Structure determination to calculate nonlinear optical coefficients in a class of organic material

A class of molecules with intramolecular two-dimensional charge transfer upon excitation has been synthesized. It is expected that these materials should be prospective candidates for nonlinear optical (NLO) applications such as second harmonic (SHG) generation. In order to optimize the macroscopic NLO properties of the compounds, it is necessary to relate their first hyperpolarizability tensors at a molecular level to those at a crystal bulk level. This requires a complete structure determination and refinement. However, the growth of sufficiently large single crystals, which are needed for structural analysis and refinement by x-ray methods, is a time-consuming and sometimes impossible task. Even larger crystals are required for NLO measurements. Single crystals of a considerably smaller size may be effectively used for complete structural analysis by electron diffraction combined with simulation methods. In addition the structure has been confirmed structure solution from electron diffraction intensities using maximum entropy and log likelihood methods. When the crystal structure of a given compound is known, its NLO properties may be estimated using quantum-mechanical methods for calculation of the molecular nonlinearity tensor and these may be related to the macroscopic coefficients of the crystalline nonlinearity tensor. In the present work, both ab initio and semiempirical quantum-mechanical calculations were employed. [S0163-1829(99)09305-4].