Second order nonlinearity in oligoamides

We report the synthesis and the first hyperpolarizabilities of a series of water-soluble sodium salts of oligoamides of all the three isomeric aminobenzoic acids. The first hyperpolarizabilities of the ortho and para isomers with different number of repeat units are comparable but higher than that of the corresponding meta isomer. The β value increases with the number of repeat units until a point and then decreases. This unexpected behaviour is attributed to the aggregation due to the presence of a hydrophobic backbone containing a water-soluble polar head group. Dilution experiments using the HRS confirm the formation of aggregates in water.