Structural Characteristics of a Partially Planarized Triarylmethyl Cation with Unsymmetrical Push-Pull Structure.

We report the physical properties of a new class of triarylmethyl-based carbocations containing both an electron-donating diphenyl ether moiety and an electron-accepting carbonyl group with a helical plane framework. Their unique packing patterns were clarified by X-ray crystallographic analysis, which depend on the counter anions to influence their photophysical properties in the solid states. Notably, the interactions between π-cation species and planar anion species lead to a unique panchromatic property, accompanying a near-infrared absorption with a λonset value of 1030 nm, which can be assigned to intermolecular charge transfer transition.

Direct evidence for a carbon-carbon one-electron σ-bond.

Covalent bonds share electron pairs between two atoms and make up the skeletons of most organic compounds in single, double and triple bonds. In contrast, examples of one-electron bonds remain scarce, most probably due to their intrinsic weakness. Although several pioneering studies have reported one-electron bonds between heteroatoms, direct evidence for one-electron bonds between carbon atoms remains elusive.

Crystallographic and spectroscopic studies on persistent triarylpropargyl cations.

By acid treatment of precursor alcohols, mesitylethynyl-substituted diarylmethyl cations were isolated as stable solids, X-ray structural analyses of which revealed a planar geometry. Furthermore, the ion pairs including these triarylpropargyl cations form charge-segregated assemblies in the crystal, and effective intermolecular interaction induces a red-shift of absorption in the crystal.

Structural-Change-Induced Two-Stage Redox Behavior of Pentacenebisquinodimethane with Tricolor Chromism.

Tricolor electrochromism was realized through the interconversion among the neutral (yellow), dicationic (green), and tetracationic (blue) states, even though only one kind of chromophore is generated upon oxidation. Both dicationic and tetracationic states were isolated as stable salts, and their different colors come from the effective inter-chromophore interaction only in the tetracationic state but not in the dicationic state. Despite the negligible Coulombic repulsion in the tetracationic state with four cyanine-type chromophores, pentacenebisquinodimethane undergoes stepwise two-stage two-electron oxidation when radical-stabilizing 5-(4-octyloxyphenyl)-2-thienyl groups are attached on the exomethylene bonds.

Carbon-based Biradicals: Structural and Magnetic Switching.

Sterically hindered C═C double bonds often deform into a bent or twisted geometry. Thus, many overcrowded ethylenes or anthraquinodimethanes can adopt multiple conformations, such as a folded form or a twisted form, which are interconvertible under the application of external stimuli. A perpendicular form with biradical character can also be adopted when designed to incorporate a stable carbon-based radical unit, which is involved in stimuli-responsive magnetic switching accompanied by a structural change.

Near-Infrared Electrochromic Behavior of Dibenzothiepin Derivatives Attached with Two Michler's Hydrol Blue Units.

10,11-Bis[bis(4-dimethylaminophenyl)methylene]dibenzo[bf]thiepin (1) and -oxepin (2) were prepared as stable yellow crystalline compounds, which are the cyclic analogues of electron-donating hexaarylbutadienes. Upon two-electron oxidation, they are reversibly transformed into the title dications (1 and 2 ) exhibiting near-infrared (NIR) absorptions, which were also isolated as stable salts. These redox pairs can serve as new entries into less well-explored organic NIR-electrochromic systems, and the separation of redox peaks (electrochemical bistability) was attained for 1/1 and 2/2 , thanks to drastic geometrical changes between neutral and dicationic states, as revealed by a series of X-ray analyses.

Solid-State Assembly by Chelating Chalcogen Bonding in Quinodimethane Tetraesters Fused with a Chalcogenadiazole.

In contrast to p-quinodimethane tetraesters, which undergo facile polymerization due to their diradical character, newly synthesized 1 and 2 consisting of a chalcogenadiazole fused to a p-naphthoquinodimethane tetraester are thermodynamically stable due to butterfly-shaped deformation. Such a folded molecular structure is also favorable for chalcogen bond (ChB) formation through intermolecular close contacts between a chalcogen atom (E: Se or S) and the oxygen atoms of ester groups in a crystal. The less-explored chelating-ChB through a C=O⋅⋅⋅E⋅⋅⋅O=C contact [Se⋅⋅⋅O: 2.

Flexible C-C Bonds: Reversible Expansion, Contraction, Formation, and Scission of Extremely Elongated Single Bonds.

Since carbon-carbon (C-C) covalent bonds are rigid and robust, the bond length is, in general, nearly constant and depends only on the bond order and hybrid orbitals. We report herein direct visualization of the reversible expansion and contraction of a C(sp )-C(sp ) single bond by light and heat. This flexibility of a C-C bond was demonstrated by X-ray analysis and Raman spectroscopy of hexaphenylethane (HPE)-type hydrocarbons with two spiro-dibenzocycloheptatriene units, the intramolecular [2+2] photocyclization of which and thermal cleavage of the resulting cyclobutane ring both occur in a single-crystalline phase.

Expandability of a long C-O bond by a scissor effect in acenaphthofuran.

The angle strain induced by ring annulation on a spiro-type naphthofuran weakens its C(sp3)-O bond at the opposite peri-position and endows expandability, so that quite different bond lengths [1.493(3)-1.526(6) Å] are determined for the elongated C(sp3)-O bond of the corresponding spiro-acenaphthofuran derivative upon X-ray analyses of its pseudopolymorphs.

9,10-Dihydrophenanthrene with Two Spiro(dibenzocycloheptatriene) Units: A Highly Strained Caged Hydrocarbon Exhibiting Reversible Electrochromic Behavior.

The title dispiro hydrocarbon 1 was designed as a new electrochromic material. This multiply clamped hexaphenylethane-type electron donor was prepared from 2,2'-diiodobiphenyl via biphenyl-2,2'-diylbis(dibenzotropylium) 2 salt. X-ray analysis of 1 revealed a highly strained structure as reflected by an elongated "ethane" bond [bond length: 1.