SECOND-HARMONIC GENERATION STUDIES OF CHIRAL ORGANIC SALTS. Brian O. Patrick, Bozena Borecka-Bednarz, John R. Scheffer, James Trotter & Alan Bree. Department of Chemistry, University of British Columbia, Vancouver, B.C., Canada, V6T 1Z1.

Nonlinear optical (NLO) materials designed for second-harmonic generation (SHG) are governed by two symmetry related requirements. They are: (i) the molecule must not possess a center of symmetry; and (ii) the crystalline packing arrangement must be non-centrosymmetric. If either condition is not met the material becomes SHG-inactive. As well, electrostatic forces between adjacent, highly polar molecules often causes them to pack in an anti-parallel (head-to-tail) fashion, decreasing the bulk nonlinearity. The introduction of chirality satisfies both symmetry conditions; however this does not ensure that the NLO chromophores will orient themselves inside the unit cell so as to produce SHG efficiently. Derivatives of p-nitroaniline and 2,4-dinitroaniline (NLO materials that are SHG-inactive) with the amino acid glycine have been coupled with derivatives of L-proline and with (1S,2S)-pseudoephedrine to form chiral salts. In both series of materials the uncoupled NLO acid is SHG-inactive, while a majority of the salts have become significantly SHG-active. X-ray crystal structures of both acids and ten of the twelve salts reveal that hydrogen bonding and shielding from the counterion play important roles in discouraging the NLO chromophores from packing in the inefficient head-to-tail fashion described above.