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We analyze the structural relationship between microscopic and macroscopic tensors which characterize the linear- and nonlinear-optical properties of molecular crystals exhibiting a strong donor-acceptor intramolecular interaction, with particular reference to methyl-(2,4-dinitrophenyl)-aminopropanoate (MAP). The quasiplanar structure of the active part of these molecules results in a strong and characteristic anisotropy of the optical hyperpolarizabilities, which can be traced up to the macroscopic level when taking into account the crystal symmetry as well as the orientation of the molecules in the unit cell. The experimental data on MAP are thoroughly analyzed on this basis, and it is found that a two-dimensional model of the lowest-order hyperpolarizability tensor results in a structural relation between the macroscopic tensor components, which is in agreement with experimental data. In addition, the principal dielectric axes of this monoclinic crystal are determined by the orientation of the aromatic plane in the unit cell. The overall analysis also enables the determination of four independent components of the molecular hyperpolarizability tensor from experimental data only, and the results have been compared to those of a semiempirical intermediate neglect of differential overlap calculation. The anisotropy of this tensor reveals that the intramolecular charge transfer responsible for the large optical nonlinearity is predominantly from the amino group to the nitro group in para position, rather than towards the nitro group in ortho position. Finally the overall analysis provides a basis for discussing what should be the best orientation of the molecules in the unit cell for maximizing the crystal nonlinearity. The result is that phase-matchable nonlinear coefficients up to six times larger than in MAP could be observed in compounds with similar molecular hyperpolarizabilities but an optimum crystal structure.
Oudar et al. (Fri,) studied this question.