Over the past few decades, many studies have been conducted on ultralong C-C bonds (bond length greater than 1.7 angstrom). 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 angstrom). 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.