Nonresonant Third-Order Nonlinear Polarizability in Linear Conjugated Molecules

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Abstract
Crude pictures relating molecular features and enhancement of nonresonant second-order hyperpolarizability have been known and successfully utilized in selection and preparation of materials for nonlinear-optical applications for over a decade. On the other hand, other than the requirement of “electron delocalization”, such a useful picture does not exist for nonresonant third-order hyperpolarizability. In pursuit of such a picture, and to obtain a better view of the range of hyperpolarizability magnitudes which might be obtainable, simple linear conjugated molecules have been synthesized and characterized. Liquid solutions of these model compounds were studied using recently developed high precision optical third harmonic generation techniques. Results of these characterizations and a new addition to the picture of structure-property relationships are reported.