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Nagatoshi Koumura

Publications and source records attributed to Nagatoshi Koumura.

9 recordsLinked to original sources
PubMed2006

Alkyl-functionalized organic dyes for efficient molecular photovoltaics.

We designed and synthesized new alkyl-functionalized organic dyes, MK-1 and MK-2, for dye-sensitized solar cells (DSSCs). Based on the MK-2 dye, a high performance of efficiency (eta, 7.7%; short-circuit current density Jsc = 14.0 mA cm-2, open-circuit voltage Voc = 0.74 V, and fill factor FF = 0.74) was achieved under AM 1.5 G irradiation (100 mW cm-2). Remarkably, the relatively higher Voc for DSSCs based on MK-1 and MK-2 dyes, which have long alkyl chains, were observed among the organic dyes caused by the increasing of the electron lifetime in the conduction band of TiO2. Our molecular design of alkyl-functionalized dyes strongly suggests the promising performance of molecular photovoltaics based on organic dyes.

Journal Article↗
PubMed2005

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 degrees rotation of the propeller with respect to the surface-mounted lower half of the system.

Journal Article↗
PubMed2004

Photocontrolled gel-to-sol-to-gel phase transitioning of meta-substituted azobenzene bisurethanes through the breaking and reforming of hydrogen bonds.

An azobenzene derivative substituted at its meta positions with two urethane moieties linked to two cholesteryl ester units forms a gel that exhibits sol-gel phase transitions upon photoirradiation as a result of trans-cis isomerization of the azobenzene unit. During the sol-gel phase transitions, hydrogen bonds, which are partly responsible for stabilizing the gels, are broken or reformed. Using circular dichroism spectroscopy, we detected asymmetrical fibrous networks in the aggregates of these gelators.

Journal Article↗
PubMed2002

Second generation light-driven molecular motors. Unidirectional rotation controlled by a single stereogenic center with near-perfect photoequilibria and acceleration of the speed of rotation by structural modification.

Nine new molecular motors, consisting of a 2,3-dihydro-2-methylnaphtho[2,1-b]thiopyran or 2,3-dihydro-3-methylphenanthrene upper part and a (thio)xanthene, 10,10-dimethylanthracene, or dibenzocycloheptene lower part, connected by a central double bond, were synthesized. A single stereogenic center, bearing a methyl substituent, is present in each of the motors. MOPAC93-AM1 calculations, NMR studies, and X-ray analysis revealed that these compounds have stable isomers with pseudoaxial orientation of the methyl substituent and less-stable isomers with pseudoequatorial orientation of the methyl substituent. The photochemical and thermal isomerization processes of the motors were studied by NMR and CD spectroscopy. The new molecular motors all show two cis-trans isomerizations upon irradiation, each followed by a thermal helix inversion, resulting in a 360 degrees rotation around the central double bond of the upper part with respect to the lower part. The direction of rotation is controlled by a single stereogenic center created by the methyl substituent at the upper part. The speed of rotation, governed by the two thermal steps, was adjusted to a great extent by structural modifications, with half-lives for the thermal isomerization steps ranging from t(1/2)(theta) 233-0.67 h. The photochemical conversions of two new motors proceeded with near-perfect photoequilibria of 1:99.

Journal Article↗
PubMed2002

Unidirectional rotary motion in a liquid crystalline environment: color tuning by a molecular motor.

Life could not exist without motion induced by a variety of molecular motors. The construction of artificial motors by chemical synthesis, which can power motions that lead to macroscopic detectable effects in a system, is a major endeavor in contemporary science. To move toward this goal, a host-guest system, composed of a nematic liquid crystal film doped with a chiral light-driven molecular motor, is assembled. Irradiation of the film results in unidirectional rotary motion of the molecular motor, which induces a motion of the mesogenic molecules leading to a molecular reorganization and, as a consequence, a change in the color of the film. In this way, by control of the rotary motion at the molecular level, color tuning over the entire visible spectrum is achieved. These findings demonstrate that a molecular motor can exert a visually observable macroscopic change in a material.

Crystallization↗
PubMed1999

Chemistry of Unique Chiral Olefins. 4. Theoretical Studies of the Racemization Mechanism of trans- and cis-1,1',2,2',3,3',4,4'-Octahydro-4,4'-biphenanthrylidenes.

The minimum energy conformations and racemization barriers for the chiral sterically overcrowded helical alkenes, trans- and cis-1,1',2,2',3,3',4,4'-octahydro-4,4'-biphenanthrylidenes (1 and 2), are reported. The trans-1 and cis-2 isomers can each adapt three different conformations, (P,P) and (M,M) (an enantiomeric pair) and an achiral (P,M) meso form, of which only the chiral isomers were obtained by synthesis. The conformations and heats of formation of (M,M)-(E)-1, (P,M)-(E)-1, (M,M)-(Z)-2, and (P,M)-(Z)-2 isomers were determined by MOPAC AM1 calculations. The racemization process for both the trans- and cis- isomers is postulated to occur via the (P,M) isomers by two successive inversions of the cyclohexenyl ring; (M,M) <--> (P,M) <--> (P,P). The (M,M) --> (P,M) and reverse (P,M) --> (M,M) isomerizations were simulated by reaction path calculations, providing the molecular structure and the activation energy of the transition state for each isomerization. For each racemization process, the activation enthalpy (DeltaH()) was calculated as 23.9 and 19.9 kcal mol(-)(1) for trans-olefin 1 and cis-olefin 2, respectively. These values reasonably agree with the experimental values obtained by temperature-dependent circular dichroism, optical rotation, and (1)H NMR magnetization transfer measurements: DeltaH() = 24.6 and 20.8 kcal mol(-)(1) for trans-olefin 1 and cis-olefin 2, respectively. While the racemization of cis-isomer 2 is controlled by the steric interaction of H5 with C4'a and C4'b, the surprisingly high barrier for trans-olefin 1 is due to the severe steric interaction between H5 and H3'alpha and/or H3'beta protons.

Journal Article↗