Local field and aggregation dependence of the micro- and macroscopic optical non-linearity of zwitterionic molecules

Abstract

When a dipolar-conjugated molecule can lower its energy through gaining aromaticity, there can be a quite dramatic evolution of its polarisation properties dependent on the local reaction field generated in its environment. We have studied conjugated dipolar derivatives of tetracyano-p-quinodimethane (TCNQ) that would have quinoidal structure were it not for even the modest reaction fields produced in any solvent medium or polymer host environment. Measurements of dipole moment, μ, and of the first hyperpolarisability, β, have been undertaken in solvent media of varying polarity. We discuss the evolution of these properties as evidenced by solvatochromism and hyper Rayleigh scattering. We also discuss one major consequence of the planarity and high polarity in these molecules, association between them, which can seriously undermine practical implementation. Nevertheless, we refer to work demonstrating that this common problem can be overcome by suitable steric control of molecular shape and show that the prospects for achieving unprecedented electro-optic coefficients from polymer media are good.