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

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF₆ ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC₈ results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

Halogen⋅⋅⋅Halogen and Halogen⋅⋅⋅π Interactions Enabled Reversible Photo-oligomerization of Conjugated Dienones: Visible Light Triggered Single-Crystal-to-Single-Crystal Transformation

The synthesis of reversible oligomer/polymers is fascinating both from the perspective of the fundamental understanding as well as their applications, ranging from biomedical to self-healing smart materials. On the other hand, the reactions that occur in single-crystal-to-single-crystal (SCSC) fashion offer great details of the structure, geometry and stereochemistry of the product. However, SCSC [2+2] oligomerization is rather difficult and rare. Further, till date there are no reports for a reversible [2+2] oligomerization in SCSC fashion. In this work, four halogen-substituted acrylic dienone molecules were deliberately designed and their ability to participate in [2+2] cycloaddition reaction in solid state was studied under visible light. Despite of having the required alignment of double bonds of dienes in all four crystal structures, they were found to exhibit variable reactivities given the differences in their weak intermolecular interactions such as halogen⋅⋅⋅halogen, halogen⋅⋅⋅π and C-H⋅⋅⋅O interactions. Notably, one of these materials exhibits reversible oligomerization in a SCSC manner.

Synthesis of Anthracene-Based Cyclic π-Clusters and Elucidation of their Properties Originating from Congested Aromatic Planes

Synthesis and properties of anthracene-based cyclic π-clusters which possess two and four anthracene units are discussed. The optimal cyclization conditions were determined based on a nickel(0)-mediated reaction that afforded a cyclic anthracene dimer as the major product. Bringing two anthracene planes in close proximity in a face-to-face manner resulted in red-shifted absorption owing to the narrowing of the HOMO-LUMO gap. The cyclic anthracene dimer exhibits multi-stimuli responsiveness due to high π-congestion. For example, photoirradiation on the anthracene dimer affords its photoisomer having C-C bonds that are longer than 1.65 Å, which can undergo thermal reversion under gentle heating. This enabled mechanochromism of the photoisomer (colorless) to the original anthracene dimer (red). Photoisomerization was also observed in the crystalline state, accompanied by crystal jumping or collapsing, that is, the photosalient effect.