Chemistry, Symmetry and Optics

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Abstract
The remarkably high quadratic optically nonlinear efficiency of some organic molecular crystals can be ascribed to properly “tailored” structural features at both molecular and crystalline levels. Introduced within the framework of an oriented gas model, symmetry considerations play a foremost part, as detailed in the case of a monoclinic P21 crystal composed of planar molecular units, and lead to experimentally confirmed structural relations between macroscopic susceptibility coefficients. A hierarchy of point groups in connection with nonlinear efficiency can be established. This helps define the optimum molecular packing for each crystal point group. Different engineering strategies, such as those based on chirality, cancellation or dipole-dipole interaction forces and hydrogen bonding are defined and exemplified in specific cases. Except in the case of chiral molecular units, it is impossible to infer the crystalline symmetry type. Statistical correlations between molecular and crystalline symmetry types are nevertheless highly desirable to help guide chemical synthesis towards the right crystalline objective.