Strategies for optimising the second-order nonlinear optical response in zwitterionic merocyanine dyes

Abstract The molecular linear and nonlinear optical (NLO) properties of a series of seven merocyanine dyes have been studied in solvents covering a broad range of polarity (dioxane to dimethylsulfoxide). The benchmark for the series was the “Right hand side” zwitterionic chromophore 1 , with a short conjugation path and 4-pyridinylidene as the donor group. Optimization strategies to improve the nonlinear response involved an extension of the conjugation path (with one or two ethenyl groups), annelation (pyridine to quinoline), variation of the solvent polarity and partial ring locking of the π-conjugated system. All chromophores have as the acceptor moiety the cyanodicyanomethylidenedihydrofuran heterocycle. Optimising the NLO response of these zwitterionic dyes by decreasing the polarity of the solvent is only possible for the parent chromophore 1 . This is because the three other successful strategies employed to further improve the second-order NLO response in polar media, result in detrimental aggregation in nonpolar media.