Nagatoshi Koumura

Light-driven monodirectional molecular rotor

Attempts to fabricate mechanical devices on the molecular level have yielded analogues of rotors, gears, switches, shuttles, turnstiles and ratchets. Molecular motors, however, have not yet been made, even though they are common in biological systems. Rotary motion as such has been induced in interlocked systems and directly visualized for single molecules, but the controlled conversion of energy into unidirectional rotary motion has remained difficult to achieve. Here we report repetitive, monodirectional rotation around a central carbon–carbon double bond in a chiral, helical alkene, with each 360° rotation involving four discrete isomerization steps activated by ultraviolet light or a change in the temperature of the system. We find that axial chirality and the presence of two chiral centres are essential for the observed monodirectional behaviour of the molecular motor. Two light-induced cis-trans isomerizations are each associated with a 180° rotation around the carbon–carbon double bond and are each followed by thermally controlled helicity inversions, which effectively block reverse rotation and thus ensure that the four individual steps add up to one full rotation in one direction only. As the energy barriers of the helicity inversion steps can be adjusted by structural modifications, chiral alkenes based on our system may find use as basic components for ‘molecular machinery’ driven by light.

Unidirectional molecular motor on a gold surface

Molecules capable of mimicking the function of a wide range of mechanical devices have been fabricated, with motors that can induce mechanical movement attracting particular attention. Such molecular motors convert light or chemical energy into directional rotary or linear motion, and are usually prepared and operated in solution. But if they are to be used as nanomachines that can do useful work, it seems essential to construct systems that can function on a surface, like a recently reported linear artificial muscle. Surface-mounted rotors have been realized and limited directionality in their motion predicted. Here we demonstrate that a light-driven molecular motor capable of repetitive unidirectional rotation can be mounted on the surface of gold nanoparticles. The motor design uses a chiral helical alkene with an upper half that serves as a propeller and is connected through a carbon–carbon double bond (the rotation axis) to a lower half that serves as a stator. The stator carries two thiol-functionalized ‘legs’, which then bind the entire motor molecule to a gold surface. NMR spectroscopy reveals that two photo-induced cis-trans isomerizations of the central double bond, each followed by a thermal helix inversion to prevent reverse rotation, induce a full and unidirectional 360° rotation of the propeller with respect to the surface-mounted lower half of the system.

Interfacial Electron-Transfer Kinetics in Metal-Free Organic Dye-Sensitized Solar Cells: Combined Effects of Molecular Structure of Dyes and Electrolytes

Electron diffusion coefficient, lifetime, and density in the TiO(2) electrode of dye-sensitized TiO(2) solar cells (DSCs) employing I(-)/I(3)(-) redox couples were measured with eight different metal-free organic dyes and three Ru complex dyes. At matched electron density, all DSCs using organic dyes (ODSCs) showed shorter electron lifetime with comparable or larger diffusion coefficients in comparison to the DSCs using the Ru dyes (RuDSC). The shorter lifetime was attributed partially to the slower dye cation reduction rate of the organic dyes by I(-), faster electron diffusion coefficient in the TiO(2), and mostly higher I(3)(-) concentration in the vicinity of the TiO(2) surface. Whereas a slight shift of the conduction band edge potential (E(cb)) of the TiO(2) was seen with a few organic dyes, no correlation was found with the dipole moment of the adsorbed dyes. This implies that the adsorbed dyes interact with cations in the electrolyte, so the direction of the dipole is altered or simply screened. The increase of [I(3)(-)] in the vicinity of the TiO(2) surface was interpreted with partial charge distribution of the dyes. Under one-sun conditions, less electron density due to shorter electron lifetime was found to be the main reason for the lower values of V(oc) for all ODSCs in comparison to that of RuDSCs. Among the organic dyes, having larger molecular size and alkyl chains showed longer electron lifetime, and thus higher V(oc). Toward higher open circuit voltage, a design guide of organic dyes controlling the electron lifetime is discussed.

Dye Regeneration Kinetics in Dye-Sensitized Solar Cells

The ideal driving force for dye regeneration is an important parameter for the design of efficient dye-sensitized solar cells. Here, nanosecond laser transient absorption spectroscopy was used to measure the rates of regeneration of six organic carbazole-based dyes by nine ferrocene derivatives whose redox potentials vary by 0.85 V, resulting in 54 different driving-force conditions. It was found that the reaction follows the behavior expected for the Marcus normal region for driving forces below 29 kJ mol(-1) (ΔE = 0.30 V). Driving forces of 29-101 kJ mol(-1) (ΔE = 0.30-1.05 V) resulted in similar reaction rates, indicating that dye regeneration is diffusion controlled. Quantitative dye regeneration (theoretical regeneration yield 99.9%) can be achieved with a driving force of 20-25 kJ mol(-1) (ΔE ≈ 0.20-0.25 V).

Visible-Light-Induced Water Splitting Based on Two-Step Photoexcitation between Dye-Sensitized Layered Niobate and Tungsten Oxide Photocatalysts in the Presence of a Triiodide/Iodide Shuttle Redox Mediator

Water splitting into H2 and O2 under visible light was achieved using simple organic dyes such as coumarin and carbazole as photosensitizers on an n-type semiconductor for H2 evolution, a tungsten(VI) oxide (WO3) photocatalyst for O2 evolution, and a triiodide/iodide (I3(-)/I(-)) redox couple as a shuttle electron mediator between them. The results on electrochemical measurements revealed that the oxidized states of the dye molecules having an oligothiophene moiety (two or more thiophene rings) in their structures are relatively stable even in water and possess sufficiently long lifetimes to exhibit reversible oxidation-reduction cycles, while the carbazole system required more thiophene rings than the coumarin one to be substantially stabilized. The long lifetimes of the oxidized states enabled these dye molecules to be regenerated to the original states by accepting an electron from the I(-) electron donor even in an aqueous solution, achieving sustained H2 and I3(-) production from an aqueous KI solution under visible light irradiation when they were combined with an appropriate n-type semiconductor, ion-exchangeable layered niobate H4Nb6O17. The use of H4Nb6O17 loaded with Pt cocatalyst inside the interlayer allowed the water reduction to proceed preferentially with a steady rate even in the presence of a considerable amount of I3(-) in the solution, due to the inhibited access of I3(-) to the reduction site, Pt particles inside, by the electrostatic repulsion between the I3(-) anions and the negatively charged (Nb6O17)(4-) layers. It was also revealed that the WO3 particles coloaded with Pt and IrO2 catalysts exhibited higher rates of O2 evolution than the WO3 particles loaded only with Pt in aqueous solutions containing a considerable amount of I(-), which competitively consumes the holes and lowers the rate of O2 evolution on WO3 photocatalysts. The enhanced O2 evolution is certainly due to the improved selectivity of holes toward water oxidation on IrO2 cocatalyst, instead of undesirable oxidation of I(-). Simultaneous evolution of H2 and O2 under visible light was then achieved by combining the Pt/H4Nb6O17 semiconductor sensitized with the dye molecules having an oligothiophene moiety, which can stably generate H2 and I3(-) from an aqueous KI solution, with the IrO2-Pt-loaded WO3 photocatalyst that can reduce the I3(-) back to I(-) and oxidize water to O2.

Substituted carbazole dyes for efficient molecular photovoltaics: long electron lifetime and high open circuit voltage performance

We designed and synthesized new substituted carbazole dyes, MK-14 and -16, for dye-sensitized solar cells (DSSCs) employing the I−/I3−redox couple. By the addition of a hexyloxyphenyl substituent to previously reported carbazole dyes MK-1 and -2, the electron lifetime and open circuit voltage of the DSSCs employing these dyes were increased, showing comparable values with those using a conventional Ru complex dye. This result was achieved by the retardation of the charge recombination, caused by more effective blocking of the I3− ion in the electrolyte than that in the cases of MK-1 and -2. The result shows the importance of the position of alkyl chains attached to the main framework of dye molecules.

Long-term stability of organic–dye-sensitized solar cells based on an alkyl-functionalized carbazole dye

We investigated the long-term stability of performance for dye-sensitized solar cells (DSSCs) based on an alkyl-functionalized carbazole dye (MK-2) and used in conjunction with ionic liquid-based electrolytes. We observed good long-term stability of the performance of the DSSCs during 60 days under visible-light irradiation at ca. 50 °C. The performance of the DSSC decreased gradually under white-light irradiation including UV light or at 80 °C under dark conditions. However, no decomposition or detachment of the dye molecule from the TiO2electrode was observed after these measurements. These results indicate that the MK-2dye molecule in the cell was stable even under white-light irradiation and at 80 °C under dark conditions.

Aqueous Dye‐Sensitized Solar Cell Electrolytes Based on the Ferricyanide–Ferrocyanide Redox Couple

Solar energy conversion efficiencies of over 4% have been achieved in DSCs constructed with aqueous electrolytes based on the ferricyanide-ferrocyanide redox couple, thereby avoiding the use of expensive, flammable and toxic solvents. This paradigm shift was made possible by the use of a hydrophobic organic carbazole dye.

Stepwise construction of head-to-tail-type oligothiophenes via iterative palladium-catalyzed CH arylation and halogen exchange.

The well-defined head-to-tail oligothiophenes were synthesized via palladium-catalyzed CH arylation and halogen exchange sequentially through the 2-halothiophene derivatives.

Total synthesis of hemibrevetoxin B

Abstract The total synthesis of Hemibrevetoxin B is described. A new cyclization approach, based on the Lewis acid mediated intramolecular cyclization of the γ-oxo-substituted allylic tin having an aldehyde group, produced the 6-6-7-7 polycyclic ether skeleton of the natural product with high stereoselectivity. The 1 H and 13 C-NMR spectra of synthetic hemibrevetoxin B was identical with those of natural product.