Double-helix pattern in a model compound of non-linear optical polymers

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 inter­digitated anti­parallel arrangement of the chromophore units.