Hayato Tsuji

Electron transfer through rigid organic molecular wires enhanced by electronic and electron–vibration coupling

Electron transfer (ET) is a fundamental process in a wide range of biological systems, photovoltaics and molecular electronics. Therefore to understand the relationship between molecular structure and ET properties is of prime importance. For this purpose, photoinduced ET has been studied extensively using donor-bridge-acceptor molecules, in which π-conjugated molecular wires are employed as bridges. Here, we demonstrate that carbon-bridged oligo-p-phenylenevinylene (COPV), which is both rigid and flat, shows an 840-fold increase in the ET rate compared with the equivalent flexible molecular bridges. A 120-fold rate enhancement is explained in terms of enhanced electronic coupling between the electron donor and the electron acceptor because of effective conjugation through the COPVs. The remainder of the rate enhancement is explained by inelastic electron tunnelling through COPV caused by electron-vibration coupling, unprecedented for organic molecular wires in solution at room temperature. This type of nonlinear effect demonstrates the versatility and potential practical utility of COPVs in molecular device applications.

Modular synthesis of benzo[b]phosphole derivatives via BuLi-mediated cyclization of (o-alkynylphenyl)phosphine.

Treatment of an (o-alkynylphenyl)phosphine with a stoichiometric amount of BuLi effects a cyclization reaction to produce a 3-lithiobenzo[b]phosphole, which affords a variety of 3-substituted benzophospholes upon reaction with electrophiles. An example is given for the synthesis of a bis-benzo[b]phosphole, which can be further converted to the corresponding benzo[b]phosphole oxide possessing high electron affinity.

Air- and Heat-Stable Planar Tri-p-quinodimethane with Distinct Biradical Characteristics

A heptacyclic carbocycle possessing three p-quinodimethane units conjugated in one plane has been synthesized and shown to exhibit distinct biradical characteristics. The molecule has a HOMO/LUMO band gap of ca. 1 eV and a S(0)-T(1) energy gap of 2.12 kcal/mol, and it absorbs and emits near-IR light at room temperature. It is air-stable under ambient light for several months and thermally stable up to 160 °C under nitrogen, and it undergoes reversible two-electron oxidation and reduction. The synthetic approach is such that a smaller and larger oligo-p-quinodimethane can be synthesized.

Benzo[b]phosphole sulfides. Highly electron-transporting and thermally stable molecular materials for organic semiconductor devices

A benzo[b]phosphole sulfide derivative DBPSB was found to function as a new class of n-type amorphous organic material, which features high electron drift mobility and high thermal stability, and the utility of this compound was demonstrated in an organic light-emitting diode.

Modular Synthesis of 1H-indenes, Dihydro-s-Indacene, and Diindenoindacene―a Carbon-Bridged p-Phenylenevinylene Congener

A variety of 1H-indenes, dihydro-s-indacenes, and diindenoindacenes, carbon-bridged phenylenevinylene derivatives, can be synthesized in good to high yields using as a synthetic module a 3-lithioindene compound made available by reductive cyclization of an alkynylbenzene derivative. The planar analogues of oligophenylenevinylene compounds thus synthesized show physical properties beneficial for use as ambipolar organic semiconductor materials.

Modular Synthesis of Functionalized Benzosiloles by Tin-Mediated Cyclization of (o-Alkynylphenyl)silane

Trimethylstannyllithium promotes cyclization of (o-alkynylphenyl)silane into a 3-stannylbenzosilole, via addition to the triple bond followed by intramolecular cyclization in a cascade fashion. This intermediate can be functionalized with either electrophiles or nucleophiles to allow modular synthesis of 2,3-disubstituted benzosiloles. One of these compounds, a phenylene-bis(benzosilole) compound, shows electron drift mobility as high as 6 x 10-4 cm2/Vs in an amorphous film, making this class of compounds promising electronic materials for organic light emitting devices and organic photovoltaic cells.

Construction of a Chiral Quaternary Carbon Center by Indium-Catalyzed Asymmetric α-Alkenylation of β-Ketoesters

Construction of a nonracemic all-carbon quaternary stereocenter at the alpha-position of beta-ketoesters was achieved by way of an indium(III)-catalyzed diastereoselective alpha-alkenylation reaction of chiral enamines with 1-alkynes. The enamine bearing a chiral auxiliary derived from l-isoleucine was added to the alkyne to give an alpha-alkenylated product in excellent yield and with a stereoselectivity better than 90% ee. One can ascribe the high selectivity to a chelate intermediate involving the auxiliary and the metal atom and the high yield to efficient interactions between the indium(III) atom and the alkyne. The selectivity increased as the reaction temperature was raised to 120 degrees C and decreased at higher temperatures.

Recent experimental and theoretical aspects of the conformational dependence of UV absorption of short chain peralkylated oligosilanes

Recent experimental and theoretical aspects of the conformation dependence of UV absorption of short chain peralkylated oligosilanes are surveyed. In the current interpretation of the conformational effect on tetrasilane electronic transitions it is not the excitation energy but the intensity of the lowest valence transitions that changes significantly as the SiSiSiSi dihedral angle varies. The latest synthetic advances have enabled us to constrain the conformation of silicon backbone in oligosilanes to some extent to provide strong experimental evidence for the theoretical proposals. A similar conformational effect has been found for a series of hexasilane conformers.

Carbon-Bridged Oligo(phenylenevinylene)s: Stable π-Systems with High Responsiveness to Doping and Excitation

The high responsiveness of π-conjugated materials to external stimuli, such as electrons and photons, accounts for both their utility in optoelectronic applications and their chemical instability. Extensive studies on heteroatom-stabilized π-conjugated systems notwithstanding, it is still difficult to combine high performance and stability. We report here that carbon-bridged oligo(p-phenylenevinylene)s (COPV-n) are not only more responsive to doping and photoexcitation but also more stable than the conventional p-phenylenevinylenes and poly(3-hexylthiophene), surviving photolysis very well in air, suggesting that they could serve as building blocks for optoelectronic applications. Activation of the ground state by installation of bond angle strain toward the doped or photoexcited state and the flat, rigid, and hindered structure endows COPVs with stimuli-responsiveness and stability without recourse to heteroatoms. For example, COPV-6 can be doped with an extremely small reorganization energy and form a bipolaron delocalized over the entire π-conjugated system. Applications to bulk and molecular optoelectronic devices are foreseen.

Modification of Chiral Monodentate Phosphine Ligands (MOP) for Palladium-Catalyzed Asymmetric Hydrosilylation of Cyclic 1,3-Dienes

Several MOP ligands 5 containing aryl groups at 2′ position of (R)-2-(diphenylphosphino)-1,1′-binaphthyl skeleton were prepared and used for palladium-catalyzed asymmetric hydrosilylation of cyclic 1,3-dienes 6 with trichlorosilane. Highest enantioselectivity was observed in the reaction of 1,3-cyclopentadiene (6a) catalyzed by a palladium complex (0.25 mol %) coordinated with (R)-2-(diphenylphosphino)-2′-(3,5-dimethyl-4-methoxyphenyl)-1,1′-binaphthyl (5f), which gave (S)-3-(trichlorosilyl)cyclopentene of 90% ee.