Synthesis and linear/nonlinear optical properties of a new class of ‘RHS’ NLO chromophore

Abstract

Examples of a new class of zwitterionic, “right-hand side” merocyanines containing a cyanodicyanomethylidenedihydrofuran electron acceptor have been prepared. As well as allowing for the facile synthesis of these chromophores, our synthetic methodology enables considerable variation in both the donor moiety as well as the extent of conjugation between the donor and acceptor systems. As expected, all of these “right-hand side” systems are negatively solvatochromic, with the difference between λ_max (polar vs. nonpolar solvents) increasing with the extent of conjugation. In accord with expectations, hyper-Raleigh scattering (HRS) measurements confirm that molecules with the greatest conjugation pathway (for e.g.21c, 23c) have the largest first hyperpolarisabilites, β_o. In addition, our HRS evaluation indicates that the 4-quinolinylidene donor nucleus is superior to both the 4-pyridinylidene and benzothiazolylidene systems. The figures of merit, μ_(calc).β_o(measured), that we obtain for some of these compounds, are of a similar magnitude to the best “left hand side” examples reported in the literature. In order to demonstrate the versatility of our synthetic technique, representative polymer-tetherable derivatives of these compounds have been prepared, as have the corresponding TDI-based polyurethanes.