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