties. Thermodynamic parameters for the electron transfer processes were evaluated and compared. In benzonitrile, the lifetime of the photo-generated biradical was very long, in spite of being around the top region in Marcus theory. The decay rate of the biradicals was retarded in the presence of magnetic fields. The decay rate constant decreased quickly with increasing the magnetic field and became constant above about 0.2 T. The magnetic field effects verified that the triplet biradical was generated by the intramolecular electron transfer from PH to the triplet excited state of C60. The long lifetime is most probably ascribed to the spin multiplicities of the biradical.

Introduction of a specific binding domain on myoglobin surface by new chemical modification

A new myoglobin, reconstituted with a modified zinc protoporphyrin, having a total of four ammonium groups at the terminal of the two propionate side chains was constructed to introduce a substrate binding site. The protein with a positively charged patch on the surface formed a stable complex with negatively charged substrates, such as hexacyanoferrate(III) and anthraquinonesulfonate via an electrostatic interaction. The complexation was monitored by fluorescence quenching due to singlet electron transfer from the photoexcited reconstituted zinc myoglobin to the substrates. The binding properties were evaluated by Stern-Volmer plots from the fluorescence quenching of the zinc myoglobin by a quencher. Particularly, anthraquinone-2,7-disulfonic acid showed a high affinity with a binding constant of 1.5 x 10(5) M(-1) in 10 mM phosphate buffer, pH 7.0. In contrast, the plots upon the addition of anthraquinone-2-sulfonic acid at different ionic strengths indicated that the complex was formed not only by an electrostatic interaction but also by a hydrophobic contact. The findings from the fluorescence studies conclude that the present system is a useful model for discussion of electron transfer via non-covalently linked donor-acceptor pairing on the protein surface.

Effect of π-system on long-range photoinduced electron transfer in through-ring α-cyclodextrin complexes of carbazole-viologen linked compounds

Abstract Carbazole-viologen linked compounds, with long spacers (ca. 2 nm), were incorporated into two α-cyclodextrins to afford rotaxane-type complexes. In the presence of a biphenyl unit in the spacer, the fluorescence lifetime of the carbazole moiety was reduced by 2 ns as compared with the case with a simple alkyl spacer ( τ = 12 ns). Contribution of the superexchange mechanism to long-range electron transfer from the carbazole- to the viologen moiety was suggested.