Flattened 1D fragments of fullerene C60 that exhibit robustness toward multi-electron reduction.

Fullerenes are compelling molecular materials owing to their exceptional robustness toward multi-electron reduction. Although scientists have attempted to address this feature by synthesizing various fragment molecules, the origin of this electron affinity remains unclear. Several structural factors have been suggested, including high symmetry, pyramidalized carbon atoms, and five-membered ring substructures. To elucidate the role of the five-membered ring substructures without the influence of high symmetry and pyramidalized carbon atoms, we herein report the synthesis and electron-accepting properties of oligo(biindenylidene)s, a flattened one-dimensional fragment of fullerene C60. Electrochemical studies corroborated that oligo(biindenylidene)s can accept electrons up to equal to the number of five-membered rings in their main chains. Moreover, ultraviolet/visible/near-infrared absorption spectroscopy revealed that oligo(biindenylidene)s exhibit enhanced absorption covering the entire visible region relative to C60. These results highlight the significance of the pentagonal substructure for attaining stability toward multi-electron reduction and provide a strategy for the molecular design of electron-accepting π-conjugated hydrocarbons even without electron-withdrawing groups.

Stereoselective Synthesis and Characterization of Indenone Azine-Based Electron-Accepting π-Conjugated Systems.

Invited for the cover of this issue is the group of Aiko Fukazawa at Kyoto University. The image depicts a N-N component replacing the one originally located between the exocyclic C=C bond of a cross-conjugated dibenzofulvalene. Read the full text of the article at 10.1002/chem.202300181.

Stereoselective Synthesis and Characterization of Indenone Azine-Based Electron-Accepting π-Conjugated Systems.

Indenone azines, in which the exocyclic C=C bond in dibenzopentafulvalene is replaced by an azine moiety (C=N-N=C), have been synthesized as novel electron-accepting π-conjugated scaffolds. Structural modulation at the 7,7'-positions of indenone azines enabled stereoselective syntheses of diastereomers in which the configurations of the two C=N bonds are E,E or Z,Z. X-ray crystallographic analyses revealed that all the indenone azines exhibit high coplanarity in contrast to the twisted frameworks of dibenzopentafulvalene derivatives, resulting in the formation of densely π-stacked structures. Electrochemical measurements and quantum chemical calculations revealed the electron-accepting character of indenone azines comparable to isoindigo dyes. In particular, the intramolecular hydrogen bonds of 7,7'-dihydroxy-substituted derivatives impart enhanced electron-accepting character and significantly red-shifted photoabsorption. This study demonstrates that indenone azines represent a promising candidate as electron-accepting building blocks for optoelectronic materials.