Diazadibora[1.1.1.1]m,p,m,p-cyclophanes: Ambipolar Conjugated Macrocycles with Different B-π-N Embedded Patterns.

Are different B(boron)-π-N(nitrogen) embedded patterns to bring about significant different (opto)electronic properties for the same macrocyclic molecular backbone? A series of B-π-N-embedded alternate-meta-para-linked cyclophanes 1-3 have been prepared and characterized as a new class of ambipolar π-conjugated B-π-N macrocycles. The answer to the opening question is yes. These macrocycles revealed the intramolecular charge transfer in the oxidized states and the intriguing photophysical proprerties in accordance with the embedded patterns, suggesting the electronic structures are tunable by introducing multiple B-π-N moieties.

A stable free tetragermacyclobutadiene incorporating fused-ring bulky EMind groups.

The first free cyclobutadiene (CBD) germanium analogue was obtained as room-temperature stable dark red crystals via the reaction of the bulky EMind-substituted 1,2-dichlorodigermene with lithium naphthalenide. The cyclic 4π-electron antiaromaticity is essentially stabilized by the polar Jahn-Teller distortion in the germanium CBD producing a planar rhombic-shaped charge-separated structure.

Induced-Dipole-Directed, Cooperative Self-Assembly of a Benzotrithiophene.

A benzotrithiophene derivative possessing phenylisoxazoles self-assembled to form stacks. The molecule isodesmically self-assembled in chloroform, whereas it self-assembled in a cooperative fashion in decalin and in methylcyclohexane. Thermodynamic studies based on isodesmic, van der Schoot, and Goldstein-Stryer mathematical models revealed that the self-assembly processes are enthalpically driven and entropically opposed. An enthalpy-entropy compensation plot indicates that the assembly processes in chloroform, decalin, and methylcyclohexane are closely related. The enthalpic gains in less-polar solvents are greater than those in more-polar solvents, resulting in the formation of large assemblies in decalin and in methylcyclohexane. The formation of large assemblies leads to cooperative assemblies. The elongation process is enthalpically more favored than the nucleation process, which drives the cooperativity of the self-assembly. DFT calculations suggested that a hexameric assembly is more stable than tetrameric or dimeric assemblies. Cooperative self-assemblies based on intermolecular interactions other than hydrogen bonding have rarely been reported. It is demonstrated herein that van der Waals interactions, including induced dipole-dipole interactions, can drive the cooperative assembly of planar π-conjugated molecules.

(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.

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 Meier's 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.

Luminescent Coordination Glass: Remarkable Morphological Strategy for Assembled Eu(III) Complexes.

Syntheses of novel luminescent Eu(III) coordination glasses 1 ([Eu(hfa)3(o-dpeb)]2), 2 ([Eu(hfa)3(m-dpeb)]3), and 3 ([Eu(hfa)3(p-dpeb)]n) are reported. They are composed of Eu(III) ions, hexafluoroacetylacetonato (hfa) ligands, and unique bent-angled phosphine oxide (o-, m-, p-dpeb) ligands with ethynyl groups. Their coordination structures and glass formability are dependent on the regiochemistry of substitution in regard to the internal benzene core. Single-crystal X-ray analyses and DFT calculation reveals dinuclear, trinuclear, and polymer structures for Eu(III) coordination glasses 1, 2, and 3, respectively. Those compounds show characteristic glass-transition (Tg = 25-96 °C) and strong luminescence properties (ΦLn = 72-94%).

Synthesis of conjugated polymers containing gallium atoms and evaluation of conjugation through four-coordinate gallium atoms.

The synthesis and analysis of the electronic states of the main-chain type-organogallium polymers are presented. We synthesized the polymers containing four-coordinate gallium atoms by organometal coupling reactions. The synthesized polymers showed good solubility in common organic solvents and enough stability for measuring a series of properties under ambient conditions. In the UV-vis absorption spectra, the electronic interaction through four-coordinate gallium atoms was suggested from the peak shifts of the polymer compared to the model compounds. Theoretical calculation of these molecules supports the extended electronic interaction through the polymer main-chain involving gallium atoms.

A stable germanone as the first isolated heavy ketone with a terminal oxygen atom.

The carbon-oxygen double bond of ketones (R(2)C=O) makes them among the most important organic compounds, but their homologues, heavy ketones with an E=O double bond (E = Si, Ge, Sn or Pb), had not been isolated as stable compounds. Their unavailability as monomeric molecules is ascribed to their high tendency for intermolecular oligomerization or polymerization via opening of the E=O double bond. Can such an intermolecular process be inhibited by bulky protecting groups? We now report that it can, with the first isolation of a monomeric germanium ketone analogue (Eind)(2)Ge=O (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), stabilized by appropriately designed bulky Eind groups, with a planar tricoordinate germanium atom. Computational studies and chemical reactions suggest the Ge=O double bond is highly polarized with a contribution of a charge-separated form (Eind)(2)Ge(+)-O(-). The germanone thus exhibits unique reactivities that are not observed with ordinary ketones, including the spontaneous trapping of CO(2) gas to provide a cyclic addition product.

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.

Air- and moisture-stable p-monothiobenzoquinones incorporated in an octaalkyl-s-hydrindacene skeleton.

p-Monothiobenzoquinones incorporated in a fused-ring octaalkyl-s-hydrindacene skeleton have been synthesized as air- and moisture-stable reddish orange crystals by the oxidation of mercaptophenol derivatives with DDQ, which have been characterized by X-ray crystallography to show a planar quinoid framework.

A planar rhombic charge-separated tetrasilacyclobutadiene.

The cyclobutadiene (CBD) molecule C(4)H(4) deviates from a high-symmetry square geometry to compensate for its antiaromatic electronic structure. Here, we report a CBD silicon analog, Si(4)(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)SiBr(3) with three equivalents of lithium naphthalenide. X-ray crystallography reveals a planar and rhombic structure of the Si(4) four-membered ring, with alternating pyramidal and planar configurations at the silicon atoms. The large (29)Si 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.

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.

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.

Pi-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.

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.