A diacetylene monomer with a rigid backbone and capable of forming hydrogen bonds was synthesized and found to polymerize forming two-dimensional supramolecular assemblies. The two-dimensional structure self-assembles when UV light generates polydiacetylene comb polymers, and hydrogen bonds are established within molecular layers. The two-dimensional assemblies have been characterized by X-ray diffraction and infrared spectroscopy and found to consist of highly ordered bilayers. The material forms blue solid thin films which generate third-order nonlinear optical signals and have remarkable photochemical stability to 1064 nm radiation from a Q-switched Nd:YAG laser. Upon heating to 62 degrees C, the material turns bright red reversibly while maintaining its two-dimensional structure, and this thermochromic process is accompanied by endothermic and exothermic signatures detected by differential scanning calorimetry. Most importantly, however, variable temperature sum frequency generation experiments show that the third-harmonic generation signals retain much of their original intensity through the thermochromic transitions. These results do not conform in a consistent manner to both the theory of third-order effects and the previously suggested connection between intramolecular conjugation and optical absorption of polydiacetylenes. It is therefore possible that intermolecular interactions in these highly ordered structures play a role in defining optical properties.