Toshi Nagata

Highly Pure Synthesis, Spectral Assignments, and Two-Photon Properties of Cruciform Porphyrin Pentamers Fused with Benzene Units

Tetrameric porphyrin formation of 2-hydroxymethylpyrrole fused with porphyrins through a bicyclo[2.2.2]octadiene unit gave bicyclo[2.2.2]octadiene-fused porphyrin pentamers. Thermal conversion of the pentamers gave fully pi-conjugated cruciform porphyrin pentamers fused with benzene units in quantitative yields. UV/Vis spectra of fully pi-conjugated porphyrin pentamers showed one very strong Q absorption and were quite different from those of usual porphyrins. From TD-DFT calculations, the HOMO level is 0.49 eV higher than the HOMO-1 level. The LUMO and LUMO+1 levels are very close and are lower by more than 0.27 eV than those of other unoccupied MOs. The strong Q absorption was interpreted as two mutually orthogonal single-electron transitions (683 nm: 86 %, HOMO-->LUMO; 680 nm: 86 %, HOMO-->LUMO+1). The two-photon absorption (TPA) cross section value (sigma((2))) of the benzene-fused porphyrin pentamer was estimated to be 3900 GM at 1500 nm, which is strongly correlated with a cruciform molecular structure with multidirectional pi-conjugation pathways.

Long-lived charge separated states from distance fixed triads consisting of zinc porphyrin, free-base porphyrin, and pyromellitimide

Abstract Photoexcitation of distance restricted triads consisting of zinc porphyrin (ZnP), free-base porphyrin (H 2 P), and pyromellitimide (Im) at 532 nm in THF at room temperature led to long-lived charge separated states (ZnP) + -H 2 P-(Im) − with a lifetime of 0.16–80 μs via electron transfer from the 1 (H 2 P)* to the Im followed by electron transfer from the ZnP to the (H 2 P) + . The lifetime of (ZnP) + -H 2 P-(Im) − state is dependent upon distances between the charges and the structures of linkage between the ZnP and H 2 P.

Intramolecular photoinduced electron transfer in fixed distance triads consisting of free-base porphyrin, zinc porphyrin, and electron acceptor

Abstract Intramolecular photoinduced electron transfer reaction of a series of fixed distance triads consisting of free-base porphyrin, zinc porphyrin, and quinone or pyromellitimide have been investigated by means of steady-state fluorescence spectroscopy, picosecond time-resolved fluorescence spectroscopy, and picosecond time-resolved transient absorption spectroscopy. The singlet excited state of the distal free-base porphyrin was found to be quenched by the attached electron acceptor at 298 K in THF and at 77 K in EPA, indicating long distance electron transfer from the free-base porphyrin to the acceptor. However, long-lived charge separated states were not formed in these model compounds.

Switching of Single‐Molecule Magnetic Properties of TbIII–Porphyrin Double‐Decker Complexes and Observation of Their Supramolecular Structures on a Carbon Surface

Double-decker complexes based on single-molecule magnets (SMMs) are a class of highly promising molecules for applications in molecular spintronics, wherein control of both the ligand oxidative states and the 2D supramolecular structure on carbon materials is of great importance. This study focuses on the synthesis and study of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP)-Tb(III) double-decker complexes with different electronic structures comprising protonated, anionic, and radical forms. Magnetic susceptibility measurements revealed that only the anionic and radical forms of the OEP-Tb(III) double-decker complexes exhibited SMM properties. The barrier heights for magnetic moment reversal were estimated to be 207 and 215 cm(-1) for the anionic and radical forms, respectively. Scanning tunneling microscopy (STM) investigations revealed that these OEP-Tb(III) complexes form well-ordered monolayers upon simple dropcasting from dilute dichloromethane solutions. All three complexes form an isomorphic pseudo-hexagonal 2D pattern, regardless of the differences in the electronic structures of their porphyrin-Tb cores. This finding is of interest for SMM technology as ultrathin films of these materials undergoing chemical transformations will not require any detrimental reorganization. Finally, we demonstrate self-assembly of the protonated 5,15-bisdodecylporphyrin (BDP)-Tb(III) double-decker complex as an example of successful supramolecular design to achieve controlled alignment of SMM-active sites.

Photosynthetic Energy Conversion: Hydrogen Photoproduction by Natural and Biomimetic Means

The main function of the photosynthetic process is to capture solar energy and to store it in the form of chemical fuels. Many fuel forms such as coal, oil and gas have been intensively used and are becoming limited. Hydrogen could become an important clean fuel for the future. Among different technologies for hydrogen production, oxygenic natural and artificial photosynthesis using direct photochemistry in synthetic complexes have a great 3 Biomimetics, Learning from Nature 50 potential to produce hydrogen as both use clean and cheap sources water and solar energy. Photosynthetic organisms capture sunlight very efficiently and convert it into organic molecules. Artificial photosynthesis is one way to produce hydrogen from water using sunlight by employing biomimetic complexes. However, splitting of water into protons and oxygen is energetically demanding and chemically difficult. In oxygenic photosynthetic microorganisms water is splitted into electrons and protons during primary photosynthetic processes. The electrons and protons are redirected through the photosynthetic electron transport chain to the hydrogen-producing enzymes-hydrogenase or nitrogenase. By these enzymes, eand H+ recombine and form gaseous hydrogen. Biohydrogen activity of hydrogenase can be very high but it is extremely sensitive to photosynthetic O2. At the moment, the efficiency of biohydrogen production is low. However, theoretical expectations suggest that the rates of photon conversion efficiency for H2 bioproduction can be high enough (> 10%). Our review examines the main pathways of H2 photoproduction using photosynthetic organisms and biomimetic photosynthetic systems and focuses on developing new technologies based on the effective principles of photosynthesis.

Reconstitution of the water-oxidizing complex in manganese-depleted photosystem II preparations using synthetic binuclear Mn(II) and Mn(IV) complexes: production of hydrogen peroxide.

Reconstitution of Mn-depleted PSII particles with synthetic binuclear Mn complexes (one Mn(II)2 complex and one Mn(IV)2 complex) was examined. In both cases the electron-transfer rates in the reconstituted systems were found to be up to 75–82% of that measured in native PSII but the oxygen evolution activity remained lower (<5–40%). However, hydrogen peroxide was also produced by the reconstituted samples. These samples therefore represent a new type of reconstituted PSII that generates hydrogen peroxide as the final product in reconstituted PSII centers.

Non-catalytic O2 evolution by [(OH2)(Clterpy)Mn(μ-O)2Mn(Clterpy)(OH2)]3+ (Clterpy = 4′-chloro-2,2′:6′,2″-terpyridine) adsorbed on mica with CeIV oxidant

It was earlier reported that [(OH2)(terpy)Mn(μ-O)2Mn(terpy)(OH2)]3+ (terpy = 2,2′:6′,2″-terpyridine) (1) adsorbed on layer compounds catalyzes water oxidation to O2 (J. Am. Chem. Soc., 2004, 126, 8084). The derivative with 4′-chloro-2,2′:6′,2″-terpyridine (Clterpy), [(OH2)(Clterpy)Mn(μ-O)2Mn(Clterpy)(OH2)](NO3)3 (2(NO3)3) was synthesized and characterized by UV-visible absorption spectroscopic and magnetic susceptibility measurements. 2 is instable in aqueous solution at room temperature, but the stability of 2 in solution significantly increased at 5 °C. The reaction of a 2–mica adsorbate with CeIV in water produced a significant amount of O2, although the reaction of 2 with CeIV in a homogenous solution did not. However, the maximum turnover number (TN = 0.52) of 2 on the mica adsorbate was less than unity, indicating the non-catalytic O2 evolution by 2 on mica in contrast to the cooperative catalysis by 1 on mica with TN = 15. The kinetic analysis showed that O2 evolution follows first order kinetics with respect to 2 adsorbed on mica, with the first-order rate constant given to be 6.8 × 10−5 s−1. The first order kinetics can be explained by O2 evolution involved in the unimolecular decomposition of 2 adsorbed on mica, which might be ascribed to the destabilized higher oxidation state of 2 due to the electron-withdrawing chloro-substitution.

The fabrication and single electron transport of Au nano-particles placed between Nb nanogap electrodes.

We have fabricated Nb nanogap electrodes using a combination of molecular lithography and electron beam lithography. Au nano-particles with anchor molecules were placed in the gap, the width of which could be controlled on a molecular scale (approximately 2 nm). Three different anchor molecules which connect the Au nano-particles and the electrodes were tested to investigate their contact resistance, and a local gate was fabricated underneath the Au nano-particles. The electrical transport measurements at liquid helium temperatures indicated single electron transistor (SET) characteristics with a charging energy of about approximately 5 meV, and a clear indication of the effect of superconducting electrodes was not observed, possibly due to the large tunnel resistance.

Synthesis of a dinucleating ligand xanthene-bis(tris(2-pyridylmethyl)amine) and its manganese complex

Abstract A new dinucleating ligand, xanthene-bis(tris(2-pyridylmethyl)amine) ( 1 ), and its manganese complex were synthesized. The ligand has two metal-binding sites that are located in a well-defined geometry determined by the rigid xanthene spacer. The manganese complex was found to have an [Mn 2 (μ-O) 2 ] core. This type of ligand will be useful for designing dinuclear metal complexes.

Design of synthetic molecular units including quinones towards the construction of artificial photosynthesis

Quinones are essential components in many biological systems, notably in photosynthesis. This is largely due to the characteristic proton-coupled redox chemistry of quinones. This review article overviews the use of quinones in studies on artificial photosynthesis, as one-electron electron acceptors, reversible proton/electron carriers, and replacements for sacrificial oxidant and reductants in photosynthetic chemical conversion. Topics included are the early attempts on intramolecular photoinduced electron transfer involving quinones, subsequent reactions after photoinduced electron transfer between pigments and quinones, photochemistry in molecular assemblies containing quinones, and photochemical quinone/hydroquinone interconversion.