Molecules of the title compound, 3, 3′-[4-(4-nitrophenyldiazenyl) phenylimino] dipropionic acid, C18H18N4O6, a model compound of second-order non-linear optically active polymers, form helicoidal rows via hydrogen bonding between carboxy groups. Pairs of helices are wrapped around the common axis in a double-helix arrangement unprecedented in dicarboxylic acids. The lateral packing of the helices shows an ...