Mechanistic study of photocatalytic CO2 reduction using a Ru(ii)–Re(i) supramolecular photocatalyst

Supramolecular photocatalysts comprising [Ru(diimine)₃]²⁺ photosensitiser and fac-[Re(diimine)(CO)₃{OC(O)OC₂H₄NR₂}] catalyst units can be used to reduce CO₂ to CO with high selectivity, durability and efficiency. In the presence of triethanolamine, the Re catalyst unit efficiently takes up CO₂ to form a carbonate ester complex, and then direct photocatalytic reduction of a low concentration of CO₂, e.g., 10% CO₂, can be achieved using this type of supramolecular photocatalyst. In this work, the mechanism of the photocatalytic reduction of CO₂ was investigated applying such a supramolecular photocatalyst, RuC2Re with a carbonate ester ligand, using time-resolved visible and infrared spectroscopies and electrochemical methods. Using time-resolved spectroscopic measurements, the kinetics of the photochemical formation processes of the one-electron-reduced species RuC2(Re)−, which is an essential intermediate in the photocatalytic reaction, were clarified in detail and its electronic structure was elucidated. These studies also showed that RuC2(Re)− is stable for 10 ms in the reaction solution. Cyclic voltammograms measured at various scan rates besides temperature and kinetic analyses of RuC2(Re)− produced by steady-state irradiation indicated that the subsequent reaction of RuC2(Re)− proceeds with an observed first-order rate constant of approximately 1.8 s⁻¹ at 298 K and is a unimolecular reaction, independent of the concentrations of both CO₂ and RuC2(Re)−.

Formation processes and reactivity of an important intermediate of photocatalytic CO₂ reduction, one-electron reduced species of a Ru(ii)–Re(i) supramolecular photocatalyst with a carbonate ester ligand, were investigated in detail.     

An estimation of the baseband transfer function of multimode optical fibers

This paper discusses an alternative criterion for estimating the baseband transfer function of multimode optical fibers. The optical fibers having little mode conversions show that the baseband frequency characteristics are affected strongly by the launching condition of an input optical pulse. In the criterion presented here, the baseband transfer function in such fibers is estimated by considering the launching condition that excites all fiber modes uniformly in amplitudes. In order to obtain such a transfer function, the actual responses affected by the launching conditions are measured together with the modal power distributions for different kinds of launching conditions. A numerical procedure is then presented that enables us to derive the baseband transfer function independently of modal power distributions. Experimental results at λ x= 633 nm and 856 nm are presented for step index fibers (0.1 and 0.2 km long) having a core radius of 30 pm (its refractive index is 1.452) and a refractive index difference of 0.7%.     

Chemical Interactions of Cryptic Actinomycete Metabolite 5‐Alkyl‐1,2,3,4‐tetrahydroquinolines through Aggregate Formation

Organisms often produce secondary metabolites as a mixture of biosynthetically related congeners. However, why are metabolites with minor chemical variations produced simultaneously? 5‐Alkyl‐1,2,3,4‐tetrahydroquinolines (5aTHQs) are small, lipophilic metabolites produced by Streptomyces nigrescens HEK616 when cultured with Tsukamurella pulmonis TP‐B0596. A mixture of 5aTHQs forms aggregates that show enhanced membrane affinity and biological activity. The ability to form aggregates and membrane‐binding activity is regulated by the length of the alkyl chains. Aggregates with long alkyl chains were too stable to fuse with lipid membranes. However, if inactive 5aTHQ congener was mixed with active congener, the mixture showed increased membrane affinity, enabling cellular entry and biological activity. Therefore, it is shown that sloppiness in a biosynthetic pathway, by which minor structural variations can be produced, is functionally rational, as the metabolites show synergistic action.     

Intermolecular vs. intramolecular photoinduced electron transfer from nucleotides in DNA to acridinium ion derivatives in relation with DNA cleavage

Photoirradiation of various 10-methylacridinium ions (AcrR⁺, R = H, ⁱPr, and Ph) intercalated in DNA results in ultrafast intramolecular electron transfer, followed by rapid back electron transfer between AcrR⁺ and nucleotides in DNA. The electron-transfer dynamics in DNA were monitored by femtosecond time-resolved transient absorption spectroscopy. Both acridinyl radical and nucleotide radical cations, formed in the photoinduced electron transfer in DNA, were successfully detected in an aqueous solution. These transient absorption spectra were assigned by the comparison with those of DNA nucleotide radical cations, which were obtained by the intermolecular electron-transfer oxidation of nucleotides with the electron-transfer state of 9-mesityl-10-methylacridinium ion (Acr–Mes⁺) produced upon photoexcitation of Acr⁺–Mes. Photoinduced cleavage of DNA with various acridinium ions (AcrR⁺, R = H, ⁱPr, Ph, and Mes) has also been examined by agarose gel electrophoresis, which indicates that the rapid intramolecular back electron transfer between acridinyl radical and nucleotide radical cation in DNA suppresses the DNA cleavage as compared with the intermolecular electron-transfer oxidation of nucleotides with Acr–Mes⁺.     

Photoinduced Processes of Supramolecular Nanoarrays Composed of Porphyrin and Benzo[ghi]perylenetriimide Units through Triple Hydrogen Bonds with One‐Dimensional Columnar Phases

One‐dimensional supramolecular columnar phases composed of porphyrins (electron donor: D) and benzo[ghi]perylenetriimides (electron acceptor: A) through triple hydrogen bonds have been successfully constructed to perform sequential light‐harvesting and electron‐transfer processes. A series of benzo[ghi]peryleneimide derivatives have been synthesized to examine the substituent effects such as imide and nitrile groups on the spectroscopic and electrochemical properties. Then, formation of the 1:1 supramolecular complex between zinc porphyrin and benzo[ghi]perylenetriimide derivatives through triple hydrogen bonds was confirmed by Job's plot of ¹H NMR titration. Next, the one‐dimensional supramolecular nanoarrays were successfully prepared in a mixed solvent. X‐ray diffraction (XRD) measurement suggested that these nanoarrays contained one‐dimensional columnar phases composed of stacked donor and acceptor layers. Finally, femtosecond transient absorption and electron spin resonance (ESR) measurements clearly indicated that photoinduced electron transfer occurred via the singlet excited states in the supramolecular columns.     

The use of a [4 + 2] cycloaddition reaction for the preparation of a series of 'tethered' Ru(II)-diamine and aminoalcohol complexes

A series of catalysts have been prepared for use in the asymmetric transfer hydrogenation of ketones. The complexes were prepared using a [4 + 2] cycloaddition reaction at a key step in the reaction sequence. This provides a means for the synthesis of catalysts with modifications at specific sites.