Second-order nonlinearities and crystal structure of 2-methoxy-4'-nitro-(E)-stilbene

Second-order nonlinear optical (NLO) properties of crystalline materials depend both on the magnitude of the molecular hyperpolarizability (β) and on the orientation of the chromophores in the crystal lattice. If the molecular hyperpolarizability is to result in a nonzero macroscopic nonlinearity, the molecule must crystallize in a noncentrosymmetric space group. The design of new second-order NLO materials requires one both to address the structure-property relationships that govern β and to develop methodologies that favor noncentrosymmetric crystallization. Methods used to encourage noncentrosymmetric crystallization include the use of chiral molecules, the incorporation of functional groups that encourage asymmetric intermolecular hydrogen bonding, the synthesis of molecules with very small ground-state dipole moments but larger excited-state dipole moments, and the variation of counterion with ionic chromophore.