Comparison of electron diffraction data from non-linear optically active organic DMABC crystals obtained at 100 and 300 kV

During the recent past, we have synthesized a new class of molecules with i ntramolecular two-dimensional charge transfer upon excitation. The present report presents such a molecule. 2,6-bis(4-dimethylamino-benzylidene)-cyclo hexanone (DMABC), with an unusually high value of the second-order non-line ar optical (NLO) coefficients. In order to optimize the macroscopic NLO pro perties of the compounds, it is necessary to relate their first hyperpolari zability tensors at a molecular level to those at a crystal bulk level. Thi s requires a complete structure determination and refinement. However, the growth of sufficiently large single crystals, which are needed for structur al analysis and refinement by X-ray methods, is a time consuming and someti mes impossible task. We have performed a complete structural analysis by el ectron diffraction combined with simulation methods and with maximum entrop y and log likelihood statistics. In order to improve the quantitative analy sis, a 300 kV data set using an on-line CCD camera was added and the best a ttainable R-values were compared with those from 100 kV data using film emu lsions. Details regarding the maximum attainable resolution for both data s ets are discussed as well as the problems which arise from the limited dyna mic range in photographic emulsions as compared to a 14 bit CCD camera. Onc e the crystal structure was known, quantum-chemical methods were used to ca lculate non-linear optical susceptibility tensor components and these were related to the macroscopic coefficients of the crystalline quadratic non-li nearity tensor. In the present work, both ab initio and semi-empirical quan tum-chemical calculations were employed. (C) 2000 Elsevier Science B.V. All rights reserved.