MEASUREMENTS OF KLEINMAN-DISALLOWED HYPERPOLARIZABILITY IN CONJUGATEDCHIRAL MOLECULES

We have designed a hyper-Rayleigh scattering scheme to measure six sca lar invariants of the squared hyperpolarizability tensor beta(2). OUT theoretical approach expresses the rotational invariants of the irredu cible beta components as scalars, which eliminates the need for diffic ult frame transformations. We applied our scheme to several conjugated chiral molecules and found that there are significant Kleinman-disall owed pseudotensor contributions to their hyperpolarizability. These co mponents, along with a large optical rotation and the results of quant um-chemical calculations, indicate a handed nonplanar delocalization o f the charge-transfer system in such molecules as predicted by quantum -chemical calculations and are expected to lead to macroscopic second- harmonic generation in axially aligned polymer materials. Pseudotensor contributions to the hyperpolarizability in chiral molecules were fou nd to be as large as the vector contribution in p-nitroaniline. We qua litatively investigated the dispersion in the Kleinman-disallowed comp onents and confirmed that these components are smaller at longer wavel engths. (C) 1998 Optical Society of America.