| Summary: | [Chemical model]
Second-harmonic generation (SHG) in the solid-state is restricted to materials that crystallize in non-centrosymmetric space groups. Unfortunately, the vast majority of solids crystallize in centrosymmetric space
groups and are therefore SHG-inactive. The
requirement for solid-state asymmetry is addressed in two series of salts. Acid (1), SHG-inactive due to its centrosymmetric (P1) packing, was coupled to six optically pure amines to form salts and/or complexes that, by virtue of their chiral counterion, crystallized in non-centrosymmetric
space groups. The 1064 nm output from a Nd:YAG laser produced 532 nm SHG from each of the six salts, with three of the salts producing SHG-intensities at least an order of magnitude greater than that of our standard, urea. X-ray crystallographic
analysis was carried out on five of the six salts, and an attempt was made to rationalize
each salt's SHG-intensity based on the orientation of its molecular charge-transfer axis in the unit cell and on its chromophore density. A second series of salts and/or complexes was formed by coupling acid (2) to the same set of optically pure amines as were used in the first series. The second electron-withdrawing group in (2) permits additional directions of charge-transfer, rather than the unidirectional charge-transfer along the para amine-nitro axis of (1). As such, the macroscopic second-order susceptibilities of salts made from (2) depend on the orientations of the aromatic ring as a whole and not simply the above-mentioned chargetransfer
axis. Once again, the parent acid of the series of salts crystallized in a
centrosymmetric space group {P2\lc) and was SHG-inactive, while each of the six salts
produced varied amounts of SHG. While none of the salts formed from (2) produce SHG
with the same intensity as the best of the salts formed from acid (1), four of the six salts
produced SHG-intensities that were at least twice that of urea. X-ray crystallographic
analysis was carried out on each of the salts and an attempt was once again made to
rationalize the SHG-intensities measured from each salt, this time based on the
orientations of the respective aromatic rings and on the different chromophore densities.
The SHG-intensities of the salts and complexes formed in the first two series
appear to be limited, in part, by the decrease in NLO chromophore density produced by
the salt formation. A means of increasing the chromophore density was achieved in a
third series of salts, where acids (1) and (2) were coupled to amines (3) and (4). While
the chromophore densities of the salts were indeed greater relative to those found in the
first two series of materials, the salts were no longer constrained to pack in noncentrosymmetric
space groups due to absence of optical purity in any of the acids or
bases. As a result, none of the salts packed in non-centrosymmetric space groups and
thus each salt was SHG-inactive.
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