CHARGE DENSITY STUDIES OF ORGANIC NON-LINEAR OPTICAL MATERIALS J. M. Cole^+,*, J. A. K. Howard⁺, G.J. McIntyre^*, R. C. B. Copley⁺, A. E. Goeta⁺, M. Szablewski⁺, G. H. Cross^{+ +}University of Durham, U.K.; ^*Institut Laue Langevin, France.

Recent developments in organic non-linear optical (NLO) materials over the last 20 years have triggered a prolific interest into this area of research. Second harmonic generation (SHG) measurements of organic NLO materials have already produced results which, by far, supersede those obtained from all known inorganic alternatives.

It has been found^1-2 that a deeper understanding of the nature of (a) charge transfer (CT) processes and (b) intermolecular forces in these compounds is the key to the optimization of the SHG effect. In light of these findings, we are currently pursuing an extensive program of charge density (CD) analysis since this provides an ideal route to improved understanding in both of these areas. Moreover, recent work³ has shown that one can use CD experimental results in order to determine the values of all (hyper)polarizability coefficients in the solid state. No other accurate solid state method to calculate these values exists.

A series of five zwitterionic tetracyanoquinodimethane (TCNQ) derivatives have been studied by single-crystal X-ray diffraction at 150K.⁴ More recently, neutron experiments at 20K have been performed in conjunction with more extensive single-crystal X-ray diffraction experiments at 20K using our in-house F_ddd cryodiffractometer (10-300K). Results of such 20K X/N CD studies will be the prime subject of this presentation.

References

1. Oudar J. L., Zyss J., Phys. Rev. A., (1982), 2016.

2. Garito A. F., Lalama S. J., Phys. Rev. A, 20, (1979), 1179.

3. Fkyerat A., Guelzim A., Baert F., Paulus W., Heger G., Zyss J., Perigaud A., Acta Crystallogr., B51, (1995), 197.

4. Cole J. C., Cole J. M., Cross G. H., Farsari M., Howard J. A. K., Szablewski M., Acta Crystallogr. B (in press).