Hiroyuki Fueno

A Planar Rhombic Charge-Separated Tetrasilacyclobutadiene

A stable silicon analog of a reactive, antiaromatic hydrocarbon has been synthesized and characterized. The cyclobutadiene (CBD) molecule C4H4 deviates from a high-symmetry square geometry to compensate for its antiaromatic electronic structure. Here, we report a CBD silicon analog, Si4(EMind)4 (1), stabilized by the bulky 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl (EMind) groups, obtained as air- and moisture-sensitive orange crystals by the reduction of (EMind)SiBr3 with three equivalents of lithium naphthalenide. X-ray crystallography reveals a planar and rhombic structure of the Si4 four-membered ring, with alternating pyramidal and planar configurations at the silicon atoms. The large 29Si chemical shift differences (Δδ > 350 parts per million) in the solid-state nuclear magnetic resonance spectra suggest a contribution of an alternately charge-separated structure. The rhombic-shaped charge-separated singlet state of compound 1 thus stabilizes its cyclic 4π-electron antiaromaticity in a manner that contrasts sharply with the bond-length alternation, characterizing the rectangular distortion of carbon-based CBD.

π-Conjugated Phosphasilenes Stabilized by Fused-Ring Bulky Groups

Pi-conjugated phosphasilenes with a variety of aryl substituents on the silicon atom have been synthesized by the use of a 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) group. X-ray structural analysis shows the highly coplanar pi-framework stabilized by the perpendicularly fixed Eind groups. The strong pi-pi* absorptions have been observed, demonstrating the extension of pi-conjugation over the skeleton. The DFT calculations indicate that the LUMO involves the substantial contribution of the 3p(pi)*(Si-P)-2p(pi)*(carbon pi-electron system) conjugation. The electrochemical properties of the phosphasilens are also presented.

A Stable Doubly Hydrogen-Bridged Butterfly-Shaped Diborane(4) Compound

In contrast to the common multiple bonding between carbon atoms, multiply bonded boron compounds have still been a synthetic challenge due to the electron deficiency of boron. We now report that a stable doubly hydrogen-bridged diborane(4), EindB(mu-H)(2)BEind, is produced by the two-electron oxidation of a hydrogen-substituted diborane(4) dianion [Li(+)(thf)](2)[Eind(H)BB(H)Eind](2-), where Eind denotes the 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl. The X-ray crystallography reveals a short B-B distance of 1.4879(7) A in comparison with the normal B-B single bond length (1.72 A), the presence of two hydrogen atoms bridged perpendicular to the B-B bond with a butterfly shape having a dihedral angle of the two BHB triangles of 113(1) degrees, and a linear geometry around the B-B bond with a C-B-B bond angle of 178.92(4) degrees. These structural data, experimental electron density analysis, and computational studies confirm the 3-fold bonding (a sigma and two pi-like bonds) between the two boron atoms incorporating the two bridging hydrogen atoms.

Theoretical study of the dechlorination reaction pathways of octachlorodibenzo-p-dioxin.

The dechlorination reaction pathways of 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin (OCDD) by the hydrogen atom are investigated by the density-functional theory B3PW91 method. The dechlorination reactions have large exothermicity and small activation energies. The activation energies (approximately 5 kcal/mol) of the sigma-complex formation due to the hydrogen addition are lower than those (approximately 9 kcal/mol) of the direct chlorine abstraction. It is suggested that the sigma-complex plays an important role in the reactions, although it has scarcely been shown in previous studies of the dechlorination of dioxins. The sigma-complex formation is favored at low temperatures and the chlorine abstraction is favored at high temperatures. Furthermore, it is found that the lateral positions have a marginal preference over the longitudinal positions. The dechlorination of OCDD by the hydrogen atom is thus not likely to result in a dominant formation of the laterally substituted toxic congeners.

Air-Stable, Room-Temperature Emissive Disilenes with π-Extended Aromatic Groups

π-Conjugated disilenes with 2-naphthyl or 2-fluorenyl groups on the silicon atoms have been synthesized as air-stable emissive red solids using the bulky 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) groups. The strong π-π* absorptions and distinct emission at room temperature, both in solution and in the solid state, have been observed due to the substantial contribution of the 3p(π)*(Si-Si)-2p(π)*(carbon π-electron system) conjugation.

Boron–Boron σ-Bond Formation by Two-Electron Reduction of a H-Bridged Dimer of Monoborane

Diborane(6) as a H-bridged dimer of monoborane can be converted cleanly by two-electron reduction into diborane(6) dianion, which is isoelectronic with ethane, through B-B σ-bond formation when each boron atom has a bulky ligand on it. The existence of the B-B σ bond is supported by the X-ray molecular structure [B-B bond length of 1.924(3) Å], NMR studies, magnetic susceptibility measurements, and DFT calculations. Stepwise hydride abstraction reactions of the diborane(6) dianion produce the corresponding H-bridged diborane(5) anion and doubly H-bridged diborane(4) without B-B bond scission.

(Z)-1,2-Di(1-pyrenyl)disilene: Synthesis, Structure, and Intramolecular Charge-Transfer Emission.

(Z)-1,2-Di(1-pyrenyl)disilene containing bulky 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) groups has been obtained as purple crystals by the reductive coupling reaction of the corresponding dibromosilane with lithium naphthalenide. An X-ray crystallographic analysis revealed an Eind- and pyrenyl-meshed molecular gear around the disilene core adopting the Z configuration, in which the two pyrenyl groups intramolecularly interact through the π-π stacking with a distance of 3.635 Å between the centers of the two pyrene rings. The disilene π-system exhibits a π(Si-Si) → π*(pyrene) intramolecular charge-transfer (ICT) fluorescence at room temperature, whose wavelengths depend on the solvent polarity. The photophysical properties are theoretically supported by computational studies including excited-state calculations.

para-Bisvinylhexaisobutyl-substituted T8 caged monomer: synthesis and hydrosilylation polymerization

The selective introduction of functional groups at the para-position in mono-functionalized T8-caged silsesquioxanes would enable us to design new organic inorganic hybrid polymers. Here, a para-substituted bisvinylhexaisobutyl-T8 caged monomer was successfully synthesized via a selective corner-opening reaction of the vinylheptaisobutyl-T8 cage followed by a subsequent corner-capping reaction.

Coplanar Oligo(p-phenylenedisilenylene)s as Si═Si Analogues of Oligo(p-phenylenevinylene)s: Evidence for Extended π-Conjugation through the Carbon and Silicon π-Frameworks

A series of oligo(p-phenylenedisilenylene)s (Si-OPVs 1-4), silicon analogues of oligo(p-phenylenevinylene)s, up to the tetramer have been synthesized and isolated by the introduction of a newly developed protecting group [(HexO)MEind] for improving their solubility. The experimental and theoretical studies of the Si-OPVs 1-4 demonstrate the fully extended π-conjugation of the Si-OPV main chains. Single crystal X-ray analyses of the monomer 1 and the dimer 2 revealed the highly coplanar Si-OPV backbones facilitating the effective extension of the π-conjugation, which has further been validated by the significant increases in the absorption maxima from 465 nm for the monomer 1 to 610 nm for the tetramer 4. The absorption maxima exhibit an excellent fit to Meiers equation, leading to the estimation of an effective conjugation length (ECL) of 9 repeat units (nECL = 9) and the absorption maximum of 635 nm for the infinite chain (λ∞ = 635 nm). In sharp contrast to other nonemissive disilenes, the Si-OPVs 2-4 show an intense fluorescence from 613 to 668 nm at room temperature with the quantum yields up to 0.48. All the data presented here provide the first evidence for the efficient extended π-conjugation between the Si═Si double bonds and the carbon π-electron systems over the entire Si-OPV skeleton. This study reveals the possibility for developing the conjugated disilene π-systems, in which the Si═Si double bonds would be promising building blocks, significantly optimizing the intrinsic photophysical and electrochemical properties of the carbon-based π-conjugated materials.

Preparation of a D–A polymer with disilanobithiophene as a new donor component and application to high-voltage bulk heterojunction polymer solar cells

A low band gap polymer composed of alternating (tetrabutyldisilano)bithiophene-benzothiadiazole units, whose absorption edge reached approximately 760 nm as a film, was prepared. A bulk heterojunction polymer solar cell was fabricated using the present polymer to provide a power conversion efficiency of 6.38% with a high Voc of 0.82 V.