A two-state, four-orbital, independent electron analysis of the first optical molecular hyperpolarizability, β, leads to the prediction that |β| maximizes at a combination of donor and acceptor strengths for a given conjugated bridge. Molecular design strategies that focus on the energetic manipulations of the bridge states are proposed for the optimization of β. The limitations of molecular classes based on common bridge structures are highlighted and more promising candidates are described. Experimental results supporting the validity of this approach are presented.

https://science.sciencemag.org/content/sci/252/5002/103.full.pdf

Approaches for Optimizing the First Electronic Hyperpolarizability of Conjugated Organic Molecules

Third-order optical susceptibilities of liquids and solids

An internal or external electric field F can drive the chemical structure, bond order alternation, and electronic structure of linear polymethine dyes from a neutral, bond-alternated, polyene-like structure, through a cyanine-like structure, and ultimately to a zwitterionic (charge-separated) bond-alternated structure. As the structure evolves under the influence of F, the linear polarizability α, the first hyperpolarizability β, and the second hyperpolarizability γ are seen to be derivatives, with respect to F, of their next lower order polarization (for α) or polarizability (for β and γ). These derivative relations provide a unified picture of the dependence of the polarizability and hyperpolarizabilities on the structure in linear polymethine dyes. In addition, they allow for predictions of structure-property relations of higher order hyperpolarizabilities.

http://science.sciencemag.org/content/sci/265/5172/632.full.pdf

A Unified Description of Linear and Nonlinear Polarization in Organic Polymethine Dyes

Stable polymeric materials for nonlinear optics: a review based on azobenzene systems

We have used the technique of static field induced second harmonic generation in liquids to determine the second order hyperpolarizability β, and the third order hyperpolarizability γ, for a wide range of conjugated and nonconjugated organic molecules. The adequacy of the Onsager local field formulation was tested as was the accuracy of the bond additivity approximation. The large π‐electron nonlinearities which are possible in substituted benzene molecules were investigated by measuring a large number of benzene derivatives and subtracting out the σ‐electron contributions.

Second and third order hyperpolarizabilities of organic molecules

The magnitude and sign (positive) of the molecular hyperpolarizability γ has been determined for several mixtures of nitrobenzene and benzene. This has allowed the testing of the adequacy of Onsager's local field factors, which are found to be valid. By comparing γ of nitrobenzene with that of nitromethane, we find that the delocalized conjugated electrons are an order of magnitude more effective in contributing to γ than are the localized nonconjugated electrons.

C. G. Bethea

Papers

Molecular hyperpolarizabilities determined from conjugated and nonconjugated organic liquids