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The search for organic materials for non-linear optics (NLO) has revealed
quite many compounds with high efficiency for frequency doubling of laser
light, but they suffer from a lack of transparency. This is due to the
generally accepted design of a NLO material which consists in connecting
a donor and an acceptor group by a conjugated bridge which leads to highly
coloured materials. To avoid the problems with this approach, we started
to develop compounds whose hyperpolarizability relies on the existence
of weak bonds. Here, the silatranes represent a well-studied class
of stable molecules with a weak Si-N bond. We introduced chirality as
only compounds which crystallize in non-centrosymmetric space groups give
non-vanishing hyperpolarizabilities as bulk materials, e.g. in crystalline
form, which is important for applications in devices. Starting with beta-pinene
as the source of chirality, we have prepared such a chiral silatrane and
characterized it by X-ray structure analysis.
As the properties of the Si-N bond are of central importance for the NLO property we decided to use the Bader analysis to obtain more information on the elctron density distribution in our silatrane, and to see what other atoms might contribute to the (hyper)polarizability. The Bader analysis provides a quantum chemical basis for the definition of bonds and allows to separate individual atoms in a molecule. A surprising result is that the interaction between nitrogen and silicon is rather elctrostatic and less covalent, envolving a negative charge at nitrogen and a positive one at silicon, in contrast to what one would naively expect.