Hexaphenylethanes with an Ultralong C–C Bond: Expandability of the C–C Bond in Highly Strained Tetraarylpyracenes

Abstract

Over the past few decades, many studies have been conducted on ultralong C–C bonds (bond length greater than 1.7 Å). This highlight review discusses the molecular design of ultralong C–C bonds and their bonding properties, especially their “expandability.” In particular, the ultralong C–C bonds in tetraarylpyracene derivatives can change in length by adopting a slightly different conformation in the crystalline state. This expandability of ultralong C–C bonds could be due to the smaller decrease in bond energy at a longer interatomic distance (>1.7 Å). This interesting property can be applied to stimulus-responsive materials, as illustrated in the thermochromism of a bis(methylacridan)-substituted pyracene crystal, since the thermally induced change in bond-length modifies the HOMO–LUMO gap.