Approximate Symmetry in the Design of Non-Linear Optical Materials

Abstract

Abstract : Pairs of molecules that cocrystallize to form predictable noncentrosymmetric packing patterns have been identified. These systems, termed quasiracemates, consist of isosteric molecular components that are chemically unique, but differ in chirality. The direction of these studies centered on establishing factors that influence quasiracemate formation by inspection of crystallographic data of full sets of related crystals. Design strategies for these systems included molecular components with functional groups that varied in electronic and steric properties. Various molecular pairs were synthesized and comparative studies of crystallographic data allowed assignment of the role of molecular environment on quasiracemate formation. Unlike the enantiomeric structures, the true racemates mimic the quasiracemate structures. The alignment of quasiracemate building blocks in predictable approximately centrosymmetric assemblages stems from the propensity for organic compounds, in general, to form centrosymmetric arrangements in crystals. This topological effect was demonstrated and exploited as a tool to fabricate a wide variety of molecular systems with rigorously noncentrosymmetric crystal motifs.