2-DIMENSIONAL SUPRAMOLECULAR ASSEMBLIES OF A POLYDIACETYLENE .1. SYNTHESIS, STRUCTURE, AND 3RD-ORDER NONLINEAR-OPTICAL PROPERTIES

A diacetylene monomer with a rigid backbone and capable of forming hyd rogen bonds was synthesized and found to polymerize forming two-dimens ional supramolecular assemblies. The two-dimensional structure self-as sembles when UV light generates polydiacetylene comb polymers, and hyd rogen bonds are established within molecular layers. The two-dimension al assemblies have been characterized by X-ray diffraction and infrare d spectroscopy and found to consist of highly ordered bilayers. The ma terial forms blue solid thin films which generate third-order nonlinea r optical signals and have remarkable photochemical stability to 1064 nm radiation from a Q-switched Nd:YAG laser. Upon heating to 62 degree s C, the material turns bright red reversibly while maintaining its tw o-dimensional structure, and this thermochromic process is accompanied by endothermic and exothermic signatures detected by differential sca nning calorimetry. Most importantly, however, variable temperature sum frequency generation experiments show that the third-harmonic generat ion signals retain much of their original intensity through the thermo chromic transitions. These results do not conform in a consistent mann er to both the theory of third-order effects and the previously sugges ted connection between intramolecular conjugation and optical absorpti on of polydiacetylenes. It is therefore possible that intermolecular i nteractions in these highly ordered structures play a role in defining optical properties.