Crystal engineering shape factors

According to Kitaigorodskii s close-packing principle, molecules will pack in a manner that minimizes void space or, in other words, in a way to maximize van der Waals interactions [20]. Hence, effects of both molecular shape and size are important in crystal engineering. For example, since racemic crystals tend to pack into centrosymmetric space groups, this statistical preference was utilized to prepare cocrystal g in Appendix 2, which is a rare molecular compound called a quasiracemate. The formation of cocrystal g was made possible due to the isosteric nature of the isoptopenyl and dimethylamino substituents, indicating that shapes rather than dipoles can be in fact the dominant factor (cf. Ref. 31). 

Logically connected to supramolecular chemistry is crystal engineering, i.e. the rational planning and execution of a crystal structure from its building blocks, molecules, or ions [97, 98], Factors such as the size, shape, or the propensity of the... 

Thus, in both cases, the molecular unit can be tailored to meet a specific requirement. A second crucial step in engineering a molecular structure for nonlinear applications is to optimize the crystal structure. For second-order effects, a noncentrosymmetrical geometry is essential. Anisotropic features, such as parallel conjugated chains, are also useful for third-order effects. An important factor in the optimization process is to shape the material for a specific device so as to enhance the nonlinear efficiency of a given structure. A thin-film geometry is normally preferred because nonlinear interactions, linear filtering, and transmission functions can be integrated into one precise monolithic structure. 

---