Toshifumi Konishi

Photosensitized Reduction of Methyl Viologen Using Eosin-Y in Presence of a Sacrificial Electron Donor in Water–Alcohol Mixture

Abstract Photoreduction of methyl viologen (MV2+) by eosin-Y (EY2−) in the presence of triethanolamine (TEOA) has been investigated in water–methanol mixture by means of steady-state photolysis and laser-flash photolysis in the visible/near-infrared regions. The complete conversion to the persistent methyl viologen radical cation (MV·+) was observed in the presence of lower concentrations of EY2− and excess TEOA. By laser-flash photolysis measurements, electron transfer was confirmed to occur from the triplet state of EY2− [3(EY2−)*] to MV2+ in the rate constants of ca 2.0 × 1010 M−1 s−1. The rates and efficiencies of production of MV·+ were found to be dependent on solvent compositions and concentrations of MV2+ ionic salt and TEOA. The back electron transfer reaction from MV·+ to EY·− was retarded in the presence of TEOA, which supports that EY2− is reproduced by accepting an electron from TEOA. In the presence of excess TEOA, the indirect formation of MV·+ from EY·3− which was produced by accepting an electron from TEOA, was confirmed. The contributions of both the oxidative and reductive routes of 3(EY2−)* for the MV·+ formation have been confirmed.

Preparation of Highly Photosensitizing Liposomes with Fullerene-Doped Lipid Bilayer Using Dispersion-Controllable Molecular Exchange Reactions

Fullerene-containing liposomes with high photosensitization ability were prepared. Disaggregated fullerenes were efficiently injected into the bilayer of liposomes by a phototriggered molecular exchange reaction. These liposomes showed far higher photoreactivity than liposomes thermally produced by heating and microwave irradiation. This result indicates that control of self-aggregation of fullerene leads to a high quantum yield for the photoreaction because of the suppression of self-quenching of photoexcited fullerenes.

Cyclodextrin complexed [60]fullerene derivatives with high levels of photodynamic activity by long wavelength excitation.

We have evaluated the photodynamic activities of C60 derivative·γ-cyclodextrin (γ-CDx) complexes and demonstrated that they were significantly higher than those of the pristine C60 and C70·γ-CDx complexes under photoirradiation at long wavelengths (610-720 nm), which represent the optimal wavelengths for photodynamic therapy (PDT). In particular, the cationic C60 derivative·γ-CDx complex had the highest photodynamic ability because the complex possessed the ability to generate high levels of (1)O2 and provided a higher level of intracellular uptake. The photodynamic activity of this complex was greater than that of photofrin, which is the most widely used of the known clinical photosensitizers. These findings therefore provide a significant level of information toward the optimization of molecular design strategies for the synthesis of fullerene derivatives for PDT.

Solubilization and debundling of purified single-walled carbon nanotubes using solubilizing agents in an aqueous solution by high-speed vibration milling technique

Purified single-walled carbon nanotubes (SWNTs) and cyclodextrins (CDs) mixed by a mechanochemical ‘high-speed vibration milling technique’ (HSVM) are soluble in an aqueous solution because of the formation of SWNT·CDs complexes and the debundling of SWNTs.

Energy Transfer from Photoexcited State of Water Soluble Hexa(Sulfobutyl)fullerene (C60((CH2)4SO3Na)6)

Abstract Photophysical properties and reactivities of water‐soluble micelle‐like C60 derivative, C60((CH2)4SO3Na)6 (abbreviated as FC4S), at the excited triplet state were investigated using laser flash photolysis. A new transient absorption band appearing at ca. 700 nm was attributed to the triplet–triplet (T‐T) absorption band of FC4S. This absorption was quenched by O2 via energy transfer forming singlet oxygen, which was confirmed by its luminescence emission band; the quantum yield was evaluated to be 0.36.

Direct Detection of Superoxide Anion Generated in C60-Photosensitized Oxidation of NADH and an Analogue by Molecular Oxygen

Visible-light irradiation of poly(vinylpyrrolidone) (PVP)-solubilized C60 in water in the presence of NADH (dihydronicotinamide adenine dinucleotide) and molecular oxygen (O2) results in formation of superoxide anion (O2˙−). Formation of O2˙− having a characteristic g‖ value of 2.18 was evidenced by the direct observation with use of a low-temperature EPR technique at 77 K. Photoinduced O2˙− formation was also observed for an N-methyl-2-pyrrolidone (NMP) solution of C60 and 1-benzyl-1,4-dihydronicotinamide (BNAH) in the presence of O2, whereas C60 radical anion (C60˙−) was formed in the absence of O2 under otherwise the same experimental conditions. These results suggest that C602− formed in the photoinduced electron-transfer reduction of C60 by BNAH acts as an electron donor to O2 to give O2˙− in NMP.

Supramolecular photocurrent-generating systems using porphyrin composite materials

Supramolecular design principles for a porphyrin-sensitized, wet-type solar cell are described. To construct efficient organic photocurrent-generating systems, the following two important targets exist: (i) kinetic control of photoinduced electron-transfer processes by spatial, three-dimensional alignment of photo-functional molecules (sensitizers, electron donors, acceptors, and mediators) and (ii) highly dense deposition of composites of the photo-functional molecules on an electrode. These objectives can be achieved by tailoring a photoactive multilayer using supramolecular interactions, such as molecular adsorption, inclusion, coordination, and recognition. Using these interactions, it is expected that the reduction of the costs of synthesis and the combinational fabrication of a simplified-molecular device will become possible. In addition, recent approaches toward the construction of supramolecular porphyrin-sensitized photovoltaic cells are introduced.

Electron transfer and energy transfer of photoexcited C60 in the presence of retinols

Electron transfer and energy transfer between the photo-excited triplet state of C60 (3C60*) and retinols, retinal and retinoic acid have been investigated by laser flash photolysis. In polar solvent, with the decay of the transient absorption band of 3C60* at 740 nm, the rise of the anion radical of C60 (C60-•) at 1070 nm and cation radicals of retinols [(retinols)+•] at 600 and 940 nm were observed in addition to the triplet state of retinols [3(retinols)*] at 400 nm. For retinal and retinoic acid, energy transfer occurs predominantly even in polar solvent, nevertheless the electron donor abilities of retinal and retinoic acid evaluated from the oxidation potentials are similar to those of retinols. The quantum yield and rate constant of electron transfer for trans-retinol are slightly faster than that for cis-retinol. After electron transfer, the cation radicals of retinols disappeared mainly by back electron transfer; however, some reactions of (retinols)+• such as intra- and inter-molecular ring closing reactions also occur, yielding persistent C60-•.

Photoinduced electron-transfer reaction between C60 and cyclic silicon compounds

Electron transfer from cyclic silicon compounds to photo-excited C60 in polar solvents has been investigated by laser photolysis with observation of the transient absorption bands in the near-IR region. For three-, four- and five-membered cyclic silicon compounds in benzonitrile, the rise of the radical anion of C60 [C60−] was observed with the rapid decay of the triplet state of C60 [3C60*], indicating that electron transfer takes place via 3C60*. The rate constant (ket) and quantum yield (Φet) of electron transfer decrease with an increase in the number of silicon units. The ket and Φet values of the cyclic silicon compounds are smaller than those of the corresponding cyclic germanium compounds; an especially prominent difference was observed for four-membered cyclic compounds.